111111111111111111111111111111111111111111111111111111111111111111111111111
US006284005Bl
(12) United States Patent
Hazen et al.
(10) Patent No.:
(45) Date of Patent:
US 6,284,005 BI
Sep.4,2001
(54) SODIUM CARBONATE
RECRYSTALLIZATION
(73) Assignee: Environmental Projects, Inc., Casper,
WY(US)
(75) Inventors: Wayne C. Hazen, Denver; Dale Lee
Denham, Jr., Arvada, both of CO (US);
Rudolph Pruszko, Green River, WY
(US); David R. Baughman, Golden,
CO (US); Ralph B. Tacoma, Evanston,
WY(US)
(57) ABSTRACT
46 Claims, 2 Drawing Sheets
The present invention provides a process for producing
sodium carbonate monohydrate crystals by introduction of
anhydrous sodium carbonate into a saturated sodium carbonate
brine solution under conditions in which sodium
carbonate monohydrate formation is favored. As the anhydrous
sodium carbonate dissolves, the brine becomes supersaturated
resulting in relief of supersaturation by formation
of sodium carbonate monohydrate crystals. The process
includes controlling supersaturation and its relief to achieve
growth of existing sodium carbonate monohydrate crystals
rather than nucleation and formation of new sodium carbonate
monohydrate crystals. The resulting crystals are
separated from insoluble impurities on a size separation
basis.
* cited by examiner
Clay, S.E., "Kinetic Study of the Dissolution of Calcined
Trona Ore in Aqueous Solutions", Minerals and Metallurgical
Processing, Nov. 1985, 236-40.
Muraoka, D., "Monohydrate Process for Soda Ash from
Wyoming Trona," Minerals and Metallurgical Processing,
May 1985, 102-03.
American Society for Testing and Materials, "Standard Test
Methods for Chemical Analysis of Soda Ash (Sodium Carbonate)",
E-359-90, Mar. 1990,403-410.
FOREIGN PATENT DOCUMENTS
661071 7/1965 (BE).
0073085B1 12/1986 (EP).
OTHER PUBLICATIONS
Primary Examiner~tuart L. Hendrickson
(74) Attorney, Agent, or Firm~heridan Ross Pc.
U.S. PATENT DOCUMENTS
9/1943 Kermer 23/295
5/1957 Pike 23/38
11/1960 Seglin et al. 23/31
1/1961 Caldwell et al. 23/63
(List continued on next page.)
References Cited
Related U.S. Application Data
Continuation-in-part of application No. 09/225,805, filed on
Jan. 5, 1999, now abandoned
Provisional application No. 60/072,805, filed on Jan. 28,
1998.
Int. CI? COlD 15/08
U.S. CI. 23/302 T; 423/203.2; 423/421
Field of Search 423/206.2, 421,
423/426; 23/302 T
2,330,221
2,792,282
2,962,348
2,970,037
(63)
(60)
(51)
(52)
(58)
(56)
Subject to any disclaimer, the term of this
patent is extended or adjusted under 35
U.S.c. 154(b) by 0 days.
(21) Appl. No.: 09/239,441
(22) Filed: Jan. 28, 1999
( *) Notice:
MAKE-UP WATER
NoOH
FEED STREAM
SEED CRYSTALS
10
14
18
22
/- SATURATED BRINE
,.....,
26
INSOLUBLE
IMPURITIES
TO WASTE
PRODUCT
US 6,284,005 BI
Page 2
U.S. PATENT DOCUMENTS
3,061,409
3,233,983
3,236,590 *
3,244,476
3,273,959
3,314,748 *
3,425,795
3,479,133
3,498,744 *
3,653,848
3,705,790
3,717,698
3,796,794
3,819,805
3,836,628
3,845,119
3,904,733
10/1962
2/1966
2/1966
4/1966
9/1966
4/1967
2/1969
11/1969
3/1970
4/1972
12/1972
2/1973
3/1974
6/1974
9/1974
10/1974
9/1975
Robson et al. 23/63
Bauer et al. 23/300
Sopchak et al. 723/426
Smith 23/63
Miller 23/63
Howard et al. 423/426
Howard et al. 23/63
Warzel 23/63
Frint et al. 723/206.2
Port et al. 23/202
Garofano et al. 23/302
Ilardi et al. 423/206
Ilardi et al. 423/421
Graves et al. 423/206
Ilardi et al. 423/206
Duke et al. 260/527
Ganey et al. 423/206
3,933,977
3,956,457
4,021,527
4,022,868
4,083,939
4,138,312
4,183,901
4,202,667
4,260,594
4,283,277
4,286,967 *
4,288,419
4,299,799
4,374,102
4,472,280
4,781,899
5,300,123
5,396,863
1/1976
5/1976
5/1977
5/1977
4/1978
2/1979
1/1980
5/1980
4/1981
8/1981
9/1981
9/1981
11/1981
2/1983
9/1984
11/1988
4/1994
3/1995
Ilardi et al. 423/206
Port et al. 423/206
Baadsgaard 423/206
Poncha 423/184
Lobunez et al. 423/421
Gill et al. 162/30
Ilardi et al. 423/206
Conroy et al. 23/302
Verlaeten et al. 423/421
Brison et al. .. 209/166
Booth et al. 23/302 T
Copenhafer et al. 423/190
Ilardi et al. 423/206
Connelly et al. 423/206
Keeney 210/666
Rauh et al. 423/206
Groll 23/303
Ninane et al. . 117/206
... MAKE-UP WATER ..
NoOH -..
,,- SATURATED BRINE ,
FEED STREAM , , ,oC, .. -... .-- I\ ' , I \ Il Il ~t , ~,
SEED CRYSTALS CRYSTALLIZER - THICKENER V-26 .. - ~ ..
,
- INSOLUBLE 10 , .~
IMPURITIES
14 -----
DISPERSION " TO WASTE -
,.
~ PRODUCT _
18 -----
PRODUCT SEPARATOR ..
~
• ---- 22...........-- SEED SEPARATOR .-. FIG. 1 ---
d•
'JJ.
•
~
~.....
~=.....
'JJ.
~
'?
~,J;;,.
N
CC
'""'"
'JJ. =~
~....
o.'"."..'",
N
e
\Jl
0'1
N
00
~
b
Q
(I)
~
1-0"
u.s. Patent Sep.4,2001 Sheet 2 of 2 US 6,284,005 BI
30 35 40 45
WEIGHT PERCENT SODIUM CARBONATE
No2C03 ---A r------. -------
UNSATURATED Nal03· H2O
SOLUTION
\
L!---------- ---8 Na2C03·7 H2O v---- ----- ._-"1'----
Nal03 ·1 0H20 !
80 -, :
25
160
180
240
220
120
100
140
200
FIG. 2
US 6,284,005 Bl
1
SODIUM CARBONATE
RECRYSTALLIZATION
CROSS-REFERENCE TO RELATED
APPLICATIONS
This application is a continuation-in-part of U.S. patent
application Ser. No. 09/225,805, filed Jan. 5, 1999. This
application claims priority under 35 U.S.c. §119(e) from
U.S. Provisional Application No. 601072,805, filed Jan. 28,
1998.
2
than about 150 mesh and the particle size of the seed crystals
is from about 100 mesh to about 150 mesh.
Relief of supersaturation preferentially by rapid growth of
existing sodium carbonate monohydrate crystals over nucle-
5 ation can alternatively be achieved by a variety of methods.
Such methods can include maintaining a solids content of at
least about 40% in the crystallizer, agitating the brine
solution at an agitation index of at least about 6, periodically
lowering the temperature of the brine solution by at least
10 about 5° c., or pausing feedstream addition at least about
60% of the time of crystallization.
FIELD OF THE INVENTION
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention relates to the production of sodium
carbonate monohydrate crystals from anhydrous sodium 15
carbonate containing impurities.
FIG. 1 is a schematic flow diagram of one embodiment of
the process of the present invention.
FIG. 2 is a phase diagram for sodium carbonate.
DETAILED DESCRIPTION OF THE
INVENTION
1.0 Introduction
The present invention is based in part on the discovery
that under certain conditions sodium carbonate has an unexpectedly
high stable supersaturation capacity that can be
rapidly relieved by the introduction of sodium carbonate
monohydrate crystal surface to produce relatively large
crystals of sodium carbonate monohydrate at high rates of
crystal growth. Significant production efficiencies can be
attained at high rates of crystal growth. The resulting crystals
can be readily separated from insoluble impurities on a
size separation basis. Relief of supersaturation is controlled
such that crystal formation primarily occurs on existing
crystals, rather than occurring as nucleation or growth of
newly formed crystals. In this manner, the particle size
distribution of crystals is controlled to achieve a desired
35 distribution of product crystal size.
Processes of the present invention achieve supersaturation
of sodium carbonate by adding an anhydrous sodium carbonate
feed, e.g., calcined trona, to a saturated sodium
carbonate brine solution under temperature conditions in
40 which sodium carbonate monohydrate crystals are formed.
