6,173,840 Beneficiation of saline minerals

Patent Number/Link:

111111111111111111111111111111111111111111111111111111111111111111111111111

US006173840Bl

(12) United States Patent

Pruszko et al.

(10) Patent No.:

(45) Date of Patent:

US 6,173,840 Bl

Jan. 16,2001

(52) U.S. CI. 209/214; 209/3; 209/218;

209/11

(51) Int. CI? B03C 1/00; B07B 1/00;

B03B 1/00

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

WY(US)

(21) Appl. No.: 09/027,043

(22) Filed: Feb. 20, 1998

Related U.S. Application Data

(60) Provisional application No. 60/038,759, filed on Feb. 21,

1997.

4,054,513 * 10/1977 Windle 209/223.1

4,214,984 * 7/1980 MacElvain 209/214

4,236,640 12/1980 Knight 209/587

4,277,329 * 7/1981 Cavanagh 209/223.1

4,294,690 * 10/1981 Kollenz 209/214

4,324,577 * 4/1982 Sepehri-Nik 209/214

4,341,744 * 7/1982 Brison 423/206

4,363,722 12/1982 Dresty, Jr. et al. 209/3

4,375,407 * 3/1983 Kronick 209/8

4,375,454 * 3/1983 Imperto et al. 209/214

4,388,179 6/1983 Lewis 208/177

4,512,879 4/1985 Attia et al. 209/3

4,609,109 * 9/1986 Good 209/223.1

4,668,591 * 5/1987 Minemura et al. 209/223.1

4,737,294 * 4/1988 Kukuck 209/223.1

4,772,383 * 9/1988 Christensen 209/223.1

4,781,298 * 11/1988 Hemstock et al. 209/214

4,814,151 * 3/1989 Beuke 423/206

4,874,508 10/1989 Fritz 209/214

4,902,428 * 2/1990 Cohen 209/214

4,943,368 7/1990 Gilbert et al. 209/2

5,238,664 * 8/1993 Frint et al. 423/206.2

(List continued on next page.)

Primary Examiner-Donald P. Walsh

Assistant Examiner-Daniel K Schlak

(74) Attorney, Agent, or Firm---8heridan Ross Pc.

Under 35 U.S.c. 154(b), the term of this

patent shall be extended for 0 days.

(54) BENEFICIATION OF SALINE MINERALS

(75) Inventors: Rudolph Pruszko, Green River, WY

(US); Roland Schmidt, Lakewood;

Dale Lee Denham, Jr., Arvada, both of

CO (US)

( *) Notice:

(58) Field of Search 209/3, 4, 7, 9,

209/10, 11,214,218,223.1,225,226,

227,908

U.S. PATENT DOCUMENTS

References Cited

(57) ABSTRACT

Processes for purification of saline minerals using magnetic

separation are disclosed. In particular, saline minerals can

include trona, borates, potash, sulfates, nitrates and chlorides.

The magnetic separation can include high intensity

magnetic separation which can be conducted at greater than

about 20,000 Gauss and up to greater than about 50,000

Gauss. Other embodiments of the invention include calcination

of a saline mineral in an inert atmosphere or in an

oxygen-containing atmosphere at a high temperature prior to

magnetic separation. A further embodiment of the invention

includes pre-alignment of particles on a surface to align the

particles of high magnetic force during a magnetic separation

step. Also disclosed are various embodiments of magnetic

separation which include subjecting an ore to a preliminary

magnetic field prior to magnetic separation.

8 Claims, 9 Drawing Sheets

Uhlig.

Koren 209/223

Cavanagh et al. 241/24

Haseman .. 241/24

Laurila 209/214

Mayer et al. 209/111.8

Fraas 209/214

Coglaiti 209/12.2

Seglin et al. 23/63

Lee et al. 209/8

Sproul et al. 423/206.2

Frangiskos 209/3

Boom et al. 209/39

Noll 209/214

3/1921

5/1955

6/1961

2/1962

4/1966

10/1966

5/1968

9/1970

4/1972

1/1975

3/1975

2/1976

6/1976

* 11/1976

1,371,825

2,708,034

2,990,124

3,022,956

3,246,753 *

3,276,581

3,382,977 *

3,528,766 *

3,655,331 *

3,860,514

3,869,538 *

3,936,372 *

3,966,590

3,990,642

(56)

Feedstream/

:~ ~16__26

~24

US 6,173,840 Bl

Page 2

U.S. PATENT DOCUMENTS

5,470,554 * 11/1995 Schmidt et al. 423/206.2

5,651,465 * 7/1997 Schmidt et al. 209/12.2

5,736,113 * 4/1998 Hazen et al. 423/206.2

5,911,959 * 6/1999 Wold et al. 423/206.2

* cited by examiner

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US 6,173,840 B1

SUMMARY OF THE INVENTION

One embodiment of the invention is a process for recovering

a saline mineral from an ore which contains saline

mineral and impurities. The method includes separating a

first portion of impurities from the ore by magnetic separation

which includes subjecting the ore to a magnetic flux

density of greater than about 20,000 Gauss. In a preferred

embodiment, the saline mineral selected from trona, borates,

potash, sulfates, nitrates and chlorides, and most preferably

is trona. In other embodiments, the process includes such a

method of magnetic separation of impurities, wherein an

impurity selected from shortite and pyrite are separated. In

these embodiments, at least about 25% of the impurity and

up to more than 75% of the impurity is removed.

