U.S. patent application number 09/791433 was filed with the patent office on 2002-01-24 for precisely sized precipitated calcium carbonate (pcc) crystals of preselected crystal habit, manufactured using pressure carbonation.
This patent application is currently assigned to G. R. INTERNATIONAL, INC.. Invention is credited to Mathur, Vijay K..
Application Number | 20020009410 09/791433 |
Document ID | / |
Family ID | 23410922 |
Filed Date | 2002-01-24 |
United States Patent
Application |
20020009410 |
Kind Code |
A1 |
Mathur, Vijay K. |
January 24, 2002 |
Precisely sized precipitated calcium carbonate (PCC) crystals of
preselected crystal habit, manufactured using pressure
carbonation
Abstract
Precipitated calcium carbonate crystals of selected crystal
habit. A pressure carbonation process is used to manufacture
precipitated calcium carbonate. A slurry of calcium hydroxide is
agitated in a pressurized reactor, and carbon dioxide is provided
to a reactor to produce calcium ions and carbonate ions that react
under pressure to produce precipitated calcium carbonate at a high
reaction rate. Control of process conditions such as pressure,
temperature, and hydroxide slurry concentration, enables production
of (1) a desired crystal habit (including sclenohedral,
rhombohedral, stacked rombohedral, or aragonite calcium carbonate
crystal structures), (2) a desired crystal size, or (3) a desired
crystal aspect ratio. The precipitated calcium carbonate produced
by the process is useful as either a paper filler or as an
ingredient in paper coatings, and provides a paper product with
improved properties.
Inventors: |
Mathur, Vijay K.; (Federal
Way, WA) |
Correspondence
Address: |
R REAMS GOODLOE JR
10725 SE 256TH STREET
SUITE 3
KENT
WA
980316426
|
Assignee: |
G. R. INTERNATIONAL, INC.
|
Family ID: |
23410922 |
Appl. No.: |
09/791433 |
Filed: |
February 22, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09791433 |
Feb 22, 2001 |
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09358759 |
Jul 21, 1999 |
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6251356 |
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Current U.S.
Class: |
423/430 |
Current CPC
Class: |
C01P 2004/03 20130101;
C01P 2006/60 20130101; C01P 2004/39 20130101; C01F 11/181 20130101;
C01P 2004/30 20130101; D21H 17/675 20130101; C01P 2006/12
20130101 |
Class at
Publication: |
423/430 |
International
Class: |
C01F 011/18 |
Claims
1. A precipitated calcium carbonate having sclenohedral crystal
structure, said sclenohedral crystal structure, said rhomboheral
crystal structure precipitated at a pressure greater than
atmospheric pressure, and comprising a surface area of
approximately 28,200 cm.sup.2/gram (measured by Blaine method), and
a GE brightness of at least 96.7.
2. A precipitated calcium carbonate having rhombohedral crystal
structure, said rhombohedral crystal structure precipitated at a
pressure greater than atmospheric pressure, and having an aspect
ratio of approximately 1:1, said rhombohedral crystal structure
comprising a surface area of approximately 40,000 cm.sup.2/gram
(measured by Blaine method), and a GE brightness of at least
92.1.
3. A precipitated calcium carbonate having rhombohedral crystal
structure, said rhombohedral crystal structure precipitated at a
pressure greater than atmospheric pressure, and having an aspect
ratio of approximately 1:1.5, said rhombohedral crystal structure
comprising a surface area of approximately 21,500 cm.sup.2/gram
(measured by Blaine method), and a GE brightness of at least
98.6.
4. A precipitated calcium carbonate having rhombohedral crystal
structure, said rhombohedral crystal structure precipitated at a
pressure greater than atmospheric pressure, and having an aspect
ratio from approximately 1:1 to approximately 1:1.5, said
rhombohedral crystal structure comprising a surface area of
approximately from approximately 40,000 cm.sup.2/gram to
approximately 21,500 cm.sup.2/gram (measured by Blaine method), and
a GE brightness of at least 92.1.
5. A precipitated calcium carbonate produced having stacked
rhombohedral crystal structure, said stacked rhombohedral crystal
structure precipitated at a pressure greater than atmospheric
pressure, and comprising a surface area of approximately 16,400
cm.sup.2/gram (measured by Blaine method), and a GE brightness of
at least 87.3.
6. A precipitated calcium carbonate having an aragonite crystal
structure, said aragonite crystal structure precipitated at a
pressure greater than atmospheric pressure, and comprising a
surface area of approximately 23,500 cm.sup.2/gram (measured by
Blaine method), and a GE brightness of at least 95.0.
Description
[0001] A portion of the disclosure of this patent document contains
material that is subject to copyright protection. The owner has no
objection to the facsimile reproduction by anyone of the patent
document or the patent disclosure, as it appears in the Patent and
Trademark Office patent file or records, but otherwise reserves all
copyright rights whatsoever.
[0002] This application is a divisional of prior pending
application Ser. No. 09/358,759, filed on Jul. 22, 1999.
TECHNICAL FIELD
[0003] This invention is related to methods for the production of
calcium carbonate via precipitation from solubilized calcium ions
and carbonate ions, and to the products of the method, and to paper
products produced using the products of the process.
BACKGROUND
[0004] The manufacture and supply of high quality calcium carbonate
for paper filler and for paper coatings is now widely practiced
around the world. Relatively recently, particularly as alkaline
papermaking has become popular, the on-site manufacture of
precipitated calcium carbonate ("PCC") from aqueous solution in
atmospheric tanks has also been developed and implemented at a
variety of locations. These on-site plants have developed because
transportation costs of either a dry powder or of a liquid slurry
of calcium carbonate was generally prohibitive. However, the
variability of product quality from the heretofore-employed on-site
PCC plants has been problematic at times. Such problems are
especially acute in those locales where relatively impure sources
of carbon dioxide have been employed, such as from boilers burning
a variety of solid or liquid waste fuels. Also, the particle size
distribution of the PCC obtained from various prior art processes
has been less than optimum, and consequently, it would be
advantageous to provide a process in which the particle size
distribution could be more effectively controlled.
[0005] In processes employed for the manufacture of precipitated
calcium carbonate, several fundamental chemical reaction steps are
normally employed, which steps can be generally summarized as
follows:
[0006] (1) Calcination--heating limestone (calcium carbonate) and
driving the carbon dioxide out, resulting in the formation of lime
(calcium oxide).
[0007] (2) Slaking--reacting lime with water to form a lime slurry
(calcium hydroxide; Ca(OH).sub.2); this reaction is accompanied by
the evolution of heat.
[0008] (3) Carbonation--reacting the lime slurry with carbon
dioxide so that the solubilized calcium from the calcium hyroxide
is reacted with the carbonate produced by bubbling the carbon
dioxide in water, to form the desired calcium carbonate; this
reaction is also exothermic.
[0009] Various prior art techniques disclose methods of preparing
different PCC crystal morphologies, shapes, sizes, and size
distribution of for the precipitated calcium carbonate. Although
the prior art known to me teaches the use of process variables such
as carbon dioxide concentration, calcium hydroxide concentration,
temperature, and the use of chemical additives, none of such prior
art processes known to me utilizes the step of carbonation under
pressure, either alone or in combination with other heretofore
utilized variables, as a technique for increasing the reaction
rate, carbonation efficiency, or for making finer PCC particles.
The prior art has also not employed pressurization of the
carbonation reaction as a method for increasing the rate of
formation of carbonate and calcium ions, the formation of which
(and especially the latter) are the primary limitation in
increasing the rate of carbonation reaction.
[0010] Moreover, the various prior art methods utilized for
production of precipitated calcium carbonate in papermaking
operations can be characterized in that the carbonation reaction
has been carried out in an atmospheric pressure vented or open
vessel. This means that the partial pressure of carbon dioxide
available in the carbonation reactor has been limited based on the
concentration of carbon dioxide available in an incoming gas
stream.
[0011] It is in the carbonation reaction that the soluble calcium
from the calcium hydroxide is converted to calcium carbonate. Then,
more solubilization of the calcium ion takes place as the calcium
hydroxide (lime slurry) is dissolved, and this proceeds until all
of the available calcium hydroxide is converted into calcium
carbonate. In this reaction, the reaction rate of calcium ions
combining with carbonate ions is almost instantaneous.
Consequently, the slow kinetic step which controls the overall
reaction rate is believed to be the rate of dissolution of calcium
hydroxide in the lime slurry, so that calcium ions are available
for reaction. In conventional industrial processes for the
manufacture of calcium carbonate, a slurry of approximately 200
gm/L of calcium hydroxide placed in an atmospheric reactor, and a
gas containing from about 15% to about 20% by volume of carbon
dioxide is bubbled through the slurry. In general, such prior art
processes have a reaction rate such that calcium carbonate is
formed at the rate of from about 0.5 grams per liter of slurry per
minute to about 1.5 grams per liter of slurry per minute. Thus, for
a batch charge of 200 grams per liter of calcium hydroxide, about
200 minutes is required to complete the reaction, per liter of
slurry.
[0012] In general, the currently utilized manufacturing processes
are slow, with low carbonation efficiencies. Thus, manufacturing
plants utilizing such prior art processes require large equipment,
resulting in high capital costs per unit of calcium carbonate
production.
