Sulfide versus hydroxide precipitation

From BioMineWiki

Reasons for precipitating sulfides and not hydroxides

Figur 1. Comparison between the equilibrium concentrations of metal hydroxides and metal sulfides

When it comes to sludge volume, reusability of the sludge and effluent quality, precipitation of metals with sulfide is superior to precipitation as hydroxides. It has many advantages over lime precipitation:

• High reactivity of sulfides with heavy metal ions and very low solubility of metal sulfides over a broad pH range resulting in lower effluent concentrations (See Figure 1).
• Sulfide precipitation, unlike hydroxide precipitation, is relatively insensitive to the presence of complexes and most chelating agents.
• Sulfide removes chromates and dichromates without preliminary reduction of the chromium to the trivalent state.
• A high degree of selective metal precipitation is possible with sulfide, as opposed to hydroxide precipitation.
• Metal sulfide sludges generally are more dense and stable than metal hydroxide sludges , exhibiting better thickening and dewatering characteristics than corresponding metal hydroxide sludge which facilitates further processing.

Previous objections against the use of sulfide, i.e. that it is toxic and corrosive, do not hold anymore because adequate safety measures and the use of modern corrosion-resistant construction materials (plastics) eliminate these disadvantages. Sulfide precipitation would be the method of choice, if the high cost of transporting, storing or producing sulfide on site (as NaSH or H₂S) would not have hampered its widespread application in the metal and mining industry.