6,284,005 Sodium carbonate recrystallization

Patent Number/Link:

111111111111111111111111111111111111111111111111111111111111111111111111111

US006284005Bl

(12) United States Patent

Hazen et al.

(10) Patent No.:

(45) Date of Patent:

US 6,284,005 BI

Sep.4,2001

(54) SODIUM CARBONATE

RECRYSTALLIZATION

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

WY(US)

(75) Inventors: Wayne C. Hazen, Denver; Dale Lee

Denham, Jr., Arvada, both of CO (US);

Rudolph Pruszko, Green River, WY

(US); David R. Baughman, Golden,

CO (US); Ralph B. Tacoma, Evanston,

WY(US)

(57) ABSTRACT

46 Claims, 2 Drawing Sheets

The present invention provides a process for producing

sodium carbonate monohydrate crystals by introduction of

anhydrous sodium carbonate into a saturated sodium carbonate

brine solution under conditions in which sodium

carbonate monohydrate formation is favored. As the anhydrous

sodium carbonate dissolves, the brine becomes supersaturated

resulting in relief of supersaturation by formation

of sodium carbonate monohydrate crystals. The process

includes controlling supersaturation and its relief to achieve

growth of existing sodium carbonate monohydrate crystals

rather than nucleation and formation of new sodium carbonate

monohydrate crystals. The resulting crystals are

separated from insoluble impurities on a size separation

basis.

* cited by examiner

Clay, S.E., "Kinetic Study of the Dissolution of Calcined

Trona Ore in Aqueous Solutions", Minerals and Metallurgical

Processing, Nov. 1985, 236-40.

Muraoka, D., "Monohydrate Process for Soda Ash from

Wyoming Trona," Minerals and Metallurgical Processing,

May 1985, 102-03.

American Society for Testing and Materials, "Standard Test

Methods for Chemical Analysis of Soda Ash (Sodium Carbonate)",

E-359-90, Mar. 1990,403-410.

FOREIGN PATENT DOCUMENTS

661071 7/1965 (BE).

0073085B1 12/1986 (EP).

OTHER PUBLICATIONS

Primary Examiner~tuart L. Hendrickson

(74) Attorney, Agent, or Firm~heridan Ross Pc.

U.S. PATENT DOCUMENTS

9/1943 Kermer 23/295

5/1957 Pike 23/38

11/1960 Seglin et al. 23/31

1/1961 Caldwell et al. 23/63

(List continued on next page.)

References Cited

Related U.S. Application Data

Continuation-in-part of application No. 09/225,805, filed on

Jan. 5, 1999, now abandoned

Provisional application No. 60/072,805, filed on Jan. 28,

1998.

Int. CI? COlD 15/08

U.S. CI. 23/302 T; 423/203.2; 423/421

Field of Search 423/206.2, 421,

423/426; 23/302 T

2,330,221

2,792,282

2,962,348

2,970,037

(63)

(60)

(51)

(52)

(58)

(56)

Subject to any disclaimer, the term of this

patent is extended or adjusted under 35

U.S.c. 154(b) by 0 days.

(21) Appl. No.: 09/239,441

(22) Filed: Jan. 28, 1999

( *) Notice:

MAKE-UP WATER

NoOH

FEED STREAM

SEED CRYSTALS

/- SATURATED BRINE

,.....,

INSOLUBLE

IMPURITIES

TO WASTE

PRODUCT

US 6,284,005 BI

Page 2

U.S. PATENT DOCUMENTS

3,061,409

3,233,983

3,236,590 *

3,244,476

3,273,959

3,314,748 *

3,425,795

3,479,133

3,498,744 *

3,653,848

3,705,790

3,717,698

3,796,794

3,819,805

3,836,628

3,845,119

3,904,733

10/1962

2/1966

4/1966

9/1966

4/1967

2/1969

11/1969

3/1970

4/1972

12/1972

2/1973

3/1974

6/1974

9/1974

10/1974

9/1975

Robson et al. 23/63

Bauer et al. 23/300

Sopchak et al. 723/426

Smith 23/63

Miller 23/63

Howard et al. 423/426

Howard et al. 23/63

Warzel 23/63

Frint et al. 723/206.2

Port et al. 23/202

Garofano et al. 23/302

Ilardi et al. 423/206

Ilardi et al. 423/421

Graves et al. 423/206

Ilardi et al. 423/206

Duke et al. 260/527

Ganey et al. 423/206

3,933,977

3,956,457

4,021,527

4,022,868

4,083,939

4,138,312

4,183,901

4,202,667

4,260,594

4,283,277

4,286,967 *

4,288,419

4,299,799

4,374,102

4,472,280

4,781,899

5,300,123

5,396,863

1/1976

5/1976

5/1977

4/1978

2/1979

1/1980

5/1980

4/1981

8/1981

9/1981

11/1981

2/1983

9/1984

11/1988

4/1994

3/1995

Ilardi et al. 423/206

Port et al. 423/206

Baadsgaard 423/206

Poncha 423/184

Lobunez et al. 423/421

Gill et al. 162/30

Ilardi et al. 423/206

Conroy et al. 23/302

Verlaeten et al. 423/421

Brison et al. .. 209/166

Booth et al. 23/302 T

Copenhafer et al. 423/190

Ilardi et al. 423/206

Connelly et al. 423/206

Keeney 210/666

Rauh et al. 423/206

Groll 23/303

Ninane et al. . 117/206

... MAKE-UP WATER ..

NoOH -..

,,- SATURATED BRINE ,

FEED STREAM , , ,oC, .. -... .-- I\ ' , I \ Il Il ~t , ~,

SEED CRYSTALS CRYSTALLIZER - THICKENER V-26 .. - ~ ..

- INSOLUBLE 10 , .~

IMPURITIES

14 -----

DISPERSION " TO WASTE -

~ PRODUCT _

18 -----

PRODUCT SEPARATOR ..

• ---- 22...........-- SEED SEPARATOR .-. FIG. 1 ---

d•

'JJ.

•

~.....

~=.....

'JJ.

~,J;;,.

'""'"

'JJ. =~

~....

o.'"."..'",

\Jl

0'1

(I)

1-0"

u.s. Patent Sep.4,2001 Sheet 2 of 2 US 6,284,005 BI

30 35 40 45

WEIGHT PERCENT SODIUM CARBONATE

No2C03 ---A r------. -------

UNSATURATED Nal03· H2O

SOLUTION

L!---------- ---8 Na2C03·7 H2O v---- ----- ._-"1'----

Nal03 ·1 0H20 !

80 -, :

160

180

240

220

120

100

140

200

FIG. 2

US 6,284,005 Bl

SODIUM CARBONATE

RECRYSTALLIZATION

CROSS-REFERENCE TO RELATED

APPLICATIONS

This application is a continuation-in-part of U.S. patent

application Ser. No. 09/225,805, filed Jan. 5, 1999. This

application claims priority under 35 U.S.c. §119(e) from

U.S. Provisional Application No. 601072,805, filed Jan. 28,

1998.

than about 150 mesh and the particle size of the seed crystals

is from about 100 mesh to about 150 mesh.

Relief of supersaturation preferentially by rapid growth of

existing sodium carbonate monohydrate crystals over nucle-

5 ation can alternatively be achieved by a variety of methods.

Such methods can include maintaining a solids content of at

least about 40% in the crystallizer, agitating the brine

solution at an agitation index of at least about 6, periodically

lowering the temperature of the brine solution by at least

10 about 5° c., or pausing feedstream addition at least about

60% of the time of crystallization.

FIELD OF THE INVENTION

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention relates to the production of sodium

carbonate monohydrate crystals from anhydrous sodium 15

carbonate containing impurities.

FIG. 1 is a schematic flow diagram of one embodiment of

the process of the present invention.

FIG. 2 is a phase diagram for sodium carbonate.

