PRODUCTION OF POTASSIUM CHLORIDE IN N E W MEXICO T. M. CRAMER United States Potash Company, Carlsbad, N. Mex.
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of the more concentrated potash salts could well have taken place in less than the life span of an imaginary Permian chemist. I n other words, the happy combination of conditions that allowed the deposition of potash may have been accomplished in a period that almost disappears in dimension when compared with the uncounted centuries of sea evaporation preceding and following the potash deposition. The slight recession in the rate of sinking of the sea bed allowed an old Permian lagoon to evaporate, and provided more time for a slight film of clays and wind-blown sand to accumulate. These no doubt protected the soluble potash salts when subsidence was resumed, and the influx of more water caused the deposition of rock salt to proceed. One cannot help drawing a comparison with Owens Lake, Calif., where residents can well remember the little steamer that once plied its broad waters. With the diversion of Owens River this lake has dried up and deposited a great body of crystal salts. The rate of evaporation and crystal deposi-
T IS NO LONGER news that a potash industry has been established in the United States. From two widely separated localities potassium salts are now reaching the market a t a daily rate of hundreds of tons, and these salts are of a quality to satisfy the particular customer. With the coming of the World War in Europe in 1914 and the great scarcity of potash which ensued, it was inevitable that the United States should turn to the development of her own resources in this respect. Saline lakes in Nebraska, cement dust, and industrial residues contributed their share to the starved potash market, but it remained for the great brine and salt body a t Searles Lake, Calif., to furnish the largest tonnage of domestic potash produced in this country from war days until the beginning of the present decade. Federal and private enterprise was responsible for the exploration that was carried on until potash of commercial quality was discovered in New Mexico in 1931. The United States Geological Survey had drilled a number of core tests in Texas and New Mexico with encouraging results, and an oil test by the Snowdenand McSweeney Company, near Carlsbad, N. Mex., disclosed the presence there of sylvite or potassium chloride. An intensive prospecting program lasting several years established a likely site, and a four-compartment shaft was started in 1929. Several major obstacles were met and overcome, including soft ground, gas pockets, and an abundance of water. It was essential that the water be absolutely controlled, since the underlying strata are water soluble. The shaft was completed in 1931, and in that year a substantial body of sylvite, intermixed with halite or common salt, was first disclosed a t a depth of 986 feet. Drifts in this potash body soon revealed the character of the material with which we had to deal. The deposit was found to be dry and free from gas, and the roof was competent. As development progressed, the extracted product was ground, sized, and sent to market in the form of a manure salts containing 25 per cent potassium oxide. The first carload shipment was made early in 1931. The sylvite body represents an interlude in the history of the Permian Sea that once occupied this area. Great thicknesses of anhydrite and salt were laid down prior to the arrival of the exact conditions which would cause the deposit of the “potash bed” in a layer 10 feet thick. It is interesting to think that, while the deposition of the whole salt body required thousands of years, and we postulate the slow subsidence of the bed of a shallow sea, the actual final deposition
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U N I T E D S T A T E S POTASH COMPANY’S
MINE AT
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INDUSTRIAL AND ENGINEERING CHEMISTRY
REFINERY OF
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VOL. 30, NO. 8
UNITEDSTATES POTASH COMPANY AT CARLSBAD N. MEX.
tion by heavy brines under a hot sun is surprising, and any theoretical calculations with respect to evaporation rates based on variable vapor pressures must not ignore the high temperatures which shallow brine bodies attain, sufficient to be painful in some cases to those who would wade about. The sylvite and salt are present in a mechanical mixture, with small amounts of carnallite, polyhalite, langbeinite, clay, sand, and water. The latter may well be a residue of the Permian Sea.
Refining Process In 1932 construction of a refinery was started. I t s location is close to the Pecos River, where ample water is obtained by gravity canal, This water is largely used for cooling purposes and is returned to the river. The first technical problem was the provision of soft water. The water of the Pecos River is notoriously hard; much of the time it is nearly saturated with gypsum, and it contains magnesium salts as well. A Permutit water softener was installed, which has proved entirely satisfactory. It was obvious, however, that even the softened water would be unsuitable for boiler feed, except as a make-up source, so provision was made to return condensate to the boilers. This meant the use of closed coils for heat exchange systems and closed coils in condensers. Since the refining process involves a cycle of heating and cooling, and since power is required for both mining and refining operations, the power plant was designed to care automatically for a variable heat and power load. This was accomplished by the use of bleeder type turbogenerators for the generation of electric power. Energy a t low voltage is stepped up for transmission over a distance of 13 miles to the mine. Some of the larger unit drives, particularly exhausters on the filters, are turbine-driven, and are of a back-pressure type to supplement the exhaust steam from the powerhouse turbines. The refining process is based upon a simple solubility relation between sodium and potassium chlorides. Since no double salts are formed in the temperature range employed, and since the chloride ion is common to both salts, we have only to deal in theory with the depression of solubility in water which the two chlorides exert upon each other. The solubility of sodium chloride alone in water increases by only a small amount with increments of temperature, and the direction of its saturation curve is actually reversed in solutions saturated with potassium chloride. The refinery was designed to operate under a cycle of temperatures from 27" to 100" C. Under favorable conditions the temperature range can be increased beyond these figures. In theory we would attempt to attain a hot solution a t lOO", saturated
