4,341,744 Soda ash production

Patent Number/Link:

United States Patent [19]

Brison et al.

[11]

[45]

4,341,744

Jul. 27, 1982

[54] SODA ASH PRODUCTION

[75] Inventors: Robert J. Brison, Arvada, Colo.;

Michael E. Webber, Martinez, Calif.

[73] Assignee: Stauffer Chemical Company,

Westport, Conn.

[21] Appl. No.: 95,035

[22] Filed: Nov. 16, 1979

10 Claims, 3 Drawing Figures

OTHER PUBLICATIONS

Fraas, F., Effect of Temperature on the Electrostatic

Separation of Minerals, Bureau of Mines Report of

Investigation 5213 Apr. 1956.

Applied Electrostatic Separation Efficient Ore Dressing

Technique, The Chemical Age Metallurgical Section,

pp. 237-239, Sep. 1943.

Primary Examiner-Gary P. Straub

Attorney, Agent, or Firm-Paul R. Martin; M. Henry

Heines

Soda ash is produced from crUde trona ore in a novel

process which comprises

(a) reducing the ore particle size to a maximum of about

4.0 millimeters in diameter,

(b) removing fines from the ore to produce a minimum

particle size of about 0.1 millimeter in diameter,

to differences in conductance,

(d) segregating the ore particles by electrostatic separation

into at least two fractions according to the differences

in electrical charge resulting from the electrification

of step (c), and

(e) calcining the fraction of least conductance to convert

the trona contained therein to soda ash,

steps (a) through (d) occurring at a temperature not to

exceed about 100· C.

[57] ABSTRACT

Related U.S. Application Data

Continuation-in-part of Ser. No. 5,644, Jan. 22, 1979,

abandoned.

Int. Cl.3 C22B 26/10; B03C 7/00;

B03C 1/00; COlD 31/24

U.S. Cl 423/206 T; 23/302 T;

209/127 R; 209/127 A; 209/128; 209/214

Field of Search 423/206 T; 204/153,

204/164; 209/127 R, 127 A, 127 B, 128, 127 C,

129, 130, 214; 23/298, 302 T

References Cited

U.S. PATENT DOCUMENTS

1,872,591 8/1932 Homan 209/128

2,071,460 2/1937 Grave 209/128

2,197,864 4/1940 Johnson 209/129

2,548,771 4/1951 Carpenter 209/127

2,765,074 10/1956 Diamond 209/127 A

3,022,890 2/1962 Snow 209/11

3,244,476 4/1966 Smith 423/206 T

3,819,805 6/1974 Graves et aI. 423/206 T

[56]

[51]

[52]

[58]

[63]

+30 MESH

MAGNETIC

REJECT

ELECTROSTATIC

REJECT

CONDUCTOR

FRACTION

100 MESH

MAGNETIC

FRACTION

u.s. Patent Jul. 27,1982 Sheet 1 of 3

+30 MESH

4,341,744

VIBRATING SCREEN

-30 MESH

HEATED

AIR ---I AIR CLASSIFIER I----.,.;-I:..=.O=-O..:.::M~E.::;.:SH:..:...__..IDUST COLLECTOR

45°C

FINES TREATMENT

DIFFERENTIAL ELECTRIFICATION CONDUCTOR

AND I----_--...:.F..:.;R:.:..;A:..=.C...:..;TI:..::O.::,N _

ELECTROSTATIC SEPARATION

NONCONDUCTOR

FRACTION

MAGNETIC SEPARATION

NONMAGNETIC

FRACTION

MAGNETIC

FRACTION

MAGNETIC

REJECT

ELECTROSTATIC

REJECT

FIGURE I

u.S. Patent Jut 'Z7, 1982 Sheet 2 of 3 4,341,744

FIGURE 2

u.S. Patent Jui. 27, 1982 Sheet 3 of 3 4,341,744

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FIGURE 3

SUMMARY OF THE INVENTION

this type are disclosed in U.S. Pat. Nos. 2,346,140;

2,639,217; 2,798,790; and 3,028,215.

In an alternative method, the mined trona is crushed,

calcined, dissolved in an aqueous solution, clarified and

5 filtered. The clear filtered solution is then evaporated to

form sodium carbonate monohydrate crystals which are

separated from the mother liquor. This mother liquor is

either recycled to the crystallizers or a portion thereof

is returned to dissolve more calcined trona. The mono-

10 hydrate crystals are then calcined to dense ash. This

route for producing soda ash has a heavy evaporation

load, entailing a high capital cost for evaporative equipment.

Processes of this type are disclosed in U.S. Pat.

Nos. 2,343,080; 2,343,081; 2,962,348; 3,131,996; and

15 3,260,567.

A further alternative involves the preparation of anhydrous

sodium carbonate by maintaining the temperature

in the crystallization units above about 109· C.,

which is the transition temperature at which anhydrous

sodium carbonate is formed as the stable crystal phase.

See U.S. Pat. No. 2,770,524. Still another method involves

the preparation of sodium bicarbonate which in

tum may be calcined and converted to dense sodium

carbonate.A procedure of this type is disclosed in U.S.

Pat. No.2,704,239.

