U.S. patent application number 15/543670 was filed with the patent office on 2018-01-25 for a controlled process for precipitating calcium carbonate.
The applicant listed for this patent is Imerys USA, Inc.. Invention is credited to Gavin BUTLER-LEE, Parvin GOLBAYANI, Nigel Victor JARVIS, Christopher PAYNTER, Ricardo M. PEREZ, Graham M. PRING, Kalena STOVALL, David TAYLOR, Douglas WICKS.
Application Number | 20180022614 15/543670 |
Document ID | / |
Family ID | 56406402 |
Filed Date | 2018-01-25 |
United States Patent
Application |
20180022614 |
Kind Code |
A1 |
PAYNTER; Christopher ; et
al. |
January 25, 2018 |
A CONTROLLED PROCESS FOR PRECIPITATING CALCIUM CARBONATE
Abstract
A process for converting gypsum into precipitated calcium
carbonate including reacting a mixture comprising gypsum and a
seed, a mineral acid, or both with at least one carbonate source,
whereby precipitated calcium carbonate is produced in the form of
calcite and/or aragonite directly without conversion from a
vaterite polymorph. Also, a process for converting gypsum into
precipitated calcium carbonate including providing a mixture
comprising i) gypsum ii) a seed, a mineral acid, or both iii) at
least one additive selected from the group consisting of ammonium
sulfate, an organic acid, or an iron material, and reacting the
mixture with at least one carbonate source to produce precipitated
calcium carbonate in the form of vaterite.
Inventors: |
PAYNTER; Christopher;
(Atlanta, GA) ; STOVALL; Kalena; (Atlanta, GA)
; WICKS; Douglas; (Johns Creek, GA) ; BUTLER-LEE;
Gavin; (St. Austell, Cornwall, GB) ; GOLBAYANI;
Parvin; (Kennesaw, GA) ; JARVIS; Nigel Victor;
(St. Austell, Cornwall, GB) ; PRING; Graham M.;
(Lostwithiel, GB) ; TAYLOR; David; (Marietta,
GA) ; PEREZ; Ricardo M.; (Cumming, GA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Imerys USA, Inc. |
Roswell |
GA |
US |
|
|
Family ID: |
56406402 |
Appl. No.: |
15/543670 |
Filed: |
January 14, 2016 |
PCT Filed: |
January 14, 2016 |
PCT NO: |
PCT/US16/13469 |
371 Date: |
July 14, 2017 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
62103425 |
Jan 14, 2015 |
|
|
|
62127687 |
Mar 3, 2015 |
|
|
|
62132385 |
Mar 12, 2015 |
|
|
|
62206594 |
Aug 18, 2015 |
|
|
|
Current U.S.
Class: |
423/431 |
Current CPC
Class: |
C01F 5/24 20130101; C01F
11/182 20130101; C01P 2006/80 20130101; C04B 14/28 20130101; C01F
11/468 20130101; C01P 2004/62 20130101; C01P 2006/12 20130101; B01D
53/502 20130101; B01D 2251/606 20130101; C01F 11/183 20130101; C01P
2004/61 20130101; D21H 17/675 20130101; B01D 2251/404 20130101;
D21H 17/67 20130101; C01P 2004/03 20130101; C01F 11/18 20130101;
C01P 2006/60 20130101; C08K 3/26 20130101; C01F 11/185 20130101;
C01F 11/181 20130101; C01P 2004/51 20130101; B01D 53/501 20130101;
C01F 5/40 20130101; C09C 1/021 20130101; C01P 2002/76 20130101;
C08K 2003/265 20130101; C01F 11/464 20130101 |
International
Class: |
C01F 11/18 20060101
C01F011/18 |
Claims
1. A process for converting gypsum into precipitated calcium
carbonate, comprising: reacting a mixture comprising gypsum and a
seed, a mineral acid, or both with at least one carbonate source to
produce precipitated calcium carbonate; wherein the reactants and
the seed and/or the mineral acid control the crystalline polymorph
and/or particle size of the precipitated calcium carbonate; and
wherein the precipitated calcium carbonate is in the form of
calcite and/or aragonite, and the calcite and/or aragonite is
produced directly from the reaction without conversion from a
vaterite polymorph.
2. The process of claim 1, wherein the carbonate source is at least
one selected from the group consisting of ammonium carbonate,
ammonium bicarbonate, ammonium carbamate, calcium carbonate,
dolomite, a metal carbonate, and carbon dioxide.
3. The process of claim 1, wherein the seed is at least one
selected from the group consisting of calcium carbonate, dolomite,
dolomitic carbonate, magnesium sulfate, magnesium hydroxide,
titania, silica, strontium and zinc oxide.
4. The process of claim 1, wherein the amount of the seed present
in the mixture is equal to or less than 10 wt % relative to the
gypsum.
5. The process of claim 1, wherein the amount of the seed present
in the mixture is at least 10 wt % relative to the gypsum.
6. The process of claim 1, wherein the mineral acid is at least one
selected from the group consisting of nitric acid, sulfuric acid,
and phosphoric acid.
7. The process of claim 1, wherein the gypsum comprises carbonate
impurities, and the mineral acid is added with a molar equivalence
that is greater than or equal to the molar equivalence of the
carbonate impurities in the gypsum.
8. The process of claim 1, wherein the mixture further comprises at
least one additive selected from the group consisting of a buffer,
a dispersant, a thickener, an anticaking agent, a defoamer, a
rheology agent, a wetting agent, a co-solvent, a brightness
enhancer or brightness dampener, and a pigment.
9. The process of claim 8, wherein the additive is citric acid,
phosphoric acid, ammonium sulfate, or sodium thiosulfate.
10. The process of claim 8, wherein the weight % of the additive
ranges from 0.5% to 10% relative to the gypsum.
11. The process of claim 1, wherein the carbonate source is a
carbonate mixture of ammonium carbonate, ammonium carbamate, and
ammonium bicarbonate, and the amount of ammonium bicarbonate is
greater than or equal to the amount of ammonium carbamate or
ammonium carbonate in the carbonate mixture.
12. The process of claim 1, wherein the carbonate source is a
carbonate mixture of ammonium carbonate, ammonium carbamate, and
ammonium bicarbonate, and the amount of ammonium bicarbonate is
less than the amount of ammonium carbamate or ammonium carbonate in
the carbonate mixture.
13. The process of claim 1, wherein the molar ratio of the gypsum
to the carbonate source ranges from 1:1.1 to 1:3.
14. The process of claim 1, wherein the carbonate source is carbon
dioxide and the carbon dioxide is reacted with ammonia or ammonium
hydroxide prior to or during reacting with the mixture comprising
gypsum and a seed, a mineral acid, or both.
15. The process of claim 1, wherein the carbonate source is an
alkali metal carbonate.
16. The process of claim 1, wherein the gypsum is filtered, sieved,
or centrifuged to remove impurities prior to the reaction.
17. The process of claim 1, further comprising processing the
precipitated calcium carbonate by at least one method selected from
the group consisting of dewatering, drying, ageing, surface
treating, size reducing, and beneficiating.
18. A process for converting gypsum into precipitated calcium
carbonate, comprising: providing a mixture comprising i) gypsum ii)
a seed, a mineral acid, or both iii) at least one additive selected
from the group consisting of ammonium sulfate, an organic acid, or
an iron material; reacting the mixture with at least one carbonate
source to produce precipitated calcium carbonate in the form of
vaterite; wherein the reactants and the seed, the mineral acid,
and/or the additive control the particle size of the vaterite.
19-28. (canceled)
29. A process for converting gypsum into precipitated calcium
carbonate, comprising: providing a mixture comprising i) gypsum and
ii) a seed, a mineral acid, or both; reacting the mixture with at
least one carbonate source to produce precipitated calcium
carbonate in the form of vaterite, such that the pH of the mixture
is less than or equal to 8; wherein the reactants, reaction
conditions and the seed and/or the mineral acid control the
particle size of the vaterite.
30-38. (canceled)
Description
CLAIM FOR PRIORITY
[0001] This PCT International Application claims the benefit of
priority of U.S. Provisional Patent Application Nos. 62/103,425,
filed Jan. 14, 2015, 62/127,687, filed Mar. 3, 2015, 62/132,385,
filed Mar. 12, 2015, and 62/206,594, filed Aug. 18, 2015, the
subject matter of all of which is incorporated herein by reference
in its entirety.
BACKGROUND OF THE DISCLOSURE
Technical Field
[0002] The present disclosure relates to a controlled process for
converting gypsum into a precipitated calcium carbonate having
desired polymorph and crystal size characteristics.
Description of the Related Art
[0003] The "background" description provided herein is for the
purpose of generally presenting the context of the disclosure. Work
of the presently named inventors, to the extent it is described in
this background section, as well as aspects of the description
which may not otherwise qualify as prior art at the time of filing,
are neither expressly or impliedly admitted as prior art against
the present disclosure.
[0004] A power plant is an industrial facility for the generation
of electric power. Each power station contains one or more
generators, a rotating machine that converts mechanical power into
electrical power by creating relative motion between a magnetic
field and a conductor. The energy source harnessed to turn the
generator varies widely. Most power stations in the world burn
fossil fuels such as coal, oil, and natural gas to generate
electricity. Fossil fuel power plants are commonly coal-fired power
stations. These coal powered plants produce heat by burning coal in
a steam boiler. The steam drives a steam turbine and generator that
then produces electricity. A biomass-fueled power plant may be
fueled by waste from sugar cane, municipal solid waste, landfill
methane, or other forms of biomass. The waste products from these
processes include ash, sulfur dioxide, nitrogen oxides and carbon
dioxide. Some of the gases can be removed from the waste stream to
reduce pollution.
[0005] Flue gas is the gas exiting to the atmosphere via a flue,
which is a pipe or channel for conveying exhaust gases from a
fireplace, oven, furnace, boiler or steam generator. Quite often,
the flue gas refers to the combustion exhaust gas produced at power
plants. The removal of waste products from flue gas, such as
SO.sub.2. is mandated by air quality regulatory agencies to reduce
the acid rain caused by coal burning. To reduce the emissions of
SO.sub.2 from coal fired power plants the post-combustion flue gas
is treated with limestone that sequesters the SO.sub.2 in the form
of gypsum (e.g. calcium sulfate).
[0006] Coal plants that use flue gas desulfurization (herein
referred to as "FGD") to reduce sulfur content set very high
specifications for the calcium content of the limestone they use.
There is a large market for calcium sulfate from FGD for reuse in
construction (e.g. Dry Wall), additionally as a solid with limited
water solubility it can be effectively isolated from the process
water and landfilled as a solid if required or converted into
calcium carbonate by known processes.
[0007] Gypsum resulting from the FGD of coal fired power plants is
an impure form of calcium sulfate. These impurities may have a
major impact on the quality and polymorph of precipitated calcium
carbonate obtained when the gypsum is reacted with carbonates.
Magnesium sulfate, for example, is a co-product with the gypsum
from the FGD process and is problematic as its high water
solubility increases the difficulty of cleaning up the FGD process
water. Overcoming these effects would be advantageous in providing
a consistent industrial calcium carbonate.
[0008] Producing precipitated calcium carbonate (PCC) by use of
calcined natural calcium carbonate is well-known and widely used in
the industry. Calcining forces formation of calcium oxide from
which a precipitated calcium carbonate is produced upon consecutive
exposure to water and carbon dioxide. The energy consumed in
calcining natural calcium carbonate is a large part of the
production cost of precipitated calcium carbonate.
[0009] In view of the foregoing, one aspect of the present
disclosure is to provide a controlled process for converting gypsum
into precipitated calcium carbonate with desired polymorph and
crystal size while avoiding the aforementioned disadvantages.
[0010] In other industries, such as drilling wells for hydrocarbon
extraction, fluids may be used for a variety of reasons, such as
lubrication and cooling of drill bit cutting surfaces while
drilling, controlling formation fluid pressure to prevent blowouts,
maintaining well stability, suspending solids in the well,
minimizing fluid loss into and stabilizing the formation through
which the well is being drilled, fracturing the formation in the
vicinity of the well, displacing the fluid within the well with
another fluid, cleaning the well, testing the well, emplacing a
packer, abandoning the well or preparing the well for abandonment,
and otherwise treating the well or the formation. These fluids
should be capable of suspending additive weighting agents (to
increase specific gravity of the mud), generally finely ground
barites (barium sulfate ore), and transport clay and other
substances capable of adhering to and coating the borehole surface.
Conventional weighting agents, however, may have undesirable
properties. For example, the weighting agents, such as iron
oxide-based weighting agents may be overly abrasive and can cause
damage or corrosion to well equipment. Similarly, barium sulfate
agents may fill cracks in the well formation, thereby inhibiting
flow of the hydrocarbon to be extracted. It may be desirable,
therefore, to provide improved additives, such as weighting agents,
with improved properties.
SUMMARY
[0011] According to a first aspect, the present disclosure relates
to a process for converting gypsum into precipitated calcium
carbonate, involving reacting a mixture comprising gypsum and a
seed, a mineral acid, or both with at least one carbonate source to
produce precipitated calcium carbonate. The reactants and the seed
and/or the mineral acid control the crystalline polymorph and/or
particle size of the precipitated calcium carbonate, the
precipitated calcium carbonate is in the form of calcite and/or
aragonite and/or vaterite, and the calcite and/or aragonite may be
produced directly from the reaction without conversion from a
vaterite polymorph.
[0012] The carbonate source is at least one selected from the group
consisting of ammonium carbonate, ammonium bicarbonate, ammonium
carbamate, calcium carbonate, dolomite, a metal carbonate, and
carbon dioxide. In one embodiment, the carbonate source is a
carbonate mixture of ammonium carbonate, ammonium carbamate, and
ammonium bicarbonate, and the amount of ammonium bicarbonate is
greater than or equal to the amount of ammonium carbamate or
ammonium carbonate in the carbonate mixture. In an alternative
embodiment, the carbonate source is a carbonate mixture of ammonium
carbonate, ammonium carbamate, and ammonium bicarbonate, and the
amount of ammonium bicarbonate is less than the amount of ammonium
carbamate or ammonium carbonate in the carbonate mixture. In the
present disclosure, the amount of carbonate source added is greater
than the amount of gypsum present in the mixture, where the molar
ratio of the gypsum to the carbonate source ranges from 1:1.1 to
1:5. In one embodiment, the carbonate source is carbon dioxide
reacted with ammonia or ammonium hydroxide prior to or during
reacting with the mixture comprising gypsum and/or a seed, a
mineral acid, or both. In an alternative embodiment, the carbonate
source is an alkali metal carbonate.
[0013] The seed is at least one selected from the group consisting
of calcium carbonate, dolomite, dolomitic carbonate, magnesium
sulfate, magnesium hydroxide, titania, silica, and zinc oxide. In
one embodiment, the amount of the seed present in the mixture is
equal to or less than 10 wt % relative to the gypsum. In an
alternative embodiment, the amount of the seed present in the
mixture is at least 10 wt % relative to the gypsum.
[0014] In the process of the present disclosure, a mineral acid may
be present in the mixture, and the mineral acid is at least one
selected from the group consisting of nitric acid, citric acid, and
phosphoric acid. The gypsum starting material may include carbonate
impurities, and, in this scenario, the mineral acid is added with a
molar equivalence that is greater than or equal to the molar
equivalence of the carbonate impurities in the gypsum.
[0015] In one embodiment, the mixture further comprises at least
one additive selected from the group consisting of a buffer, a
dispersant, a thickener, an anticaking agent, a defoamer, a
rheology agent, a wetting agent, a co-solvent, a brightness
enhancer, and a pigment. According to certain embodiments, the
additive is citric acid, phosphoric acid, ammonium sulfate, or
sodium thiosulfate. In one embodiment, the weight % of the additive
ranges from 0.5% to 10% relative to the gypsum.
[0016] The gypsum used in the present process may be purified
gypsum or unpurified gypsum. In one embodiment, the gypsum is
filtered, sieved, or centrifuged to remove impurities prior to the
reacting.
[0017] The process for converting gypsum into precipitated calcium
carbonate further includes processing the precipitated calcium
carbonate by at least one method selected from the group consisting
of dewatering, drying, ageing, surface treating, size reducing, and
beneficiating.
[0018] According to a second aspect, the present disclosure relates
to a process for converting gypsum into precipitated calcium
carbonate, including providing i) gypsum and ii) a seed, or at
least one process condition selected from the group consisting of a
reaction temperature less than 45.degree. C.; and reacting the
gypsum with at least one carbonate source to produce precipitated
calcium carbonate in the form of vaterite. The reactants and the
seed and/or process conditions control the particle size of the
vaterite.
[0019] The carbonate source is at least one selected from the group
consisting of ammonium carbonate, ammonium bicarbonate, ammonium
carbamate, calcium carbonate, dolomite, a metal carbonate, and
carbon dioxide. In an alternative embodiment, the carbonate source
is a carbonate mixture of ammonium carbonate, ammonium carbamate,
and ammonium bicarbonate, and the amount of ammonium bicarbonate is
less than the amount of ammonium carbamate in the carbonate
mixture. In yet another embodiment, the carbonate source is carbon
dioxide and the carbon dioxide is reacted with ammonia or ammonium
hydroxide prior to or during reacting with the mixture comprising
gypsum and a seed, a mineral acid, or both.
[0020] The seed is at least one selected from the group consisting
of calcium carbonate, precipitated calcium carbonate, dolomite,
dolomitic carbonate, magnesium sulfate, magnesium hydroxide,
titania, silica, and zinc oxide.
