Phosphoric acid prepared by the digestion of phosphate rock commonly contains cationic impurities such as calcium, magnesium, aluminum and iron.
It is known to purify phosphoric acid by solvent extraction techniques. The organic solvent displays a greater preference for phosphoric acid than it does for the metal salt impurities in the crude phosphoric acid. Nevertheless, an undesirable residue of polyvalent cationic impurities amounting to 25 to 250 parts per million usually remains in the solvent phase.
U.S. Pat. No. 3,993,733 describes a process for preparing alkaline metal phosphates by neutralizing a solvent acid phase to obtain a precipitate and thereafter neutralizing the resultant aqueous acid phase to form a solution of soluble phosphate salts.
U.S. Pat. No. 3,993,736 describes a process for preparing purified phosphoric acid by first neutralizing an acid containing solvent phase to obtain a precipitate, neutralizing the resultant aqueous phase to form a solution of soluble phosphate salts, and thereafter reconverting the phosphate salts to phosphoric acid.
U.S. Pat. No. 4,112,118 extracts phosphoric acid containing solvent with an alkaline solution to remove substantially all of the P.sub.2 O.sub.5 value of the solvent. The aqueous extract contains phosphoric acid and phosphate salts in combination.
U.S. Pat. No. 4,024,225 describes a sequential neutralization procedure for preparing phosphate salts.
U.S. Pat. No. 3,081,151 prepares a purified phosphate salt liquor using a two-step neutralization.
Prior art processes are not adapted to the concurrent production of purified phosphoric acid and purified phosphate salt.