In a further embodiment of the present invention, a

process is provided for recovering saline mineral from an

ore which includes saline mineral and impurities. The

method includes calcining the saline mineral in an inert

atmosphere and the separating a portion of the impurities

from the ore by magnetic separation. In a preferred

embodiment, the inert atmosphere is any nonoxygencontaining

atmosphere and can be selected from carbon

dioxide, nitrogen, and water vapor. In an alternative

embodiment, calcination is conducted in an oxygencontaining

atmosphere at a temperature of greater than about

1500 C. and subsequently separating a portion of the impurities

by magnetic separation. A further embodiment of the

present invention includes a process for the purification of

saline minerals in an ore which includes saline mineral and

50 impurities by magnetic separation in a first magnetic field,

wherein the first magnetic field has positions of higher

intensity and lower intensity. The process includes prealigning

the ore on a surface with respect to one or more of

said positions of higher intensity of said first magnetic field

before separation. The process further includes separating a

portion of the impurities from the ore by magnetic separation

in the first magnetic field.

In a further embodiment of the present invention, a

process is provided for removing magnetic impurities from

an ore which contains a saline mineral and magnetic impurities

and has a particle size of less than 100 mesh. The

process includes subjecting the ore to a magnetic flux

density of greater than about 20,000 Gauss whereby magnetic

impurities are separated from the saline mineral. This

process can include recovering greater than about 25 wt. %

of the magnetic impurities and up to greater than about 75

wt. % of the magnetic impurities.

beneficiating trona or soda ash that generally comprises the

step of density separation, and optionally includes electrostatic

separation and/or magnetic separation. U.S. Pat. No.

5,470,554 is incorporated herein by reference in its entirety.

5 The resulting product can have trona or soda ash purities on

the order of 97-98% or more. Although this process works

quite satisfactorily, there is always a desire to reduce the

number of steps, and thereby reduce the costs, associated

with separation processes. In addition, there is always a

10 desire to improve the recovery of beneficiation processes.

As such, it can be appreciated that it would be desirable

to be able to beneficiate trona or soda ash utilizing a low cost

dry beneficiation process to obtain trona or soda ash purities

of about 97-98% or higher and recoveries on the order of

15 90-95%. Accordingly, it is an object of the present invention

to provide a dry process for the beneficiation of trona or soda

ash resulting in higher purities and lower costs than many

existing dry beneficiation processes, and which is simpler

and less expensive than known wet beneficiation processes.

FIELD OF THE INVENTION

CROSS-REFERENCE TO RELATED

APPLICATIONS

BACKGROUND OF THE INVENTION

BENEFICIATION OF SALINE MINERALS

The present invention relates to the beneficiation of saline

minerals and, more specifically, trona, by methods of magnetic

separation.

This application claims priority from U.S. Provisional

Application Ser. No. 60/038,759, filed Feb. 21, 1997,

entitled "BENEFICIATION OF SALINE MINERALS."

Many saline minerals are recognized as being commercially

valuable. For example, trona, borates, potash and

chlorides are mined commercially. After mining, these minerals

need to be beneficiated to remove naturally occurring

impurities. 20

With regard to trona (Na2C03.NaHC03 .2H2 0), highpurity

trona is commonly used to make soda ash, which is

used in the production of glass, paper and other goods.

Naturally-occurring trona, or crude trona, is found in large 25

deposits in the western United States, such as in Wyoming

and California, and also in Egypt, Kenya, Botswana, Tibet,

China, Venezuela and Turkey. The largest deposit of trona in

the world is located in the Green River Basin in Wyoming.

Crude trona ore from Wyoming is typically between about 30

80% and about 90% trona, with the remaining components

including shortite, pyrite, quartz, dolomite, mudstone, oil

shale, kerogen, mica, nahcolite and clay minerals.

Many areas of the glass and paper making industries

require soda ash produced from trona having a purity of 99% 35

or more. In order to obtain such a high purity, wet beneficiation

processes have been used. Such processes generally

involve crushing the crude trona, solubilizing the trona,

treating the solution to remove insolubles and organic

matter, crystallizing the trona, and drying the trona which 40

may subsequently be calcined to produce soda ash.

Alternatively, the crude trona can be calcined to yield crude

sodium carbonate, which is then solubilized, treated to

remove impurities, crystallized and dried to produce sodium

carbonate monohydrate or calcined to produce soda ash. 45

While the above-described wet processes can produce with

high purity, they tend to be time consuming and expensive

to perform, and therefore result in a product which is much

more expensive than products produced using known dry

processes.

Not all industries which use trona or soda ash require such

a highly purified form. For example, certain grades of glass

can be produced utilizing soda ash having less than 97%

purity. For this purpose, U.S. Pat. No. 4,341,744 discloses a

dry beneficiation process which is less complex and less 55

expensive than the above-described wet beneficiation process.

Such a dry beneficiation process generally includes

crushing the crude trona, classifying the trona by particle

size, electrostatically separating certain impurities, and

optionally magnetically separating other impurities. Such a 60

process can yield trona or soda ash having up to 95% to 97%

purity with a 60-74% recovery, depending on the quantity

and type of impurities present in the crude trona ore.