[0021] In one embodiment, the process further comprises providing
an additive before or during the step of reacting, wherein the
additive is ammonium sulfate, and ammonium sulfate is present in
the mixture from 0.5% to 10% by weight, relative to the weight of
gypsum. In yet another embodiment, the iron material is iron
oxide.
[0022] The process for converting gypsum into precipitated calcium
carbonate further includes processing the vaterite by at least one
method selected from the group consisting of dewatering, drying,
ageing, surface treating, size reducing, and beneficiating, wherein
the processing converts the vaterite into a calcite or aragonite
polymorph or a blend of polymorphs.
[0023] According to a third aspect, the present disclosure relates
to a process for converting gypsum into precipitated calcium
carbonate involving providing a gypsum; and reacting the gypsum
with at least one carbonate source to produce precipitated calcium
carbonate in the form of vaterite, such that i) the pH of the wet
vaterite is less than or equal to 8, or ii) the vaterite is dried.
In one embodiment, the carbonate source is ammonium carbamate or
ammonium carbonate.
[0024] In another aspect, the process further comprises providing
an additive before or during the step of reacting, wherein the
additive is ammonium sulfate.
[0025] According to another aspect of this disclosure, a method of
precipitating calcium carbonate may include providing a core or
seed material in solution, adding calcium sulfate to the solution,
adding a carbonate source to the solution, and precipitating
calcium carbonate onto the core or seed material.
[0026] According to another aspect, the carbonate source may
include ammonium carbonate.
[0027] According to another aspect, the core material or seed
material may be dissolvable in dilute acid, such as, for example,
hydrochloric acid (HCl).
[0028] According to another aspect, the core material or seed
material may include a weighting agent. According to some
embodiments, the core material may include iron oxide, such as, for
example hematite. According to some embodiments, the core material
may include barium sulfate. According to another aspect, the core
materials or seed material may include at least one of AgI, AgCl,
AgBr, AgCuS, AgS, Ag.sub.2S, Al.sub.2O.sub.3, AsSb, AuTe.sub.2,
BaCO.sub.3, BaSO.sub.4, BaCrO.sub.4, BaO, BeO, BiOCl,
(BiO).sub.2CO.sub.3, BiO.sub.3, Bi.sub.2S.sub.3, Bi.sub.2O.sub.3,
CaO, CaF.sub.2, CaWO.sub.4, CaCO.sub.3, (Ca,Mg)CO.sub.3, CdS, CdTe,
Ce.sub.2O.sub.3, CoAsS, Cr.sub.2O.sub.3, CuO, Cu.sub.2O, CuS,
Cu.sub.2S, CuS.sub.2, Cu.sub.9S.sub.5, CuFeS.sub.2,
Cu.sub.5FeS.sub.4, CuS.Co.sub.2S.sub.3, Fe.sup.2+Al.sub.2O.sub.4,
Fe.sub.2SiO.sub.4, FeWO.sub.4, FeAs.sub.2, FeAsS, FeS, FeS.sub.2,
FeCO.sub.3, Fe.sub.2O.sub.3, .alpha.-Fe.sub.2O.sub.3,
.alpha.-FeO(OH), Fe.sub.3O.sub.3, FeTiO.sub.3, HgS,
Hg.sub.2Cl.sub.2, MgO, MnCO.sub.3, Mn.sub.2S, MnWO.sub.4, MnO,
MnO.sub.2, Mn.sub.2O.sub.3, Mn.sub.3O.sub.3, Mn.sub.2O.sub.7,
MnO(OH), CaMoO.sub.4, MoS.sub.2, MOO.sub.2, MOO.sub.3, NbO.sub.4,
NiO, NiAs.sub.2, NiAs, NiAsS, NiS, PbTe, PbSO.sub.4, PbCrO.sub.4,
PbWO.sub.4, PbCO.sub.3, (PbCl).sub.2CO.sub.3,
Pb.sup.2+.sub.2Pb.sup.4O.sub.4, Sb.sub.2SnO.sub.5, Sc.sub.2O.sub.3,
SnO, SnO.sub.2, SrO, SrCO.sub.3, SrSO.sub.4, TiO.sub.2, UO.sub.2,
V.sub.2O.sub.3, VO.sub.2, V.sub.2O.sub.5, VaO, Y.sub.2O.sub.3,
YPO.sub.4, ZnCO.sub.3, ZnO, ZnFe.sub.2O.sub.4, ZnAl.sub.2O.sub.4,
ZnS, ZrSiO.sub.4, ZrO.sub.2, ZrSiO.sub.4, of combinations thereof.
According to a further aspect, the seed material or core material
may include two or more homogeneous domains, such as, for example,
(Ba,Sr)SO.sub.4, (Ba,Sr)CO.sub.3, or Ba(SO.sub.4,CrO.sub.3).
[0029] According to another aspect, the precipitated calcium
carbonate may be between about 10% and about 25% by weight of the
combined calcium carbonate and seed material or core material, such
as, for example, between about 10% and about 15% by weight, between
about 15% and about 20% by weight, between about 20% and about 25%
by weight, between about 12% by weight and about 18% by weight, or
between about 18% by weight and about 23% by weight of the combined
calcium carbonate and seed material or core material.
[0030] According to another aspect, the precipitated calcium
carbonate-core composition may have a specific gravity in a range
having a lower limit of about 2.6, 3, 4, 4.5, 5, or 5.5 to an upper
limit of about 20, 15, 10, 9, 8, or 7, and permutations
thereof.
[0031] The foregoing paragraphs have been provided by way of
general introduction, and are not intended to limit the scope of
the following claims. The described embodiments, together with
further advantages, will be best understood by reference to the
following detailed description taken in conjunction with the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0032] A more complete appreciation of the disclosure and many of
the attendant advantages thereof will be readily obtained as the
same becomes better understood by reference to the following
detailed description when considered in connection with the
accompanying drawings, wherein:
[0033] FIG. 1 is an illustration of a flue gas desulfurizing
apparatus.
[0034] FIG. 2 is an SEM image of a rhombic calcite, 300-500 nm.
[0035] FIG. 3 is an SEM image of a calcite and vaterite blend.
[0036] FIG. 4 is an SEM image of a rhombic calcite, .about.5
.mu.m.
[0037] FIG. 5 is an SEM image of a vaterite and aragonite
blend.
[0038] FIG. 6 is an SEM image of vaterite.
[0039] FIG. 7 is an SEM image of a vaterite and calcite blend
produced from gypsum and sodium carbonate.
[0040] FIG. 8 is an SEM image of a rhombic calcite, 300-500 nm from
calcite-seeded gypsum.
[0041] FIG. 9 is an SEM image of a rhombic calcite, 1-3 .mu.m from
dolomite seeded-gypsum.
[0042] FIG. 10 is an SEM image of a rhombic calcite, 5 .mu.m from
calcite seeded-gypsum.
[0043] FIG. 11 is an SEM image of a rhombic calcite, 300 nm-1 .mu.m
from magnesite-seeded gypsum.
[0044] FIG. 12 is an SEM image of a calcite .about.300-500 nm with
2% calcite seeding and low theoretical ammonium bicarbonate content
in ammonium carbonate.
[0045] FIG. 13 is an SEM image of a vaterite .about.300-500 nm with
no seeding and low theoretical ammonium bicarbonate content in
ammonium carbonate.
[0046] FIG. 14 is an SEM image of a calcite .about.300-500 nm with
2% calcite seeding and high theoretical ammonium bicarbonate
content in ammonium carbonate.
[0047] FIG. 15 is an SEM image of a Rhombic PCC from Crystal
Seeding with calcite seed.
[0048] FIG. 16 is an SEM image of a Rhombic PCC from Crystal
Seeding with magnesite seed.
[0049] FIG. 17 is an SEM image of a Rhombic PCC from Crystal
Seeding with dolomite seed.
[0050] FIG. 18 is an SEM image of a Non-Rhombic Polymorph from
Partial-Crystal Seeding of vaterite+aragonite from
[calcite+MgCO.sub.3 (1:3 molar ratio)] seeding.
[0051] FIG. 19 is an SEM image of a Non-Rhombic Polymorph from
Non-Crystal Seeding of vaterite+aragonite from MgCO.sub.3
seeding.
[0052] FIG. 20 is an SEM image of a Non-Rhombic Polymorph from
Non-Crystal Seeding of vaterite rhombic calcite and aragonite from
dolomitic quicklime seeding.
[0053] FIG. 21 is an SEM image of a Calcite, Vaterite from FGD
Gypsum+Ammonium Carbonate.
[0054] FIG. 22 is an SEM image of a Jamaican ore Seed GCC.
[0055] FIG. 23 is an SEM image of a Rhombic Calcite from
Calcite-Seeded Gypsum.
[0056] FIG. 24 is an SEM image of a bluegrass ore Dolomite Seed
GCC.
[0057] FIG. 25 is an SEM image of a Rhombic Calcite from
Dolomite-Seeded Gypsum.
[0058] FIG. 26 is an SEM image of a Vaterite from Pure Gypsum (No
Seeding) at Low Temp (12 C).
[0059] FIG. 27 is an SEM image of a Calcite, Vaterite Blend from
Pure Gypsum (No Seeding).
[0060] FIG. 28 is an SEM image of a Rhombic Calcite from
Calcite-Seeded Gypsum.
[0061] FIG. 29 is an SEM image of a Large Rhombic Calcite from
Calcite-Seeded Gypsum Reacted with Ammonium Carbonate Heated >46
C.
[0062] FIG. 30 is an SEM image of a Vaterite, Aragonite from Gypsum
seeded with MgCO.sub.3.
[0063] FIG. 31 is an SEM image of MgCO.sub.3 from
MgSO.sub.4+Ammonium Carbonate.
[0064] FIG. 32 is an SEM image of MgCO.sub.3 from
MgSO.sub.4+Ammonium Carbonate.
[0065] FIG. 33 is an SEM image of a Carbonate Blend (est.
.about.7.3% MgCO.sub.3, 22.4% MgCa(CO.sub.3).sub.2 and 61.8%
CaCO.sub.3) from 1:1 [CaSO.sub.4:MgSO.sub.4]+Ammonium
Carbonate.
[0066] FIG. 34 is an SEM image of MgCO.sub.3 from MgSO.sub.4+Sodium
Carbonate.
[0067] FIG. 35 is an SEM image of a Vaterite, Calcite from
Gypsum+Sodium Carbonate.
[0068] FIG. 36 is an SEM image of Gypsum, 99% (Sigma Gypsum) Used
for Most Trials (Except Where Noted).
[0069] FIG. 37 is an SEM image of a US Gypsum.
[0070] FIG. 38 is an SEM image of a US Gypsum.
[0071] FIG. 39 shows SEM images of vaterite to calcite conversion
for various feed concentrations and aging processes.
[0072] FIG. 40 shows SEM images for different feed
concentrations.
[0073] FIG. 41 shows an SEM image and an exemplary measurement of
squareness.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0074] Embodiments of the present disclosure will now be described
more fully hereinafter.
A Process for Desulfurizing Flue Gas to Form Gypsum
[0075] The present disclosure relates to a process for
desulfurizing a flue gas comprising i) scrubbing a flue gas
comprising sulfur dioxide with a SO.sub.2 sequestrating agent to
yield a suspension comprising flue gas desulfurized gypsum and an
aqueous solution comprising magnesium sulfate ii) separating the
flue gas desulfurized gypsum from the magnesium sulfate solution
iii) reacting at least one carbonate salt with the magnesium
sulfate solution to yield magnesium carbonate, which may be of the
form magnesite and iv) isolating the magnesium carbonate or
magnesite.
[0076] In each of the particular embodiments herein, it is
envisioned that the flue gas for desulfurization is obtained from a
coal power plant, an oil power plant, a natural gas power plant,
and/or a biomass fueled plant.
[0077] Calcium carbonate used in the flue gas desulfurization
process can be pure calcium carbonate or have added components,
such as magnesium. Depending on the calcium and magnesium levels,
calcium carbonate may be classified as calcite or dolomite, among
others.
[0078] Dolomite is an anhydrous carbonate mineral composed of
calcium magnesium carbonate, e.g. CaMg(CO.sub.3).sub.2. The word
dolomite is also used to describe the sedimentary carbonate rock,
which is composed predominantly of the mineral dolomite. The
mineral dolomite crystallizes in the trigonal-rhombohedral system.
It forms white, tan, gray, or pink crystals. Dolomite is a double
carbonate, having an alternating structural arrangement of calcium
and magnesium ions.
[0079] In terms of the present disclosure, magnesite may refer to
the crystalline form or the amorphous form of magnesium carbonate,
MgCO.sub.3.
[0080] In some embodiments, the SO.sub.2 sequestrating agent can
be, but is not limited to, calcium-containing carbonate minerals
including dolomite, calcite, vaterite, aragonite, ankerite,
huntite, minrecordite, barytocite, ikaite, amorphous calcium
carbonate, hydrates thereof, or combinations thereof. In a certain
embodiment, the SO.sub.2 sequestrating agent is dolomite, or a
hydrate thereof. In one embodiment, the SO.sub.2 sequestrating
agent is vaterite of high surface area.
[0081] After scrubbing with dolomite or dolomitic limestone, the
magnesium sulfate rich supernatant of a spent carbonate stream is
separated from the solid gypsum co-product by any conventional
means, such as filtration or centrifugation, and optionally
recrystallizing the solid gypsum to obtain a product with a higher
purity. Filtration methods may be, but are not limited to, vacuum
filtration. The purity of isolated desulfurized gypsum may be
variable, depending on the starting purity of the sulfur-containing
flue gas.
[0082] According to some embodiments, a magnesium sulfate solution
from the extraction of dolomite may be seeded with gypsum and/or a
carbonate (such as calcium carbonate) to control the dolomite
composition. For example, increasing the relative amount of gypsum
may decrease dolomite precipitation, whereas relatively more
ammonium carbonate may increase dolomite precipitation.
[0083] The supernatant is then treated with an appropriate
carbonate salt to yield a value added precipitated magnesium
carbonate, such as magnesite. In one embodiment, the carbonate salt
comprises a carbonate or bicarbonate anion and at least one cation
selected from the group consisting of sodium, calcium, cobalt,
copper, potassium, ammonium, chromium, iron, aluminum, tin, lead,
magnesium, silver, titanium, vanadium, zinc, lithium, nickel,
barium, strontium, and hydronium. The added carbonate salt may be
in solid form, in aqueous solution, or a suspension or slurry.
Under conditions which the carbonate salt is added as a solution or
suspension, the pH of the solution or suspension may be acidic (pH
less than 6.5), neutral (pH 6.5-7.5), or basic (pH greater than
7.5).
[0084] Magnesite is a mineral with the chemical formula MgCO.sub.3
(magnesium carbonate). Naturally occurring magnesite generally is a
trigonal-hexagonal scalenohedral crystal system. Similar to the
production of lime, magnesite can be burned in the presence of
charcoal to produce MgO, which in the form of a mineral is known as
periclase. Large quantities of magnesite are burnt to make
magnesium oxide, which is a refractory material used as a lining in
blast furnaces, kilns and incinerators. Magnesite can also be used
as a binder in flooring material. Furthermore it is being used as a
catalyst and filler in the production of synthetic rubber and in
the preparation of magnesium chemicals and fertilizers. The
isolated magnesite from the present disclosure, in particular, has
value as a filler pigment especially for applications which require
some degree of acid resistance (food packaging, synthetic marble).
In one embodiment, the isolated magnesite may be amorphous with a
surface area greater than 20 m.sup.2/g.
[0085] In one embodiment, the flue gas desulfurized gypsum is
separated from the magnesium sulfate solution by filtration or
centrifugation.
[0086] In the present disclosure, the gypsum isolated by filtration
or centrifugation can optionally be 1) used in the manufacture of
wall board or other construction materials, 2) landfilled, or 3)
converted into calcium carbonate by known processes in the prior
art or by processes disclosed hereinafter.
[0087] As to the gypsum for use in the present disclosure, there is
no particular limitation and the gypsum may be natural gypsum,
synthetic (e.g., chemically produced) gypsum, FGD gypsum, and/or
phosphogypsum. However, FGD gypsum and chemically produced gypsum
are mentioned as examples.
[0088] According to some embodiments, the FGD gypsum may be ground
or milled. The grinding or milling of the FGD gypsum may be
followed by one or more of magnetic separation, bleaching, acid
washing, or other beneficiation processes.
[0089] Pure gypsum is a soft sulfate mineral composed of calcium
sulfate dihydrate, with the chemical formula CaSO.sub.4.2H.sub.2O.
It can be used as a fertilizer, is the main constituent in many
forms of plaster and is widely mined. Gypsum resulting from the
flue gas desulfurization (FGD) of coal fired power plants is an
impure form of calcium sulfate. When converted into calcium
carbonate, these impurities may have a major impact on the quality
and polymorph of precipitated calcium carbonate obtained when the
gypsum is reacted with ammonium sulfate. Magnesium sulfate, for
example, is a co-product with the gypsum from the FGD process and
is problematic as its high water solubility increases the
difficulty of cleaning up the FGD process water. In this
disclosure, US gypsum refers to impure gypsum, which generally is
80-90% pure. This gypsum typically contains impurities such as
calcite and MgCO.sub.3 and these impurities may be in about a 1:3
ratio. Raw gypsum, or non-purified gypsum may have variable purity
depending on the source.
A Process for Converting Gypsum into Precipitated Calcium
Carbonate
[0090] As used herein, "precipitated calcium carbonate" or "PCC"
refers to a synthetically manufactured calcium carbonate material
that can be tailor-made with respect to its compositional forms,
purity, morphology, particle size, and other characteristics (e.g.
particle size distribution, surface area, cubicity, etc.) using
various precipitation techniques and methods. Precipitated calcium
carbonate thus differs from natural calcium carbonate or natural
calcium carbonate-containing minerals (marble, limestone, chalk,
dolomite, shells, etc.) or ground calcium carbonate (natural
calcium carbonate which has been ground) in terms of both methods
of manufacture as well as the various composition/characteristics
mentioned above, and which will be described more fully
hereinafter.
