5,911,959
Jun. 15,1999
[11]
[45]
111111111111111111111111111111111111111111111111111111111111111111111111111
US005911959A
Patent Number:
Date of Patent:
United States Patent [19]
Wold et al.
[54] METHOD FOR PURIFICATION AND
PRODUCTION OF SALINE MINERALS
FROM TRONA
[75] Inventors: John S. Wold, Casper, Wyo.; Wayne
C. Hazen, Denver, Colo.; Rudolph
Pruszko, Green River, Wyo.; Roland
Schmidt, Lakewood; Dale Lee
Denham, Jr., Louisville, both of Colo.
[73] Assignee: Environmental Projects, Inc.
[21] Appl. No.: 08/967,281
[22] Filed: Nov. 7, 1997
4,202,667
4,283,277
4,288,419
4,299,799
4,341,744
4,363,722
4,374,102
4,375,454
4,472,280
4,512,879
4,781,899
4,814,151
4,943,368
5,096,678
5,238,664
5,470,554
5/1980 Conroy et al. 23/302
8/1981 Brison et al. 209/166
9/1981 Copenhafer et al. 423/190
11/1981 Ilardi et al. 423/206
7/1982 Brison et al. 423/206
12/1982 Dresty et al. .. .... 209/3
2/1983 Connelly et al. 423/206
3/1983 Imperto et al. 423/206
9/1984 Keeney 210/666
4/1985 Attia et al. 209/3
11/1988 Rauh et al. 423/206
3/1989 Beuke 423/206
7/1990 Gilbert et al. . 209/2
3/1992 Mackie 423/27
8/1993 Frint et al 423/206.2
11/1995 Schmidt et al. 423/206.2
References Cited
Related U.S. Application Data
Continuation of application No. 08/373,955, Jan. 17, 1995,
abandoned, which is a continuation-in-part of application
No. PCT/US94/05918, May 25, 1994, and application No.
08/066,871, May 25, 1993, Pat. No. 5,470,554.
Int. CI.6 COlD 7/00; C22B 26/10
U.S. CI. 423/206.2; 423/421; 23/302 T
Field of Search 423/206.2, 421;
23/302 T
19 Claims, 3 Drawing Sheets
ABSTRACT
OTHER PUBLICATIONS
[57]
Disclosed are a variety of processes for the purification of
saline minerals and in particular, trona. Some of the processes
include a combination wet and dry recovery process
which results in high recovery and purification at relatively
low cost. The dry separation methods can include density
separation, magnetic separation and electrostatic separation.
Other processes include a modified slush process for the
purification of calcined trona (sodium carbonate) by the
introduction of anhydrous sodium carbonate into a saturated
brine solution and subsequent separation of insoluble impurities.
Trona Soda Ash, Chemical Engineering, May, 1953, pp. 118,
120.
Perry, Chilton and Kirkpatrick, Chemical Engineers Handbook,
4th Ed. 1963, pp. 21-61 to 21-70, no month.
Soda Ash Production Keeps Booming, Chemical Engineering,
Jul. 3, 1967, pp. 60, 62.
Perry & Chilton and Kirkpatrick, Chemical Engineers'
Handbook, 5th Ed. 1973, pp. 8-31, no month.
D. Muraoka, Monohydrate Process for Soda Ash from
Wyoming Trona, Minerals and Metallurgical Processing,
May, 1985, pp. 102-103.
S. E. Clay, Kinetic Study of the Dissolution of Calcined
Trona Ore in Aqueous Solutions, Minerals and Metallugical
Processing, Nov., 1985, pp. 236-240.
American Society for Testing and Materials, Standard Test
Methods for Chemical Analysis of Soda Ash (Sodium Carbonate),
E-359-90, Mar., 1990, pp. 403-410.
Primary Examiner~teven Bos
Attorney, Agent, or Firm~heridan Ross P.e.
