製塩化学石膏の精製について
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概要
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The quality, crystal form and amounts of combined water of the chemical gypsum byproduced from salt making are very variable with the crystallization condition of calcium sulfate in a concentrated solution of brine and therefore the utilization of the gypsum is often limited.<BR>The raw gypsum materials generally contain dihydrate and hemihydrate of calicum sulfate for the principal constituent, and also sodium chloride and alkalline double salts for impurities. These hemihydrate and anhydrate are easily hydrated to dihydrate by water or sea water, and the most of impurities are easily soluble in water, it is probable to improve crystal form and develop crystal size with dissolving the impurities by aging simply in water.<BR>The present authors studied on the most brief, effective and economical purification process of these raw gypsum materials and the physical properties of the calcined gypsum prepared from the purified crystalline gypsum. Sample gypsum materials are a scale rakaed out from multiple effect evaporator and a pan scale deposited in the concentrating pan over the hot spring heater. For the purification process, washing by and steeping in cold water, hot water, boiling water, a dilute solution of sodium chloride and of sulfuric acid are taken alone or together. Microcrystals of sodium chloride are almost removed by washing, and hemihydrate and anhydrate completely convert to dihydrate. Crystal size of gypsum makes difference to each process and is developed to the gross prismatic crystal by steeping in water firstly and washing with a dilute solution of sulfuric acid secondly.<BR>The physical properties of the calcined gypsum prepared from salt making process are far superior to them of the calcined from the double decomposition of rock phosphate and sulfuric acid to phosphoric acid and gypsum, but it has tendency to hasten the initial setting time for a little sodium chloride remained within crystalline gypsum.
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