Each of these processing techniques involves dissolution,

clarification, filtration and crystallization, with

expensive reagents and recovery steps, adding substantially

to the cost of the final product.

Accordingly, it is an object of the present invention

to provide a relatively simple and inexpensive process

for the production of soda ash from trona ore.

Further objects will be apparent from the following

description.

4,341,744

SODA ASH PRODUCTION

BACKGROUND OF THE INVENTION

Na2C03.NaHC03.2H20

The naturally occurring mineral, sodium sesquicarbonate,

is a well known source of sodium compounds, particularly

useful for the production of soda ash for the glass

industry. This mineral, commonly referred to as trona,

occurs in large deposits in Wyoming. These deposits are 20

approximately 85% to 95% sodium sesquicarbonate,

with the remainder consisting of insoluble impurities

such as oil shales,shortite (Na2C03.2CaC03), and miscellaneous

sedimentary materials. The water soluble

constituents other than sodium sesquicarbonate are 25

mostly other types of sodium compounds, such as sodium

chloride and sodium sulfate.

The sodium content can be readily extracted from the

ore by dissolution, separation of the insoluble materials

from the solution, and evaporation and cooling of the 30

solution to crystallize therefrom the sodium carbonate

values in different forms. The crystals are then calcined

to produce soda ash. This process produces a highly

refined product by a complicated and expensive technique.

Many industries, such as the glass industry, do 35

not always require such a high grade of soda ash. Certain

levels of impurities are acceptable, depending on

the quality of glass to be produced. In fact, the major It has now been discovered that trona ore can be

raw materials used in the manufacture of clear glass beneficiated by electrical means, in which particles of

other than soda ash all have substantially the same im- 40 the ore are segregated according to differences in elecpurities

associated with crude trona. Beneficiation, or trical conductivity. Whereas techniques of electrostatic

purification, of the trona is necessary, however, before separation have not heretofore been applied to trona

it can be used for glass manufacture, since the amount of ore, the present invention entails the discovery that

impurities varies and products of consistent quality there is a sufficient difference in conductivity between

cannot otherwise be obtained. 45 trona and the impurities in trona ore to permit a separa-

Various methods have been used to purify crude tion by electrostatic means. By virtue of this discovery,

trona. In one such method, mined trona is crushed to commercial grade soda ash can be produced using subabout

8-mesh and dissolved in a hot recirculating trona stantially fewer processing steps, eliminating the need

mother liquor carrying more normal carbonate than for storing and processing large quantities of dissolving

bicarbonate so that the sodium carbonate and bicarbon- 50 liquors and avoiding much of the energy required for

ate in the mined trona are both dissolved. The insoluble crystallization.

material is settled out of the solution in clarifiers and the In particular, this invention consists of a process for

solution polished by filtration. Sodium sesquicarbonate the production of soda ash from trona ore which comis

crystallized and separated from the hot solution, then prises

calcined to soda ash. The mother liquor remaining after 55 (a) reducing the ore particle size to a maximum of

the crystallization can be recycled to the dissolving about 4.0 millimeters in diameter,

tanks to dissolve more crude trona. A portion of the (b) removing fines from the ore to produce a minimother

liquor can be passed to the crystallizers to form mum particle size of about 0.1 millimeters in diameter,

a second crop ofsodium sesquicarbonate crystals. This (c) differentially electrifying the ore particles accordprocessing

route contains several drawbacks. First, the 60 .ing to differences in conductance,

carbonate-bicarbonate ratio in the recycling mother (d) segregating the ore by electrostatic separation

liquor must be maintained. Second, the relatively low into at least two fractions according to the differences in

concentration of salts per unit of solution entails the use electrical charge resulting from the electrification of

ofcostly large scale processing equipment to recover step (c), and

the sesquicarbonate. Sequestrants are required to facili- 65 (e) calcining the fraction of least conductance to

tate filtration, and crystallizing aids are needed to pro- convert the trona contained therein to soda ash,

vide particles of the desired size and shape. Both of steps (a) through (d) occurring at a temperature not to

these additives contaminate the product. Processes of exceed about 100· C.

CROSS REFERENCE TO RELATED

.APPLICATIONS

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

application Ser. No. 005,644, filed Jan. 22, 1979,

now abandoned.

the basis of particle size. Although the classifier shown

is external to the size reduction unit, internal classifiers

can also be used. In the figure, the classifier separates

the ore into two fractions, one being a nominal30X 100

5 mesh fraction, and the other nominally smaller than

loo-mesh. The latter is· directed to a dust collector.

Once collected in this manner, the fines may either be

discarded, or converted to soda ash by one of the conventional

techniques mentioned to the "Background of

10 the Invention" above.

Greater separation efficiency in the electrostatic separation

unit can be achieved by further narrowing the

particle size range fed to the unit, independent of the

average size of each range. Thus, if the size ranges

15 stated above are further classified into two or more size

fractions, an overall improvement in separation efficiency

will result if each fraction is fed separately to the

separation unit. Such size classification can be performed

by the same type of equipment described above.

20 Temperature control of the ore to avoid excessive

temperatures is necessary during all process steps up to

and including the electrification and segregation steps.

