Phosphoric acid/lime hazardous waste detoxification treatment process

Claim 1

A process for producing a non-hazardous sludge from an aqueous solution which contains heavy metals comprising the steps of: (a) adding phosphoric acid or an acid phosphate salt to the aqueous solution which contains a heavy metal or an aggregate of heavy metals at a hazardous level to precipitate the heavy metals as metal phosphates, the amount of phosphate added being at least one mol per mol of heavy metals; (b) lowering the pH of the solution to less than about 5.0; (c) adding a coagulant to the solution to coagulate the precipitated metal phosphates; (d) raising the pH of the solution to above about 7.0 by the addition of an alkaline calcium source capable of raising the pH to above about 7.0 to form a non-hazardous sludge including the coagulated metal phosphates; and (e) dewatering the resulting non-hazardous sludge to obtain a sludge which leaches less than the following concentrations of metals employing the test of 40 CFR §261, Appendix II: ______________________________________ Metal Concentration in ppm ______________________________________ Pb 5 Cr 5 Cd 1 As 5 Hg 0.2 Se 1 Ag 5 Ba 100. ______________________________________