[0013] Relatively recently, approximately eighty percent (80%) of
the world paper production has been converted to an alkaline
papermaking process. In that process, precipitated calcium
carbonate ("PCC") is employed as the primary filler. An average
papermill may require from about 20,000 to about 100,000 tons per
year of PCC. To meet such demands, the production of PCC has
shifted from off-site to on-site. One important advantage of
on-site PCC production has been the saving of transportation costs.
Also, a primary raw material for PCC production, namely carbon
dioxide, is available free at many mills, as a waste product from
lime kiln flue gas. Such gas normally contains from about twelve
percent to about twenty five percent (12%-25%) of carbon dioxide.
However, one limitation encountered was that variability and
fluctuation in the carbon dioxide concentration in the flue gas
produced variability in the resulting PCC. Moreover, some mills do
not have lime kilns, and free on-site sources of carbon dioxide are
limited to flue gas from gas fired boilers, which only have seven
to ten percent (7-10%) carbon dioxide concentration. In such
situations, it has not heretofore been economical to place an
"on-site" PCC plant at the mill location.
[0014] Thus, in order to manufacture large quantities calcium
carbonate as required in papermaking operations, it has heretofore
been necessary to provide very large reactors (for example,
reactors in the 18,000 gallons to 20,000 gallons range are common).
Thus it is evident that it would be desirable to provide a process
in which the overall production rate of calcium carbonate is
increased, thereby reducing the reactor size for a desired PCC
production rate. It would also be advantageous to develop a process
which (a) can utilize low CO2 containing gas, and (b) in which the
effects of fluctuation in CO2 concentration on particle size
distribution of PCC can be minimized.
[0015] Several prior art processes are known which superficially
resemble portions of my process to some limited extent. In U.S.
Pat. No. 3,304,154 issued on Feb. 14, 1967 to Dimitrios
Kiouzes-Pezas for a Process for Producing Spheroidal Alkaline Earth
Metal Carbonates, carbon dioxide gas is bubbled through a
cylindrical autoclave reactor having a calcium hydroxide suspension
therein. Pressure in the reactor was accumulated until a pressure
from about 4 to 6 atmospheres gauge, and preferably about 5
atmospheres gage, was built up. Then, the reactor was rotated,
while keeping the temperature between 60.degree. to 90.degree.
Centigrade. However, that process has some practical limitations
and thus is not well suited to the on-site production of PCC.
First, it is difficult to produce the needed quantities (up to
100,000 tons per year) from such reactors, and starting at the low
calcium hydroxide concentrations taught therein. Second, the
process only produces spheroidal crystal structures. Finally, the
rotation of the reactor presents various practical mechanical
problems, and would result in undesirable cost and expense.
[0016] In U.S. Pat. No. 5,164,006 issued on Nov. 17, 1992 to Vasant
Chapnerkar et al, for a Method for Preparing Acid Resistant Calcium
Carbonate Pigments, gaseous carbon dioxide is added to a slurry of
calcium hydroxide under atmospheric conditions. This conventional
prior art process has a calculated reaction rate of approximately
1.0 grams per liter of slurry per minute, to produce a PCC product
having a sclenohedral crystal habit with a surface area of 27,000
cm.sup.2/gram (Blaine method). However, pressure carbonation was
not utilized in that prior art process.
[0017] In U.S. Pat. No. 5,215,734 issued on Jun. 1, 1993 to Charles
Kunesh et al, for Rhombohedral Calcium Carbonate and Accelerated
Heat-Aging Process for the Production Thereof, a method of
hydro-thermal post treatment of PCC is described. In that process,
PCC produced under conventional process conditions is "heat aged"
in a hydrothermal bomb at temperatures of up to 300.degree. C. for
from 1 to about 24 hours, to cause the crystal structure to change
to a rhombohedral PCC having a surface area of from about 3 to
about 15 m.sup.2/gram. So, this prior art technique uses
conventional atmospheric PCC production, at relatively low reaction
rates, before pressurization is utilized.
[0018] In summary, there continues to be a need for a high
efficiency, simple method of production of PCC that is capable of
efficiently producing large quantities of precipitated calcium
carbonate. And, it would be advantageous to be able to employ such
a process for on-site production of PCC at locations where only
relatively dilute gas streams containing low percentages of carbon
dioxide are available. Finally, it would be advantageous to employ
such a process with flexible manufacturing capability, so that
desired crystal shapes and sizes can be produced when and where
required to meet the manufacturing requirements of a paper mill.
Importantly, it would be desirable that PCC produced from a new
method of on-site production of PCC would improve the properties of
paper produced when utilizing the product from such a novel PCC
manufacturing process.
OBJECTS, ADVANTAGES, AND NOVEL FEATURES
[0019] My novel manufacturing process for producing precipitated
calcium carbonate can be advantageously applied to a variety of
paper mill or manufacturing plant locations. This is because my
process can advantageously employ low concentrations of carbon
dioxide in reaction gas, such as may be found in stack gas from
package boilers, or from other "low grade" carbon dioxide sources.
My novel process is simple, easily applied to automated
manufacturing process methods, and is otherwise superior to those
PCC manufacturing methods heretofore used or proposed.
[0020] From the foregoing, it will be apparent to the reader that
one important and primary object of the present invention resides
in providing an improved method for producing precipitated calcium
carbonate.
[0021] Another objective of my process, and of the apparatus for
carrying out the process, is to simplify the manufacturing
procedures, which importantly, simplifies and improves quality
control in the manufacture of high purity precipitated calcium
carbonate.
[0022] Another objective of my process is to produce a novel, high
purity, uniformly sized, calcium carbonate product via use of the
process
[0023] Other important but more specific objects of the invention
reside in the provision of an improved manufacturing process for
the manufacture of precipitated calcium carbonate, as described
herein, which:
[0024] significantly increases the rate of the carbonation reaction
and thus the production of precipitated calcium carbonate;
[0025] significantly reduces the size of equipment and the
building, thus reduces capital costs of on-site plants;
[0026] increases the efficiency of carbon dioxide utilization, or
carbonation efficiency;
[0027] utilizes low concentration carbon dioxide sources, so that
it can be effectively applied in a variety of locations where
on-site precipitated calcium carbonate production has not
heretofore been economically feasible;
[0028] provides a low cost precipitated calcium carbonate;
[0029] reduces the effect of fluctuations in CO2 concentration in
flue gas and thus provides a high degree of particle size
uniformity, to met optical quality requirements for use in paper
manufacturing operations;
[0030] provides a high quality precipitated calcium carbonate for
filler in alkaline papermaking;
[0031] provides a high degree of particle size uniformity, to meet
optical quality requirements for use in paper manufacturing
operations;
[0032] enables the production of a variety of distinct crystal
morphologies, including calcite scalenohedral, calcite
rhombohedral, and aragonite;
[0033] enables the efficient production of small calcium carbonate
crystals;
[0034] enables process control to be established using reliable and
batch reproducible process parameters, thus enhancing quality
assurance;
[0035] enables the lime slaking production rate to be matched with
the precipitated calcium carbonate production rate, thus
significantly increasing operating rates and thereby reducing
equipment size requirements;
[0036] Other important objects, features, and additional advantages
of my invention will become apparent to the reader from the
foregoing and from the appended claims and the ensuing detailed
description, as the discussion below proceeds in conjunction with
examination of the accompanying drawing.
SUMMARY
[0037] I have now invented, and disclose herein, a novel process
for the manufacture of precipitated calcium carbonate ("PCC"). This
manufacturing process does not have the above-discussed drawbacks
common to heretofore-employed on-site PCC production methods of
which I am aware. The process increases the carbon dioxide
utilization efficiency, and thus overcomes the
heretofore-encountered shortcomings with respect to utilization of
gas streams containing low concentrations of carbon dioxide. Also,
it enables effective process control, providing a method for
creating relatively uniform particle sizes, and thus reliably
controlling crystal quality. And, because the PCC production rate
and the lime slaking rate can be effectively matched, the equipment
employed in the process achieves a high utilization rate, thus
decreasing capital costs on an installed cost per unit of
production capacity basis.
[0038] My method for the production of precipitated calcium
carbonate involves providing lime, either as calcium oxide or
calcium hydroxide, and mixing the calcium oxide or calcium
hydroxide with a solvent until a calcium hydroxide slurry is
formed, with the slurry containing an undissolved solute comprising
a calcium containing molecule, preferably calcium hydroxide, and a
solution comprising calcium ions. Preferably, the solvent is water,
and an aqueous slurry is provided by slaking the lime. Also, lime
slurry can be manufactured in batches that are sized to match a
desired charge volume for a carbonation reactor, or more
preferably, the lime slurry can be continuously manufactured. In
this way, a sequential or semi-continuous operation can be provided
wherein lime slaking is matched to utilization of a slurry in a
carbonation reaction batch. The lime slurry is charged to the
carbonation reactor, which reactor is maintained at a pressure
above the prevailing atmospheric pressure at the plant locale,
while passing a gas stream containing carbon dioxide through the
reactor. Carbonate ions are produced from dissociation or
dissolution of the carbon dioxide in aqueous slurry, which
carbonate ions react with calcium ions available from the solution
carrying the lime slurry, to form a calcium carbonate precipitate.
