U.S. patent application number 15/091666 was filed with the patent office on 2017-10-12 for production of high strength hydrochloric acid from calcium chloride feed streams by crystallization.
This patent application is currently assigned to THE ROYAL INSTITUTION FOR THE ADVANCEMENT OF LEARNING/MCGILL UNIVERSITY. The applicant listed for this patent is THE ROYAL INSTITUTION FOR THE ADVANCEMENT OF LEARNING/MCGILL UNIVERSITY. Invention is credited to George DEMOPOULOS, Thomas FELDMANN.
Application Number | 20170291826 15/091666 |
Document ID | / |
Family ID | 59981968 |
Filed Date | 2017-10-12 |
United States Patent
Application |
20170291826 |
Kind Code |
A1 |
DEMOPOULOS; George ; et
al. |
October 12, 2017 |
PRODUCTION OF HIGH STRENGTH HYDROCHLORIC ACID FROM CALCIUM CHLORIDE
FEED STREAMS BY CRYSTALLIZATION
Abstract
The present relates to a method for producing calcium sulfate
solid crystals and hydrochloric acid (HCl) from a calcium chloride
solution comprising the steps of feeding a continuous stirred-tank
reactor with a calcium chloride solution, sulfuric acid and water;
mixing the calcium chloride solution, sulfuric acid and water in
the reactor; and maintaining the reactor a temperature of less than
about 70.degree. C., converting the calcium chloride solution,
sulfuric acid and water into HCl and calcium sulfate solid
crystals. The method described herein can be incorporated as a
means for regenerating HCl from CaCl.sub.2 solutions which are
generated in the metallurgical industry when processing
calcium-bearing ores for recovering metals like rare earth
elements.
Inventors: |
DEMOPOULOS; George;
(Montreal, CA) ; FELDMANN; Thomas; (Montreal,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
THE ROYAL INSTITUTION FOR THE ADVANCEMENT OF LEARNING/MCGILL
UNIVERSITY |
Montreal |
|
CA |
|
|
Assignee: |
THE ROYAL INSTITUTION FOR THE
ADVANCEMENT OF LEARNING/MCGILL UNIVERSITY
Montreal
CA
|
Family ID: |
59981968 |
Appl. No.: |
15/091666 |
Filed: |
April 6, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C01P 2004/03 20130101;
C22B 59/00 20130101; C01P 2002/88 20130101; C01F 11/46 20130101;
C22B 3/44 20130101; Y02P 10/234 20151101; C01P 2002/72 20130101;
Y02P 10/20 20151101; C22B 3/10 20130101; C01B 7/035 20130101 |
International
Class: |
C01F 11/46 20060101
C01F011/46; C01B 7/03 20060101 C01B007/03; C22B 3/44 20060101
C22B003/44; C22B 26/20 20060101 C22B026/20; C22B 3/10 20060101
C22B003/10 |
Claims
1. A method for producing calcium sulfate solid crystals and
azeotropic hydrochloric acid (HCl) from a calcium chloride solution
comprising the steps of: feeding a continuous-stirred tank reactor
with a calcium chloride solution, sulfuric acid and water; mixing
the calcium chloride solution, sulfuric acid and water in the
reactor; and maintaining the reactor at a temperature of less than
about 70.degree. C., converting the calcium chloride solution,
sulfuric acid and water into azeotropic HCl and calcium sulfate
solid crystals.
2. The method of claim 1, wherein the calcium sulfate solid
crystals are crystals of at least one of calcium sulfate dihydrate,
calcium sulfate .alpha.-hemihydrate and mixture thereof.
3. (canceled)
4. The method of claim 1, wherein up to 30 wt % (9.5 mol/L) of
super-azeotropic HCl is obtained.
5. The method of claim 1, wherein the ratio of sulfate to calcium
in the reactor is 0.90 to 0.98.
6. The method of claim 1, wherein the temperature of the reactor is
about less than 60.degree. C.
7. The method of claim 1, wherein the temperature of the reactor is
about 40.degree. C.-70.degree. C.
8. The method of claim 1, wherein the temperature of the reactor is
about 40.degree. C. or less.
9. The method of claim 1, wherein the reactor is continuously fed
with calcium chloride solution, sulfuric acid and water,
continuously producing azeotropic HCl and calcium sulfate solid
crystals.
10. The method of claim 1, wherein the calcium chloride solution is
a feed stream from the processing of calcium-bearing ores.
11. The method of claim 1, wherein calcium chloride solution,
sulfuric acid and water are fed in multiple parallel reactors.
12. A process of extracting metals from calcium-bearing ores
comprising the steps of: leaching the ores with HCl, producing a
leachate containing a calcium chloride solution and metals;
separating the metals from the calcium chloride solution; feeding a
continuous-stirred tank reactor with the calcium chloride solution,
sulfuric acid and water; mixing the calcium chloride solution,
sulfuric acid and water in the reactor; maintaining the reactor at
a temperature of less than about 70.degree. C., converting the
calcium chloride solution, sulfuric acid and water into azeotropic
HCl and calcium sulfate solid crystals; and recycling the HCl to
the leaching of the ores.
13. The process of claim 12, wherein the calcium sulfate solid
crystals are crystals of at least one of calcium sulfate dihydrate,
calcium sulfate .alpha.-hemihydrate and mixture thereof.
14. (canceled)
15. The process of claim 12, wherein the metals are rare earth
metals.
16. A construction board comprising calcium sulfate
.alpha.-hemihydrate produced by the method of claim 1.
Description
TECHNICAL FIELD
[0001] The present relates to a process for the production of high
strength hydrochloric acid from calcium chloride feed streams.
BACKGROUND ART
[0002] Hydrochloric acid (HCl)-based leaching of calcium-based
rock, with the goal of extracting valuable metals such as rare
earth elements, can produce a CaCl.sub.2 solution by-product. The
extraction of these metals relies on the use of concentrated HCl,
typically of azeotropic strength. The CaCl.sub.2 solutions cannot
be released into the environment due to their high concentration of
chloride ions and have to be processed further. This is ideally
done in a way that offers the possibility to reclaim the HCl used
by the process (acid recycling). Therefore, in order to enable the
leaching of minerals with HCl on an industrial scale, the acid
needs to be recovered. Conventional HCl recovery techniques, such
as pyrohydrolysis for iron chloride solutions, are used at
industrial scale, but are not economically feasible for the
treatment of CaCl.sub.2 solution for fundamental thermodynamic
reasons. The pyrohydrolysis of CaCl.sub.2 requires temperatures of
up to 1000.degree. C. to recover the HCl.
