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

Abstract

Abstract The reduction of Pb2+ and Ni2+ ions to the metallic state with the Ca–Si alloy powder can be stoichiometrically interpreted by the following elementary reactions 1–4 including the decomposition of water, a side reaction: d Ca0+Me2+=Ca2++Me0(1) (Ca0+2Me2+=Ca2++2Mez−1)(1′) Si0+2Me2++3H2O=H2SiO3+2Me0+4H+(2) (Si0+4Mez++3H2O=H2SiO3+4Mez−1+4H+)(2′) Ca0+2H2O=Ca2++H2+2OH−(3) Si0+3H2O=H2SiO3+2H2(4) The occurrence of a reaction other than the above ones was suggested for the reduction of Fe3+ to Fe2+ ion under strongly acidic conditions. The reduction of Pb2+ and Ni2+ 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 Fe3+ ion. Based on the above elementary reactions, the reduction of Hg2+ ion was discussed stoichiometrically. Almost the same equivalents of Hg0 and HgI species formed in a 20-matom dm−3 Hg2+ solution, with an initial pH of 4.0. The contribution of Si to the reaction was greater than that of Ca.