In a preferred embodiment, the lime slurry is fed to the
carbonation reactor at a pH of 12 or more, and the carbonation
reaction is carried out until substantially all available calcium
is reacted, as indicated by reduction in pH to a pre-selected
endpoint, which occurs when no further hydroxide ions become
available via solvating of calcium hydroxide. When the desired
endpoint pH is reached, which endpoint is normally at least as low
as 8.5, the precipitated calcium carbonate is discharged from the
carbonation reactor, and thereafter, another lime slurry charge is
fed to the reactor, and the carbonation reaction is resumed. For
commonly encountered temperatures and pressures, such as normal
temperature and pressure (25.degree. C. and atmospheric pressure)
the lime slurry (at about 200 grams per liter) comprising calcium
ions contains about 1.6 grams per liter of soluble calcium
hydroxide, as ion. Preferably, the carbonation reaction is carried
out in a continuous stirred tank reactor with a high shear
mechanical mixer, in order to increase the reaction rate.
[0039] In my novel process, the partial pressure of carbon dioxide
available for the carbonation reaction is increased by way of
pressurization of the incoming gas stream to the carbonation
reactor. This can normally be conveniently accomplished by
quenching (cooling) and scrubbing an available stack gas, and then
compressing the cleaned and cooled incoming gas in a gas
compressor, before sending the compressed gas to the carbonation
reactor.
BRIEF DESCRIPTION OF THE DRAWING
[0040] FIG. 1 is a graphical depiction of the reaction rate in
grams of calcium hydroxide (expressed as calcium carbonate) per
liter of slurry per minute, showing the increase in carbonation
reaction rate as the pressure at which the carbonation reaction
takes place is increased.
[0041] FIG. 2 is graphical depiction of the increase in carbonation
efficiency as the pressure at which the carbonation reaction takes
place is increased.
[0042] FIG. 3 is a graphical depiction of the change in surface
area of precipitated calcium carbonate, showing the change as the
pressure at which the carbonation reaction takes place is
increased.
[0043] FIG. 4 is a graphical depiction of the change in density of
a paper sheet made by utilizing the PCC produced by my novel method
as a filler, with the sheet density shown as a function of the
pressure at which the carbonation reaction takes place.
[0044] FIG. 5 is a graphical depiction of the change in porosity of
a paper sheet made by utilizing the PCC produced by my novel method
as a filler, with the sheet porosity shown as a function of the
pressure at which the carbonation reaction takes place.
[0045] FIG. 6 is a graphical depiction of the change in brightness
of a paper sheet made by utilizing the PCC produced by my novel
method as a filler, with the sheet brightness shown as a function
of the pressure at which the carbonation reaction takes place.
[0046] FIG. 7 is a graphical depiction of the change in opacity of
a paper sheet made by utilizing the PCC produced by my novel method
as a filler, with the sheet opacity shown as a function of the
pressure at which the carbonation reaction takes place.
[0047] FIG. 8 is a graphical representation of the light scattering
coefficient of the precipitated calcium carbonate produced by the
present process as a function of the pressure at which the
carbonation reaction takes place.
[0048] FIG. 9 is a graphical comparison of the reaction rate of the
carbonation reaction as a function of the temperature at which the
carbonation reaction takes place, showing the reaction rate for a
gas stream containing 20 percent carbon dioxide, at 0 psig
(atmospheric pressure) and at 30 psig.
[0049] FIG. 10 is a graphical comparison of the carbon dioxide
usage efficiency as function of the temperature at which the
carbonation reaction is carried out, showing the efficiency for a
gas stream containing 20 percent carbon dioxide, at 0 psig
(atmospheric pressure), and at 30 psig.
[0050] FIG. 11 is a graphical representation of the surface area
(shown as Blaine) of PCC as a function of the temperature at which
the carbonation reaction is carried out, showing the PCC surface
area for a gas stream containing 20 percent carbon dioxide at 0
psig (atmospheric pressure), and when using my novel process at 30
psig.
[0051] FIG. 12 is a graphical depiction of the change in density of
a paper sheet made by utilizing the PCC produced by my novel method
as a filler, with the sheet density shown as a function of the
temperature at which the carbonation reaction takes place.
[0052] FIG. 13 is a graphical depiction of the change in porosity
of a paper sheet made by utilizing the PCC produced by my novel
method as a filler, with the sheet porosity shown as a function of
the temperature at which the carbonation reaction takes place.
[0053] FIG. 14 is a graphical depiction of the change in brightness
of a paper sheet made by utilizing the PCC produced by my novel
method as a filler, with the sheet brightness shown as a function
of the temperature at which the carbonation reaction takes
place.
[0054] FIG. 15 is a graphical depiction of the change in opacity of
a paper sheet made by utilizing the FCC produced by my novel method
as a filler, with the sheet opacity shown as a function of the
temperature at which the carbonation reaction takes place.
[0055] FIG. 16 is a graphical representation of the light
scattering coefficient of the precipitated calcium carbonate
produced by the present process as a function of the temperature at
which the carbonation reaction takes place.
[0056] FIG. 17 is a graphical representation of the reaction rate
of the carbonation reaction as a function of the percentage of
carbon dioxide a gas stream provided to the carbonation reactor,
showing the efficiency for a gas stream at 0 psig (atmospheric
pressure), and clearly showing the increased reaction rate when
using my novel process at 30 psig.
[0057] FIG. 18 is a graphical representation of the carbonation
reaction efficiency as a function of the percentage of carbon
dioxide in a gas stream provided to the carbonation reactor,
showing the efficiency for a gas stream at 0 psig (atmospheric
pressure), and clearly showing the increased efficiency when using
my novel process at 30 psig.
[0058] FIG. 19 is a graphical representation of the surface area of
precipitated calcium carbonate (as indicated by Blaine) a function
of the percentage of carbon dioxide in a gas stream provided to the
carbonation reactor, showing the efficiency for a gas stream at 0
psig (atmospheric pressure), and showing the increased surface area
when using my novel process at 30 psig, for low (5% by volume) to
moderate (20% by volume) carbon dioxide concentrations.
[0059] FIG. 20 is a graphical depiction of the change in density of
a paper sheet made with precipitated calcium carbonate, with the
sheet density shown as a function of the concentration of carbon
dioxide in the carbonation reactor incoming gas stream, for 0 psig
(atmospheric pressure), and showing the density when using my novel
process at 30 psig.
[0060] FIG. 21 is a graphical depiction of the change in porosity
of a paper sheet made by utilizing precipitated calcium carbonate,
with the sheet porosity shown as a function of the concentration of
carbon dioxide in the carbonation reactor incoming gas stream, for
0 psig (atmospheric pressure), and showing the porosity when using
my novel process at 30 psig.
[0061] FIG. 22 is a graphical depiction of the change in brightness
of a paper sheet made by utilizing PCC as a filler, with the sheet
brightness shown as a function of the concentration of carbon
dioxide in the carbonation reactor incoming gas stream, for 0 psig
(atmospheric pressure), and showing the brightness when using my
novel process at 30 psig for the production of PCC.
[0062] FIG. 23 is a graphical depiction of the change in opacity of
a paper sheet made utilizing PCC as a filler, with the sheet
opacity shown as a function concentration of carbon dioxide in the
carbonation reactor incoming gas stream, for 0 psig (atmospheric
pressure), and showing the opacity when using my novel process at
30 psig for the production of PCC.
[0063] FIG. 24 is a graphical representation of the light
scattering coefficient of PCC as a function concentration of carbon
dioxide in the carbonation reactor incoming gas stream, for 0 psig
(atmospheric pressure), and showing the light scattering
coefficient when using my novel process at 30 psig for the
production of PCC.
[0064] FIG. 25 is a graphical representation of the reaction rate
of the carbonation reaction, in terms of the grams per liter per
minute of calcium hydroxide converted, as a function of the
concentration of calcium hydroxide in the lime slurry (expressed as
grams of calcium hydroxide as calcium carbonate, per liter of
slurry), for a reaction according to my invention, carried out at
30 psig and 100.degree. F. using a gas stream to the carbonation
reaction which contains 20% carbon dioxide by volume.
[0065] FIG. 26 is a graphical representation of the carbon dioxide
efficiency, in the carbonation reaction as a function of the
concentration of calcium hydroxide in the lime slurry (expressed as
grams of calcium hydroxide as calcium carbonate, per liter of
slurry), for a reaction according to my invention, carried out at
30 psig and 1 00.degree. F. using a gas stream which contains 20%
carbon dioxide by volume upon entry to the carbonation reactor.
[0066] FIG. 27 is a graphical representation of the surface area
(Blaine) of PCC produced by my process as a function of the
concentration of calcium hydroxide in the lime slurry (expressed as
grams of calcium hydroxide as calcium carbonate, per liter of
slurry), for a reaction according to my invention carried out at 30
psig and 100.degree. F. using a gas stream to the carbonation
reaction which contains 20% carbon dioxide by volume.
[0067] FIG. 28 is a graphical representation of the reaction rate
of the carbonation reaction as a function of the speed of the
agitator used to stir the lime slurry in the reactor, for a
reaction carried out at 30 psig and 100.degree. F. using a gas
stream entering the carbonation reactor which contains 20% carbon
dioxide by volume.
[0068] FIG. 29 is a graphical representation of the carbon dioxide
utilization efficiency as a function of the speed of the agitator
used to stir the lime slurry in the reactor, for a reaction carried
out at 30 psig and 100.degree. F. using a gas stream entering the
carbonation reactor which contains 20 percent carbon dioxide by
volume.