There are uses for trona or soda ash, for example in certain

applications in the glass industry, requiring a purity of at 65

least 97%, yet not needing a purity over 99%. To accomplish

this, U.S. Pat. No. 5,470,554 discloses a dry method for

US 6,173,840 B1

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of the magnetic properties of pyrite from

trona calcined at low temperature.

FIG. 2 is a graph of the magnetic properties of pyrite from

trona calcined at low temperature and washed with hydrochloric

acid.

FIG. 3 is a graph of the magnetic properties of pyrite from

uncalcined trona.

FIG. 4 is a graph of the magnetic properties of pyrite from

an ore body other than trona from Climax, Colo.

FIG. 5 is a graph of the magnetic properties of pyrite from

an ore body other than trona from Vulcan, Colo.

FIG. 6 is a graph of the magnetic properties of pyrite from

an ore body other than trona in Argentina.

FIG. 7 is a graph of the magnetic properties of shortite

from a non-magnetic fraction of uncalcined trona.

FIG. 8 is a graph of the magnetic properties of shortite

from a magnetic fraction of uncalcined trona.

FIG. 9 is an illustration of a design for an open gradient

magnetic separator.

DETAILED DESCRIPTION

The present invention is a dry beneficiation process for

recovering saline minerals from an ore containing the saline

mineral and impurities, and takes advantage of previously

unrecognized characteristics of such ore. As used herein and

in the mineral processing industry, the term "saline mineral"

refers generally to any mineral which occurs in evaporite

deposits. Saline minerals that can be beneficiated by the

present process include, without limitation, trona, borates,

potash, sulfates, nitrates, chlorides, and preferably trona.

The purity of saline minerals within an ore depends on the

deposit location, as well as on the area mined at a particular

deposit. In addition, the mining technique used can significantly

affect the purity of the saline minerals. For example,

by selective mining, higher purities of saline minerals can be

achieved. Regarding the saline mineral trona, deposits are

located at several locations throughout the world, including

Wyoming (Green River Basin), California (Searles Lake),

Egypt, Kenya, Venezuela, Botswana, Tibet, China and Turkey

(Beypazari Basin). For example, a sample of trona ore

from Searles Lake has been found to have from about 50%

to about 90% by weight (wt. %) trona and a sample taken

from the Green River Basin in Wyoming has been found to

have from about 70 wt. % to about 92 wt. % trona. The

remaining 8 wt. % to 30 wt. % of the ore in the Green River

Basin sample comprised impurities including shortite,

pyrite, shale consisting predominantly of dolomite, clay,

quartz and kerogen, and traces of other impurities. Other

samples of trona ore can include different percentages of

trona and impurities, as well as include other impurities.

Particular aspects of the present invention will be

described in the context of the saline mineral trona. Such

descriptions are not intended to limit the general applicability

of the present invention to only trona. Moreover, as

discussed herein, trona is often processed by calcination to

produce soda ash (Na2C03 ). It should be noted that, unless

specifically indicated otherwise, use of the term trona herein

can refer to both trona (i.e., Na2C03.NaHC03 .2H20) and

trona which has been processed by calcination to form soda

ash. In addition, it should be recognized that descriptions of

other processing steps herein (e.g., magnetic separation)

should be construed to include processing of calcined and

uncalcined trona. More particularly, it should be noted that,

in preferred embodiments, calcined trona is used for magnetic

separations and density separations described herein.

It has now been discovered that saline minerals, such as

trona, do not have a constant distribution of impurities

5 throughout the saline mineral particles. That is, crushed

saline mineral ore particles have a variety of impurity

contents ranging from no impurities in some particles to

almost total impurities in other particles. It is believed that

this characteristic of saline minerals was not previously

10 recognized. Therefore, until the recognition of the present

invention, there has been no suggestion or teaching to

conduct aspects of the present process. It has been discovered

that the impurities in saline mineral ores, such as trona

containing ores, are typically concentrated in a relatively

15 small percentage of the particles, while the rest of the saline

mineral particles can be of relatively high purity. For

example, under microscopic examination, most of the particles

of crushed trona after calcination are almost completely

white (i.e., substantially white), indicating highly

20 pure trona, while others are distinctly yellowish-brown,

indicating the presence of interstitial impurities. Such particles

can be selectively separated (e.g., manually sorted) to

yield a high purity fraction (e.g., about 97% or more saline

mineral) and a low purity fraction (e.g., less than about 97%

25 saline mineral).

It can be appreciated that, while the above-described

manual sorting process produces a suitable product, the

process can be impractical for high volume separation. The

process of the present invention, however, includes all

30 methods for selectively separating high and low purity

products whether manual or automated. That is, more

commercially-viable processes for separating the high purity

saline mineral from the low purity saline mineral also fall

within the scope of this invention. Thus, the process of the

35 present invention includes a process for selectively separating

a low purity fraction having more than about 3%

impurity content from a high purity fraction having less than

about 3% impurity content. Such separation is based upon

differences in the properties of the particles at the level of

40 impurities present in the particles.

