Precipitation of Phosphate by Iron Oxide and Aluminum Hydroxide from Solutions Containing Calcium and Potassium

Abstract

When acidic calcium phosphate solutions such as those formed by the dissolution of fertilizers react with aluminum hydroxide in the presence of little potassium, all the potassium is precipitated as taranakite, K₃Al₅H₆(PO₄)₈·18H₂O; most of the rest of the phosphate is precipitated as aluminum and calcium aluminum phosphates. In the presence of considerable potassium, the precipitation of phosphate is governed by the initial composition of the solution; as much as 25% of the phosphate is precipitated as dicalcium phosphate and most of the rest is precipitated as taranakite. Iron oxide reacts much more slowly with phosphate than aluminum hydroxide and has much less effect on the precipitation of phosphate.