DETAILED DESCRIPTION OF THE

INVENTION

1.0 Introduction

The present invention is based in part on the discovery

that under certain conditions sodium carbonate has an unexpectedly

high stable supersaturation capacity that can be

rapidly relieved by the introduction of sodium carbonate

monohydrate crystal surface to produce relatively large

crystals of sodium carbonate monohydrate at high rates of

crystal growth. Significant production efficiencies can be

attained at high rates of crystal growth. The resulting crystals

can be readily separated from insoluble impurities on a

size separation basis. Relief of supersaturation is controlled

such that crystal formation primarily occurs on existing

crystals, rather than occurring as nucleation or growth of

newly formed crystals. In this manner, the particle size

distribution of crystals is controlled to achieve a desired

35 distribution of product crystal size.

Processes of the present invention achieve supersaturation

of sodium carbonate by adding an anhydrous sodium carbonate

feed, e.g., calcined trona, to a saturated sodium

carbonate brine solution under temperature conditions in

40 which sodium carbonate monohydrate crystals are formed.

Thus, the tendency of the anhydrous sodium carbonate feed

to convert to the monohydrate form within the brine solution

causes the anhydrous sodium carbonate feed to dissolve,

thereby creating supersaturation, before forming sodium

45 carbonate monohydrate. Further, it has been surprisingly

found that, under appropriate conditions, sodium carbonate

has a supersaturation capacity of about 30 gil, which is about

an order of magnitude higher and more stable in the absence

of sodium carbonate monohydrate crystal surfaces than

50 would be expected by one skilled in the art. Therefore, the

present invention includes achieving and maintaining high

levels of supersaturation near the supersaturation capacity of

sodium carbonate to create a high driving force for supersaturation

relief which results in rapid crystallization.

Supersaturation created in this manner is relieved by

formation of sodium carbonate monohydrate. Sodium carbonate

monohydrate can form as a result of exceeding the

supersaturation limit, which causes primary nucleation

resulting in formation of clouds of small nuclei of sodium

60 carbonate monohydrate. The term "supersaturation limit" is

used to describe a condition where the level of supersaturation

of sodium carbonate in the brine solution is unstable

and results in a relatively spontaneous formation of crystals

by primary andlor secondary nucleation. This type of super-

65 saturation relief is unproductive because the small nuclei

cannot easily be grown to a size large enough to be separated

from insoluble impurities. Supersaturation relief can also

SUMMARY OF THE INVENTION

BACKGROUND OF THE INVENTION

One common method of purifying a compound is to

crystallize the compound in a solution. Methods of crystal- 20

lization typically involve controlling macroscopic external

variables such as evaporating solvent to create supersaturation

or adjusting the temperature of the solvent to affect

solubility. These crystallization methods are generally

directed to achieving maximum solids recovery andlor puri- 25

fication without any regard to the size or shape of the

crystals.

Therefore, there is a need for a crystallization process that

can effectively control or influence the ratio of crystal

growth to formation of new crystals at low energy costs. 30

The present invention is based on the discovery that

sodium carbonate has an unexpectedly high stable supersaturation

capacity under appropriate conditions that can be

rapidly relieved by the introduction of sodium carbonate

monohydrate crystal surfaces to produce relatively large

crystals of sodium carbonate monohydrate at high rates of

crystal growth. The resulting crystals can be readily separated

from insoluble impurities on a size separation basis.

More particularly, the process of the present invention is

for producing sodium carbonate monohydrate from a feedstream

which includes anhydrous sodium carbonate and

insoluble impurities. The process includes adding the feedstream

to a saturated sodium carbonate brine solution under

conditions to create supersaturation of at least about 5 gil.

The process further includes processing within parameters

that preferentially relieve the supersaturation by rapid

growth of existing sodium carbonate monohydrate crystals

rather than by nucleation. In this manner, the particle size

distribution of crystals is controlled to achieve a desired

distribution of crystal size product. The sodium carbonate

monohydrate crystals produced by the process are recovered

from the saturated brine solution.

The process can include the use of a high feed rate of at

least about 100 grams of feedstream per minute for each liter

of solution in the crystallizer. The process can also include

relieving the supersaturation preferentially by rapid growth

of existing sodium carbonate monohydrate crystals over

nucleation by adding sodium carbonate monohydrate seed

crystals to the saturated sodium carbonate brine solution.

Such seed crystals can be produced by removing sodium

carbonate monohydrate crystals from the brine solution and

sizing the removed crystals to produce a seed crystal size

fraction for reintroduction to the brine solution. In a preferred

embodiment, the particle size of the feedstream is less

US 6,284,005 Bl

occur by growth of existing sodium carbonate monohydrate

crystals, which is desired in the present invention.

Processes of the present invention are based on the

recognition that since supersaturation is created by the

introduction of anhydrous feed, the supersaturation limit can

be exceeded in a localized area at the point of introduction

of the feed. Therefore, control of supersaturation and its

relief in the local environment near where the feed is

introduced is critical. The present invention provides the

proper thermodynamic environment wherein it is easier to

preferentially relieve supersaturation by growth of existing

crystals than by nucleation.

Processes of the present invention include a multi-faceted

approach to control local supersaturation and its relief to

achieve the desired mechanism for supersaturation relief,

preferably the growth of existing crystals. One of the elements

of processes of the present invention is to use high

agitation to rapidly disperse areas of local high supersaturation

to avoid exceeding local supersaturation limits, and to

bring the surfaces of existing crystals into contact with such

areas of supersaturation. The use of high agitation is quite

contrary to standard crystallization practice and technology.

Processes of the present invention also provide a large

amount of available sites for relief of supersaturation on

existing crystals so that if the degree of supersaturation in a

localized area is approaching the maximum level, i.e., the

supersaturation limit, the supersaturation can be quickly

relieved by sodium carbonate monohydrate formation on an

existing crystal surface instead of by nucleation. Sites for

crystallization are provided by the use of seed crystals

and/or by maintaining a high solids content in the crystallizer.

The present invention can also include pausing during

the introduction of feed to allow for dispersion of local areas

of very high supersaturation by agitation and/or productive

relief of supersaturation on existing crystals in local areas of

very high supersaturation. Control of temperature in the

crystallizer is also used to control the rate of relief of

supersaturation.

The terms "recrystallization" and "crystallization" are

used interchangeably herein and refer to the step of adding

anhydrous sodium carbonate to a saturated sodium carbonate

brine solution and crystallizing sodium carbonate monohydrate

from the saturated brine solution, i.e., the anhydrous

sodium carbonate dissolves in the saturated brine solution,

forms a supersaturated solution which then causes growth of

sodium carbonate monohydrate crystals because the temperature

of the saturated brine solution is in the range of

sodium carbonate monohydrate stability. A "saturated brine

solution" refers to a solution which is saturated with sodium

carbonate.

2.0 Feedstream Composition and Introduction

2.1 Composition

As noted above, a feedstream of the present invention

comprises anhydrous sodium carbonate. For example, processes

of the present invention can be used for purifying

anhydrous sodium carbonate (such as calcined trona) containing

impurities or for producing dense soda ash from light

soda ash. Moreover, the present invention is particularly

well adapted for use with feedstreams having high contents

of insoluble impurities. In particular, the present invention

can be used for purifying anhydrous sodium carbonate

feedstreams in which impurities are included within the

crystal structure even when the particles are finely ground.

Thus, although the present invention can be used with a

substantially pure anhydrous sodium carbonate, the present

invention is particularly suitable for use with feedstreams

having greater than about 15% by weight insoluble

impurities, and even more particularly, having greater than

about 30% by weight insoluble impurities. Although any

anhydrous sodium carbonate including synthetic anhydrous

sodium carbonate or calcined trona can be used, processes of

5 the present invention will now be described in detail in

reference to purification of calcined trona containing impurities

and FIG. 1. And as such, the terms "calcined trona" and

"anhydrous sodium carbonate" will hereinafter be used

interchangeably.

