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,
(c) differentially electrifying the ore particles according
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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-~
- ~
- ~
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- ~\
- \\
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- \ \
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- \ \
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15 14
FIGURE 3
SUMMARY OF THE INVENTION
2
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
1
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.
4
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
3
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
5
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-
6
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.
3
8
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
7
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.
10
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.
11
TABLE I
4,341,744
12
Example
No.
2
3
4
6
7
9
10
II
Feed Rate
(kglh/m)
22
22
745
186
372
745
745
745
745
745
22
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.
12
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
13
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-
14
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.
IS
16
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,
(c) differentially electrifying the ore particles according
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
15
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.
16
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. • • • • •
25
30
35
40
45
50
55
60
65