Recovery of alkali values from trona ore

Alkali values are recovered in a cyclic method from mechanically mined trona ore or from trona deposits by a solution mining technique utilizing a solvent comprising an aqueous solution of sodium sulfide. The ore is solubilized as sodium carbonate which is precipitated as sodium sesquicarbonate and/or sodium bicarbonate by treating the solution with hydrogen sulfide and the precipitated salt separated from the mother liquor. In the process, sodium sulfide and hydrogen sulfide become converted into sodium hydrosulfide which is contained in the mother liquor. Heat treatment of the mother liquor converts the sodium hydrosulfide into sodium sulfide thereby regenerating a solution of sodium sulfide which is recycled to the trona ore and into hydrogen sulfide which is evolved and recycled to the sodium carbonate solution.

This invention relates to the recovery of alkali values from trona ore. 
More particularly, the trona is solubilized with an aqueous solvent 
containing sodium sulfide and the alkali values contained in the solution 
are ultimately recovered as soda ash. 
More soda ash (sodium carbonate) produced in the United States is obtained 
from naturally-occurring subterranean trona ore deposits in Wyoming, which 
consist mainly of sodium sesquicarbonate, Na.sub.2 
CO.sub.3.NaHCO.sub.3.2H.sub.2 O. At the present time, these trona deposits 
are mechanically mined and the trona converted to soda ash by either the 
sesquicarbonate process or the monohydrate process, whose features are 
summarized in U.S. Pat. No. 3,528,766. 
Currently-employed soda ash processes utilize only those trona deposits 
which are recoverable by mechanical mining and which are relatively low in 
soluble, nonsesquicarbonate impurities, such as chlorides and sulfates. 
Large trona deposits exist in the same area which are associated with 
substantial amounts of sodium chloride, containing up to 10% or more by 
weight NaCl. By contrast, trona deposits presently being worked ordinarily 
contain less than 0.1% by weight sodium chloride, 0.04%-0.08% NaCl being 
typical. Soda ash has not previously been prepared from salt-containing 
subterranean Wyoming trona deposits because of the depth of the deposits, 
which precludes their being mechanically mined. The subterranean trona 
beds located in southwestern Wyoming constitute the world's largest known 
reserves of trona and have heretofore remained unexploited because of 
their substantial salt content. 
The existence of problems associated with the presence of sodium chloride 
in trona deposits is evidenced by the fact that no salt-containing 
underground Wyoming trona deposits are being mined in commercial soda ash 
operations, as well as by the disclosures in U.S. Pat. Nos. 3,119,655 and 
3,273,959 which relate to the dissolution of low salt (&lt;0.1% NaCl) trona. 
The present invention provides an economical method of preparing a 
relatively pure soda ash product by utilizing an aqueous solvent to 
recover the alkali values from trona regardless of the salt content of the 
trona. 
In accordance with the present invention, alkali values are recovered from 
underground trona ore deposits in a method which comprises introducing 
into the region of the trona deposit an aqueous mining solvent having 
dissolved therein sodium sulfide, maintaining the solvent in the region to 
solubilize trona as sodium carbonate until the solvent comes essentially 
into an equilibrium with the sodium sesquicarbonate, withdrawing at least 
a portion of the resulting mining solution from the region and recovering 
alkali values from the withdrawn solution. 
The trona ore in an alternative procedure may be mechanically mined and the 
mined ore treated with an aqueous solvent containing from about 3 to about 
8% by weight sodium sulfide in a manner analogous to the method employed 
with underground trona deposits.

A preferred recovery method of this invention involves introducing into the 
region of the trona deposit an aqueous mining solvent having dissolved 
therein from about 3 to about 8% by weight sodium sulfide; maintaining the 
solvent in the region to solubilize trona as sodium carbonate until the 
solvent comes essentially into an equilibrium with the sodium 
sesquicarbonate and withdrawing from the region at least a portion of the 
resulting mining solution. The withdrawn mining solution is treated with 
hydrogen sulfide so as to convert the sodium carbonate to sodium 
sesquicarbonate and sodium bicarbonate which are crystallized and the 
crystallized solid is separated from the mother liquor. The mother liquor 
contains sodium carbonate, sodium sesquicarbonate, sodium hydrosulfide and 
hydrogen sulfide. 