In a preferred embodiment of the present invention, the

step of separating impurities from an ore containing saline

mineral and impurities includes the step of ultra-high magnetic

separation. The ultra-high magnetic separation step

45 subjects the ore to conditions such that materials of different

magnetic susceptibilities (e.g., trona and shale) separate

from each other into a recovered stream and an impurity

stream. In accordance with the present invention, the ultrahigh

intensity of the magnetic flux during the magnetic

50 separation step is at least about 20,000 Gauss, preferably at

least about 30,000 Gauss, and more preferably at least about

50,000 Gauss. It should be noted that 10,000 Gauss is

equivalent to 1 tesla and these units can be used interchangeably.

Such ultra-high magnetic fluxes are in contrast to the

55 above-described standard intensity magnetic separation at

less than about 20,000 Gauss because such standard intensities

are not sufficient to separate particles having both

magnetic (e.g., shale) and non-magnetic (e.g., saline

mineral) components. In order to generate such an ultra-high

60 magnetic flux, superconducting electromagnetic field generators

can be utilized. For example, an open gradient

superconducting magnet can be used. It should be appreciated

that other types of magnetic separators may be used, as

long as the required ultra-high magnetic flux can be

65 obtained. For example, it is recognized that some rare earth

magnets can now achieve a magnetic flux of up to about

28,000 Gauss. With regard to the beneficiation of trona,

US 6,173,840 B1

5 6

intensity separation as described herein. Alternatively, water

can be added to such a stream to dissolve the small crystals

therein. In addition, the liquid medium can be an alcohol or

other suitable medium.

In a further aspect of the present invention, it has been

surprisingly found that during high intensity magnetic

separation, as described above, impurities not previously

recognized as being susceptible to magnetic separation can

be separated. In one such embodiment, this process includes

10 subjecting a saline mineral containing ore to high intensity

magnetic separation, whereby the impurity shortite is separated

from the saline mineral. In another such embodiment,

the impurity pyrite is separated. In this process, at least about

25 wt. % of the impurity is separated, more preferably at

15 least about 50 wt. %, and most preferably at least about 75

wt.%.

As noted above, magnetic separation at under about

20,000 Gauss is known. It has been appreciated, however,

that there are limits on the effectiveness of such processes.

20 It has previously been thought that at 20,000 Gauss the

primary separation is between liberated impurities and trona

containing particles. As noted above, it has now been

identified that impurities in saline mineral are typically not

evenly distributed throughout the saline mineral particles.

25 However, it has been determined that, in performing a

magnetic separation on crushed trona ore using a 20,000

Gauss magnetic field, no appreciable separation or difference

in magnetic susceptibilities between high and low

purity particles occurred. At 23,000 Gauss, an identifiable

30 difference in magnetic susceptibility was noted between the

high purity particles and the low purity particles. In a

subsequent test, a 28,000 Gauss magnetic field was found to

give remarkable separation between the high purity particles

and the low purity particles. The high purity particles were

35 subsequently subjected to density separation at a 2.3 specific

gravity cut-off. The resulting purity of the product was on

the order of about 97%-98%, and the recovery was as high

as about 90%-95%. Therefore, in addition to discovering

that crushed trona ore actually comprises a large percentage

40 of high purity particles, it has also been discovered that a

high purity, high recovery trona product can be obtained by

subjecting crushed trona ore to ultra-high intensity magnetic

separation. (See, for example, Examples 1, 4 and 8.) Such a

process provides significant advantages because acceptable

45 purity levels and recoveries can be achieved without the

significantly greater expense associated with recrystallization

processes. It should be noted that high intensity magnetic

separation processes are more effective at separating

pyrite from trona ore having a wide size fraction (e.g., 6x20

50 mesh or 20x100 mesh) than, for example, density separations

such as air tabling. Therefore, a dry separation process

using only high intensity separation could be more useful

than a dry separation process using density separation when

the desired product must be low in pyrite so long as

55 moderate amounts of shortite can be tolerated. For example,

it is contemplated by the present inventors that some amount

of shortite may be desirable in soda ash in certain

applications, such as the production of glass. Alternatively,

high intensity magnetic separations can be conducted in

60 combination processes which also include density

separations, electrostatic separations or recrystallization

processes.

In a further aspect of the present invention it has been

surprisingly found that saline mineral particles of -100 mesh

65 can be efficiently separated by high intensity magnetic

separation, as described above. It is generally recognized

that conventional magnetic separation for such small partypical

impurities that can be removed during the magnetic

separation step include shale and impure trona with interstitial

dolomite and paramagnetic clay-type materials, which

have higher magnetic susceptibilities than pure trona. In

addition, other impurities may be separated by the present 5

process due to their association with the magnetic impurities.

As noted, any type of high intensity magnetic separator is

suitable for use in the present invention. More particularly,

either dry or wet high intensity magnetic separators can be

used. Dry separation refers to any process in which dry

particles are subjected to a high intensity magnetic field for

separation of magnetic impurities. Wet separation refers to

any process in which magnetic impurities are separated

from, e.g., crystals of a saline mineral in a saturated brine in

a high intensity magnetic field. Alternatively, the wet

medium can include other liquids, such as an alcohol.

