111111111111111111111111111111111111111111111111111111111111111111111111111
US006464736Bl
(12) United States Patent
Hazen et al.
(10) Patent No.:
(45) Date of Patent:
US 6,464,736 BI
*Oct. 15, 2002
(List continued on next page.)
(54) RECRYSTALLIZATION PROCESS
(75) Inventors: Wayne C. Hazen, Denver; Dale Lee
Denham, Jr., Arvada; David R.
Baughman, Golden, all of CO (US);
Rudolph Pruszko, Dubuque, IA (US)
3,314,748 A
3,425,795 A
3,479,133 A
3,498,744 A
4/1967 Howard et al. .
2/1969 Howard et al. .
11/1969 Warzel .
3/1970 Frint et al. .
23/63
23/63
23/63
23/63
(73) Assignee: Environmental Projects, Inc., Casper,
WY(US) BE
EP
FOREIGN PATENT DOCUMENTS
661071 7/1965
0073085 B1 12/1986
(56) References Cited
U.S. PATENT DOCUMENTS
(21) Appl. No.: 09/521,828
(22) Filed: Mar. 9, 2000
Related U.S. Application Data
(60) Provisional application No. 60/147,532, filed on Aug. 5,
1999.
(51) Int. CI? BOlD 9/00; C13K 1/10;
C30B 17/00; COlD 7/24; COlD 7/40
(52) U.S. CI. 23/295 R; 23/298; 23/301;
23/302 T; 23/302 R
(58) Field of Search 23/301, 302 T,
23/302 R, 295 R, 298; 423/206.2, 421,
425, 422, 427, 426
37 Claims, 2 Drawing Sheets
ABSTRACT
OTHER PUBLICATIONS
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.
The present invention provides a process for producing
crystals of a polymorphic compound in a first crystal structure
by introduction of the compound in a second crystal
structure into a saturated brine solution of the compound
under conditions in which formation of the first crystal
structure is favored and without evaporation or changes in
temperature. As the second crystal structure dissolves, the
brine becomes supersaturated resulting in relief of supersaturation
by formation of crystals of the first crystal structure.
The process includes controlling supersaturation and its
relief to achieve growth of existing crystals of the first
crystal structure rather than nucleation and formation of new
crystals. The resulting crystals are separated from insoluble
impurities on a size separation basis.
(57)
Primary Examiner-Wayne A. Langel
Assistant Examiner-Jonas N. Strickland
(74) Attorney, Agent, or Firm---Sheridan Ross Pc.
9/1943 Kermer 23/295
9/1957 Pike .. ... 23/38
11/1960 Seglin et al. 23/31
1/1961 Caldwell et al. 23/63
4/1961 Porter 23/143
10/1962 Robson et al. 23/63
2/1966 Bauer et al. 23/300
2/1966 Sopchak et al. 23/63
4/1966 Smith ... 23/63
9/1966 Miller .. 23/63
This patent is subject to a terminal disclaimer.
Subject to any disclaimer, the term of this
patent is extended or adjusted under 35
U.S.c. 154(b) by 0 days.
2,330,221 A
2,792,282 A
2,962,348 A
2,970,037 A
2,981,600 A
3,061,409 A
3,233,983 A
3,236,590 A
3,244,476 A
3,273,959 A
( *) Notice:
MAKE-UP WATER
NoOH
FEED STREAM
SEED CRYSTALS
10
14
18
22
26
INSOLUBLE
IMPURITIES
TO WASTE
PRODUCT
US 6,464,736 BI
Page 2
U.S. PATENT DOCUMENTS
3,653,848 A *
3,705,790 A
3,717,698 A
3,796,794 A
3,819,805 A
3,836,628 A
3,845,119 A
3,904,733 A
3,933,977 A
3,956,457 A
4,021,527 A
4,022,868 A
4,083,939 A
4,138,312 A
4/1972 Port et al. 23/302
12/1972 Garofano et al. 23/302
2/1973 Ilardi et al. 423/206
3/1974 Ilardi et al. 423/421
6/1974 Graves et al. 423/206
9/1974 Ilardi et al. 423/206
10/1974 Duke et al. 260/527
9/1975 Ganey et al. 423/206
1/1976 Ilardi et al. 423/206
5/1976 Port et al. 423/206
5/1977 Baadsgaard 423/206
5/1977 Poncha 423/184
4/1978 Lobunez et al. 423/421
2/1979 Gill et al. 162/30
4,183,901 A
4,202,667 A
4,260,594 A
4,283,277 A
4,286,967 A
4,288,419 A
4,299,799 A
4,374,102 A
4,472,280 A
4,781,899 A
5,300,123 A *
5,396,863 A
6,284,005 B1 *
* cited by examiner
1/1980
5/1980
4/1981
8/1981
9/1981
9/1981
11/1981
2/1983
9/1984
11/1988
4/1994
3/1995
9/2001
Ilardi et al. 423/206
Conroy et al. 23/302
Verlaeten et al. 423/421
Brison et al. 209/166
Booth, Jr. et al. 23/298
Copenhafer et al. 423/190
Ilardi et al. 423/206
Connelly et al. 423/206
Keeney 210/666
Rauh et al. 423/206
Grott .... ... 23/303
Ninane et al. 117/206
Hazen et al. 23/302
MAKE-UP WATER ~
. NoOH .
