Effect of Sintering Atmosphere on Clinker Made from Chromium-Bearing Sludge

摘要

The purpose of this study was to investigate the effects of sintering atmosphere (oxidizing and reducing) on the polymorphs of dicalcium silicates (Ca_2SiO_4, C_2S) and on the chromium leaching of the belite-rich clinkers made from a chromium-bearing sludge. This sludge was generated in an electroplating factory, and in addition to chromium, it contained nickel, copper, and zinc. In the clinker production, air was used as the oxidizing atmosphere, and carbon monoxide, which was produced by burning graphite with an insufficient amount of oxygen, was employed as the reducing atmosphere. Dicalcium silicates were substantially formed under both kinds of sintering atmosphere, but there was some non-hydraulic γ-C_2S in the clinkers produced under the oxidizing atmosphere. In addition, the amount of γ-C_2S decreased with the chromium-bearing sludge addition, while that of β-C_2S increased. The clinkers produced under the reducing atmosphere had less residual chromium, a finding which shows that more chromium was evaporated. However, the reducing atmosphere can decrease the proportion of hexavalent chromium (Cr(VI)) in the resulting clinkers. For other heavy metals, the residual amounts of nickel and copper generally increased with the sludge addition, but zinc was absent in most of the clinkers produced under the reducing atmosphere. This implies that the evaporation of zinc is much more significant than the other heavy metals under a reducing atmosphere. In the leaching tests, the concentrations of nickel, copper, and zinc were below the detection limit in all the leachates. In terms of chromium, the total leaching concentration was highly related to Cr(VI). The clinkers produced under the oxidizing atmosphere had high leaching concentrations of chromium, and thus failed to meet the regulatory limit. In contrast, the reducing atmosphere was effective in decreasing the chromium leaching, and it therefore makes the resulting cement clinkers more environmentally-sound.