Suitable high intensity magnetic separators can be of any

known design. For example, magnetic separators can be of

a type wherein a feedstream is fed in free fall in proximity

to a high intensity magnet so that magnetic particles tend to

migrate toward the magnet and non-magnetic particles do

not. In this manner, as the particles are collected on either

side of a splitter, the magnetic particles will physically

separate from the non-magnetic particles. With particular

regard to FIG. 9, a particular apparatus is illustrated. The

feedstream 10 is fed to a dropoff point 12 in free fall. At the

dropoff point 12, the feedstream is in close proximity to the

magnet 14. As the feedstream 10 falls, the magnetic particles

16 migrate toward the magnet 14 and the non-magnetic

particles 18 do not migrate toward the magnet 14. After the

particles are separated and fall further toward the splitter 20,

the magnetic particles 16 fall on one side of the splitter,

while the non-magnetic particles 18 fall on the other side of

the splitter 20. The horizontal distance between the dropoff

point 12 and the magnet 14 defines the gap width 22. The

horizontal distance between the splitter and a point on the

magnet face which is closest to the arc of the feedstream

defines the splitter gap width 24. The magnet angle 26 is

defined by the angle between the bottom surface of the

magnet and the horizontal. In an alternative high intensity

magnetic separator design for dry separations, particles can

be fed onto a matrix, such as a steel wool or wire mesh

screen matrix in proximity to a high intensity magnet. Such

a matrix can be made of any metal, such that in the presence

of a magnetic flux, the metal can act as a magnet. The metal

can be in any configuration having a high surface area such

as a wool or screen. The magnetic particles in the feed

stream will become attached to the matrix, whereas nonmagnetic

particles will migrate through the matrix and be

removed from the bottom of the matrix. This apparatus

design is particularly well suited for fine particles, such as

feedstreams having a particle size of less than 100 mesh.

In a further apparatus design for use with wet high

intensity magnetic separations, a matrix in a vessel filled

with a liquid is provided. The matrix is subjected to the

magnetic field of a high intensity magnet. The liquid can be,

for example, a saturated brine solution of a saline mineral.

For example, such a liquid can be from a saturated brine

recrystallization process, such as is described in PCT application

PCT/US96/00700, which is incorporated herein by

reference in its entirety. Such a stream can include the entire

size range of crystals generated by the process or can be

generated by separating large crystals from insoluble impurities

and smaller crystals on a size separation basis. The

resulting stream of a saturated brine solution with insoluble

impurities and small crystals can be treated by wet high

US 6,173,840 B1

7 8

density separation effect. For example, it has been found

that, in order to get adequate density separation using an air

table, the particle size distribution ratio should be about 3.0

or less, preferably about 2.8 or less, and more preferably

5 about 2.2 or less.

As discussed above, density separation is conducted by

subjecting the ore to conditions such that materials of

different densities separate from each other. The mineral

stream having materials of varying densities is separated by

10 a rougher pass into a denser and a lighter stream, or into

more than two streams of varying densities. Typically, in the

case of beneficiating trona, the separation is made at a

specific gravity of about 2.3, and trona is recovered in the

lighter stream. The purity of a saline mineral recovered from

15 density separation can be increased by reducing the weight

recovery of the recovered stream from the feed stream. At

lower weight recoveries, the recovered stream will have a

higher purity, but the rougher pass will also have a reduced

yield because more of the desired saline mineral will report

20 to the impurity stream. Such a "high purity" process may be

beneficial in that it requires less subsequent processing (e.g.,

separation) of the ore and, in addition, may be of higher

value because it can be used in other applications where high

purity saline minerals are required.

In an alternative embodiment, the impurity stream from

density separation can go through one or more scavenger

density separation step(s) to recover additional saline mineral

to improve the overall recovery. The scavenger separation

is similar to the above-described density separation

30 step. The scavenger step treats the impurity stream from the

rougher pass and recovers a portion of the saline mineral

therefrom. The recovered scavenger portion is combined

with the above-described recovered stream to increase the

overall recovery from density separation, or recycles it to

35 other steps in the process, with or without further size

reduction.

In a further alternative embodiment, the recovered stream

from the rougher pass density separation can go through one

40 or more cleaning density separation steps to further remove

impurities from the recovered stream and improve the purity

of the final product. The cleaning step is similar to the

above-described density separation process in that impurities

are removed from the stream by density separation. In

45 both scavenging and cleaning passes, the feed stream into

those passes can undergo further size reduction, if desired,

for example, to achieve higher liberation.

With regard to the beneficiation of trona, which, prior to

calcination, has a density of 2.14 and after calcination has an

50 apparent density of about 1.5, impurities that are removed

during the density separation step of the present invention

include shortite, having a density of 2.6, dolomite, having a

density of 2.8-2.9 and pyrite, having a density of 5.0. Each

of these is separable from the trona ore because of differ-

55 ences in density from trona. By practice of the present

invention, of the total amount of shortite, dolomite, pyrite

and, if present, potentially valuable heavy minerals in the

trona ore, the density separation step can remove at least

about 10 wt. % and more preferably, about 50 wt. %, and

60 most preferably, about 90 wt. % of the heavy impurity.

