Recovery of Heavy Metals with Calcium Silicon Alloy. IV. Stoichiometric Discussion of the Reduction of Lead(II), Nickel(II), Iron(III), and Mercury(II) Ions

Article abstract of DOI:10.1246/bcsj.58.1524

The reduction of Pb(2+) and Ni(2+) ions to the metallic state with Ca-Si alloy powder can be stoichiometrically interpreted by the following elementary reactions 1-4 including the decomposition of water, a side reaction: Ca(0) + Me(2+) = Ca(2+) +Me(0) (1); (Ca(0) + 2Me(z+) = Ca(2+) +2Me(z-1)) (1'); Si(0) + 2Me(2+) + 3H2O = H2SiO3 + 2Me(0) + 4H(1+) (2); (Si(0) + 4Me(z+) +3H2O = H2SiO3 +4Me(z-1) + 4H(1+)) (2'); Ca(0) + 2H2O = Ca(2+) +H2 + 2OH(1-) (3); Si(0) + 3H2O = H2SiO3 + 2H2 (4).The occurence of a reaction other than the above ones was suggested for the reduction ofFe(3+) to Fe(2+) ion under strongly acidic conditions.The reduction of Pb(2+) and Ni(2+) ions proceeded very slowly.On viewing the equivalents oxidized, the contribution of Ca in the alloy to the overall reaction was greater than that of Si; Si scarcely participated at all in the reduction of Fe(3+) ion.Based on the above elementary reactions, the reduction of Hg(2+) ion was discussed stoichiometrically.Almost the same equivalents of Hg(0) and Hg(I) species formed in a 20-matom dm^-3 Hg(2+) solution, with an initial pH of 4.0.The contribution of Si to the reaction was greater than that of Ca.