In the preferred method, the mother liquor is treated to regenerate a 
mining solvent which is recycled to the region of the trona ore. In the 
solubilization of the sodium sesquicarbonate to sodium carbonate the 
sodium sulfide is converted to sodium hydrosulfide. During the treatment 
of the withdrawn mining solution the major portion of the dissolved sodium 
carbonate reacts with the hydrogen sulfide to form sodium sesquicarbonate 
which is precipitated and sodium hydrosulfide. Some sodium carbonate will 
react with the hydrogen sulfide to form sodium bicarbonate which is 
precipitated and sodium hydrosulfide. Following the separation of the 
crystallized solids, the mother liquor is subjected to distillation. 
Hydrogen sulfide dissolved in the liquor is evolved. Any sodium 
bicarbonate present in the liquor will react with sodium hydrosulfide to 
form sodium carbonate and hydrogen sulfide which is evolved. The sodium 
hydrosulfide is decomposed to reconstitute the sodium sulfide and to form 
hydrogen sulfide which is evolved. The evolved hydrogen sulfide is 
recycled to the withdrawn mining solution. To reconstitute the mining 
solvent, sufficient water is added to compensate for water losses 
resulting from crystallization and separation of the crystallized solids, 
for that which may be lost during distillation and for the volume of 
solvent remaining in the region of the trona deposit. Sodium sulfide is 
added to compensate for that remaining in the mining solution which 
occupies the volume of trona removed. 
The resulting reconstituted mining solvent is reintroduced into the region 
of the trona deposit and the cycle of recovery repeated. 
Soda ash is preferably recovered as the alkali product from the 
crystallized sodium sesquicarbonate and sodium bicarbonate by calcination 
of the crystallized solids in a conventional calciner. 
The method of this invention is well suited for solution mining of the 
large trona deposits that exist in southwestern Wyoming whether or not the 
deposits are associated with sodium chloride. Although the salt-free 
deposits are generally located about 1500 feet or less below the surface 
and are presently mined, the method of this invention may be utilized for 
solution mining these deposits. The deposits associated with high amounts 
of salt are generally found in beds from about 2000 to 4000 feet below the 
surface. The depth of these beds precludes the use of conventional 
mechanical methods to recover the salt-containing trona. 
Where sodium chloride is associated with trona it varies in amount and its 
degree of intermixture with the trona. Deposits containing 3% up to 10% or 
more by weight sodium chloride are generally considered to be associated 
with substantial amounts of salt. Because the interlayered trona seams in 
the deeper beds of salt-containing trona are thick, numerous and 
relatively close together, the recovery of alkali values therefrom is 
facilitated by the method of this invention which utilizes an aqueous 
solvent introduced to the region of the trona deposits by solution mining 
techniques. 
It is well known that sodium chloride reduces the solubility of sodium 
carbonate in water slightly, but that it reduces substantially the 
solubility of sodium sesquicarbonate and sodium bicarbonate. Thus, the 
yield of recovered alkali values by precipitation per unit of recovered 
mining solution is not decreased significantly by the presence of sodium 
chloride associated with the trona deposit. 
Although the aqueous solvent is preferably introduced into the region of 
the trona deposits, especially where the deposit is associated with 
substantial amounts of sodium chloride, the alternative embodiment of the 
invention provides for contacting mechanically mined ore with the aqueous 
solvent. After the aqueous solvent has been maintained in contact with the 
trona ore for a period of time sufficient to come into an equilibrium with 
the sodium sesquicarbonate, the solution is withdrawn from the region of 
the underground trona deposit (solution mining technique) or from the 
mined trona and the separated solution treated to recover the alkali 
values. 
