PLANT OF THE AMERICAN POTASH AND CHEMICAL CORPORATION
q. RWR* University of California, Los Angeles, Calif.
I
N AN EARLIER article (11) the main project of the
Searles Lake industry, dealing with potash and borax, was outlined in an illustrative manner. The processes have subseauentlv been discussed in more technical detail bv Gale (6) a i d Mirnford (9). The development of severa1"newer products now makes it appropriate to present a sequel to the original report, Since the previous writing, activities of all sorts at the isolated desert town of Trona, Calif., have more than doubled. Twelve hundred tons of refined alkali-metal salts, instead of 400, roll out daily over the company railway, destined for domestic and foreign markets. Population has grown to 2200, an ultramodern high school has just been completed, and two excellent highways give convenient entry. It is still true, however, that the Searles Lake settlement, including its Westend neighbor, stands a little off-center in an area of 5000 square miles containing no other town or village. After contemplating all this lebensraum, one discovers that Main Street, Trona, now displays a one-hour parking sign. Such is the gregarious nature of man. Since the launching of the potash-borax enterprise, the Trona researuh staff has wrested a t least five new ingredients from the complex Searles Lake brine. These include sodium sulfate, sodium carbonate, potassium sulfate, sodium bromide, and bromine. Lithium concentrates are already avail-
able, with the possibility of pure salts later. Far from admitting any terminus to the problem, the desert chemists are not dead sure that there is any chemical element in the whole Deriodic table not remesented in the Aw brine.
Salvage of Sodium Salts The original potash-borax process involved the rejection of huge quantities of sodium sulfate and carbonate. I n the presence of an excess of sodium chloride these components crystallize readily as the anhydrous double salt, burkeite, normally designated as Na2COs.2Na2S04. I n actual practice with solutions of varying concentration, several modifications of the standard burkeite formula are recognized, including 2Na2COs.3Na2S04 or sesquiburkeite. Despite the apparent adaptability of purified burkeite to the glass industry, the market will have none of it. Only separate single salts are acceptable. Accordingly, in earlier years more than a million tons of burkeite were rejected along with almost fabulous amounts of sodium chloride. The task of cracking the double salt by the phase rule was troublesome, but the carbonate and sulfate products are now recovered to whatever degree plant development and current market may justify. The Trona processes are so extensively cyclic in character that it is impossible to begin an explanation simply and pre133
cisely with any one solution or solid mass as the “starting material”. Even the raw brine entering the plant is promptly employed as condensing and counterwashing media, and is further altered by addition of recycled mother liquors or wash solutions so that its behavior by no means agrees with a laboratory experiment starting anew. Furthermore, the chemical engineer constantly plays little tricks with temperature, pressure, and concentration which are not quite in agreement with the formal phase diagrams. To describe all these with scientific justification would plunge this account into confusion. For illustrative purposes, therefore, it is assumed here that one starts with lake brine, which is first evaporated at a temperature far above the wellknown thermal region in which both sodium sulfate and carbonate form decahydrates. This is merely the first step in the old potash-borax process. A gross precipitation a t this stage yields sodium chloride, burkeite, and a small quantity of sodium carbonate monohydrate. It is just here that the old process, which threw these salts t o waste, is interrupted by the new technique.
Isolation of Sulfate and Carbonate
Above. Soda Products Digesting Tanks L e f t . Evaporator Unit No. 3
Below. Sal Soda Coolers
The sodium chloride just mentioned appears for the most part as coarse crystals, 50 mesh per inch (20 mesh per cm.) and larger. The burkeite and sodium carbonate come out as fine crystals, mostly under 50 mesh but not colloidal. The whole mixture passes as a hot slurry to huge conical separators, known as salt traps, in which the coarse salt settles rapidly. Countercurrent washing leaves the sodium chloride for rejection to the sewer, but is sufficiently vigorous to carry not only the valuable mother liquor but also the fine sulfate-carbonate crystals to another conical receiver known as a clarifier. Some of the recent equipment used in the operations just described is reported to be the largest of its kind in the world. Entering the clarifier a t a mid-position, the suspension remains long enough to settle partially and permit a continuous decantation of clear hot concentrated liquor over the top. At the bottom the thickened slurry passes out to Oliver filters. Possible flashing of the hot liquor under the partial vacuum is obviated a t this point by the addition of some cold end liquor just before filtration. The filter cake, consisting of burkeite, sodium carbonate monohydrate, and a little sodium chloride, is washed with lake brine and now becomes the principal raw material for the soda products plant.