5/1957 Pike 23/38
5/1959 Hoekje 423/192
1/1961 Caldwell et al. 23/63
4/1961 Porter 23/143
2/1966 Bauer et al. 23/300
4/1966 Smith 23/63
11/1969 Warzel 23/63
3/1970 Frint et al. 423/206.2
4/1972 Seglin et al. 423/206.2
12/1972 Garofano et al. 23/302
2/1973 Ilardi et al. 423/206
3/1974 Ilardi et al. 423/421
6/1974 Graves et al. 423/206
9/1974 Ilardi et al. 423/206
10/1974 Duke et al. 260/527
3/1975 Sproul et al. 423/206
9/1975 Ganey et al. 423/206
1/1976 Ilardi et al. 423/206
5/1976 Port et al. 423/206.2
5/1977 Ganey et al. 423/206.2
5/1977 Baadsgaard 423/206
4/1978 Lobunez et al. 423/421
2/1979 Gill et al. 162/30
1/1980 Ilardi et al. 423/206
U.S. PATENT DOCUMENTS
2,792,282
2,887,360
2,970,037
2,981,600
3,233,983
3,244,476
3,479,133
3,498,744
3,655,331
3,705,790
3,717,698
3,796,794
3,819,805
3,836,628
3,845,119
3,869,538
3,904,733
3,933,977
3,956,457
4,021,526
4,021,527
4,083,939
4,138,312
4,183,901
[51]
[52]
[58]
[56]
[63]
u.s. Patent Jun. 15,1999 Sheet 1 of 3 5,911,959
INCOMING FEED
1
CRUSH
~
CALCINE
~
LARGE SIZE SMALL
SEPARATION
RECOVERED DRY IMPURITY
SEPARATION
RECOVERED WET IMPURITY
SEPARATION
DRY
RECOVERED
WET
RECOVERED
Fig. 1
WET
IMPURITY
u.s. Patent Jun. 15,1999 Sheet 2 of 3 5,911,959
RECYCLE
INCOMING FEED
1
ADD TO
SATURATED
SOLUTION
FILTER
SCREEN
SOLID
SOLUBILIZE
CRYSTALS
FILTER
LARGE PARTICLES
RECOVERED
CRYSTALS
SOLID
IMPURITIES
Fig. 2
u.s. Patent Jun. 15,1999 Sheet 3 of 3 5,911,959
RECYCLE
INCOMING FEED
1
ADD TO
SATURATED
SOLUTION
NONMAGNETIC
MAGNETIC MAGNETIC
SEPARATION
IMPURITIES
FILTRATE FILTER SOLID
RECOVERED
CRYSTALS
Fig. 3
5,911,959
2
SUMMARY OF THE INVENTION
The present invention includes a process for the purification
of saline minerals having insoluble impurities (e.g.,
iron-bearing materials, dolomite, shale, searlesite and
northupite). In one aspect, the process includes the steps of
calcining trona to form sodium carbonate, sizing the feedstream
into a large size fraction and a small size fraction,
separating the large size fraction into a first recovered
portion and a first impurity portion (e.g., comprising halite
and shortite) by a dry separation method, separating the first
impurity portion into a second recovered portion and second
impurity portion by a wet separation method, and separating
the small size fraction into a third recovered portion and
third impurity portion by a wet separation method. The dry
separation method is selected from the group consisting of
density separation, magnetic separation, electrostatic
separation, and combinations thereof. In addition, the wet
separation methods include a dissolution and crystallization
process.
In another aspect, the process of the present invention
includes the steps of calcining the trona to form sodium
carbonate, separating a first portion of the feedstream into a
first recovered portion and a first impurity portion by a dry
separation method, and separating a second portion of the
feedstream into a second recovered portion and second
impurity portion by a wet separation method, wherein the
second portion comprises particles having a particle size
larger than about 100 mesh.
In yet another aspect, the process of the present invention
includes the steps of calcining the trona to form sodium
carbonate, separating a first portion of impurities by a dry
separation method, and separating a second portion of
impurities by a wet separation method, wherein at least
about 15 wt. % of the feedstream is processed by the wet
separation method.
In yet another aspect, the process of the present invention
includes the steps of calcining the trona to form sodium
carbonate, separating a first portion of impurities by a dry
separation method, and separating a second portion of
impurities by a wet separation method, wherein an output of
the dry separation method comprises at least about 97
weight percent soluble material and at most about 0.1 wt. %
Iron.
5
ciation processes typically do not consistently produce such
a purity. Consequently, these industries generally use trona
purified by the more expensive and complex wet beneficiation
processes.
Commonly-assigned U.S. patent application Ser. No.
08/066,871, filed May 25, 1993, now U.S. Pat. No. 5,470,
554, which is incorporated herein by reference in its entirety,
discloses a dry process for beneficiating saline minerals and
which achieves purities on the order of about 97% or more.
10 The disclosed process significantly enhances the saline
mineral recovery process by producing a low cost, high
purity product. However, dry processes may have difficulty
in producing higher purities due to the problem of processing
fines and/or removing interstitial impurities by a dry
15 process.
Accordingly, it is an object of the present invention to
provide a process for the beneficiation of saline minerals and
in particular, trona, resulting in higher purities than existing
dry beneficiation processes and which is simpler and less
20 expensive than known wet beneficiation processes. It is
another object of the present invention to provide an
enhanced wet beneficiation process which has advantages
over known wet processes.