Trona ore is susceptible to calcination at high temperature,

and since calcined trona is considerably less responsive

to electrostatic separation than trona which

has not been calcined, high temperatures must be

avoided.

The primary source of high temperatures is external

heat optionally applied to the ore prior to separation for

the purpose of removing moisture. A low moisture

content is essential to the optimum separation of the ore

components, since water itself acts as a conductor and

renders particles conductive which are otherwise non-

35 conductive. Depending on mining conditions, of

course, it may not be necessary to apply any heat at all

to achieve the desired moisture level. I.e, the ore may be

sufficientlydry as mined, or it may be made so by exposure

to dry air for a period of time if the air is drier than

the ore. External heat, however, serves to hasten the

drying process when the ore is damp, and is particularly

helpful when the atmosphere has a high relative humidity.

The danger of encountering excessive temperatures

will vary, therefore, depending on the extent to which

external heating is applied. In any event, it is essential

that the temperature of the ore in steps (a) through (d)

of the above-described process be maintained below

about 100° C. Temperatures within this range are fully

sufficient to dry the ore for optimum separation, particularly

after the particle size has been reduced. It is preferable

to dry the ore particles at a temperature not to

exceed about 60° C. for a period of time ranging from

about I minute to about 20 minutes, depending on the

55 dampness of the ore and the surrounding air. In the

figure, drying is accomplished by feeding air heated to

45° C. to the air classifier. Thus, drying and fines removal

are combined in a single operation. The dried

and appropriately sized ore particles are then collected

in a surge bin from which they are fed to the electrification

and electrostatic separation units.

The particles are then differentially electrified according

to differences in conductance. The phrase "differential

electrification" is used herein to denote any

charging technique which produces charged particles

in which the polarity and/or amount of charge are

different for particles of different conductance. Differential

electrification is generally achieved by conduc-

4,341,744

DESCRIPTION OF THE PREFERRED

EMBODIMENTS

FIG. 1

BRIEF DESCRIPTION OF THE DRAWINGS

This invention is further illustrated by the accompanying

drawings in which:

FIG. 1 represents a generalized flow sheet showing

the complete process in which mined trona ore is converted

to commercial grade soda ash. All elements of

the present invention are included in this flow sheet.

FIG. 2 represents an embodiment of the portion of

the process involving the differential electrification and

electrostatic separation steps. In this embodiment, these

steps are both performed in a single apparatus and differential

electrification is achieved by ion bombardment

and conductance.

FIG. 3 represents a further embodiment ofthe electrification

and electrostatic separation steps, performed on

a single apparatus. Here, however, electrification is

achieved by conductance without ion bombardment.

The mined ore must be reduced to particulate form

before a substantial separation between the conductor

and non-conductor particles can occur. Thus, size re- 25

duction by crushing or grinding is generally necessary

prior to feeding the ore to the separator. The size reduction

must be sufficient to substantially break up aggregates

which contain both trona and impurities, so that

these components can be substantially divided into sep- 30

arate fractions. Effective separation can be achieved

with a maximum size of about 4.0 millimeters (mm)

(5-mesh, Tyler Standard Sieve Series). The preferred

maximum size is about 2.0 mm (9-mesh) and the most

preferred is about 0.6 mm (30-mesh).

The size reduction for the instant invention can be

accomplished by any conventional technique. An undesirable

feature of size reduction, however, is the generation

of fines which accompanies every size reduction

technique. Fines are detrimental to the electrostatic 40

separation process, and are removed in a separate step

subsequent to the size reduction..For maximum efficiency,

however, the size reduction technique should be

carefully selected such that a minimum proportion of

fines is produced. Impact crushing, notably that which 45

occurs in cage or hammer mills, is a preferred technique.

In the flow sheet of FIG. 1, a multistage size

reduction is shown, which consists of a preliminary

coarse crushing, followed· by finer crushing in a hammer

mill, from which the emerging particles are passed 50

through a 30-mesh vibrating screen. Those particles

which are too large to pass through the screen are recycled

to the hammer mill for further crushing. The use of

multiple stages is helpful in maintaining a. minimum

amount of fines generation.

As indicated above, fines must be eliminated from the

ore prior to feeding the ore to the separator. Preferably,

fines should be removed as soon as practical after the

size reduction step in order to minimize the adhesion of

fme particles to coarse particles, since such adhesion 60

interferes with electrostatic separation. The minimum

particle size fed to the separator should be about 0.75

mm (2oo-mesh), preferably about 0.15 mm (loo-mesh).

Conventional screening operations are suitable for this

purpose, as are other common types of sizing equip- 65

ment. An air classifier, as shown in FIG. 1, is a typical

example. Air classifiers are pneumatic devices forseparating

material into two or more fractions primarily on

4,341,744

tion, or conduction in conjunction with ion bombardment.

In electrification by conduction, the particles are

placed in contact with a grounded electrode in the

presence of an electric field. The field induces a surface 5

charge on the grounded electrode which is opposite in

sign to that of the electrode generating the field, and the

conductor particles undergo an electron transfer with

the ground. In the instant process, the trona impurities

behave as conductors. Their ability to transfer an elec- 10

trical charge to or from the ground by conductance

produces a charge differential between them and the

trona particles, which are relatively unable to make

such a transfer, behaving as non-conductors. The field

can be generated by an electrode which is either posi- 15

tively or negatively charged, with similar results.

