HEMIHYDRATE TYPE PHOSPHORIC ACID PROCESS

摘要

Phosphate rock and sulfuric acid are reacted under conditions which result in the formation of calcium sulfate hemihydrate and phosphoric acid of about 35% to about 55% P205. A two vessel reaction system maybe used in which the reaction slurry undergoes inter and intra- vessel circulation. The solution portion of the slurry in the first vessel (the "dissolver") is preferably maintained at a negative sulfate concentration (i.e. excess Ca+2) and the solution in the second vessel (the "crystallizer") is preferably maintained at a positive sulfate ion concentration. Also preferred is that the second vessel be maintained at a reduced pressure, (e.g. to provide evaporative cooling). Better filtration rates can thus be obtained due to the favorable shape, size and distribution of the hemihydrate crystals. Most preferred is that a crystal modifier (e.g. a sulfonic acid, a sulfonic acid salt, tall oil fatty acids or esterified tall oil fatty acids) be present in crystallizer. Surprisingly, the sulfuric acid used in the process can be less (e.g., 0.5 - 10%) than the stoichiometric amount to convert the calcium in the feed phosphate rock to calcium sulphate.