Thus, the tendency of the anhydrous sodium carbonate feed
to convert to the monohydrate form within the brine solution
causes the anhydrous sodium carbonate feed to dissolve,
thereby creating supersaturation, before forming sodium
45 carbonate monohydrate. Further, it has been surprisingly
found that, under appropriate conditions, sodium carbonate
has a supersaturation capacity of about 30 gil, which is about
an order of magnitude higher and more stable in the absence
of sodium carbonate monohydrate crystal surfaces than
50 would be expected by one skilled in the art. Therefore, the
present invention includes achieving and maintaining high
levels of supersaturation near the supersaturation capacity of
sodium carbonate to create a high driving force for supersaturation
relief which results in rapid crystallization.
Supersaturation created in this manner is relieved by
formation of sodium carbonate monohydrate. Sodium carbonate
monohydrate can form as a result of exceeding the
supersaturation limit, which causes primary nucleation
resulting in formation of clouds of small nuclei of sodium
60 carbonate monohydrate. The term "supersaturation limit" is
used to describe a condition where the level of supersaturation
of sodium carbonate in the brine solution is unstable
and results in a relatively spontaneous formation of crystals
by primary andlor secondary nucleation. This type of super-
65 saturation relief is unproductive because the small nuclei
cannot easily be grown to a size large enough to be separated
from insoluble impurities. Supersaturation relief can also
55
SUMMARY OF THE INVENTION
BACKGROUND OF THE INVENTION
One common method of purifying a compound is to
crystallize the compound in a solution. Methods of crystal- 20
lization typically involve controlling macroscopic external
variables such as evaporating solvent to create supersaturation
or adjusting the temperature of the solvent to affect
solubility. These crystallization methods are generally
directed to achieving maximum solids recovery andlor puri- 25
fication without any regard to the size or shape of the
crystals.
Therefore, there is a need for a crystallization process that
can effectively control or influence the ratio of crystal
growth to formation of new crystals at low energy costs. 30
The present invention is based on the discovery that
sodium carbonate has an unexpectedly high stable supersaturation
capacity under appropriate conditions that can be
rapidly relieved by the introduction of sodium carbonate
monohydrate crystal surfaces to produce relatively large
crystals of sodium carbonate monohydrate at high rates of
crystal growth. The resulting crystals can be readily separated
from insoluble impurities on a size separation basis.
More particularly, the process of the present invention is
for producing sodium carbonate monohydrate from a feedstream
which includes anhydrous sodium carbonate and
insoluble impurities. The process includes adding the feedstream
to a saturated sodium carbonate brine solution under
conditions to create supersaturation of at least about 5 gil.
The process further includes processing within parameters
that preferentially relieve the supersaturation by rapid
growth of existing sodium carbonate monohydrate crystals
rather than by nucleation. In this manner, the particle size
distribution of crystals is controlled to achieve a desired
distribution of crystal size product. The sodium carbonate
monohydrate crystals produced by the process are recovered
from the saturated brine solution.
The process can include the use of a high feed rate of at
least about 100 grams of feedstream per minute for each liter
of solution in the crystallizer. The process can also include
relieving the supersaturation preferentially by rapid growth
of existing sodium carbonate monohydrate crystals over
nucleation by adding sodium carbonate monohydrate seed
crystals to the saturated sodium carbonate brine solution.
Such seed crystals can be produced by removing sodium
carbonate monohydrate crystals from the brine solution and
sizing the removed crystals to produce a seed crystal size
fraction for reintroduction to the brine solution. In a preferred
embodiment, the particle size of the feedstream is less
3
US 6,284,005 Bl
4
occur by growth of existing sodium carbonate monohydrate
crystals, which is desired in the present invention.
Processes of the present invention are based on the
recognition that since supersaturation is created by the
introduction of anhydrous feed, the supersaturation limit can
be exceeded in a localized area at the point of introduction
of the feed. Therefore, control of supersaturation and its
relief in the local environment near where the feed is
introduced is critical. The present invention provides the
proper thermodynamic environment wherein it is easier to
preferentially relieve supersaturation by growth of existing
crystals than by nucleation.
Processes of the present invention include a multi-faceted
approach to control local supersaturation and its relief to
achieve the desired mechanism for supersaturation relief,
preferably the growth of existing crystals. One of the elements
of processes of the present invention is to use high
agitation to rapidly disperse areas of local high supersaturation
to avoid exceeding local supersaturation limits, and to
bring the surfaces of existing crystals into contact with such
areas of supersaturation. The use of high agitation is quite
contrary to standard crystallization practice and technology.
Processes of the present invention also provide a large
amount of available sites for relief of supersaturation on
existing crystals so that if the degree of supersaturation in a
localized area is approaching the maximum level, i.e., the
supersaturation limit, the supersaturation can be quickly
relieved by sodium carbonate monohydrate formation on an
existing crystal surface instead of by nucleation. Sites for
crystallization are provided by the use of seed crystals
and/or by maintaining a high solids content in the crystallizer.
The present invention can also include pausing during
the introduction of feed to allow for dispersion of local areas
of very high supersaturation by agitation and/or productive
relief of supersaturation on existing crystals in local areas of
very high supersaturation. Control of temperature in the
crystallizer is also used to control the rate of relief of
supersaturation.
The terms "recrystallization" and "crystallization" are
used interchangeably herein and refer to the step of adding
anhydrous sodium carbonate to a saturated sodium carbonate
brine solution and crystallizing sodium carbonate monohydrate
from the saturated brine solution, i.e., the anhydrous
sodium carbonate dissolves in the saturated brine solution,
forms a supersaturated solution which then causes growth of
sodium carbonate monohydrate crystals because the temperature
of the saturated brine solution is in the range of
sodium carbonate monohydrate stability. A "saturated brine
solution" refers to a solution which is saturated with sodium
carbonate.
2.0 Feedstream Composition and Introduction
2.1 Composition
As noted above, a feedstream of the present invention
comprises anhydrous sodium carbonate. For example, processes
of the present invention can be used for purifying
anhydrous sodium carbonate (such as calcined trona) containing
impurities or for producing dense soda ash from light
soda ash. Moreover, the present invention is particularly
well adapted for use with feedstreams having high contents
of insoluble impurities. In particular, the present invention
can be used for purifying anhydrous sodium carbonate
feedstreams in which impurities are included within the
crystal structure even when the particles are finely ground.
Thus, although the present invention can be used with a
substantially pure anhydrous sodium carbonate, the present
invention is particularly suitable for use with feedstreams
having greater than about 15% by weight insoluble
impurities, and even more particularly, having greater than
about 30% by weight insoluble impurities. Although any
anhydrous sodium carbonate including synthetic anhydrous
sodium carbonate or calcined trona can be used, processes of
5 the present invention will now be described in detail in
reference to purification of calcined trona containing impurities
and FIG. 1. And as such, the terms "calcined trona" and
"anhydrous sodium carbonate" will hereinafter be used
interchangeably.
10 2.2 Size
As noted, supersaturation is achieved by adding calcined
trona to a saturated brine solution under temperature conditions
at which sodium carbonate monohydrate forms.
Thus, the calcined trona dissolves, thereby creating super-
15 saturation and also releasing impurities, before forming
sodium carbonate monohydrate. The rate and completeness
of calcined trona dissolving in a saturated brine solution is
determined by, among other factors, its particle size. Since
the presence of undissolved hydrated calcined trona can
20 compete with seed crystals of monohydrate as a substrate for
relieving supersaturation, the calcined trona added to the
saturated brine solution should dissolve substantially completely
to ensure that the majority of supersaturation relief is
by growth of seed crystals, not by growth on undissolved
25 anhydrous feed, and to ensure that at least a portion of
impurities present within the crystal lattice of sodium carbonate
is released. If the feedstream of calcined trona
dissolves only partially, the remaining particles can have
undesired effects such as forming agglomerates or relieving
30 supersaturation to form mixed particles of calcined trona and
sodium carbonate monohydrate. Thus, to ensure a substantially
complete dissolution of the particles the particle size of
calcined trona ore in the feedstream, whether in a slurry
form or a dry form, is preferably less than about 100 mesh
35 (Tyler), more preferably less than about 150 mesh, still more
preferably less than about 200 mesh, and most preferably
less than about 400 mesh. It should be appreciated that when
the particle size of calcined trona ore is within the above
described range, any insoluble impurities present in the
40 calcined trona ore will also be within the confines of the
above described particle size.
The above particle size limitations allow calcined trona
ore to dissolve relatively quickly and completely in a
saturated brine solution in the crystallizer 10.