with both sodium and potassium chlorides. It sometimes happens that this is not attained, and therefore on cooling we find that sodium chloride is the first salt precipitated. We then depend on further cooling to redissolve the sodium chloride. All the crystallization paths end with a cool mother liquor that is saturated with respect to potassium chloride and capable of dissolving sodium chloride. The result is that the potassium chloride alone is in the crystal deposit, and the task remains to remove it from the mother liquor with a-minimum of effort and in as pure a state as practicable. %Te start with a cold mother liquor, saturated with respect to potassium chloride and unsaturated with respect to sodium chloride. This liquor is brought to a temperature of perhaps 110" C., first by circulating through condenser coils on the coolers, and then by the use of simple heat exchangers which make use of exhaust steam. The dissolvers are vessels with false bottoms. The salts are charged from the top, the vessel is closed, and the heated mother liquor is passed through the body of ground salts. Potassium chloride is dissolved, together with a small amount of sodium chloride. The enriched liquor passes out of the dissolver, saturated with potassium chloride. Cooling is accomplished in vacuum evaporators. A high vacuum is maintained by means of large steam ejectors, which compress the vapors rising from the boiling liquor so that they can be condensed in the summer by the warm river water. I n winter the water of the river is cold enough to provide condensation without the aid of the ejectors. Inert gases are removed by small steam ejectors. The cooled liquor, carrying potassium chloride in suspension, is pumped to settling tanks, where a large part of the liquor is decanted through launders. The thickened crystal mass is fed to top-feed Oliver filters where the potassium chloride is washed and dried. Products of combustion from natural gas furnaces are used for the final drying on the filter. The dried crystal cake is crushed, screened, and conveyed . either to warehouse or direct to cars. The refining process is made up of the following steps: (a) Cold mother liquor is heated. ( b ) The hot liquor is passed through a stationary bed of crushed crude salts. (c) The hot potash-enriched liquor is cooled. (d) Much of the mother liquor is decanted and the thickened crystal magma is filtered and dried. (e) The waste sodium chloride is stored in a natural reservoir.
Problems Encountered For the buildings a steel framework, transite covering, and concrete floors were chosen. Provision was made for adequate daylight illumination and for the abatement of both dust and steam.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
The materials of equipment construction include plain steel, cast iron, chrome-nickel steels, nonferrous alloys, rubber, and wood. Although the theoretical solution contains sodium and potassium chlorides, the liquors actually contain magnesium sulfate and chloride, calcium sulfate, and some potassium sulfate. These add to the corrosive nature of the hot solutions. Electrolysis plays a large part in the affairs of a potash refinery, and the symptoms would be amusing were they not annoying. A steel valve interior will receive a bright copper coating, but when one looks for it a week later the coating has gone, perhaps to appear a hundred feet away on an agitator propeller. Natural gas, from eastern New Mexico fields, is burned under the boilers and about the plant. The chemical and designing engineer must give thought to the effect of altitude. It must be considered in many placesfor example, in psychrometric studies, calorimetry, boiling points, compressors, and exhauster design. The altitude of 3000 feet has appreciable effects in these respects. Weird problems were encountered by us when operations started. One week, several years ago, we were plagued with moths. They appeared literally by millions, and we even
W. A. GALE American Potash & Chemical Corporation, Trona, Calif.
The Searles Lake system of salts and brine is regarded as having nine components, three degrees of freedom, and six solid phases. Equilibrium diagrams are presented to show the behavior of major components upon isothermal evaporation and to illustrate the Trona process. At low temperatures the precipitation of the double salt glaserite [K3Na(S0J2] prevents a separation of potassium and sodium salts, whereas at high temperatures only sodium salts are precipitated. The hot concentrated liquor is removed and cooled rapidly to crystallize potassium chloride ; at the same time supersaturation is maintained with respect to borax, which is crystallized before returning the mother liquor to the cycle.
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considered operating the refinery in the dark. A thoughtful foreman conceived the idea of putting out lights in the refinery and letting them burn in the power plant. When the moths were attracted there in droves, he concentrated them further by bright lights a t the boiler doors, then putting out all lights he opened the fire chamber doors and the moths poured in to their destruction. The traditions of potash have resulted in many popular misbeliefs regarding the product. The early and substantial trade of our country in refined wood ashes has left the impression in many quarters that all potash is caustic or corrosive in character. There is widespread belief that potash is largely used in munitions manufacture. Of course this conception started with the use of potassium nitrate in black powder. The idea was given further impetus by the publicity attendant upon the scarcity of potash during the late war. Our agricultural lands felt the pinch of scarcity, and the cotton plant is a greedy consumer of potash. Cotton is used to make nitrocellulose. So potash does help in the defense of our country, but the farmer stands between the potash industry and the present-day munitions factory. RECEIVED May 23, 1938.
HE phase rule is an expression of a simple fundamental relation existing between the number of components of any system in equilibrium, the number of phases and the number of conditions which we must arbitrarily fix in order to define the entire system. These conditions we call the “degrees of freedom” or variance of the system. (“Components” are taken to mean the least number of chemical substances necessary to express the composition of every phase present in the system; “phases” are the physically distinct, mechanically separable substances, each of homogeneous composition, which are present in a system; “conditions” are defined as the temperature, pressure, and concentration of the components.) The phase rule in its simplest form states that the sum of the number of phases P , plus F, the number of degrees of freedom, is always greater by 2 than C, the number of components making up the system, or
T
P+F=C+P
To many this would seem an unimportant and meaningless generalization in comparison with the more obvious laws of nature. However, during the formation of the earth’s crust, the separation of the various rock minerals from magma, and later the deposition of secondary minerals from water solutions, have all been governed by the principles underlying the phase rule. We find this rule a valuable guide in the study of any process of separating salts by fractional crystallization. In such a case it is first necessary to investigate the equilibrium conditions and phase relations of the system in question. This may take months or even years of patient laboratory work to obtain the desired solubility data and to identify the solid phases involved. Then we represent this information on