[0003] It is thus desirable to have a means to treat the
concentrated calcium chloride solutions generated from the
processing of calcium-based rock, thereby allowing recovery of the
chloride units as high strength HCl for re-use.
SUMMARY
[0004] In accordance with the present disclosure there is now
provided a method for producing calcium sulfate solid crystals and
hydrochloric acid (HCl) from a calcium chloride solution comprising
the steps of feeding a continuous-stirred tank reactor with a
calcium chloride solution, sulfuric acid and water; mixing the
calcium chloride solution, sulfuric acid and water in the reactor;
and maintaining the reactor at a temperature of less than about
70.degree. C., converting the calcium chloride solution, sulfuric
acid and water into HCl and calcium sulfate solid crystals.
[0005] In an embodiment, the calcium sulfate solid crystals are
crystals of at least one of calcium sulfate dihydrate, calcium
sulfate .alpha.-hemihydrate and mixture thereof.
[0006] In another embodiment, the azeotropic HCl is obtained with
calcium sulfate solid crystals.
[0007] In another embodiment, up to 30 wt % (9.5 mol/L) of
super-azeotropic HCl is obtained.
[0008] In an additional embodiment, the ratio of sulfate to calcium
in the reactor is 0.90 to 0.98.
[0009] In a further embodiment, the temperature of the reactor is
about less than 60.degree. C.
[0010] In another embodiment, the temperature of the reactor is
about 40.degree. C.-70.degree. C.
[0011] In an embodiment, the temperature of the reactor is about
40.degree. C. or less.
[0012] In another embodiment, the reactor is continuously fed with
calcium chloride solution, sulfuric acid and water, continuously
producing HCl and calcium sulfate solid crystals.
[0013] In an embodiment, the calcium chloride solution is a feed
stream from the processing of calcium-bearing ores.
[0014] In another embodiment, calcium chloride solution, sulfuric
acid and water are fed in multiple parallel reactors.
[0015] Also encompassed is a process of extracting metals from
calcium-bearing ores comprising the steps of leaching the ores with
HCl, producing a leachate containing a calcium chloride solution
and metals; separating the metals from the calcium chloride
solution; feeding a continuous-stirred tank reactor with the
calcium chloride solution, sulfuric acid and water; mixing the
calcium chloride solution, sulfuric acid and water in the reactor;
maintaining the reactor at a temperature of less than about
70.degree. C., converting the calcium chloride solution, sulfuric
acid and water into HCl and calcium sulfate solid crystals; and
recycling the HCl to the leaching of the ores.
[0016] In an embodiment, the metals are rare earths.
[0017] It is also provided a construction board comprising calcium
sulfate .alpha.-hemihydrate produced by the method or process
described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Reference will now be made to the accompanying drawings.
[0019] FIG. 1 illustrates an experimental set-up for a continuous
process as encompassed herein.
[0020] FIG. 2A illustrates the evolution of HCl and CaCl.sub.2
concentration over operating time for experiments run at 30.degree.
C. (CT4) and 60.degree. C. (CT6) under different start-up
modes.
[0021] FIG. 2B illustrates the evolution of HCl concentration over
time for experiments producing azeotropic and super-azeotropic
strength HCl.
[0022] FIG. 3A illustrates a model-based estimation of water
activity a.sub.w as a function of CaCl.sub.2 concentration and HCl
concentration at 40.degree. C., wherein (*) indicates the steady
state composition, the solid observed to form experimentally is
given in ( ); - - - - (dashed line) signifies the water activity
line at which DH and HH have the same metastability; above the line
HH is relatively more stable than DH and below the line the reverse
is true.
[0023] FIG. 3B illustrates a model-based estimation of water
activity a.sub.w as a function of CaCl.sub.2 concentration and HCl
concentration at 60.degree. C., wherein (*) indicates the steady
state composition, the solid observed to form experimentally is
given in ( ); - - - - (dashed line) signifies the water activity
line at which DH and HH have the same metastability; above the line
HH is relatively more stable than DH and below the line the reverse
is true.
[0024] FIG. 4 illustrates a comparison of particle size
distributions obtained from experiments CT1, CT2 and CT3 after 8 h
of experiment.
[0025] FIG. 5A illustrates the crystal morphologies of steady-state
products (after 8 h) from experiment CT2 calcium sulfate
hemihydrate.
[0026] FIG. 5B illustrates the crystal morphologies of steady-state
products (after 8 h) from experiment CT3 calcium sulfate
dihydrate.
[0027] FIG. 6A illustrates a scanning electron microscope (SEM)
image showing the crystal morphology and size of .alpha.-HH
crystals produced at a steady-state concentration of 3.4 mol/L
(CT2) of CaCl.sub.2.
[0028] FIG. 6B illustrates a SEM image showing the crystal
morphology and size of .alpha.-HH crystals produced at a
steady-state concentration of 1.2 mol/L (CT7) of CaCl.sub.2.
[0029] FIG. 6C illustrates a SEM image showing the crystal
morphology and size of .alpha.-HH crystals produced at a
steady-state concentration of 0.8 mol/L (CT6) of CaCl.sub.2.
[0030] FIG. 6D illustrates a SEM image showing the crystal
morphology and size of .alpha.-HH crystals produced at a
steady-state concentration of 0.1 mol/L (CT12) of CaCl.sub.2.
[0031] FIG. 7A illustrates the effect of steady-state
crystallization conditions on .alpha.-HH crystal size distributions
at CT2 conditions and CT12 conditions.
[0032] FIG. 7B illustrates the effect of steady-state
crystallization conditions on .alpha.-HH crystal size quality as
characterized by differential scanning calorimetry (DSC) at CT2
conditions and CT12 conditions.
[0033] FIG. 8A illustrates the relationship between the amount of
fine particles and the exothermic peak in a DSC scan.