Phase Separations Manipulations of temperature, pressure, and concentration alone do not suffice to isolate both sulfate and carbonate from the mixture. The Trona operator is compelled to resort to the device of addition of a component. Accordingly, the discussion of plant practice is interrupted here to present, in abbreviated form, the main phase diagrams which show the theoretical way out. Continuous curve AEB of Figure 1represents solubilities of sodium carbonate in all possible concentrations of sodium sulfate, and vice-versa, a t 23” C.; solid phases are the two decahydrates. The figure is virtually a cross section of the three-dimensional diagram for the system as depicted by Allen, Gale, and Ritchie (1). 134
Arrow CD follows the increasing concentration of salts when burkeite, the essential raw material, is dissolved in water, Since the temperature of this operation is several degrees above 23", the arrow is enabled to pass well beyond boundary EB. Terminus D sets itself arbitrarily, despite established phase "laws" to the contrary, for which the graphic details are readily shown in a three-dimensional figure ( 1 ) but omitted here to avoid confusion in a plane figure. Unfortunately for clarity of a plane illustration, solubilities retreat after the region of 30" C. is well passed, and the new phase boundaries would fall awkwardly in the E-D-B region. Since D falls well below E (an outpost on the boundary between carbonate and sulfate), the burkeite solution designated D, upon cooling, will deposit a substantial yield of one compound-namely, Glauber salt. I n this manner the simpler part of the present task was solved a t Trona. D E represents the progressive deposition of Glauber salt. It proceeds to the left because of depletion of water as decahydrate. At E the boundary of the sodium carbonate field is reached. Barring possible supersaturation, further cooling would precipitate both sal soda and Glauber salt. The phase operator removes the Glauber salt and is now compelled to add a new componentnamely, sodium chloride. Simultaneously the temperature is raised to 50" C. or above. These two changes cause a complete disappearance of the decahydrate curves, and a great lowering of solubilities of carbonate and especially sulfate. The result is shown in Figure 2, where LJGH replaces AEB of Figure 1. The new center section, JG, indicates double salt formation. Point E is transferred from Figure 1 for reference. But E , the mother liquor from the Glauber salt crystallization, is for the moment left figuratively stranded far afield. A large quantity of burkeite is necessarily deposited, since E now comes within the burkeite phase field. The solution is thus rapidly stripped of sulfate and, fortunately, only moderately of carbonate, with J as the immediate destination. The stopover at K is caused by a special Trona trick of using first a certain sodium chloride-carbonate mixture which happens to be available, followed by straight sodium chloride. After removal of the precipitated burkeite at the strategic point J,the temperature is lowered to 5" C. Arrow J F indicates a substantial crystallisation of sal soda, and finally mother liquor F , containing mainly sodium chloride, is discarded.
Plant Practice Burkeite is dissolved in distilled water (plant condensate) to avoid even traces of calcium or magnesium, which would contaminate the products and make much scale trouble in pipe lines. The temperature is 27" C. Glauber salt is precipitated as the resulting saturated solution is cooled to 22", a value slightly below the theoretical minimum point E in Figure 1, Actually, the undesired carbonate is tardy in appearing; the chemical engineer is enabled to "fudge" a little and thereby get an enhanced yield of Glauber salt. A 5" drop seems at first glance to be unfavorably small for the attainment of high yield. One must remember, however, that the sodium sulfate is taking 135
Above. Pyroborate Molding Machine
Right. CentrifugalFilters Below. Potash Storage
INDUSTRIAL A N D ENGINEERING CHEMISTRY
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Vol. 34, No. 2
30
a
f 20 w
s
I § 10 z2
35 C
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GRAMS Na2SO, F€
00 G WATER
FIGURE1. SOLUBILITYOF SODIUM CARBONATE AND SODIUM SULFATE IN WATER AT 2 3 C.
out ten moles of water of crystallization as it falls, thus tending to increase the concentration of the residual liquor, and helping the process. The slurry of Glauber salt plus sulfatecarbonate liquor is finally delivered to Oliver flters. The mother liquor is heated to a t least 50" C. (in recent plant practice to about 70°), and treated with sodium chloride containing a lot of burkeite and a little sodium carbonate monohydrate, which happens to be available from a near-by process. A large crop of burkeite drops out and is separated on more Oliver filters to be sent back to the beginning of the cycle. The new liquor (at point J of Figure 2) is cooled to 30" and thereby loses a little crystalline sodium chloride. This loss of salt gives the opposite of a "salting-out" effect, and raises the solubility of sodium sulfate just enough so that the latter component, now undesired but already reduced to low quantity, will not crystallize in the next operation. Cooling of the liquor from 30" to 5' C. provides the large yield of sodium carbonate decahydrate at which the whole scheme has been aiming for some time. I n this refrigeration process the Trona operators employ a particularly efficient technique. Liquefied ammonia is delivered directly to the outside of the vessel containing the carbonate liquor. Only the single layer of steel separates refrigerant from liquor, in contrast to conventional technique of chilling brine in a separate machine and transporting to the soda crystallizer. The crude sal soda is collected on Oliver filters, washed, and recrystallized by evaporation of solution a t high temperature. The resulting monohydrate is separated on Oliver filters, washed, put through rotary dryers, and emerges as a high-grade granular solid in many ways superior to the standard "dense soda ash" of commerce. Analysis, 58.2 to 58.3 per cent NazO; theory for anhydrous NazCOs, 58.49 per cent; trade practice, 58.0 per cent NazO. To meet the demand for light soda ash, part of the above product is further processed to give a material of low apparent density.