The aqueous solvent employed in the method of this invention contains from 
about 3 to about 8% by weight sodium sulfide, preferably from 4 to 7%. 
Recycled aqueous solvent contains sodium sulfide and sodium carbonate 
produced during regeneration of the solvent. In solution mining, other 
soluble impurities such as sulfates, chlorides, borates and phosphates do 
not accumulate in appreciable concentrations in the regenerated aqueous 
solvent. This is accounted for by the fact that the volume of solvent 
which remains in the cavity to replace the dissolved trona functions as a 
purge stream. 
The temperature of the solvent is not critical, temperatures of from 
20.degree. to 80.degree. C. being satisfactory. In surface treatment of 
mechanically mined trona, the solvent temperature preferably is between 
30.degree. and 50.degree. C. to provide for maximum solubility of sodium 
carbonate. Higher temperatures may be used but are less practical because 
of the decreased solubility of sodium carbonate and because of the energy 
costs required. In solution mining, the solvent may be introduced at 
temperatures around 30.degree. C. without significant heat losses. Trona 
deposits that are 2,000 feet and more below the surface generally have a 
temperature of about 25.degree.-35.degree. C. and the ground temperature 
increases for deeper deposits. 
Upon startup, an aqueous solution containing the proper sodium sulfide 
content is employed as the solvent until sufficient regenerated solvent 
can be prepared from the solution following recovery of the alkali values. 
The aqueous mining solvent is desirably introduced into the region of the 
underground trona deposits by means of one or more wells using 
conventional solution mining techniques. An alternative to the preferred 
solution mining technique involves mechanical mining of the subterranean 
trona ore, followed by treatment of the mined ore with a solvent similar 
to the aqueous mining solvent in a surface treatment operation. The 
economics of solution mining with the aqueous mining solvent, however, 
favor this procedure over presently employed mechanical mining methods. 
A single solution mining well ordinarily has an injection pipe and 
withdrawal pipe. Separate injection and withdrawal wells may be used, the 
two types desirably being spaced apart, located from a few hundred to one 
thousand feet apart, and being connected via underground fractures in the 
trona formation through which the aqueous mining solvent may pass. 
Introduction of the aqueous mining solvent to the region of a 
salt-containing trona ore deposit results in the formation of a solution 
which comes into equilibrium with the sodium sesquicarbonate and possibly 
with sodium chloride if present. The concentration of sodium chloride will 
be dependent primarily upon the amount present with the trona. 
Although sodium chloride reduces substantially the solubility of sodium 
sesquicarbonate, the aqueous mining solvent overcomes the marginal 
solubility of trona by the reaction with sodium sulfide to form soluble 
sodium carbonate. The sodium chloride reduces the solubility of sodium 
carbonate but slightly and thus has no significant affect on the recovery 
of the alkali values. 
The dissolution of the trona by its reaction with sodium sulfide results in 
the release of water of hydration from the trona which dilutes the aqueous 
solvent and may improve slightly the dissolving rate of the trona. Also, 
the reaction of the sodium sulfide in the solvent with the trona is mildly 
exothermic, and in the case of solution mining, provides a source of 
localized heating which maintains the desired temperature and also 
promotes convective circulation of the solution to increase the rate of 
dissolution of the sodium sesquicarbonate. 
In the practice of solution mining, at least a portion of the aqueous 
mining solvent that is introduced into the region of the trona deposits is 
withdrawn as mining solution, having a composition and characteristics as 
noted above. It should be apparent that recovery of mining solution in an 
amount or rate equivalent to the amount or rate of solvent introduced may 
not be feasible in a continuous, sustained operation. It is estimated that 
approximately onetenth of the introduced solvent will remain behind in the 
cavity left by dissolved trona since such solvent replaces dissolved trona 
which is withdrawn in the mining operation. The contribution of the water 
from the hydrated water of the dissolved sodium sesquicarbonate and as a 
byproduct of the sodium sulfide-sodium sesquicarbonate reaction does not 
appreciably offset these losses of mining solvent which replace dissolved 
trona. 
