Partition of heavy and alkali metals during sewage sludge incineration

In this paper, two approaches are used to investigate the transformation of heavy and alkali metals during sewage sludge incineration: a thermodynamic equilibrium calculation and experiments using a bench-scale combustor. The distribution of heavy and alkali metals at different temperatures (800, 700, 600, 400, and 200 degrees C) is studied in these experiments. The chemical equilibrium calculation shows that vaporized metallic compounds form a condensed phase at different temperatures. The calculation results show that vaporized lead compounds begin to transfer to the solid phase at 500 degrees C. The main species of solid lead in the flue gas is PbCl2(s) below 500 degrees C. Zn and K have the same temperature range (300-500 degrees C), while the conversion temperature for As is 600 degrees C and that for Na and Cu is above 800 degrees C. The experimental results also prove the feasibility of separating heavy and alkali metals according to their gas-solid transformation temperature zones. On the basis of the experimental results, the optimum separation temperature is < 400 degrees C for Pb and K, 600 degrees C for As, and 800 degrees C for Cu, Zn, and Na.