[0091] The method of producing precipitated calcium carbonate (PCC)
by use of calcined natural calcium carbonate is well-known and
widely used in the industry. Calcining generates calcium oxide from
which a precipitated calcium carbonate is produced upon consecutive
exposure to water and carbon dioxide. However, the energy consumed
in calcining natural calcium carbonate is a large part of the
production cost. Therefore, converting impure FGD gypsum, a bulk
byproduct from power plant energy production, or other non-calcined
gypsum sources into PCC would provide a low energy and economical
method for PCC manufacture. The present disclosure relates to a
process for converting gypsum into precipitated calcium carbonate
without a calcination step, and controlling the morphology, size,
and properties of the precipitated calcium carbonate thus
obtained.
[0092] Calcium carbonate can be precipitated from aqueous solution
in one or more different compositional forms: vaterite, calcite,
aragonite, amorphous, or a combination thereof. Generally,
vaterite, calcite, and aragonite are crystalline compositions and
may have different morphologies or internal crystal structures,
such as, for example, rhombic, orthorhombic, hexagonal,
scalenohedral, or variations thereof.
[0093] Vaterite is a metastable phase of calcium carbonate at
ambient conditions at the surface of the earth and belongs to the
hexagonal crystal system. Vaterite is less stable than either
calcite or aragonite, and has a higher solubility than either of
these phases. Therefore, once vaterite is exposed to water, it may
convert to calcite (e.g., at low temperature) or aragonite (at high
temperature: .about.60.degree. C.). The vaterite form is uncommon
because it is generally thermodynamically unstable.
[0094] The calcite form is the most stable form and the most
abundant in nature and may have one or more of several different
shapes, for example, rhombic and scalenohedral shapes. The rhombic
shape is the most common and may be characterized by crystals
having approximately equal lengths and diameters, which may be
aggregated or unaggregated. Calcite crystals are commonly
trigonal-rhombohedral. Scalenohedral crystals are similar to
double, two-pointed pyramids and are generally aggregated.
[0095] The aragonite form is metastable under ambient temperature
and pressure, but converts to calcite at elevated temperatures and
pressures. The aragonite crystalline form may be characterized by
acicular, needle- or spindle-shaped crystals, which are generally
aggregated and which typically exhibit high length-to-width or
aspect ratios. For instance, aragonite may have an aspect ratio
ranging from about 3:1 to about 15:1. Aragonite may be produced,
for example, by the reaction of carbon dioxide with slaked
lime.
[0096] In the present disclosure, the methods of producing a PCC
composition may be varied to yield different polymorphs of calcium
carbonate, such as, for example, vaterite, calcite, aragonite,
amorphous calcium carbonate, or combinations thereof. The methods
may be modified by varying one or more of the reaction rate, the pH
of the mixtures, the reaction temperature, the carbonate species
present in the reaction (e.g., ammonium carbonate, ammonium
bicarbonate), the concentration of the different carbonate species
present in the reaction (e.g., ammonium carbonate and/or ammonium
bicarbonate concentrations), the purity of the feed materials
(e.g., purity of the feed gypsum), and the concentrations of the
feed materials (e.g., gypsum and/or carbonate concentrations and/or
seeds and other additives).
Exemplary Methods
[0097] In the present disclosure, converting gypsum into
precipitated calcium carbonate is accomplished with Methods A-I, or
a combination thereof. Method and process parameter selection
enables control of the precipitated calcium carbonate structure,
such as crystalline polymorph and particle size. The following is a
general description of the Methods A-I.
Method A
[0098] The method includes:
[0099] i. Treating raw gypsum with a mineral acid including, but
not limited to, nitric, sulfuric or phosphoric acid, to consume any
unreacted calcium or magnesium carbonate remaining from the
desulfurization process. The amount of mineral acid added to the
FGD gypsum is optionally a molar equivalent of or in excess of the
amount of unreacted carbonate.
[0100] ii. Reacting the mineral acid treated FGD gypsum with
ammonium carbonate at low temperature ranging from 0-60.degree. C.
or from 8-50.degree. C., for 3-300 min or for 5-250 min, to produce
calcium carbonate in a vaterite crystal structure, calcite crystal
structure, aragonite crystal structure, amorphous calcium
carbonate, or mixtures or blends thereof.
[0101] iii. Optionally annealing the resulting calcium carbonate in
a dry or wet state to form a desired polymorph or polymorph
mixture.
Method B
[0102] The method includes:
[0103] i. Treating raw gypsum with a mineral acid including, but
not limited to, nitric, sulfuric or phosphoric acid, to consume any
unreacted calcium or magnesium carbonate remaining from the
desulfurization process. The amount of mineral acid added to the
FGD gypsum is equimolar to the amount of unreacted carbonate.
[0104] ii. Adding calcite (or aragonite) from either (ground
calcium carbonate) GCC or PCC to the mineral acid treated FGD
gypsum as a seed, and reacting with ammonium carbonate at low
temperature ranging from 0-60.degree. C. or from 8-50.degree. C.,
for 3-300 min or for 5-250 min, to produce a calcium carbonate with
different dominant morphologies (e.g., crystalline or amorphous)
from that obtained without the added calcium carbonate. The calcite
can be rhombohedral or scalenohedral.
Method C
[0105] The method includes:
[0106] i. Adding calcium carbonate to raw FGD gypsum to afford a
well-defined mixture of calcium carbonate and calcium sulfate.
[0107] ii. Reacting the FGD gypsum and calcium carbonate mixture
with ammonium carbonate at low temperature ranging from
0-60.degree. C. or from 8-50.degree. C., for 3-300 min or for 5-250
min, to produce calcium carbonate with a different dominant crystal
structure from that obtained without the added calcium
carbonate.
Method D
[0108] The method includes:
[0109] i. Preparing a seeded FGD gypsum by the process of Method B
or Method C above, where dolomite, dolomitic carbonate, magnesium
sulfate, magnesium hydroxide, titania (TiO.sub.2), silica
(SiO.sub.2), or zinc oxide (e.g., ZnO), or mixtures thereof, is
added as a seed instead of calcium carbonate.
[0110] ii. Reacting the seeded FGD gypsum with ammonium carbonate
to produce calcium carbonate in a vaterite crystal structure,
calcite crystal structure, aragonite crystal structure, amorphous
calcium carbonate, or mixtures or blends thereof.
[0111] According to some embodiments, the seed may result in a
hybrid morphology having morphologies related to both the seed
morphology and the PCC morphology.
Method E
[0112] The method includes:
[0113] i. The process of Methods A, B, C, or D where an additive is
added to the gypsum to yield other defined calcium carbonate
polymorphs and particle sizes, including but not limited to,
rhombic or scalenohedral calcite, vaterite, aragonite, amorphous
calcium carbonate, or blends thereof.
Method F
[0114] The method includes
[0115] i. The process of Methods A through E wherein the ammonium
carbonate comprises a mixture of ammonium carbonate, ammonium
carbamate and ammonium bicarbonate, such that the amount of
ammonium bicarbonate is greater than or equal to the ammonium
carbamate concentration, and upon reaction yields a calcite or
calcite-vaterite blend.
Method G
[0116] The method includes:
[0117] i. The process of Methods A through E wherein the ammonium
carbonate comprises a mixture of ammonium carbonate, ammonium
carbamate and ammonium bicarbonate, such that the amount of
ammonium bicarbonate is less than or equal to the ammonium
carbamate concentration, and upon reaction yields a vaterite or
vaterite-calcite blend.
Method H
[0118] The method includes:
[0119] i. The process of Methods A-G where the ammonium carbonate
is produced by the reaction of ammonium hydroxide with CO.sub.2.
The ammonium carbonate introduced to the gypsum slurry is of a
tailored pH, and/or excess CO.sub.2 is employed to influence the
polymorph and particle size obtained.
Method I
[0120] The method includes:
[0121] i. The process of Methods A through E where a metal
carbonate is used in place of the ammonium carbonate for reaction
with the FGD gypsum.
[0122] ii. Reacting the FGD gypsum of Methods A through E with the
metal carbonate to produce calcium carbonate in a vaterite crystal
structure, calcite crystal structure, or calcite-vaterite-aragonite
crystal structure blend.
[0123] Tables 1 and 2 below identify product characteristics
obtained from various examples of the foregoing methods.
TABLE-US-00001 TABLE 1 Reaction Surface Area Reaction Reaction Time
Product Geometry PSD (d50), D30/d70 .times. Surface Area (StA
uptake), Details Temperature (minutes) (FTIR) (SEM) microns 100
(BET), m.sup.2/g m.sup.2/g 99% pure n/a 20 vaterite spherical 5.32
68.49 11.25 10.63 gypsum + "coral" + ammonium some rhombic
carbonate (DI water) 99% pure 46.degree. C. 10 vaterite spherical
5.02 63.29 11.67 10.43 gypsum + "coral" + ammonium some rhombic
carbonate 99% pure 32-37.degree. C. 10 vaterite spherical 3.96
67.11 -- 13.51 gypsum + "coral" ammonium carbonate 99% pure n/a 95
vaterite spherical 4.72 62.50 13.9 12.89 gypsum + "coral" +
ammonium some large carbonate @ rhombic room temp 98% pure
20-22.degree. C. 60 vaterite spherical 4.02 62.50 14.17 -- gypsum +
"coral" + ammonium some rhombic carbonate @ room temp (redo of
Trial 15) 99% pure 32.degree. C. 30 vaterite spherical 3.08 65.79
n/a 14.89/ gypsum, "coral" + 14.82 2% "pure" some rhombic calcium
carbonate + 2% H2SO4 (excess) w ammonium carbonate 99% pure
35-36.degree. C. 12 calcite rhombic 4.61 60.24 4.95 4.56 gypsum, 2%
Supermite + ammonium carbonate 99% pure 32.degree. C. 10 calcite
rhombic 5.24 59.52 4.15 4.41 gypsum, 2% "pure" calcium carbonate +
ammonium carbonate 99% pure 30.degree. C. 20 calcite rhombic 5.51
59.52 3.65 3.74 gypsum, 2% "pure" calcium carbonate + ammonium
carbonate US gypsum + 36.degree. C. 10 calcite rhombic -- -- 4.65
3.03 ammonium carbonate 99% pure 32.degree. C. 30 vaterite +
spherical 3.23 59.88 n/a 24.32 gypsum, 2% small "coral" + MgCO3 w
amount needles ammonium calcite carbonate 99% pure 32-34.degree. C.
10 calcite/ spherical 4.84 60.98 n/a 6.33 gypsum, 2% vaterite
"coral" magnesite w ~9:1 ammonium carbonate 99% pure 31.degree. C.
13 vaterite, Rhombic, 2.89 51.00 n/a 13.73 gypsum, 2% calcite,
spherical dolomitic aragonite "coral" + quicklime + needles
ammonium carbonate 99% pure 30.degree. C. 25 calcite rhombic 4.91
60.98 n/a 2.62 gypsum, 2% dolomite + ammonium carbonate 99% pure
32.degree. C. 10 vaterite spherical 4.26 68.97 14.25 12.69 gypsum,
~10% "coral" + ammonium some large sulfate rhombic solution +
ammonium carbonate 99% pure 31-33.degree. C. 10 calcite rhombic -
27.1 72.46 -- 2.78 gypsum + large ammonium carbonate (amm carb temp
fluctuated >46 C. during dissolution, but cooled to 43 C. prior
to gypsum addition) Sigma 12.degree. C. 120 vaterite + spherical
13.76 71.43 -- 15.25/ gypsum + ~8% "coral" 16.04 ammonium gypsum
hydroxide w CO.sub.2 at 12 C. Sigma 29-30.degree. C. 12 vaterite
elliptical 2.66 43.29 10.71 10.66 gypsum + "coral" + sodium some
large carbonate rhombic dolomitic 35.degree. C. 40 carbonate +
various shapes: 3.5 26 32.29 n/a quicklime + 15-20% balls, other
ammonium Mg(OH)2 carbonate MgSO4 + 25-28.degree. C. 10 MgCO3
undefined 0.74 could not be could not be could not be sodium
determined determined determined carbonate
TABLE-US-00002 TABLE 2 Reaction Surface Area Reaction Reaction Time
Product Geometry PSD (d50), d30/d70 .times. Surface Area (StA
update), Details Temperature (minutes) (FTIR) (SEM) microns 100
(BET), m2/g m2/g 99% gypsum + 33.degree. C. 10 calcite large 27.1
72.46 n/a 2.78 ammonium rhombic carbonate (at elevated ammonium
carbonate temperature) 99% pure 36.degree. C. 12 calcite Rhombic
4.61 60.24 4.95 4.56 gypsum, 2% calcite + ammonium carbonate 99%
pure 34.degree. C. 10 calcite Rhombic 4.84 60.24 n/a 6.33 gypsum,
2% w small magnesite + amount of ammonium vaterite carbonate 99%
pure 35.degree. C. 25 calcite Rhombic 4.91 60.24 n/a 2.62 gypsum,
2% dolomite + ammonium carbonate
[0124] In the present disclosure, the term "reaction" may refer to
any complete or partial reaction. A partial reaction refers to any
reaction where some amount of a reagent or substrate remains in the
reaction mixture after the reaction takes place.
[0125] In regards to methods A-I, precipitation of the PCC may be
influenced or controlled by one or more of the reaction rate, pH,
the ionic strength, reaction temperature, carbonate species present
in the reaction, seed species composition, seed species
concentration, purity of the feed materials (e.g., gypsum),
concentration of the feed materials, ratio of the feed materials,
or aging of the reaction components.
[0126] In terms of methods A-I, the pH of the reacting mixture may
be controlled. In one embodiment, the reacting mixture may be
acidic (pH less than 6.5), neutral (pH 6.5-7.5), or basic (pH
greater than 7.5). In regards to methods A-I, the ionic strength of
the reacting mixture may also be controlled. The ionic strength, I,
of a solution is a function of the concentration of all ions
present in that solution.
I = 1 2 i = 1 n c i z i 2 ##EQU00001##
[0127] where c.sub.i is the molar concentration of ion i (M,
mol/L), z.sub.i is the charge number of that ion, and the sum is
taken over all ions in the solution. In one embodiment, the ionic
strength is controlled by the stoichiometry of ionizable reactants.
In another embodiment, the ionic strength is controlled by the
addition of ionic additives. These ionic additives may be a
participating reactant, a spectator ion (i.e. a non-participating
reactant), and/or a total ionic strength adjustment buffer. In
another embodiment, the ionic strength is controlled by the use of
deionized (DI) water.
[0128] In regards to methods A-I, a solvent may be added to gypsum
to form a gypsum solution, slurry or suspension prior to reacting
with the carbonate source. Suitable solvents that may be used for
forming a gypsum solution, slurry, or suspension include aprotic
polar solvents, polar protic solvents, and non-polar solvents.
Suitable aprotic polar solvents may include, but are not limited
to, propylene carbonate, ethylene carbonate, butyrolactone,
acetonitrile, benzonitrile, nitromethane, acetonitrile,
nitrobenzene, sulfolane, dimethylformamide, N-methylpyrrolidone, or
the like. Suitable polar protic solvents may include, but are not
limited to, water, nitromethane, and short chain alcohols. Suitable
short chain alcohols may include, but are not limited to, one or
more of methanol, ethanol, propanol, isopropanol, butanol, or the
like. Suitable non-polar solvents may include, but are not limited
to, cyclohexane, octane, heptane, hexane, benzene, toluene,
methylene chloride, carbon tetrachloride, or diethyl ether.
Co-solvents may also be used. In a certain embodiment, the solvent
added to gypsum is water. Gypsum is moderately water-soluble
(2.0-2.5 g/l at 25.degree. C.). Therefore, to form a gypsum
solution, enough water is added to fully dissolve all of the gypsum
prior to reaction. To form a slurry or suspension, an amount of
water is added to partially dissolve the gypsum, such that some of
the gypsum is fully dissolved and some of the gypsum remains in
solid form. In another embodiment, water is added to gypsum to form
a slurry, wherein the percent of solids in the slurry is 10-50%,
20-40%, or 30-35%. In methods A-I, the concentration of the
reacting mixture is also controlled. In a certain embodiment, the
concentration is controlled by the addition or subtraction of water
from the reacting solution, mixture, or slurry.
[0129] In terms of methods A-I, the mineral acid, ammonium
carbonate, calcite, aragonite, calcium carbonate, dolomite,
ammonium bicarbonate, ammonium carbamate, ammonium hydroxide,
carbon dioxide, or any other additive or combination thereof, may
be added to the gypsum in bulk, portion-wise, or by a slow-addition
process to control the PCC product characteristics. The rate of
addition of these components also controls the reacting mixture
concentration. In terms of methods A-I, the mineral acid, ammonium
carbonate, calcite, aragonite, calcium carbonate, dolomite,
ammonium bicarbonate, ammonium carbamate, ammonium hydroxide,
carbon dioxide, or any other additive or carbonate source or
combination thereof is added as a solution, a solid, a suspension
or slurry, a gas, or a neat liquid. In terms of adding a gas, the
gas may be bubbled into a solution to an effective concentration,
or may be used to purge or pressurize the reaction vessel until a
desired effective concentration is reached.