45 2.3 Feed Rate
As noted above, it has been surprisingly found that, under
appropriate conditions, sodium carbonate has a supersaturation
capacity of about 30 gil, which is about an order of
magnitude higher than would be expected by one skilled in
50 the art. Therefore, the present invention includes achieving
and maintaining high levels of supersaturation near the
supersaturation capacity of sodium carbonate to create a
high driving force for supersaturation relief which results in
rapid crystallization. For example, the process includes
55 creating supersaturation of at least about 5 gil, more preferably
at least about 10 gil, more preferably at least about 20
gil and up to 30 gil. Supersaturation can be calculated within
a localized volume in a crystallizer or within the entire
volume of a crystallizer. For example, supersaturation can be
60 calculated as follows. A volume of saturated brine, which
can include sodium carbonate monohydrate crystals and
calcined trona, can be withdrawn from a crystallization
vessel through a screen and filter to remove solid materials.
Water in the withdrawn brine is then evaporated and the
65 amount of sodium carbonate per volume of brine can be
gravimetrically determined. The amount of sodium carbonate
in excess of the known solubility level is the amount of
5
US 6,284,005 Bl
6
supersaturation. Because of the high capacity for supersaturation
and the very rapid relief of supersaturation, the rate of
introduction of the feedstream or feed rate can be very high
in the present invention. More particularly, the feed rate can
be at least about 100 grams per minute for each liter of 5
volume (gil/min), preferably at least about 200 gil/min, more
preferably at least about 400 gil/min, and even more preferably
at least about 800 gil/min. These feed rates are
significantly higher than feed rates expected to be useful by
one skilled in the art and those utilized by previous crystal- 10
lization methods.
2.4 Method of Introduction
The feedstream, which includes anhydrous sodium
carbonate, can be introduced to the saturated brine solution
using any of the known methods including by a direct 15
injection, a screw feeder and gravity. The feedstream can be
a slurry of anhydrous sodium carbonate in a saturated brine
solution or dry anhydrous sodium carbonate.
A dry anhydrous sodium carbonate feedstream must be
dispersed and dissolved quickly in the saturated brine 20
solution, otherwise particles may become hydrated and form
agglomerates. If the particles in the feedstream are too
coarse, they will not dissolve completely, thus possibly
reducing the purity of the product; therefore, the particle size
of the feedstream should be within the range discussed 25
above. On the other hand, fine particles tend to "float" on top
of the saturated brine solution and become hydrated and
form agglomerates. Generally, at a high feedstream addition
rate discussed above, it is difficult to quickly disperse and
dissolve the anhydrous sodium carbonate into the saturated 30
brine solution. It has been found by the present inventors
that these problems can be overcome by using high
agitation, as discussed below.
One can also avoid these problems, such as agglomerate
formation and floatation of fines, by adding a feedstream of 35
anhydrous sodium carbonate in a slurry form. A slurry of
anhydrous sodium carbonate can be prepared by mixing
calcined trona ore and the saturated sodium carbonate solution
at atmospheric pressure and transferring the mixture
into a slurry feedstream vessel having a desired temperature 40
at increased pressure. Alternatively, calcined trona ore and
the saturated sodium carbonate solution can be fed directly
into the slurry feedstream vessel at a desired temperature
and pressure to form a slurry feedstream. At a temperature
above the transition temperature of sodium carbonate mono- 45
hydrate to anhydrous sodium carbonate (108.5° C. for a pure
system of water and sodium carbonate at one atmosphere of
pressure), solids in the slurry include anhydrous sodium
carbonate crystals and insoluble materials originally present
in the calcined trona ore. It is recognized by those skilled in 50
the art that the transition temperature can be adjusted by
various means, including by adding sodium chloride.
One method of preparing a slurry of feedstream involves
mixing anhydrous sodium carbonate with a saturated sodium
carbonate brine solution at a temperature at least above the 55
transition temperature of anhydrous sodium carbonate to
sodium carbonate monohydrate preferably at least about 5°
C. above the transition temperature, and more preferably at
least about 2° C. above the transition temperature. A "transition
temperature" refers to a temperature at which stable 60
anhydrous sodium carbonate changes its morphology to
stable sodium carbonate monohydrate. See for example, line
Ain FIG. 2, the transition of anhydrous to sodium carbonate
monohydrate. Line B in FIG. 2 represents the transition
temperature between sodium carbonate heptahydrate and 65
sodium carbonate monohydrate. It will be appreciated that
this step of producing a slurry feedstream must be conducted
at above atmospheric pressures and must use a feeding
mechanism that maintains a continuous pressure seal
between the environment of the feed slurry and of the brine
solution.
It should be further appreciated that this method of
introduction of anhydrous sodium carbonate can be used for
processing in any aqueous solution.
2.5 Calcination
When trona is used as a feedstream, it must be converted
into anhydrous sodium carbonate by calcination prior to
being added to the saturated brine solution. Trona can be
calcined using any known calcination technology. For
example, calcination can be conducted with a fluidized bed
calciner. When a fluidized bed calciner is used to calcine
trona ore, the trona ore is comminuted and is generally
separated into three size ranges: 6x20 mesh, 20x100 mesh
and -100 mesh. Each size can then be separately calcined in
a fluidized bed calciner. Calcined trona is then combined and
comminuted to provide a feedstream having above mentioned
particle size. Further, trona in the feedstream can be
calcined using indirect heat calcination as disclosed in
commonly assigned U.S. patent application Ser. No. 09/151,
694 that was filed on Sep. 11, 1998, which is incorporated
herein by reference in its entirety.
3.0 Crystallization
As shown in FIG. 1, a feedstream comprising calcined
trona is added to a saturated sodium carbonate brine solution
in a crystallizer 10 to generate supersaturation within the
saturated brine solution. The feedstream and the saturated
brine solution can be added simultaneously and/or sequentially.
The present method controls crystallization conditions
so that relief of supersaturation created by introduction of
the anhydrous sodium carbonate feed primarily occurs on
existing sodium carbonate monohydrate crystals rather than
by nucleation.
3.1 Seed Crystals
In one embodiment of the present invention, supersaturation
relief on existing crystals is achieved by the introduction
of seed crystals of sodium carbonate monohydrate to
the crystallizer 10. Thus, in contrast to other crystallization
methods in which a major amount of crystal growth is by
nucleation or on crystals newly formed by nucleation, processes
of this particular embodiment of the present invention
provide supersaturation relief primarily by growing seed
crystals to crystals that are large enough to be separable from
insoluble impurities on a size separation basis. Moreover,
the size distribution of the product crystal population can
also be controlled by adding seed crystals of a desired
particle size range. By the use of seed crystals in this
manner, crystal growth is productive in the sense that it
occurs on crystals which will be large enough to recover on
a size separation basis, rather than occurring on small
particles which cannot practically be grown large enough to
be separated from insoluble impurities.
Seed crystals can be prepared separately or can be prepared
as a part of the process flow of the present crystallization
process, as described below. For example, seed crystals
can be produced by removing crystals from the
crystallizer and sizing the crystals to produce a seed crystal
size portion for reintroduction to the crystallizer.
Furthermore, at least a portion of the product of the present
process can be comminuted, e.g., ground, to a desired seed
crystal size and used as a source of the seed crystals.
In a batch process, the seed crystals are typically added
prior to the addition of the feedstream, whereas in a continuous
process, the seed crystals are typically added continuously
during the operation of the present invention. As
US 6,284,005 Bl
7 8
TABLE 1
rolling surface with quick turnover and quick absorption
of dry material into mass of slurry.
turnover of slurry, but not all solids held in suspension
mild turnover of slurry with all solids held in suspension
Description
static, no movement or mixing
violent turbulent movement of all slurry in entire vessel
degradation of mechanical fracturing of material
1
2
3
4
5
6
7
8
9
10
Agitation
Index
Preferably, the mixture in the crystallizer 10 is stirred at an
agitation index of at least about 4, more preferably at least
about 7, still more preferably at least about 8, and most
preferably at least about 9.
Evidence of insufficient agitation can be readily determined
by examining crystal structures of the product. The
product resulting from insufficient agitation may include the
presence of agglomerates, long needle-like crystals or dendrites.
In contrast to other methods, agitation in the present
invention preferably does not produce a typical vortex
associated with using a single propeller non-baffled agitation
system. In a particular embodiment of the present invention,
agitation of the monohydrate slurry is achieved by using at
least two propellers having a counter pitch or other suitable
65 agitation methods including using an attrition scrubber and
any other impeller configurations which achieve the desired
agitation index discussed above.
supersaturation is created by the anhydrous feed dissolving
to in the saturated brine solution. Therefore, control of
supersaturation and its relief in the local environment, for
example, by sufficiently high agitation, where the feed is
5 introduced is critical. The term "local" refers to the immediate
environment of a small portion of the brine solution in
the crystallizer 10 and not the overall amount of sodium
carbonate within the total volume of the crystallizer 10.
