6,464,736 Recrystallization process

Patent Number/Link:

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

(73) Assignee: Environmental Projects, Inc., Casper,

WY(US) BE

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

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

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.

•

~.....

~=.....

U'""'"l

'JJ. =~

o.'"."..'",

\Jl

0'1

':J;.

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

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 :

FIG. 2

US 6,464,736 Bl

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

RECRYSTALLIZATION PROCESS

BACKGROUND OF THE INVENTION

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

US 6,464,736 Bl

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

9 10

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

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

11 12

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

13 14

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.

US 6,464,736 Bl

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

17 18

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

# (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

(b) preferentially relieving supersaturation by growth of

crystals over nucleation, wherein the crystals are the

first crystal structure form of the compound; and

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.

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.

US 6,464,736 Bl

* * * * *

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

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

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