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 (CaS[O.sub.4] x 2[H.sub.2]O 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[degrees]C-1450[degrees]C with addition of sulfur. Sulfur promotes the volatilization of Pb and Cd at the temperature from 950[degrees]C to 1250[degrees]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[degrees]C and 1450[degrees]C, respectively. Both forms of sulfur (CaS[O.sub.4] x 2[H.sub.2]O and CaS) have similar effect on the Cu/Pb/Cd volatilization. Sulfur concentrated in interstitial phases of the clinkers mainly as [Ca.sub.4][Al.sub.6][O.sub.12]S[O.sub.4] and CaS[O.sub.4]. 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 [Ca.sub.2]Cu[O.sub.3] and [Ca.sub.2]Cu[O.sub.3] 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.
Due to the rapid development of modern industry and city, significant amount of solid wastes (SW) are being produced every day, for instance, 3.28 billion tons of industrial SW, 173 million tons of municipal solid waste (MSW) and 25 million tons of sludge were produced in China in 2016 . About 77% of the solid wastes were transported to the landfills and waste incineration facilities . However, solid wastes can be considered as secondary raw fuels and/or materials in the cement industry, for example, in co-processing of solid wastes in the cement kiln [3-6]. By the implementation of co-processing, the natural resources can be saved and carbon dioxide emissions can be reduced [7-10]. Nevertheless, most of the solid wastes contain heavy metals that could be volatilized and move with flue gas into the atmosphere during clinkerization, polluting the environment and threatening human health [11-13], Therefore, it is important to study the volatilization of heavy metals during co-processing of solid wastes in the cement kiln.
The previous studies carried on heavy metal emissions from solid wastes co-processing indicated that the fate of heavy metals strongly depended on the waste composition, particularly on the content of chlorine and sulfur in the solid wastes and cement raw meal [14,15], The content of sulfur in the cement kiln is about 0.5 wt%-2.0 wt%, that mainly come from raw fuels (e.g., coal) and materials (e.g., limestone containing sulfide...