M2aq H2S MSs 2Haq

where M = nickel, cobalt, zinc. Metals can be selectively removed from solution by controlling pH of the sulfide precipitation reaction. The pH is controlled by the addition of ammonia to neutralize the acid generated in the precipitation reaction:

The nickel and cobalt sulfides are then returned to the leach, the zinc sulfide (which is precipitated independently of nickel and cobalt sulfide) is sold as a by-product, and the barren liquor (metal free ammonium sulfate solution) proceeds to ammonium sulfate recovery.

Ammonium Sulfate Recovery. The solution obtained after removal of the metal values as metal sulfides, contains only ammonium sulfate. This solution, referred to as "barren solution" is then evaporated and ammonium sulfate, which is formed as an overall refinery by-product, is recovered in a crystalline form. The ammonium sulfate recovery consists of crystallization followed by centrifuging, drying, and screening. The crystalline ammonium sulfate is then sold as a fertilizer.

Applications. Most hydrometallurgically produced nickel powder is compacted into briquettes or rondelles for consumption as an alloying additive in the steel industry. Smaller amounts of powder are roll compacted into nickel strip or dissolved to produce various nickel salts.

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