In an alternative embodiment, impurities removed during

the density separation process can be recovered as a product

for commercial use. For example, in the beneficiation of

trona, the impurities removed during the air tabling step can

65 comprise as much as 90% shortite. Such shortite may be

acceptable, for example, for certain applications where it can

be used for neutralization of acids or removal of sulfur from

ticles is not effective. It is believed that static charges on the

particles prevent effective separation between magnetic and

non-magnetic particles. In addition, it is believed that such

small particles do not efficiently separate because of a weak

magnetic force. However, such small particles can be effectively

separated by high intensity magnetic separation. High

intensity magnetic separators using a matrix, such as steel

wool, are believed to be particularly effective for such small

particles. Without intending to be bound by theory, it is

believed that the metal matrix functions to physically break

up associations between magnetic and non-magnetic particles.

In addition, it is believed that the metal matrix is able

to dissipate static charges which otherwise hold such particles

together. In this aspect of the invention, at least about

25 wt. % of magnetic impurities are separated from the -100

mesh particles, more preferably at least about 50 wt. %, and

most preferably at least about 75 wt. %.

Other processes may be utilized to practice the present

invention to selectively separate high and low purity portions

instead of ultra-high intensity magnetic separation. For

example, the process of selectively separating low purity

saline mineral from high purity saline mineral comprise

performing colorimetric separation. More specifically, for

example in the separation of trona, the low purity particles

may be separated based upon their darker color than the high 25

purity particles utilizing an automated colorimetric sorting

process. Such a process may utilize a video imager in

conjunction with appropriately-timed blasts of pressurized

gas to deflect the darker colored particles away from the high

purity particles.

The impurity stream from the selective separation step,

such as ultra-high intensity magnetic separation, can go

through one or more scavenger steps to improve the overall

recovery. The scavenger step recovers a portion of the

impurity stream from the rougher pass through magnetic

separation and combines it with the above-described recovered

stream or recycles it to the process with or without

further size reduction to increase the overall recovery of the

magnetic separation step. Furthermore, the recovered stream

from magnetic separation can go through one or more

magnetic cleaning steps to further remove impurities from

the recovered stream and improve the purity of the final

product.

The present process may further include removing impurities

from the ore containing saline minerals by a density

separation method. Density separation methods are based on

subjecting an ore to conditions such that materials of different

densities physically separate from each other.

Thereby, certain impurities having a different density than

the desired saline mineral can be separated. The density

separation step of the present invention is most preferably a

dry process; however, wet density separation processes,

such as heavy media separation, can be used as well. In dry

density separation processes, the need for processing in a

saturated brine solution, solid/brine separation, and drying

of the product is eliminated. Consequently, dry processes

according to the present invention tend to be cheaper and

less complex than wet processes. Any known density separation

technique could be used for this step of the present

invention, including air tabling or dry jigging.

During such dry separation steps, it is important to

maintain a narrow particle size distribution with a particular

fraction. That is, the ratio of the largest particle within a

fraction of the smallest particle within that fraction should

be relatively small. Without such a small ratio of particle

size distribution, the differences in particle sizes could tend

to dominate the separation process, thereby reducing the

US 6,173,840 B1

9 10

three size fractions of 6 mesh by 20 mesh, 20 mesh by 100

mesh, and minus 100 mesh, the two larger size fractions are

preferably treated using an apparatus in which particles free

fall while subject to a high intensity magnet and are col-

5 lected on either side of a splitter. Such an apparatus is also

known as an open gradient magnetic separator ("OGMS").

The smallest size fraction is preferably treated by a high

intensity magnetic separator having a metal matrix.

In yet another embodiment of the present invention, the

10 ore is dried prior to the separation processes set forth above.

The drying step removes surface moisture from the ore to

better enable the ore to be separated. Drying can be accomplished

by any conventional mineral drying technique,

including rotary kiln, fluid bed or air drying. The ore can be

dried to less than about 2%, and preferably less than about

15 1% surface moisture content. During the drying process, it

is preferred that the saline mineral is not raised to such a

temperature for such a period of time that it is calcined. In

the case of trona, the drying temperature should remain

below about 40 degrees centigrade to avoid calcination.

In still another embodiment of the present invention, a

de-dusting step is added to the basic beneficiation process to

remove fines before the selective separation step (e.g.,

magnetic separation). Such a de-dusting step can be conducted

before, during or after one or more of the crushing

25 and sizing steps, but preferably before the electrostatic

separation, magnetic separation and density separation

steps. The fines produced during the processing of trona are

relatively high purity trona and are useful in several industrial

applications. For example, trona recovered by

30 de-dusting can have a purity of greater than about 94%,

preferably greater than about 96% and more preferably

greater than about 98%. Fines can be collected in de-dusting

steps by use of a baghouse, or other conventional filtering

device, and sold as purified trona without further processing.

35 In a further aspect of the present invention, the magnetic

separation achieved by any magnetic separation step of the

present invention, including high intensity magnetic separation

and conventional intensity magnetic separation, can

be improved by subjecting the ore that is being processed to

a preliminary magnetic field prior to separation and subse-

40 quently separating a portion of impurities from the ore by

magnetic separation. Without being bound by theory, it is

believed that by subjecting particles being processed to a

preliminary magnetic field before physical separation of

magnetic from non-magnetic particles, the magnetic tractive

45 force on particles during magnetic separation is greater. In

this manner, more efficient separations can be achieved

during magnetic separation steps.