Thus, the term "local supersaturation limit" refers to the
10 degree of supersaturation within any volume of a crystallizer
in which formation of a crystal nucleus by primary and/or
secondary nucleation can occur. It will be appreciated
therefore, that within the crystallizer 10, while the average
degree of supersaturation can be below the supersaturation
limit, a localized region of high supersaturation can occur
15 and thereby exceed the supersaturation limit in that localized
region, resulting in undesired nucleation. To reduce or avoid
this undesired nucleation, processes of the present invention
can also include control of local supersaturation by using
high agitation to rapidly disperse areas of high local super-
20 saturation. High agitation brings the surfaces of existing
crystals into contact with areas of high local supersaturation
and thereby, increases the effective net surface area available
for supersaturation relief by increasing the probability of an
existing crystal particle coming into contact with an area of
25 local high supersaturation area. One measure of agitation is
a qualitative agitation index as described below. The term
"agitation index" refers to a scale of agitation in a crystallizer.
An agitation index of 0 means that there is no perceptible
stirring or movement within the mixture, whereas an
30 agitation index of 10 means the mixture in the crystallizer is
stirred at a very high and rapid degree of mixing and
agitation such that degradation or mechanical fracturing of
crystals occurs. Table 1 shows the qualitative characteristics
of the 0-10 agitation index.
used in this invention, a "continuous addition" can include
both non-interrupted addition as well as interval addition
throughout the process as needed.
The particle size of the seed crystals is selected such that
a product having an acceptable particle size range is produced.
For example, the seed crystals need to be large
enough that, given the amount of growth achieved in a given
crystallization, the resulting product crystals will be large
enough to be separable from insoluble impurities on a size
separation basis. Preferably, the particle size of the seed
crystals is in the range from about 100 mesh (Tyler) to about
400 mesh, more preferably from about 100 mesh to about
200 mesh and most preferably from about 100 mesh to about
150 mesh. Alternatively, the range of the particle size of seed
crystals is about 2 standard sieve sizes or less. A "standard
sieve size" is denoted by increasing or decreasing the
opening in a sieve size by the ratio of the square root of 2
or 1.414, i.e., taking a screen opening and multiplying or
dividing it by the square root of 2 or 1.414. The seed crystal
size range utilized is determined by the desired product
particle size range. For example, a narrow seed crystal size
range results in a narrow product particle size range.
The amount of seed crystals and feedstream added to the
saturated brine solution depends on the volume of the
saturated brine solution in the crystallizer 10. However, as
noted below, the total amount of seed crystals and feedstream
added to the saturated brine solution typically results
in a monohydrate slurry having a solids content in accordance
with the parameters discussed below. As used herein,
a "monohydrate slurry" refers to a saturated brine solution
containing solid sodium carbonate monohydrate crystals.
Such a high solids content ensures that sufficient surface area
is available for supersaturation relief on existing crystals
before any significant amount of nucleation can occur in the
brine solution. In another embodiment, for the above mentioned
particle sizes of seed crystals and products, the ratio 35
of seed crystals added to the feedstream added is at least
about 1:1 by weight, preferably at least about 5:1 by weight,
and more preferably at least about 10: 1. Generally, about an
equal amount of the solids content by weight of the seed
crystals and the feedstream is added to the saturated brine 40
solution.
3.2 Solids Content
A further aspect of the present invention to control
supersaturation relief on existing crystals of sodium carbonate
monohydrate is to maintain a high solids content in the 45
crystallizer 10. In this manner, if the degree of supersaturation
in a localized area is approaching the maximum level,
supersaturation can be quickly relieved by sodium carbonate
monohydrate formation on an existing crystal surface
instead of by nucleation. As will be appreciated, the solids 50
content in the crystallizer 10 depends on a variety of factors
including the amount of seed crystals added and the amount
and solids density of the feedstream added to the saturated
brine solution, as well as the desired density for optimal
crystallizer operation. These variables are controlled such 55
that the monohydrate slurry has a solids content of at least
about 17% by weight, more preferably at least about 35% by
weight, even more preferably at least about 40% by weight,
and most preferably at least about 60% by weight.
Alternatively, the particle surface area density, i.e., the total 60
amount of surface area of crystals present per volume, is at
least about 40 cm2/ml, preferably at least about 75 cm2/ml,
more preferably at least about 95 cm2/ml, and most preferably
at least about 125 cm2/ml.
3.3 Crystallizer Agitation
As noted above, the supersaturation limit of the brine
solution can be exceeded in a small localized area because
US 6,284,005 Bl
9 10
calciner. By adding a freshly calcined, i.e., hot, trona to the
saturated brine solution, the amount of energy and the cost
required to maintain the mixture at the above described
temperature can be significantly reduced compared to pro-
S cesses where calcined trona is reheated prior to being added
to the saturated brine solution or where the saturated brine
solution is at a higher temperature then the feedstream.
As noted above, the present invention includes controlling
supersaturation relief to achieve crystal growth on existing
10 crystals of sodium carbonate monohydrate rather than initiating
nucleation. A further aspect of the present invention
is the control of supersaturation relief by modifying the
temperature of the crystallization solution in a temperature
cycling process to control the amount of fines as discussed
15 in more detail below.
3.5 Feed Addition Pause
Crystal formation in the form of nucleation occurs when
the local supersaturation level exceeds the supersaturation
limit. When the rate of supersaturation generation exceeds
20 the rate of supersaturation relief, eventually the supersaturation
level somewhere in the crystallizer will exceed the
supersaturation limit resulting in nucleation (sometimes
referred to as "snowing-out"). Thus, to prevent the supersaturation
level in a local area from exceeding the super-
25 saturation limit, the addition of the feedstream to the saturated
brine can be stopped briefly or intermittently to
decrease the supersaturation level by allowing growth of
existing crystals. More particularly, the break or pause in
feedstream addition can be conducted at least about 60% of
30 the time of crystallization. More preferably, the pause can be
conducted at least about 30%, and most preferably, at least
about 5% of the time of crystallization. For example, if the
pause is 10% of the crystallization time, the feedstream
would be paused 6 minutes during every hour of operation.
35 It should be noted that when pausing is used, it is preferably
conducted frequently, such as by switching between feeding
and pausing every several minutes, or about every five
minutes.
3.6 Crystal Growth Rate
It is believed that the conventional recommended crystal
growth rates for good crystal quality is from about 2
microns/minute to about 5 microns/minute. A "good crystal
quality" refers to crystals which are generally hexagonal,
roughly equi-dimensional, slightly elongated with an aspect
45 ratio of WxLxH of about 1:1.5:0.75. See for example,
Goldschmidt, Atlas der Krystallformen, p. 128 (Carl Winters
Universitatbuchhandlung, Heidelberg 1922), which is incorporated
herein by reference in its entirety. The crystal
growth rate of the present invention is significantly higher
50 than the conventional recommended crystal growth rates
while providing a similar crystal quality. Preferably the
crystal growth rate of the present invention is at least about
5 microns/minute, more preferably at least about 10
microns/minute, and most preferably at least about 20
55 microns/minute. It has been found that the crystal growth
rate of the present invention does not decrease significantly
by having a higher solids to saturated brine solution ratio.
However, it is believed the crystal growth rate does depend
on the size of the seed crystals. The reason for the higher
60 growth rate of coarser crystals is the mass transfer of sodium
carbonate monohydrate crystals from finer crystals to
coarser crystals. The operation of this mechanism at high
crystal growth rates such as in the current invention is
contrary to what would be expected by one of skill in the art.
An average crystal growth rate can be determined by a
variety of methods including by a statistical analysis of a
sample product crystal. For example, the average crystal
Preferably, the solution is agitated at greater than about 10
horsepower/lOOO gallons (hp/lOOO gal), more preferably at
least about 100 hp/lOOO gal, and most preferably at least
about 200 hp/lOOO gal. Alternatively, when a propeller
system is used for agitating the monohydrate slurry, the
propeller tip speed is at least about 8 feet/sec (ftlsec),
preferably at least about 10 ftlsec, and more preferably at
least about 22 ft/sec.
Adequate agitation can be achieved by use of any vessel
providing agitation as described above. For example, such a
vessel can include a one impeller system; two impellers
having counter pitch, such as is used in an attrition scrubber;
multiple impellers having alternating counter pitch in the
crystallizer 10, or other configurations providing the desired
agitation index. Thus, in such agitation, it is important to
create a rapid exchange of solid particles and the solution
portion of the saturated brine solution.
It should be noted, however, that while high agitation is
beneficial, it should be conducted in a manner without a
significant amount of impact destruction. The term "impact
destruction" refers to a process where two or more particles
collide and result in a particle size reduction for one or more
particles.