BACKGROUND OF THE INVENTION
1
METHOD FOR PURIFICATION AND
PRODUCTION OF SALINE MINERALS
FROM TRONA
REFERENCE TO RELATED APPLICATIONS
This is a continuation of application Ser. No. 08/373,955,
filed on Jan. 17, 1995 now abandoned, which is a
continuation-in-part of U.S. patent application Ser. No.
08/066,871, filed May 25, 1993, now U.S. Pat. No. 5,470,
554, and a continuation-in-part of PCT Application No.
US94/05918, filed May 25, 1994.
FIELD OF THE INVENTION
The present invention relates generally to the beneficiation
of saline minerals and, more specifically, trona. The
invention further relates to a combination dry and wet
process for recovering saline minerals from an ore containing
saline minerals and impurities, and an enhanced wet
process for beneficiating saline minerals.
Many saline minerals are recognized as being commercially
valuable. For example, trona, borates, potash and
sodium chloride are mined commercially. After mining, 25
these minerals typically need to be beneficiated to remove
naturally occurring impurities.
With regard to trona (Na2C03.NaHC03 .2H2 0), highpurity
trona is commonly used to make soda ash, which is 30
used in the production of glass and paper. Naturallyoccurring
trona, or crude trona, is found in large deposits in
the western United States, such as in Wyoming and
California, and also in Egypt, Kenya, Botswana, Tibet,
Venezuela and Turkey. Crude trona ore from Wyoming is 35
typically between about 80% and about 90% trona, with the
remaining components including shortite, halite, quartz,
dolomite, mudstone, oil shale, kerogen, mica, nahcolite and
clay minerals.
The glass and paper making industries generally require 40
soda ash produced from trona having a purity of 99% 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, crystal- 45
lizing the trona, and drying the trona which 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 mono- 50
hydrate.
Not all industries which use trona require such a highly
purified form of trona. For example, certain grades of glass
can be produced using trona having less than 97% purity.
For this purpose, U.S. Pat. No. 4,341,744 discloses a dry 55
beneficiation process which is less complex and less 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 60
magnetically separating other impurities. Such a process can
yield trona having up to about 95% to 97% purity, depending
on the quantity and type of impurities present in the crude
trona ore.
There are uses for trona, for example, in certain applica- 65
tions in the glass industry, requiring a purity of at least 97%,
yet not needing a purity over 99%. The known dry benefi3
5,911,959
4
Another aspect of the process includes the steps of
calcining trona to produce sodium carbonate, and magnetically
separating (e.g., dry magnetic separation) a first portion
of impurities from the sodium carbonate. The process
can further include the step of density separating a second
portion of impurities from the sodium carbonate, preferably
also after the calcining step.
In another aspect of the present invention, a modified
slush process is provided to enhance recovery of sodium
carbonate during wet separation methods. The modified
slush process includes the steps of introducing a portion of
the sodium carbonate to a saturated sodium carbonate brine
solution at a temperature of between about 35° C. and about
112° c., converting the sodium carbonate to monohydrate
crystals, and separating at least a portion of the monohydrate
crystals from at least a portion of the insoluble impurities,
wherein the portion of separated monohydrate crystals
includes crystals having a particle size about equal to an
average particle size of the insoluble impurities. Preferably,
the temperature of the brine solution is between about 35° C.
and about 50° C.
In various embodiments of the modified slush process, the
step of separating can be a size separation step (e.g.,
screening), a magnetic separation step, and flotation separation
step.
In another aspect, the process includes the steps of introducing
sodium carbonate to a saturated sodium carbonate
brine solution at a temperature less than about 35° c.,
converting the sodium carbonate to decahydrate crystals,
and separating at least a portion of the decahydrate crystals
from at least a portion of the insoluble impurities.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flow diagram illustrating a combined wet and
dry separation process of the present invention.
FIG. 2 is a flow diagram illustrating a modified slush
process of the present invention for the production of
sodium carbonate using size separation.
FIG. 3 is a flow diagram illustrating a modified slush
process of the present invention for the production of
sodium carbonate using magnetic separation.
DETAILED DESCRIPTION OF THE
INVENTION
Processes of the present invention are designed to recover
saline minerals from naturally occurring ores to produce
commercially valuable purified minerals. As used 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, sodium chloride, 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. Deposits of trona ore are located at several locations
throughout the world, including Wyoming (Green
River Formation), California (Searles Lake), Egypt, Kenya,
Venezuela, Botswana, Tibet and Turkey (Beypazari Basin).
