Volatilization of Heavy Metals (As, Pb, Cd) During Co-Processing in Cement Kilns

Abstract

Heavy metals emitted by flue gas during co-processing hazardous waste (HW) in cement kilns presents a serious threat to the environment. Volatilization characteristics of three semivolatile metals (As, Pb, and Cd) in different chemical forms (As2S3, NaAsO2, Na2HAsO4, PbS, PbO, PbCl2, CdO, CdS, CdCl2) during co-processing with cement raw materials were studied. Thermogravimetric experiments were performed to tentatively predict the lower limits of volatilization temperatures. Experiments involving co-processing of these compounds with cement raw meals was carried out at different temperatures over different time periods in an electric heated tube furnace to gain more insight into the macroscopic kinetic characteristics of volatilization. Volatilization of As, Pb, and Cd during co-processing with cement raw materials increases with time initially, and after 25 min, they do not volatilize. Temperature and compound speciation significantly affect metal volatilization. For Pb and Cd, volatilization was higher at higher temperatures. Volatilization of As, however, decreased as temperature increased at temperatures above 1,000°C, which can be explained by three parallel reactions. Kinetic models on the volatilization of these compounds during co-processing with cement raw materials were obtained. These models can provide a preliminary theory basis for predicting the release of heavy metals (Pb, Cd, As) and be used to control the environmental risk posed by heavy metals through restricting their feeding rate during co-processing HW in cement kilns.