Phosphate recovery from residual acidic waters by consecutive neutralisation first with chalk or lime and then lime, with pptn. of calcium phosphate

Abstract

Phosphates are recovered from residual acidic waters, contg. over 50 mg/1 phosphate ions, as obtd. in the prodn. of H3PO4 and of chemical fertilizers with P. The process consists in 2 consecutive neutralization steps of the waste waters. The 1st step uses chalk, ground below 3 mm, as neutralizing reagent, as a suspension or powder, or lime, as milk of lime or powder, to pH 4-5.5, followed by clarifying by decantation, filtration, etc. The SO4 (textreads 4(2-) only) F and SiF6 ions, etc., are then eliminated. In the 2nd step, the reagent is lime, as milk of lime or powder, in the presence of F, SiF6, NH4 ions, followed by the pptn. of Ca phosphates, e.g. by decantation or filtration. The Ca phosphates can be reused as raw materials. The addn. of flocculants, e.g. polyacrylamide, increases decantation velocity.