For example, a sample of trona ore from Searles Lake has
been found to have between about 50% and about 90% by
weight (wt. %) trona and a sample taken from the Green
River Formation in Wyoming has been found to have
between about 80 and about 90 wt. % trona. The remaining
10 to 20 wt. % of the ore in the Green River Formation
sample comprised impurities including shortite (1-5 wt. %)
and halite, and the bulk of the remainder comprises shale
5 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.
In various embodiments of the present invention, combi-
10 nation dry and wet processes for the production of sodium
carbonate are provided. The dry processes can include any
known or hereafter developed processes for the dry beneficiation
of ores containing saline minerals and in particular,
sodium carbonate. Such processes can include density
15 separation, magnetic separation and/or electrostatic separation.
The wet processes can include any process which
includes dissolution and crystallization.
Referring to FIG. 1, one embodiment includes includes
the steps of calcining the saline mineral, e.g., trona to form
20 sodium carbonate, and separating the feedstream into a large
size fraction (e.g., larger than about 150 mesh, preferably
about 100 mesh, and more preferably about 65 mesh) and a
small size fraction (e.g., smaller than about 150 mesh,
preferably about 100 mesh, and more preferably about 65
25 mesh). The large size fraction is subsequently separated into
a first recovered portion and a first impurity portion by a dry
separation method. The first impurity portion from the dry
separation method is then separated into a second recovered
portion and a second impurity portion by a wet separation
30 method. Similarly, the small size fraction from the size
separation process is separated into a third recovered portion
and a third impurity portion by a wet separation method.
Prior to various processes of the present invention, ore can
35 be treated by a crushing step to reduce particle size. 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
40 crushing. Autogenous and semi-autogenous crushing are
particularly beneficial because the coarse particles of ore
partially act as the crushing medium, thus requiring less cost
in obtaining grinding media. Moreover, because saline minerals
are typically soft, these methods are suitable for use in
45 the present process. In addition, these two crushing methods
allow for the continuous removal of crushed material.
In general, crushing to smaller particle size achieves
better liberation of impurities and thus, improved recovery.
However, if the particle size after crushing is too fine, there
50 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
55 by crushing the ore to less than about 6 mesh.
The calcining step can be performed using any appropriate
calcining process. For example, direct heating using a
rotary kiln or fluidized bed reactor can be utilized. In a
preferred embodiment, the calcining step is performed by an
60 indirect heating process in a calcining vessel such as a
fluidized bed reactor at a temperature of at least about 120°
C. In the indirect heating process, the combustion gases
from the heat source are not in direct fluid communication
with the ore containing the saline mineral, but rather provide
65 heat to the ore by conduction through, for example, heating
coils. In one embodiment, the calcining occurs at about 140°
C. with a 20 minute residence time within a fluidized bed
5,911,959
5
reactor. Other calcining times and temperatures may also be
utilized as is known in the art. It should be noted that the
calcining step can be performed either before or after the dry
separation methods. When magnetic separation and/or density
separation is utilized as the separation step, preferably 5
calcination is performed first. When electrostatic separation
is utilized as the separation step, preferably calcination is
performed after the electrostatic separation step.
The size separation step according to the present invention
can include any appropriate size separation process 10
wherein particles larger than a predetermined cut-off size are
separated from particles smaller than the cut-off size. For
example, the process may include screening or air
classification, and preferably includes screening. The
desired cut-off size can be determined based upon a number 15
of factors, including the desired final product purity and the
relative costs associated with performing the wet separation
and dry separation methods. In general, it is desirable to
design the process such that the material that goes to wet
separation (i.e., the small size fraction and the first impurity 20
stream) comprises at least about 15 weight percent of the
original feedstream. Preferably, such weight percent is at
least about 20 weight percent, and more preferably at least
about 30 weight percent. Suitable cut-off sizes may include
150 mesh, 100 mesh, and preferably 65 mesh. Of course, the 25
actual cut-off size will depend on the desired weight percent
going to wet separation and on the size to which the original
feed was crushed.
As noted above, the large size fraction from the size
separation step is separated into a first recovered portion and 30
a first impurity portion by a dry separation method. For
example, the dry separation method may include density
separation, magnetic separation, electrostatic separation,
and combinations thereof. The dry separation method preferably
results in a large recovered portion having a soluble 35
content of at least about 97 weight percent, and further
having at most about 0.1 wt. % iron. More preferably, the
iron content is at most about 0.08 wt. %, and even more
preferably at most about 0.05 wt. %. Each of the abovenoted
dry separation methods (i.e., density separation, mag- 40
netic separation, and electrostatic separation) are described
below in more detail.