[0034] FIG. 8B illustrates the relationship between the amount of
fine particles and the steady-state CaCl.sub.2 content.
[0035] FIG. 9 illustrates the comparison of x-ray diffraction (XRD)
patterns of solid produced in the continuous process as encompassed
herein (t=8 h) (CT12: 2; CT10: 3; CT9: 4; and CT6: 5) with
.alpha.-HH reference material produced in the autoclave
process.
[0036] FIG. 10 illustrates a comparison of DSC patterns of solid
produced in the continuous process (t=8 h) (CT12: 2; CT10: 3; CT9:
4; and CT6: 5) with .alpha.-HH reference material produced in the
autoclave process.
[0037] FIG. 11A illustrates the crystal morphology of .alpha.-HH
produced by the process described herein according to one
embodiment under CT6 conditions (t=8 h).
[0038] FIG. 11B illustrates the crystal morphology of .alpha.-HH
produced by the process described herein according to one
embodiment under CT9 conditions (t=8 h).
[0039] FIG. 11C illustrates the crystal morphology of .alpha.-HH
produced by the process described herein according to one
embodiment under CT10 conditions (t=8 h).
[0040] FIG. 11D illustrates the crystal morphology of .alpha.-HH
produced by the process described herein according to one
embodiment under CT12 conditions (t=8 h).
[0041] FIG. 11E illustrates the crystal morphology of .alpha.-HH
produced with a process according to current commercially-available
material.
[0042] FIG. 12 illustrates a flow sheet of the acid regeneration
process described herein incorporated in a metallurgical
process.
[0043] It will be noted that throughout the appended drawings, like
features are identified by like reference numerals.
DETAILED DESCRIPTION
[0044] It is provided a process for the production of high strength
(sub-azeotropic, azeotropic or super-azeotropic) hydrochloric acid
from calcium chloride feed streams.
[0045] Using a low temperature (40-70.degree. C.) hydrochemical
process, the process described herein produces a solid calcium
sulfate product (calcium sulfate .alpha.-hemihydrate; .alpha.-HH or
calcium sulfate dihydrate; DH). Especially, .alpha.-HH is currently
only produced by an energy intensive and labor intensive autoclave
process that operates at approximately 115.degree. C.
[0046] It is demonstrated that the process described herein
resulted in a production rate of approximately 0.5
kg.sub..alpha.-HH/h and 0.9 kg.sub.conc. HCl solution/h. The
process described herein can be embedded in any
hydrometallurgical/chemical operation that generates a concentrated
CaCl.sub.2 solution as a by-product of its processes and where such
solution cannot be used/treated internally or sold externally and
HCl needs to be regenerated. The regenerated HCl can be reused
internally in the production process or sold on the external
market.
[0047] Also encompassed herein is the use of the solid product
(.alpha.-HH) described herein in the production of plaster board
used in the drywall construction industry for example.
[0048] It is provided a process for regenerating HCl from
CaCl.sub.2 solutions, which has not been previously economically
feasible and hence not been practiced on an industrial scale so
far. Furthermore, it is provided a means for the metallurgical
industry to recycle CaCl.sub.2 waste solution streams and hence
make these operations more environmentally-friendly.
[0049] The process described herein provides a significant
improvement over the commercially-existing autoclave-based
production of .alpha.-HH, a construction material that is already
in use. The process described herein utilizes a continuous mode of
operation and uses much lower temperatures, such as at room
temperature for example, preferably at 40-70.degree. C. compared to
known processes (115-130.degree. C. for conventional processes). In
addition, the processing time is reduced from several hours for
known conventional processes to less than 3 h, preferably 1-2
h.
[0050] Contrary to the salt solution method, which uses
concentrated CaCl.sub.2 solutions or mixtures of
CaCl.sub.2--MgCl.sub.2--KCl to convert calcium sulfate dihydrate
from flue gas desulfurization operations into .alpha.-HH, the
temperature used in the present process is lower and the processing
time is shorter. Furthermore, the salt solution process cannot be
integrated into the larger context of a hydrometallurgical
extraction process that continuously produces CaCl.sub.2 solution.
This is because the salt solution is not transformed/consumed
during the process of calcium sulfate dihydrate to .alpha.-HH
conversion. Hence, the salt solution method does not provide an
answer to the problem of spent CaCl.sub.2 solution treatment and
acid regeneration in the same way as the present process does.
[0051] Laboratory crystallization studies are carried out in batch
or semi-batch reactors. The latter has been the system of choice as
it allows operation in constant composition mode. This concept
achieves a constant supersaturation level via regulated reagent
addition. This technique has proven particularly successful with
relatively dilute reagent solutions when precipitation of sparingly
soluble compounds at a controlled pH level is practiced. As such,
the semi-batch constant composition crystallization system can also
simulate steady-state crystallization as encountered in continuous
stirred-tank crystallizers that are widely used in large-scale
industrial operations. However, in many cases the semi-batch
crystallization results may not be entirely transferable to the
continuous crystallizer operation mainly because of differences in
the residence time distribution and mixing effects. It has been
discovered that crystallization of metastable phases in
concentrated acidic solutions not amenable to pH control, due to
excessive acidity levels for pH probes, constitutes another case
where employment of a continuous stirred-tank reactor (CSTR) during
the development stage offers distinct superiority over the
semi-batch precipitation approach.
[0052] It is provided herein the results of continuous
crystallization experiments in the context of HCl regeneration from
concentrated calcium chloride solutions. Such solutions are
generated in chloride-based hydrometallurgical processes. One
example is the HCl leaching of calcium-based ores for rare earth
extraction. However, regeneration of HCl via pyrohydrolysis of
CaCl.sub.2 solutions is not feasible due to the very high
temperatures (.about.800-1000.degree. C.) required. This has
prompted exploration of alternative HCl regeneration routes. To
this end, a reactive crystallization process involving stage-wise
reaction of concentrated calcium chloride solution (up to 5 mol/L)
with concentrated sulfuric acid (up to 18 mol/L) at temperatures
below 100.degree. C. in a semi-batch configuration has been
investigated previously, based on the following equation
CaCl.sub.2+H.sub.2SO.sub.4+xH.sub.2O.fwdarw.2HCl+CaSO.sub.4.xH.sub.2O
(1)
[0053] In principle, depending on the process operating window
defined in terms of temperature, solution composition, [HCl] and
[CaCl.sub.2], and time, the reaction can be controlled to produce
metastable calcium sulfate dihydrate (DH, x=2) or metastable
calcium sulfate .alpha.-hemihydrate (.alpha.-HH, x=0.5), while
avoiding formation of the thermodynamically stable, but
undesirable, calcium sulfate anhydrite (AH, x=0). The process is
feasible due to the low solubility of calcium sulfates, especially
if a small amount of calcium chloride is still present in solution.
