Preparation of phosphates

In a continuous process for the preparation of potassium dihydrogen phosphate by the steps involving the reaction of phosphate rock or solubilized form thereof with sulfuric acid, potassium hydrogen sulfate, and recycle phosphoric acid at a temperature of about 40.degree.-90.degree.C. in a main reactor to form a resulting slurry containing KH.sub.2 PO.sub.4, H.sub.3 PO.sub.4 and precipitated calcium sulfate, removing the calcium sulfate solid from the mixture, precipitating KH.sub.2 PO.sub.4 from the resultant solution by addition of an organic solvent, recovering the KH.sub.2 PO.sub.4 from the phosphoric acid solution, removing the organic solvent from the phosphoric acid solution, and recycling at least a portion of the phosphoric acid solution to the main reactor, the improved steps which comprise:

• a. conducting the initial step of the reaction by initially charging a mixture of phosphate rock or solubilized form thereof, sulfuric acid and recycle phosphoric acid to a reactor and adding to the material at a controlled rate a 10-50 wt. percent aqueous solution of KHSO.sub.4 until substantially complete reaction is obtained for the formation of filterable calcium sulfate crystals;
• b. continuously passing the resulting slurry to a first separator where the solid calcium sulfate precipitate is removed to leave a filtrate solution;
• c. continuously washing the solid in the separator with water and recycling the strong wash to the main reactor;
• d. removing the filtrate solution from said first separator and passing to a second reactor and reacting with a member selected from the group consisting of calcium oxide or equivalent calcium bearing material, phosphate rock, solubilized forms thereof and mixtures thereof, at a temperature of about 40.degree. to 90.degree.C.;
• e. passing the resulting slurry to a separator to provide solids and a filtrate, removing the resulting solids;
• f. passing the filtrate to an evaporator and evaporating to obtain a solids concentration of greater than about 40 weight percent;
• g. passing the resulting concentrated mixture to a solvent treatment precipitation step and contacting with a sufficient amount of an organic solvent selected from the group consisting of lower alkyl alcohols and lower alkyl ketones to precipitate solid potassium dihydrogen phosphate;
• and
• h. passing the slurry resulting form the organic solvent precipitation step to a separator to obtain solid potassium dihydrogen phosphate and an organic solvent solution, removing the solids and drying to provide a solid KH.sub.2 PO.sub.4 product, passing the solvent solution to a solvent recovery stage, removing the solvent and recycling to the precipitation step and recovering a solvent-free phosphoric acid solution suitable for recycle to the main reactor as recycle phosphoric acid.