Basis Study Of Lime-iron Salt Solid Waste Containing Arsenic

The lime-iron salt solid waste containing arsenic studied in this study wasoriginated from the treatment of wastewater with sulfuric acid contamination fromone non-ferrous metals manufacture in Hubei province, China. The basic propertiesand leaching behavior under various conditions were studied to provide a good basisfor hazard assessment and subsequent treatment of this waste. Cement and chemicalreagents acted as different stabilizers were tested for stabilizing this waste. Theleaching kinetics of copper and zinc were also explored for reclaiming copper andzinc from the studied solid waste. It will provide basic data for the decontaminationand recovery of this waste.The morphology of the target solid waste was characterized by SEM, anddistribution of heavy metals was analyzed by Tessier sequential extraction method inthis thesis. The results showed that composition of the target lime-iron salt solid wastecontaining arsenic was very complex. The particle sizes spanned a wide range and itsstructure was relatively unconsolidated. Arsenic species in the solid waste weredominant with freely exchangeable ions and associated residuals; cadmium, copperand zinc in the solid waste were dominant with Fe-Mn oxides and associated residuals;lead was dominant with associated residuals. This indicated that the migration ofsome heavy metals in this solid waste was very strong and they could directly bereleased into the environment to threat organisms. In addition, some of heavy metalscould be transformed with the presence of acid rain, thus penetrating into theenvironment and potentially affecting organisms. Therefore, lime-iron salt solid wastecontaining arsenic must be disposed appropriately to avoid harming organisms.The leaching potentials of As、Cd、Zn、Cu and Pb from lime-iron salt solid wastecontaining arsenic in specific leachant and simulated acid rain was studied to reflecttheir potential impact on the environment. It was found that the pH of leachantaffected strongly on the leached concentrations of arsenic and other heavy metals. Theleached concentrations of arsenic and other heavy metals reached equilibrium within avery short time. It suggested that the arsenic and other heavy metals in this solid wastewere easy to be leached. In the simulated acid rain experiment, the leachedconcentrations of arsenic were11.00mg/L （pH=5） and13.70mg/L （pH=4）, so theuntreated lime-iron salt solid waste with arsenic could be highly toxic. Leachingcharacteristics of calcinated lime-iron salt solid waste containing arsenic indicated that calcination of the waste could stabilize many heavy metals, but it not the case forarsenic.Two methods have been evaluated for the stabilization/solidification of lime-ironsalt solid waste containing arsenic in this paper and the best conditions for stabilizingtreatment have been determined. The parameters for stabilizing treatment have beenoptimized. The optimum parameters for stabilizing including35%cement,10%rectorite,5%fly ash,5%sand and45%arsenic residues with an aging time of2hours.When chemical reagent （ferrous sulfate） was employed as stabilizer, the target solidwaste was maximally stabilized with the molar ratio of ferrous sulfate and arsenic（Fe:As） of3.0and60min of reaction time at room temperature. Leachedconcentrations of arsenic were less0.5mg/L after stabilizing treatment with abovetwo methods.The leaching kinetics of copper and zinc with the ammonia-ammonium carbonatewere also studied for providing the basis for the comprehensive utilization of thissolid waste. The results showed that more than60%of copper and zinc waseffectively recovered at room temperature when the total ammonia concentration was5mol/L, the molar ratio of NH₃/NH₄⁺was2:3, liquid to solid ratio was4:1. Adiffusion-controlled kinetic model was established according to the obtainedexperimental data. Based on the Arrhenius empirical formula, the apparent activationenergy for copper and zinc was calculated to be41.125kJ/mo1and32.245kJ/mo1,respectively. The corresponding leaching kinetics of copper and zinc could bedescribed by the following two equations:1-2α/3-（1-α）^2/3=10297×exp（-41125/RT）t（copper）1-2α/3-（1-α）^2/3=232×exp（-32245/RT）t（zinc）The basic properties and leaching behavior of lime-iron salt solid waste containingarsenic under various conditions were studied in detail. The solid waste was stabilizedeffectively with cement and chemical reagent. Ammonia-ammonium carbonate wasused as leachant for the reuse of copper and zinc in the target waste, thecorresponding leaching kinetic models of copper and zinc were established. Thisstudy will provide basic data for the decontamination and recovery of the studiedwaste.