", SATURATED BRINE
FEED STREAM
, , ,
. , ,'", ~+ + I I ...
'... ,.' . ,
SEED CRYSTALS CRYSTALLIZER THICKENER V 26
•
10 .. INSOLUBLE IMPURITIES
14 ----
DISPERSION TO WASTE .
---- PRODUCT ...
18 ----
PRODUa SEPARATOR ...
~
22_______ ---S-EED SEPARATO-R-- . FIG. 1
d•
'JJ.
•
~
~.....
~=.....
o~
U'""'"l
~
N
CS
'JJ. =~
~
o.'"."..'",
N
e
\Jl
0'1
':J;.
0'1
~
~
0'1
~
1-0"
u.s. Patent Oct. 15, 2002 Sheet 2 of 2 US 6,464,736 BI
240
""-----
No{03 ----------A
220 1-----1-1-----+----+------+----1
200 t--------+---II-----+----t-----t----i
180 1-----1-+----+---+-----+----1
160 1------+-+------+----+-----+---
140 .----+--+----+---+-----r-- UNSATURATED N0
2
C0
3
"H
2
0
SOlUTION
30 35 40 45
WEIGHT PERCENT SODIUM CARBONATE
120 1---------+-\-\--+---+----+---100
1-----1---l----+------.1e-------- 1-----
V~~02~~:~~~~--. B
I ----- Na{03"10 H20 I
80 V--- 1 :
25
FIG. 2
US 6,464,736 Bl
2
DETAILED DESCRIPTION OF THE
INVENTION
BRIEF DESCRIPTION OF THE DRAWINGS
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.
1.0 Introduction
The present invention is based in part on the discovery
that under certain conditions some chemical compounds
having multiple crystal forms (i.e., polymorphic
compounds) have unexpectedly high dissolution rates and
produce highly stable supersaturation capacity that can be
rapidly relieved by the introduction of crystal surfaces of the
polymorphic compound in a first crystal structure to produce
relatively large crystals of the compound in a first crystal
structure 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 distribution of product crystal size.
Processes of the present invention achieve supersaturation
of the polymorphic compound by adding feed of the compound
in the second crystal structure to a saturated solution
under conditions in which crystals of the polymorphic
compound in the first crystal structure are formed. Thus, the
tendency of the feed to convert from the second crystal
structure to the first crystal structure within the brine solution
causes the feed in the second crystal structure to
dissolve, thereby creating supersaturation, before forming
the first crystal structure. Further, it has been found that
some chemical compounds have a surprisingly high and
stable supersaturation capacity. For example, under appropriate
conditions, sodium carbonate has a supersaturation
capacity of about 30 gil, which is about an order of mag-
65 nitude higher and more stable in the absence of sodium
carbonate monohydrate crystal surfaces than would be
expected by one skilled in the art. Therefore, the present
of solution in the crystallizer. The process can also include
relieving the supersaturation preferentially by rapid growth
of existing crystals of the compound in the first crystal
structure over nucleation by adding seed crystals of the
compound in the first crystal structure to the saturated brine
solution of the compound. Such seed crystals can be produced
by removing crystals of the compound in the first
crystal structure from the brine solution and sizing the
removed crystals to produce a seed crystal size fraction for
10 reintroduction to the brine solution, by producing the seed
crystals separately, and/or by grinding or partial dissolution
of part of the product. In a preferred embodiment, the
particle size of the feedstream is less than about 150 mesh
and the particle size of the seed crystals is from about 100
15 mesh to about 150 mesh.
Relief of supersaturation preferentially by rapid growth of
existing crystals of the compound in the first crystal structure
over nucleation can alternatively be achieved by a
variety of methods. Such methods can include maintaining
20 a solids content of at least about 40% in the crystallizer,
agitating the brine solution at an agitation index of at least
about 4, periodically lowering the temperature of the brine
solution by at least about 5° c., or pausing feedstream
addition at least about 60% of the time of crystallization.
FIELD OF THE INVENTION
SUMMARY OF THE INVENTION
CROSS-REFERENCE TO RELATED
APPLICATIONS
1
RECRYSTALLIZATION PROCESS
BACKGROUND OF THE INVENTION
30
The present invention is based on the discovery that some
chemical compounds having multiple crystal forms (i.e.,
polymorphic compounds) have unexpectedly high dissolution
rates and have unexpectedly stable supersaturation
capacities under appropriate conditions that can be rapidly 35
relieved by the introduction of crystal surfaces of the polymorphic
compound in a first crystal structure to produce
relatively large crystals of the compound at high rates of
crystal growth. Some of these compounds occur naturally in
deposits, such as evaporite deposits. The resulting crystals 40
which are larger than insoluble impurities can be readily
separated from insoluble impurities on a size separation
basis.