Density separation methods are based on subjecting an
ore to conditions such that materials of different densities
physically separate from each other. Thereby, certain impu- 45
rities having a different density than the desired saline
mineral can be separated. Any known density separation
technique could be used for this step of the present
invention, including air tabling or dry jigging. In density
separation, the incoming mineral stream is separated into a 50
denser and a lighter stream, or into more than two streams
of varying densities. Typically, in the case of beneficiating
trona, trona is recovered in the lighter stream. The purity of
a saline mineral recovered from density separation can be
increased by reducing the weight recovery of the recovered 55
stream from the incoming feed stream. At lower weight
recoveries, the recovered stream will have a higher purity,
but the process will also have a reduced yield because more
of the desired saline mineral will report to the impurity
stream. Such a "high purity" process may be beneficial in 60
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.
The impurity stream from density separation can go 65
through one or more scavenger density separation step(s) to
recover additional trona to improve the overall recovery. The
6
scavenger separation is similar to the above-described density
separation step. The scavenger step recovers a portion of
the impurity stream from the rougher pass having the saline
mineral in it and combines that portion with the abovedescribed
recovered stream to increase the overall yield
from density separation or recycles it to other steps in the
process, with or without further size reduction.
The recovered stream from the initial density separation
can go through one 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 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 has an
uncalcined density of about 2.14, impurities that are
removed during the density separation step may include
shortite, having a density of about 2.6, dolomite, having a
density of about 2.8-2.9 and pyrite, having a density of
about 5.0. Each of these is separable from the trona ore
because of differences in density from trona. In addition, in
the calcined state, the apparent density of trona decreases,
thereby allowing for the removal of impurities such as halite
having a density of about 2.17.
Electrostatic separation methods are based on subjecting
the ore to conditions such that materials of different electrical
conductivities separate from each other. The electrostatic
separation of the present invention can be accomplished
by any conventional electrostatic separation
technique. U.S. Pat. No. 4,341,744 to Brison et al.
("Brison") discloses standard electrostatic separation processes
suitable for use in the present invention in col. 4, line
62 through col. 6, line 32, which is incorporated herein by
reference in its entirety. As discussed in Brison, saline
mineral ore particles are first differentially electrified and
then separated into a recovered stream from an impurity
stream by various electrostatic separation processes.
As noted above for the density separation step, the impurity
stream from the initial pass of an electrostatic separation
process can go through a scavenger step to improve the
overall recovery. The scavenger step recovers a saline
mineral-containing portion of the impurity stream from the
rougher pass through electrostatic separation and combines
it with the above-described recovered stream to increase the
overall yield of the electrostatic separation step or otherwise
cycles it to other steps in the process. Furthermore, the
recovered stream from the initial pass of the electrostatic
separation can go through one or more electrostatic cleaning
steps to further remove impurities from the recovered stream
and improve the purity of the final product.
For example, electrostatic separation can be used to
separate trona from impurities having a higher electrical
conductivity, such as shale, mudstone or pyrite. It should be
appreciated, however, that electrostatic separation could also
be used to separate impurities that have a lower electrical
conductivity than the saline mineral being recovered.
Magnetic separation methods subject the ore to conditions
such that materials of different magnetic susceptibilities
separate from each other into a recovered stream and an
impurity stream. The magnetic separation step can be
accomplished by any conventional technique, such as
induced roll, cross-belt, high intensity rare earth magnetic
separation methods, or ultra high intensity magnetic sepa7
5,911,959
8
ration. Induced roll may be used in the present invention for
the finer fractions and high intensity rare earth magnets are
used for the coarser fractions. With regard to the beneficiation
of trona, typical impurities that can be removed during
the magnetic separation step include shale which has a 5
higher magnetic susceptibility than trona.
As noted above for the density and electrostatic separation
steps, the impurity stream from the initial pass of the
magnetic separation process can go through one or more
scavenger steps to improve the overall recovery. The scav- 10
enger step recovers a portion of the impurity stream from the
initial 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 yield of the magnetic separation step. 15
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.
In another embodiment of the present invention, the ore is 20
sized into multiple 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. On the other hand, a larger number of fractions 25
may also increase the cost of the overall process. The
number of size fractions used will likely depend on the size
cutoff used for the above-noted size separation step. In
general, the smaller the size cutoff, the more fractions that
will be utilized. 30
In another aspect of the present invention, the magnetic
separation method has been found to work particularly well
with respect to trona if performed after the calcination step
as compared to magnetic separation performed before the 35
calcination step. Without being bound to any particular
theory, it is believed that the improved performance has to
do with the decrease in apparent density of trona that results
from the calcination process. For example, when a cross belt
magnetic separator is used, this decrease in apparent density 40
appears to cause the non-magnetic material (i.e., the sodium
carbonate) to be thrown further horizontally than it would if
it were uncalcined trona, thereby resulting in a better separation
from the magnetic material.