[0069] FIG. 30 is a graphical representation of the surface area
(Blaine) of the PCC made in my novel process, expressed as a
function of the speed of the agitator used to stir the lime slurry
in the reactor, for a reaction carried out at 30 psig and
100.degree. F. using a gas stream entering the carbonation reactor
which contains 20% carbon dioxide by volume.
[0070] FIG. 31 is a photograph of the sclenohedral crystals of
precipitated calcium carbonate obtained in the process of the
present invention; the photographs were taken with a scanning
electromicroscope (SEM).
[0071] FIG. 32 is a photograph of the rhombohedral crystals of
precipitated calcium carbonate obtained in the process of the
present invention, where the crystals have an aspect ratio of
approximately 1:1; the photographs were taken with a scanning
electromicroscope (SEM).
[0072] FIG. 33 is a photograph of the rhombohedral crystals of
precipitated calcium carbonate obtained in the process of the
present invention, where the crystals have an aspect ration of
approximately 1:1.5; the photographs were taken with a scanning
electromicroscope (SEM).
[0073] FIG. 34 is a photograph of the stacked rhombohedral crystals
of precipitated calcium carbonate obtained in the process of the
present invention; the photographs were taken with a scanning
electromicroscope (SEM).
[0074] FIG. 35 is a photograph of the aragonite crystals of
precipitated calcium carbonate obtained in the process of the
present invention; the photographs were taken with a scanning
electromicroscope (SEM).
[0075] FIG. 36 is a process flow diagram showing one convenient
arrangement for lime slaking, to prepare a calcium hydroxide slurry
for feed to a pressurized carbonation reactor, in order to carry
out the process of the present invention.
[0076] FIG. 37 is a process flow diagram showing one convenient
configuration for gas compression, or alternate carbon dioxide
preparation, for feed of pressurized carbon dioxide to a
pressurized reactor, in order to carry out the process of the
present invention.
[0077] FIG. 38 is a process flow diagram showing one convenient
arrangement for reaction of lime slurry with carbon dioxide under
pressurized conditions, and for final preparation of the product
produced in the carbonation reactor.
[0078] In general, the information depicted in the various figures
represents data developed from schlenohedral precipitated calcium
carbonate crystals, unless otherwise indicated.
DESCRIPTION
[0079] A novel process for producing precipitated calcium carbonate
is provided which enables the efficient use of "free" carbon
dioxide found in flue gas, and more particularly, from flue gas
containing relatively low concentrations of carbon dioxide. This
process is capable of providing a variety of PCC morphologies, and
the use of such PCC produced by this process has some unique
properties for use as a filler in papermaking operations. This in
turn results in some unusual and beneficial paper properties for
the superior paper products made with the PCC provided according to
the inventive process disclosed herein. Importantly, the
precipitated carbonates that can be manufactured by this process
include distinct crystal morphologies including calcite
scalenohedral, calcite rhombohedral of various aspect ratios, and
aragonite.
[0080] The basic chemistry for producing precipitated calcium
carbonates is well known, and the basic steps of calcination,
slaking, and carbonation, were noted above. The following chemical
reactions describe such basic steps: 1
[0081] The final carbonation reaction is an equilibrium reaction.
Therefore, as the soluble calcium ion is converted to calcium
carbonate precipitate, more dissolution of the calcium hydroxide
takes place from the lime slurry to increase the concentration of
the calcium ion up to the solvent solubility limits (inverse
temperature dependent phenomenon), until all of the available
calcium hydroxide is dissolved, and all available calcium ions have
been converted into calcium carbonate.
[0082] The carbonation reaction is accompanied by the evolution of
heat (i.e., it is an exothermic reaction). The pH of the lime
slurry decreases during the course of the reaction of a batch of
lime with carbon dioxide, and such pH changes from approximately
12.4 to the equilibrium pH of 8, plus or minus about 1 pH unit.
[0083] However in my process the endpoint of the carbonation
reaction is indicated when the pH reaches 7,+/-0.5 pH units. The
reaction rate of the carbonation step is effected by (a) the
concentration of soluble calcium ion, which is controlled by the
rate of dissolution of calcium hydroxide, and (b) rate of carbon
dioxide dissolution or mass transfer into water to form an
available carbonate ion.
[0084] It is important to note, for purposes of my invention, that
the rate of dissolution of Ca(OH).sub.2 is a function of the
temperature and of the pressure at which the dissolution takes
place. This is important since the controlling reaction in the
overall calcium carbonate production process is the dissolution of
the available calcium hydroxide, which is only sparingly soluble in
aqueous solution, and which is inversely dependent upon temperature
in aqueous solution.
[0085] The conventional industrial process for production of
precipitated calcium carbonate is performed by providing a slurry
of approximately 200 g/L of calcium hydroxide in an atmospheric
reactor, and bubbling a gas stream containing carbon dioxide at
about 15-20% by volume into the reactor. In commercially employed
PCC production processes, reaction rates in the range of from about
0.5 grams per liter of slurry per minute to about 1.5 grams per
liter of slurry per minute are commonly seen. Thus, in prior art
PCC production processes, the time required to complete the
reaction in the carbonation reactor is approximately 200 minutes.
That relatively slow overall reaction rate results in a requirement
for large carbonation reactors ( reactors in the 18,000 to 20,000
gallon range are common), with the associated high capital
costs.
[0086] The following ionic reactions describe the overall PCC
production process: 2
[0087] Calcium carbonate is produced by the combination of
equations (6) and (10): 3
[0088] The reactions described by equations (4), (6), (8), and (9)
are slow reactions. Thus the rate controlling reactions could be
considered to be reactions (4) and (9). On the other hand, the
reactions described by the equations (7) and (10) are
instantaneous. In any event, the overall reaction for formation of
PCC, based on raw materials supplied to the process, is as follows:
4
[0089] Also, the calcium carbonate formed by equations (11 or 12)
is also partially soluble in the presence of weak carbonic acid as
follows: 5
[0090] For purposes of my invention, it is important to understand
that the equilibrium of the overall reaction (12) is controlled by
the following primary process variables:
[0091] 1) Reaction Temperature
[0092] 2) Concentration of carbon dioxide
[0093] 3) Partial pressure of carbon dioxide
[0094] 4) Rate of flow of carbon dioxide
[0095] 5) Concentration of Ca(OH).sub.2
[0096] 6) Solubility of Ca(OH).sub.2
[0097] 7) Rate of agitation
[0098] 8) Crystalline habit of the calcium carbonate (calcite vs.
aragonite)
[0099] 9) Chemical additives
[0100] Importantly, carrying out the carbonation reaction at a
pressure greater than atmospheric increases the solubility of
carbon dioxide in aqueous solution, which thus provides a higher
concentration of carbonate ions in solution, for reaction with
available calcium ions. Also, the elevated pressure is believed to
increase the Ca.sup.++ formation. Consequently, an overall increase
in the rate of the carbonation reaction is experienced, thus
leading to an increase in carbonation efficiency, as well as in the
production of finer PCC particles.
[0101] Turning now to FIGS. 36, 37, and 38, process flow schemes
for my novel PCC production process are illustrated. Calcium oxide
(lime) 54 is normally delivered from a rail car (not shown) via
hopper or conveyor or other transport device such as pneumatic tube
powered by blower to an incoming lime storage silo 56. A feeder
sends stored lime via conveyor to a slaker tank 62 which is stirred
by high sheer mixing agitator 64. Slaking water is added to slaker
tank 62 via line 66 from mill water storage tank 68. Storage tank
68 can be fed with water and steam to provide a desired water
temperature in storage tank 68. Slaked lime is pumped via pump 69
to screen 70 to remove oversize materials. Grit 72 is captured and
sent via screw conveyor 74 to grit bin 75.
[0102] The stirred slaked lime slurry is dropped into a mixer 80
stirred surge tank 84 and then pumped 86 via heat exchanger 88 to a
storage tank 90 which is stirred by agitator 92. Preferably, the
volume of slurry stored in storage tank 90 matches the charge
required by carbonation reactor 100 (see FIG. 38), so that once a
batch of slurry is sent from tank 90 to carbonation reactor 100,
the storage tank 90 can be refilled with another batch of slurry.
By matching the size (in terms of throughput) of the slaker 62 to
the size (in terms of thruput) of the carbonation reactor 100,
equipment can be optimized, and both the slaker 62 and the
carbonation reactor 100 can be almost continuously utilized. When
desired to produce the proper crystal habit product, the lime
slurry discharged from tank 90 via pump 101 can be cooled via
chiller 102, or other convenient heat exchange apparatus or process
in order to increase the solubility of calcium in aqueous liquid in
the carbonation reactor 100.
[0103] Any convenient source of carbon dioxide can be utilized in
my novel process, ranging from fresh carbon dioxide provided from
storage tanks 103, or more commonly, flue gas 104 which is sent to
quencher 106 for cooling by a cooling water stream 108. The
quenched gases flow via line 110 to compressor 112 which increases
the pressure of the gas stream, thus increasing the partial
pressure of carbon dioxide supplied to the carbonation reactor 100.
The compressed gas stream 114 is sent to a heat exchanger 116 for
cooling of the gas stream via water stream 118 which is returned to
sewer 120. Alternately, carbon dioxide from tank 103 is heated via
steam supply 122 to heat exchanger 124. The conditioned carbon
dioxide stream, i.e., at a pre-selected temperature and pressure
suitable to assist in producing the desired crystal habit PCC
product, is sent via line 130 to the reactor 100. The cooled,
compressed gas stream 130 containing carbon dioxide under pressure
is sent to the carbonation reactor 100.