A common procedure for processing such ores is to
dissolve the mineral to be recovered and separate the 45
insoluble impurities by clarification methods, such as thickening
and/or filtration. The water added in the process must
be removed, generally by expensive methods, such as
evaporation.
More particularly, the process of the present invention is 50
for producing a polymorphic compound in a first crystal
structure from a feedstream which includes in a second
crystal structure and insoluble impurities. The process
includes adding the feedstream to a saturated brine of the
compound under conditions to create supersaturation of at 55
least about 5 gil. The process further includes processing
within parameters that preferentially relieve the supersaturation
by rapid growth of existing crystals of the compound
in the first crystal structure rather than by nucleation. In this
manner, the particle size distribution of crystals is controlled 60
to achieve a desired distribution of crystal size product
which is coarse enough that it can be separated from the
insoluble impurities on a size basis. The crystals of the
compound in the first crystal structure 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
5 This application claims priority from U.S. Provisional
Application Serial No. 60/147,532, filed Aug. 5, 1999,
entitled "RECRYSTALLIZATION PROCESS."
The present invention relates to the production of purified
crystals from a polymorphic compound containing impurities.
One common method of purifying a compound is to
crystallize the compound in a solution. Methods of crystallization
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 and/or purification
without any regard to the size or shape of the
crystals.
Therefore, there is a need for a crystallization process that 25
can effectively control or influence the ratio of crystal
growth to formation of new crystals at low energy costs.
US 6,464,736 Bl
3 4
brine solution" refers to a solution which is in equilibrium
with the compound.
2.0 Feedstream Composition and Introduction
2.1 Composition
As noted above, a feedstream of the present invention
comprises a polymorphic compound in a second crystal
form which is to be converted to a desired first crystal form.
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 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
feedstreams, the present invention is particularly suitable for
use with feedstreams having greater than about 15% by
20 weight insoluble impurities, and even more particularly,
having greater than about 30% by weight insoluble impurities.
Polymorphic compounds of the present invention include
compounds which have two or more crystal structures
wherein the crystal structure of the compound is dependent
on some operational variable in a crystallization process. For
example, the different crystal structures can be different
states of hydration and the operational variable can be
temperature. Thus, in the instance of sodium carbonate,
30 sodium carbonate occurs in a monohydrate crystal structure
and in an anhydrous crystal structure. Among other
variables, temperature will determine what state of hydration
sodium carbonate is in.
Resources containing polymorphic compounds of the
present invention are typically brine and evaporite chemicals.
Such minerals can be found as bedded salts, playa
deposits (dry lake beds) and solar evaporation ponds. Bedded
salts are usually recovered by shaft mining or in situ
solution mining. Playa deposits can be recovered by solution
or open pit mining. Examples are Atacama caliche in Chile
and brine chemicals recovered by solution mining at Searles
Lake. Some examples of suitable polymorphic compounds,
in addition to sodium carbonate, include kernite
(Na2B404.4H20) which is typically contaminated with
gangue and some borax; calcined colemanite
(2CaO.3B203); langbeinite (2MgS04.K2S04); carnallite
(MgCI2.KCl.6H20) which typically includes NaCI (about
25%) and gangue (about 3-4%); mixed salts from solar
ponds or shallow dry lakes such as Searles Lake (KCIK2S04-
Na2S04-Na2C03) and the Great Salt Lake
(Na2S04-K2S04); and Chile caliche ore (NaCI-Na2S04)
which typically includes a few percent of NaC03 with about
25% gangue.
2.2 Size
As noted, supersaturation is achieved by adding the
compound in the second crystal structure to a saturated brine
solution under temperature and/or other conditions at which
the first crystal structure forms. Thus, the compound in the
feedstream dissolves, thereby creating supersaturation and
also releasing impurities, before forming the first crystal
structure. The rate and completeness of the feedstream
dissolving in a saturated brine solution is determined by,
among other factors, its particle size. Since the presence of
undissolved feedstream in the second crystal structure can
compete with seed crystals of the compound in the first
crystal structure as a substrate for relieving supersaturation,
the feedstream added to the saturated brine solution should
invention includes achieving and maintaining high levels of
supersaturation near the supersaturation capacity of the
polymorphic compound to create a high driving force for
supersaturation relief which results in rapid crystallization.
Supersaturation created in this manner is relieved by 5
formation of the first crystal structure of the compound. The
first crystal structure can form as a result of exceeding the
supersaturation limit, which causes primary nucleation
resulting in formation of clouds of small nuclei of the first
crystal structure of the compound. The term "supersatura- 10
tion limit" is used to describe a condition where the level of
supersaturation of the compound in the brine solution is
unstable and results in a relatively spontaneous formation of
crystals by primary and/or secondary nucleation. This type
of supersaturation relief is unproductive because the small 15
nuclei cannot easily be grown to a size large enough to be
separated from insoluble impurities. Supersaturation relief
can also occur by growth of existing crystals of the compound
in the first crystal structure, 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 feed in the second crystal structure, the
supersaturation limit can be exceeded in a localized area at
the point of introduction of the feed. Therefore, control of 25
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 35
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. 40
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 45
relieved by formation of the first crystal structure 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 50
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 55
supersaturation.
