“Hemihydrate Process” as one of wet processes for making phosphoric acid was experimentally examined in the authors laboratory and then evaluated in view of technical points. The process is essentially consisted of the decomposition of phosphate rock with sulfuric acid and the filtration and the washing of hemihydrate. Some economical and technological benefications are estimated on the industrial practice. These are a reduction of initial investment, a simplification in operation of the process, production of more concentratrated phosphoric acid than that produced from conventional processes and a reduction in energies required for by-produced gypsum applications.
The results of present approarches to “Hemihydrate Process” are as follows;
(1) In producing phosphoric acid with concentration 30% to 35% P₂O₅, the decompositions of powdered Florida and Kola phosphate rocks (-200 mesh) with sulfuric acid are completed in a short time, 2 hours.
(2) Size of hemihydrate crystals from Kola phosphate rock is relatively larger than that of hemihydrate crystals from Florida phosphate rock.
(3) As the concentrations of phosphoric acid obtained by decomposition of phosphate rock with sulfuric acid increase and the suction pressures on the filtration of slurries consisted of hemihydrate and phosphoric acid decrease, the amounts of phosphoric acid separated from the slurry decrease.
(4) In the final filtration step, the amount of water required for washing hemihydrate as pure as possible, is a key on the industrial succsses of “Hemihydrate Process”. It was shown that the amount of water determined in present approarches, which is required for washing hemihydrate in final filtration step of the process, in which produces phosphoric acid with concentration 30% to 35% P₂O₅ in the case of Florida phosphate rock and 30% to 40% P₂O₅ in the case of Kola phosphate rock, are suitable for keep constant the concentration of product phosphoric acid.
(5) Since the hydration and hardening of hemihydrate during filtration and washing tend to blind the filter cloth, hot washing solutions and hot warter with temperatures over 80°C Should be used in both operetions and the operation system too Should be Keep always in temperatures over 80°C. But, as the hemihydrate from Kola phosphate rock is relatively stable in aqueous solutions, it may be allowed to operate the filtration and washing in temperature range 60-80°C.

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