Summary for sulfuric acid leaching Discussion
sulfuric acid leaching of a former thiocyanate tank was discussed in the context of a zinc-nickel tank that had contained zinc chloride, nickel chloride, and ammonium thiocyanate. Thomas Christian asked whether 10% sulfuric acid was safe to use. Mark Baker replied that it is acceptable as long as all spent solution is removed first, and he recommended filling the tank about three-quarters with water before slowly adding acid while mixing. Another answerer added that there is no need to be anxious about hydrogen cyanide: thiocyanate does not liberate HCN when its solutions are acidified, based on his experience with thiocyanate titrations in strongly acidic nitric acid. He did caution, however, that thiocyanate can react with chlorine or hypochlorite to form cyanogen chloride, so those oxidizers should be avoided.
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