The terms "recrystallization" and "crystallization" are
used interchangeably herein and refer to the step of adding
the polymorphic compound in the second crystal structure to
a saturated brine solution and crystallizing the polymorphic 60
compound in the first crystal structure from the saturated
brine solution, i.e., the compound in the second crystal
structure dissolves in the saturated brine solution, forms a
supersaturated solution which then causes growth of crystals
in the first crystal structure because the temperature and/or 65
other condition of the saturated brine solution are in the
range of stability for the first crystal structure. A "saturated
5
US 6,464,736 Bl
6
dissolve substantially completely to ensure that the majority
of supersaturation relief is by growth of seed crystals, not by
growth on undissolved feed, and to ensure that at least a
portion of impurities present within the crystal lattice of the
feedstream is released. If the feedstream dissolves only 5
partially, the remaining particles can have undesired effects
such as forming agglomerates or relieving supersaturation to
form mixed particles of the compound in the first and second
crystal structure. Thus, to ensure a substantially complete
dissolution of the particles the particle size of in the 10
feedstream, whether in a slurry form or a dry form, is
preferably less than about 100 mesh (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 15
the feedstream is within the above described range, any
insoluble impurities present in the feedstream will also be
within the confines of the above described particle size.
The above particle size limitations allow the polymorphic
compound in the second crystal structure in the feedstream 20
to dissolve relatively quickly and completely in a saturated
brine solution in the crystallizer 10.
2.3 Feed Rate
As noted above, it has been surprisingly found that, under
appropriate conditions, some chemical compounds have 25
surprisingly high supersaturation capacities. For example,
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 the art. Therefore, the present
invention includes achieving and maintaining high levels of 30
supersaturation near the supersaturation capacity of the
compound in question to create a high driving force for
supersaturation relief which results in rapid crystallization.
For example, the process includes creating supersaturation
of at least about 5 gil, more preferably at least about 10 gil, 35
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 calculated as follows. A
volume of saturated brine, which can include the compound 40
in the first and second crystal structures, 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 amount of dissolved compound per
volume of brine can be gravimetrically determined. The 45
amount of compound in excess of the known solubility level
is the amount of 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 50
particularly, the feed rate can be at least about 100 grams per
minute for each liter of 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 55
expected to be useful by one skilled in the art and those
utilized by previous crystallization methods.
2.4 Method of Introduction
The feedstream, which includes the polymorphic compound
in the second crystal structure, can be introduced to 60
the saturated brine solution using any of the known methods
including by a direct injection, a screw feeder and gravity.
The feedstream can be a slurry of the compound in the
second crystal structure in a saturated brine solution or dry.
Adry feedstream must be dispersed and dissolved quickly 65
in the saturated brine 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 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 compound in the second
crystal structure into the saturated 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 in
a slurry form. A slurry feedstream can be prepared by mixing
the compound in the second crystal structure and the saturated
brine solution at atmospheric pressure and transferring
the mixture into a slurry feedstream vessel having a desired
temperature and pressure to maintain the compound in the
second crystal structure. Alternatively, the compound in the
second structure and the saturated brine solution can be fed
directly into the slurry feedstream vessel at a desired temperature
and pressure to form a slurry feedstream.
For example, at a temperature above the transition temperature
of sodium carbonate monohydrate 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 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 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 anhydrous
sodium carbonate changes its morphology to stable sodium
carbonate monohydrate. See for example, line A in FIG. 2,
the transition of anhydrous to sodium carbonate monohydrate.
Line B in FIG. 2 represents the transition temperature
between sodium carbonate heptahydrate and 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 in a sodium carbonate
system, 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
US 6,464,736 Bl
7 8
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 feed-
S stream added to the saturated brine solution typically results
in a slurry of the first crystal structure having a solids content
in accordance with the parameters discussed below. As used
herein, a"slurry of the first crystal structure" refers to a
saturated brine solution containing solid crystals of the first
10 crystal structure. 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
15 crystals and products, the ratio 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
20 added to the saturated brine solution.
3.2 Solids Content
A further aspect of the present invention to control
supersaturation relief on existing crystals of the first crystal
structure is to maintain a high solids content in the crystal-
25 lizer 10. In this manner, if the degree of supersaturation in
a localized area is approaching the maximum level, supersaturation
can be quickly relieved by the first crystal structure
formation on an existing crystal surface instead of by
nucleation. As will be appreciated, the solids content in the
30 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 that the
35 slurry of the first crystal structure 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
40 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
supersaturation is created by the feed dissolving to in the
saturated brine solution. Therefore, control of supersaturation
and its relief in the local environment, for example, by
50 sufficiently high agitation, where the feed is 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 compound within the
total volume of the crystallizer 10. Thus, the term "local
55 supersaturation limit" refers to the 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
60 can be below the supersaturation limit, a localized region of
high supersaturation can occur 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
65 include control of local supersaturation by using high agitation
to rapidly disperse areas of high local supersaturation.