10 2.2 Size

As noted, supersaturation is achieved by adding calcined

trona to a saturated brine solution under temperature conditions

at which sodium carbonate monohydrate forms.

Thus, the calcined trona dissolves, thereby creating super-

15 saturation and also releasing impurities, before forming

sodium carbonate monohydrate. The rate and completeness

of calcined trona dissolving in a saturated brine solution is

determined by, among other factors, its particle size. Since

the presence of undissolved hydrated calcined trona can

20 compete with seed crystals of monohydrate as a substrate for

relieving supersaturation, the calcined trona added to the

saturated brine solution should dissolve substantially completely

to ensure that the majority of supersaturation relief is

by growth of seed crystals, not by growth on undissolved

25 anhydrous feed, and to ensure that at least a portion of

impurities present within the crystal lattice of sodium carbonate

is released. If the feedstream of calcined trona

dissolves only partially, the remaining particles can have

undesired effects such as forming agglomerates or relieving

30 supersaturation to form mixed particles of calcined trona and

sodium carbonate monohydrate. Thus, to ensure a substantially

complete dissolution of the particles the particle size of

calcined trona ore in the feedstream, whether in a slurry

form or a dry form, is preferably less than about 100 mesh

35 (Tyler), more preferably less than about 150 mesh, still more

preferably less than about 200 mesh, and most preferably

less than about 400 mesh. It should be appreciated that when

the particle size of calcined trona ore is within the above

described range, any insoluble impurities present in the

40 calcined trona ore will also be within the confines of the

above described particle size.

The above particle size limitations allow calcined trona

ore to dissolve relatively quickly and completely in a

saturated brine solution in the crystallizer 10.

45 2.3 Feed Rate

As noted above, it has been surprisingly found that, under

appropriate conditions, sodium carbonate has a supersaturation

capacity of about 30 gil, which is about an order of

magnitude higher than would be expected by one skilled in

50 the art. Therefore, the present invention includes achieving

and maintaining high levels of supersaturation near the

supersaturation capacity of sodium carbonate to create a

high driving force for supersaturation relief which results in

rapid crystallization. For example, the process includes

55 creating supersaturation of at least about 5 gil, more preferably

at least about 10 gil, more preferably at least about 20

gil and up to 30 gil. Supersaturation can be calculated within

a localized volume in a crystallizer or within the entire

volume of a crystallizer. For example, supersaturation can be

60 calculated as follows. A volume of saturated brine, which

can include sodium carbonate monohydrate crystals and

calcined trona, can be withdrawn from a crystallization

vessel through a screen and filter to remove solid materials.

Water in the withdrawn brine is then evaporated and the

65 amount of sodium carbonate per volume of brine can be

gravimetrically determined. The amount of sodium carbonate

in excess of the known solubility level is the amount of

US 6,284,005 Bl

supersaturation. Because of the high capacity for supersaturation

and the very rapid relief of supersaturation, the rate of

introduction of the feedstream or feed rate can be very high

in the present invention. More particularly, the feed rate can

be at least about 100 grams per minute for each liter of 5

volume (gil/min), preferably at least about 200 gil/min, more

preferably at least about 400 gil/min, and even more preferably

at least about 800 gil/min. These feed rates are

significantly higher than feed rates expected to be useful by

one skilled in the art and those utilized by previous crystal- 10

lization methods.

2.4 Method of Introduction

The feedstream, which includes anhydrous sodium

carbonate, can be introduced to the saturated brine solution

using any of the known methods including by a direct 15

injection, a screw feeder and gravity. The feedstream can be

a slurry of anhydrous sodium carbonate in a saturated brine

solution or dry anhydrous sodium carbonate.

A dry anhydrous sodium carbonate feedstream must be

dispersed and dissolved quickly in the saturated brine 20

solution, otherwise particles may become hydrated and form

agglomerates. If the particles in the feedstream are too

coarse, they will not dissolve completely, thus possibly

reducing the purity of the product; therefore, the particle size

of the feedstream should be within the range discussed 25

above. On the other hand, fine particles tend to "float" on top

of the saturated brine solution and become hydrated and

form agglomerates. Generally, at a high feedstream addition

rate discussed above, it is difficult to quickly disperse and

dissolve the anhydrous sodium carbonate into the saturated 30

brine solution. It has been found by the present inventors

that these problems can be overcome by using high

agitation, as discussed below.

One can also avoid these problems, such as agglomerate

formation and floatation of fines, by adding a feedstream of 35

anhydrous sodium carbonate in a slurry form. A slurry of

anhydrous sodium carbonate can be prepared by mixing

calcined trona ore and the saturated sodium carbonate solution

at atmospheric pressure and transferring the mixture

into a slurry feedstream vessel having a desired temperature 40

at increased pressure. Alternatively, calcined trona ore and

the saturated sodium carbonate solution can be fed directly

into the slurry feedstream vessel at a desired temperature

and pressure to form a slurry feedstream. At a temperature

above the transition temperature of sodium carbonate mono- 45

hydrate to anhydrous sodium carbonate (108.5° C. for a pure

system of water and sodium carbonate at one atmosphere of

pressure), solids in the slurry include anhydrous sodium

carbonate crystals and insoluble materials originally present

in the calcined trona ore. It is recognized by those skilled in 50

the art that the transition temperature can be adjusted by

various means, including by adding sodium chloride.

One method of preparing a slurry of feedstream involves

mixing anhydrous sodium carbonate with a saturated sodium

carbonate brine solution at a temperature at least above the 55

transition temperature of anhydrous sodium carbonate to

sodium carbonate monohydrate preferably at least about 5°

C. above the transition temperature, and more preferably at

least about 2° C. above the transition temperature. A "transition

temperature" refers to a temperature at which stable 60

anhydrous sodium carbonate changes its morphology to

stable sodium carbonate monohydrate. See for example, line

Ain FIG. 2, the transition of anhydrous to sodium carbonate

monohydrate. Line B in FIG. 2 represents the transition

temperature between sodium carbonate heptahydrate and 65

sodium carbonate monohydrate. It will be appreciated that

this step of producing a slurry feedstream must be conducted

at above atmospheric pressures and must use a feeding

mechanism that maintains a continuous pressure seal

between the environment of the feed slurry and of the brine

solution.

It should be further appreciated that this method of

introduction of anhydrous sodium carbonate can be used for

processing in any aqueous solution.

2.5 Calcination

When trona is used as a feedstream, it must be converted

into anhydrous sodium carbonate by calcination prior to

being added to the saturated brine solution. Trona can be

calcined using any known calcination technology. For

example, calcination can be conducted with a fluidized bed

calciner. When a fluidized bed calciner is used to calcine

trona ore, the trona ore is comminuted and is generally

separated into three size ranges: 6x20 mesh, 20x100 mesh

and -100 mesh. Each size can then be separately calcined in

a fluidized bed calciner. Calcined trona is then combined and

comminuted to provide a feedstream having above mentioned

particle size. Further, trona in the feedstream can be

calcined using indirect heat calcination as disclosed in

commonly assigned U.S. patent application Ser. No. 09/151,

694 that was filed on Sep. 11, 1998, which is incorporated

herein by reference in its entirety.

3.0 Crystallization

As shown in FIG. 1, a feedstream comprising calcined

trona is added to a saturated sodium carbonate brine solution

in a crystallizer 10 to generate supersaturation within the

saturated brine solution. The feedstream and the saturated

brine solution can be added simultaneously and/or sequentially.

The present method controls crystallization conditions

so that relief of supersaturation created by introduction of

the anhydrous sodium carbonate feed primarily occurs on

existing sodium carbonate monohydrate crystals rather than

by nucleation.