3.4 Temperature Control
As discussed above, the temperature of the saturated brine
solution is maintained such that the formation of sodium
carbonate monohydrate is favored as determined by the
phase diagram, as shown in FIG. 2. The temperature of the
saturated brine solution in the crystallizer 10 is maintained
at between about 40° C. and the transition temperature of
anhydrous sodium carbonate to sodium carbonate monohydrate
to ensure formation of sodium carbonate monohydrate,
preferably between about 70° C. and the transition temperature
of anhydrous sodium carbonate to sodium carbonate
monohydrate, more preferably between about 90° C. and
the transition temperature of anhydrous sodium carbonate to
sodium carbonate monohydrate and most preferably
between about 98° C. and the transition temperature of
anhydrous sodium carbonate to sodium carbonate monohydrate
It has been discovered by the present inventors that 40
keeping the temperature in the crystallizer as close as
possible to but below the transition temperature of sodium
carbonate monohydrate to anhydrous sodium carbonate
reduces the "drive", i.e., the rate of conversion, of anhydrous
sodium carbonate in the feedstream to change morphologically
to sodium carbonate monohydrate. This discovery
allows the processes of the present invention to be controlled
easily and results in larger, better formed crystals as discussed
in detail below.
To maintain a substantially constant temperature of the
saturated brine solution within the crystallizer 10, the temperature
difference between the saturated brine solution and
the feedstream should be small enough such that no significant
cooling or heating of the saturated brine solution occurs
during the addition of the feedstream. Preferably, the temperature
difference between the feedstream and the saturated
brine solution is about 20° C. or less, more preferably about
15° C. or less, and most preferably about 10° C. or less.
In another embodiment, the temperature of the dry feed
particles in the feedstream is at least about 95° c., preferably
at least about 120° c., and more preferably at least about
150° C.
Still in another embodiment, freshly calcined trona can be
added directly to the crystallizer 10 along with a saturated
brine solution to maintain the temperature of the mixture in 65
the crystallizer 10 as disclosed above. Freshly calcined trona
has a high particle temperature as it comes out of the
US 6,284,005 Bl
11 12
one aspect of the present invention, the crystallization process
is conducted by maintaining the amount of solids in the
monohydrate slurry in the form of agglomerates and/or
aggregates at about 10% by weight or less of the total
5 sodium carbonate solids in the monohydrate slurry, more
preferably at about 5% by weight or less of the total sodium
carbonate solids in the monohydrate slurry, and most preferably
at about 0.5% by weight or less of the total sodium
carbonate solids in the monohydrate slurry.
As used herein, the term "aggregate" refers to a collection
of particles or crystals in clusters or clumps. The particles
can be held together as a result of the attraction of weak
forces, such as van der Waals forces. The term "agglomerate"
refers to particles or feed held together by forces
15 stronger than van der Waals forces, which can be formed, for
example, by anhydrous feed particles which are not fully
dissolved acting as a site for crystallization of monohydrate
crystals, or anhydrous feed that was not dispersed or dissolved
absorbing water to hydrate.
20 3.9 Crystallizer Pressure
The crystallizer 10 can be equipped to be operated at a
wide range of pressure. In one embodiment, the crystallizer
10 is operated at atmospheric pressure. In another
embodiment, the crystallizer 10 can be operated at any
25 desired pressure of up to about 35 pounds per square inch
(psia), more preferably up to about 30 psia, and most
preferably up to about 25 psia. Unless otherwise noted, the
pressure refers to an absolute pressure and not a relative, i.e.,
gauge, pressure. Whether operated under atmospheric pres-
30 sure or higher pressure, the temperature of the saturated
brine solution in the crystallizer 10 is maintained to favor the
formation of sodium carbonate monohydrate. When the
crystallizer 10 is operated under pressure, the introduction of
the feedstream is preferably at a similar pressure. A pres-
35 surized pump such as a Fuller Kinyon pump (not shown) or
any other type of pump which can achieve a desired pressure
can be used to introduce the dry or slurry feedstream into the
crystallizer 10. However, it should be recognized that the
feedstream can be at a variety of pressures independent of
40 the crystallization itself.
3.10 Multiple Crystallization Vessels
In a further embodiment, the crystallization is conducted
in a series of two or more crystallizers. In this manner, the
initial feedstream can be used to generate fines by nucleation
45 in a first crystallizer. The fines are then transferred to a
second crystallizer and used as seed crystals for subsequent
crystallization where they are grown to a larger size. Thus,
in either the second or some subsequent crystallizer, the
crystals are grown large enough for a size separation from
50 insoluble impurities. By using a multiple tank system which
allows successive crystal growth conditions, the need for a
separate seed crystals as discussed above in Section 3.1 can
be eliminated.
4.0 Dispersion
Referring again to FIG. 1, at least a portion of sodium
carbonate monohydrate crystals and saturated brine solution
are separated from the crystallizer 10. The sodium carbonate
monohydrate product is eventually recovered in a product
separator 18, preferably on a size separation basis. However,
60 as noted above, crystallizations are conducted at high solids
content, such as at least about 17% solids content. Product
separation with such a viscous mixture can be difficult.
Therefore, as shown in FIG. 1, the separation process can
also include transferring at least a portion of the monohy-
65 drate slurry from the crystallizer 10 to a dispersion tank 14
to decrease the solids content of the monohydrate slurry in
order to, inter alia, facilitate the separation process. It should
growth rate can be obtained by dividing the total amount of
crystal growth in the sample by the total crystallization time
and the total crystal surface area.
3.7 Nucleation Control
Processes of the present invention involve controlling
crystallization conditions as discussed in Sections 3.1-3.6 to
provide conditions for relieving the supersaturation in the
crystallizer 10 by growing existing crystals rather than by
nucleation. If a significant amount of primary and/or secondary
crystal nucleation occurs in the crystallizer 10, then 10
a large amount of fines is generated. Production of fines
limits productive crystal growth because fines have a large
ratio of surface area to volume compared to larger crystals.
Since fines are small, even significant growth of them will
not make them large enough to be separated from insoluble
impurities on a size separation basis. Therefore, such growth
is unproductive. However, it should be appreciated that
some formation of new crystals by nucleation may be
necessary when the process includes generating new seed
crystals. Thus, processes of the present invention may be
used to allow formation of new crystals by nucleation in a
relatively controlled amount for this purpose.
Thus, in a further aspect of the present invention, the
amount of solids in the saturated sodium carbonate brine
formed by primary and/or secondary nucleation in the
crystallizer 10 is maintained at about 10% by weight or less
of the total sodium carbonate solids in the saturated brine,
more preferably at about 5% by weight or less of the total
sodium carbonate solids in the saturated brine, still more
preferably at about 1% by weight or less of the total sodium
carbonate solids in the saturated brine, and most preferably
at about 0.5% by weight or less of the total sodium carbonate
solids in the saturated brine. For example, given a defined
crystal population at a point in time, one can determine
whether new crystals have been formed by primary and/or
secondary nucleation by determining whether the crystal
population at a later point in time has smaller crystals or an
increase in smaller crystals compared to the earlier point in
time. One can also determine whether new crystals have
been formed by primary and/or secondary nucleation by
identifying whether a drop in yield of +200 mesh crystals
occurs. One can also determine whether new crystals have
been formed by primary and/or secondary nucleation in a
continuous process by identifying fluctuations in the size
distribution of crystals at a point in time at which a stable
population would be expected.
In a further aspect of the invention, control of the crystallization
conditions can maintain or reduce the portion of
the solid material in the monohydrate slurry which has a
small particle size. More particularly, the processes of the
present invention can include maintaining the amount of
solids in the monohydrate slurry having a particle size of less
than about 400 mesh at less than about 10% by weight of the
total sodium carbonate solids in the monohydrate slurry,
more preferably at less than about 2% by weight of the total 55
sodium carbonate solids in the monohydrate slurry, and most
preferably at less than about 0.5% by weight of the total
solids in the monohydrate slurry.
3.8 Agglomerate/Aggregate Control
Processes of the present invention for controlling crystallization
conditions as discussed above in Sections 3.1-3.6
can also substantially avoid formation of a significant
amount of agglomerates and/or aggregates. If a significant
amount of agglomerates and/or aggregates are formed, the
purity of any recovered product may be significantly
decreased because insoluble and soluble impurities can be
trapped within the agglomerates and aggregates. Thus, in
US 6,284,005 Bl
13 14
bulk density of at least about 0.95 glml, preferably at least
about 1.0 g/ml, and more preferably at least about 1.1 g/ml.
In another embodiment of the present invention, the product
has a packed density of at least about 1.0 g/ml, preferably at
5 least about 1.1 g/ml, and more preferably at least about 1.2
g/ml.
The product of the present invention also has a lower
amount of dust, i.e., fines, than crystals produced by the
conventional crystallization processes. Without being bound
10 by any theory, this low amount of dust present in the product
is believed to be due to a variety of novel features of the
present invention including the use of seed crystals, the
relief of supersaturation primarily by crystal growth rather
than by formation of new crystals, and the block-like shape
15 of the product crystals which is more resistance to abrasion
than other crystal shapes.