High agitation brings the surfaces of existing crystals into
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 is added to a saturated
brine solution in a crystallizer 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 feed primarily occurs on existing crystals in
the first crystal structure 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 the first crystal structure 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, or partially
dissolved, 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 45
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.
US 6,464,736 Bl
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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 the first
crystal structure is formed. For example, formation of
sodium carbonate monohydrate is favored as determined by
the phase diagram, as shown in FIG. 2. In this example, 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
15 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
20 temperature of anhydrous sodium carbonate to sodium carbonate
monohydrate.
It has been discovered by the present inventors that
keeping the temperature in the crystallizer as close as
possible to but below the transition temperature between
25 first and second crystal structures reduces the "drive", i e.,
the rate of conversion, of the feedstream to change morphologically
to the first crystal structure. This discovery allows
the processes of the present invention to be controlled easily
and results in larger, better formed crystals as discussed in
30 detail below.
It should be noted that temperature of the saturated brine
solution is preferably actively controlled to maintain a fairly
constant temperature. For example, if the crystallization
reaction is exothermic, the crystallizer will need to be cooled
35 to avoid overheating. In addition, temperature differences
between the saturated brine solution and the feedstream can
cause fluctuations in temperature unless the crystallizer is
cooled or heated to maintain a constant temperature.
Alternatively, temperature differences between the saturated
40 brine solution and the feedstream can be kept small enough
such that no significant cooling or heating of the saturated
brine solution occurs during the addition of the feedstream.
In this instance, preferably, the temperature difference
between the feedstream and the saturated brine solution is
45 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
50 150° C.
In the embodiment of sodium carbonate, 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 the crystallizer 10 as disclosed above. Freshly
55 calcined trona has a high particle temperature as it comes out
of the 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-
60 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
65 crystals 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
TABLE 1
mild turnover of slurry with all solids held in suspension
turnover of slurry, but not all solids held in suspension
Description
rolling surface with quick turnover and quick absorption of
dry material into mass of slurry.
static, no movement or mixing
violent turbulent movement of all slurry in entire vessel
degradation or 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 slurry of the first crystal structure is achieved
by using at least two propellers having a counter pitch or
other suitable agitation methods including using an attrition
scrubber and any other impeller configurations which
achieve the desired agitation index discussed above.
Preferably, the solution is agitated at greater than about 10
horsepower/l 000 gallons (hp/1000 gal), more preferably at
least about 100 hp/1000 gal, and most preferably at least
about 200 hp/1000 gal. Alternatively, when a propeller
system is used for agitating the slurry of the first crystal
structure, the propeller tip speed is at least about 8 feet/sec
(ft/sec), preferably at least about 10 ft/sec, 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
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 local
high supersaturation area. One measure of agitation is a 5
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
agitation index of 10 means the mixture in the crystallizer is 10
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.
US 6,464,736 Bl
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nucleation. If a significant amount of primary and/or secondary
crystal nucleation occurs in the crystallizer 10, then
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 brine formed by primary
and/or secondary nucleation in the crystallizer 10 is maintained
at about 10% by weight or less of the total solids of
the compound in the saturated brine, more preferably at
about 5% by weight or less of the total solids of the
compound in the saturated brine, still more preferably at
about 1% by weight or less of the total solids of the
compound in the saturated brine, and most preferably at
about 0.5% by weight or less of the total solids of the
compound 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
30 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 +100
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 slurry of the first crystal structure
which has a small particle size. More particularly, the
processes of the present invention can include maintaining
the amount of solids in the slurry having a particle size of
less than about 400 mesh at less than about 10% by weight
of the total solids of the compound in the slurry, more
preferably at less than about 2% by weight of the total solids
of the compound in the slurry, and most preferably at less
than about 0.5% by weight of the total solids of the compound
in the 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
one aspect of the present invention, the crystallization process
is conducted by maintaining the amount of solids in the
slurry in the form of agglomerates and/or aggregates at
about 10% by weight or less of the total solids of the
compound in the slurry, more preferably at about 5% by
weight or less of the total solids of the compound in the
slurry, and most preferably at about 0.5% by weight or less
of the total solids of the compound in the slurry.
