Removal of metal ions from waste water

Claim 1

A method of treating an acid aqueous effluent of pH less than 4 obtained in the hydrometallurgy of zinc for removing metal ion impurities therefrom comprising the sequential steps of: passing said aqueous effluent containing at least 20 ppm total of the metal ions Zn, Cd, Pb, Mn and Fe, each of said metal ions being present in amounts of at least about 2 ppm, said effluent also containing at least 1 ppm of the metal ions Mg, Ca, Hg, Na, K, Cu, Ni, Cr and As to a first reactor, preparing an amount of a lime slurry sufficient to neutralize said aqueous effluent and mixing an aliquot portion thereof comprising about 50% to 95% of said lime slurry with said aqueous effluent in a first stage treatment in said first reactor sufficient to raise the pH thereof to about 7 to 8 and precipitate metal hydroxides therein. then in a second stage treatment passing said limewater-precipitate mixture to a second reactor to which the remainder of said lime slurry is added to provide a higher pH of about 8.5 to 9.5 effective to complete the precipitation of said metal hydroxides and provide an aqueous effluent containing residual metal ions other than Ca, Mg, Na and K ions selected from the group consisting of at least two elements of the group Zn, Cd, Mn, Hg and As, the total amount of lime (CaO) added ranging from about 5% to 20% excess of that required stoichiometrically to precipitate said hydroxides. passing said reacted mixture to a clarifier to provide a thickened underflow of said metal hydroxide precipitate and an overflow of said aqueous effluent, filtering said underflow of thickened precipitate to form a filter cake and recycling the filtrate thereof to said clarifier, treating said overflow of aqueous effluent with an amount of sodium silicate solution corresponding to about 5 to 30 times that amount required stoichiometrically to remove said residual metal ions from solution as a precipitate, adding a flocculating agent to agglomerate the precipitate formed, disposing of said filter cake, and passing said sodium silicate treated effluent to a settler, the overflow of said settler being discharged substantially free of said residual metal ion impurities.

Claim 2

A method of treating an acid aqueous effluent of pH less than 4 obtained in the hydrometallurgy of zinc for removing metal ion impurities therefrom comprising the sequential steps of: passing said aqueous effluent containing at least 20 ppm total of the metal ions Zn, Cd, Pb, Mn and Fe, each of said metal ions being present in amounts of at least about 2 ppm, said effluent also containing at least 1 ppm of the metal ions Mg, Ca, Hg, Na, K, Cu, Ni, Cr and As to a first reactor, preparing an amount of a lime slurry sufficient to neutralize said aqueous effluent and mixing an aliquot portion thereof comprising about 50% to 95% of said lime slurry with said aqueous effluent in a first stage treatment in said first reactor sufficient to raise the pH thereof to about 7 to 8 and precipitate metal hydroxides therein. then in a second stage treatment passing said limewater-precipitate mixture to a second reactor to which the remainder of said lime slurry is added to provide a higher pH of about 8.5 to 9.5 effective to complete the precipitation of said metal hydroxides and provide an aqueous effluent containing residual metal ions other than Ca, Mg, Na and K ions selected from the group consisting of at least two elements of the group Zn, Cd, Mn, Hg and As, the total amount of lime (CaO) added ranging from about 5% to 20% excess of that required stoichiometrically to precipitate said hydroxides. passing said reacted mixture to a clarifier to provide a thickened underflow of said metal hydroxide precipitate and an overflow of said aqueous effluent, filtering said underflow of thickened precipitate to form a filter cake and recycling the filtrate thereof to said clarifier, treating said overflow of aqueous effluent with an amount of sodium silicate solution corresponding to about 5 to 30 times that amount required stoichiometrically to remove said residual metal ions from solution as a precipitate, adding a flocculating agent to agglomerate the precipitate formed, disposing of said filter cake, and passing said sodium silicate treated effluent to a settler, the overflow of said settler being discharged substantially free of said residual metal ion impurities.