The solution mining of trona ore deposits with an aqueous solvent 
containing the preferred 4 to 8% by weight sodium sulfide can result in 
about 0.2 lb. sodium carbonate per gallon of solution withdrawn from the 
ground. 
The operations involved in the solution mining of subterranean trona 
deposits to recover sodium sesquicarbonate and sodium bicarbonate and 
ultimately soda ash are illustrated in the flow diagram of FIG. 1. 
The mining solution withdrawn from the region of the trona deposit is 
introduced into suitable vessel (MIXER-CRYSTALLIZER) wherein the solution 
is treated with hydrogen sulfide. The vessel preferably constitutes a 
gas-liquid mixer. The mining solution comprises an aqueous solution of 
sodium carbonate and sodium hydrosulfide. As the hydrogen sulfide is added 
and mixed with the mining solution, the pH of the solution is lowered and 
the sodium carbonate is converted into sodium sesquicarbonate and sodium 
bicarbonate which are precipitated. In the reaction between hydrogen 
sulfide and sodium carbonate the hydrogen sulfide is converted into sodium 
hydrosulfide. The nature of the precipitated sodium salt is dependent upon 
the pH of the solution. At pH's above about 9.70, sodium sesquicarbonate 
is precipitated. At pH's below about 9.70, sodium sesquicarbonate and 
sodium bicarbonate are precipitated. In general, as the amount of hydrogen 
sulfide introduced is increased and the pH of the solution decreases, the 
greater the proportion of sodium bicarbonate produced. Any other soluble 
salts, if present, such as sodium chloride, borates, sulfates and the like 
derived from the impurities associated with the trona remain in solution. 
The precipitated crystalline material is separated from the mother liquor 
as illustrated (SEATOR) in FIG. 1. Separation may be effected with a 
centrifuge, gravity separator and filter or other suitable conventional, 
solid-liquid separation equipment. The separated crystalline material is 
preferably washed with water and the wash water added to the mother 
liquor. 
The recovered sodium sesquicarbonate and sodium bicarbonate are converted 
into soda ash in a CALCINER as shown in FIG. 1. The calciner may be gas 
fired, steam tube or fluid bed conventional calciner. 
The mother liquor after separation of the crystalline material is 
regenerated to form an aqueous mining solvent which may then be recycled 
to the region of the trona deposit. The mother liquor contains sodium 
carbonate, sodium bicarbonate, sodium hydrosulfide and hydrogen sulfide. 
The mother liquor is subjected to distillation, as in STILL, FIG. 1, 
whereby the hydrogen sulfide is evolved. Any sodium bicarbonate present in 
the liquor will react with sodium hydrosulfide to form sodium carbonate 
and hydrogen sulfide which is evolved. Sodium hydrosulfide is decomposed 
to form sodium sulfide and hydrogen sulfide which is evolved. The evolved 
hydrogen sulfide is recycled to the MIXER-CRYSTALLIZER. 
To the resulting aqueous solution of sodium sulfide make-up water and 
sodium sulfide are added to provide the desired sodium sulfide 
concentration, (SOLVENT MAKE-UP) FIG. 1. Wash water from the crystal 
separation step may constitute a portion of the make-up water. A portion 
of the added water serves to replace water losses occurring during 
crystallization and water losses from separation of the crystallized 
material from the mother liquor. Additional water and sodium sulfide are 
added to compensate for the aqueous solvent which replaces the volume of 
dissolved trona. The reconstituted mining solvent is reintroduced to the 
region of the trona deposit, (INJECTION WELL) FIG. 1. 