in a temperature cycling process to control the amount of
fines as discussed 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 5
limit. When the rate of supersaturation generation exceeds
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 super- 10
saturation level in a local area from exceeding the supersaturation
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. In addition, such a feed addition pause 15
allows any very fine material, which has been unintentionally
formed and which would be the thermodynamically
unstable, to dissolve. In this manner, such fine material is not
available as a site for relief of supersaturation. More
particularly, the break or pause in feedstream addition can be 20
conducted at least about 60% of 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 25
minutes during every hour of operation. 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. With regard to
sodium cabonate monohydrates, a "good crystal quality"
refers to crystals which are generally roughly equi- 35
dimensional, slightly elongated with an aspect ratio of
WxLxH of about 1:1.5:0.75. See for example, Goldschmidt,
Atlas der Krystallformen, p. 128 (Carl Winters Universit
atbuchhandlung, Heidelberg 1922), which is incorporated
herein by reference in its entirety. The crystal growth rate of 40
the present invention is significantly higher 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, 45
and most preferably at least about 20 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 50
the seed crystals. The reason for the higher growth rate of
coarser crystals is the mass transfer of crystals in the first
crystal structure 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 55
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
growth rate can be obtained by dividing the total amount of 60
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 65
provide conditions for relieving the supersaturation in the
crystallizer 10 by growing existing crystals rather than by
US 6,464,736 Bl
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45
10
5.0 Recovery
The present invention also includes recovering product
from the slurry of the first crystal structure. The recovery
process can include separating a particular particle size
5 range of crystals of the first crystal structure from the 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 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
15 product recovery has to be larger than the particle size of the
feed so that insoluble impurities initially in the feed are not
recovered with product.
Separation of crystals of the first crystal structure is
generally conducted by screening or cycloning and avoiding
20 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 eliminat-
25 ing 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 crystals are separated from the saturated brine
solution, they can be dehydrated (i.e., dried) using known
techniques, if desired.
The purity of crystals produced by the processes of the
present invention is at least about 99%, more preferably at
35 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
40 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. In the
instance of sodium carbonate, the majority of the product is
50 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 crystals are respon-
55 sible for a high bulk density observed in the product of the
present invention. The product of the present invention has
a poured bulk density of at least about 0.95 g/ml, 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
60 product has a packed density of at least about 1.0 g/ml,
preferably at 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
65 conventional crystallization processes. Without being bound
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
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
stronger than van der Waals forces, which can be formed, for
example, by feed particles which are not fully dissolved
acting as a site for crystallization of crystals of the first
crystal structure, or feed that was not dispersed or dissolved
absorbing water to hydrate.
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
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 pressure
or higher pressure, the temperature of the saturated
brine solution in the crystallizer 10 is maintained to favor the
formation of the first crystal structure of the compound.
When the crystallizer 10 is operated under pressure, the
introduction of the feedstream is preferably at a similar
pressure. A pressurized 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 pres- 30
sures independent of 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
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
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 crystals of
the first crystal structure and saturated brine solution are
separated from the crystallizer 10. The product is eventually
recovered in a product separator 18, preferably on a size
separation basis. However, 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 slurry of the first crystal structure from the
crystallizer 10 to a dispersion tank 14 to decrease the solids
content of the slurry in order to, inter alia, facilitate the
separation process. It should 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.
15
US 6,464,736 Bl
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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
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 blocklike
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 dendritic or needle
like crystal shapes. Without being bound by any theory, it is
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.
6.0 Seed Separation
Again referring to FIG. 1, an undersize fraction of the
slurry of the first crystal structure 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
fraction for use as seed crystals. The undersize fraction
will include 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 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 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.
Alternatively, the fines can be further sized at an intermediate
size to remove the insoluble particles and crystals of
the first phase which are too small for productive growth.
Other methods of producing seed crystals include the
following: wet comminution of crystals of the first crystal
structure; dry comminution of crystals; dissolution of a
portion of crystals of the first crystal structure by water
addition; and dissolution of crystals in a slurry by changing
the slurry temperature to increase the solubility of the
compound in the brine.
7.0 Thickening
The undersize fraction from the seed separator 22, containing
saturated brine solution, insoluble impurities and/or
crystals of the first crystal structure 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 impurities
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
during the settling process, the brine solution can be diluted
with water or a non-saturated brine solution to dissolve fine
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
5 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 solu-
10 tion 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. In the event of a
chemical compound in which the crystallization reaction is
highly exothermic, there is not a need to be concerned about
15 the temperature of the brine solution in the thickener.
8.0 Bicarbonate Control
When the compound is sodium carbonate, 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
20 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
25 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.
30 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
35 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
40 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
45 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
50 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
55 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 slur of the first crystal
structure from the crystallizer 10 and/or at least a portion of
60 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
65 a crystal growth, i.e., some mass of fine particles is converted
to coarse particles by a process of dissolving and
recrystallizing. This phenomenon occurs because extremely
US 6,464,736 Bl
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cycling process. In this embodiment, the temperature of the
slurry is typically changed by at least about 5° c., more
preferably at least about 10° c., and most preferably at least
about 20° C.
5 10.0 Fines Scavenging
As a means for improving the product yield, the slurry of
fine particles remaining after the product size crystals have
been removed can be further processed to recover the
mineral values present in the slurry of fines. The slurry of
10 fines can also include impurities which were present in the
feedstream and any fine crystals of the first crystal structure
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
This process will be described in terms of a sodium
carbonate system. 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
20 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
25 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
30 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
35 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
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 crystalliza-
45 tion process can be readily justified economically. The use of
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 prod-
50 uct crystals of a purity level as described above, in some
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.
These crystals are mechanically separated from the brine
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.