In order to obtain high strength HCl, both reagents (CaCl.sub.2,
H.sub.2SO.sub.4) need to be used at high concentration, which
causes the water activity of the solution to be significantly
reduced. It has been shown that the availability of free water,
reflected by the magnitude of water activity, is a key parameter
controlling calcium sulfate phase (meta-)stability in addition to
the effect of temperature. Over the temperature range 55.degree.
C.-95.degree. C., .alpha.-HH was found to be the favored metastable
phase while DH is the metastable phase at lower temperature. AH is
always the stable phase in the relevant composition range. It has
been shown that the lifetime of DH and .alpha.-HH primarily depends
on the level of HCl concentration, which has the greatest impact on
the water activity in the system by lowering it. This makes the
production of the metastable phases very challenging, as their
conversion to the undesirable stable AH phase is a constant
threat.
[0054] So far, nothing exists showing a successfully operating
continuous HCl regeneration process from concentrated CaCl.sub.2
solutions. Up to now, the production of DH and/or .alpha.-HH in a
continuous crystallization reactor cascade via adaptation of a
semi-batch staged crystallization scheme failed to generate
super-azeotropic strength HCl without forming the undesirable
anhydrite (AH) phase. The present provides a newly-developed
continuous process that yields high strength HCl (up to 9.5 mol/L,
super-azeotropic strength) by reactive crystallization of
.alpha.-HH without forming anhydrite. The present process was
designed on the basis of water activity calculations made with the
OLI Stream Analyzer software. It was demonstrated using a
single-stage CSTR crystallizer (see FIG. 12), which can be
implemented in a parallel reactor scheme upon industrial scale up,
rather than the more conventional in-series approach.
[0055] Accordingly, the process described herein consists of a
single, continuous, stirred tank reactor (CSTR) in which the
following reaction is performed:
CaCl.sub.2+H.sub.2SO.sub.4+xH.sub.2O.fwdarw.2HCl+CaSO.sub.4.xH.sub.2O
(2) [0056] where x=0.5 or 2.
[0057] This reaction is performed at a stoichiometric sulfate to
calcium ratio of the inflow solutions of 0.90 to 0.98 in a single
step and in continuous fashion. It results in the generation of
super-azeotropic HCl with up to .apprxeq.30 wt. % (9.5 mol/L). The
temperature of the reaction inside the stirred reactor can be about
.ltoreq.60.degree. C. for the production of .alpha.-HH crystals or
.ltoreq.40.degree. C. for the production of DH crystals. The
nominal residence time of the crystals in the reaction tank is
preferably 1 h. CaCl.sub.2 and H.sub.2SO.sub.4 solution is
constantly fed by pumps to the reactor and the reaction product
(slurry consisting of HCl acid and .alpha.-HH or DH solid) is
continuously removed.
[0058] Depending on the concentration of the inflow solutions and
the stoichiometric ratio, as summarized in Table 1, it can be seen
that different HCl concentrations can be obtained in a stable
manner for several hours.
TABLE-US-00001 TABLE 1 Summary of experimental parameters and
properties Experimental parameter and solution compositions
Steady-state product properties Feed Feed Steady state solution
solution CaCl.sub.2 flow Steady state CaCl.sub.2 Solid Median
Reaction extent, CaCl.sub.2 H.sub.2SO.sub.4 rate, actual
H.sub.2SO.sub.4 flow HCl conc. .+-. conc. .+-. Solid produced
particle Exp. Temperature (SO.sub.4.sup.2-/Ca.sup.2+ ratio)* conc.
conc. (target) rate (target) standard dev standard dev content at
steady size no. .degree. C. %, -- mol/L mol/L mol/h mol/h mol/L
mol/L % state .mu.m Experiments with target product DH CT2 40 33.3,
(0.95) 5.06 12.54 3.99 (4.00) 1.25 (1.25) 2.8 .+-. 0.1 3.4 .+-. 0.2
17 HH 16 CT3 40 100, (0.90) 3.03 5.40 1.79 (1.80) 1.63 (1.62) 4.0
.+-. 0.1 0.4 .+-. 0.0 24 DH 97 CT4 30 100, (0.90) 5.04 7.71 2.94
(3.00) 2.65 (2.70) 6.7 .+-. 0.2 0.5 .+-. 0.0 35 DH & HH 40
traces Experiments with target product HH CT6 60 100, (0.90) 4.94
7.71 2.97 (3.00) 2.49 (2.70) 5.9 .+-. 0.1 0.8 .+-. 0.1 31 HH 25 CT9
60 100, (0.95) 5.06 17.87 3.95 (3.95) 3.95 (3.75) 8.9 .+-. 0.1 0.5
.+-. 0.1 36 HH 27 CT10 60 100, (0.95) 5.11 17.87 3.89 (3.95) 3.88
(3.75) 9.0 .+-. 0.2 0.3 .+-. 0.1 36 HH 33 CT12 56 .+-. 1** 100,
(0.95) 5.11 17.87 3.61 (3.95) 3.95 (3.75) 9.5 .+-. 0.2 0.1 .+-. 0.0
n.a. HH 61
[0059] The process encompassed herein was tested on a lab scale
using a temperature-controlled reactor with a working volume of 1 L
over a period of 10 h, during which the process was operated at
stable steady state conditions for 6.5 h. An example of an
experimental set up can be seen in FIG. 1.
