Stabilization of arsenic sulfide sludge by hydrothermal treatment

Arsenic sulfide sludge (ASS) generated during the treatment of strongly acidic wastewater with sulfide precipitation method has long been a problem for the nonferrous smelting enterprises. In this study, stabilization of ASS from a lead and zinc smelter by hydrothermal treatment was investigated. The ASS was characterized with high contents of arsenic (46.9%) and sulfur (32.6%), and leaching concentration of arsenic was determined to be 702.0 mg/L. The parameters of the hydrothermal treatment were studied in detail. It was found that the leaching concentrations of As, Pb and Cd, dehydration ratio, density and morphology of ASS changed dramatically during the hydrothermal treatment. Comprehensive consideration of these changes, the optimal conditions were determined to be temperature of 200 degrees C, reaction time of 4 h, liquid-to-solid (L/S) ratio of 1:1 and the initial pH of 2. Under the optimal conditions, the leaching concentration of As for the treated ASS decreased to < 5 mg/L. The yellow muddy ASS changed into a big bulk with metallic luster. The density of the ASS increased from 1.20 g/cm(3) to 2.29 g/cm(3), the moisture content decreased from 62.59% to 6.50%, and the volume reduction ratio reached 91.67%. After the hydrothermal treatment, the speciation transformation of As, Pb and Cd occurred. Some of the As, Pb and Cd in ASS transformed into a more stable speciation after the hydrothermal treatment. As a result, the stabilization of As, Pb and Cd attributed to both of the microstructure transformation of ASS and the speciation transformation of As, Pb and Cd.