The wet separation method of the various embodiments of 45
the present invention includes a dissolution and crystallization
process. Such a process takes advantage of the fact that
the solubilization and crystallization of saline minerals
results in more pure crystals because impurities are excluded
as crystals are formed after solubilization. In accordance 50
with wet separation processes of the present invention the
product recovered thereby is highly pure and can contain
greater than about 97 percent by weight (wt. %) soluble
material, more preferably greater than about 98 wt. %
soluble material and most preferably greater than about 99 55
wt % soluble material. Further, such product has less than
about 0.1 wt. % iron. More preferably, the iron content is at
most about 0.08 wt. %, and even more preferably at most
about 0.05 wt. %.
In one embodiment of the dissolution and crystallization 60
process, saline mineral crystals are dissolved in water or an
unsaturated saline solution. For example, in the case of
trona, trona (the sesquicarbonate form of sodium carbonate)
or anhydrous sodium carbonate (calcined trona) can be
dissolved. Once in solution, water is driven off, and the 65
saline mineral crystallizes. For example, the water can be
driven off by heating the solution. However, such a process
can be expensive and time-consuming due to the energy
required to heat the water and the amount of time required
to fully dry the crystals.
Another method to perform dissolution and crystallization
in the instance of trona is to perform a process known as the
"slush" process. In the slush process, trona is first calcined
to produce anhydrous sodium carbonate crystals which are
added to a saturated sodium carbonate brine solution. As
anhydrous crystals go into solution and recrystallize, they
crystallize in the monohydrate form if the temperature is
between 35° C. and about 112° C. Accordingly, there is a
continuous crystallization process which tends to significantly
reduce the impurities in the crystals. Crystals can then
be recovered from the brine solution. A specific example of
the slush process is disclosed in U.S. Pat. No. 2,887,360
issued May 19, 1959 (Hoekje).
In another embodiment of the combination wet and dry
process of the present invention, a process for the production
of saline mineral from a feedstream having impurities is
provided. The process includes the steps separating a first
portion of said feedstream into a first recovered portion and
a first impurity portion by a dry separation method, and
separating a second portion of said feedstream into a second
recovered portion and second impurity portion by a wet
separation method, wherein said second portion comprises
particles having a particle size larger than about 100 mesh
and more preferably, larger than about 65 mesh. The dry
separation method is selected from the group consisting of
density separation, magnetic separation, electrostatic
separation, and combinations thereof. In addition, the wet
separation method includes a dissolution and crystallization
process. In the instance of trona, this embodiment can
further include the step of calcining the trona to form sodium
carbonate.
In another embodiment of the combination wet and dry
process of the present invention, a process for the production
of saline mineral from a feedstream having impurities is
provided. This embodiment includes the steps of separating
a first portion of impurities by a dry separation method, and
separating a second portion of impurities by a wet separation
method, wherein at least about 15 weight percent of said
feedstream is processed by said wet separation method. The
dry separation method is selected from the group consisting
of density separation, magnetic separation, electrostatic
separation, and combinations thereof. In addition, the wet
separation method includes a dissolution and crystallization
process. Preferably, the weight percent processed by said
wet separation process is at least about 20 percent, and more
preferably at least about 30 percent. In the instance of trona,
this embodiment can further include the step of calcining the
trona to form sodium carbonate.
In yet another aspect, the combination wet and dry
process includes separating a first portion of impurities by a
dry separation method, and separating a second portion of
impurities by a wet separation method, wherein an output of
said dry separation method comprises at least about 97
weight percent soluble material and at most about 0.1 weight
percent iron. The dry separation method is selected from the
group consisting of density separation, magnetic separation,
electrostatic separation, and combinations thereof. In
addition, the wet separation method includes a dissolution
and crystallization process. Preferably, the weight percent of
iron present in the output of said dry separation process is at
most about 0.08 weight percent iron, and more preferably at
most about 0.05 weight percent iron. In the instance of trona,
this embodiment can include the step of calcining the trona
to produce sodium carbonate.
5,911,959
9 10
When the method of separation of impurities in the
modified slush process is froth flotation, the sodium carbonate
crystal-containing stream is subjected conditions suitable
for selectively causing sodium carbonate crystals to float to
the surface of the stream, to the exclusion of the impurities.
The floating sodium carbonate crystals are then recovered.
The remaining stream can then be treated to remove the
impurities such as by filtration.