In ion bombardment electrification, a stream of mobile

ions is provided by a corona discharge resulting

from. a pointed or small surfaced electrode, such as a

wire, a surface with a sharp edge, or a series of sharp 20

points. The ore particles are placed in contact with a

grounded conductor in the path of these mobile ions. A

charge differential occurs between the trona particles

and the impurities by virtue of the greater ability of the

impurities to transfer the charge resulting from the 25

bombarding ions to the ground, and alsoto accept from

the ground the opposite charge induced thereon by the

, electric field. The term "high tension" is generally used

to characterize electrostatic separators using ion bombardment

for electrification. Although ion bombard- 30

ment is not an electrostatic' method of charging, the

term "electrostatic separation" is commonly used to

include systems charged by high tension discharges as

well as true electrostatic fields.

The grounded conductor upon which the' particles 35

are placed in the electrification step may either be stationary

or mobile. The particles may either be in motion

relative to the surface, or in motion with the surface.

For example, the configuration may range from an

inclined surface down which the particles slide under 40

the force of gravity, to a moving conveyor belt or a

rotating cylinder to carry the particles through the

electric field. It is not necessary for the particles to be in

continuous contact with the surface in order to obtain

the differential charging effect. The surface may be of 45

any conductive material, such as zinc, copper, iron,

aluminum, tin, chromium, and alloys of the above.

Either following or simultaneous to the differential

electrification step, the ore particles are segregated into

at least two fractions according to differences in con- 50

ductance. This is accomplished by electrostatic separation.

Although frequently used in the art to encompass

both the electrification and segregation steps, the term

"electrostatic separation" is used herein to denote the

segregation step alone. In this specification, the term 55

refers to the segregation resulting from the electrical

attraction of a portion of the ore mixture toward an

electrode or the repulsion of the portion from an electrode

as a result of the charge differential existing between

the portion and the rest of the ore mixture. 60

A variety of techniques can be applied to accomplish

this segregation, depending on the relative charges of

the conductor and· non-conductor particles and the

manner of electrification. In one embodiment, the differentially

charged particles are placed in free-fall be- 65

tween two oppositely. charged electrodes, and a positively

charged portion is deflected laterally toward the

negative electrode, or vice versa. In other embodi-

ments, the differentially charged particles remain in

contact with the grounded conductive surface upon

which the electrification took place, and are selectively

held to the surface, repelled from the surface, or attracted

to a charged electrode, depending on the electrical

charge on each particle. Thus, as indicated in the

figure, the electrification and electrostatic separation

steps can be combined in a single unit. Conductor particles

can be displaced by a non-electrical force such as

gravity or centifugal force. Non-conductor particles

can subsequently be displaced by mechanical means,

such as a brush. In addition, electrical means of neutralizing

the charge on the non-conductor particles can be

used to facilitate their removal. A variety of applicable

techniques is well documented in F. Fraas, "Electrostatic

Separation of Granular Materials," U.S. Dept. of

the Interior, Bureau of Mines, Bulletin #603, 1962.

Regardless of the technique used, however, the result

is a conductor fraction, anon-conductor fraction, and

optionally one or more middlings fractions. Only the

conductor and non-conductor fractions are shown in

the figure. Separation efficiency can be significantly

enhanced by recycling the middlings fraction. Greater

separation can also be achieved with an array of separator

units connected in parallel, in series, or both. In this

case, one or more intermediate products can be recycled

to improve the efficiency of separation. The nonconductor

fraction, i.e., the fraction of least conductance,

contains the highest proportion of trona. Once

isolated from the remainder of the ore, this fraction can

be forwarded directly to the calcining unit.

Magnetic separation is an optional additional processing

step which can be used to supplement the electrostatic

separator and provide a product of greater purity.

The magnetic separation unit is shown in FIG. 1 subsequent

to the electrostatic separation unit and prior to

the calciner. Any of the several high intensity dry magnetic

separation units known in the art will be suitable

for this purpose. Examples include the induced-roll and

cross-belt separators. The preferred unit is the highintensity

induced-roll magnetic separator. As in the case

of the electrostatic separator, the magnetic separator

produces at least two fractions, ranging from a magnetic

fraction to a non-magnetic fraction. Only these

two fractions are shown in the figure, although middlings

can be taken for recycle. Multiple units arranged

in parallel, series, or both, and recycling of intermediate

products are appropriate here as in the electrostatic

separation step. The least magnetic fraction contains the

highest proportion of trona.

Any conventional calciner can be used to convert the

trona to soda ash. The reaction which occurs is as follows:

2(Na2C03.NaHC03.2H20}->3Na2C03 + 5H20

t +C02 t

This can be accomplished in general by any method of

transferring heat to the particles. Included among commercial

calcining units are direct heat units, indirect

heat units, and fluidized bed systems.