60 The recrystallization that takes place in this second stage
will produce 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 illustration
and is not intended to limit the scope of the present
invention.
small particles appear to be less thermodynamically stable
than larger particles.
As used in this invention, "aging" refers to a process of
allowing time for dissolution of some of the small fine
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 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 example, in the instance of sodium
carbonate, when the temperature of the saturated brine
solution in the crystallizer 10 is from about 80° C. to about 15
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
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 changing the
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. to increase solubility.
Alternatively, in the instance of sodium carbonate, the
temperature of the saturated brine solution is reduced to less
than about 70° c., more preferably less than about 60° c.,
and most preferably less than about 50° c., but always above
35° c., the transition point between sodium carbonate
decahydrate and monohydrate. 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 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. Thus, in one embodiment, the
fines are classified into coarse fines and small fines. The 40
cutoff for the coarse fines would be a size that would not
dissolve completely in the temperature cycle. Then, the
small fines fraction is cooled to achieve complete dissolution
of the small fines. The fractions are then rejoined. When the
temperature of the saturated brine solution is increased, the
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 secondary
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 slurry of the first crystal structure in the 55
crystallizer or to a slip stream, i.e., a portion, of the slurry
such that a portion of the slurry is cycled through an external
heat exchanger to change the temperature of the slurry.
When the temperature cycling is applied to the entire
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 slurry
through an external heat exchanger, such temperature
cycling is conducted on a continuous basis while a portion
of the slurry is continuously circulated through the heat 65
exchanger. In one particular embodiment of a temperature
cycling process, a heat exchanger is used for the temperature
US 6,464,736 Bl
TABLE 2
Test Feed Time to Add Feed % Solids Grams/liter Supersaturation
30
# (gil) (seconds) at End omin. 5 min.
1 30 10 12.2 15.8 7.1
2 60 10 15.7 22.5 5.9
3 120 15 25.5 26.3 1.5
35
20
60
(b) preferentially relieving supersaturation by growth of
crystals over nucleation, wherein the crystals are the
first crystal structure form of the compound; and
(c) recovering crystals from the saturated solution.
2. The crystallization process of claim 1, wherein said
polymorphic compound is selected from the group consisting
of sodium carbonate, kernite, colemanite, langbeinite,
anhydrite, carnallite, KCI-K2SO4-Na2SO4-Na2C03
mixed salts, Na2S04-K2mixed salts, and Chile caliche ore.
10 3. The crystallization process of claim 1, wherein the first
crystal structure is a hydrated form of the polymorphic
compound.
4. The crystallization process of claim 1, wherein the
second crystal structure is an a lower hydrated form of the
polymorphic compound.
15 5. The crystallization process of claim 1, wherein whether
the compound takes the form of the first crystal structure or
the second crystal structure is temperature dependent.
6. The crystallization process of claim 1, wherein the
process further comprises introducing seed crystals in the
20 first crystal structure form of the compound to the supersaturated
solution.
7. The crystallization process of claim 1, wherein the step
of relieving supersaturation comprises relief of supersaturation
on seed crystals of the compound in the first crystal
25 structure form.
8. The crystallization process of claim 7, further comprising
sizing the recovered crystals to produce a seed crystal
size fraction.
9. The crystallization process of claim 7, wherein a
particle size of the feedstream is less than a particle size of
the seed crystals.
10. The crystallization process of claim 7, wherein the
range of a particle size of the seed crystals is about 3
standard sieve sizes or less.
11. The crystallization process of claim 7, wherein a
particle size of the seed crystals is from about 100 mesh to
about 150 mesh.
12. The crystallization process of claim 1, wherein a
particle size of the feedstream is about 150 mesh or less.
13. The crystallization process of claim 1, wherein the
step of relieving the supersaturation preferentially by growth
of the compound in the first crystal form over nucleation
comprises maintaining a solids content of at least about
17%.
14. The crystallization process of claim 1, wherein the
step of relieving the supersaturation preferentially by growth
of the compound in the first crystal form over nucleation
comprises agitating the solution at an agitation index of at
least about 4.
15. The crystallization process of claim 1, wherein the
step of relieving the supersaturation preferentially by growth
of the compound in the first crystal form over nucleation
comprises periodically changing the temperature of the
solution by at least about 5° C.
16. The crystallization process of claim 1, wherein the
step of relieving the supersaturation preferentially by growth
of the compound in the first crystal form over nucleation
comprises pausing feedstream addition at least about 5% of
the time of crystallization.
17. The crystallization process of claim 1, wherein an
amount of solids in the solution formed by nucleation is
maintained at about 5% by weight or less of the total solids
of the compound in the solution.
18. The crystallization process of claim 1, wherein an
65 amount of solids in the solution having a particle size of less
than about 400 mesh is maintained at about 10% by weight
or less of the total solids of the compound in the solution.