[0130] In one embodiment, the carbonate source is selected from the
group consisting of ammonium carbonate, ammonium bicarbonate,
ammonium carbamate, calcium carbonate, dolomite, a metal carbonate,
and carbon dioxide, wherein the metal carbonate comprises a
carbonate or bicarbonate anion and at least one cation selected
from the group consisting of sodium, calcium, cobalt, copper,
potassium, ammonium, chromium, iron, aluminum, tin, lead,
magnesium, silver, titanium, vanadium, zinc, lithium, nickel,
barium, strontium, and hydronium. In a certain embodiment, water is
added to the carbonate source to form a slurry prior to the
reaction with gypsum. The carbonate source slurry is then added to
the gypsum to give a molar ratio of reaction of gypsum:carbonate
source of 1:1.1 to 1:5, 1:1.3 to 1:2.5, or 1:1.5 to 1:2. The
carbonate selected should, after reaction with the gypsum, yield a
sulfate product with higher solubility than the calcium carbonate
generated. Therefore, the generated sulfate may be separated from
the calcium carbonate precipitate by removal of the aqueous phase
from the reaction slurry. Exemplary metal carbonates include sodium
carbonate and magnesium carbonate. The sulfate product can be
treated as needed, and used for an appropriate, separate
application. For example, if ammonium carbonate is used in the
reaction, ammonium sulfate will be generated, which can be
separated and used in applications such as fertilizer. In addition,
a portion can be fed into the starting gypsum slurry to aid in
control of the reaction rate, and consequently, control the PCC
produced. After separating the PCC slurry from the aqueous solution
to form a PCC cake, the cake can be washed with water to remove
remaining sulfate. The carbonate source can be pre-formed or
generated during the reaction. For example, CO.sub.2 may be bubbled
with ammonia gas to generate ammonium carbonate used for the
reaction. In general, ammonium carbonate exists as a mixed salt
comprising a mixture of ammonium carbonate, ammonium carbamate, and
ammonium bicarbonate. The amount of each species may depend on the
reaction conditions used to manufacture the ammonium carbonate.
Furthermore, ammonium carbamate quickly converts to ammonium
carbonate in the presence of water. In general, ammonium
bicarbonate dissolves slower and reacts slower with gypsum than
ammonium carbamate or ammonium carbonate.
[0131] The PCC production process of methods A-I of the present
disclosure aims to produce only one form of PCC. However, a small
amount of an alternative polymorph is often present, and can be
readily tolerated in most end uses. Thus, the PCC compositions
comprising mixtures of crystalline forms (e.g., aragonite and
calcite) can be readily employed in coating formulations. Even in
the case of PCC compositions predominantly comprising one form
(predominately vaterite, for example), the compositions are likely
to contain a small amount of at least one other crystal PCC
structure (e.g., calcite). As a result, the PCC compositions of the
present disclosure may optionally comprise at least one second PCC
form that differs from the main PCC form.
[0132] In some embodiments, the size, surface area, and cubicity of
the PCC products may be influenced by feed concentrations or aging
of the reaction components. For example, a lower feed concentration
may result in a larger particle size distribution. A larger
particle size distribution may have a lower surface area. Aging,
for example, may reduce the surface area of the PCC or improve the
cubicity of the PCC particles. According to some embodiments, the
aging may convert some or all of a polymorph to a different
polymorph. For example, aging may convert a vaterite phase to
calcite. According to some embodiments, including ammonium sulfate
in a gypsum slurry feed may aid in controlling the PCC polymorph
and particle size.
[0133] In certain embodiments of the present disclosure, a stage of
drying the PCC product may also be carried out in any of methods
A-I subsequent to dewatering. The drying of the product may also
contribute to the resulting crystal product polymorph. In certain
embodiments of the present disclosure, the PCC reaction product is
a first composition after the reaction, prior to drying, with a
solids content of at least 70%. The PCC reaction product may
convert to a second composition after the drying stage. The drying
stage may convert any amorphous PCC product of a first composition
to a crystalline polymorph of a second composition (and different
drying methods may make different polymorphs). The product of a
first composition may be aged and seeded. A dried product may also
be aged. Similar to the drying process, aging may also change the
polymorph composition. The reaction to form PCC, the seeding, the
drying, and the aging may all be employed in a batch process, or a
continuous process (e.g. in a tubular reactor with inline static
mixers or cascade mixers). In one embodiment, the drying is
performed at a temperature range of 30-150.degree. C. for 1-15
hours.
[0134] In some embodiments, the addition of additives or seed
materials may affect the structure of the PCC. For example, adding
citric acid to the PCC formation step may increase the surface area
of a formed PCC product. Altering the pH, such as through the use
of an acidic additive, such as an acid (e.g., phosphoric acid), may
be used to control or vary the shape, particle size, or surface
area of the PCC, and in particular to vary the morphology of a PCC.
In some embodiments, the seed composition may be used to control
the resulting PCC morphology. For example, using greater than about
5 wt % coarse scalenohedral PCC (relative to the weight of the feed
material) as a seed material may yield a larger or coarser PCC
product, and may result in a greater surface area. For example,
using less than about 5% of a fine rhombohedral PCC as a seed
material yields a PCC product with a finer crystal size within a
PCC aggregate, whereas greater than about 5% of the fine
rhombohedral PCC seed material yields a finer-sized aggregate of
the PCC produced. Further, under seeding conditions where a pure
calcite seed (where dolomite or magnesium levels are <2%) is
added to the reacting mixture, the resulting PCC product is formed
with a rhombic geometry. Similarly, seeding with magnesite or
dolomite also yields rhombic PCC.
[0135] In the present disclosure, a "hybrid structure" refers to a
PCC component bound to at least a portion of a surface of a seed
component. For example, the PCC component may be chemically bound
to the seed component, such as, for example, through ionic,
coordinate covalent (dative), or van der Waals bonds. According to
some embodiments, the PCC component may physically bond or attach
to the seed component. According to some embodiments, the PCC
component may be adsorbed or physisorbed to the seed component.
According to some embodiments, the PCC component may form a
carbonate layer over the seed component during the carbonate
addition step. For example, the PCC component may form a carbonate
layer, shell, or coating that covers at least a portion of,
majority of, or substantially all of the seed component. According
to some embodiments, the PCC component may coat, enclose, or
encapsulate substantially all of the seed component. According to
some embodiments, the hybrid structure may include a PCC component,
a seed component, and/or an interfacial component. The interfacial
component may be, for example, a boundary region between the PCC
component and the seed component. The interfacial component may
include a chemical composition containing elements of the carbonate
component and the second component. For example, when the hybrid
structure includes a calcium carbonate as the PCC component and a
magnesium carbonate as the seed component, an interfacial region
may include calcium and/or magnesium diffusing into the other
component, or a region containing a mixture of calcium carbonate
and magnesium carbonate. An interfacial region may occur, for
example, upon thermal treatment (e.g., sintering) of the hybrid
structure.
[0136] A structure described as "amorphous" herein refers to no
short or long chain order and a crystalline structure refers to at
least some level of order. Materials that may be described as
semi-crystalline may therefore be considered crystalline in the
present disclosure. The products herein are typically not 100%
crystalline or 100% amorphous or non-crystalline, but rather exist
on a spectrum between these points. In some embodiments, the PCC
may be predominantly amorphous or a combination of an amorphous
phase and a crystalline phase (such as calcite, vaterite, or
aragonite).
[0137] "Produced directly" as used herein, refers to a process
where a product polymorph (e.g. calcite) is formed without the
formation of an intermediate polymorph (e.g. vaterite) that is
isolated, and in a second independent step, converted into the
product polymorph using a processing technique mentioned herein
(e.g. wet aging). In other words, "produced directly" refers to a
process whereby the product is formed in one reaction operation. An
example of a reaction product that is not "produced directly" may
include forming an intermediate polymorph using a reaction method
referred to herein, and subsequently isolating the intermediate
polymorph, and subjecting it to a processing condition that
converts the intermediate polymorph into a different product
polymorph.
[0138] The gypsum used in the present process may be purified
gypsum or unpurified gypsum. In one embodiment, the gypsum is
filtered, sieved, or centrifuged to remove impurities prior to the
reacting.
Exemplary Processes for Converting Gypsum into Calcite
[0139] According to a another aspect, the present disclosure
relates to a process for converting gypsum into precipitated
calcium carbonate, involving reacting a mixture comprising gypsum
and a seed, a mineral acid, or both with at least one carbonate
source to produce precipitated calcium carbonate. The reactants and
the seed and/or the mineral acid control the crystalline polymorph
and/or particle size of the precipitated calcium carbonate, the
precipitated calcium carbonate is in the form of calcite, and the
calcite is produced directly from the reactants without conversion
from a vaterite polymorph.
[0140] A. In one embodiment, a calcite polymorph is produced
directly when the mineral acid is citric acid, nitric acid, or
phosphoric acid. In alternate embodiments, the mineral acid is an
acid with a pKa less than or equal to 3. In one embodiment, the
gypsum starting material may include carbonate impurities, and, in
this scenario, the mineral acid is added with a molar equivalence
that is greater than or equal to the molar equivalence of the
carbonate impurities in the gypsum. In certain embodiments, the
mineral acid may be present in an amount ranging from 0.1 wt % to
20 wt %, or 0.5 wt % to 10 wt %, or 1 wt % to 5 wt % based on the
dry weight of gypsum.
[0141] B. In method B, a calcite PCC product may be formed when the
gypsum is treated with a mineral acid as described in reference to
method A and seeded with a calcite seed. In one embodiment, less
than 25% by weight of a calcium carbonate seed is added to the
gypsum, for example less than 10%, less than 5%, or less than 1% by
weight based on the gypsum. For instance, the seed may be present
in an amount ranging from 0.1% to 25%, or 0.5 to 15%, or 1% to 10%
by weight based on the gypsum. In one embodiment, the PCC produced
has a dominant crystal polymorph consistent with calcite, with a
geometry comprising rhombic. In another embodiment, the PCC
produced has a PSD (d.sub.50) ranging from 1-28 microns. In another
embodiment, the PCC has a steepness (d.sub.30/d.sub.70.times.100)
in a range from 30-100, or 53-71, or 59-63. In another embodiment,
the PCC has a surface area (BET and/or stearic acid uptake) ranging
from 1-30, or from 3-9 m.sup.2/g. According to some embodiments,
the PCC may have a relatively steep particle size distribution, for
example, a steepness greater than about 46. According to some
embodiments, the PCC may have a relatively broad particle size
distribution, for example, a steepness less than about 40.
[0142] C. In one embodiment, a calcite polymorph PCC is produced
when the seed is a calcite seed. In some embodiments, at least 25%
by weight of calcium carbonate is added to the gypsum, at least
10%, at least 5%, at least 2%, or at least 1% by weight based on
the gypsum. For instance, the seed may be present in an amount
ranging from 1% to 25%, or 2 to 30%, or 5% to 40% by weight based
on the gypsum. In one embodiment, the PCC produced has a dominant
crystal polymorph consistent with calcite, with a geometry
comprising rhombic. In another embodiment, the PCC has a surface
area ranging from 0.1-8, or from 2.5-30 m.sup.2/g.
[0143] D. The seed may also be at least one selected from the group
consisting of dolomite, dolomitic carbonate, magnesium sulfate,
magnesium hydroxide, titania, silica, and zinc oxide. For example,
the seed may be dolomitic calcium carbonate. In some embodiments,
the PCC produced may have a crystal geometry including needle forms
of rhombic calcite, as well as other forms. Further, the PCC may
have a hybrid structure when seeded with a non-PCC seed material,
such as, for example, titania, silica, zinc oxide, or mixtures
thereof. In some embodiments, the gypsum may be seeded with
magnesium sulfate and/or magnesium hydroxide instead of calcium
carbonate.
[0144] E. The additive may be, but is not limited to a buffer, a
dispersant, a thickener, an anticaking agent, a defoamer, a
rheology agent, a wetting agent, a crystal seed, a co-solvent, a
brightness enhancer, or any agent that affects crystal
morphology/geometry of the product. Examples of additives include,
but are not limited to, citric acid, phosphoric acid, a sugar,
BaCl.sub.2, MgO, MgCO.sub.3, H.sub.2SO.sub.4, H.sub.3PO.sub.4, HCl,
various phosphates, sodium hexametaphosphate, ammonium sulfate,
sodium thiosulfate, and NO.sub.3 compounds. Examples of brightness
enhancers include, but are not limited to, fluorescent brightening
agents. According to some embodiments, when the additive is an
acid, such as, for example, citric acid, the surface area of a
resulting PCC morphology may be increased. The selection of the
acid, such as, for example, phosphoric acid, may be used in varying
amounts to control the shape, particle size, and/or surface area of
the PCC. In one embodiment, a calcite polymorph is produced when
the mixture further comprises citric acid, phosphoric acid,
ammonium sulfate, or sodium thiosulfate. In one embodiment, the
weight % of the additive ranges from 0.1% to 20%, or 0.5% to 10%,
or 1% to 6% relative to the gypsum. In some embodiments, ammonium
sulfate is added to the reaction mixture to control the reaction
rate. In some embodiments, sodium thiosulfate is added instead of
ammonium sulfate. For example, the ammonium sulfate may be added to
a gypsum slurry. The concentration of ammonium sulfate may be
varied to control the PCC polymorph type and particle size.
[0145] F. The carbonate source may be at least one selected from
the group consisting of ammonium carbonate, ammonium bicarbonate,
ammonium carbamate, calcium carbonate, dolomite, a metal carbonate,
and carbon dioxide. When a mixture of ammonium carbonate is in
solution with gypsum, and the reaction takes place in solution, the
PCC product tends to be calcite. In one embodiment, the carbonate
source is a carbonate mixture of ammonium carbonate, ammonium
carbamate, and ammonium bicarbonate, and the amount of ammonium
bicarbonate is greater than or equal to the amount of ammonium
carbamate or ammonium carbonate in the carbonate mixture. In one
embodiment, ammonium bicarbonate is added to the ammonium carbonate
(or vice versa) to generate a mixture, and the mixture is then
added to the gypsum. In another embodiment, CO.sub.2 gas is bubbled
into a slurry containing ammonium hydroxide, and the bubbling
results in the formation of ammonium carbonate, and ammonium
bicarbonate and/or ammonium carbamate in situ, and the resulting
mixture of ammonium carbonate, ammonium carbamate and ammonium
bicarbonate is then added to the gypsum.
[0146] In certain embodiments, the reaction temperature may be
equal to or greater than 30.degree. C. and the polymorph formed is
calcite.
[0147] In other embodiments, the gypsum is a natural gypsum and the
polymorph formed is a calcite and/or vaterite product. For
instance, the calcium carbonate product may comprise at least 30%
calcite and vaterite.
[0148] In certain embodiments, the reaction is carried out at a pH
equal to or greater than 10 and the polymorph formed is
calcite.
[0149] H. In one embodiment, the carbonate source is carbon dioxide
and the carbon dioxide is reacted with ammonia or ammonium
hydroxide prior to or during reacting with the mixture comprising
gypsum and a seed, a mineral acid, or both. The pH of the mixture
is tailored to a pH equal to or greater than 10, which may result
in the formation of a calcite polymorph. In the process, the pH can
be adjusted by adjusting the concentration of the carbon dioxide or
the ammonia. In the present disclosure, the amount of carbonate
source added may be greater than the amount of gypsum present in
the mixture, where the molar ratio of the gypsum to the carbonate
source ranges from about 1:1.1 to 1:5, 1:1.3 to 1:2.5, or from
about 1:1.5 to 1:2. The carbon dioxide can be pure carbon dioxide
gas, flue gas containing 15-90% carbon dioxide gas, or flue gas
with enriched carbon dioxide gas (e.g., greater than 90% CO.sub.2).
In one embodiment, the FGD gypsum is mixed with ammonia prior to
the addition of CO.sub.2. Ammonium hydroxide may be pre-formed by
addition of ammonia to water, and the ammonium hydroxide may be fed
into slurried gypsum prior to addition of CO.sub.2. Alternatively,
ammonium carbonate may be fully generated, then introduced to the
slurried gypsum for reaction. Reacting ammonium hydroxide with
CO.sub.2 at room temp to 40.degree. C. gives a mixture of ammonium
carbonate and ammonium bicarbonate, which may react as anticipated
with seeded gypsum to produce calcite. In presence of gypsum,
ammonium hydroxide with CO.sub.2 yields a mixture of ammonium
carbonate and ammonium bicarbonate, which begins to react with
gypsum and yield ammonium sulfate during ammonium bicarbonate and
ammonium carbonate generation. In an alternative embodiment,
ammonia and CO.sub.2 are first mixed and reacted, and then the
reacted mixture is mixed with the FGD gypsum. In one embodiment,
the CO.sub.2 is added by bubbling into solution. In an alternative
embodiment, CO.sub.2 is added as dry ice. During the preparation,
the nucleation rate and crystal size of calcium carbonate can be
controlled through controlling of the reaction time and
temperature. In a certain embodiment, the carbon dioxide, or carbon
dioxide equivalent is equimolar or greater to the gypsum reactant.
The reaction time may be 0.2-10 hours, or 0.5-3 hours, and the
temperature may be in a range from 8-98.degree. C., or from
10-90.degree. C. According to some embodiments, a
CO.sub.2-containing gas, such as a flue gas, may be continuously
added during the reaction period with the ammonia. According to
some embodiments, the addition of a CO.sub.2-containing gas may be
stopped during the reaction period with the ammonia. When the
CO.sub.2 addition is stopped, it may be optionally restarted prior
to a filtration step. According to some embodiments, the reaction
products may be stored before separating the carbonate from
ammonium sulfate to allow for ripening of the reaction products.