The temperature required for full calcination of the

trona depends on the particle size and the amount of

time the particles are exposed to the heat. Thus, there is

no critical range. For practical operating conditions,

however, calcination is conducted between about 90° C.

and about 200° C., preferably between about 120° C.

and about 1800 C. In a direct heat calciner, full converFIG.

brush 9 removes the residual particles from the cylinder

surface. This final fraction, containing the highest concentration

ofnon-conductor particles, and consequently

the highest concentration of trona, is collected in bin 10,

from which it is taken to a calcining unit.

FIG. 3 illustrates a plate-type separator, again combining

the electrification and segregation functions. The

crushed and screened crude ore particles are delivered

from a feed hopper 11 to a stationary grounded conductive

plate 12. The particles flow down the plate by

gravitational force, entering an electrostatic field induced

by a curved electrode 13. Although a positive

15 charge is shown in the drawing, the electrode 13 may be

of either positive or negative charge. The electrostatic

field created by the electrode 13 induces a surface

charge on the ground plate 12 of a sign opposite to that

of the electrode. The conductor particles then accept a

charge from the plate surface 12 by conduction. These

particles, now bearing a charge of the same sign as plate

12 but opposite to that of the electrode 13, are repelled

from plate 12 and attracted toward the electrode 13 by

electrostatic force. The resulting electrostatic force

induces the conductor particles to veer outward from

the plate 12. They are subsequently collected in a bin 14.

The non-conductor particles are relatively unaffected

by the electrostatic field created by the electrode 13.

They thus remain generally in contact with the

grounded surface 12 by gravitational force until they

fall into collector bin 15. As in FIG. 2, middlings fractions

can be collected between the conductor and nonconductor

fractions, although they are not shown in

FIG. 3.

The separators shown in FIGS. 2 and 3 are well

known and readily available from commercial suppliers.

However, there are many variations of these two configurations

available. The two shown here were selected

as typical examples.

A significant difference between the rotor and platetype

separators shown in the drawings is the extent to

which the non-conductor particles adhere to the surface

in each case. Gravity alone is sufficient to separate the

non-conductor particles from the surface in FIG. 3,

whereas a wiper electrode and a tensioned brush are

shown in FIG. 2. This is one demonstration of the structural

and operational differences among the various

types of electrostatic separators which can be used in

the process of the invention. Many other variations are

also possible within the scope of the invention. The

common operative feature, however, is the segregation

of the conductor particles from the non-conductor particles

by virtue of the difference in conductivity between

them. Specifically, the ability of the conductor

particles in the trona ore to accept a charge from or

transmit or relinquish a charge to a grounded surface by

conductance, in conjunction with the inability of the

non-conducting trona particles to do so, permits the

separation to occur.

The high tension separator shown in FIG. 2 generally

affords a' more efficient separation between conductor

and non-conductor particles. However, wide variations

in separation efficiency in any given apparatus can be

achieved by adjustment of the various system parameters,

such as rotor diameter, rotation speed, electrode

potential, and spatial configurations. In addition, recycling

of the conductor, non-conductor, or middlings

fractions will produce a more complete separation in

4,341,744

FIG. 2

FIG. 2 illustrates a high tension rotor-type separator

which combines the electrification and segregation

steps. The crude ore, which has already been crushed 20

and screened, is delivered in particulate form from a

feed hopper 1 onto the surface of a grounded conductive

rotating cylinder 2. The rotation of the cylinder,

shown clockwise in the drawing, carries the particles

into the field of influence of an electrode 3 which 25

carries a DC charge. While the electrode charge is

shown as positive in the drawing, a negative charge can

be used to similar effect. This charged electrode induces

an opposite charge (negative in this case) on the surface

of the grounded rotor 2. A wire 4 attached to the elec- 30

trode 3 creates a corona discharge, resulting in a stream

of ions bearing the same charge as the electrode 3, and

traveling toward the grounded cylinder 2. All particles

in the path of these ion accept a strong electrical charge

from them, although the conductor particles quickly 35

lose their charge to the grounded cylinder 2 and may

acquire the same charge as the cylinder surface.

The conductor particles bear no electrical attraction

to the cylinder 2 and are selectively displaced from the

cylinder surface by centrifugal and gravitational forces, 40

as well as by electrostatic repulsion. In contrast, the

nonconductor particles adhere to the surface by virtue

of the electrical attraction between the positive charge

they bear and the negative charge induced on the cylinder

surface. Under most conditions, a greater degree of 45

selectivity can be achieved when a static or non-ionizing

electrode 5 is used following the ionizing electrode

3.. Bearing the same charge as the ionizing electrode 3,

the static electrode 5 serves to attract the oppositely

charged conductor particles, causing a more distinct 50

separation. Note that in this embodiment, both ion bombardment

and conductive charging occur in a single

apparatus. The static electrode 5 is optional, however,

since substantial selectivity is obtained using the ionizing

electrode 3 alone. In either case, the fraction con- 55

taining the highest concentration of conductor particles

is displaced the greatest distance from the rotating cylinder

2, and falls into a collector bin 6.

As in nearly all physical mineral separation processes,

a complete separation is not made in a single cycle. 60

Segregation of conductor particles from non-conductor

particles can be enhanced, however, by taking one or

more middlings fractions. A single middlings fraction is

shown in the figure, and is collected in a separate bin 7.

