Oxidative control of post precipitation in phosphoric acid

Claim 1

In the method of making wet process phosphoric acid analyzing about 57-63% P 2 O 5 prepared by steps including (a) clarifying crude feed acid in a clarifier to provide a clarified acid and a sludge-containing underflow, said feed acid analyzing about 26-36% P 2 O 5 , having an Fe (as Fe 2 O 3 ):P 2 O 5 weight ratio of at least 0.04; and at least some of said Fe being present as Fe ++ ; (b) treating the clarified acid with aluminum silicate in an aluminum silicate treatment zone; (c) concentrating the treated acid to about 46-51% P 2 O 5 ; (d) settling solids in the concentrated acid in a crystallizer-settler providing an overflow and an underflow; (e) sending crystallizer-settler underflow to the clarifier in (a) and sending clarifier underflow to the reaction train; (f) further concentrating the overflow in (d) to about 57-63% P 2 O 5 ; the improvement comprising adding an oxidant to the acid at a point between the clarification step in (a) and the concentration step in (c) to oxidize substantially all Fe ++ to Fe +++ , whereby the post-precipitation characteristics of the acid are improved.