The ripening could be performed with or without the addition of
CO.sub.2 during the storage. According to some embodiments, the
CO.sub.2 may be added after the conversion to calcium carbonate and
ammonium sulfate. According to some embodiments, the CO.sub.2 may
be added after isolating the calcium carbonate. According to some
embodiments, the introduction of CO.sub.2, such as, for example,
after isolating the calcium carbonate or after a reslurrying step,
may be used to control the particle size of the calcium
carbonate.
[0150] I. In an alternative embodiment, the carbonate source is an
alkali metal carbonate, which is sodium, potassium, cesium,
lithium, rubidium, francium. In one embodiment, the metal of the
metal carbonate is a monovalent ion (e.g., an alkali metal). In one
embodiment, the PCC produced may have a crystal geometry including
needle forms of aragonite, rhombic calcite, and other forms. In one
embodiment, the metal of the metal carbonate is a divalent ion,
such as, for example, magnesium, strontium, beryllium, barium, or
radium. Magnesium carbonate may also be yielded under conditions
where magnesium cation is present in the gypsum or in the metal
carbonate (e.g. magnesium carbonate, dolomite, etc.).
[0151] The process for converting gypsum into precipitated calcium
carbonate further includes processing the precipitated calcium
carbonate by at least one method selected from the group consisting
of dewatering, drying, ageing, surface treating, size reducing, and
beneficiating. Processing PCC with a calcite polymorph may change
the aforementioned properties of the calcite. However, the
processing does not convert calcite into another polymorph.
Exemplary Processes for Converting Gypsum into Aragonite
[0152] According to a first aspect, the present disclosure relates
to a process for converting gypsum into precipitated calcium
carbonate, involving reacting a mixture comprising gypsum and a
seed, a mineral acid, or both with at least one carbonate source to
produce precipitated calcium carbonate. The reactants and the seed
and/or the mineral acid control the crystalline polymorph and/or
particle size of the precipitated calcium carbonate, the
precipitated calcium carbonate is in the form of aragonite, and the
aragonite is produced directly from the reaction without conversion
from a vaterite polymorph.
[0153] C. In one embodiment, an aragonite polymorph PCC is produced
when the seed is aragonite or a blended seed. The blended seed may
comprise magnesium carbonate and calcium carbonate, for instance,
blended at a ratio of 5:1 or more preferably 3:1. In some
embodiments, at least 10% by weight of seed is added to the gypsum,
at least 5%, at least 2%, or at least 1% by weight based on the
gypsum. For instance, the seed may be present in an amount ranging
from 1% to 25%, or 2 to 30%, or 5% to 40% by weight based on the
gypsum. In one embodiment, the PCC produced has a dominant crystal
polymorph consistent with aragonite. In another embodiment, the PCC
has a surface area (BET and/or stearic acid uptake) ranging from 2
to 30 m.sup.2/g, or from 5 to 15 m.sup.2/g, or from 2 to 8
m.sup.2/g.
[0154] D. The seed may also be at least one selected from the group
consisting of dolomitic carbonate, magnesium sulfate, magnesium
hydroxide, titania, silica, strontium and zinc oxide. For example,
the seed may be dolomitic calcium carbonate of high magnesium
content. Further, the PCC may have a hybrid structure when seeded
with a non-PCC seed material, such as, for example, titania,
silica, zinc oxide, strontium or mixtures thereof. In some
embodiments, the gypsum may be seeded with magnesium sulfate and/or
magnesium hydroxide instead of a carbonate.
[0155] E. The additive may be, but is not limited to a buffer, a
dispersant, a thickener, an anticaking agent, a defoamer, a
rheology agent, a wetting agent, a crystal seed, a co-solvent, a
brightness enhancer, or any agent that affects crystal
morphology/geometry of the product. Examples of additives include,
but are not limited to, citric acid, phosphoric acid, a sugar,
BaCl.sub.2, MgO, MgCO.sub.3, H.sub.2SO.sub.4, H.sub.3PO.sub.4 HCl,
various phosphates, sodium hexametaphosphate, ammonium sulfate,
sodium thiosulfate, and NO.sub.3 compounds. Examples of brightness
dampeners include, but are not limited to, Fe.sub.2O.sub.3, MnO,
and Pb.sup.+2. According to some embodiments, when the additive is
an acid, such as, for example, citric acid, the surface area of a
resulting PCC morphology may be increased. The selection of the
acid, such as, for example, phosphoric acid, may be used in varying
amounts to control the shape, particle size, and/or surface area of
the PCC. In one embodiment, the weight % of the additive ranges
from 0.1% to 25%, or 0.5 to 15%, or 1% to 10% relative to the
gypsum. In some embodiments, ammonium sulfate is added to the
reaction mixture to control the reaction rate. In some embodiments,
sodium thiosulfate is added instead of ammonium sulfate. For
example, the ammonium sulfate may be added to a gypsum slurry. The
concentration of ammonium sulfate may be varied to control the PCC
polymorph type and particle size.
[0156] F. The carbonate source may be at least one selected from
the group consisting of ammonium carbonate, ammonium bicarbonate,
ammonium carbamate, calcium carbonate, dolomite, a metal carbonate,
and carbon dioxide. In one embodiment, ammonium bicarbonate is
added to the ammonium carbonate (or vice versa) to generate a
mixture, and the mixture is then added to the gypsum. In another
embodiment, CO.sub.2 gas is bubbled into a slurry containing
ammonium hydroxide, and the bubbling results in the formation of
ammonium carbonate, and/or ammonium bicarbonate and/or ammonium
carbamate in situ, and the resulting mixture of ammonium carbonate,
ammonium carbamate and ammonium bicarbonate is then added to the
gypsum.
[0157] H. In one embodiment, the carbonate source is carbon dioxide
and the carbon dioxide is reacted with ammonia or ammonium
hydroxide prior to or during reacting with the mixture comprising
gypsum and a seed, a mineral acid, or both. In the process, the pH
can be adjusted by adjusting the concentration of the carbon
dioxide or the ammonia. In the present disclosure, the amount of
carbonate source added may be greater than the amount of gypsum
present in the mixture, where the molar ratio of the gypsum to the
carbonate source ranges from about 1:1.1 to 1:5, 1:1.3 to 1:2.5, or
from about 1:1.5 to 1:2. The carbon dioxide can be pure carbon
dioxide gas, flue gas containing 15-90% carbon dioxide gas, or flue
gas with enriched carbon dioxide gas (e.g., greater than 90%
CO.sub.2). In one embodiment, the FGD gypsum is mixed with ammonia
prior to the addition of CO.sub.2. Ammonium hydroxide may be
pre-formed by addition of ammonia to water, and the ammonium
hydroxide may be fed into slurried gypsum (or vice versa) prior to
addition of CO.sub.2. Alternatively, ammonium carbonate or ammonium
carbonate with ammonium bicarbonate may be fully generated, then
introduced to the slurried gypsum for reaction. In the presence of
gypsum, ammonium hydroxide with CO.sub.2 yields ammonium
bicarbonate and ammonium carbonate, which begins to react with
gypsum and yield ammonium sulfate during ammonium bicarbonate and
ammonium carbonate generation. In an alternative embodiment,
ammonia and CO.sub.2 are first mixed and reacted, and then the
reacted mixture is added to the FGD gypsum. In one embodiment, the
CO.sub.2 is added by bubbling into solution. In an alternative
embodiment, CO.sub.2 is added as dry ice. During the preparation,
the nucleation rate and crystal size of calcium carbonate can be
controlled through control of the reaction time and temperature. In
a certain embodiment, the carbon dioxide, or carbon dioxide
equivalent is equimolar or greater to the gypsum reactant. The
reaction time may be 0.2-10 hours, or 0.5-3 hours, and the
temperature may be in a range from 8-90.degree. C., or from
10-98.degree. C. According to some embodiments, a
CO.sub.2-containing gas, such as a flue gas, may be continuously
added during the reaction period with the ammonia. According to
some embodiments, the addition of a CO.sub.2-containing gas may be
stopped during the reaction period with the ammonia. When the
CO.sub.2 addition is stopped, it may be optionally restarted prior
to a filtration step. According to some embodiments, the reaction
products may be stored before separating the carbonate from
ammonium sulfate to allow for ripening of the reaction products.
The ripening could be performed with or without the addition of
CO.sub.2 during the storage. According to some embodiments, the
CO.sub.2 may be added after the conversion to calcium carbonate and
ammonium sulfate. According to some embodiments, the CO.sub.2 may
be added after isolating the calcium carbonate. According to some
embodiments, the introduction of CO.sub.2, such as, for example,
after isolating the calcium carbonate or after a reslurrying step,
may be used to control the particle size of the calcium
carbonate.
[0158] I. In an alternative embodiment, the carbonate source is an
alkali metal carbonate, which is sodium, potassium, cesium,
lithium, rubidium, francium. In one embodiment, the metal of the
metal carbonate is a monovalent ion (e.g., an alkali metal). In one
embodiment, the PCC produced may have a crystal geometry including
needle forms of aragonite, and other forms. In one embodiment, the
metal of the metal carbonate is a divalent ion, such as magnesium.
Magnesium carbonate may also be yielded under conditions where
magnesium cation is present in the gypsum or in the metal carbonate
(e.g. magnesium carbonate, dolomite, etc.).
[0159] The process for converting gypsum into precipitated calcium
carbonate further includes processing the precipitated calcium
carbonate by at least one method selected from the group consisting
of dewatering, drying, ageing, surface treating, size reducing, and
beneficiating.
Exemplary Processes for Converting Gypsum into Vaterite
[0160] According to a second aspect, the present disclosure relates
to a process for converting gypsum into precipitated calcium
carbonate, including providing i) gypsum and ii) a seed, or at
least one process condition selected from the group consisting of a
reaction temperature between 10 and 60.degree. C., or between 18 to
45.degree. C., or more preferably 25 to 35.degree. C.; and reacting
the gypsum with at least one carbonate source to produce
precipitated calcium carbonate in the form of vaterite. The
reactants and the seed and/or process conditions control the
particle size of the vaterite.
[0161] The PCC produced has a dominant crystal polymorph consistent
with vaterite, with a geometry comprising spherical "coral" as well
as some rhombic. In some embodiments, the vaterite PCC may have a
flower-shaped geometry, a rose-shaped geometry, a needle-shaped
geometry, a ball-shaped or spherical-shaped geometry, or a
hexagonal geometry. The geometry or structure of the vaterite may
be varied by varying one or more of the reaction rate, pH, reaction
temperature, or purity of the feed gypsum. For example, a feed of
high-purity gypsum (e.g., 99%) yields vaterite with a ball-shaped
geometry, whereas co-generating an ammonia-based carbonate
precursor and the PCC tends to yield a more flower-shaped vaterite.
According to some embodiments, ball-shaped vaterite PCC is produced
with pre-formed ammonia-based carbonates and lower purity gypsum
during the PCC reaction. The PCC produced has a PSD (d.sub.50)
ranging from 2.0-12.0, or from 3.0-7.0 microns. In another
embodiment, the PCC has a steepness (d.sub.30/d.sub.70.times.100)
ranging from 30-100, or 50-100, or 56-83, or 59-71. In another
embodiment, the PCC has a surface area (BET and/or stearic acid
uptake) ranging from 5-75 m.sup.2/g, or from 5-20 m.sup.2/g, or
from 7-15 m.sup.2/g.
[0162] In other embodiments, a reaction temperature of less than
30.degree. C. results in a vaterite polymorph.
[0163] E. The additive may be, but is not limited to a buffer, a
dispersant, a thickener, an anticaking agent, a defoamer, a
rheology agent, a wetting agent, a crystal seed, a co-solvent, a
brightness enhancer or dampener, or any agent that affects crystal
morphology/geometry of the product. Examples of additives include,
but are not limited to, citric acid, phosphoric acid, a sugar,
BaCl.sub.2, MgO, MgCO.sub.3, H.sub.2SO.sub.4, H.sub.3PO.sub.4 HCl,
various phosphates, sodium hexametaphosphate, ammonium sulfate,
sodium thiosulfate, and NO.sub.3 compounds. Examples of brightness
dampeners include, but are not limited to, Fe.sub.2O.sub.3, MnO,
and Pb.sup.+2 and other lead compounds. According to some
embodiments, when the additive is an acid, such as, for example,
citric acid, the surface area of a resulting PCC morphology may be
increased. The selection of the acid, such as, for example,
phosphoric acid, may be used in varying amounts to control the
shape, particle size, and/or surface area of the PCC. In one
embodiment, the additive is ammonium sulfate, and ammonium sulfate
is present in the mixture from 0.5 wt/vol % to 50 wt/vol %, or 2
wt/vol % to 35 wt/vol %, or 4% to 20% by weight. In some
embodiments, ammonium sulfate is added to the reaction mixture to
control the reaction rate. In some embodiments, sodium thiosulfate
is added instead of ammonium sulfate. For example, the ammonium
sulfate may be added to a gypsum slurry. The concentration of
ammonium sulfate may be varied to control the PCC polymorph type
and particle size. Alternatively, the organic acid is citric acid,
and the wt % of citric acid in the mixture is greater than or equal
to 0.1%, greater than or equal to 10%, or greater than or equal to
25% relative to the weight of gypsum.
[0164] G. The carbonate source is at least one selected from the
group consisting of ammonium carbonate, ammonium bicarbonate,
ammonium carbamate, calcium carbonate, dolomite, a metal carbonate,
and carbon dioxide. In one embodiment, the carbonate source is a
carbonate mixture of ammonium carbonate, ammonium carbamate, and
ammonium bicarbonate, and the amount of ammonium bicarbonate is
less than the amount of ammonium carbamate or ammonium carbonate in
the carbonate mixture. In general, under conditions in which the
reaction between the carbonate source and gypsum takes place in a
slurry, the PCC yielded is vaterite. In one embodiment, ammonium
bicarbonate is added to the ammonium carbonate (or vice versa) to
generate a mixture, and the mixture is then added to the gypsum. In
another embodiment, CO.sub.2 gas is bubbled into a slurry
containing ammonium hydroxide, and the bubbling results in the
formation of ammonium carbonate and/or ammonium carbamate, and
ammonium bicarbonate in situ, and the resulting mixture of ammonium
carbonate, ammonium carbamate and ammonium bicarbonate is then
added to the gypsum.
[0165] H. In yet another embodiment, the carbonate source is carbon
dioxide and the carbon dioxide is reacted with ammonia or ammonium
hydroxide prior to or during reacting with the mixture comprising
gypsum and a seed, a mineral acid, or both. The pH of the mixture
is tailored to a pH of less than 10, which results in formation of
a vaterite polymorph. The carbon dioxide can be pure carbon dioxide
gas, flue gas containing 15-90% carbon dioxide gas, or flue gas
with enriched carbon dioxide gas (e.g., greater than 90% CO.sub.2).
In one embodiment, the FGD gypsum is mixed with ammonia prior to
the addition of CO.sub.2. Ammonium hydroxide may be pre-formed by
addition of ammonia to water, and the ammonium hydroxide may be fed
into slurried gypsum prior to addition of CO.sub.2. In an
alternative embodiment, ammonia and CO.sub.2 are first mixed and
reacted, and then the reacted mixture is added to the FGD gypsum.
Reacting ammonium hydroxide with CO.sub.2 at room temp or up to at
least 40.degree. C. gives a mixture of ammonium carbonate and
ammonium bicarbonate, which reacts as anticipated with unseeded
gypsum to give vaterite. Alternatively, ammonium carbonate may be
fully generated, then introduced to the slurried gypsum for
reaction. In presence of gypsum, ammonium hydroxide with CO.sub.2
yields ammonium carbonate and ammonium bicarbonate, which begins to
react with gypsum and yield ammonium sulfate during ammonium
bicarbonate and ammonium carbonate generation. In one embodiment,
the CO.sub.2 is added by bubbling into solution. In an alternative
embodiment, CO.sub.2 is added as dry ice. During the preparation,
the nucleation rate and crystal size of calcium carbonate can be
controlled through controlling of the reaction time and
temperature. In a certain embodiment, the carbon dioxide, or carbon
dioxide equivalent is equimolar or greater to the gypsum reactant.
The reaction time is 0.2-10 hours, or 0.5-3 hours, and the
temperature is in a range from 8-90.degree. C., or from
10-98.degree. C. According to some embodiments, a
CO.sub.2-containing gas, such as a flue gas, may be continuously
added during the reaction period with the ammonia. According to
some embodiments, the addition of a CO.sub.2-containing gas may be
stopped during the reaction period with the ammonia. When the
CO.sub.2 addition is stopped, it may be optionally restarted prior
to a filtration step. According to some embodiments, the reaction
products may be stored before separating the carbonate from
ammonium sulfate to allow for ripening of the reaction products.
The ripening could be performed with or without the addition of
CO.sub.2 during the storage. According to some embodiments, the
CO.sub.2 may be added after the conversion to calcium carbonate and
ammonium sulfate. According to some embodiments, the CO.sub.2 may
be added after isolating the calcium carbonate. According to some
embodiments, the introduction of CO.sub.2, such as, for example,
after isolating the calcium carbonate or after a reslurrying step,
may be used to control the particle size of the calcium
carbonate.
[0166] I. In an alternative embodiment, the carbonate source is an
alkali metal carbonate, such as sodium carbonate, potassium,
cesium, lithium, rubidium, or francium based carbonate. In one
embodiment, the metal of the metal carbonate is a monovalent ion
(e.g., an alkali metal). In one embodiment, the PCC produced may
have a crystal geometry including, for example, spherical vaterite.
In one embodiment, the metal of the metal carbonate is a divalent
ion, such as magnesium, strontium, beryllium, barium, or radium.