3.1 Seed Crystals

In one embodiment of the present invention, supersaturation

relief on existing crystals is achieved by the introduction

of seed crystals of sodium carbonate monohydrate to

the crystallizer 10. Thus, in contrast to other crystallization

methods in which a major amount of crystal growth is by

nucleation or on crystals newly formed by nucleation, processes

of this particular embodiment of the present invention

provide supersaturation relief primarily by growing seed

crystals to crystals that are large enough to be separable from

insoluble impurities on a size separation basis. Moreover,

the size distribution of the product crystal population can

also be controlled by adding seed crystals of a desired

particle size range. By the use of seed crystals in this

manner, crystal growth is productive in the sense that it

occurs on crystals which will be large enough to recover on

a size separation basis, rather than occurring on small

particles which cannot practically be grown large enough to

be separated from insoluble impurities.

Seed crystals can be prepared separately or can be prepared

as a part of the process flow of the present crystallization

process, as described below. For example, seed crystals

can be produced by removing crystals from the

crystallizer and sizing the crystals to produce a seed crystal

size portion for reintroduction to the crystallizer.

Furthermore, at least a portion of the product of the present

process can be comminuted, e.g., ground, to a desired seed

crystal size and used as a source of the seed crystals.

In a batch process, the seed crystals are typically added

prior to the addition of the feedstream, whereas in a continuous

process, the seed crystals are typically added continuously

during the operation of the present invention. As

US 6,284,005 Bl

7 8

TABLE 1

rolling surface with quick turnover and quick absorption

of dry material into mass of slurry.

turnover of slurry, but not all solids held in suspension

mild turnover of slurry with all solids held in suspension

Description

static, no movement or mixing

violent turbulent movement of all slurry in entire vessel

degradation of mechanical fracturing of material

Agitation

Index

Preferably, the mixture in the crystallizer 10 is stirred at an

agitation index of at least about 4, more preferably at least

about 7, still more preferably at least about 8, and most

preferably at least about 9.

Evidence of insufficient agitation can be readily determined

by examining crystal structures of the product. The

product resulting from insufficient agitation may include the

presence of agglomerates, long needle-like crystals or dendrites.

In contrast to other methods, agitation in the present

invention preferably does not produce a typical vortex

associated with using a single propeller non-baffled agitation

system. In a particular embodiment of the present invention,

agitation of the monohydrate slurry is achieved by using at

least two propellers having a counter pitch or other suitable

65 agitation methods including using an attrition scrubber and

any other impeller configurations which achieve the desired

agitation index discussed above.

supersaturation is created by the anhydrous feed dissolving

to in the saturated brine solution. Therefore, control of

supersaturation and its relief in the local environment, for

example, by sufficiently high agitation, where the feed is

5 introduced is critical. The term "local" refers to the immediate

environment of a small portion of the brine solution in

the crystallizer 10 and not the overall amount of sodium

carbonate within the total volume of the crystallizer 10.

Thus, the term "local supersaturation limit" refers to the

10 degree of supersaturation within any volume of a crystallizer

in which formation of a crystal nucleus by primary and/or

secondary nucleation can occur. It will be appreciated

therefore, that within the crystallizer 10, while the average

degree of supersaturation can be below the supersaturation

limit, a localized region of high supersaturation can occur

15 and thereby exceed the supersaturation limit in that localized

region, resulting in undesired nucleation. To reduce or avoid

this undesired nucleation, processes of the present invention

can also include control of local supersaturation by using

high agitation to rapidly disperse areas of high local super-

20 saturation. High agitation brings the surfaces of existing

crystals into contact with areas of high local supersaturation

and thereby, increases the effective net surface area available

for supersaturation relief by increasing the probability of an

existing crystal particle coming into contact with an area of

25 local high supersaturation area. One measure of agitation is

a qualitative agitation index as described below. The term

"agitation index" refers to a scale of agitation in a crystallizer.

An agitation index of 0 means that there is no perceptible

stirring or movement within the mixture, whereas an

30 agitation index of 10 means the mixture in the crystallizer is

stirred at a very high and rapid degree of mixing and

agitation such that degradation or mechanical fracturing of

crystals occurs. Table 1 shows the qualitative characteristics

of the 0-10 agitation index.

used in this invention, a "continuous addition" can include

both non-interrupted addition as well as interval addition

throughout the process as needed.

The particle size of the seed crystals is selected such that

a product having an acceptable particle size range is produced.

For example, the seed crystals need to be large

enough that, given the amount of growth achieved in a given

crystallization, the resulting product crystals will be large

enough to be separable from insoluble impurities on a size

separation basis. Preferably, the particle size of the seed

crystals is in the range from about 100 mesh (Tyler) to about

400 mesh, more preferably from about 100 mesh to about

200 mesh and most preferably from about 100 mesh to about

150 mesh. Alternatively, the range of the particle size of seed

crystals is about 2 standard sieve sizes or less. A "standard

sieve size" is denoted by increasing or decreasing the

opening in a sieve size by the ratio of the square root of 2

or 1.414, i.e., taking a screen opening and multiplying or

dividing it by the square root of 2 or 1.414. The seed crystal

size range utilized is determined by the desired product

particle size range. For example, a narrow seed crystal size

range results in a narrow product particle size range.

The amount of seed crystals and feedstream added to the

saturated brine solution depends on the volume of the

saturated brine solution in the crystallizer 10. However, as

noted below, the total amount of seed crystals and feedstream

added to the saturated brine solution typically results

in a monohydrate slurry having a solids content in accordance

with the parameters discussed below. As used herein,

a "monohydrate slurry" refers to a saturated brine solution

containing solid sodium carbonate monohydrate crystals.

Such a high solids content ensures that sufficient surface area

is available for supersaturation relief on existing crystals

before any significant amount of nucleation can occur in the

brine solution. In another embodiment, for the above mentioned

particle sizes of seed crystals and products, the ratio 35

of seed crystals added to the feedstream added is at least

about 1:1 by weight, preferably at least about 5:1 by weight,

and more preferably at least about 10: 1. Generally, about an

equal amount of the solids content by weight of the seed

crystals and the feedstream is added to the saturated brine 40

solution.

3.2 Solids Content

A further aspect of the present invention to control

supersaturation relief on existing crystals of sodium carbonate

monohydrate is to maintain a high solids content in the 45

crystallizer 10. In this manner, if the degree of supersaturation

in a localized area is approaching the maximum level,

supersaturation can be quickly relieved by sodium carbonate

monohydrate formation on an existing crystal surface

instead of by nucleation. As will be appreciated, the solids 50

content in the crystallizer 10 depends on a variety of factors

including the amount of seed crystals added and the amount

and solids density of the feedstream added to the saturated

brine solution, as well as the desired density for optimal

crystallizer operation. These variables are controlled such 55

that the monohydrate slurry has a solids content of at least

about 17% by weight, more preferably at least about 35% by

weight, even more preferably at least about 40% by weight,

and most preferably at least about 60% by weight.

Alternatively, the particle surface area density, i.e., the total 60

amount of surface area of crystals present per volume, is at

least about 40 cm2/ml, preferably at least about 75 cm2/ml,

more preferably at least about 95 cm2/ml, and most preferably

at least about 125 cm2/ml.

3.3 Crystallizer Agitation

As noted above, the supersaturation limit of the brine

solution can be exceeded in a small localized area because

US 6,284,005 Bl

9 10

calciner. By adding a freshly calcined, i.e., hot, trona to the

saturated brine solution, the amount of energy and the cost

required to maintain the mixture at the above described

temperature can be significantly reduced compared to pro-

S cesses where calcined trona is reheated prior to being added

to the saturated brine solution or where the saturated brine

solution is at a higher temperature then the feedstream.

As noted above, the present invention includes controlling

supersaturation relief to achieve crystal growth on existing

10 crystals of sodium carbonate monohydrate rather than initiating

nucleation. A further aspect of the present invention

is the control of supersaturation relief by modifying the

temperature of the crystallization solution in a temperature

cycling process to control the amount of fines as discussed

15 in more detail below.