The product of the present invention has improved
flowability and decreased bridging compared to products
produced by conventional methods. It is believed the block-
20 like crystal shape and the absence of fine crystals produces
higher flowability and lower bridging in storage vessels.
This block-like crystal shape has smoother crystal surfaces
compared to other crystal shapes such as jack-like or needle
like crystal shapes. Without being bound by any theory, it is
25 believed that the smooth surface of block-like shaped crystals
has a lower frictional force than other crystal shapes. In
addition, larger particles have a reduction in specific surface
area, and thereby the cohesiveness between particles is
reduced.
30 6.0 Seed Separation
Again referring to FIG. 1, an undersize fraction of the
monohydrate slurry from the product separator 18 can be
transferred to a seed crystal separation apparatus 22 to
separate at least a portion of crystals from the undersize
35 fraction for use as seed crystals. The undersize fraction will
include sodium carbonate monohydrate crystals smaller than
the size cutoff in the product separator 18 and insoluble
impurities. To effectively produce a seed crystal population,
the undersize fraction from the product separator 18 must
40 include an upper size range which is larger than the size of
the insoluble impurities. In this manner, by conducting a size
separation in the seed separator 22, seed crystals which are
free of insoluble impurities can be recovered as an oversize
fraction, and the insoluble impurities with small sodium
45 carbonate monohydrate crystals are generated as the undersize
fraction. The seed crystal separation can be accomplished
by any of the appropriate known methods as discussed
above. As discussed above, a seed crystal population
produced in this manner is then used in a crystallizer.
Other methods of producing seed crystals include the
following: wet comminution of monohydrate crystals; dry
comminution of monohydrate crystals; dissolution of a
portion of monohydrate crystals by water addition; dissolution
of crystal in a slurry by cooling the slurry to increase the
55 solubility of sodium carbonate in the brine; and controlled
cooling of a slurry of anhydrous sodium carbonate.
7.0 Thickening
The undersize fraction from the seed separator 22, containing
saturated brine solution, insoluble impurities and/or
60 sodium carbonate monohydrate crystals which are smaller
than the desired seed crystal size is then further processed.
As shown in FIG. 1, the undersized fraction from the seed
separator 22 is transferred to a thickener 26 to allow for
settling of insoluble impurities. The settled insoluble impu-
65 rities are then purged from the system, while the clear
overflow and/or the resulting clarified saturated brine solution
can be recycled and reused. It should be appreciated that
be noted that the dispersion step should not dilute the
solution below saturation. Otherwise, product loss can occur
by dissolution of product. Typically, a saturated brine solution
having a substantially negligible solids content is added
to the dispersion tank 14 to reduce the solids content of the
monohydrate slurry to about 25% by weight or less, more
preferably to about 15% by weight or less solids content, and
most preferably to about 10% by weight or less solids
content.
5.0 Recovery
The present invention also includes recovering product
from the monohydrate slurry. The recovery process can
include separating a particular particle size range of sodium
carbonate monohydrate crystals from the monohydrate
slurry. Size separation is conducted in a separation apparatus
18 and can be affected by any of the appropriate known
methods. For example, screening, cyclones (such as
hydrocyclones) or elutriation can be used.
The sodium carbonate monohydrate crystal product
which is recovered typically has a particle size of greater
than at least about 150 mesh. Preferably, the product has a
particle size of greater than at least about 100 mesh, and
more preferably greater than at least about 80 mesh. More
particularly, the size cutoff for product recovery has to be at
least as large as or larger than the particle size of the feed so
that insoluble impurities initially in the feed are not recovered
with product.
Separation of sodium carbonate monohydrate crystals is
generally conducted by screening or cycloning and avoiding
drying of the crystals. Drying of the crystals at this stage
may result in cementing, or agglomerate formation, of
crystals and/or impurities, thereby reducing the purity of the
product (but not the purity of the crystals). Drying of the
crystals can be avoided or reduced by reducing or eliminating
evaporation of the solvent, or by covering the screen
with solvent or solvent vapors to maintain solvent saturation.
Alternatively, a pressurized and/or submerged size
separation process can be used, which ensures that local
evaporation of solvent is minimized or eliminated.
Once sodium carbonate monohydrate crystals are separated
from the saturated brine solution, they can be dehydrated
(i.e., dried) using known techniques to provide anhydrous
sodium carbonate.
The purity of crystals produced by the processes of the
present invention is at least about 99%, more preferably at
least about 99.5% and most preferably at least about 99.8%.
The term "purity of product" refers to the overall purity of
the product and can include impurities which can be present
on the surface of the crystals or which can be trapped within
agglomerates. The term "purity of crystals" refers to the 50
presence or lack of impurities within the crystal lattice
structure. In other words, the purity of product refers to the
purity of a particular batch of the product produced by the
process of the present invention, whereas the purity of
crystals refers to the purity of crystals within the product.
5.1 Physical Property of the Product
Unlike some of the current crystallization processes, the
process of the present invention does not utilize a crystal
modifier to affect the crystal shape of the product. The
majority of the product is block-like in shape, as discussed
above, and is surprisingly resistant to abrasion. Preferably at
least about 55% of the particles in the product is block-like
in shape, more preferably at least about 75%, and most
preferably at least about 95%.
It is believed that these block-like crystal are responsible
for a high bulk density observed in the product of the present
invention. The product of the present invention has a poured
15
US 6,284,005 Bl
16
during the settling process, the brine solution can be diluted
with water or a non-saturated brine solution to dissolve fine
sodium carbonate monohydrate crystals which may be
present. Furthermore, makeup water can be added as
required by the overall mass balance of the system.
Prior to being purged from the system, settled insoluble
impurities can be further concentrated, e.g., by filter press,
to recover at least a portion of the saturated brine solution.
In addition, the clear overflow and/or the clarified saturated
brine solution can be further clarified by filtration to remove
any fine insoluble impurities that may be present.
When the saturated brine solution is reused, it is desirable
that the temperature of the saturated brine solution in the
thickener is kept at no more than about 20° C. different than
the temperature of the saturated brine solution in the crystallizer
tank to minimize the energy cost of reheating the
saturated brine solution from the thickener. Preferably, the
difference in temperature between the saturated brine solution
and the saturated brine solution in the crystallizer tank
is about 15° C. or less, more preferably about 10° C. or less,
and most preferably about 5° C. or less.
8.0 Bicarbonate Control
It has been found that the crystal size and/or the shape can
be affected by the presence of sodium bicarbonate in the
saturated brine solution. Therefore, the process of the
present invention can further include maintaining the concentration
of sodium bicarbonate below about 10 gil in the
saturated brine solution which is added to the crystallizer 10,
more preferably below about 5 gil, and most preferably
about 0 gil. Larger sodium carbonate crystals can be grown
in crystallization processes when the amount of bicarbonate
present in the brine solution is maintained within these
limits. One method of controlling the sodium bicarbonate
level in the saturated brine solution is disclosed in a commonly
assigned, U.S. patent application Ser. No. 09/167,
627, filed on Oct. 6, 1998, which is incorporated by reference
herein in its entirety.
A further advantage of the present process which has been
recognized is that, in the absence of bicarbonate, crystals
which are grown have a more beneficial shape, e.g., a
well-formed block-like shape. In contrast, crystals grown in
the presence of significant amounts of sodium bicarbonate
can have a needle-like, dendritic or jack-shaped structure
and/or cloudy centers. Thus, crystals produced in accordance
with the present invention, having a more compact
and block-like shape, produce a material having a higher
bulk density and a lower friability than those produced in the
presence of a relatively large amount of bicarbonate.
In a preferred embodiment of the present invention, a
sufficient amount of base is used to reduce the concentration
of sodium bicarbonate to within the parameters discussed
above. Preferably, after neutralizing any initial sodium
bicarbonate in the crystallizer, base is added to the crystallization
process to maintain a concentration of at least about
0.75 molell of equivalent base, more preferably at least
about 0.50 molell, and most preferably at least about 0.25
molell. When sodium hydroxide is used as the base, after
neutralizing any initial sodium bicarbonate in the
crystallizer, the amount of sodium hydroxide used is preferably
at least about 6 gil, more preferably at least about 4
gil, and most preferably at least about 2 gil.
9.0 Aging
Processes of the present invention can also include transferring
at least a portion of the monohydrate slurry from the
crystallizer 10 and/or at least a portion of the screened
saturated brine solution into an aging apparatus (not shown).
The aging apparatus allows growth of at least a portion of
the crystals in the saturated brine solution by dissolving at
least a portion of fines and then promoting crystal growth by
relieving the supersaturation in the form of a crystal growth,
i.e., some mass of fine particles is converted to coarse
5 particles by a process of dissolving and recrystallizing.
