1994 Volume 110 Issue 15 Pages 1214-1220
This study is a fundamental research for using the rhodan waste solution to remove nickel from nickel alloy scraps. The dissolution of nickel in basic ammonium thiocynate was studied, and its dissolution mechanism was discussed. The results obtained are surmmarized as follows: The dissolution of nickel can be effectively carried out by the blowing oxygen gas with the presence of Cu (II). The largest rate was obtained in the pH range of 9 to 9.5, but the favorable value of pH in the solution is 9, because, it is necessary to suppress the evaporation of NH3. Nickel is scarcely dissolved in the solution of Cu (II) ammine alone, however, the dissolution of nickel is sharply increased by adding of a small amount of SCN-ions to the solution of Cu (II) ammine. And the dissolution rate is considerably higher than that in the solution containing ammonium alone. Nickel is chiefly dissolved as the amine complex Ni (NH3) n2+, and the CuSCN films is formed on the surface of nickel as the nickel is dissolved. However, it can be gradually dissolved with increasing NH3and SCN-ions, and subsequently the dissolution of nickel proceeds. The dissolution rate of nickel increases proportionally with the concentration of ammonium thiocyanate at its lower concentration. The activation energy of dissolution in this region was 68.5 kJ/mol. The rate-determining step is assumed to be the chemical reaction on an anodic site of the surface of nickel. At the higher concentration of ammonium thiocyanate, the dissolution rate of nickel is constant independently of the concentration, but it is proportional to the concentration of Cu (II). The activation energy of dissolution in this region was 16.3 kJ/mol. The rate-determining step is assumed to be the diffusion of Cu (II) on the cathodic site of nickel. The reactions are considered as follows: Ni+2SCN-+2Cu (NH3) 42+=Ni (NH3) n2++2CuSCN (s) + (8-n) NH3
CuSCN (s) +SCN-=Cu (SCN) 2-
Cu (SCN) 2-+4NH3+H++1/4O2=Cu (NH3) 42++2SCN-+1/2H2O
CuSCN (s) +4NH3+H++1/4O2=Cu (NH3) 42++SCN-+1/2H2O