3.5 Feed Addition Pause

Crystal formation in the form of nucleation occurs when

the local supersaturation level exceeds the supersaturation

limit. When the rate of supersaturation generation exceeds

20 the rate of supersaturation relief, eventually the supersaturation

level somewhere in the crystallizer will exceed the

supersaturation limit resulting in nucleation (sometimes

referred to as "snowing-out"). Thus, to prevent the supersaturation

level in a local area from exceeding the super-

25 saturation limit, the addition of the feedstream to the saturated

brine can be stopped briefly or intermittently to

decrease the supersaturation level by allowing growth of

existing crystals. More particularly, the break or pause in

feedstream addition can be conducted at least about 60% of

30 the time of crystallization. More preferably, the pause can be

conducted at least about 30%, and most preferably, at least

about 5% of the time of crystallization. For example, if the

pause is 10% of the crystallization time, the feedstream

would be paused 6 minutes during every hour of operation.

35 It should be noted that when pausing is used, it is preferably

conducted frequently, such as by switching between feeding

and pausing every several minutes, or about every five

minutes.

3.6 Crystal Growth Rate

It is believed that the conventional recommended crystal

growth rates for good crystal quality is from about 2

microns/minute to about 5 microns/minute. A "good crystal

quality" refers to crystals which are generally hexagonal,

roughly equi-dimensional, slightly elongated with an aspect

45 ratio of WxLxH of about 1:1.5:0.75. See for example,

Goldschmidt, Atlas der Krystallformen, p. 128 (Carl Winters

Universitatbuchhandlung, Heidelberg 1922), which is incorporated

herein by reference in its entirety. The crystal

growth rate of the present invention is significantly higher

50 than the conventional recommended crystal growth rates

while providing a similar crystal quality. Preferably the

crystal growth rate of the present invention is at least about

5 microns/minute, more preferably at least about 10

microns/minute, and most preferably at least about 20

55 microns/minute. It has been found that the crystal growth

rate of the present invention does not decrease significantly

by having a higher solids to saturated brine solution ratio.

However, it is believed the crystal growth rate does depend

on the size of the seed crystals. The reason for the higher

60 growth rate of coarser crystals is the mass transfer of sodium

carbonate monohydrate crystals from finer crystals to

coarser crystals. The operation of this mechanism at high

crystal growth rates such as in the current invention is

contrary to what would be expected by one of skill in the art.

An average crystal growth rate can be determined by a

variety of methods including by a statistical analysis of a

sample product crystal. For example, the average crystal

Preferably, the solution is agitated at greater than about 10

horsepower/lOOO gallons (hp/lOOO gal), more preferably at

least about 100 hp/lOOO gal, and most preferably at least

about 200 hp/lOOO gal. Alternatively, when a propeller

system is used for agitating the monohydrate slurry, the

propeller tip speed is at least about 8 feet/sec (ftlsec),

preferably at least about 10 ftlsec, and more preferably at

least about 22 ft/sec.

Adequate agitation can be achieved by use of any vessel

providing agitation as described above. For example, such a

vessel can include a one impeller system; two impellers

having counter pitch, such as is used in an attrition scrubber;

multiple impellers having alternating counter pitch in the

crystallizer 10, or other configurations providing the desired

agitation index. Thus, in such agitation, it is important to

create a rapid exchange of solid particles and the solution

portion of the saturated brine solution.

It should be noted, however, that while high agitation is

beneficial, it should be conducted in a manner without a

significant amount of impact destruction. The term "impact

destruction" refers to a process where two or more particles

collide and result in a particle size reduction for one or more

particles.

3.4 Temperature Control

As discussed above, the temperature of the saturated brine

solution is maintained such that the formation of sodium

carbonate monohydrate is favored as determined by the

phase diagram, as shown in FIG. 2. The temperature of the

saturated brine solution in the crystallizer 10 is maintained

at between about 40° C. and the transition temperature of

anhydrous sodium carbonate to sodium carbonate monohydrate

to ensure formation of sodium carbonate monohydrate,

preferably between about 70° C. and the transition temperature

of anhydrous sodium carbonate to sodium carbonate

monohydrate, more preferably between about 90° C. and

the transition temperature of anhydrous sodium carbonate to

sodium carbonate monohydrate and most preferably

between about 98° C. and the transition temperature of

anhydrous sodium carbonate to sodium carbonate monohydrate

It has been discovered by the present inventors that 40

keeping the temperature in the crystallizer as close as

possible to but below the transition temperature of sodium

carbonate monohydrate to anhydrous sodium carbonate

reduces the "drive", i.e., the rate of conversion, of anhydrous

sodium carbonate in the feedstream to change morphologically

to sodium carbonate monohydrate. This discovery

allows the processes of the present invention to be controlled

easily and results in larger, better formed crystals as discussed

in detail below.

To maintain a substantially constant temperature of the

saturated brine solution within the crystallizer 10, the temperature

difference between the saturated brine solution and

the feedstream should be small enough such that no significant

cooling or heating of the saturated brine solution occurs

during the addition of the feedstream. Preferably, the temperature

difference between the feedstream and the saturated

brine solution is about 20° C. or less, more preferably about

15° C. or less, and most preferably about 10° C. or less.

In another embodiment, the temperature of the dry feed

particles in the feedstream is at least about 95° c., preferably

at least about 120° c., and more preferably at least about

150° C.

Still in another embodiment, freshly calcined trona can be

added directly to the crystallizer 10 along with a saturated

brine solution to maintain the temperature of the mixture in 65

the crystallizer 10 as disclosed above. Freshly calcined trona

has a high particle temperature as it comes out of the

US 6,284,005 Bl

11 12

one aspect of the present invention, the crystallization process

is conducted by maintaining the amount of solids in the

monohydrate slurry in the form of agglomerates and/or

aggregates at about 10% by weight or less of the total

5 sodium carbonate solids in the monohydrate slurry, more

preferably at about 5% by weight or less of the total sodium

carbonate solids in the monohydrate slurry, and most preferably

at about 0.5% by weight or less of the total sodium

carbonate solids in the monohydrate slurry.

As used herein, the term "aggregate" refers to a collection

of particles or crystals in clusters or clumps. The particles

can be held together as a result of the attraction of weak

forces, such as van der Waals forces. The term "agglomerate"

refers to particles or feed held together by forces

15 stronger than van der Waals forces, which can be formed, for

example, by anhydrous feed particles which are not fully

dissolved acting as a site for crystallization of monohydrate

crystals, or anhydrous feed that was not dispersed or dissolved

absorbing water to hydrate.

20 3.9 Crystallizer Pressure

The crystallizer 10 can be equipped to be operated at a

wide range of pressure. In one embodiment, the crystallizer

10 is operated at atmospheric pressure. In another

embodiment, the crystallizer 10 can be operated at any

25 desired pressure of up to about 35 pounds per square inch

(psia), more preferably up to about 30 psia, and most

preferably up to about 25 psia. Unless otherwise noted, the

pressure refers to an absolute pressure and not a relative, i.e.,

gauge, pressure. Whether operated under atmospheric pres-

30 sure or higher pressure, the temperature of the saturated

brine solution in the crystallizer 10 is maintained to favor the

formation of sodium carbonate monohydrate. When the

crystallizer 10 is operated under pressure, the introduction of

the feedstream is preferably at a similar pressure. A pres-

35 surized pump such as a Fuller Kinyon pump (not shown) or

any other type of pump which can achieve a desired pressure

can be used to introduce the dry or slurry feedstream into the

crystallizer 10. However, it should be recognized that the

feedstream can be at a variety of pressures independent of

40 the crystallization itself.

3.10 Multiple Crystallization Vessels

In a further embodiment, the crystallization is conducted

in a series of two or more crystallizers. In this manner, the

initial feedstream can be used to generate fines by nucleation

45 in a first crystallizer. The fines are then transferred to a

second crystallizer and used as seed crystals for subsequent

crystallization where they are grown to a larger size. Thus,

in either the second or some subsequent crystallizer, the

crystals are grown large enough for a size separation from

50 insoluble impurities. By using a multiple tank system which

allows successive crystal growth conditions, the need for a

separate seed crystals as discussed above in Section 3.1 can

be eliminated.