[0104] The lime slurry at a preselected temperature is sent from
storage tank 90 to carbonation reactor 100 via line 132. During the
reaction of a batch of slurry in reactor 100, the pH of the liquid
in the carbonation reactor 100 is measured by pH probe 134 or by
other suitable method or means, until the pH falls and ultimately
reaches a desired endpoint that indicates that available calcium
has been consumed. During reaction, agitator 136 maintains high
shear agitation in reactor 100. Agitator 136 therefore has a high
tip speed, ranging from about 260 feet per minute up to about 780
feet per minute, depending upon the design configuration.
[0105] The present invention involves carrying out the carbonation
reaction between CO.sub.2 and Ca(OH).sub.2 under pressure in a
carbonation reactor 100 which is a pressure vessel. This novel
process involves bubbling CO.sub.2 into the Ca(OH).sub.2 slurry in
reactor 100 where the pressure can range from above atmospheric
pressure to as much as about 100 psig. Preferably, the pressure in
the reactor 100 is maintained at up to about 30 psig, and more
preferably, the pressure in the reactor is maintained in the range
from about 15 psig to about 30 psig. Inert gas and any residual
carbon dioxide not utilized (such loss is kept to an absolute
minimum) in reactor 100 is routed via vent line 138 to the
atmosphere.
[0106] By carrying out the carbonation reaction under pressure
according to this invention, the reaction rate can be increased
from the rate of about 1.0 grams of calcium hydroxide per liter of
slurry per minute to up to 10 grams of calcium hydroxide per liter
of slurry per minute. Thus, a production rate increase of as much
as 10 fold can be achieved by utilizing my novel process. This
dramatic increase in reaction rate, even when employed at moderate
pressures or with lower concentrations of carbon dioxide, results
in a decrease in carbonation time from the prior art range of 180
to 200 minutes per batch (when conducted at atmospheric pressure
conditions) to as low as 30 to 40 minutes per batch (when conducted
under pressurized conditions in carbonation reactor 100).
Importantly, the carbonation reactor can be sized less than 200
gallons capacity per ton per day of PCC output, and more
preferably, less than 100 gallons capacity per ton per day, and
most preferably, less than 50 gallons per ton per day of PCC
output.
[0107] Importantly, in my novel process, key process parameters,
such as reaction temperature, carbon dioxide partial pressure, flow
rate of carbon dioxide, lime slurry concentration in the
carbonation reactor, agitator speed in the carbonation reactor, can
be more effectively employed, in order to (a) increase the rate of
carbonation reaction, (b) increase the carbonation efficiency,
i.e., carbon dioxide utilization, and (c) to produce CaCO.sub.3
particles of different morphology, shape, size, and size
distribution.
[0108] In any event, the precipitated calcium carbonate produced in
carbonation reactor 100 is discharged, preferably a PCC batch tank
182 which is stirred by agitator 184. Each PCC batch is then
discharged via pump 186 to final screens 188, where any remaining
oversize material is removed and sent via chute 190 to conveyor 72
and ultimately to grit bin 74. The produced PCC is received in tank
200. Optionally, line 154 supplies additional selected chemicals
from tank 202 to via metering pump 203 to tank 200, to minimize any
pH rise and associated loss of product, as well as to provide
further product quality attributes as may be desired in a
particular on-site situation. Tank 200 is preferably, but need not
be, atmospheric. Finally, the product PCC is stored in tank 202,
and mixed with agitator 204, before being sent via pump 206 to the
papermill.
[0109] The pressure carbonation drives the overall reaction, by
improving the CO.sub.2 mass transfer (CO.sub.3.sup..dbd.
formation). The higher reaction pressure evidently also increases
the solubilization of Ca(OH).sub.2 slurry into calcium ions
(Ca.sup.++). This results in a higher reaction rate, due to
increased calcium ion availability, which in turn reduces the
reaction time of calcium carbonate formation.
[0110] Since much higher reaction rates are achievable, for the
same production rate, my novel PCC manufacturing can be carried out
using much smaller equipment and building size than is the case
with prior art atmospheric PCC production equipment. Overall, even
considering the additional equipment required in my process, such
as the gas compressor, an overall lower capital and operating cost
is achievable.
[0111] Another distinct advantage of this "Pressure Carbonation"
invention is that it increases the efficiency of CO.sub.2
utilization. Moreover, of industrial significance is the ability to
use carbon dioxide in concentrations as low as 5 percent by volume.
Because the incoming gas stream is pressurized, and the partial
pressure of carbon dioxide is increased in the aqueous solution,
the pressurized carbonation reaction provides higher concentrations
of CO.sub.3.sup..dbd. ions, since the dissolution of CO.sub.2 is
proportional to the partial pressure of CO.sub.2. Importantly, low
grade carbon dioxide containing gases (including those in the 10.0%
carbon dioxide by volume range) such as are available from gas
fired boilers, can be advantageously employed in on-site PCC
production plants.
[0112] Even when utilizing low concentrations of CO.sub.2,
utilization of the same may exceed 90%, and more preferably, exceed
95%,and most preferably, exceed 99%.
[0113] My novel pressure carbonation process for the production of
PCC can also produce a wide variety of crystal habits, including
like calcite, rhombohedral, and aragonite in different sizes,
shapes, and aspect ratios.
[0114] Finally, and most importantly, the PCC provided by the
instant invention produces crystals which improve key paper
properties, including porosity, density, brightness, and
opacity.
[0115] My novel process has been thoroughly investigated in
experimental laboratory apparatus, in three main steps:
[0116] (1) Slaking
[0117] Market quality lime in a quantity from about 50 to about 300
grams (90% CaO), size at {fraction (1/2)}" rotary pebble type lime,
was added slowly to approximately 1.2 liters of water, under
constant stirring. The time taken for slaking was approximately 30
minutes. Due to the exothermic nature of the reaction, the final
temperature is elevated over starting conditions. The actual final
temperature rise is dependent on the initial water temperature and
on the "reactivity" of the lime. In general, the initial
temperature of water provided in the process is in the range of
80.degree. F. to about 100.degree. F. The final temperature, after
slaking, is normally in the range from about 150.degree. F. to
about 160.degree. F. In any event, the resulting calcium hydroxide
slurry is screened, preferably through a 140 mesh screen.
[0118] 2) Pressure Carbonation--Lab Reactor Design
[0119] Experimentally, the screened Ca(OH).sub.2 slurry is then
transferred into a reaction vessel of 1.6L total capacity. The
reactor is capable of being heating with outside jacketed heaters.
The system can be sealed and operated at super atmospheric
pressures (i.e. at pressures greater than atmosphere). The reactor
is also fitted with a cooling coil to maintain isothermal
temperature, when desired necessary. In the experimental vessel,
the agitator impeller used is a Rustin 200. The agitator/impeller
is connected to a magnetic, variable speed, drive. The particular
vessel is also fitted with a dip or a sample tube. The primary
purpose of the dip tube is to obtain samples of
Ca(OH).sub.2/CaCO.sub.3 slurry periodically and to follow the
conversion of calcium hydroxide to calcium carbonate by measuring
pH and/or by titration. The experimental reactor is also connected
to a temperature controller via a transducer
[0120] (3) Pressure Carbonation--Process and Process Variables
[0121] Experimentally, the slaked lime slurry was placed in a
reactor capable of withstanding pressures greater than 1
atmosphere. The carbon dioxide was supplied from pressure cylinders
with a pressure in the range of 0 to about 180 psig. In order to
simulate flue gas from lime kilns, the primary source of CO.sub.2
containing gasses for commercial onsite PCC plants, nitrogen
(N.sub.2) from pressure cylinders was also supplied along with the
CO.sub.2. Each of the gases are passed through a separate mass flow
meter. The flow of gases was further verified by a CO.sub.2/water
displacement process and/or by actually weighing the mass of
CO.sub.2 lost from the CO.sub.2 cylinder. The reaction conditions
were varied to meet the specific requirement of reaction rate,
particle size, shape and morphologies. The Ca(OH).sub.2
concentration used ranged from 50 grams per liter of slurry (90%
CaO) to a high of about 300 grams per liter of slurry. The
preferred concentration was about 250 grams of calcium hydroxide
per liter of slurry. Experimentally, the carbonation temperature
was varied from 60.degree. F. to 130.degree. F. In generally, the
selected temperature was chosen based on the need to obtain a
desired crystal morphology and particle size. For example,
sclenohedral PCC was manufactured in the range of 90.degree. F. to
106.degree. F. carbonation temperature. The preferred carbonation
temperature for rhombohedral PCC was 30.degree. F. to 50 .degree.
F. Finally, the threshold carbonation temperature for an aragonite
structure was approximately 120.degree. F.
[0122] The carbon dioxide concentration was also varied from a low
of 5.0% CO.sub.2/95% N.sub.2 by volume to a high of 100%
CO.sub.2/0% N.sub.2 by volume. The preferred CO.sub.2 concentration
fraction was 20% CO.sub.2, with the remainder 80% N.sub.2, by
volume.
[0123] Another important variable is the flow rate of the carbon
dioxide through the carbonation reactor. Experimentally, the flow
of carbon dioxide was varied from a conventional flow rate of 0.5
L/min to 4.0 L min. The preferred flow rate was 1.5 liters per
minute for the above noted size reactor.