The presence of salt with the trona deposit has no significant affect on 
the recovery of alkali values by the practice of the present method. While 
the solubility of sodium sesquicarbonate is reduced substantially by the 
presence of sodium chloride, the present method involves the 
solubilization of trona as sodium carbonate the solubility of which is 
decreased but slightly. The salt concentration in the mining solution will 
remain relatively constant and will be in equilibrium with the salt 
associated with the trona. The salt will remain dissolved in the mining 
solution as it travels through the cycle and will not build up since upon 
reinjection to the region of the trona that protion which replaces 
dissolved trona functions as a purge stream. 
In the modification illustrated in FIG. 2, the mining solution withdrawn 
from the well is treated in the MIXER-CRYSTALLIZER with an additional 
quantity of hydrogen sulfide supplied from an external source. In the 
Wyoming area of trona deposits and adjacent thereto, are found gas wells 
from which sour gas evolves. This gas may be used as the source of the 
additional hydrogen sulfide. As excess hydrogen sulfide is mixed with the 
withdrawn mining solution, the pH of the solution is lowered and the 
relative proportion of sodium bicarbonate increases and increases the 
amount of sodium hydrosulfide formed. 
As in the method depicted in FIG. 1, following treatment of the solution 
with hydrogen sulfide and precipitation of sodium sesquicarbonate and 
sodium bicarbonate, the crystallized material is separated from the mother 
liquor. After washing the separated crystallized material it is calcined 
to form soda ash. 
The mother liquor is regenerated to form an aqueous mining solvent which is 
recycled to the region of the trona deposit. The mother liquor is 
subjected to distillation, as in the STILL, FIG. 2, whereby hydrogen 
sulfide is evolved. Sodium hydrosulfide is decomposed to sodium sulfide 
and hydrogen sulfide which is evolved. The evolved hydrogen sulfide is 
recycled to the MIXER-CRYSTALLIZER. 
The amount of hydrogen sulfide added from the external hydrogen sulfide 
source, such as sour gas, is sufficient to form an amount of sodium 
hydrosulfide during the conversion of the sodium carbonate in the mining 
solution to provide for the required sodium sulfide in the mining solvent. 
This type of operation eliminates the need of supplying added sodium 
sulfide to compensate that remaining in the mining solution as replacement 
for the dissolved trona. Make-up water is added to replace water losses 
during crystallization and water losses from separation of the 
crystallized material from the mother liquor. Water is also added to 
compensate for the solvent which replaces dissolved trona. 
In an alternative practice of the method of the invention, mechanically 
mined trona is utilized. The operations are illustrated diagrammatically 
in FIG. 3. 
Trona ore is fed to a suitable CRUSHER, such as a hammer mill, and the 
crushed ore transferred to a suitable vessel, DISSOLVER, where it is 
treated with an aqueous solvent containing sodium sulfide. The 
concentration of sodium sulfide may be as described above. The temperature 
may be from about 30.degree. to 50.degree. C., preferably at least 
30.degree. C. 
The resulting solution comparable to the withdrawn mining solution is 
passed to a suitable gas-liquid mixer, MIXER-CRYSTALLIZER, wherein 
hydrogen sulfide is mixed with the solution. As described above, as the 
hydrogen sulfide is mixed with the solution, the pH of the solution is 
decreased and the sodium carbonate is converted into sodium 
sesquicarbonate and sodium bicarbonate which precipitate. The relative 
proportions of sodium sesquicarbonate and sodium bicarbonate produced may 
be controlled by the amount of added hydrogen sulfide. 
The precipitated salts are separated from a mother liquor (SEATOR) as 
illustrated in FIG. 3. Preferably, the separated crystalline material is 
washed with water. The recovered sodium sesquicarbonate and sodium 
bicarbonate are converted into soda ash in a CALCINER which may be gas 
fired, steam tube or fluid bed conventional calciner. 
The mother liquor separated from the crystalline material is subjected to 
distillation, as in a STILL, FIG. 3, whereby hydrogen sulfide is evolved 
which is recycled to the MIXER-CRYSTALLIZER and mixed with additional 
solution of trona. The sodium hydrosulfide is decomposed to form hydrogen 
sulfide which is evolved and sodium sulfide thereby regenerating the 
solvent.