Magnesium carbonate may also be yielded under conditions where
magnesium cation is present in the gypsum or in the metal carbonate
(e.g. magnesium carbonate, dolomite, etc.).
[0167] The process for converting gypsum into precipitated calcium
carbonate further includes processing the vaterite by at least one
method selected from the group consisting of dewatering, drying,
ageing, surface treating, size reducing, and beneficiating, wherein
the processing converts the vaterite into a calcite or aragonite
polymorph. In one embodiment, seeding a vaterite PCC with ground
calcium carbonate (GCC) in water with a pH ranging from 4 to 9, or
5 to 8, or 6 to 7.7 converts the vaterite into a calcite polymorph.
For this conversion, a GCC seed is provided in 0.1 wt % to 25 wt %,
or 0.5 wt % to 15 wt %, or 1 wt % to 10 wt % relative to the
vaterite, and the temperature is maintained at less than 35.degree.
C. Further, vaterite can be converted to a calcite polymorph by
adding ammonium hydroxide or other bases, such as sodium hydroxide,
potassium hydroxide or calcium hydroxide to a mixture of vaterite
and water, and adjusting the pH to be greater than or equal to 10,
or greater than or equal to 10.5, or greater than or equal to 11.
For this conversion process, the temperature is maintained at a
temperature in a range between about 23.degree. C. and about
80.degree. C., between about 25.degree. C. and about 50.degree. C.,
or between about 30.degree. C. and about 40.degree. C. According to
some embodiments, the conversion process temperature may be
maintained for a time period in a range from about 30 minutes to
about 12 hours. According to some embodiments, substantially all of
the vaterite may be converted into calcite. In one embodiment, the
addition of 2 to 10%, 4 to 9%, or 6 to 8% of ammonium bicarbonate
to vaterite PCC also aids in conversion to a calcite polymorph. In
terms of converting vaterite into calcite, the vaterite and water
mixture may be in the form of a wet cake or a slurry. In contrast,
drying of the vaterite stabilizes the vaterite polymorph and
prevents it from converting to calcite.
[0168] Alternatively, the vaterite may be stabilized by the
presence of ammonium sulfate, and/or the presence of iron
materials, or the absence of ammonium bicarbonate and ammonium
sulfate. For instance, the vaterite may be stabilized by the
presence of ammonium sulfate even when ammonium bicarbonate is
present such that the ammonium bicarbonate to ammonium sulfate
ratio is between [0:100] and [1:15]. In certain embodiments, the
vaterite may be stabilized for 1 day, or 1 week, or a month or
longer.
[0169] According to a third aspect, the present disclosure relates
to a process for converting gypsum into precipitated calcium
carbonate involving providing a gypsum, and reacting the gypsum
with at least one carbonate source to produce precipitated calcium
carbonate in the form of vaterite, such that i) the pH of the wet
vaterite is less than or equal to 8, ii) the vaterite is dried.
Precipitated Calcium Carbonate (PCC)--Morphologies and
Properties
[0170] In the present disclosure, the crystalline content of a PCC
composition may be readily determined through visual inspection by
use of, for example, a scanning electron microscope or by x-ray
diffraction. Such determination may be based upon the
identification of the crystalline form and is well known to those
of skill in the art.
[0171] The PCC compositions of the present disclosure are
characterized by a single crystal polymorph content of greater than
or equal to 30% by weight relative to the total weight of the
composition, greater than or equal to 40% by weight, greater than
or equal to 60% by weight, greater than or equal to about 80% by
weight, or greater than or equal to about 90% by weight.
[0172] The PCC compositions may also be characterized by their
particle size distribution (PSD). As used herein and as generally
defined in the art, the median particle size (also called d.sub.5)
is defined as the size at which 50 percent of the particle weight
is accounted for by particles having a diameter less than or equal
to the specified value.
[0173] The PCC compositions may have a do in a range from about 0.1
microns to about 30 microns, for example, from about 2 microns to
about 14 microns, from about 2 to about 8 microns, from about 1
micron to about 4 microns, or from about 0.1 micron to about 1.5
microns. The d.sub.50 may vary with the morphology of the PCC. For
example, calcite PCC may have a d.sub.50 in a range from about 1 to
about 28 microns, such as, for example, from about 1 to about 2
microns, from about 1 to about 5 microns, from about 2 to about 4
microns, or from about 4 to about 6 microns. Vaterite PCC may have
a d.sub.50 in a range from about 0.1 microns to about 30 microns,
such as, for example, from about 0.1 to about 2 microns, from about
1 to about 5 microns, or from about 2 to about 8 microns.
[0174] According to some embodiments, between about 0.1 percent and
about 60 percent of the PCC particles are less than about 2 microns
in diameter. In other embodiments, between about 55 percent and
about 99 percent of the PCC particles are less than 2 microns in
diameter. According to some embodiments, less than about 1 percent
of the PCC particles are greater than 10 microns in diameter, such
as, for example, less than 0.5 percent of the PCC particles are
greater than 10 microns in diameter, or less than 0.1% of the PCC
particles are greater than 10 microns in diameter.
[0175] The PCC compositions may be further characterized by their
aspect ratio. As used herein, aspect ratio refers to a shape factor
and is equal to the largest dimension (e.g. length) of a particle
divided by the smallest dimension of the particle orthogonal to it
(e.g. width). The aspect ratio of the particles of a PCC
composition may be determined by various methods. One such method
involves first depositing a PCC slurry on a standard SEM stage and
coating the slurry with platinum. Images are then obtained and the
particle dimensions are determined, using a computer based analysis
in which it is assumed that the thickness and width of the
particles are equal. The aspect ratio may then be determined by
averaging fifty calculations of individual particle length-to-width
aspect ratios.
[0176] The PCC compositions may also be characterized in terms of
their cubicity, or the ratio of surface area to particle size
(i.e., how close the material is to a cube, rectangular prism, or
rhombohedron). In certain embodiments of the present disclosure, a
lower surface area is advantageous. Smaller particles typically
have much higher surface area, but small particle size is
advantageous for many different applications. Thus PCC products
with small particle size material and lower than "normal" surface
area are particularly advantageous. Rhombic crystal forms are
generally preferred in terms of cubicity.
[0177] According to some embodiments, the cubic nature of the PCC
compositions may be determined by the "squareness" of the PCC
particles. A squareness measurement generally describes the angles
formed by the faces of the PCC particle. Squareness, as used
herein, can be determined by calculating the angle between adjacent
faces of the PCC, where the faces are substantially planar.
Squareness may be measured using SEM images by determining the
angle formed by the edges of the planar faces of the PCC particle
when viewed from a perspective that is parallel to the faces being
measured. FIG. 41 shows an exemplary measurement of squareness.
According to some embodiments, the PCC compositions may have a
squareness in a range from about 70 degrees to about 110
degrees.
[0178] In the present disclosure, the monodispersity of the product
refers to the uniformity of crystal size and polymorphs. The
steepness (d.sub.30/d.sub.70.times.100) refers to the particle size
distribution bell curve, and is a monodispersity indicator. d.sub.x
is the equivalent spherical diameter relative to which x % by
weight of the particles are finer. According to some embodiments,
the PCC compositions may have a steepness in a range from about 30
to about 100, such as, for example, in a range from about 33 to
about 100, from about 42 to about 76, from about 44 to about 75,
from about 46 to about 70, from about 50 to about 66, from about 59
to about 66, or from about 62 to about 65. According to some
embodiments, the PCC compositions may have a steepness in a range
from about 20 to about 71, such as, for example, in a range from
about 25 to about 50. In some embodiments, the steepness may vary
according to the morphology of the PCC. For example, calcite may
have a different steepness than vaterite.
[0179] According to some embodiments, the PCC compositions may have
a top-cut (d.sub.90) particle size less than about 25 microns, such
as, for example, less than about 17 microns, less than about 15
microns, less than about 12 microns, or less than about 10 microns.
According to some embodiments, the PCC compositions may have a
top-cut particle size in a range from about 2 microns to about 25
microns, such as, for example, in a range from about 15 microns to
about 25 microns, from about 10 microns to about 20 microns, or
from about 3 microns to about 15 microns.
[0180] According to some embodiments, the PCC compositions may have
a bottom-cut (d.sub.10) particle size less than about 4 microns,
such as, for example, less than about 2 microns, less than about 1
micron, less than about 0.7 microns, less than about 0.5 microns,
less than 0.3 microns, or less than 0.2 microns. According to some
embodiments, the PCC compositions may have a bottom-cut particle
size in a range from about 0.1 micron to about 4 microns, such as,
for example, in a range from about 0.1 micron to about 1 micron,
from about 1 micron to about 4 microns, or from about 0.5 microns
to about 1.5 microns.
[0181] The PCC compositions may additionally be characterized by
their BET surface area. As used herein, BET surface area refers to
the Brunauer-Emmett-Teller (BET) explaining the physical adsorption
of gas molecules on a solid surface. It refers to multilayer
adsorption, and usually adopts non-corrosive gases (i.e. nitrogen,
argon, carbon dioxide and the like) as adsorbates to determine the
surface area data. The BET surface area may vary according to the
morphology of the PCC. According to some embodiments, the PCC may
have a BET surface area less than 80 m.sup.2/g, such as, for
example, less than 50 m.sup.2/g, less than 20 m.sup.2/g, less than
15 m.sup.2/g, less than 10 m.sup.2/g, less than 5 m.sup.2/g, less
than 4 m.sup.2/g, or less than 3 m.sup.2/g. In some embodiments,
the calcite PCC composition particles may have a BET surface area
in a range from 1 to 30 m.sup.2/g, such as, for example, from 2 to
10 m.sup.2/g, from 3 to 6.0 m.sup.2/g, from 3 to 5.0 m.sup.2/g. In
other embodiments, calcite PCC may have a BET surface area in a
range from 1 to 6 m.sup.2/g, from 1 to 4 m.sup.2/g, from 3 to 6
m.sup.2/g, or from 1 to 10 m.sup.2/g, from 2 to 10 m.sup.2/g, or
from 5 to 10 m.sup.2/g. According to some embodiments, calcite PCC
may have a BET surface area less than or equal to 30 m.sup.2/g.
Vaterite PCC may have a BET surface area in a range from 5 to 75
m.sup.2/g. In certain embodiments, the vaterite PCC composition
particles have a BET surface area in a range from 7 to 18
m.sup.2/g, from 5 to 20 m.sup.2/g, or from 7 to 15 m.sup.2/g. In
some embodiments, aragonite PCC may have a BET surface area in the
range from 2 to 30 m.sup.2/g.
[0182] The PCC compositions may additionally be characterized by
the ratio of BET surface area to d.sub.50. In a certain embodiment,
the vaterite PCC composition particles have a ratio of BET surface
area to d.sub.50 of 1-6.5, 2-5.5, or 2.5-5. In another embodiment,
the calcite PCC composition particles have a ratio of BET surface
area to d.sub.50 of 0.6-2, 0.7-1.8, or 0.8-1.5.
[0183] The PCC compositions may additionally be characterized by
their stearic acid uptake surface area. As used herein, stearic
acid uptake surface area refers to a surface treatment of the PCC
compositions with stearic acid. Under controlled conditions, the
stearic acid may form a monolayer on the surface of the PCC and
thus provide information regarding the surface area via adsorption
or uptake of stearic acid. The stearic acid uptake surface area may
vary according to the morphology of the PCC. According to some
embodiments, the PCC may have a stearic acid uptake surface area
less than 80 m.sup.2/g, such as, for example, less than 75
m.sup.2/g, less than 50 m.sup.2/g, less than 20 m.sup.2/g, less
than 15 m.sup.2/g, less than 10 m.sup.2/g, less than 5 m.sup.2/g,
less than 4 m.sup.2/g, or less than 3 m.sup.2/g.
[0184] In some embodiments, the yield of PCC using the process
herein is greater than 50%, greater than 60%, greater than 80%, or
greater than 90%.
[0185] The PCC compositions of the present disclosure may be in any
desired form, including but not limited to, powders, crystalline
solids, or in dispersed form, i.e., the PCC compositions may be
dispersed in a liquid, such as in an aqueous medium. In one
embodiment, the dispersed PCC composition comprises at least about
50% PCC by weight relative to the total weight of the dispersion,
at least about 70% PCC by weight. The dispersed PCC composition may
comprise at least one dispersing agent, which may be chosen from
dispersing agents now known in the art or hereafter discovered for
the dispersion of PCC. Examples of suitable dispersing agents
include, but are not limited to: polycarboxylate homopolymers,
polycarboxylate copolymers comprising at least one monomer chosen
from vinyl and olefinic groups substituted with at least one
carboxylic acid group, and water soluble salts thereof. Example of
suitable monomers include, but are not limited to, acrylic acid,
methacrylic acid, itaconic acid, crotonic acid, fumaric acid,
maleic acid, maleic anhydride, isocrotonic acid, undecylenic acid,
angelic acid, and hydroxyacrylic acid. The at least one dispersing
agent may be present in the dispersed PCC composition in an amount
ranging from about 0.01% to about 2%, from about 0.02% to about
1.5% by weight relative to the total weight of the dispersion.
[0186] FGD gypsum typically contains contaminants and is of low
whiteness and brightness. Major contributors to discoloration may
include insoluble impurities, such as pyrite and various organic
species. In the present disclosure, gypsum may be pretreated prior
to reaction with a carbonate source. In one embodiment, this
pretreatment includes, but is not limited to, a filtration or
sieving step and/or a mineral acid treatment step. The filtration
method may be, but is not limited to vacuum filtration.
[0187] Fully dissolved gypsum may be filtered or centrifuged to
remove the impurities that result in low whiteness and brightness.
The filtration method may be, but is not limited to vacuum
filtration, but may refer to any dewatering process common to the
art. Furthermore, large contaminants may be removed from the gypsum
by sieving. Alternatively, a mineral acid, such as nitric acid, may
be employed to improve whiteness and brightness of the gypsum by
removing species causing discoloration. A mineral acid may also be
employed to remove remaining carbonate species in the gypsum.
Therefore, in terms of methods A and B wherein mineral acid is
added to remove excess carbonate, additional mineral acid may be
added to remove non-carbonate contaminants. Additionally, the
mineral acid addition step of methods A and B to remove carbonate
impurities may be different from the mineral acid used to remove
contaminants resulting in low whiteness and brightness, and the
step to remove carbonate impurities may take place prior to the
step to remove contaminants resulting in low whiteness and
brightness, and vice versa. Thus, it is envisioned within the scope
of the present disclosure that in methods A-I, the method may be
modified to include a step for removing impurities and/or improving
the whiteness and brightness of the gypsum by filtering from the
fully dissolved gypsum, sieving, and/or utilizing a mineral acid.
According to some embodiments, the PCC compositions may have low
ionic impurities. According to some embodiments, the low ionic
impurities may improve the electrical properties of the PCC or a
finished product containing the PCC. It is envisioned that this
pretreatment step is performed prior to reacting gypsum with the
carbonate source, additive, or seed and that a gypsum of improved
whiteness and brightness may yield, after reaction with a carbonate
source, a PCC product of whiteness and brightness similar to that
of the cleaned gypsum. Polymorph and particle size of the PCC
yielded may be controlled using methods disclosed herein, in one or
more of their embodiments.
[0188] Brightness refers to a measure of directional reflectance
from a material of light at a certain wavelength or certain
wavelengths. As used herein, ISO brightness refers to an ISO
standard that quantifies the brightness of a substantially white or
near-white material (i.e. paper) as it would be perceived in an
environment that is illuminated with a mixture of cool-white
fluorescence and some filtered daylight, specifically blue light of
specific spectral and geometric characteristics, generally
.about.457 nm. In some embodiments, the ISO brightness may be
determined by a standard test for brightness, such as ASTM D985-97.
According to some embodiments, the PCC compositions or paper or
paperboard materials comprising them may have an ISO brightness
greater than or equal to 54, such as, for example, greater than or
equal to 65, greater than or equal to 85, greater than or equal to
88, greater than or equal to 90, greater than or equal to 92, or
greater than or equal to 95. According to some embodiments, the PCC
compositions have a consistent or homogeneous brightness across the
PCC particles.
[0189] Rhombic precipitated calcium carbonate of a particular size
distribution can be yielded from FGD gypsum by reacting with
ammonium carbonate in the presence of a calcium carbonate crystal
seed and by controlling reaction parameters as disclosed herein.
Rhombic PCC generated from this method has similar properties to
either rhombic produced by traditional methods or ground calcium
carbonate (GCC).
[0190] Reaction of gypsum with carbonate in the presence or absence
of additives to yield the rhombic PCC can be carried out in a batch
or continuous process. Specific selection of reaction conditions
aid in fine-tuning the properties of the PCC generated. The
following may be controlled for rhombic PCC production disclosed
herein: concentration of gypsum and other reactants, starting
temperature for each reactant, reaction temperature and reaction
time, drying temperature, annealing temperature for generated PCC
where employed, selection and maintenance of pH and ionic strength
for each solution, addition rate of each added component, and rate
of CO.sub.2 addition, where employed.
[0191] PCC may be surface treated with stearic acid, other stearate
or hydrocarbon species to yield a specific level of hydrophobicity.
Hydrophobicity may be measured using a moisture uptake (MPU)
technique, in which a PCC powder is exposed to a high relative
humidity atmosphere for 24 h or longer and the weight change due to
water sorption is recorded. In general, the maximum reduction in
MPU achievable by surface treatment is particularly advantageous.