[0060] The crystallization experiments as described herein were
carried out in a continuous-stirred tank reactor 5. Encompassed
herein is the use of multiple reactors in parallel. The working
volume of the reactor 5 was 1 L, which was maintained by the
overflow of the slurry through a 1 cm diameter tube at the side of
the reactor. A turbulent mixing regime was achieved with a
two-level pitched blade impeller with 3 blades at each level,
having a hydrodynamic diameter of 6 cm (3 cm apart vertically)
controlled by a speed motor 3. The stirrer speed was adjusted as
necessary to maintain good mixing and slurry flow inside the
reactor 5; it was set to 300 rpm at the beginning of the experiment
(low solids content) and subsequently increased up to 700 rpm at
steady state condition. The sulfuric acid and calcium chloride
solutions 1 were fed drop-wise with peristaltic pumps 2 at
different flow rates in order to maintain a residence time of 60
min and account for the stoichiometry of the experiment. The
temperature was maintained by either heating the reactor with a
temperature controlled hot-plate 4 or by cooling the reactor with
compressed air to stabilize it at a certain temperature (in the
case of DH experiments only). The latter was necessary to offset
the heat released from acid (H.sub.2SO.sub.4) mixing and the
reaction. All solutions were prepared with deionized water (DIW),
and ACS reagent grade chemicals. The produced HCl-solid mixture was
collected in a receiving container 6.
[0061] Since a continuous reactor goes through a start-up phase
before it reaches steady state conditions, the effect of different
starting solution compositions was tested. In some experiments, the
start-up was done with water and in some experiments a synthetic
solution with a composition closer to the expected steady-state
composition was used (Table 2).
TABLE-US-00002 TABLE 2 Detailed experimental results Experimental
parameter and solution compositions Feed Feed CaCl.sub.2 flow
Reaction extent solution solution rate, target (SO.sub.4.sup.2-/Ca
CaCl.sub.2 H.sub.2SO.sub.4 actual H.sub.2SO.sub.4 flow Retention
Exp. Temperature ratio) conc. conc. (target) rate (target) time no.
.degree. C. %, -- mol/L mol/L mol/h mol/h min Experiments with DH
as target CT1 40 33.3, (0.95) 5.32 12.54 3.94 (4.0) 1.19 (1.25) 61
CT2 40 33.3, (0.95) 5.06 12.54 3.99 (4.0) 1.25 (1.25) 60 CT3 40
100, (0.90) 3.03 5.40 1.79 (1.8) 1.63 (1.62) 60 CT4 30 100, (0.90)
5.04 7.71 2.94 (3.0) 2.65 (2.70) 64 CT5 13 100, (0.90) 5.00 18.00
3.98 (4.0) 3.42 (3.60) 64 Experiments with .alpha.-HH as target CT6
60 100, (0.90) 4.94 7.71 2.97 (3.0) 2.49 (2.70) 66 CT7 60 100,
(0.90) 4.96 18.00 3.90 (4.0) 3.05 (3.60) 64 CT9 60 100, (0.95) 5.06
17.87 3.95 (3.95) 3.95 (3.75) 56 CT10 60 100, (0.95) 5.11 17.87
3.89 (3.95) 3.88 (3.75) 56 CT12 56 .+-. 1** 100, (0.95) 5.11 17.87
3.61 (3.95) 3.95 (3.75) 60 Steady-state product properties Steady
Steady state state CaCl.sub.2 Solid produced HCl conc. .+-. conc.
.+-. at steady state Start-up condition standard standard Solid
(Median Start-up Exp. dev dev content particle size) Seed type
solution no. mol/L mol/L % .mu.m used type Experiments with DH as
target CT1 2.88 .+-. 0.07 3.55 .+-. 0.07 14 HH lab made DH
synthetic (12) CT2 2.75 .+-. 0.12 3.41 .+-. 0.19 17 HH lab made DH
DI water (16) CT3 4.02 .+-. 0.10 0.38 .+-. 0.02 24 DH lab made DH
DI water (97) CT4 6.74 .+-. 0.19 0.48 .+-. 0.02 35 DH, HH traces
lab made DH DI water (40) CT5 8.13 .+-. 0.18 0.92 .+-. 0.04 36
DH/HH mix lab made DH DI water (9) Experiments with .alpha.-HH as
target CT6 5.94 .+-. 0.12 0.77 .+-. 0.05 31 HH lab made .alpha.-HH
synthetic (25) CT7 6.74 .+-. 0.17 1.22 .+-. 0.08 32 HH lab made
.alpha.-HH synthetic (7) CT9 8.88 .+-. 0.12 0.45 .+-. 0.04 36 HH
lab made .alpha.-HH synthetic (27) CT10 8.97 .+-. 0.2 0.33 .+-.
0.07 36 HH Knauf FGD* .alpha.- synthetic (33) HH CT12 9.52 .+-.
0.21 0.11 .+-. 0.04 n.a. HH lab made .alpha.-HH synthetic (61)
[0062] The reason for this investigation was to determine if the
time to reach steady state or if the stability of the seed crystals
would be affected. With reference to the latter, emphasis was
placed on ensuring that the DH or HH crystals used as seed would
not undergo phase transformations during the start-up period. In
other words, the strategy was to have the right crystal phase (DH
or HH depending on the desired end product) present during the
unsteady-state start-up of the process.
[0063] FIG. 2A shows the evolution of acid concentration and
CaCl.sub.2 concentration over time for two typical experiments
(CT4, CT6). It can be seen that steady state was reached after
approximately 3.5 h in both cases and that the process is
reasonably stable. As can be seen in the experiment that was
started with deionized water (CT4), a fast build-up of acid
concentration within the first few hours was observed. On the other
hand, in an experiment started with a synthetic solution of near
steady-state composition (CT6), steady-state was also reached after
3-3.5 h.
[0064] The choice of the start-up mode is dictated by the targeted
product, DH or .alpha.-HH. This is because it is important to avoid
phase transformation of the seed crystals during start-up. For
example, if .alpha.-HH seed crystals are added to DIW at the
beginning of a test targeting .alpha.-HH as product, this will not
work as after the ramp-up phase .alpha.-HH will have converted to
DH as the latter is the thermodynamically stable phase in water at
temperatures below 42.degree. C. These results allow the selection
of suitable start-up conditions, thus avoiding complications.
