Effect of coexisting inorganic chlorides on lead volatilization from CaO-SiO2-Al2O3 molten slag under municipal solid waste gasification and melting conditions

The effect of inorganic chlorides on volatilization of lead compounds from CaO-SiO2-Al2O3 molten slag under simulating conditions of gasification and melting of municipal solid waste was studied. It was found from the experiments that the volatilization of lead compounds from CaO (30 wt.%)-SiO2 (50 wt.%)-Al2O3 (20 wt.%) molten slag in a lab-scale electric furnace at 1773 K in N-2, N-2-O-2 or N-2-CO-CO2 atmosphere was promoted by the addition of NaCl, KCl and CaCl2 to the slag containing PbO, in the order of CaCl2 > NaCl > KCl. The rate of volatilization and the amount of volatilized lead compounds were the highest by employing CaCl2 in N-2-CO-CO2 atmosphere, due to combined reduction of PbO to metallic lead (Pb) and chlorination of PbO to PbCl2. By heating the molten slag at 1773 K for about 10 min, almost 100% of PbO was removed by adding CaCl2 to the slag at a molar ratio of (Cl)/(Pb) = 27. However, the heating of molten slag at 1773 K for 10 min was not sufficient for complete removal of PbO, even by adding an excessive amount of NaCl and KCl to the slag at a higher molar ratio of (Cl)/(Pb) = 110. From the equilibrium calculations on the main elements involved in municipal solid waste in shaft-type gasification/melting furnace, almost 100% of lead compounds were predicted to volatilize as Pb, PbS and PbCl2. The ratio of PbCl2 increased with the increase in chlorine (Cl) content in the waste. It was considered by equilibrium calculations that increased volatilization of lead compounds was attributed to the pronounced formation of PbCl2 by the reaction of PbO with HCl which was released from NaCl, KCl and CaCl2. (C) 2011 Elsevier Ltd. All rights reserved.