Study On Oxidation And Stabilization Of High Concentration Arsenic Waste Residue And Pilot-scale

In the past two decades, with the rapid development of China's sulphic acid industry, the treatment of acidic wastewater has produced a large amount of waste residue with a lot of heavy metals including As, Cd, Cr, Cu, Zn, Pb, F. For a long time, these waste residues have been discarded or stacked, making great threat to human health and ecological environment. In this study, the waste residue with high concentration arsenic from sulphic acid wastewater treatment system was studied. The basic property of the arsenic waste residue were investigated to provide a good interpretation for the hazardous waste assessment and stabilization treatment of the arsenic waste residue. And then stabilization experiments were conducted. Based on the laboratory experiments, a pilot scale treatment of arsenic waste residue was achieved to provide basic theory and technical guidelines for the decontamination treatment and engineering applications.(1) X-Ray fluorescence(XRF), X-Ray diffraction(XRD) and sequential extraction method analyses were used in an attempt to elucidate the composition, morphology and species distribution of arsenic waste residue. The results showed that the composition of arsenic waste residue was very complex. Arsenic species were dominant with carbonate fraction and very sensitive to the p H change of environment. This indicated its migration was very strong and the leaching concentration of arsenic was much higher than the “identification standards for hazardous wastes identification for extraction toxicity”(GB5085.3-2007) and it belonged to hazardous waste and had a great threat to environment. The main contents and achievements are as follows:(2) Compared with iron and sulfur compounds, alkaline calcium compounds have a good effect on arsenic stabilization and finally calcium hypochlorite and lime were chosen to explore the stabilization mechanism. With the addition of calcium hypochlorite, the Ca/As molar ratio was up to 2.0, and after the addition of lime, Ca/As molar ratio was up to 2.4 and the p H ranged from 11.5 to 12.0, the leaching concentration of arsenic was lowering the “Standard for pollution control on the security landfill site for hazardous waste level”(GB18598-2001).X-ray photoelectron spectroscopy(XPS) and XRD analyses showed that As(III) was dominated in the arsenic waste residue and then some changed to As(V) after the addition of calcium hypochlorite, informing insoluble materials Ca HAs O4·2H2O.(3) On the basis of laboratory experiment, a pilot-scale process to treat arsenic waste residue was achieved using calcium hypochlorite and lime. It included field investigation and the design of schematic diagram and equipment for pilot-scale. The mass ratio of materials for the oxidation and stabilization was determined. The mass ratio of arsenic waste residue: water: calcium hypochlorite: lime=1:(0.05~0.10):0.05:(0.05~0.15)and the p H should be controlled at 11.5~12.0. A unit treatment cost of the chemical agents for treating arsenic waste residue was 120 RMB/t,which was a very low cost. It can provide some references for treatment of arsenic waste residue in engineering applications.