[0065] It has been shown previously that apart from the activity of
water (governed by solution composition), the temperature at which
the crystallization-based acid regeneration process is performed
influences the crystal phase that is formed. The heat released by
the reaction system needs to be taken into account in the process
design as it is significant, especially with high reactant stream
concentrations. For example, a calculation with OLI Stream Analyzer
showed that the heat released from mixing and the reaction between
the reagents was 310 kJ/h. This calculation was done for experiment
CT5 (5 mol/L CaCl.sub.2 at 3.98 mol/h and 18 mol/L H.sub.2SO.sub.4
at 3.42 mol/h). As a result of the high level of heat released, a
steady-state temperature was attained in the non-insulated,
single-walled glass reactor in the case of experiment CT12. A
temperature of 56.degree. C. was measured while the ambient
temperature was 23.degree. C. during the experiment, which was run
without any temperature control. This means that the process can
essentially run autogenously if concentrated feed solutions are
used.
[0066] Previous semi-batch crystallization work with a similar
system showed that calcium sulfate dihydrate (DH) could exist in a
solution with a composition of .about.2.8 mol/L HCl and .about.3.5
mol/L CaCl.sub.2 at 40.degree. C., as was the case in CT1 and CT2,
for a time sufficiently longer than the residence time of 60 min.
Surprisingly, DH was not obtained as the steady state product, but
rather .alpha.-HH (see Table 2). This underlines the importance of
performing continuous crystallization tests in the case of
metastable crystal phases.
[0067] The observed behavior is explained with the help of FIG. 3,
which shows estimations of the water activity (a.sub.w) as a
function of HCl and CaCl.sub.2 concentration. The calculations were
made with OLI Stream Analyzer. A value for a.sub.w close to 1 means
that the "lifetime" of the metastable phase in contact with the
solution will be significantly extended. From Equation 2 and FIG.
3, a concentrated CaCl.sub.2 feed solution, for example 5 mol/L,
will be converted to a certain HCl concentration (approximately two
times the CaCl.sub.2 concentration). Therefore, the steady state
condition will always be diagonally (left and up) from the initial
calcium chloride concentration in such a diagram, since calcium is
consumed/precipitated as calcium sulfate and HCl is left in
solution.
[0068] This situation is illustrated with the results of
experiments CT1, CT2 and CT3 conducted at 40.degree. C. (FIG. 3A).
Although CT1 and CT2 should have formed DH (as CT3 did), .alpha.-HH
was formed instead. The reason for the formation of .alpha.-HH is
that the steady state composition of CT1 and CT2 in comparison to
CT3 was in a less favorable region in terms of water activity
despite the lower acid concentration (2.75 mol/L HCl in CT2 vs. 4
mol/L HCl in CT3). This realization, that it is the water activity
that governs the crystal production process, prompted the idea to
opt for a single CSTR operating at low steady-state CaCl.sub.2
concentration that is associated with a relatively higher water
activity. In this way, the relative metastability of the target
phase is enhanced, allowing for high HCl concentrations to be
achieved. The dashed line in FIG. 3 denotes the change of relative
metastability, i.e., below the line DH is relatively more stable
than .alpha.-HH and above the line .alpha.-HH is relatively more
stable than DH. In all conditions investigated, calcium sulfate
anhydrite is the thermodynamically stable phase.
[0069] This water activity-based single-stage crystallization
approach also proved to have beneficial effects on crystal growth
as reflected by the particle size distribution data in Table 1,
FIGS. 4 and 5. Thus the DH crystals produced at the relatively high
acid concentration of 4 mol/L HCl but low (0.38 mol/L) CaCl.sub.2
concentration (CT3 test) were much larger than the HH crystals
produced at the same temperature (40.degree. C.) but lower HCl
concentration (2.75 mol/L HCl) and correspondingly higher (3.4
mol/L) CaCl.sub.2 concentration (CT2 test). There are two
influences to distinguish, namely that of water activity and that
of calcium chloride/calcium ion concentration in solution. The
water activity in the former case (CT3) was higher than the
corresponding one in the latter case. This caused a different phase
to form DH, which has a higher relative metastability.
[0070] A comparison of the particle size data shows in the case of
CT2 the fraction of crystals being smaller than 10 .mu.m was 43%
while in the case of CT3 it was only 6%. It is proposed that this
difference is due to high CaCl.sub.2 concentration in CT2. In this
situation, homogeneous nucleation prevails as a result of high
supersaturation, hence the smaller crystal size. By contrast,
crystallization at low CaCl.sub.2 concentration favors crystal
growth of existing crystals over nucleation, due to lower
supersaturation. It should be noted that large crystals have better
filtration characteristics than small ones.
[0071] In essence, it is demonstrated that water activity will
determine the crystal phase that is formed, while the residual
CaCl.sub.2 concentration will influence the particle growth/size
characteristics of that solid phase.
[0072] Experiments CT1 and CT2, which were run at 40.degree. C.,
yielded .alpha.-HH crystals as a result of operating at low water
activity. In both experiments, the .alpha.-HH crystals were rather
fine and of poor quality as evident by their SEM morphology (see
FIG. 5A), particle size distribution (FIG. 6A), and differential
scanning calorimetry scans (DSC in FIG. 6B). Furthermore, the
obtained acid concentration in this case (CT2) was quite low and
not of interest from an industrial process point of view
(only.about.2.7 mol/L, see Table 1). However, by increasing the
temperature to 60.degree. C. and lowering the steady-state
CaCl.sub.2 concentration below 1 mol/L, larger .alpha.-HH crystals
were produced (see Table 1, CT6) at a higher HCl concentration of
roughly azeotropic strength of .about.5.9 mol/L. The crystal size
of .alpha.-HH was observed to be very dependent on the steady-state
CaCl.sub.2 concentration level. For example, the size distribution
of CT7 (median particle size given in Table 2) that was run at 1.2
mol/L CaCl.sub.2 showed the presence of a significant amount of
fines. By reducing the level of CaCl.sub.2, the generation of fines
was eliminated and crystal growth was promoted as demonstrated by
the size distribution data of CT9, CT10 and CT12. It can be seen in
the case of the CT12 test that the .alpha.-HH had grown to large,
elongated prismatic shape crystals (FIG. 6D) while the generated
HCl acid concentration was of super-azeotropic strength, .about.9
mol/L (or 30%), while at higher CaCl.sub.2 concentrations much
finer HH crystals were produced (FIGS. 6A-C).