In a position further along the direction of rotation 65

the decay of the charges on the non-conductor particles

is optionally assisted by a high voltage wiping electrode

8 of alternating or direct current. Finally, a tensioned

sion can be accomplished in a residence time of about 5

to 10 minutes. In an indirect heat calciner, such as a

steam tube calciner, slightly longer residence times are

generally required.

Following calcination, the soda ash can be cooled by 5

a stream of cold air, from which the warm effluent can

be fed to the air classifier to supplement the drying

process prior to the electrostatic separation. The cooled

soda ash is then stored until ready for shipping, at which

time it can be transferred directly to a shipping vessel by 10

conveyor belt, hoppers, feeders, etc.

The process as described above can be operated in

continuous or batch-wise fashion. Many further variations

are possible, as will be apparent to one skilled in

the art.

rial remained in the middlings fraction. The conductor

fraction was then set aside.

The non-conductor fraction was then fed back

through the hopper, and the resulting middlings fraction

.was again recycled until a negligible quantity remained

in the middlings bin. Three fractions then remained:

the original conductor fraction (referred to

hereinafter as "conductor"), the second conductor fraction

(referred to hereinafter as "intermediate"), and the

final non-conductor fraction (referred to hereinafter as

"non-conductor"). Each of these fractions was weighed

and analyzed for its iron content. The iron content was

determined by dissolving the sample in concentrated

15 hydrochloric, nitric, perchloric, and hydrofluoric acids.

The dissolved sample was then analyzed for its iron

content by atomic absorption.

Several of the samples were also analyzed for the

content of material insoluble in water and the content of

material insoluble in aqueous hydrochloric acid. Since

trona is water-soluble, the quantity insoluble in water

substantially represents the portion of the trona ore

other than trona itself. Of the water-insoluble material,

the portion soluble in aqueous HCl is primarily shortite.

Thus, the quantity insoluble in aqueous HCI substantially

represents the portion of the ore other than shortite

and trona.

Table I lists the conditions and results obtained from

30 ten individual samples of trona ore taken from the same

location in a mine at Green River, Wyo. The ore particles,

originally of diameter less than one inch (2.54 em),

were hammer-milled and screened. The particles finally

used for the hopper feed were those remaining between

3D-and lOO-mesh screens, which had aperture sizes of

0.0203 inch (0.516 em) and 0.0055 inch (0.0140 em),

respectively. The feed analysis of an average sample of

this material is shown above the data for these tests. The

data for Example 11 was derived from a sample taken

from a different location in the mine, although crushed

and screened in the same manner as the first sample.

Again, the feed analysis is shown above the test results

for this test.

The data generally shows that the iron content of the

non-conductor fraction is sharply reduced from that

originally present in the feed. It is also evident that the

amounts of water-insoluble and aqueous HCI insoluble

materials in the non-conductor fractions are substantially

reduced. Comparison of tests I and 2 with tests 3,

4, and 5 shows that improved separation results when

both the feed rate and rotor speed are increased.

In Runs 6-10, the ore particles were tumbled in a

blender prior to being fed to the hopper in an effort to

simulate the effects of normal ore handling to be expected

in a commercial operation. The samples were

tumbled for 2, 4, 8, and 16 minutes, respectively. The

result was only a slight increase in the iron assay of the

non-conductor fraction.

. In general, the data clearly show that a non-conductor

fraction of sharply reduced iron content was obtained

in each case. In addition, the figures for tests 3, 4,

5, and 10 further indicate that the quantity of material

which was insoluble in either water or aqueous HCI was

also sharply reduced. These indicators show that the

non-conductor fraction consisted of a substantially beneficiated

trona product.

4,341,744

EXAMPLES 1-11

Rotor-Type Separator Test Results

A seriesofbatch~wiseseparations were performed on

a high tension rotary separator of the type shown in

FIG. 2. The apparatus used was a Carpco@ Research

Model High Tension Separator. The separator contains

a rotating cylinder, 6 inches (15.2 em) in diameter and 6 35

inches (15.2 em) in length, as the grounded surface. A

pinning (Le., ionizing) electrode and a lifting electrode,

corresponding to electrodes 3 and 5 of FIG. 2, were

used. Both were cylindrical in shape, with a wire, at~ 40

tached, extending a small.distance from' the cylinder

surface and running the length of the cylinder,parallel

to the cylinder axis. No wiper electrode, corresponding

to electrode 8 on FIG. 2, was used. The pinning electrode

was situated at an angle of 31.so from the horizon- 45

tal using the rotor center line as the apex.The electrode

was arranged such that the wire was positioned directly

between the electrode and the rotor surface, at a distance

of 3. em from the rotor surface. The.lifting elec~

trode was placed at an angle of 7.5" above the horizon- 50

tal, with the wire positioned on the side of the electrode

away from therotor surface, essentially eliminating the.

effect of this wire. The distance between the electrode

surface and the rotor surface was 4.1 em. A voltage of

22 kilovolts was imposed on each electrode. 55

Three bins were placed below the rotor, with adjustable

splitter panels positioned above the walls dividing

the bins.

