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Can we safely leach a former thiocyanate tank with sulfuric acid?

Q. We are leaching a tank that was zinc-nickel . Is it ok to use 10% sulfuric acid to leach the tank? The tank used to contain 10 grams per liter zinc chloride, nickel chloride, and ammonium thiocyanate.

Thomas Christian
plating engineer - Sanford, North Carolina
September 15, 2008

A. Thomas,
Yes, as long as all the spent solution is removed from the tank. I would fill the tank 3/4 with water then add the acid slowly while mixing.

Mark Baker
process engineer - Malone, New York

A. You need not be anxious. Thiocyanate will not liberate HCN when its solutions are acidified. I routinely do silver metal analysis in plating solutions via thiocyanate titration, in a matrix of about 5 N nitric acid, and no HCN is released even when a large excess of titrant is used.

Thiocyanate will react with chlorine/hypochlorite to form cyanogen chloride, however.

Dave Wichern
Consultant - The Bronx, New York

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