Selective Recovery of Nickel and Cobalt from Electromachining Sludge Materials

ABSTRACT The recovery of nickel and cobalt from electromachining sludge in the form of relatively high-purity marketable products has been investigated. The investigation has shown that a series of hydrometallurgical unit operations can be performed to recover nickel and cobalt from electromachining sludge in the form of nickelic hydroxide and cobalt sulfate/cobalt metal, respectively. The recovery unit operations for nickel include high pH cyanide complexation, followed by oxidation and precipitation of nickelic hydroxide. The resultant filtrate can be treated for cobalt concentration and recovery by precipitation of a cobaltous/cobaltic double cyanide salt and cobaltous hydroxide. This concentrated form of cobalt can then be converted to cobalt sulfate by acid baking; then redissolved and electrowon as high purity cobalt metal. Product purity rivals that of conventional multistage solvent extraction processes. First order cost estimation indicates the possibility of excellent return on investment.