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Anhydrous Sodium Sulfate The well-known solubility curve for the system Glauber salt-sodium sulfate-water, breaking sharply at 32.38" C., is unfavorable for a rapid, economical output of the anhydrous salt. Although the slope is negative above 32.38", it is not steep enough to permit a large precipitation with rise of temperature. Still worse, there is considerable absorption of heat a t the transition temperature with resultant economic loss.
__--"----~_--Y
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l
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uneconomical. Converting two marketable salts into products of which only one has real market value does not pay in this lowprice field. As usual, t h e ubiquitous burkeite was then drafted into service; this time to replace the
February, 1942
INDUSTRIAL AND ENGINEERING CHEMISTRY
encouraged to crystallize out with the potassium chloride. This mixture, containing up to 1.8 per cent bromide, is neutral and therefore workable. The Trona plant kills two birds with one stone; bromine is expelled and salvaged, while a n equivalent quantity of new potassium chloride replaces the bromide and thus effects the desired purification of the chloride product as described by Gale and Pearson (7). The hot crude potassium chloride-potassium bromide liquor, saturated a t 87' C., is treated in Kubierschky towers made of Vermont granite. Chlorine enters a t the middle, steam at the bottom. The changing mixture of chlorine, bromine, and steam, bubbling upward, is retarded in circuitous passageways until the final, nearly pure output of bromine is delivered through Pyrex pipe to tantalum-lined condensers. The crude bromine is double-fractionated to eliminate all water, chlorine, and traces of organic bromides, and eventually delivered to storage containers of chemical stoneware. Transfer of the dangerous liquid is by siphon rather than natural drainage, as a safety measure in the event a pipe line should fail. The final product, delivered to 6.5-pound bottles, rates 99.9 per cent pure, with chlorine (0.05 per cent or less) as the principal contaminant. Iodine is totally absent. This unusually high purity is the natural result of using a crystallized, purely inorganic product as raw material instead of the brines and bitterns usually employed, together with a double fractionation of the liquid bromine. During the prevailing emergency the demand for elemental bromine has been so great that it is not possible to manufacture bromides. Later it is expected that the manufacture of sodium, potassium, and ammonium bromides will be resumed. I n this enterprise a recent modification of the van der Meulen process (8) will be used.
Borax The Trona plant today accounts for nearly half of the redly competitive world production of borax, while the operations at Boron, Calif., 60 miles distant, furnish nearly all of the remainder from the minerals tincal and rasorite. Apparently all other known world sources of boron either are trivial in quantity or occur as compounds requiring a costly chemical process. Ulexite and colemanite (calcium borates) are illustrations of ores which cannot compete with those of the Mohave Desert in spite of low wage levels elsewhere. The main processes at Trona still follow standard technique
137
(6,9,I l ) , but in recent years great attention has been directed to anhydrous borax. Judging by competing patents, i t is necessary to use the novel scheme of melting borax, to effect dehydration, on a bed of its own material, heat being directed from above. Instead of forming a glass, at Trona (8) the molten borax is caused to crystallize continuously in an endless chain of small ingots which, in turn, are crushed to size for sacking and shipping. The crystalline, anhydrous product (3)is especially attractive to manufacturers of enamel and borosilicate glass. Being easily ground, it is remarkably free of iron from the mill.
Living Conditions Operations at Trona have now been conducted for sufficient time to permit adequate appraisal of hygienic conditions. The impression of veteran residents is that children reared in the desert community are of somewhat higher physical level than those of the milder regions of southern California; the latter, in turn, have a good record, judging by recent military medical reports. An ample temperature range is experienced; 119" E'. is the summer record, 10" the winter's worst so far. The normal diurnal maximum in July runs from 100" to 105",but fall, winter, and spring are delightful.
Acknowledgment Acknowledgment is given of valuable assistance in the preparation of this report from R. W. Mumford, W. A. Gale, and C. F. Ritchie. The processes described here, together with novel equipment for their use, are covered by various patents and pending applications, a few of which are mentioned (1-6,7,8, IO).
Literature Cited Allen, W. H., Gale, W. A., and Ritchie, C. F. (to Am. Potash & Chemical Corp.), U. 8. Patent 1,836,426(Dec. 15,1931). Black, L. G.,Ibid., 2,064,337(Dec. 15,1936). Ibid., 2,146,051 (Feb. 7,1939). Ibid., 2,251,317(Aug. 5,1941). Black, L. G.,and Rich, M. M., Ibid., 1,996,053(April 2, 1935). Gale, W. A., IND.ENG.CHEM.,30,867(1938). Gale, W. A., and Pearson, E. P. (to Am. Potash & Chemical Corp.), U. S. Patent 1,775,598(Sept. 9,1940). Meulen, J. H.van der, U. S. Patent 1,775,598(Sept. 9, 1930). Mumford, R. W., IND.ENG.CHEM.,30,872(1938). Ritchie, C. F.,and Warren, G. E. (to Am. Potash & Chemical Corp.), U. S. Patent 1,936,070(Nov. 21,1933). Robertson, G. R., IND.ENQ.CHEM.,21,520(1929).
Borax Crystallizing House and Evaporator Buildings