19
EXAMPLE 1
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 40
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 45
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 50
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 55
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 crystallization process for producing a first crystal
structure form of a polymorphic compound from a feedstream
comprising a second crystal structure form of the
compound comprising:
(a) adding the feedstream to a saturated solution of the
compound to create supersaturation of at least about 5
gil;
This example illustrates the high capacity for supersaturation
of sodium carbonate and a technique for measuring
the same. 5
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
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
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
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
solids, and the amount of supersaturation at 0 minutes and
at 5 minutes are shown below in Table 2.
21
US 6,464,736 Bl
22
* * * * *
comprises periodically lowering the temperature of the
solution by at least about 5° C.
30. The process of claim 23, wherein the step of relieving
the supersaturation preferentially by rapid growth of crystals
in the first crystal structure over nucleation comprises pausing
feedstream addition at least about 10% of the time of
crystallization.
31. The process of claim 23, wherein the amount of
crystals in the solution formed by nucleation is maintained
10 at about 20% by weight or less of the total solids of the
compound in the solution.
32. The process of claim 23, wherein the amount of solids
in the solution having a particle size of less than about 400
mesh is maintained at less than about 25% by weight of the
total solids of the compound in the solution.
33. The process of claim 23, wherein the amount of solids
in the solution in the form of agglomerates and/or aggregates
is maintained at about 20% by weight or less of the total
solids of the compound in the solution.
34. The process of claim 23, wherein the saturated solution
is at a temperature above the atmospheric boiling point
of the solution.
35. A process for producing a first crystal structure of a
polymorphic compound from a feedstream comprising a
second crystal structure form of the compound and
impurities, the process comprising:
(a) adding a feedstream comprising the second crystal
structure form of the compound and impurities having
a particle size of less than about 100 mesh to a saturated
solution of the compound at a rate of at least about 400
gil/min under a condition to create supersaturation of at
least about 5 gil;
(b) preferentially relieving the supersaturation by growth
of crystals in the first crystal structure over nucleation,
wherein the step of relieving comprises adding seed
crystals of the compound in the first crystal structure
having a particle size of from about 150 mesh to about
100 mesh to the saturated solution, maintaining a solids
content of at least about 17% and agitating the solution
at an agitation index of at least about 4; and
(c) recovering a portion of the crystals in the first crystal
structure from the saturated solution, wherein said
recovering step comprises removing a portion of the
crystals in the first crystal structure from the solution,
dispersing the crystals in the first crystal structure to a
solids content of less than about 25% by weight by the
addition of solution and recovering crystals in the first
crystal structure having a particle size of greater than
about 100 mesh from insoluble impurities on a size
separation basis.
36. The process of claim 35, wherein the particle size of
said feedstream is less than about 150 mesh.
37. The process of claim 36, wherein the saturated solution
is at a temperature above the atmospheric boiling point
of the solution.
19. The crystallization process of claim 1, wherein an
amount of solids in the solution in the form of agglomerates
and/or aggregates is maintained at about 10% by weight or
less of the total solids of the compound in the solution.
20. The crystallization process of claim 1, wherein the 5
feedstream further comprises insoluble impurities.
21. The crystallization process of claim 20, wherein the
step of recovering comprises:
removing a portion of the first crystal structure form of the
compound from the solution;
dispersing the first crystal structure form of the compound
in a second solution; and
separating the first crystal structure form of the compound
from insoluble impurities on a size separation basis. 15
22. The crystallization process of claim 1, wherein the rate
of adding the feedstream is at least about 100 giL/min.
23. A process for producing a first crystal structure form
of a polymorphic compound from a feedstream comprising
a second crystal structure form of the compound and 20
impurities, the process comprising:
(a) adding the feedstream to a saturated solution of the
compound at a rate of at least about 100 gil/min to
create supersaturation of at least about 5 gil;
(b) preferentially relieving the supersaturation by growth 25
of crystals in the first crystal structure over nucleation,
wherein the step of relieving comprises adding seed
crystals of the compound in the first crystal structure to
the saturated solution, maintaining a solids content of at
least about 17% and agitating the solution at an agita- 30
tion index of at least about 4; and
(c) recovering a portion of the crystals in the first crystal
structure from the saturated solution, wherein said
recovering step comprises removing a portion of the
crystals in the first crystal structure from the solution, 35
dispersing the crystals in the first crystal structure by
the addition of saturated solution and separating crystals
in the first crystal structure from insoluble impurities
on a size separation basis.
24. The process of claim 23, wherein the seed crystals are 40
produced by removing crystals in the first crystal structure
from the solution and sizing the removed crystals to produce
a seed crystal size fraction for reintroduction to the solution.
25. The process of claim 23, wherein a particle size of the
feedstream is less than a particle size of the seed crystals 45
after growth on the seed crystals.
26. The process of claim 23, wherein a range of a particle
size of the seed crystals is not greater than about 3 standard
SIeve sIzes.
27. The process of claim 23, wherein a particle size of the 50
feedstream is less than about 150 mesh.
28. The process of claim 23, wherein a particle size of the
seed crystals is from about 100 mesh to about 150 mesh.
29. The process of claim 23, wherein the step of relieving
the supersaturation preferentially by rapid growth of existing 55
crystals in the first crystal structure over nucleation further