[0073] DSC measurements further confirmed the high quality of the
.alpha.-HH crystals, as revealed by the sharp
endothermic/exothermic peaks (FIG. 7B). High quality .alpha.-HH
provides for a better binder material than the more commonly used
.beta.-HH phase.
[0074] Further analysis shows a direct relationship between the
fraction of very fine crystals (<1 .mu.m) and the exothermic
peak signal in the DSC measurements (see FIG. 8A). The latter was
quantified by measuring the area underneath the peak to the
baseline of the signal. In addition, there is a direct relationship
between the amount of fines and the concentration of CaCl.sub.2 at
steady-state (see FIG. 8B). The differently-sized crystals and the
relationship to the steady-state CaCl.sub.2 concentration were also
confirmed by the SEM images presented in FIG. 6. It can be clearly
seen that .alpha.-HH crystals produced under conditions of lower
steady-state calcium chloride concentration are much larger than
those produced at much higher CaCl.sub.2 concentration. This
behavior relates to the prevailing supersaturation condition, which
is directly influenced by the magnitude of the CaCl.sub.2
concentration.
[0075] The effect of CaCl.sub.2 on crystal growth--as contrasted
between CT2 ("high" CaCl.sub.2 concentration, fine crystals) and
CT12 ("low" CaCl.sub.2 concentration, large crystals) tests--can be
explained by considering the local supersaturation generated upon
the entry of a droplet of concentrated sulfuric acid into the
receiving solution in the reactor. Although the reactor was well
and turbulently mixed, some short finite amount of time is
necessary to evenly distribute the viscous H.sub.2SO.sub.4 drop.
Since the crystallization reaction of calcium sulfates proceeds
without any noticeable induction time, it is apparent that calcium
sulfate crystals nucleate instantaneously in the immediate vicinity
around the droplet, before they have the chance to be dispersed
into the bulk solution.
[0076] Thus, the process described herein provides a means for the
efficient regeneration of high strength HCl (up to .about.9 mol/L,
.about.30% super-azeotropic) that is critically needed in the
implementation of modern chloride hydrometallurgical processes and
which was not possible until the present disclosure from calcium
chloride solutions. By making use of water activity and
supersaturation control concepts, a novel acid regeneration process
is provided featuring the crystallization of well-grown .alpha.-HH
or DH crystals in a single CSTR without formation of the
undesirable AH. The process described herein involves reaction of
concentrated CaCl.sub.2 solution with concentrated sulfuric acid
which is flexible with respect to the concentration of the
regenerated HCl as well as the type of calcium sulfate phase
(.alpha.-HH or DH) that is produced.
[0077] The selective production of DH or .alpha.-HH crystals with
simultaneous regeneration of HCl in a continuous reactor is
demonstrated for the first time. The production of DH is possible
only at sub-azeotropic HCl concentration (6 mol/L) and a preferred
temperature .ltoreq.40.degree. C. The regeneration of >9 mol/L
HCl with simultaneous production of .alpha.-HH was achieved with
little to no heating at 60.degree. C., due to the strongly
exothermic nature of the dilution of the H.sub.2SO.sub.4 feed and
the reaction itself. The concentration of CaCl.sub.2 at
steady-state influenced the crystal size distribution of
.alpha.-HH. Lower concentrations, e.g., 0.5 mol/L or below, led to
larger crystals with narrower particle size distribution,
effectively lowering the fraction of fines (crystals of <10
.mu.m). This behavior is attributed to a lower local
supersaturation environment. In contrast, the high calcium chloride
concentration and low water activity that is encountered in
multistage reactor set-ups severely reduces the life-time of
.alpha.-HH crystals, making the regeneration of high-concentration
acid unfeasible.
[0078] When solid .alpha.-HH is produced by the process described
herein, characterization by X-ray diffraction, Differential
Scanning calorimetry and Scanning Electron Microscopy compared to
commercially available .alpha.-HH produced with the state of the
art autoclave method (FIGS. 9-11) respectively, demonstrates that
the solid produced by the process described herein produces
crystals that are similar in size and crystal shape.
[0079] The process described herein provides a means for
regenerating HCl from CaCl.sub.2 solutions which are generated in
the metallurgical industry when leaching calcium-bearing ores to
extract rare earths, for example. Accordingly, it is provided a
means to recycle CaCl.sub.2 waste solution streams and hence make
these operations more environmentally-friendly.
[0080] Calcium bearing ores are, for example, sedimentary rock
deposits of gypsum, limestone, skarn and shale. Some common
calcium-bearing minerals include apatite, calcite, dolomite,
fluorite, and gypsum (calcium sulfate).
[0081] As depicted in FIG. 12, normal extraction metallurgical
processes incorporating the process described herein consist in
leaching 12 using HCl ores, preferably calcium-bearing ores 10 in
order to produce a leachate containing metals and a calcium
chloride solution. After separation/purification 14, the calcium
solution collected can be integrated in the process (HCl
regeneration process described herein 16) in a reactor in order to
extract calcium sulfate crystals and HCl which is recycled back to
the leaching step 12.
[0082] The present disclosure will be more readily understood by
referring to the following examples which are given to illustrate
embodiments rather than to limit its scope.
Example I
Crystallization Experiments
[0083] The crystallization experiments were carried out in a
continuous stirred-tank reactor. The working volume of the reactor
was 1 L, which was maintained by the overflow of the slurry through
a 1 cm diameter tube at the side of the reactor. A turbulent mixing
regime was achieved with a two-level pitched blade impeller with 3
blades at each level, having a hydrodynamic diameter of 6 cm (3 cm
apart vertically). The stirrer speed was adjusted as necessary to
maintain good mixing and slurry flow inside the reactor; it was set
to 300 rpm at the beginning of the experiment (low solids content)
and subsequently increased up to 700 rpm at steady state condition.
A nearly ideal residence time distribution was achieved in all
tests.