[0124] The rate of agitation of the impeller speed is important, in
order to maintain high rates of mass transfer of CO.sub.2 (gas)
into dissolved CO.sub.2 (aqueous), i.e., the rate of carbonic acid
formation. Experimentally, the agitator speed was varied from 500
rpm to 1500 rpm. The preferred rpm was 1470.
[0125] Importantly, operating the carbonation reaction under
isothermal conditions resulted in unique PCC products.
[0126] Experimentally, since the carbonation reaction is an
exothermic reaction, the progress of the reaction was accompanied
by an observed increase in temperature. The reaction kinetics were
determined using temperature to indicate the endpoint of the
carbonation reaction. As the conversion of Ca(OH).sub.2 into
CaCO.sub.3 was completed, the temperature reached a maximum, and
then dropped. A temperature probe controller connected to the
reaction vessel was used to follow the rise and fall of the
reaction temperature. The temperature profile was used to indicate
the reaction end point. The chemical analysis of the final product,
and pH, confirmed the finding of the carbonation reaction end
point. If the pH drifts higher, then the carbon dioxide can be
applied sequentially until stable pH is achieved.
[0127] Experimentally, the calcium carbonate formed under the novel
pressure carbonation technique was filtered through a Whatman #212
filter paper using a vacuum pump, and was washed to remove
impurities. One portion of the sample was dried, and the other
portion was reslurried for end use in performance testing via
preparation of paper handsheets.
[0128] Specific examples which set forth novel process parameters,
or products, include the following examples:
EXAMPLE 1
The Effect of Pressure in a Pressure Carbonation System on Reaction
Rate Carbonation Efficiency, and Surface Area
[0129] In a series of experiments, the carbonation reaction
pressure was raised from 0 psig (as done with a conventional open
tank PCC system) to 70 psig. The reaction temperature was kept
constant at 100.degree. F. and the % CO.sub.2 was kept constant at
20% CO.sub.2/ 80% N.sub.2 by volume. The resulting experimental
data is given in Table 1B. The resulting reaction rate at 0 psig
was 4.6 grams per liter of slurry per minute. In the pressure
carbonation system operating at 70 psig, it was 6.1 grams per liter
per minute. The increase in reaction rate was approximately 33%.
The carbonation efficiency, i.e., carbon dioxide utilization,
increased from 76% to 100%. The surface area (Blaine) of the PCC
produced by the process increased from 31,400 cm.sup.2/gram at 0
psig, to a maximum of 40,200 cm.sup.2/g at 50 psig, and then
decreased slightly to 35,500 cm.sup.2/g at 70 psig. See FIGS. 1, 2,
and 3.
1TABLE 1A Effect of Pressure on Reaction Rate, Carbonation
Efficiency, and Surface Area Pressure Reaction Carbonation Surface
Area Batch # (psig) Rate Efficiency (%) (cm.sup.2/g) 135 0 4.6
77.379 31,400 136 10 5.2 86.308 33,200 146 20 5.6 88.000 38,700 137
30 5.6 93.500 37,200 143 30 5.6 95.489 36,800 138 50 6.0 97.565
40,200 148 60 5.9 102.000 36,300 139 70 6.1 97.565 35,500
Effect of Carbonation Reaction Temperature on Key Paper
Properties
[0130] The PCC batches prepared in Example 1 were then used to
prepare paper handsheets. Some of the key paper properties,
including sheet density, sheet porosity, sheet brightness, and
sheet opacity, were then measured in each of the handsheets which
were formed. The data from tests on handsheets is provided in Table
1B below. A graphical representation of the data is also provided
in FIGS. 4 through 9 below. It is important to note some of the key
characteristics of paper made from PCC under different carbonation
pressures. The sheet density of paper handsheets containing PCC
produced under increasing pressure is shown in FIG. 4. The sheet
porosity of paper handsheets containing PCC produced under
increasing pressure is shown in FIG. 5. As shown in FIG. 6, the
sheet brightness of paper handsheets containing PCC produced under
pressure decreased as pressures increased up to about 30 psig.
Thereafter, the sheet brightness increases as the reaction pressure
was increased from 30 psig to 70 psig. As shown in FIG. 7, the
sheet opacity of paper handsheets increased as the pressure of the
carbonation reaction producing the PCC increased. Also, as
indicated in FIG. 8, the scattering coefficient of handsheets
produced using PCC manufactured under pressure carbonation proved
higher than the scattering coefficient of PCC produced at 0 psig as
in a conventional, open system.
2TABLE 1B Effect of Carbonation Reaction Pressure on Key Paper
Properties Porosity Scattering Batch Pressure Density (sec/100
Brightness Opacity Coefficient # (psig) (g/cm.sup.3) cc air) (ISO)
(ISO) (cm.sup.2/g) 135 0 0.588 11.37 90.71 87.83 2338.55 136 10
0.595 12.09 90.58 88.11 2358.83 137 30 0.595 13.11 90.48 88.35
2442.24 138 50 0.610 14.66 90.57 88.00 2321.92 139 70 0.606 14.65
90.72 88.33 2340.92
EXAMPLE 2
The Effect of Temperature in a "Pressure Carbonation" System
[0131] As in example 1, the slaked lime was placed into a reactor
at a slurry concentration of 250 grams of calcium hydroxide grams
per liter. The starting carbonation temperature was varied from
65.degree. F. to 125.degree. F. A first set of reactions was
carried out under conventional atmospheric pressure or open PCC
type system conditions at 0 psig. The next set of reactions was
carried out under a pressure of 30 psig. A gas mixture of 20%
carbon dioxide and 80% nitrogen by volume was bubbled through the
reactor. The flow of carbon dioxide was at the rate of 1.5 liters
per minute. The reaction rate was calculated by titrating
Ca(OH).sub.2 at the beginning and end of the reaction. As the
reaction proceeded, the reaction temperature increased, with the
temperature starting at 38.degree. C. and ending at 73.degree. C.
The end of the reaction was indicated when the temperature reached
a maximum and then declined. The point of inflection in the
temperature curve was taken as the completion point of the
carbonation reaction.
[0132] The carbonation reaction conditions and the experimental
data resulting is shown in Table 2A and in Table 2B. The reaction
rates at varying temperature, for a prior art atmospheric system (0
psig) are shown in FIG. 9. The corresponding reaction rates for my
"pressurized carbonation" system operating at 30 psig are also
shown in FIG. 9. The graphs indicate that in the pressurized
carbonation process, the reaction rates steadily increased as a
function of temperature. On the other hand, the data indicated that
the reaction rate as a function of temperature in an open system
(at Opsig) gradually as temperature was raised, from approximately
4.4 grams per liter per minute to 5.0 grams per liter per minute
until the temperature reached about 100.degree. F. However, FIG. 9
shows that as the temperature was increased beyond 100.degree. F.,
the rate of reaction decreased to 4.4 grams per liter per
minute.
[0133] As indicated in FIG. 10, similar results were observed with
respect to carbon dioxide utilization efficiency. The carbon
dioxide utilization efficiency in my pressurized system increased
significantly as the temperature was increased from about
60.degree. F. to about 120.degree. F. As can be seen in FIG. 10,
with carbonation occurring at 30 psig, the carbon dioxide
utilization efficiencies were in the range from slightly above 80%
to out 100%. In an atmospheric system (operating at 0 psig) the
carbonation efficiency was lower, ranging from about 74% to about
84%.
[0134] Overall, a pressurized carbonation system provided a higher
reaction rate throughout the whole range of operating temperature.
The surface areas of the produced PCC, as measured by Blaine for
both the pressurized and non-pressurized systems at different
temperatures, is provided in FIG. 11. In both the pressurized
system and in the atmospheric system cases, the surface area of the
product decreased as the reaction temperature was increased. The
surface area of calcium carbonate decreased from approximately
44,000 cm.sup.2/g to a coarse PCC of 22,000 cm.sup.2/g.
Importantly, the controllability of surface area via temperature
was more linear under pressurized carbonation conditions, at least
at the 30 psig condition which was tested.
3TABLE 2A The Effect of Temperature on Reaction Rate, Carbonation
Efficiency, and Surface Area in a Non-Pressurized System.
Temperature Pressure % Reaction Rate Carbonation Surface Batch #
(.degree. F.) (psig) CO.sub.2 (g/L/m) Efficiency (%) Area
(cm.sup.2/g) 175 70 0 20.0 4.40 77.4 42,100 174 80 0 20.0 4.20 74.8
44,700 173 90 0 20.0 4.75 81.6 42,800 172 100 0 20.0 4.96 81.6
33,800 169 106 0 20.0 4.70 84.7 34,500 170 110 0 20.0 4.60 80.14
23,900 171 120 0 20.0 4.40 76.1 23,000
[0135]
4TABLE 2B The Effect of Temperature on Reaction Rate, Carbonation
Efficiency, and Surface Area in a Pressurized System Temperature
Pressure % Reaction Rate Carbonation Surface Batch # (.degree. F.)
