PRECIPITATION
Precipitation reactions are widely used to recover metal values or to remove impurities from process streams and waste waters. Many hydrometallurgical processes contain one or more precipitation steps. Hazen has extensive expertise in developing and piloting precipitation processes.
For instance, hydroxide is used to precipitate iron from acid streams, neutralize acid streams for disposal, recover nickel and cobalt hydroxide from sulfate liquors, and remove metals from wastewater. Platinum group metals are also recovered from acidic leach solutions by precipitation.
Sulfide is another common compound used in precipitation steps. Hydrogen sulfide is used to recover copper from copper-bearing streams and nickel and cobalt from acid sulfate liquors. Sodium hydrosulfide and calcium sulfide are widely used to remove zinc, copper, lead, silver, and cadmium from waste streams.
Cementation is a specialized precipitation process in which a more reactive metal is used to remove a more noble metal from solution. Copper cementation has been used extensively to recover copper. Gold and silver are recovered using zinc dust cementation.