[0084] The sulfuric acid and calcium chloride solutions were fed
drop-wise with peristaltic pumps at different flow rates in order
to maintain a residence time of 60 min and account for the
stoichiometry of the experiment. The temperature was maintained by
either heating the reactor with a temperature controlled hot-plate
or by cooling the reactor with compressed air to stabilize it at a
certain temperature (in the case of DH experiments only). The
latter was necessary to offset the heat released from acid
(H.sub.2SO.sub.4) mixing and the reaction. All solutions were
prepared with deionized water (DIW), and reagent grade
chemicals.
[0085] Slurry samples were taken from the reactor every 30 min and
immediately filtered through 0.22 .mu.m Millipore Swinnex filters.
The recovered crystals (.about.1-1.5 g) were immediately washed
with isopropanol after filtration and stored in test tubes covered
by a layer of isopropanol before further analysis. This procedure
successfully prevents any potential phase transformation of the
crystals, especially .alpha.-HH, which is a metastable phase and
might otherwise be susceptible to reaction with ambient
moisture.
[0086] Liquid samples taken during the experiment were immediately
diluted with deionized water by a factor of 10, in order to avoid
any further precipitation of calcium sulfates due to temperature
change. The solutions were analyzed for acid concentration and
Ca.sup.2+ concentration via titration of 1 mL of the diluted
sample.
[0087] The solids were characterized by X-ray powder diffraction
performed with a Cu K.alpha. Philips PW 1710 diffractometer.
Additionally, crystal size distributions were measured in
isopropanol with a Horiba Laser Scattering Particle Size
Distribution Analyzer LA-920. Scanning electron microscopy was
performed with a Philips XL30 FEG-SEM; samples were coated with
carbon in order to avoid charging effects during imaging. In
addition, differential scanning calorimetry (DSC) was carried out
on a TGA Instruments Q2000 apparatus, with a heating rate of 10
K/min and a nitrogen gas flow of 50 mL/min in closed aluminum
crucibles. The latter two types of measurements were performed on
selected samples to obtain information about the crystal morphology
and the type of calcium sulfate hemihydrate produced, respectively.
Especially, DSC is capable of identifying .alpha.-HH, due to a
sharp exothermic peak found in the thermograms.
[0088] In order to characterize the slurry properties during steady
state conditions, its solid content was determined on the basis of
dry weight of produced solid to slurry weight at the end of the
experiment.
Example II
Effect of Temperature on Dihydrate Production at High HCl
Concentration
[0089] Reduction of the temperature at which the crystallization is
performed (from 40.degree. C. to 30.degree. C. to 13.degree. C.)
helped to increase the steady state HCl concentration, while DH
with only a minor .alpha.-HH content was produced at 30.degree. C.
or a mix of DH/.alpha.-HH at 13.degree. C. This shows that a water
activity of .gtoreq.0.5 is required to operate in the window of DH
"life-time" allowing for the precipitation of a pure solid phase,
In the case of CT5, it appears that the temperature was not low
enough to stabilize DH for a sufficiently long time, i. e., longer
than the retention time in the reactor. This is due to the fact
that the steady-state composition of the solution was in a region
with a water activity value that was too low. Therefore, it can be
seen that in order to obtain pure DH crystals at high acid
concentration (>7 mol/L HCl), it will be necessary to cool the
reactor to temperatures well below 13.degree. C.
Example III
Effect of Seed Material on Product Morphology
[0090] A comparison between a situation in which commercial seed
crystals of .alpha.-HH (CT10) were used in the process during
start-up with an experiment where lab-made .alpha.-HH (CT9)
crystals were used showed no difference in particle size of the
steady-state product after 8 h. A comparison of the seed material
with SEM images of process product shows that even after 0.5 h
almost no compact commercially-available .alpha.-HH crystals are
left. It appears that these crystals would have dissolved and
re-precipitated in the more elongated (rod-shaped) form, which is
typical of .alpha.-HH produced under the conditions of discussing
HCl regeneration process. Further support for this explanation is
given by the fact that calcium sulfates experience higher
solubility at high HCl content when only low concentrations of
calcium ions are present.
Example IV
Influence of Nominal Retention Time on Process
[0091] As shown by experiments CT9, CT10 and CT12, it was possible
to produce .alpha.-HH and simultaneously recover high-strength HCl.
In all cases the nominal retention time, defined by reactor volume
divided by reagent flow rate, was .about.60 min. In order to
investigate the effect of a shorter residence time of .about.30
min, two experiments were conducted with 5 mol/L CaCl.sub.2 and
11.1 mol/L (CT15) or 18.2 mol/L (CT14) H.sub.2SO.sub.4 solution.
The volumetric flow rates were adjusted to obtain the target
nominal retention time. In both cases, no steady state conditions
were achieved. In the case of CT14, the slurry became impossible to
mix after 30 min and in the case of CT15 this happened after 1 h.
XRD results confirmed the presence of calcium sulfate anhydrite (at
least partially). It is known that the morphology of this phase's
crystals is very fine and fibrous. Therefore, under these
conditions a phase transformation of .alpha.-HH to AH occurred. The
reason for this is that the increased molar flow rate, which
delivers more sulfuric acid per time to the same point in the
reactor than in the 60 min retention time experiments, caused a
zone of high acid concentration resulting in local low water
activity. This created a favorable environment for the formation of
AH. Therefore, it can be concluded that the nominal retention time
of the reactor should be at least one hour and preferably below 2-3
h, which showed that especially at high acid strength the
metastable life-time of .alpha.-HH is only a few hours, before
transformation to the thermodynamically stable AH phase starts.
[0092] While the present disclosure has been described with
particular reference to the illustrated embodiment, it will be
understood that numerous modifications thereto will appear to those
skilled in the art. Accordingly, the above description and
accompanying drawings should be taken as illustrative and not in a
limiting sense.
[0093] While the disclosure has been described in connection with
specific embodiments thereof, it will be understood that it is
capable of further modifications and this application is intended
to cover any variations, uses, or adaptations, including such
departures from the present disclosure as come within known or
customary practice within the art and as may be applied to the
essential features hereinbefore set forth, and as follows in the
scope of the appended claims.
* * * * *