As used in this invention, "aging" refers to a process of
dissolving some of the small crystals present in the saturated
brine solution and relieving at least a portion of the supersaturation
by growth on existing crystals. The aging can be
10 a natural equilibrium phenomena where crystals are constantly
being dissolved and recrystallized or it can be
achieved by diluting and concentrating the saturated brine
solution or simply by a temperature cycling process. The
aging process can be used to produce seed crystals or to
increase the amount and/or the size of the product. For
15 example, when the temperature of the saturated brine solution
in the crystallizer 10 is from about 80° C. to about 90°
c., it has been observed that by allowing the resulting
saturated brine solution to stir or stand for an additional
about 10 to about 15 minutes after the addition of the
20 feedstream and/or the seed crystals, the amount and/or the
size of larger sodium carbonate monohydrate crystals can be
significantly increased. This phenomena occurs at faster
rates at increased temperatures.
The temperature cycling process involves reducing the
25 temperature of the saturated brine solution at least about 10°
c., more preferably at least about 20° c., and most preferably
at least about 40° C. Alternatively, the temperature of
the saturated brine solution is reduced to less than about 70°
c., more preferably less than about 60° c., and most
30 preferably less than about 50° c., but always above 35° c.,
the top of stability range for sodium carbonate decahydrate.
As FIG. 2 shows, the solubility of sodium carbonate
increases as the temperature is reduced. Thus, reducing the
temperature of the saturated brine solution dissolves at least
35 a portion of the sodium carbonate monohydrate crystals. It
should be appreciated that while some fines may be completely
dissolved, some larger crystals may also be partially
dissolved during the temperature cycling process. When the
temperature of the saturated brine solution is increased, the
40 solubility of sodium carbonate decreases as shown in FIG.
2. This reduction in solubility causes relief of supersaturation
of the brine solution by growth of existing crystals or by
primary and/or secondary nucleation. By maintaining a
condition which limits the amount of primary and/or sec-
45 ondary nucleation as discussed above, the amount of fines
generated can be reduced and the crystal sizes can be
increased using an aging process.
As stated above, temperature cycling process can be
applied to the entire monohydrate slurry in the crystallizer or
50 to a slip stream, i.e., a portion, of the monohydrate slurry
such that a portion of the monohydrate slurry is cycled
through an external heat exchanger to reduce the temperature
of the monohydrate slurry.
When the temperature cycling is applied to the entire
55 monohydrate slurry as a whole, the process is typically
performed by cycling the crystallizer's temperature about
once an hour. If the temperature cycling is affected to a
portion of the monohydrate slurry through an external heat
exchanger, such temperature cycling is conducted on a
60 continuous basis while a portion of the monohydrate slurry
is continuously circulated through the heat exchanger. In one
particular embodiment of a temperature cycling process, a
heat exchanger is used for the temperature cycling process.
In this embodiment, the temperature of the monohydrate
65 slurry is typically lowered by at least about 5° c., more
preferably at least about 10° c., and most preferably at least
about 20° C.
US 6,284,005 Bl
17 18
25 Grams/liter
Feed Time to Add % Solids Supersaturation
Test # (gil) Feed (seconds) at End o min. S min.
1 30 10 12.2 15.8 7.1
2 60 10 15.7 22.5 5.9
30 3 120 15 25.5 26.3 1.5
TABLE 2
The results in Table 2 illustrate that high levels of supersaturation
can be obtained by practice of the present invention.
For example, in Test No.3, supersaturation of 26.3 gil
was present at the end of the feed addition. The results
further illustrate that the supersaturation is rapidly relieved.
For example, in Test No.3, the amount of supersaturation at
the end of feed addition went from 26.3 gil to 1.5 gil at 5
minutes after the end of feed addition.
The foregoing description of the present invention has
been presented for purposes of illustration and description.
Furthermore, the description is not intended to limit the
invention to the form disclosed herein. Consequently, variations
and modifications commensurate with the above
teachings, and the skill or knowledge of the relevant art, are
within the scope of the present invention. The embodiment
described hereinabove is further intended to explain the best
mode known for practicing the invention and to enable
others skilled in the art to utilize the invention in such, or
other, embodiments and with various modifications required
by the particular applications or uses of the present invention.
It is intended that the appended claims be construed to
include alternative embodiments to the extent permitted by
the prior art.
What is claimed is:
1. A process for producing sodium carbonate monohydrate
from a feedstream comprising anhydrous sodium carbonate
and impurities, the process comprising:
(a) adding the feedstream to a saturated sodium carbonate
brine solution at a rate of at least about 100 gil/min
under conditions to create supersaturation of at least
about 5 gil;
(b) processing within parameters that preferentially
relieve the supersaturation by rapid growth of existing
sodium carbonate monohydrate crystals over nucleation;
and
A four liter vessel with intense agitation was partially
filled with a slurry of 65x100 mesh sodium carbonate
monohydrate seed crystals and heated to 88° C. Minus 150
mesh calcined trona, heated to 125° C. was added rapidly to
5 the vessel. Immediately after addition of the calcined trona
was complete, the concentration of dissolved sodium carbonate
in the brine was determined by withdrawing the brine
through a screen and filter to exclude seed crystals and
calcined trona. Water was evaporated from the withdrawn
10 brine to produce a solid residue. The quantity of sodium
carbonate per gram of withdrawn brine was gravimetrically
determined. The quantity of sodium carbonate in excess of
the solubility limit of sodium carbonate is the amount of
supersaturation. A second sample was taken 5 minutes after
15 feed addition was complete to evaluate the amount of
supersaturation at that time and the amount of relief of
supersaturation in the 5 minute interval.
Three tests were run with the amount of feed being varied.
The amount of feed added, the time of addition, the percent
20 solids, and the amount of supersaturation at 0 minutes and
at 5 minutes are shown below in Table 2.
EXAMPLE 1
This example illustrates the high capacity for supersatu- 65
ration of sodium carbonate and a technique for measuring
the same.
10.0 Fines Scavenging
As a means for improving the product yield, the slurry of
fine particles remaining after the product size monohydrate
crystals have been removed can be further processed to
recover the soda ash values present in the slurry of fines. The
slurry of fines can also include impurities which were
present in the feedstream and any fine sodium carbonate
monohydrate crystals which are smaller than the product
size. One technique for processing the slurry of fines to
improve the product yield is to use a pressure slurry system
as described below.
10.1 Pressure Slurry System Crystallization
In this process, the slurry of fines is thickened to a
relatively high solids content, preferably to at least about
17% solids by weight, more preferably to at least about 25%
solids by weight, even more preferably to at least about 40%
solids by weight, and most preferably to at least about 60%
solids by weight. The slurry of fines can be thickened by a
conventional gravity thickener, a membrane filter, or any
suitable device that permits decanting saturated brine from
the slurry of fines while retaining the solids.
The thickened slurry of fines is then pumped into a
pressure vessel operating above the transition temperature of
monohydrate sodium carbonate to anhydrous sodium carbonate.
In general, this vessel is operated at a temperature of
at least about 7° C. above the transition temperature. In the
pressure vessel, the incoming slurry is heated above the
transition temperature of monohydrate sodium carbonate to
anhydrous sodium carbonate. This heating converts sodium
carbonate monohydrate to anhydrous sodium carbonate. The
resulting anhydrous sodium carbonate slurry is then added to
the feedstream or to the crystallizer directly. In this manner,
the slurry of sodium carbonate monohydrate fines is
recycled to the crystallization process of the present invention
to increase the amount of sodium carbonate recovery.
Depending on the yield of each stage of crystallization, a 35
pressure slurry system for fines scavenging can be repeatedly
used. Because the operating and capital costs in each
stage of crystallization processes of the present invention are
relatively low, having a multiple stage pressure crystallization
process can be readily justified economically. The use of 40
a multiple stage crystallization process increases the yield of
sodium carbonate from a depletable resource such as trona.
11.0 Product Purity Control
Although processes of the present invention provide product
crystals of a purity level as described above, in some 45
cases, such as when soluble impurities are present in the
feedstream, it may be necessary to utilize a multiple stage
crystallization process to achieve the product having the
above described purity level.
Crystals are produced in a first stage of crystallization. 50
These crystals are mechanically dewatered and repulped in
brine from a second stage of crystallization in the process.
This repulped slurry is fed to the second stage pressure
slurry crystallization system as described above. The recrystallization
that takes place in this second stage will produce 55
crystals containing less soluble impurities than were present
in the product of the first stage recrystallization. This process
can be repeated with as many stages as are required to get
the desired purity levels.
The following example is provided for purposes of illus- 60
tration and is not intended to limit the scope of the present
invention.
US 6,284,005 Bl
19
(c) recovering at least a portion of the sodium carbonate
monohydrate crystals from the saturated brine solution.
2. The process of claim 1, wherein the supersaturation is
at least about 10 gil.
3. The process of claim 1, wherein the supersaturation is 5
at least about 20 gil.
4. The process of claim 1, wherein the feedstream is
produced by a process comprising mixing anhydrous
sodium carbonate with a saturated sodium carbonate brine
solution at a temperature above the transition temperature 10
between sodium carbonate monohydrate and anhydrous
sodium carbonate.
