PURPOSE: To obtain high purity calcium secondary phoshate by adding hydrogen sulfide, magnesium silicate and a potassium salt to a soln., prior to treatment, obtd. by decomposing phosphate ore with hydrochloric acid.

CONSTITUTION: Phosphate ore is decomposed with hydrochloric acid, and decomposition residue is removed. To the resulting soln. are added above about 100 times as much hydrogen sulfide or a sulfide as arsenic by mol contained in the soln., above about 1% of magnesium silicate or calcium silicate to the amt. of the soln., above twice as much a calcium salt or a sodium salt as the theoretical reaction mol no. required to convert fluorine contained in the soln. into a silicofluoride, and ammonia. Primary neutralization is then carried out at pH 0.5W10, and insol. matter is removed. Ammonia is added to this soln. to carry out secondary neutralization at pH 2W5, thereby separating calcium secondary phosphate crystals. Sulfuric acid is added to the remaining soln. to precipitation-separate gypsum, and ammonia is further added to precipitation-separate ammnoium chloride.