(psig) CO.sub.2 (g/L/m) Efficiency (%) Area (cm.sup.2/g) 127 65
30.0 20 4.82 81.6 52,700 129 70 30.0 20 5.30 91.6 42,000 130 80
30.0 20 5.16 88.0 41,500 128 90 30.0 20 5.40 91.6 43,100 133 100
30.0 20 5.30 93.5 36,100 131 106 30.0 20 5.72 95.5 27,800 132 110
30.0 20 5.93 99.7 24,100 134 120 30.0 20 6.10 100.0 22,100
[0136]
5TABLE 2C Effect of Carbonation Reaction Temperature on Key Paper
Properties Scattering Temperature Density Porosity Brightness
Opacity Coefficient Batch # (.degree. F.) (g/cm.sup.3) (sec/100 cc
air) (ISO) (ISO) (cm.sup.2/g) 127 65 0.610 17.18 88.93 84.97
1539.58 129 70 0.606 14.38 89.81 85.96 1778.02 130 80 0.606 14.25
90.27 86.82 2035.03 128 90 0.613 16.96 90.33 86.95 2074.19 133 100
0.606 12.66 90.73 88.33 2340.79 131 106 0.588 9.31 90.45 87.44
2246.31 132 110 0.581 8.38 89.77 86.61 2021.03 134 120 0.592 9.41
90.03 85.67 1799.77
EXAMPLE 3
The Effect of % CO-hd 2 Concentration on Reaction Rate, Carbonation
Efficiency, and Surface Area
[0137] In this series of experiments, the concentration of CO.sub.2
was varied from 5.0% CO.sub.c/95% N.sub.2 to 100% CO.sub.2/0%
N.sub.2,, by volume. Other reaction conditions were kept constant
at the following levels:
6 Flow of CO.sub.2: 1.5 liters per minute Carbonation Reaction
Temp.: 100.degree. F. Ca(OH).sub.2 Concentration: .about.260 grams
per liter
[0138] The results of the measurements of the reaction rate,
carbonation efficiency, and PCC surface area are given in Table 3A
and 3B. The results are also graphed in Figures 17, 18, and 19.
[0139] It is evident from FIG. 17 that when the entering gas stream
contains only 5.0% CO.sub.2, the reaction rate is almost doubled by
using my "pressure carbonation" PCC production process with
pressure carbonation at 30 psig. As indicated in Figure 19 the
surface area comparison between batch #140 and batch #149 also
indicates the formation of a finer PCC particle size using the
"pressure carbonation" technique.
[0140] Also, as the concentration of carbon dioxide increased, the
reaction rates increased. Similarly, the carbonation efficiency
increased with increasing CO.sub.2 concentration. The particle
surface area also increased with CO.sub.2 concentration indicating
formation of finer PCC particles (42,000cm.sup.2/g). The reaction
rate under the pressurized system was much higher than with the
reaction carried out at atmospheric pressure. See FIGS. 17, 18, and
19.
7TABLE 3A The Effect of CO.sub.2 Concentration on Reaction Rate,
Carbonation Efficiency, and Surface Area in a Pressurized System.
Carbonation Blaine Batch # Pressure % CO.sub.2 Reaction Rate
Efficiency (%) (cm.sup.2/g) 140 30 5 4.5 76.0 25,700 141 30 10 5.3
89.0 35,700 142 30 15 5.7 93.5 29,200 143 30 20 5.6 95.5 36,800 144
30 50 6.0 100.0 39,200 145 30 100 5.6 93.5 42,800
[0141]
8TABLE 3B Comparative Example - The Effect of CO.sub.2
Concentration on Reaction Rate, Carbonation Efficiency, and Surface
Area in a Non-Pressurized System. Carbonation Blaine Batch #
Pressure % CO.sub.2 Reaction Rate Efficiency (%) (cm.sup.2/g) 149 0
5 2.3 60.0 23,100 150 0 10 3.5 64.0 27,900 151 0 15 4.2 72.0 28,100
152 0 20 4.7 77.0 27,500 153 0 50 5.4 99.7 41,400 154 0 100 5.8
97.0 40,500
Effect of % CO.sub.2 on Key Paper Properties
[0142] The PCC produced under pressure carbonation conditions at
different carbon dioxide concentrations was used to make paper
handsheets. The quality data of key paper properties is set forth
in Tables 3C, for handsheets made with PCC manufactured under
pressure carbonation conditions, and in Table 3D, for handsheets
made with PCC manufactured under atmospheric conditions. The
graphical representations of the data are shown in FIGS. 20 through
24. The key characteristics of the paper handsheets as a function
of carbon dioxide concentration are given below. In FIG. 20, in
paper produced using PCC manufactured under pressure carbonation
conditions, the sheet density is shown to increase with increasing
carbon dioxide concentration. In FIG. 21, in paper produced using
PCC manufactured under pressure carbonation conditions, the Gurley
sheet porosity increased over paper produced using PCC manufactured
under atmospheric conditions. In other words, the higher Gurley
sheet porosity seen in handsheets made from PCC manufactured under
pressure carbonation conditions means that tighter sheets were made
possible by utilizing PCC manufactured under pressure.
[0143] Turning now to FIG. 22, it is important to note that the
sheet brightness of handsheets produced from PCC made under
pressure was higher than the brightness of handsheets made from PCC
produced under atmospheric systems. However, in both cases, an
increase the % CO.sub.2 up to about 60% or more resulted in lower
sheet brightness.
[0144] In FIG. 23, it is also seen that except at low carbon
dioxide concentrations, where the sheet opacity was comparable, the
sheet opacity of handsheets produced from PCC made under pressure
was higher than the opacity of handsheets up through about 60%
pressure carbonation.
[0145] With respect to scattering coefficient, as seen in FIG. 24,
handsheets produced with PCC manufactured under pressure
carbonation conditions had higher values for the scattering of
light.
9TABLE 3C Effect of % CO.sub.2 in a Pressure Carbonation System On
Key Paper Properties Porosity Scattering % CO.sub.2 Density
(sec/100 cc Brightness Opacity Coefficient 30 Batch # (%)
(cm.sup.3/g) air) (ISO) (ISO) (cm.sup.2/g) psig 140 5 0.592 9.04
90.13 86.74 2112.18 30 141 10 0.581 11.55 89.49 88.52 2353.36 30
142 15 0.585 13.80 89.41 88.27 2308.44 30 143 20 0.595 16.95 89.40
88.53 2374.34 30 144 50 0.613 21.74 89.63 88.68 2306.65 30 145 100
0.613 20.66 89.33 87.96 2201.31 30
[0146]
10TABLE 3D Comparative Example Effect of % CO.sub.2 in a
Non-Pressurized System on Key Paper Properties Porosity Scattering
% CO.sub.2 Density (sec/100 cc Brightness Opacity Coefficient 0
Batch # (%) (cm.sup.3/g) air) (ISO) (ISO) (cm.sup.2/g) psig 149 5
0.592 12.11 89.40 88.08 2245.56 0 150 10 0.581 11.44 89.19 88.49
2224.49 0 151 15 0.588 12.17 88.97 88.13 2195.90 0 152 20 0.592
13.40 88.95 88.36 2149.23 0 153 50 0.613 25.04 89.05 87.60 2104.61
0 154 100 0.595 20.09 89.13 88.66 2313.66 0
EXAMPLE 4
The Effect of Calcium Hydroxide Concentration on Reaction Rate,
Carbonation Efficiency and Surface Area of PCC
[0147] In this example, the concentration of calcium hydroxide,
measured as calcium carbonate, was varied from a low of 35 grams
per liter to a high of 308 grams per liter. The constant reaction
conditions were as follows:
11 CO.sub.2 Flow: 1.5 liters per minute CO.sub.2 Concentration: 20%
Carbonation Temp.: 100.degree. F. Reaction Pressure: 30 psig
[0148] The experimental data for variation of calcium hydroxide
slurry concentration in a pressure carbonation reactor is shown in
Table 4. The reaction rate response, as calcium hydroxide
concentration is varied, is given in FIG. 25.
[0149] The carbonation efficiency and surface area of the PCC are
given in FIGS. 26 and 27, respectively. As shown in FIG. 25, the
reaction rate response was curvilinear, an inverse parabola. The
carbonation efficiency followed a similar trend. However, as the
calcium hydroxide concentration increased, the particle surface
area of the PCC manufactured under pressure carbonation conditions
decreased steadily from about 55,000 cm.sup.2/g to about 30,000
cm.sup.2/g, as the calcium hydroxide concentrations increased from
about 25 to about 308 grams per liter of lime slurry. FIGS. 25 and
26 indicate that the initial reaction rates and carbon dioxide
efficiency, respectively, were higher at lower calcium hydroxide
concentrations. The reaction rate and the carbon dioxide
utilization efficiency decreased as the concentration of
Ca(OH).sub.2 increased to about 125 grams per liter of calcium
hydroxide. However, beyond 150 grams per liter, the reaction rate
and the carbon dioxide utilization efficiency increased, reaching a
maximum at, or slightly less than, about .about.300 grams per liter
of Ca(OH).sub.2.
