Potassium Sulfate from Poly halite J
Extraction from Aqueous Mixtures in Which Calcium Sulfate Is Synthesized
E. P. SCHOCH The University of Texas, A u s t i n , Texas
Since the discovery of minable beds of pure polyhalite in Texas in 1926, the writer has developed four processes for the manufacture of commercial products from it which are progreqsively of better economic promise. The first three processes are reviewed. In the final method, lime slurry and polyhalite are heated separately to a desired temperature (e.g., 220" C.) and are mixed in the presence of such an amount of water as to produce potassium sulfate solutions with concentrations as high as possible, yet far enough below the equilibrium concentration to obtain iinmediately a solid practically free from pentasalt. Thus the time of treatment is reduced to 15 minutes, and solutions can be obtained containing about 13.7 grams of potassium sulfate per 100 grams of water (about onetenth of this being in the form of hydroxide), and the magnesium can be easily converted entirely to light carbonate of magnesia. Finally, as will be shown later, the large amount of anhydrite obtained can be readily converted to plaster. Thus polyhalite appears to be economically usable.
the magnesium and potassium sulfates with hot water, and separating the latter by fractional crystallization. This wap the first method investigated by the writer (18) and also by the U. S. Bureau of Mines (4, 5, 8, 14, 22, 25, 24). The writer's roasting process differs from that of the Bureau of Mines in that it employs a higher temperature (above 500" C.) in order to render the calcium sulfate so inactive as to allow of the complete extraction of the soluble salts before the calcium sulfate becomes appreciably soluble. Thus a sample of polyhalite heated a t 600" C. for one hour and then boiled with enough water to form a solution containing 12.72 grams of potassium sulfate and 9.55 grams of magnesium sulfate per 100 grams of water did not give a turbidity with ammonium oxalate until it had boiled for 45 minutes. More complete data are shown by the following two trials: In the first, 200 grams of salt-free polyhalite, which had been calcined at 550" C. for 1.5 hours, were treated with 400 grams
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HE discovery of minable beds of polyhalite in West Texas in 1926 by private parties made the problem of the extraction of potassium sulfate from polyhalite an important economic problem in the United States and led the writer to take up its study. The solution of this problem is coniplicated by the possibly extensive formation of pentasalt, K2SO4.5CaSO4.H20, which is difficult to decompose, and the formation of which hinders all reactions between potassium sulfate and calcium compounds.' A general solution of both of these problems has now been attained and is presented here. Aside from patent records, there has been no publication of this work; hence it is deemed proper to present it in its historical relation to other work on the subject. Method Based on Initial Roasting of Polyhalite The first method ever proposed for the extraction of potassium sulfate from polyhalite was that of Rose (f7); it consists of roasting the mineral a t low red heat, extracting S L C T l O N A-A
1 Byngenite, KnSO,.CaSO