5. The method of claim 4, wherein the aqueous solution
is at above atmospheric pressure and at a temperature above
the atmospheric boiling point of the aqueous solution and 15
wherein the feed slurry is introduced by a feeder that
maintains a continuous pressure seal between the environment
of the feed slurry and of the aqueous solution.
6. The process of claim 1, wherein the step of relieving the
supersaturation preferentially by rapid growth of existing 20
sodium carbonate monohydrate crystals over nucleation
comprises adding sodium carbonate monohydrate seed crystals
to the saturated sodium carbonate brine solution.
7. The process of claim 6, wherein the seed crystals are
produced by removing sodium carbonate monohydrate crys- 25
tals from the brine solution and sizing the removed crystals
to produce a seed crystal size fraction for reintroduction to
the brine solution.
8. The process of claim 6, wherein the particle size of the
feedstream is less than the particle size of the seed crystals. 30
9. The process of claim 6, wherein the range of the particle
size of the seed crystals is not greater than about 3 standard
SIeve sIzes.
10. The process of claim 6, wherein the particle size of the
feedstream is less than about 150 mesh. 35
11. The process of claim 6, wherein the particle size of the
seed crystals is from about 100 mesh to about 150 mesh.
12. The process of claim 1, wherein the step of relieving
the supersaturation preferentially by rapid growth of existing
sodium carbonate monohydrate crystals over nucleation 40
comprises maintaining a solids content of at least about
17%.
13. The process of claim 1, wherein the step of relieving
the supersaturation preferentially by rapid growth of existing
sodium carbonate monohydrate crystals over nucleation 45
comprises agitating the brine solution at an agitation index
of at least about 4.
14. The process of claim 1, wherein the step of relieving
the supersaturation preferentially by rapid growth of existing
sodium carbonate monohydrate crystals over nucleation 50
comprises periodically lowering the temperature of the brine
solution by at least about 5° C.
15. The process of claim 1, wherein the step of relieving
the supersaturation preferentially by rapid growth of existing
sodium carbonate monohydrate crystals over nucleation 55
comprises pausing feedstream addition at least about 60% of
the time of crystallization.
16. The process of claim 1, wherein the step of relieving
the supersaturation preferentially by rapid growth of existing
sodium carbonate monohydrate crystals over nucleation 60
comprises pausing feedstream addition at least about 30% of
the time of crystallization.
17. The process of claim 1, wherein the step of relieving
the supersaturation preferentially by rapid growth of existing
sodium carbonate monohydrate crystals over nucleation 65
comprises pausing feedstream addition at least about 5% of
the time of crystallization.
20
18. The process of claim 1, wherein the amount of solids
in the brine solution formed by primary and/or secondary
nucleation in the crystallizer is maintained at about 20% by
weight or less of the total sodium carbonate solids in the
brine solution.
19. The process of claim 1, wherein the amount of solids
in the brine solution having a particle size of less than about
400 mesh is maintained at less than about 25% by weight of
the total sodium carbonate solids in the brine solution.
20. The process of claim 1, wherein the amount of solids
in the brine solution in the form of agglomerates and/or
aggregates is maintained at about 20% by weight or less of
the total sodium carbonate solids in the brine solution.
21. The process of claim 1, wherein the step of recovering
comprises removing a portion of the sodium carbonate
monohydrate crystals from the brine solution, dispersing the
sodium carbonate monohydrate crystals by the addition of
brine solution and recovering sodium carbonate monohydrate
crystals from insoluble impurities on a size separation
basis.
22. The process of claim 1, wherein the temperature of the
saturated brine solution is at least about 70° C.
23. The process of claim 1, wherein the saturated sodium
carbonate brine solution is at a temperature above the
atmospheric boiling point of the solution.
24. The process of claim 1, wherein said feedstream
comprises calcined trona.
25. The process of claim 1, further comprising agitating
the brine solution at an agitation index of at least about 7.
26. The process of claim 1, further comprising agitating
the brine solution at an agitation index of at least about 9.
27. The process of claim 1, further comprising agitating
the brine solution at greater than about 10 hp/1000 gal.
28. The process of claim 1, further comprising agitating
the brine solution at greater than about 100 hp/1000 gal.
29. The process of claim 1, further comprising agitating
the brine solution at greater than about 200 hp/1000 gal.
30. The process of claim 4, wherein said feedstream
comprises calcined trona.
31. A process for producing sodium carbonate monohydrate
from a feedstream comprising anhydrous sodium carbonate
and impurities, the process comprising:
(a) adding the feedstream to a saturated sodium carbonate
brine solution at a rate of at least about 100 gil/min
under a condition to create supersaturation of at least
about 5 gil;
(b) processing within a parameter that preferentially
relieve the supersaturation by rapid growth of existing
sodium carbonate monohydrate crystals over
nucleation, wherein the step of relieving comprises
adding sodium carbonate monohydrate seed crystals to
the saturated sodium carbonate brine solution, maintaining
a solids content of at least about 40% and
agitating the brine solution at an agitation index of at
least about 4; and
(c) recovering a portion of the sodium carbonate monohydrate
crystals from the saturated brine solution,
wherein said recovering step comprises removing a
portion of the sodium carbonate monohydrate crystals
from the brine solution, dispersing the sodium carbonate
monohydrate crystals by the addition of brine
solution and recovering sodium carbonate monohydrate
crystals from insoluble impurities on a size separation
basis.
32. The process of claim 31, wherein said feedstream
comprises calcined trona.
33. The process of claim 31, wherein the seed crystals are
produced by removing sodium carbonate monohydrate crys21
US 6,284,005 Bl
22
tals from the brine solution and sizing the removed crystals
to produce a seed crystal size fraction for reintroduction to
the brine solution.
34. The process of claim 31, wherein the particle size of
the feedstream is less than the particle size of the seed 5
crystals.
35. The process of claim 31, wherein the range of the
particle size of the seed crystals is not greater than about 3
standard sieve sizes.
36. The process of claim 31, wherein the particle size of 10
the feedstream is less than about 150 mesh.
37. The process of claim 31, wherein the particle size of
the seed crystals is from about 100 mesh to about 150 mesh.
38. The process of claim 31, wherein the step of relieving
the supersaturation preferentially by rapid growth of existing 15
sodium carbonate monohydrate crystals over nucleation
further comprises periodically lowering the temperature of
the brine solution by at least about 5° C.
39. The process of claim 31, wherein the step of relieving
the supersaturation preferentially by rapid growth of existing 20
sodium carbonate monohydrate crystals over nucleation
comprises pausing feedstream addition at least about 60% of
the time of crystallization.
40. The process of claim 31, wherein the amount of solids
in the brine solution formed by primary and/or secondary 25
nucleation in the crystallizer is maintained at about 20% by
weight or less of the total sodium carbonate solids in the
brine solution.
41. The process of claim 31, wherein the amount of solids
in the brine solution having a particle size of less than about 30
400 mesh is maintained at less than about 25% by weight of
the total sodium carbonate solids in the brine solution.
42. The process of claim 31, wherein the amount of solids
in the brine solution in the form of agglomerates and/or
aggregates is maintained at about 20% by weight or less of 35
the total sodium carbonate solids in the brine solution.
43. The process of claim 31, wherein the saturated sodium
brine solution is at a temperature above the atmospheric
boiling point of the solution.
44. A process for producing sodium carbonate monohydrate
from a feedstream comprising anhydrous sodium carbonate
and impurities, the process comprising:
(a) adding a feedstream having a particle size ofless than
about 100 mesh to a saturated sodium carbonate brine
solution at a rate of at least about 400 gil/min under a
condition to create supersaturation of at least about 5
gil;
(b) processing within a parameter that preferentially
relieve the supersaturation by rapid growth of existing
sodium carbonate monohydrate crystals over
nucleation, wherein the parameter comprises adding
sodium carbonate monohydrate seed crystals having a
particle size of from about 150 mesh to about 100 mesh
to the saturated sodium carbonate brine solution, maintaining
a solids content of at least about 60% and
agitating the brine solution at an agitation index of at
least about 4; and
(c) recovering a portion of the sodium carbonate monohydrate
crystals from the saturated brine solution,
wherein said recovering step comprises removing a
portion of the sodium carbonate monohydrate crystals
from the brine solution, dispersing the sodium carbonate
monohydrate crystals to a solids content of less than
about 25% by weight by the addition of brine solution
and recovering sodium carbonate monohydrate crystals
having a particle size of greater than at least about 100
mesh from insoluble impurities on a size separation
basis.
45. The process of claim 44, wherein the particle size of
said feedstream is less than about 150 mesh.
46. The process of claim 44, wherein the saturated sodium
carbonate brine solution is at a temperature above the
atmospheric boiling point of the solution.
* * * * *