4.0 Dispersion

Referring again to FIG. 1, at least a portion of sodium

carbonate monohydrate crystals and saturated brine solution

are separated from the crystallizer 10. The sodium carbonate

monohydrate product is eventually recovered in a product

separator 18, preferably on a size separation basis. However,

60 as noted above, crystallizations are conducted at high solids

content, such as at least about 17% solids content. Product

separation with such a viscous mixture can be difficult.

Therefore, as shown in FIG. 1, the separation process can

also include transferring at least a portion of the monohy-

65 drate slurry from the crystallizer 10 to a dispersion tank 14

to decrease the solids content of the monohydrate slurry in

order to, inter alia, facilitate the separation process. It should

growth rate can be obtained by dividing the total amount of

crystal growth in the sample by the total crystallization time

and the total crystal surface area.

3.7 Nucleation Control

Processes of the present invention involve controlling

crystallization conditions as discussed in Sections 3.1-3.6 to

provide conditions for relieving the supersaturation in the

crystallizer 10 by growing existing crystals rather than by

nucleation. If a significant amount of primary and/or secondary

crystal nucleation occurs in the crystallizer 10, then 10

a large amount of fines is generated. Production of fines

limits productive crystal growth because fines have a large

ratio of surface area to volume compared to larger crystals.

Since fines are small, even significant growth of them will

not make them large enough to be separated from insoluble

impurities on a size separation basis. Therefore, such growth

is unproductive. However, it should be appreciated that

some formation of new crystals by nucleation may be

necessary when the process includes generating new seed

crystals. Thus, processes of the present invention may be

used to allow formation of new crystals by nucleation in a

relatively controlled amount for this purpose.

Thus, in a further aspect of the present invention, the

amount of solids in the saturated sodium carbonate brine

formed by primary and/or secondary nucleation in the

crystallizer 10 is maintained at about 10% by weight or less

of the total sodium carbonate solids in the saturated brine,

more preferably at about 5% by weight or less of the total

sodium carbonate solids in the saturated brine, still more

preferably at about 1% by weight or less of the total sodium

carbonate solids in the saturated brine, and most preferably

at about 0.5% by weight or less of the total sodium carbonate

solids in the saturated brine. For example, given a defined

crystal population at a point in time, one can determine

whether new crystals have been formed by primary and/or

secondary nucleation by determining whether the crystal

population at a later point in time has smaller crystals or an

increase in smaller crystals compared to the earlier point in

time. One can also determine whether new crystals have

been formed by primary and/or secondary nucleation by

identifying whether a drop in yield of +200 mesh crystals

occurs. One can also determine whether new crystals have

been formed by primary and/or secondary nucleation in a

continuous process by identifying fluctuations in the size

distribution of crystals at a point in time at which a stable

population would be expected.

In a further aspect of the invention, control of the crystallization

conditions can maintain or reduce the portion of

the solid material in the monohydrate slurry which has a

small particle size. More particularly, the processes of the

present invention can include maintaining the amount of

solids in the monohydrate slurry having a particle size of less

than about 400 mesh at less than about 10% by weight of the

total sodium carbonate solids in the monohydrate slurry,

more preferably at less than about 2% by weight of the total 55

sodium carbonate solids in the monohydrate slurry, and most

preferably at less than about 0.5% by weight of the total

solids in the monohydrate slurry.

3.8 Agglomerate/Aggregate Control

Processes of the present invention for controlling crystallization

conditions as discussed above in Sections 3.1-3.6

can also substantially avoid formation of a significant

amount of agglomerates and/or aggregates. If a significant

amount of agglomerates and/or aggregates are formed, the

purity of any recovered product may be significantly

decreased because insoluble and soluble impurities can be

trapped within the agglomerates and aggregates. Thus, in

US 6,284,005 Bl

13 14

bulk density of at least about 0.95 glml, preferably at least

about 1.0 g/ml, and more preferably at least about 1.1 g/ml.

In another embodiment of the present invention, the product

has a packed density of at least about 1.0 g/ml, preferably at

5 least about 1.1 g/ml, and more preferably at least about 1.2

g/ml.

The product of the present invention also has a lower

amount of dust, i.e., fines, than crystals produced by the

conventional crystallization processes. Without being bound

10 by any theory, this low amount of dust present in the product

is believed to be due to a variety of novel features of the

present invention including the use of seed crystals, the

relief of supersaturation primarily by crystal growth rather

than by formation of new crystals, and the block-like shape

15 of the product crystals which is more resistance to abrasion

than other crystal shapes.

The product of the present invention has improved

flowability and decreased bridging compared to products

produced by conventional methods. It is believed the block-

20 like crystal shape and the absence of fine crystals produces

higher flowability and lower bridging in storage vessels.

This block-like crystal shape has smoother crystal surfaces

compared to other crystal shapes such as jack-like or needle

like crystal shapes. Without being bound by any theory, it is

25 believed that the smooth surface of block-like shaped crystals

has a lower frictional force than other crystal shapes. In

addition, larger particles have a reduction in specific surface

area, and thereby the cohesiveness between particles is

reduced.

30 6.0 Seed Separation

Again referring to FIG. 1, an undersize fraction of the

monohydrate slurry from the product separator 18 can be

transferred to a seed crystal separation apparatus 22 to

separate at least a portion of crystals from the undersize

35 fraction for use as seed crystals. The undersize fraction will

include sodium carbonate monohydrate crystals smaller than

the size cutoff in the product separator 18 and insoluble

impurities. To effectively produce a seed crystal population,

the undersize fraction from the product separator 18 must

40 include an upper size range which is larger than the size of

the insoluble impurities. In this manner, by conducting a size

separation in the seed separator 22, seed crystals which are

free of insoluble impurities can be recovered as an oversize

fraction, and the insoluble impurities with small sodium

45 carbonate monohydrate crystals are generated as the undersize

fraction. The seed crystal separation can be accomplished

by any of the appropriate known methods as discussed

above. As discussed above, a seed crystal population

produced in this manner is then used in a crystallizer.

Other methods of producing seed crystals include the

following: wet comminution of monohydrate crystals; dry

comminution of monohydrate crystals; dissolution of a

portion of monohydrate crystals by water addition; dissolution

of crystal in a slurry by cooling the slurry to increase the

55 solubility of sodium carbonate in the brine; and controlled

cooling of a slurry of anhydrous sodium carbonate.

7.0 Thickening

The undersize fraction from the seed separator 22, containing

saturated brine solution, insoluble impurities and/or

60 sodium carbonate monohydrate crystals which are smaller

than the desired seed crystal size is then further processed.

As shown in FIG. 1, the undersized fraction from the seed

separator 22 is transferred to a thickener 26 to allow for

settling of insoluble impurities. The settled insoluble impu-

65 rities are then purged from the system, while the clear

overflow and/or the resulting clarified saturated brine solution

can be recycled and reused. It should be appreciated that

be noted that the dispersion step should not dilute the

solution below saturation. Otherwise, product loss can occur

by dissolution of product. Typically, a saturated brine solution

having a substantially negligible solids content is added

to the dispersion tank 14 to reduce the solids content of the

monohydrate slurry to about 25% by weight or less, more

preferably to about 15% by weight or less solids content, and

most preferably to about 10% by weight or less solids

content.

5.0 Recovery

The present invention also includes recovering product

from the monohydrate slurry. The recovery process can

include separating a particular particle size range of sodium

carbonate monohydrate crystals from the monohydrate

slurry. Size separation is conducted in a separation apparatus

18 and can be affected by any of the appropriate known

methods. For example, screening, cyclones (such as

hydrocyclones) or elutriation can be used.

