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1 2002

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The interaction of two ligand containing electroplating solutions. Final decontamination

Pigaga A., Selskis A., Juškénas R., Sveikauskaité A.

Ars Separatoria Acta

2002

No. 1

138-148

We suggested previously a cheap and simple way to decontaminate two ligand-containing rinsing waters of metal finishing. Both copper diphosphate containing rinsing wastewater and acidic zinc ammonium containing rinsing wastewater may be decontaminated by mixing with each other, which results in precipitation of solid solutions of copper-zinc-potassium-ammonium diphosphates. This way of decontamination requires no expensive reagents since only a small amount of H2SO4 is needed for pH adjustment. 80-99.5 % of environmentally dangerous substances, viz. zinc, copper and diphosphate, are removed from the mixture. However, Cu2+ and Zn2+ amounts in filtrates significantly exceed discharge consent level (DCL). Moreover, high concentrations of undesirable diphosphate and ammonium ions, which cause the eutrophication of natural water reservoirs, are present in the filtrates. Depending on the waste water composition of the specific plant and the environment protection requirements the goals of the decontamination may be as follows: 1.) To remove Cu2+ and Zn2+ ions below DCL, 2.) To remove Cu2+ and Zn2+ ions below DCL and additionally to lower the amount of phosphates, 3.) To remove Cu2+ and Zn2+ ions below DCL and additionally to lower the amount of phosphates and ammonium. Our investigation has shown, that goal 1 can be easily and cheaply achieved in industry by using lime, goal 2 – by using lime and spent steel etching solution or phosphogypsum. Goal 3 was achieved by precipitating barely soluble fine crystalline MgNH4PO4 .6H2O. It has been determined, that at optimal conditions as much as 95% of NH4 + ions are precipitated, both residual c Cu2+ and c Zn2+ < DCL, and the concentration of soluble phosphates is reduced 5-40 fold.