12TABLE 4 The Effect of Calcium Hydroxide Concentration on Reaction
Rate, Carbonation Efficiency, and Surface Area. Calcium Hydroxide
Concentration Reaction Carbonation Surface Area Batch # (as
CaCO.sub.3) Rate Efficiency (%) (cm.sup.2/g) 168 25 6.3 116.64
54,800 162 49 5.4 97.28 39,400 164 94 4.7 80.64 47,400 165 151 4.7
82.20 44,400 163 194 5.5 100.11 39,400 166 206 5.9 100.11 27,300
167 266 5.9 99.73 37,300 161 308 6.2 105.17 29,600
EXAMPLE 5
Effect of Agitation (RPM of Agitator) on Pressure Carbonation
[0150] As in example 3, the slaked lime was placed in a pressurized
reactor vessel. In this set of experiments, the agitation in the
reaction vessel was successively increased from 500 RPM to about
1800 RPM on the agitator. The other reaction conditions were kept
constant at the following levels:
13 CO.sub.2 Flow: 1.5 liters per minute CO.sub.2 Concentration: 20%
Carbonation Temperature: 100.degree. F. Ca(OH).sub.2 Concentration:
.about.250 gpl Carbonation Pressure: 30 psig
[0151] The reaction rate was measured by titration of the lime
slurry at regular intervals. The experimental data is given in
Table 5. As indicated in FIG. 28, with increased agitation, the
reaction rate of carbonation increased three fold from about 2.0
grams per liter per minute of calcium hydroxide consumption to
about 6.0 grams per liter per minute of calcium hydroxide
consumption. Importantly, under pressure carbonation conditions of
30 psig, with increased agitation, the carbonation efficiency
increased from a low of 35.0% to a high of 99.6%, as indicated in
FIG. 29. As shown in FIG. 30, the particle surface area of PCC
manufactured under pressure carbonation conditions of 30 psig
increased from a low of 27,900 cm.sup.2/g as measured by Blaine, to
about to 43,400 cm.sup.2/g.
14TABLE 5 The Effect of Agitation on Reaction Rate, Carbonation
Efficiency, and Surface Area. Reaction Carbonation Surface Area
Batch # RPM Rate Efficiency (%) (cm.sup.2/g) 155 400 2.0 35.619
21,900 156 750 4.3 73.574 27,400 157 1100 5.2 91.592 35,100 158
1800 5.8 102.000 43,400 159 1500 6.0 99.600 32,600
EXAMPLE 6
Preparation of Sclenohedral PCC
[0152] A slaked lime slurry having a concentration of 246 grams per
liter of slurry was placed in a pressurized reaction vessel. A gas
mixture of 20% carbon dioxide/80% nitrogen was bubbled through the
reactor. The initial carbonation reaction temperature was at
100.degree. F. The pressure in the carbonation reaction vessel was
maintained at 30 psig. The PCC manufactured under such pressurized
carbonation conditions had the following characteristics:
[0153] Particle Surface Area=28,200 cm.sup.2/gram (Blaine
method)
[0154] Brightness=96.7 GE
[0155] Crystal Habit=Sclenohedral (calcite)
[0156] The scanning electron micrograph for this scalenohedral PCC
product is shown in FIG. 31. Among other uses, these PCC particles
are useful as fillers in paper and paper boards.
EXAMPLE 7
Preparation of Rhombohedral PCC with .about.1:1 Aspect Ratio
[0157] A slaked lime slurry having a concentration of 87 grams per
liter of calcium hydroxide slurry (expressed as calcium carbonate)
was placed in a pressurized reaction vessel. A gas mixture of 20%
carbon dioxide/80% nitrogen was bubbled through the reactor. The
initial carbonation reaction temperature was at 68.degree. F. The
increase of reaction temperature was limited to 4.0.degree. F. by
circulating cooling water through the reactor. The pressure in the
reaction vessel during pressure carbonation was maintained at 20
psig. The PCC manufactured under such pressurized carbonation
conditions had the following characteristics:
[0158] Particle Surface Area=40,900 cm.sup.2/gram (Blaine
method)
[0159] Brightness=92.1 GE Aspect Ratio.about.1:1
[0160] The scanning electron micrograph for this rhombohederal PCC
product is shown in FIG. 32. Among other uses, these PCC particles
can be effectively used for both filler and as coating material for
paper.
EXAMPLE 8
Preparation of Rhombohedral PCC with .about.1:1.5+ Aspect Ratio
[0161] By manipulation of process variables, PCC with various
aspect ratios can easily and reliably be produced using my
pressurized carbonation process. Aspect ratio is the ratio of
crystal breadth to crystal length, and is considered a
semi-qualitative number. To produce rhombohedral PCC with an aspect
ratio of 1:1.5, a slaked lime slurry having a concentration of 116
grams per liter of calcium hydroxide slurry (expressed as calcium
carbonate) was placed in a pressurized reaction vessel. A gas
mixture of 20% carbon dioxide/80% nitrogen was bubbled through the
reactor. The initial carbonation reaction temperature was at
50.degree. F. The reaction was carried out under isothermal
conditions, and thus, heat generated by the exothermic nature of
the reaction was removed with circulating cooling water to maintain
the reactor temperature. The pressure in the reaction vessel during
pressure carbonation was maintained at 30 psig. The PCC
manufactured under such pressurized carbonation conditions had the
following characteristics:
[0162] Particle Surface Area=21,500 cm.sup.2/gram (Blaine
method)
[0163] Brightness=98.6 GE
[0164] Aspect Ratio.about.1:1.5+
[0165] The scanning electron micrograph for this rhombohederal PCC
product is shown in FIG. 33. Among other uses, these rhombohederal
PCC particles can be effectively used for both filler and in
coating formulations for paper.
EXAMPLE 9
Preparation of "Stacked" Rhombohedral PCC
[0166] By manipulation of process variables, a unique "stacked" PCC
crystal structure can be reliably produced using my pressurized
carbonation process. To produce stacked rhombohedral PCC, a slaked
lime slurry having a concentration of 32 grams per liter of calcium
hydroxide slurry (expressed as calcium carbonate) was placed in a
pressurized reaction vessel. A gas mixture of 25% carbon
dioxide/75% nitrogen was bubbled through the reactor. The initial
carbonation reaction temperature was at 73.degree. F. The
carbonation pressure was maintained at 70 psig. The reaction
yielded a PCC with the following characteristics:
[0167] Surface Area=16,400 cm.sup.2 /gram (measured by Blaine)
[0168] Brightness=87.3 GE
[0169] Crystal Structure=stacked rhombohedral
[0170] The scanning electron micrograph for this rhombohederal PCC
product is shown in FIG. 34. The pressure carbonation conditions
just described provide this unique stacked rhombohedral crystal
structure. Among other uses, these stacked rhombohederal PCC
particles can be especially useful in coating.
EXAMPLE 10
Preparation of Aragonite PCC
[0171] When desired, aragonite crystal habit PCC crystal structure
can be reliably produced using my pressurized carbonation process.
To produce stacked rhombohedral PCC, a slaked lime slurry having a
concentration of 229 grams per liter of calcium hydroxide slurry
(expressed as calcium carbonate) was placed in a pressurized
reaction vessel. A gas mixture of 25% carbon dioxide/75% nitrogen
by volume was bubbled through the reactor. The initial carbonation
reaction temperature was at 120.degree. F. The carbonation pressure
was maintained at 70 psig. The reaction yielded a PCC with the
following characteristics:
[0172] Surface Area=23,500cm.sup.2/gram (measured by Blaine)
[0173] Brightness=95.0 GE
[0174] Crystal Structure=aragonite
[0175] The scanning electron micrograph for this aragonite PCC
product is shown in FIG. 35. The pressure carbonation conditions
just described provide this aragonite crystal structure. Among
other uses, these aragonite PCC particles can be useful in filler
for paper.
[0176] Generally, it should also be noted that the pressure
carbonation for production of PCC process as described herein can
be used with any convenient source of carbon dioxide, since the
pressurization of the reactor advantageously increases the partial
pressure of carbon dioxide to an extent that it can be economically
exploited. It is to be appreciated that my process for the
production of precipitated calcium carbonate is an appreciable
improvement in the state of the art for on-site production of
calcium carbonate. My novel process treats the manufacture of
calcium carbonate in a manufacturing environment from a new
perspective, to provide significantly improved production
rates.
[0177] In my improved manufacturing process, control of the pH,
temperature, and time of reaction is determined by the nature of
the progress of the reaction in a particular batch. Importantly,
the process is readily automated and can be put into an automated
process control environment. Although only a few exemplary
embodiments of this invention have been described in detail, it
will be readily apparent to those skilled in the art that my
pressurized production process for manufacture of calcium
carbonate, and the apparatus for implementing the process, may be
modified from those embodiments provided herein, without materially
departing from the novel teachings and advantages provided.
[0178] It will thus be seen that the objects set forth above,
including those made apparent from the preceding description, are
efficiently attained. Since certain changes may be made in carrying
out the method for production of precipitated calcium carbonate
according to the teachings herein, it is to be understood that my
invention may be embodied in other specific forms without departing
from the spirit or essential characteristics thereof. Many other
embodiments are also feasible to attain advantageous results
utilizing the principles disclosed herein. Therefore, it will be
understood that the foregoing description of representative
embodiments of the invention have been presented only for purposes
of illustration and for providing an understanding of the
invention, and it is not intended to be exhaustive or restrictive,
or to limit the invention only to the precise forms disclosed.
[0179] The intention is to cover all modifications, equivalents,
and alternatives falling within the scope and spirit of the
invention, as expressed herein above and in the appended claims. As
such, the claims are intended to cover the methods, apparatus,
structures (including crystal structures), and products described
herein, and not only the equivalent methods or structural
equivalents thereof, but also equivalent methods or structures. The
scope of the invention, as described herein and as indicated by the
appended claims, is thus intended to include variations from the
embodiments provided which are nevertheless described by the broad
meaning and range properly afforded to the language of the claims,
as explained by and in light of the terms included herein, or the
equivalents thereof.
* * * * *