The sodium carbonate monohydrate crystal product

which is recovered typically has a particle size of greater

than at least about 150 mesh. Preferably, the product has a

particle size of greater than at least about 100 mesh, and

more preferably greater than at least about 80 mesh. More

particularly, the size cutoff for product recovery has to be at

least as large as or larger than the particle size of the feed so

that insoluble impurities initially in the feed are not recovered

with product.

Separation of sodium carbonate monohydrate crystals is

generally conducted by screening or cycloning and avoiding

drying of the crystals. Drying of the crystals at this stage

may result in cementing, or agglomerate formation, of

crystals and/or impurities, thereby reducing the purity of the

product (but not the purity of the crystals). Drying of the

crystals can be avoided or reduced by reducing or eliminating

evaporation of the solvent, or by covering the screen

with solvent or solvent vapors to maintain solvent saturation.

Alternatively, a pressurized and/or submerged size

separation process can be used, which ensures that local

evaporation of solvent is minimized or eliminated.

Once sodium carbonate monohydrate crystals are separated

from the saturated brine solution, they can be dehydrated

(i.e., dried) using known techniques to provide anhydrous

sodium carbonate.

The purity of crystals produced by the processes of the

present invention is at least about 99%, more preferably at

least about 99.5% and most preferably at least about 99.8%.

The term "purity of product" refers to the overall purity of

the product and can include impurities which can be present

on the surface of the crystals or which can be trapped within

agglomerates. The term "purity of crystals" refers to the 50

presence or lack of impurities within the crystal lattice

structure. In other words, the purity of product refers to the

purity of a particular batch of the product produced by the

process of the present invention, whereas the purity of

crystals refers to the purity of crystals within the product.

5.1 Physical Property of the Product

Unlike some of the current crystallization processes, the

process of the present invention does not utilize a crystal

modifier to affect the crystal shape of the product. The

majority of the product is block-like in shape, as discussed

above, and is surprisingly resistant to abrasion. Preferably at

least about 55% of the particles in the product is block-like

in shape, more preferably at least about 75%, and most

preferably at least about 95%.

It is believed that these block-like crystal are responsible

for a high bulk density observed in the product of the present

invention. The product of the present invention has a poured

US 6,284,005 Bl

during the settling process, the brine solution can be diluted

with water or a non-saturated brine solution to dissolve fine

sodium carbonate monohydrate crystals which may be

present. Furthermore, makeup water can be added as

required by the overall mass balance of the system.

Prior to being purged from the system, settled insoluble

impurities can be further concentrated, e.g., by filter press,

to recover at least a portion of the saturated brine solution.

In addition, the clear overflow and/or the clarified saturated

brine solution can be further clarified by filtration to remove

any fine insoluble impurities that may be present.

When the saturated brine solution is reused, it is desirable

that the temperature of the saturated brine solution in the

thickener is kept at no more than about 20° C. different than

the temperature of the saturated brine solution in the crystallizer

tank to minimize the energy cost of reheating the

saturated brine solution from the thickener. Preferably, the

difference in temperature between the saturated brine solution

and the saturated brine solution in the crystallizer tank

is about 15° C. or less, more preferably about 10° C. or less,

and most preferably about 5° C. or less.

8.0 Bicarbonate Control

It has been found that the crystal size and/or the shape can

be affected by the presence of sodium bicarbonate in the

saturated brine solution. Therefore, the process of the

present invention can further include maintaining the concentration

of sodium bicarbonate below about 10 gil in the

saturated brine solution which is added to the crystallizer 10,

more preferably below about 5 gil, and most preferably

about 0 gil. Larger sodium carbonate crystals can be grown

in crystallization processes when the amount of bicarbonate

present in the brine solution is maintained within these

limits. One method of controlling the sodium bicarbonate

level in the saturated brine solution is disclosed in a commonly

assigned, U.S. patent application Ser. No. 09/167,

627, filed on Oct. 6, 1998, which is incorporated by reference

herein in its entirety.

A further advantage of the present process which has been

recognized is that, in the absence of bicarbonate, crystals

which are grown have a more beneficial shape, e.g., a

well-formed block-like shape. In contrast, crystals grown in

the presence of significant amounts of sodium bicarbonate

can have a needle-like, dendritic or jack-shaped structure

and/or cloudy centers. Thus, crystals produced in accordance

with the present invention, having a more compact

and block-like shape, produce a material having a higher

bulk density and a lower friability than those produced in the

presence of a relatively large amount of bicarbonate.

In a preferred embodiment of the present invention, a

sufficient amount of base is used to reduce the concentration

of sodium bicarbonate to within the parameters discussed

above. Preferably, after neutralizing any initial sodium

bicarbonate in the crystallizer, base is added to the crystallization

process to maintain a concentration of at least about

0.75 molell of equivalent base, more preferably at least

about 0.50 molell, and most preferably at least about 0.25

molell. When sodium hydroxide is used as the base, after

neutralizing any initial sodium bicarbonate in the

crystallizer, the amount of sodium hydroxide used is preferably

at least about 6 gil, more preferably at least about 4

gil, and most preferably at least about 2 gil.

9.0 Aging

Processes of the present invention can also include transferring

at least a portion of the monohydrate slurry from the

crystallizer 10 and/or at least a portion of the screened

saturated brine solution into an aging apparatus (not shown).

The aging apparatus allows growth of at least a portion of

the crystals in the saturated brine solution by dissolving at

least a portion of fines and then promoting crystal growth by

relieving the supersaturation in the form of a crystal growth,

i.e., some mass of fine particles is converted to coarse

5 particles by a process of dissolving and recrystallizing.

As used in this invention, "aging" refers to a process of

dissolving some of the small crystals present in the saturated

brine solution and relieving at least a portion of the supersaturation

by growth on existing crystals. The aging can be

10 a natural equilibrium phenomena where crystals are constantly

being dissolved and recrystallized or it can be

achieved by diluting and concentrating the saturated brine

solution or simply by a temperature cycling process. The

aging process can be used to produce seed crystals or to

increase the amount and/or the size of the product. For

15 example, when the temperature of the saturated brine solution

in the crystallizer 10 is from about 80° C. to about 90°

c., it has been observed that by allowing the resulting

saturated brine solution to stir or stand for an additional

about 10 to about 15 minutes after the addition of the

20 feedstream and/or the seed crystals, the amount and/or the

size of larger sodium carbonate monohydrate crystals can be

significantly increased. This phenomena occurs at faster

rates at increased temperatures.

The temperature cycling process involves reducing the

25 temperature of the saturated brine solution at least about 10°

c., more preferably at least about 20° c., and most preferably

at least about 40° C. Alternatively, the temperature of

the saturated brine solution is reduced to less than about 70°

c., more preferably less than about 60° c., and most

30 preferably less than about 50° c., but always above 35° c.,

the top of stability range for sodium carbonate decahydrate.

As FIG. 2 shows, the solubility of sodium carbonate

increases as the temperature is reduced. Thus, reducing the

temperature of the saturated brine solution dissolves at least

35 a portion of the sodium carbonate monohydrate crystals. It

should be appreciated that while some fines may be completely

dissolved, some larger crystals may also be partially

dissolved during the temperature cycling process. When the

temperature of the saturated brine solution is increased, the

40 solubility of sodium carbonate decreases as shown in FIG.

2. This reduction in solubility causes relief of supersaturation

of the brine solution by growth of existing crystals or by

primary and/or secondary nucleation. By maintaining a

condition which limits the amount of primary and/or sec-

45 ondary nucleation as discussed above, the amount of fines

generated can be reduced and the crystal sizes can be

increased using an aging process.

As stated above, temperature cycling process can be

applied to the entire monohydrate slurry in the crystallizer or

50 to a slip stream, i.e., a portion, of the monohydrate slurry

such that a portion of the monohydrate slurry is cycled

through an external heat exchanger to reduce the temperature

of the monohydrate slurry.

