Hemihydrate type phosphoric acid process

Abstract

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% P.sub.2 O.sub.5. A two vessel reaction system is used in which the reaction slurry undergoes intra- and inter- vessel circulation (preferably through a draft tube). The solution portion of the slurry in the first vessel (the 'dissolver') is preferably maintained at a negative sulfate concentration (i.e. excess Ca.sup.+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 evaporation cooling). Better filtration rates can thus be obtained due to the favorable shape, dominant size and size 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 alkoxylated or esterified tall oil fatty acids) be present in the 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.