Prior to the experiment, the~iied feed particles were 60

dried ina convection-type 'oven at 38° C.for thirty

minutes. After they were removed from the oven, the

particles were transferred: directly to the feed hopper

where approximately the same temperature was maintainedby

the, use of a heatlamp. The feed was then run 6S

through the separator. :After initial separation, the mid~

dlings fraction was fed back into the hopper; This pro-

.cedure was repeated until a negligible amount of mate-

.. 9

the final product. In a typical plant size configuration,

several individual separator units may be combined in

series to promote even further segregation without the

use of middlings fractions. As one example, non-conductor

fractions are taken from each of a series of sepa- 5

rator units and combined downstream, while the conductor

fractions from each unit are used as the feed for

the next unit in the series. The reverse configl,lration

may also be used, whereby the conductor fractions 10

from each unit are combined downstream while each

non-conductor fraction forms the feed for the succeeding

unit. Hybrid configurations can also be used,

whereby these two flow schemes are combined in a

single series arrangement.

Electrostatic separators of either the high tension,

plate,or free-fall types can be used in either batchwise

or continuous operations,· and they can be combined

with. magnetic. separation devices to provide further

segregation of iron-bearing particles from the trona. 20

Various magnetic separation devices known in the art

are suitable for this. purpose. One' example of such a

device is the high intensity induced roll magnetic separator.

The following examples are offered to further illus- 25

trate the process of the invention.

TABLE I

4,341,744

Example

No.

Feed Rate

(kglh/m)

745

186

372

745

Test Results Using High Tension Rotary Separator

Rotor Speed Weight Iron Content

(revolutions/min) Fraction (as % of Feed) (ppm)

Average Analysis of Feed for Examples 1-10: 910

130 N· 85.3 260

I 3.7 3070

C 11.0 6760

130 N 82.6 310

I 5.5 3860

C 11.9 5030

200 N 67.5 110

I 11.4 920

C 21.1 3580

200 N 74.1 130

I 6.4 870

C 19.5 4080

200 N 70.2 110

I 7.6 1130

C 22.2 3510

200 N 53.7 100

I 14.9 450

C 31.4 2630

200 N 55.6 110

I 13.1 ~O

C 31.3 2630

200 N 64.8 130

I 11.3 670

C 23.9 3470

200 N 68.0 120

I 10.1 850

C 21.9 4100

200 N 60.9 90

I 11.1 700

C 28.0 3120

Analysis of Feed for Example,II - 660

130 N 87.6 240

I 4.0 2430

C 8.4 4920

% Insoluble in

H20 HCl (aq)

6.34 3.00

2.55 0.46

3.04 0.48

2.74 0.46

2.45 0.31

4.25 2.14

*N = Non-conductor

] = Intermediate

C = Conductor

EXAMPLE 12

Plate-Type Separator Test Results

A plate-type separator of the configuration shown in 40

FIG. 3 was used for separation of another sample of the

same composition as that used for Example 11. The

sample was milled and screened in the same manner as

Examples 1-11 to produce a 30-100 mesh particles, and 45

similarly dried prior to being placed on the apparatus.

recycle (referred to herein as "intermediate 2"), and the

final non-conductor fraction (referred to herein as "nonconductor").

The fractions were analyzed in the same

manner as in Examples 1-11. The results are shown in

Table II.

As in Table I; it is evident in Table II that a sharp

reduction in iron content, water insoluble material and

aqueous Hel insoluble material was achieved in the

non-conductor product.

TABLE II

Example

No.

Test Results Using Plate-Type Separator

Feed Rate Weight Iron Content

(kglh/m) Fraction (as % of Feed) (ppm)

Analysis of Feed for Example Ii - 660

745 Non-conductor 87.9 200

Intermediate I 2.9 2000

Intermediate 2 3.7 2710

Conductor 5.5 5510

% Insoluble in

H20 HCI (aq)

4.25 2.14

3.13 0.53

The apparatus used was a Reichert ® Laboratory

Plate-Type Separator with a voltage of 12 kilovolts

applied to the electrode. The distance between the electrode

and the grouned plate was adjusted to give the 60

best separation. The test was run in a manner similar to

that described for Examples 1-11 above, except that an

additional recycle was done on the non-conductor

product. Thus, four fractions were collected: the original

conductor fraction (referred to herein as "conduc- 65

tor"), the conductor fraction from the first non-conductor

recycle (referred to herein as "intermediate I"), the

conductor fraction from the second non-conductor

EXAMPLES 13 AND 14

Rotor-Type Separator Polarity Tests

These examples demonstrate the effect of changing

the electrode polarity on the rotor-type separator of

Examples 1-11.

The same rotor-type separator was used and the samples

were milled and screened in the same way to produce

30-100 mesh particles, which were dried for thirty

minutes at 38° C.

4,341,744

In this experiment, only two fractions were collected

per separation-a conductor and a non-conductor fraction.

The conductor fraction from the first separation

was set aside and the non-conductor fraction recycled

fora second separation. This procedure was thenre-

re~ults, as shown in Table IV, indicate that a sharp

reduction in iron content in the non-conductor product,

and hence an effective separation of the trona from the

remainder of the ore, is achieved at a particle size in

5 excess of 4 millimeters.

TABLE IV

COARSE FEED TESTS ON ROTARY SEPARATOR

Example

No.

