Technological Study On One Step Mothod For Leaching Arsenic From Rich Arsenic Slag And Reduction And Freezing Crystalli-Zation Of Leaching Solution

Based on comprehensive analysis of the advantages and defects of different kinds of traditional approaches for treating arsenic sulfide slag, the rich arsenic slag produced in pHospHoric acid plant was used to extract arsenic by using sodium hypochlorite solution as leaching reagent in this paper. The leaching residue was treated by landfill after removing sulfur and pHospHor, and the trivalent arsenic in leaching solution was oxidized to pentavalent arsenic by secondary oxidation. After acidolysis and evaporating concentration processes, the oxidized solution was reduced by using SO2, which could reduce H3ASO4to HAsO2. Finally, the reduced solution was treated by freezing crystallization, getting HAsO2and mother solution. As2O3was obtained after drying of HAsO2, and sodium sulfate crystal was gotten after arsenic removal, evaporation and cooling crystallization. The residual liquid was recycled in evaporating concentration process. According to the experimental results, the optimum technological conditions for each procedure were obtained.(1)The optimum conditions for alkaline leaching-oxidative desulfurization: countercurrent leaching, the dosage of sodium hypochlorite solution of4times of the theoretical dosage, leaching temperature30℃, leaching time10min. Under these conditions, the total leaching efficiency of arsenic was more than99.97%, and the mass fraction of arsenic in the leaching residue was0.1396%.(2) The optimum conditions for secondary oxidation of leaching solution: the dosage of sodium hypochlorite solution of twice the theoretical dosage, pH value of solution of6, oxidation temperature30℃, oxidation time1.5h. Under these conditions, the oxidation rate of As(Ⅲ) was more than98%.(3) The optimum conditions for reduction process:reduction temperature30℃, reduction time2.5h, pH value of solution of2, arsenic concentration in solution of26.66g/L. Under these conditions, the reduction rate of As(Ⅴ) was98.15%.(4) The optimum conditions for freezing crystallization:freezing temperature-15℃, freezing time2.5h, pH value of solution of0. Under these conditions, the recovery rate of arsenic was89.72%, and the total recovery rate of arsenic was89.43%.This technology just used simple equipments, and the operation time was short. In addition, there was no emission of waste water and gas in the entire process, and the leaching rate of arsenic was quite high. However, this method required large amount of sodium hypochlorite solution, resulting to high costs of production.