Influence of sulfur on the fate of heavy metals during clinkerization

The fate of heavy metals during clinkerization is of crucial significance to the solid waste utilization, environmental management, and sustainable development. This paper presents a laboratory scale simulation that aims to investigate the effects of sulfur on the fate of Cu, Pb, and Cd during clinkerization. The sulfur-bearing phases (CaSO4·2H2O and CaS) and metal oxides were mixed with cement raw meal in appropriate ratios to produce clinkers. The volatilization and solidification of Cu, Pb and Cd were investigated using atomic absorption spectrometry, thermogravimetric analysis, X-ray diffraction analyses, electron probe X-ray microanalysis, and scanning electron microscopy. The volatilization of Cu slightly increased in the temperature range 950 °C-1450 °C with addition of sulfur. Sulfur promotes the volatilization of Pb and Cd at the temperature from 950 °C to 1250 °C by decreasing the melting point of PbO and CdO. Sulfur increased the solidification ability of clinker decreasing the volatilization ratio of Pb and Cd at the temperature of 1350-1450 °C and 1450 °C, respectively. Both forms of sulfur (CaSO4·2H2O and CaS) have similar effect on the Cu/Pb/Cd volatilization. Sulfur concentrated in interstitial phases of the clinkers mainly as Ca4Al6O12SO4 and CaSO4. Cu, Pb and Cd were mainly solidified within interstitial phases of the clinkers forming solid solutions with the variable compositions. Cu was also present in alite and belite and as crystalline phases of Ca2CuO3 and CaCu2O3 in the clinkers. This research can help to improve understanding of the fate of heavy metals and provide a guideline for risk assessment during the co-processing of solid wastes in cement kiln.