Production of pure alkali metal phosphate solutions from wet-processed

Abstract

Production of pure alkali metal phosphate solutions by neutralizing wet-processed phosphoric acid with an alkali liquor and/or carbonate. To this end, the wet-processed phosphoric acid is neutralized to a pH from 4 to 9, the resulting neutralization sludge consisting substantially of insoluble aluminum and iron phosphates is separated from the alkali metal phosphate solution and scrubbed with water, the sludge so treated is mixed with alkali liquor, alkali metal phosphate solution and water-glass so as to establish in the resulting suspension a molar ratio of SiO.sub.2 to Al.sub.2 O.sub.3 of at least 2 : 1 and a molar ratio of dissolved alkali metal oxide to dissolved P.sub.2 O.sub.5 ranging from 2.9 : 1 to 3.3 : 1, the whole is heated to temperatures from 80.degree. to 100.degree. C to effect breakup of the sludge in the suspension with the resultant formation of a solid matter residue together with a trialkali metal phosphate solution, the solid matter residue is separated from the trialkali metal phosphate solution, water-scrubbed, and discarded. More particularly, crude wet-processed phosphoric acid containing more than 45 weight % of P.sub.2 O.sub.5 is diluted with a portion of the scrubbing water obtained during operation and its P.sub.2 O.sub.5 -content is thereby reduced down to 30 to 45 weight %. The acid so treated is neutralized with an alkali liquor and/or carbonate and with the trialkali metal phosphate solution originating from the processed neutralization sludge.