The term "preliminary magnetic field" refers to a magnetic

field to which a feed stream being processed is sub-

50 jected prior to physical separation of magnetic from nonmagnetic

particles. A feed stream being processed can be

subjected to a preliminary magnetic field using conventional

apparatus known to those in the art. For example, a feed

stream can be transported along a conveyor belt and sub-

55 jected to a magnetic field during linear transport on the

conveyor belt. Subsequently, particles will reach the end of

the conveyor belt and drop off of the conveyor belt while

being subjected to the primary magnetic field for separation.

In this embodiment of the present invention, the preliminary

magnetic field typically has a magnetic flux density of

60 greater than about 2000 Gauss, more preferably greater than

about 5000 Gauss, and most preferably greater than about

20,000 Gauss. Further, the step of subjecting particles to a

preliminary magnetic field is conducted for at least about 1

second, more preferably at least about 30 seconds, and most

65 preferably at least about 60 seconds. The positive effect of

a preliminary magnetic field can be further enhanced by

increased temperatures. More particularly, the process is

flue gases. In addition, for some trona deposits, potentially

valuable heavy minerals may be present. Such minerals can

be separated by the method and recovered.

It should be appreciated that the above-identified process

steps could be performed along with other process steps. For

example, such other process steps can include low or standard

intensity magnetic separation, electrostatic separation,

or any other suitable separation technique. In addition, the

above-identified process steps could be performed in any

order. Further, calcination can be conducted at any point in

the sequence of process steps and preferably is conducted

prior to any magnetic or density separation steps.

In a further embodiment of the present invention, the

saline mineral-containing ore can be crushed to achieve

liberation of impurities prior to the separation steps. The

crushing step of the present invention can be accomplished

by any conventional technique, including impact crushing

(e.g., cage or hammer mills), jaw crushing, roll crushing,

cone crushing, autogenous crushing or semi-autogenous

crushing. Autogenous and semi-autogenous crushing are

particularly beneficial because the coarse particles of ore 20

partially act as the crushing medium. Moreover, because

saline minerals are typically soft, these methods are suitable

for use in the present process. In addition, these two crushing

methods allow for the continuous removal of crushed material

and high grade potentially saleable dust.

In general, crushing to smaller particle size achieves

better liberation of impurities and thus improves recovery.

However, if the particle size after crushing is too fine, there

may be adverse effects upon subsequent separation steps. In

addition, over-crushing is not needed for many applications

of the present invention and merely increases the costs

associated with the crushing step. It has been found that

acceptable liberation for the present process can be achieved

by crushing the ore to at least about 6 mesh. Preferably, the

particle size range after crushing is from about 6 to about

100 mesh and, more preferably, from about 6 to about 65

mesh.

In another embodiment of the present invention, the ore is

sized into size fractions prior to the separation steps. Each

size fraction is subsequently processed separately. In

general, the narrower the range of particle size within a

fraction, the higher the efficiency of removal of impurities.

This is particularly true if air tabling is used as a density

separation step, wherein small particle size distribution

ratios are desired. On the other hand, a larger number of

fractions will increase the efficiency, but may increase the

cost of the overall process. The use of from 1 to 10 fractions

has been found to be acceptable. Preferably, the number of

fractions is from 4 to 10 and, more preferably, the number

of fractions is 8. Any conventional sizing technique can be

used for the present process, including screening or air

classification. For dividing into 8 fractions, the fractions

typically have the following particle size ranges: 6 to 8

mesh; 8 to 10 mesh; 10 to 14 mesh; 14 to 20 mesh; 20 to 28

mesh; 28 to 35 mesh; 35 to 48 mesh; 48 to 65 mesh (Tyler

mesh). Utilizing these fractions, the particle size distribution

ratio for each fraction is about 1.42. It should be noted that

a smaller number of size fractions, including only one size

fraction (such as 6 meshx100 mesh or 6 meshxO) or two size

fractions (such as 6 mesh by 20 mesh and 20 mesh by 0), can

be used for high intensity magnetic separations. Without

intending to be bound by theory, it is believed that the effects

of particle size are reduced under high intensity magnetic

separator apparatus designs. Thus, high intensity magnetic

separation can be conducted on a size fraction from 6 or 8

mesh by 20, 65 or 100 mesh.

In a further embodiment, the different size fractions can

be treated with high intensity magnetic separations using

different equipment. For example, in the instance of using

US 6,173,840 B1

11 12

EXAMPLES 1-8

Eight tests were performed on trona-containing ore in

accordance with the present invention. The tests were performed

on samples generated from ore mined from the

Green River Basin in Wyoming. The ore received was

crushed to nominal 50 mesh. The ore was screened into size

fractions, and the 20x35 fraction was used as the supply for

the eight tests. Referring to Table 1, the original 20x35

fraction comprised 94.01% soluble matter (i.e., trona) and

5.99% insoluble matter (i.e., impurities). The 20x35 fraction

was subjected to standard magnetic separation utilizing a

rare earth separator at its maximum intensity of less than

about 20,000 Gauss. The standard magnetic separation step

65 resulted in removal of 3.0% of the weight of the original

sample in a magnetic fraction, and the remaining nonmagnetic

fraction comprised 95.7% trona.

magnetic separation. For example, in a preferred

embodiment, the magnetic separation is conducted by feeding

ore onto a conveyor belt which then transports the ore

toward one end of the conveyor belt. The magnet creating

the first magnetic field is positioned in the rollers at the end

of the belt towards which the ore is transported. In this

manner, there will be one or more positions along the length

of the roller which have higher or maximum intensity of

magnetic force compared to other portions of the roller.