Partizle

.. Mesh Size

12 X 30·

Feed Rate Rotor Speed Weight

(kglh/m) (revolutions/min) Fraction (as % of Feed)

395 ,250" Feed

N·· 8I.l

I 7.4

C 11.5

395 250 Feed

N 81.1

I 9.7

C 9.2

Iron Content

(ppm)

1056

613

1577

4587

1399

980

2095

4359

• Actual Screen apertures:

4-mesh: 0.47 em

12-mesh: 0.16 em

3D-mesh: 0.059 em:

··N = Non-conductor

I = Intermediate

C· = Conductor

peated a third time, after which the final non-conductor

fraction was set aside and labeled "non-conductor."

The three conductor fractions were then combined and 25

put through a series of three more separations, each

time removing the conductor fraction and recycling the

non-conductor fraction as before. The non-conductor

fraction remaining after the sixth separation is here

designated "intermediate," and the three conductor 30

fractions from the fourth, fifth, and sixth separations

were combined for analysis as the final "conductor"

fraction. Iron analysesfor these fractions were obtained

in the same·manner as in the preceding examples. The

. results are shown in Table III. 35

The two runs were identiCal except for the polarity of

the pinning and lifting electrodes. A positive potential

was imposed in Example 13 and a negative potential

was imposed in Example 14. The weight and iron contents

of the various fractions clearly indicate that a 40

separation was achieved in each case.

TABLE III

What is claimed is:

1. A process for the production of soda ash from

trona ore which comprises

(a) reducing the ore particle size to a maximum of

about 4.0 millimeters in diameter,

(b) removing fines from the ore to produce a minimum

particle size of about 0.1 millimeter in diameter,

to differences in conductance,

(d) segregating the ore particles by electrostatic separation

into at least two fractions according to the

differences in electrical charge resulting from the

electrification of step (c), and

(e) calcining the fraction of least conductance to

convert the trona contained therein to soda ash,

steps (a) through (d) occurring at a temperature not to

exceed about 100· C.

2. A process according to claim 1 in which step (c) is

Polarity Tests on Rotary Separator

Example Ele~trode Feed Rate Rotor Speed Weight Iron Content··

No. Potential (kv) (kglh/m) (revolutions/min) Fraction (as % of Feed) (ppm)

I3 +25 590 ISO N· 77.5 407

I 13.5 1021

C 9.0 3885

14 -25 590 ISO N 80.6 349

I 10.9 1757

C 8.5 4500

*N = Non-conductor

I. = Intermediate

C = Conductor

"Average feed analysis: approximately 830 ppm iron

EXAMPLES 15 AND 16

Rotor-Type Separator on Coarse Feed

These examples demonstrate the effectiveness of the

process of the present invention on relatively coarse 60

feeds.

The same rotor-type separator described in Examples

1-11 above was used, with an electrode potential of

+26 kilovolts. The samples were prepared and dried in

a manner similar to that described above, except that 65

coarser sizes were used. The separations were performed

in the same manner as described in Examples 13

and 14, and similar iron analyses were performed. The

performed by contacting the ore with a grounded conductor

in the presence of an electric field generated by

a positively charged non-corona-producing electrode,

and step (d) is performed by selectively displacing the

ore fraction of greatest conductance from the grounded

conductor.

3. A process according to claim 1 in which step (c) is

performed by contacting the ore with a grounded conductor

in the presence of an electric field generated by

a negatively charged non-corona-producing electrode,

and step (d) is performed by selectively displacing the

4,341,744

ore fraction of greatest conductance from the grounded

conductor.

4. A process according to claim 1 in which step (c) is

performed by placing the ore on a rotating cylindrical 5

grounded conductor in the path of a stream of positively

charged mobile ions generated by a positively

charged electrode producing a corona discharge, and

10 step (d) is performed by the action of gravitation and/or

centifugal force on the ore.

5. A process according to claim 1 in which step (c) is

performed by placing the ore on a rotating cylindrical 15

grounded conductor in the path of a stream of negatively

charged mobile ions generated by a negatively

charged electrode producing a corona discharge, and

step (d) is performed by the action ofgravitation and/or 20

centrifugal force on the ore.

6. A process according to claims 1, 2, 3, 4, or 5 in

which steps (a) through (d) occur at a temperature not

to exceed about 60° C.

7. A process according to claims 1, 2, 3, 4, or 5 in

which the maximum particle size of step (b) is about 2.0

millimeters in diameter and the minimum particle size of

step (b) is about 0.15 millimeter in diameter.

8. A process according to claims 1, 2, 3, 4, or 5 in

which the maximum particle size of step (a) is about 0.6

millimeter in diameter and the minimum particle size of

step (b) is about 0.15 millimeter in diameter.

9. A process according to claims 1, 2, 3, 4, or 5 in

which the fraction of least conductance referred to in

step (c) is subjected to high intensity dry magnetic separation

to further reduce the iron content either before or

after calcination.

10. A process according to claims 1, 2, 3, 4, or 5 in

which the ore prior to step (c) is separated into at least

two fractions according to particle size, and steps (c),

(d), and (e) are performed on each such fraction. • • • • •