Hydrophobicity may also be measured by contact angle, in which a
droplet of a test liquid (e.g. water) is placed on a PCC powder and
is observed to see whether the droplet is absorbed (wets) or gives
a stable droplet with a measurable contact angle. Surface
treatments may involve dry or wet coating with a C.sub.6-C.sub.22
fatty acid or fatty acid salt. Such treatments are well-known in
the art, and in addition to stearic acid, include such materials as
ammonium stearate, sodium stearate, palmitic acid, and others. The
fatty acid/fatty acid salt is provided in sufficient quantity to
coat a substantial portion of the surface of the majority of PCC
particles. The amount of hydrophobizing agent needed to coat a
substantial portion of the PCC surface is related to the PCC
surface area. In one embodiment, a calcite PCC of this disclosure
requires 0.5-1.0% hydrophobizing agent to coat the surface. In
another embodiment, a vaterite PCC requires 2.0-3.0% hydrophobizing
agent to coat the surface. Treated and untreated PCC or blends
thereof, of single or blended size distributions can be used in a
variety of applications, including adhesives and sealants as a
rheology modifier, in paints and ink for opacity and as an
extender, as a paper filler, for surface finishing and brightness,
a functional filler in plastics and as an extender. According to
some embodiments, the hydrophobizing agent may form a monolayer on
the surface of the PCC. According to some embodiments, the amount
of hydrophobizing agent may be in a range from about 0.15 m.sup.2/g
to about 18 m.sup.2/g to coat the particles, such as, for example,
in a range from about 0.15 m.sup.2/g to about 8 m.sup.2/g or from
about 10 m.sup.2/g to about 17 m.sup.2/g. The amount of
hydrophobizing agent may be dependent on the morphology of the PCC.
For example, calcite PCC may have an amount of hydrophobizing agent
in a range from about 0.15 m.sup.2/g to about 20 m.sup.2/g to coat
the particles, and vaterite PCC may have an amount of
hydrophobizing agent in a range from about 10 m.sup.2/g to about 80
m.sup.2/g to coat the particles.
[0192] In some embodiments, a size reduction method is employed
either in situ or on the product after recovery. A size reduction
method may include sonication or grinding. Since the products
appear to exhibit `substructure` that is most likely interpretable
as aggregation, a size reduction method may break apart the
aggregates into their constituent building blocks. According to
some embodiments, ultrasound may be used to break down
agglomerates.
[0193] According to some embodiments, the PCC may be beneficiated
by grinding or milling. In some embodiments, the beneficiation may
include one or more of magnetic separation, bleaching, or acid
washing. The separation, bleaching, or acid washing may occur
before the grinding/milling, after the grinding/milling, or
both.
[0194] The PCC compositions of the present disclosure may
optionally comprise at least one added pigment. Suitable pigments
are those now known or that may be hereafter discovered. Exemplary
pigments include, but are not limited to, titanium dioxide,
calcined clays, delaminated clays, talc, calcium sulfate, other
calcium carbonate, kaolin clays, calcined kaolin, satin white,
plastic pigments, aluminum hydrate, and mica.
[0195] The pigment may be present in the PCC compositions of the
present disclosure in an amount less than about 70% by weight
relative to the total weight of the composition. It is to be
understood that the skilled artisan will select any amounts of the
optional at least one second PCC form and the optional at least one
pigment in such a way so as to obtain various desired properties
without affecting, or without substantially affecting, the
advantageous properties of the PCC compositions disclosed
herein.
A Process for Converting Limestone into Precipitated Calcium
Carbonate
[0196] The present disclosure also relates to a process for
converting limestone, marble, or chalk into precipitated calcium
carbonate comprising i) treating limestone, marble, or chalk with a
mineral acid comprising sulfate anions to yield a calcium sulfate
and magnesium sulfate mixture ii) optionally adding a calcium
carbonate seed to the calcium sulfate iii) optionally adding an
additive to the calcium sulfate iv) reacting the calcium sulfate
with at least one carbonate source selected from the group
consisting of ammonium carbonate, ammonium bicarbonate, ammonium
carbamate, calcium carbonate, dolomite, a metal carbonate, and
carbon dioxide at a reaction temperature of 8-50.degree. C. and a
reaction time of 5-250 minutes, to yield precipitated calcium
carbonate and v) isolating the precipitated calcium carbonate. In a
certain embodiment, reaction conditions are used to control the
crystalline polymorph and particle size of the precipitated calcium
carbonate thus obtained.
[0197] Limestone is a sedimentary rock composed largely of the
minerals calcite and aragonite. Dolomitic quicklime is calcined
dolomite that is rehydrated (e.g., MgOH and CaOH).
[0198] Certain embodiments of the present disclosure relate to a
low energy method of producing precipitated calcium carbonate of
controlled polymorph and particle size with limestone, marble, or
chalk as the calcium source. Treating an impure calcium carbonate
source with sulfuric acid generates sulfate products, including
calcium sulfate (gypsum) and magnesium sulfate. Then, after
treatment with ammonium carbonate or a metal carbonate, a PCC and
sulfate-based solution-phase byproduct are generated. The polymorph
and particle size of PCC generated from gypsum produced in this
process can be controlled by methods disclosed within, in one or
more of their embodiments. The use of sulfuric acid and limestone
to generate gypsum is known in the art. However, the controlled
precipitation of calcium carbonate to generate one or more various
PCC polymorphs has not been previously described. Separately,
magnesium carbonate can be generated from MgSO.sub.4 formed during
dolomitic limestone reaction with sulfuric acid and dissolved in
the aqueous phase by reaction with an appropriate carbonate.
[0199] In one embodiment, the amount of sulfuric acid added to
limestone is optionally a molar equivalent of or in excess of the
amount of calcium present in the limestone.
Exemplary Precipitated Calcium Carbonate Compounds
[0200] The present disclosure relates to a precipitated calcium
carbonate compound with a vaterite polymorph. The vaterite
precipitated calcium carbonate described within has novel
structural characteristics, such as particle size distribution
(PSD), steepness, and BET surface area, as compared to heretofore
known vaterite precipitated calcium carbonate. See Table 3 below
for vaterite characteristics.
TABLE-US-00003 TABLE 3 IMERYS IMERYS Calcite Vaterite PSD (d50),
1.5-28 1.5-28.sup. d30/d70 .times. 100 73.5-59.5 69.0-43.3 Surface
Area (BET), m.sup.2/g 0.4-20 8-17 Surface Area (Stearic Acid
uptake), m.sup.2/g 0.4-20 8-17
[0201] The PCC compositions may also be characterized in terms of
their cubicity, or the ratio of surface area to particle size
(i.e., how close the material is to a perfect cube). According to
certain embodiments of the present disclosure, a lower surface area
is advantageous. Smaller particles typically have much higher
surface area, but small particle size is advantageous for many
different applications. Thus PCC products with small particle size
material and lower than "normal" surface area are particularly
advantageous. Rhombic crystal forms are generally preferred in
terms of cubicity.
[0202] An example of a qualitative understanding of cubicity can be
shown by comparing FIG. 14 with FIG. 24. FIG. 14 shows a PCC
composition having a relatively low surface area to particle size
ratio because of the relatively smooth, planar faces of the
particles. FIG. 24, by contrast, shows a PCC composition having
relatively non-planar faces because of protrusions on the faces of
the particles, and therefore, the composition shown has a higher
surface area and a lower cubicity.
[0203] FIG. 41 shows an exemplary measurement of squareness.
According to some embodiments, the PCC compositions may have a
squareness in a range from about 70 degrees to about 110 degrees.
Five measurements of angles between the planar faces of the PCC
particles in FIG. 41 were taken using IMAGE J analysis software,
one of which is shown in FIG. 41. Angles were measured by randomly
selecting particles from those having faces normal to the plane of
the image. The measured angles between the edges were 74.6 degrees,
105.2 degrees, 109.6 degrees, 82.6 degrees, and 74.3 degrees.
According to some embodiments, the squareness of the PCC particles
may be in a range from about 70 degrees to about 110 degrees, such
as, for example, in a range from about 75 degrees to about 105
degrees, or from about 80 degrees to about 110 degrees.
[0204] In the present disclosure, the monodispersity of the product
refers to the uniformity of crystal size and polymorphs. The
steepness (d.sub.30/d.sub.70.times.100), as defined above, refers
to the particle size distribution bell curve, and is a
monodispersity indicator. In the present disclosure, the preferred
PCC product is monodisperse with a steepness greater than 30, or
greater than 40, or greater than 50, and less than 60, or even less
than 65. According to some embodiments, the PCC may have a
steepness in a range from about 30 to about 100, such as, for
example, in a range from about 34 to about 100, from about 42 to
about 77. In some embodiments, the steepness may vary according to
the morphology of the PCC. For example, calcite may have a
different steepness than vaterite.
[0205] The present disclosure enables the generation of varied PSD
(d.sub.50) and polymorphs of the PCC product, which can be formed
as vaterite, aragonite, calcite (e.g., rhombic or scalenohedral
calcite), or amorphous calcium carbonate. In general, lower
reaction temperature yields smaller/finer, higher surface area
vaterite `balls`. In general, lower excess of ammonium carbonate
yields smaller/finer crystals within aggregates, and higher surface
area products often comprised of a calcite/vaterite blend.
[0206] In one embodiment, the PCC composition of the present
disclosure is characterized by a single vaterite crystal polymorph
content of greater than or equal to 30% by weight relative to the
total weight of the composition, greater than or equal to 40% by
weight, greater than or equal to 60% by weight, greater than or
equal to about 80% by weight, or greater than or equal to about 90%
by weight.
[0207] In one embodiment, the vaterite PCC has a geometry
comprising spherical coral, elliptical coral, rhombic,
flower-shaped or mixtures thereof.
[0208] In another embodiment, the vaterite PCC has a PSD (d.sub.50)
ranging from 2.0-7.0, 2.4-6.0, or 2.6-5.5 microns.
[0209] In another embodiment, the vaterite PCC has a steepness
(d.sub.30/d.sub.70.times.100) ranging from 30-100, 37-100, 40-83,
42-71.
[0210] In another embodiment, the vaterite PCC has a BET surface
area ranging from 8-18, 10-17, or 10.4-16.1 m.sup.2/g.
[0211] In another embodiment, the vaterite PCC may have an amount
of hydrophobizing agent in a range from about 10 m.sup.2/g to about
17 m.sup.2/g to coat the particles.
[0212] The present disclosure also relates to a precipitated
calcium carbonate compound with a calcite polymorph. The calcite
precipitated calcium carbonate described within has improved
structural characteristics, such as particle size distribution
(PSD), steepness, and BET surface area, as compared to heretofore
known calcite precipitated calcium carbonate. See Table 3 for a
comparison of calcite manufactured from the process herein and a
known PCC calcite.
[0213] In one embodiment, the PCC composition of the present
disclosure is characterized by a single calcite crystal polymorph
content of greater than or equal to 30% by weight relative to the
total weight of the composition, greater than or equal to 40% by
weight, greater than or equal to 60% by weight, greater than or
equal to about 80% by weight, or greater than or equal to about 90%
by weight.
[0214] In one embodiment, the calcite PCC has a rhombic geometry.
In general, seeding gypsum with crystalized calcium carbonate
consistently yields rhombic PCC. Seeding with calcite, dolomite, or
magnesite yields rhombic PCC. In general, seeding with coarse
scalenohedral PCC >5% yields a larger/coarser and a higher
surface area product. In general, seeding with fine rhombohedral
PCC <5% yields a finer crystal size within the aggregate; >5%
gives finer aggregates. In the absence of seeding, ammonium
carbonate conditions influence rhombic PCC formation.
[0215] In one embodiment, rhombic PCC yielded may be small stacked
plates of 300-500 nm, forming inconsistent or consistent particle
shapes, having a d.sub.50 of 1-6 .mu.m, a steepness of 91-56, and
surface area 2-5 m.sup.2/g.
[0216] Table 4 below identifies product characteristics obtained
from various examples of the foregoing methods.
TABLE-US-00004 TABLE 4 Individual Particle Size Surface Particle
Size Distribution (d50), Steepness Area PCC SEM (est.) agglomerates
(.mu.m) (d30/d70 .times. 100) (m.sup.2/g) Rhombic See FIG. 8
300-500 nm 4.6 60 4.6 Rhombic See FIG. 9 1-2 .mu.m 4.9 61 2.6
Rhombic See FIG. 10 ~5 .mu.m 27.1 73 2.8
[0217] In another embodiment, the calcite PCC has a PSD (d.sub.50)
ranging from 1.8-6.0, 2.2-5.8, or 2.8-5.6 microns.
[0218] In another embodiment, the calcite PCC has a steepness
(d.sub.30/d.sub.70.times.100) ranging from 30-100, 40-100, 50-83,
56-71.
[0219] In another embodiment, the calcite PCC has a BET surface
area ranging from 3.0-7.0, 3.1-6.0, or 3.5-5.0 m.sup.2/g.
[0220] In another embodiment, the calcite PCC may have an amount of
hydrophobizing agent in a range from about 0.15 m.sup.2/g to about
8 m.sup.2/g to coat the particles.
[0221] The present disclosure also relates to a precipitated
calcium carbonate compound with an aragonite polymorph. The
aragonite precipitated calcium carbonate described within has
improved structural characteristics, such as particle size
distribution (PSD), steepness, and BET surface area, as compared to
heretofore known aragonite precipitated calcium carbonate.
[0222] In one embodiment, the PCC compound of the present
disclosure is characterized by a single aragonite crystal polymorph
content of greater than or equal to 30% by weight relative to the
total weight of the composition, greater than or equal to 40% by
weight, greater than or equal to 60% by weight, greater than or
equal to about 80% by weight, or greater than or equal to about 90%
by weight.
[0223] According to some embodiments, after forming the PCC
compound of the present disclosure, the morphology may be changed
through post-processing techniques, such as aging. For example,
according to some embodiments, an amorphous PCC may be used as a
precursor to convert into a crystalline morphology, such as
vaterite, aragonite, or calcite. According to some embodiments, a
metastable PCC, such as vaterite or aragonite, may be converted to
calcite through aging, such as, for example, wet aging. The amount
of vaterite converted to calcite through aging may be varied by
adjusting the properties of the aging conditions. For example, the
aging may be varied by the presence or absence of ammonium sulfate,
including the amount of ammonium sulfate, the aging temperature,
and the concentration of wet cake solids. According to some
embodiments, when less than about 90% vaterite is present, the
vaterite will convert to calcite. When greater than or equal to
about 90% vaterite is present, the vaterite can be retained in a
dry powder or wet cake. The amount of retained vaterite may vary
depending on the aging parameters. According to some embodiments,
vaterite can be converted to calcite through a mechanical process,
such as by grinding or ball milling the vaterite.
[0224] According to some embodiments, the concentrations of gypsum
and ammonium carbonate may influence the conversion of vaterite to
calcite. For example, higher concentrations of gypsum and ammonium
carbonate may produce vaterite PCC that is more stable (e.g.,
resistant to converting to calcite) than vaterite PCC produced
using lower concentrations of gypsum and ammonium carbonate. For
example, when 10.7% gypsum and 12.5% ammonium carbonate (1:1.7
molar ratio [gypsum:ammonium carbonate]) are reacted at room
temperature, the reaction forms vaterite that converts to calcite
in the presence of ammonium sulfate within 24 hours. When higher
concentrations of gypsum and ammonium carbonate are used in the
reaction, the vaterite produced may be more stable. For example,
when 35% gypsum and 33% ammonium carbonate (1:1.7 molar ratio
[gypsum:ammonium carbonate]) are reacted at room temperature, the
reaction forms vaterite, but the vaterite is stable in the presence
of ammonium sulfate for at least 24 hours. In other embodiments,
impurities, such as, for example, iron present in either gypsum or
ammonium-based carbonates may assist stabilizing vaterite PCC
produced by the methods described herein. According to some
embodiments, the conversion of vaterite to calcite may be
controlled through the storage of the vaterite. For example,
vaterite in liquid suspension with ammonium sulfate may convert to
calcite may be inhibited relative to cakes of vaterite having about
40-60% solids without ammonium sulfate. According to some
embodiments, the conversion of vaterite to calcite may reduce the
surface area of the PCC formed. For example, the conversion of
vaterite to calcite may reduce the surface area of the PCC from
greater than 10 m.sup.2/g (vaterite) to less than 1 m.sup.2/g
(calcite). According to some embodiments, the conversion of
vaterite to calcite may increase the particle size of the PCC. In
other embodiments, the conversion of vaterite to calcite may not
significantly change the particle size of the PCC. Other additives,
such as, for example, citric acid may inhibit the conversion of
vaterite to calcite. For example, about 5% by weight citric acid on
vaterite may inhibit the conversion to calcite. According to some
embodiments, adding ammonium bicarbonate may accelerate the
conversion of vaterite to calcite, whereas ammonium sulfate in
concentrations from about 0.5% to about 10% by weight may slow or
inhibit the conversion to calcite. FIG. 39 shows exemplary effects
on morphology of the PCC by varying the feed concentrations and the
aging process. FIG. 40 shows exemplary effects of the feed
composition on PCC.
[0225] According to some embodiments, the methods of forming the
PCC compound may be performed in a continuous process, such as, for
example, using a tubular reactor. In some embodiments, in the
continuous process, the reactants may be mixed in such a way as to
cause cavitation.
Commercial Applications
[0226] According to certain embodiments, the present disclosure
relates to commercial applications of the vaterite, calcite, and/or
aragonite precipitated calcium carbonate compound described herein,
in one or more of its embodiments.
[0227] According to a first aspect, the present disclosure relates
to a polymer film or a breathable polymer film containing the
vaterite, calcite, and/or aragonite precipitated calcium carbonate
compound in one or more of its embodiments.
[0228] According to a second aspect, the present disclosure relates
to a pulp or paper material containing the vaterite, calcite,
and/or aragonite precipitated calcium carbonate compound in one or
more of its embodiments.