Thus, in order for the ore to be pre-aligned such that it is

primarily contacted with the portion of the roller having a

higher intensity magnetic field, it is necessary for the ore to

be aligned in the direction of travel of the belt in alignment

with the one or more positions of higher intensity. In this

embodiment, the step of pre-aligning can be accomplished

by a variety of means. For example, in the above example in

which an ore feed stream is fed onto a belt which is

transported toward a first end of the belt, pre-alignment can

be accomplished by subjecting the ore, for alignment purposes

only, to a second magnetic field in the vicinity of the

portion of the belt at which the ore is fed onto the belt. In this

manner, as ore particles are fed onto the belt and are settling

to a static position, magnetic particles will tend to migrate

and settle in alignment with one or more magnets placed

under the belt at the position desired for alignment. Thus, as

the ore is subsequently transported toward the end of the belt

toward the first magnetic field for purposes of magnetic

separation, the magnetic particles will be aligned with the

desired position of higher intensity of the first magnetic

field. Alternatively, alignment can be accomplished by

simple mechanical means of feeding the ore onto the belt at

one or more discrete locations across the width of the belt

such that as the belt is advanced, a line of ore is laid down

on the belt. Thus, alignment is accomplished by aligning the

discrete feed discharge point with the one or more positions

of higher intensity of the first magnetic field.

The foregoing process including pre-alignment of ore on

a surface for magnetic separation can be used alone or in

conjunction with other aspects of the present invention.

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

50 include alternative embodiments to the extent permitted by

the prior art.

conducted at temperatures of at least about 1000 c., more

preferably 1500 c., and most preferably 2000 C.

In a further embodiment of the present invention, a saline

mineral can be calcined prior to magnetic separation. More

particularly, in one aspect of this embodiment, the saline 5

mineral is calcined in an inert atmosphere prior to separating

impurities from the ore. In a second aspect of this

embodiment, the saline mineral is calcined in an oxidizing

atmosphere at a temperature of greater than about 1500 C.

prior to separating impurities from the ore. In this embodiment

of the present invention, it has been surprisingly found 10

that by calcination under the conditions identified above

(i.e., an inert atmosphere or an oxidizing atmosphere at high

temperature), the subsequent magnetic separation is more

efficient than in the absence of such calcining conditions.

As used herein, reference to the term "inert atmosphere" 15

refers to a non-oxidizing atmosphere (i.e., an atmosphere

substantially free of oxygen). More particularly, an inert

atmosphere is any non-oxygen containing atmosphere and in

particular, can comprise carbon dioxide, nitrogen, or water

vapor. In a further preferred embodiment, carbon dioxide

used for providing an inert atmosphere for calcination 20

purposes can be carbon dioxide which is recycled from an

exhaust stream from the calcination process. For example, in

the instance of calcination of trona, the byproducts of

calcination include carbon dioxide and water. Thus, the

water can be condensed from the exhaust stream and the 25

carbon dioxide recycled for use as the calcination gas, for

example, in a fluidizing bed.

When using an inert atmosphere in this embodiment, the

calcination temperatures typically range between about 430

C. and about 4000 c., more preferably between about 1000 30

C. and about 3500 c., and most preferably between about

1500 C. and about 2600 C.

In the aspect of this embodiment of the present invention

in which an oxidizing atmosphere is used during calcination,

the atmosphere most typically includes air but can include

other oxygen containing gases as well. Further, the calcina- 35

tion temperature in this aspect of the invention is at least

about 1500 c., more preferably at least about 2500 c., and

most preferably at least about 5000 C.

Subsequent to the calcination step as described above, the

saline mineral containing ore is then subjected to magnetic 40

separation. Such magnetic separation can include high intensity

magnetic separation as generally described herein, or

conventional intensity magnetic separation as described

herein and otherwise described in the art. In a further

preferred embodiment, the process of calcining saline min- 45

eral in an inert atmosphere or in an oxidizing atmosphere at

a high temperature followed by magnetic separation can also

include the step of subjecting the calcined ore to a preliminary

magnetic field prior to magnetic separation, as generally

described above.

A further embodiment of the present invention includes a

process for the purification of saline mineral by magnetic

separation in an ore comprising saline mineral and impurities.

In this embodiment, the magnetic separation is conducted

in a first magnetic field having positions of higher 55

intensity and lower intensity. The process includes prealigning

the ore on a surface with respect to one or more of

the positions of higher intensity of the first magnetic field

before separation of impurities and separating a first portion

of impurities from the ore by magnetic separation in the first

magnetic field. In this manner, higher efficiency of magnetic 60

separation is achieved because the ore is conducted through

the first magnetic field at the positions of highest magnetic

intensity. Therefore, magnetically susceptible particles are

subject to a higher magnetic tractive force and thereby are