Heavy metal removal from wastewater

There are numerous streams with relatively small concentrations of metals (10 ~ 1000 mg/l) of base metals like copper, zinc or nickel. Removal of these metals typically means bulk precipitation as hydroxide in gypsum slurry during lime precipitation. The downside: it isn’t cost-effective in recovering the heavy metal in wastewater. That seems a waste as heavy metals like zinc and chromium are becoming scarcer and more valuable.

Recovering low concentrations of f.i. zinc and chromium in wastewater with sulfide would be the method of choice, if the high cost of transporting, storing or producing sulfide on-site (as NaSH or H₂S) would not hinder its widespread application.

This is why metal sulfide precipitation is superior to metal hydroxide precipitation:

• High reactivity of sulfides with heavy metal ions and very low solubility of the resulting metal sulfides over a broad pH range resulting in lower effluent concentrations
• Metal sulfide sludges are denser and have better thickening and dewatering characteristics than metal hydroxide sludges, which facilitates further processing
• An easily marketable, compact product is formed
• High degree of selective precipitation is easily accomplished
• Relatively insensitive to the presence of complexes and most chelating agents
• Chromates and dichromates without separate reduction of the chromium

Paques THIOTEQ^™ Metal and SULFATEQ^™ technologies offer economically attractive metal recovery. These processes produce sulfide from elemental sulfur or sulfate under ambient conditions. High-rate engineered systems that provide reliable and efficient performances. Installations with metal recovery capacities of a few kilograms up to 24 ton/d have been built.