[0229] According to a third aspect, the present disclosure relates
to a diaper comprising a breathable polymer film containing the
vaterite, calcite, and/or aragonite precipitated calcium carbonate
compound in one or more of its embodiments.
[0230] According to a fourth aspect, the present disclosure relates
to a filled polymer composition comprising the PCC of the present
disclosure in one or more of its embodiments as filler, wherein the
polymer can be any desired polymer or resin.
[0231] According to some embodiments, the PCC compositions may be
used as a filler for various applications. Exemplary applications
include, but are not limited to, fillers or additives for plastics,
paper coatings, adhesives, sealants, caulks, paper, moldings,
coatings, paint, rubber products, and concrete. For example, the
PCC compositions may be used as a filler or additive for
polyvinylchloride (PVC), plasticized PVC (pPVC), polypropylene
(PP), rubber, coatings, paint, ceramics, paper, or concrete. Some
exemplary uses include use as a filler or additive for PVC pipes or
moldings, pPVC, paint (e.g., exterior paint or road paint), tile
coatings (e.g., ceiling tile coatings), decorative coatings,
moldings (e.g., PVC moldings, pPVC moldings, or PP moldings), sheet
molding compounds, bulk molding compounds, adhesives, caulks,
sealants, rubber products, paper, paper fillers, paper coatings, or
concrete. According to some embodiments, the relatively lower
surface area of the PCC compositions may be suitable as a filler
and may have improved dispersibility. The PCC compositions may, in
general, have relatively low brightness (e.g. 65 ISO brightness) to
relatively high brightness (e.g., greater than 90 ISO brightness)
and may have a consistent brightness, which may improve the color
of a given product in an application. The PCC compositions
disclosed herein may have a relatively low surface area when
compared to other calcium carbonate products, such as, for example,
ground calcium carbonate (GCC). The relatively low surface area may
contribute to low adsorption of additives by the PCC, reduced
amounts of additives to treat a surface of the PCC, and/or low
moisture pick-up by the PCC. According to some embodiments, the
relatively lower surface area may contribute to a relatively lower
viscosity of the material to which the PCC is added and/or a
greater amount of "active" particles when used as a filler or
additive, such as, for example, in polymer films. According to some
embodiments, a broad particle size distribution of the PCC may
increase particle packing, whereas a steep or narrow particle size
distribution of the PCC may decrease particle packing. According to
some embodiments, a relatively smaller PCC particle size may
improve the gloss of a coating, such as, for example, a paper
coating or paint, containing the PCC composition. A relatively
smaller particle size may also improve the impact resistance of a
material, such as, for example, a molded product or coating,
containing the PCC composition.
[0232] According to some embodiments, the steepness and/or cubicity
of the PCC particles described herein may improve the handling
properties of powders. For example, the steepness and/or cubicity
of the PCC particles may improve the flowability of powders.
[0233] According to some embodiments, the PCC compositions
described herein may have improved oil absorption properties.
Improved oil absorption may, for example, improve the flowability
of paints or powders incorporating the PCC compositions.
[0234] According to some embodiments, the PCC compositions, such as
the vaterite PCC compositions, may be used for various
applications, including but not limited to drug delivery, medical
devices, biosensing, encapsulation, tracing, polymer fillers,
cavitation enhancement in films, heavy metal sequestration, as a
nucleation agent (for example, a foam nucleation agent), an
abrasive, FGD feeds, synthetic paper component, or emulsion systems
filler. In some embodiments, the PCC, such as vaterite PCC, may be
used as a drug delivery agent or component. For example, vaterite
may be used as a platform for small molecule or protein absorption
or adsorption, such as into the pores of the vaterite. Vaterite may
also be used, in some embodiments, as a microparticle or
microcapsule for drug encapsulation or drug delivery, for example,
vaterite may be used to encapsulate molecules including, but not
limited to, insulin, bovine serum albumin, and lysozymes. In some
embodiments, encapsulation may occur during a phase transition of
the PCC from vaterite to calcite. Such encapsulation may promote
controlled release of the encapsulated molecules. In some
embodiments, encapsulation may occur through absorption or
adsorption of the molecules into the pores of the vaterite. In
other embodiments, encapsulation may occur through direct
encapsulation during the formation of the PCC particles. In other
embodiments, encapsulation may occur through hollow-centered PCC
particles.
[0235] According to some embodiments, the PCC, such as vaterite may
be used as a controlled release agent. For example, vaterite may be
exposed to highly acidic environments to control release. Vaterite
exposed to such environments may break down, thereby releasing the
encapsulant or encapsulated, absorbed, or adsorbed molecules.
According to some embodiments, the vaterite may serve as a template
protein structure to control release of a molecule. According to
some embodiments, the vaterite may be used as a template for
cross-linking polymer, such as, for example, biopolymers. In some
embodiments, the polymers may be cross-linked using the vaterite as
a template. Subsequent removal of the vaterite may result in a
cross-linked polymer having a structure similar to the vaterite
template (e.g., spherical).
[0236] According to some embodiments, the PCC, such as vaterite,
may be used in medical devices, such as, for example, implantable
medical devices. In some embodiments, vaterite may exhibit rapid
bioabsorption, for example, due to vaterite's high surface area.
Because of rapid absorption, vaterite may be used as a calcium
source for biological applications, such as, for example, bone
regeneration. Vaterite may also assist in the generation of bone
minerals, such as phosphate bone minerals, such as hydroxyapatite.
In some embodiments, the hydroxyapatite or other small molecules
may be encapsulated by the vaterite or PCC, or may be bound (either
chemically or physically) to the surface of the vaterite.
Conversion of the vaterite to calcite, in some embodiments, may
also promote binding of the PCC to bone.
[0237] According to some embodiments, the PCC, such as vaterite,
may be used in biosensing applications. For example, vaterite may
be used in biosensing of pH changes or ion sensing. In some
embodiments, a fluorescent pH sensor may be encapsulated by the
vaterite, such as, for example, in tracing applications.
[0238] According to some embodiments, the PCC, such as vaterite may
be used as a filler for polymers. For example, the vaterite may be
used in polymer films, such as, for example, cavitation
enhancement. In some embodiments, the vaterite may promote more
uniform cavitation of pores and may increase the breathability of
the film.
[0239] According to some embodiments, PCC, such as vaterite, may
encapsulate metals, such as heavy metals. For example,
encapsulation may occur through a phase change from vaterite to
calcite.
[0240] According to some embodiments, the PCC, such as vaterite,
may be used as a nucleating agent. In some embodiments, the
vaterite may act as a foam nucleating agent.
[0241] According to some embodiments, the PCC, such as vaterite,
may be used as an abrasive, such as, for example, a cleaning
abrasive.
[0242] According to some embodiments, the PCC, such as vaterite may
be used as a feed material in an FGD process. In some embodiments,
the increased surface area of the vaterite may improve reactivity
and/or increase the reaction rate. For example, the vaterite may
neutralize sulfuric acid generated in the FGD process.
[0243] According to some embodiments, the properties of PCC
compositions, such as vaterite described in this disclosure may be
beneficial for various applications. For example, a polymorph shift
may be induced under shear and/or heat. For example, the vaterite
may convert to needle-like particles or rhombic particles. A
polymorph shift may, in some embodiments, be influenced by the
presence of surfactants or macromolecules. A polymorph shift may
also be influenced by inclusions in the vaterite structure, such as
metals or other ions. For example, polymorph changes may be
mitigated through the use of surfactants, reacting the vaterite in
the presence of metals, or through the use of additives. Additives
may include, but are not limited to, acids or additives for
biomineralization, such as, for example, ovalbumin, glutamic acid,
or aspartic acid.
[0244] The examples below are intended to further illustrate
examples of a process for desulfurizing flue gas to form gypsum and
for converting gypsum thus obtained or limestone into precipitated
calcium carbonate with desired polymorph and crystal size.
Example 1
[0245] Gypsum or other sulfate was slurried in water at 35% solids
or in a solution of 30-35% ammonium sulfate. Ammonium carbonate was
dissolved in water at elevated temperature in a concentration to
give a 1:1.7 [gypsum:ammonium carbonate] molar ratio for reaction.
Alternative carbonate feeds were dissolved in water at room
temperature in an amount to give a 1:1.7 [sulfate:carbonate] molar
ratio for reaction. The sulfate slurry and carbonate solution were
mixed and allowed to react for at least 10 minutes. The slurry was
then filtered and the slurry cake washed with water. Reaction cake
and decanted liquid were chemically and physically analyzed by
FTIR, DSC, SEM, Sedigraph, and BET surface area analysis. Reactions
involving this process are described in Table 1 and 2.
Example 2
[0246] For reactions involving ammonium carbonate production from
ammonium hydroxide and CO.sub.2, sulfate was slurried at ambient
temperature and pressure as described above. Ammonium hydroxide was
added to the sulfate slurry just prior to the mixture being poured
in a reaction vessel. The reaction vessel was covered, then heated
or cooled to a selected temperature with stirring. CO.sub.2 was
bubbled through the reaction vessel with stirring for a minimum 1
hour. After 1 hour of reaction time, a small portion of the slurry
was removed and checked for full conversion to PCC by
phenolphthalein color change. Upon reaction completion, the slurry
was removed from the reaction vessel, filtered, washed and analyzed
as described above. Reactions involving this process are described
in Table 1.
Exemplary Core Materials with Precipitated Calcium Carbonate
[0247] According to some embodiments, one or more of the methods
disclosed in this disclosure may be used to precipitate PCC onto a
core material. For example, the PCC may form a surface layer or
coating that imparts beneficial properties to the core material.
The process may, in some embodiments, be used to precipitate a core
material from solution where the precipitated core material has a
PCC layer or coating. It is understood that the use of the word
"layer" or "coating" includes a precipitated calcium carbonate
phase formed on at least a portion of the surface of the core
material. In some embodiments, the PCC layer or coating may cover
half or substantially all of the surface of the core material. The
core material may form the core of a calcium carbonate-coated
composition.
[0248] According to some embodiments, the core material may include
a weighting agent for use in drilling fluids. The weighting agent
may include iron oxide, such as, for example hematite
(Fe.sub.2O.sub.3). Other weighting agents may be used, however,
such as, for example, AgI, AgCl, AgBr, AgCuS, AgS, Ag.sub.2S,
Al.sub.2O.sub.3, AsSb, AuTe.sub.2, BaCO.sub.3, BaSO.sub.4,
BaCrO.sub.4, BaO, BeO, BiOCl, (BiO).sub.2CO.sub.3, BiO.sub.3,
Bi.sub.2S.sub.3, Bi.sub.2O.sub.3, CaO, CaF.sub.2, CaWO.sub.4,
CaCO.sub.3, (Ca,Mg)CO.sub.3, CdS, CdTe, Ce.sub.2O.sub.3, CoAsS,
Cr.sub.2O.sub.3, CuO, Cu.sub.2O, CuS, Cu.sub.2S, CuS.sub.2,
Cu.sub.9S.sub.5, CuFeS.sub.2, Cu.sub.5FeS.sub.4,
CuS.Co.sub.2S.sub.3, Fe.sup.2+Al.sub.2O.sub.4, Fe.sub.2SiO.sub.4,
FeWO.sub.4, FeAs.sub.2, FeAsS, FeS, FeS.sub.2, FeCO.sub.3,
Fe.sub.2O, .alpha.-Fe.sub.2O.sub.3, .alpha.-FeO(OH),
Fe.sub.3O.sub.3, FeTiO.sub.3, HgS, Hg.sub.2Cl.sub.2, MgO,
MnCO.sub.3, Mn.sub.2S, MnWO.sub.4, MnO, MnO.sub.2, Mn.sub.2O.sub.3,
MnsO.sub.3, Mn.sub.2O.sub.7, MnO(OH), CaMoO.sub.4, MoS.sub.2,
MOO.sub.2, MOO.sub.3, NbO.sub.4, NiO, NiAs.sub.2, NiAs, NiAsS, NiS,
PbTe, PbSO.sub.4, PbCrO.sub.4, PbWO.sub.4, PbCO.sub.3,
(PbCl).sub.2CO.sub.3, Pb.sup.2+.sub.2Pb.sup.4+O.sub.4,
Sb.sub.2SnO.sub.5, Sc.sub.2O.sub.3, SnO, SnO.sub.2, SrO,
SrCO.sub.3, SrSO.sub.4, TiO.sub.2, UO.sub.2, V.sub.2O.sub.3,
VO.sub.2, V.sub.2O.sub.5, VaO, Y.sub.2O.sub.3, YPO.sub.4,
ZnCO.sub.3, ZnO, ZnFe.sub.2O.sub.4, ZnAl.sub.2O.sub.4, ZnS,
ZrSiO.sub.4, ZrO.sub.2, ZrSiO.sub.4, of combinations thereof.
According to some embodiments, the core material may include two or
more homogeneous domains, such as, for example, (Ba,Sr)SO.sub.4,
(Ba,Sr)CO.sub.3, or Ba(SO.sub.4,CrO.sub.3). In some embodiments,
barium sulfate may be used as a weighting agent onto which calcium
carbonate is precipitated.
[0249] Precipitation of calcium carbonate onto a weighting agent
may impart beneficial properties to the weighting agent. For
example, weighting agents, such as iron oxide and barium sulfate,
are denser than calcium carbonate and, therefore, provide a
desirable densifying agent when added to fluids, such as drilling
fluids. These weighting agents, on their own, also have a high
hardness, which makes them abrasive to machinery and formations in
the earth. As a result, the weighting agents may cause abrasion or
corrosion of the machinery during us. By precipitating calcium
carbonate onto the iron oxide or other weighting agent, the
relatively lower hardness of the calcium carbonate layer (e.g.,
about 3 Mohs for calcium carbonate versus about 5.5 Mohs for
hematite) reduces the abrasivity of the weighting agent, which may
reduce the abrasion caused during use of the agent.
[0250] According to some embodiments, the amount of the calcium
carbonate coating may be tailored to provide a desired specific
gravity of the resulting agent. For example, a PCC-coated hematite
weighting agent may have an amount of PCC to provide a specific
gravity of the PCC-coated hematite that is between the specific
gravity of the calcium carbonate and the core material. The
modification of the specific gravity of the weighting agent may
allow for tailoring the weighting agent to specific
applications.
[0251] The choice of weighting agent may also be determined based
on the intended application. For example, calcium carbonate and
iron oxide are dissolvable in dilute acids, such as, for example,
dilute hydrochloric acid (HCl). Choosing a weighting agent and
coating with similar dissolution properties may permit removal of
both the weighting agent and the coating to increase the flow of
hydrocarbon by "shocking" the well with dilute acid to remove the
weighting agent.
[0252] The precipitation of calcium carbonate onto various core
materials also allows for a wider distribution of particle sizes in
the weighting agent. The particle size distribution may be
controlled either by the size of the weighting agent or the amount
of PCC used to coat the particles.
[0253] According to some embodiments, a method of precipitating
calcium carbonate may include providing a core material in
solution, adding calcium sulfate to the solution, adding a
carbonate source to the solution, and precipitating calcium
carbonate onto the core material. According to some embodiments,
the carbonate source may include ammonium carbonate.
[0254] According to some embodiments, the precipitated calcium
carbonate may include a carbonate material containing calcium and
at least one other metal. For example, the precipitated calcium
carbonate may include dolomite (CaMg(CO.sub.3).sub.2).
[0255] According to some embodiments, the core material may be
dissolvable in dilute acid, such as, for example, hydrochloric acid
(HCl).
[0256] According to some embodiments, the core material may include
a weighting agent. According to some embodiments, the core material
may include iron oxide, such as, for example hematite. According to
some embodiments, the core material may include barium sulfate.
[0257] According to some embodiments, the precipitated calcium
carbonate may be between about 10% and about 25% by weight of the
combined calcium carbonate and core material, such as, for example,
between about 10% and about 15% by weight, between about 15% and
about 20% by weight, between about 20% and about 25% by weight,
between about 12% by weight and about 18% by weight, or between
about 18% by weight and about 23% by weight of the combined calcium
carbonate and core material.
[0258] According to some embodiments, the precipitated calcium
carbonate-core composition may have a specific gravity in a range
having a lower limit of about 2.6, 3, 4, 4.5, 5, or 5.5 to an upper
limit of about 20, 15, 10, 9, 8, or 7, and permutations
thereof.
[0259] According to some embodiments, the precipitated calcium
carbonate-core composition may be suitable for use in a drilling
application. For example, the precipitated calcium carbonate-core
composition may be suitable for use as a weighting agent in a
drilling fluid, such as an additive to a drilling mud.
Example 3
[0260] An exemplary calcium carbonate coated hematite was prepared
by reacting calcium sulfate with ammonium carbonate solution.
First, 6.30 grams of calcium sulfate from Sigma Aldrich were mixed
with 250 mL of water at 25.degree. C. for 30 minutes for form a
slurry. Next, 20 grams of the Hematite were added to the slurry and
mixed to keep the specific ratio of 84.5% to 15.5% hematite to PCC
by weight. Then 5.97 grams of ammonium carbonate were dissolved in
250 mL of water and immediately added to the slurry. The final
mixture was mixed for 1 hr to precipitate the PCC onto the
hematite. After mixing, the mixture was filtered and washed to
remove excess ammonium sulfate. The resulting cake containing the
PCC-coated hematite was recovered.
[0261] Nothing in the above description is meant to limit the scope
of the claims to any specific composition or structure of
components. Many substitutions, additions, or modifications are
contemplated within the scope of the present disclosure and will be
apparent to those skilled in the art. The embodiments described
herein were presented by way of example only and should not be used
to limit the scope of the claims.
* * * * *