The sodium carbonate in various embodiments of the
present invention may further include soluble impurities, in
which case such soluble impurities may enter into the brine
solution. Such soluble impurities will be excluded from the
recovered sodium carbonate crystals, but may build up to
undesireable levels in the brine solution. Thus, such soluble
impurities can be removed by a variety of suitable methods.
For example, a bleed stream can be removed from the
saturated brine solution to remove soluble impurities and
prevent build up thereof. Further, soluble impurities can be
removed from the bleed stream, for example, by use of an
activated charcoal filter.
In yet another embodiment of slush processes of the
present invention, including the modified slush process, after
the sodium carbonate has been introduced into the brine
solution and conversion to monohydrate crystals has
25 occurred, the brine solution may be increased in temperature
to above about 112° C. to convert monohydrate crystals to
anhydrous form. Subsequent cooling of the brine solution
converts the anhydrous sodium carbonate back into monohydrate
form. Such multiple recrystallization processes
30 assist in further purging the impurities and improving the
overall purification process. However, in a preferred
embodiment, no such conversion takes place since it is
believed than sufficient purity can be obtained without the
extra crystallization steps. Avoiding the above-noted heating
and crystallization steps reduces the energy required to
perform the process of the present invention.
In another aspect of the slush processes of the present
invention, including the modified slush process, the brine
solution is at a temperature of less than about 35° C. In this
aspect, anhydrous sodium carbonate will convert to sodium
decahydrate upon introduction into the brine solution. The
decahydrate crystals can then be separated from impurities
in the manner as described elsewhere herein. If desired, the
brine solution can then be heated to a temperature above 35°
C. to convert the decahydrate crystals to monohydrate
crystals.
The foregoing description of the present invention has
been presented for purposes of illustration and description.
Furthermore, the description is not intended to limit the
invention to the form disclosed herein. Consequently, variations
and modifications commensurate with the above
teachings, and the skill or knowledge of the relevant art, are
within the scope of the present invention. The embodiment
described hereinabove is further intended to explain the best
mode known for practicing the invention and to enable
others skilled in the art to utilize the invention in such, or
other, embodiments and with various modifications required
by the particular applications or uses of the present invention.
It is intended that the appended claims be construed to
include alternative embodiments to the extent permitted by
the prior art.
What is claimed is:
1. A process for the production of sodium carbonate from
a feedstream of trona having impurities comprising the steps
of:
(a) crushing said trona to a particle size of less than about
6 mesh;
Further embodiments of the present invention include a
modified slush process which provide for high recovery of
sodium carbonate compared to known processes. The modified
slush process includes introducing sodium carbonate to
a saturated sodium carbonate brine solution at a temperature 5
of between about 35° C. and about 112° C. and converting
the sodium carbonate to monohydrate crystals. The process
further includes separating at least a portion of said monohydrate
crystals from at least a portion of said insoluble
impurities, wherein the portion of monohydrate crystals 10
includes crystals having a particle size about equal to an
average particle size of said insoluble impurities. The temperature
of the brine solution for the modified slush process
is more preferably between about 35° C. and about 50° C.
Typically impurities which can be removed by the modified 15
slush process include iron-bearing materials, dolomite,
shale, searlesite and northupite.
Appropriate residence times of sodium carbonate in the
brine solution can be selected by those skilled in the art. It
should be recognized, however, that longer residence times 20
will result in larger monohydrate crystals which can have
significant advantages with respect to recovery. It is believed
that residence times of the sodium carbonate in the brine
solution could be as little as 15 minutes, but can be as long
as 1 or 1.5 hours.
The step of separating monohydrate crystals from
insoluble impurities in the modified slush process of the
present invention can be accomplished by a variety of
suitable methods. Such methods can include size separation,
magnetic separation and flotation.
When the method of separation is size separation, as
illustrated in the flow chart of FIG. 2, the process parameters
for the modified slush process are such that crystals grow to
a size larger than the impurities. For example, the monohydrate
crystals may be larger than about 150 mesh, preferably 35
about 100 mesh, and more preferably about 65 mesh. Of
course, the desired crystal size will depend in part on the size
of the impurities, which depends on the size to which the
original feedstream was crushed. In addition, the modified
slush process further treats the resulting mixture to separate 40
the impurities from the small crystals.
When the method of separation is size separation, as
illustrated in the flow chart of FIG. 2, the modified slush
process can include, for example, treating the impuritycontaining
fraction to recover crystals in the impurity- 45
containing fraction which are smaller than the size separation
cutoff. Such a treatment can include filtering the solids
from the impurity-containing fraction to form a cake of
small crystals and impurities, dissolving the small crystals in
water, screening out the impurities, and recycling the solu- 50
tion to the brine solution. Alternatively, the water in the
recycle solution could be driven off (e.g., by heating) to
recover the sodium carbonate by crystallization.