When the temperature cycling is applied to the entire

55 monohydrate slurry as a whole, the process is typically

performed by cycling the crystallizer's temperature about

once an hour. If the temperature cycling is affected to a

portion of the monohydrate slurry through an external heat

exchanger, such temperature cycling is conducted on a

60 continuous basis while a portion of the monohydrate slurry

is continuously circulated through the heat exchanger. In one

particular embodiment of a temperature cycling process, a

heat exchanger is used for the temperature cycling process.

In this embodiment, the temperature of the monohydrate

65 slurry is typically lowered by at least about 5° c., more

preferably at least about 10° c., and most preferably at least

about 20° C.

US 6,284,005 Bl

17 18

25 Grams/liter

Feed Time to Add % Solids Supersaturation

Test # (gil) Feed (seconds) at End o min. S min.

1 30 10 12.2 15.8 7.1

2 60 10 15.7 22.5 5.9

30 3 120 15 25.5 26.3 1.5

TABLE 2

The results in Table 2 illustrate that high levels of supersaturation

can be obtained by practice of the present invention.

For example, in Test No.3, supersaturation of 26.3 gil

was present at the end of the feed addition. The results

further illustrate that the supersaturation is rapidly relieved.

For example, in Test No.3, the amount of supersaturation at

the end of feed addition went from 26.3 gil to 1.5 gil at 5

minutes after the end of feed addition.

The foregoing description of the present invention has

been presented for purposes of illustration and description.

Furthermore, the description is not intended to limit the

invention to the form disclosed herein. Consequently, variations

and modifications commensurate with the above

teachings, and the skill or knowledge of the relevant art, are

within the scope of the present invention. The embodiment

described hereinabove is further intended to explain the best

mode known for practicing the invention and to enable

others skilled in the art to utilize the invention in such, or

other, embodiments and with various modifications required

by the particular applications or uses of the present invention.

It is intended that the appended claims be construed to

include alternative embodiments to the extent permitted by

the prior art.

What is claimed is:

1. A process for producing sodium carbonate monohydrate

from a feedstream comprising anhydrous sodium carbonate

and impurities, the process comprising:

(a) adding the feedstream to a saturated sodium carbonate

brine solution at a rate of at least about 100 gil/min

under conditions to create supersaturation of at least

about 5 gil;

(b) processing within parameters that preferentially

relieve the supersaturation by rapid growth of existing

sodium carbonate monohydrate crystals over nucleation;

and

A four liter vessel with intense agitation was partially

filled with a slurry of 65x100 mesh sodium carbonate

monohydrate seed crystals and heated to 88° C. Minus 150

mesh calcined trona, heated to 125° C. was added rapidly to

5 the vessel. Immediately after addition of the calcined trona

was complete, the concentration of dissolved sodium carbonate

in the brine was determined by withdrawing the brine

through a screen and filter to exclude seed crystals and

calcined trona. Water was evaporated from the withdrawn

10 brine to produce a solid residue. The quantity of sodium

carbonate per gram of withdrawn brine was gravimetrically

determined. The quantity of sodium carbonate in excess of

the solubility limit of sodium carbonate is the amount of

supersaturation. A second sample was taken 5 minutes after

15 feed addition was complete to evaluate the amount of

supersaturation at that time and the amount of relief of

supersaturation in the 5 minute interval.

Three tests were run with the amount of feed being varied.

The amount of feed added, the time of addition, the percent

20 solids, and the amount of supersaturation at 0 minutes and

at 5 minutes are shown below in Table 2.

EXAMPLE 1

This example illustrates the high capacity for supersatu- 65

ration of sodium carbonate and a technique for measuring

the same.

10.0 Fines Scavenging

As a means for improving the product yield, the slurry of

fine particles remaining after the product size monohydrate

crystals have been removed can be further processed to

recover the soda ash values present in the slurry of fines. The

slurry of fines can also include impurities which were

present in the feedstream and any fine sodium carbonate

monohydrate crystals which are smaller than the product

size. One technique for processing the slurry of fines to

improve the product yield is to use a pressure slurry system

as described below.

10.1 Pressure Slurry System Crystallization

In this process, the slurry of fines is thickened to a

relatively high solids content, preferably to at least about

17% solids by weight, more preferably to at least about 25%

solids by weight, even more preferably to at least about 40%

solids by weight, and most preferably to at least about 60%

solids by weight. The slurry of fines can be thickened by a

conventional gravity thickener, a membrane filter, or any

suitable device that permits decanting saturated brine from

the slurry of fines while retaining the solids.

The thickened slurry of fines is then pumped into a

pressure vessel operating above the transition temperature of

monohydrate sodium carbonate to anhydrous sodium carbonate.

In general, this vessel is operated at a temperature of

at least about 7° C. above the transition temperature. In the

pressure vessel, the incoming slurry is heated above the

transition temperature of monohydrate sodium carbonate to

anhydrous sodium carbonate. This heating converts sodium

carbonate monohydrate to anhydrous sodium carbonate. The

resulting anhydrous sodium carbonate slurry is then added to

the feedstream or to the crystallizer directly. In this manner,

the slurry of sodium carbonate monohydrate fines is

recycled to the crystallization process of the present invention

to increase the amount of sodium carbonate recovery.

Depending on the yield of each stage of crystallization, a 35

pressure slurry system for fines scavenging can be repeatedly

used. Because the operating and capital costs in each

stage of crystallization processes of the present invention are

relatively low, having a multiple stage pressure crystallization

process can be readily justified economically. The use of 40

a multiple stage crystallization process increases the yield of

sodium carbonate from a depletable resource such as trona.

11.0 Product Purity Control

Although processes of the present invention provide product

crystals of a purity level as described above, in some 45

cases, such as when soluble impurities are present in the

feedstream, it may be necessary to utilize a multiple stage

crystallization process to achieve the product having the

above described purity level.

Crystals are produced in a first stage of crystallization. 50

These crystals are mechanically dewatered and repulped in

brine from a second stage of crystallization in the process.

This repulped slurry is fed to the second stage pressure

slurry crystallization system as described above. The recrystallization

that takes place in this second stage will produce 55

crystals containing less soluble impurities than were present

in the product of the first stage recrystallization. This process

can be repeated with as many stages as are required to get

the desired purity levels.

The following example is provided for purposes of illus- 60

tration and is not intended to limit the scope of the present

invention.

US 6,284,005 Bl

monohydrate crystals from the saturated brine solution.

2. The process of claim 1, wherein the supersaturation is

at least about 10 gil.

3. The process of claim 1, wherein the supersaturation is 5

at least about 20 gil.

4. The process of claim 1, wherein the feedstream is

produced by a process comprising mixing anhydrous

sodium carbonate with a saturated sodium carbonate brine

solution at a temperature above the transition temperature 10

between sodium carbonate monohydrate and anhydrous

sodium carbonate.

5. The method of claim 4, wherein the aqueous solution

is at above atmospheric pressure and at a temperature above

the atmospheric boiling point of the aqueous solution and 15

wherein the feed slurry is introduced by a feeder that

maintains a continuous pressure seal between the environment

of the feed slurry and of the aqueous solution.

6. The process of claim 1, wherein the step of relieving the

supersaturation preferentially by rapid growth of existing 20

sodium carbonate monohydrate crystals over nucleation

comprises adding sodium carbonate monohydrate seed crystals

to the saturated sodium carbonate brine solution.

7. The process of claim 6, wherein the seed crystals are

produced by removing sodium carbonate monohydrate crys- 25

tals from the brine solution and sizing the removed crystals

to produce a seed crystal size fraction for reintroduction to

the brine solution.

8. The process of claim 6, wherein the particle size of the

feedstream is less than the particle size of the seed crystals. 30

9. The process of claim 6, wherein the range of the particle

size of the seed crystals is not greater than about 3 standard

SIeve sIzes.

10. The process of claim 6, wherein the particle size of the

feedstream is less than about 150 mesh. 35

11. The process of claim 6, wherein the particle size of the

seed crystals is from about 100 mesh to about 150 mesh.

12. The process of claim 1, wherein the step of relieving