Method for treating solid hazardous heavy metal-containing compositions

A method for converting a solid hazardous heavy metal-containing composition into a solid non-hazardous heavy metal-containing composition and an aqueous composition essentially free of heavy metals, comprising the steps of

• a) mixing and dissolving the solid hazardous heavy metal-containing composition with an acid solution, thereby obtaining an hazardous heavy metal acid composition, wherein the solid hazardous heavy-metal containing composition is the end product of a precipitation process of heavy metals, containing heavy metals, and leaches more than 2 ppm of cadmium, or more than 5 ppm of arsenic, or more than 50 ppm of copper, or more than 0.5 ppm of mercury, or more than 10 ppm of nickel, or more than 10 ppm of lead, or more than 50 ppm of zinc, according to a Modified EU Standard NS-EN 12457-2 using 10 l of water per kg of the solid hazardous heavy metal-containing composition;
• b) precipitating the heavy metals from the hazardous heavy metal acid composition by reacting the hazardous heavy metal acid composition with a heavy metal-precipitation agent at a pH of at least 0.9, wherein the pH of the acid composition is measured after a 13-fold dilution by volume in water, and adjusting the pH of the hazardous heavy metal acid composition to at least 0.9 if the measured pH is below 0.9;
• and reacting the hazardous heavy metal acid composition with a heavy metal-precipitation agent, thereby forming a heavy metal precipitate and an aqueous supernatant;
• and
• c) separating out the heavy metal precipitate from the aqueous supernatant, whereby the heavy metal precipitate is the solid non-hazardous heavy metal-containing composition, wherein the solid non-hazardous heavy metal-containing composition leaches 2 ppm or less of cadmium into an aqueous solution, upon treatment of the solid non-hazardous heavy metal-containing composition according to the Modified EU Standard, and the aqueous supernatant is the aqueous-composition essentially free of heavy metal and comprising less than 12 ppm cadmium;
• wherein the heavy metal-precipitation agent comprises a diorgano-dithiophosphinic acid or the alkali metals or ammonia salts thereof, represented by Formula 1:
• and wherein R is a linear or branched hydrocarbon group selected from alkyl, aryl, alkylaryl, or aralkyl, and wherein the hydrocarbon group contains 3 to 20 carbon atoms, and M is H, alkali metal or ammonia.