When the method of separation of impurities in the
modified slush process is magnetic separation, as illustrated 55
in FIG. 3, the sodium carbonate crystal-containing stream is
subjected to a magnetic flux so that impurities which are
magnetically susceptible are removed. For example, a magnetic
separation may be performed by a magnetic flux
density of at least about 15 Gauss. Preferably, the magnetic 60
flux density is at least about 20 Gauss, and more preferably
at least about 30 Gauss. The method of magnetic separation
can be wet or dry separation and is preferably wet magnetic
separation. Once the impurities are removed, the resulting
stream can then be filtered to remove most of the crystals, 65
and the remaining solution can be recycled back to the brine
solution or dried to recover the dissolved sodium carbonate.
5,911,959
11 12
* * * * *
13. A process, as claimed in claim 6, wherein said sodium
carbonate introduced into said brine solution further comprises
soluble impurities, and wherein said process further
comprises the step of treating said brine solution to remove
at least a portion of said soluble impurities.
14. A process, as claimed in claim 13, wherein said step
of treating said brine solution to remove soluble impurities
comprises the step of removing a bleed stream having said
soluble impurities from said brine solution.
10 15. A process, as claimed in claim 14, further comprising
removing said soluble impurities from said bleed stream and
reintroducing said bleed stream into said brine solution.
16. A process, as claimed in claim 10, wherein said size
15 separation step comprises separating monohydrate crystals
having a crystal size larger than a size separation cutoff from
a stream comprising said insoluble impurities and small
monohydrate crystals having a crystal size smaller than the
size separation cutoff, and wherein said process further
20 comprises the step of separating insoluble impurities and
said small monohydrate crystals from said stream.
17. A process, as claimed in claim 16, wherein said
separated insoluble impurities and small crystals form a
cake, and wherein said process further comprises the step of
dissolving said small monohydrate crystals from said cake to
produce a recycle solution.
18. A process, as claimed in claim 17, further comprising
the step of separating said insoluble impurities from said
recycle solution.
19. A process for the production of sodium carbonate from
trona having impurities, comprising the steps of:
(a) crushing said trona to less than about 6 mesh;
(b) calcining said trona to produce sodium carbonate;
(c) separating a first portion of impurities comprising
shortite from said sodium carbonate by density separation
such that materials of different densities are
separated from each other;
(d) introducing a portion of said sodium carbonate to a
saturated brine solution of sodium carbonate;
(e) converting said sodium carbonate to monohydrate
crystals; and
(f) separating at least a portion of said monohydrate
crystals from a second portion of impurities comprising
iron-bearing materials.
(b) calcining said trona to produce sodium carbonate and;
(c) magnetically separating a first portion of impurities
from said sodium carbonate.
2. A process, as claimed in claim 1, wherein said step of
magnetically separating is a dry magnetic separation pro- 5
cess.
3. A process, as claimed in claim 2, wherein said process
results in a recovery of said sodium carbonate of at least
about 97 weight percent and an iron content of at most about
0.1 weight percent.
4. A process, as claimed in claim 1, further comprising the
step of density separating a second portion of impurities
from said sodium carbonate.
5. A process, as claimed in claim 1, wherein said step of
density separating occurs after said step of calcining.
6. A process for the production of sodium carbonate from
trona having insoluble impurities comprising the steps of:
(a) crushing said trona to a particle size of less than about
6 mesh;
(b) calcining said trona to produce sodium carbonate;
(c) separating a first portion of impurities by density
separation such that materials of different densities
separate from each other;
(d) introducing a portion of said sodium carbonate to a 25
saturated sodium carbonate brine solution;
(e) converting said sodium carbonate to monohydrate
crystals; and
(f) separating at least a portion of said monohydrate
crystals from at least a portion of said insoluble impu_ 30
rities.
7. A process, as claimed in claim 6, wherein said step of
calcining said trona is conducted before said step of separating
a first portion of impurities.
8. A process, as claimed in claim 7, wherein said first 35
portion of impurities comprises an impurity selected from
the group consisting of halite and shortite.
9. A process, as claimed in claim 6, wherein said density
separation comprises a process selected from the group
consisting of air tabling and dry jigging. 40
10. A process, as claimed in claim 6, wherein said step of
separating at least a portion of said monohydrate crystals is
a size separation step.
11. A process, as claimed in claim 10, wherein said
monohydrate crystals are larger than about 150 mesh. 45
12. A process, as claimed in claim 6, wherein a temperature
of said saturated brine solution is between about 35° C.
and about 50° C.