Study On Stability Mechanism And Stabilizer Of High Levels Arsenic Waste Residue

With the rapid development of mining and smelting industry,a large number of solid wastes like mining waste rock,tailings and metallurgical slag have been produced.These residues storage in outdoor not only takes up a lot of fields,dissolution of toxic and harmful group such as arsenic and other heavy metal at the action may leaching with the rainfall,and then spread into the surrounding area,immediately destruction of soil ecosystem,or polluted surface water,even ground water,threatened the safety of drinking water and human health.According to the problem of arsenic pollution,domestic and foreign experts have carried out a lot of research,because of the high cost and treatment effect is not ideal,there is not effective method to treat the pollution of arsenic in mining area.The arsenic waste studied in this study was originated from the arsenic residue left over by history from a chemical factory in Nanhua county of Yunnan province,China.The basic properties of arsenic residue with different stockpiling points were studied to provide a good basic data and scientific basis for hazard analysis and assessment and subsequent comprehensive utilization and harmless treatment of this waste.Iron salts,manganese dioxide and activated carbon acted as single or compound stabilizers were tested for stabilizing this waste.The stabilization effect was evaluated by comparing the leaching behavior under the leaching experiment and combining with project requirements,and the "sulphuric acid&nitric acid"leaching test was chosen at last.The biological toxicity and migration transformation rule of arsenic waste were studied by different forms of arsenic with BCR continuous extraction.The stability mechanism of arsenic was investigated by phase analysis with the instrument of FT-IR,XRD,SEM,XPS and the morphology analysis of arsenic.The basic characteristics of arsenic wastes such as chemical composition was studied,and the results showed that the arsenic wastes particle size distribution was not balance and crystallization degree was not high.Basic composition elements were consist of O,Fe,Al,Ca,Si,S,K,etc.heavy metal was given priority to As.The highest content of arsenic was in number 2 slag heaps,and the,arsenic content was higher with the finer particles.Mineral composition was given mainly by SiO2,As2O3,As2S3,As4S4,Ca3(AsO4)2,FeAsO4·2H2O,Fe2O3,Fe3O4,FeO(OH).The leaching potentials of As form arsenic wastes in difference leaching experiment was studied to reflect As potential impact on the environment.The results showed that this arsenic wastes was high biological toxicity and high potential risk to the environment with the high leaching concentration of As.It was found that the pH of leachant affected strongly on the leached concentration of As,leaching of As concentration increased with pH.The leached concentration of As reached equilibrium after 2 hour,it suggested that arsenic in this solid wastes were easy to be leached and potential threats to the environment.Stabilization of arsenic waste by chemical reagents was studied in this paper and the best process parameters for stabilizing treatment have been determined by various mixing test.The parameters for stabilizing treatment by using FeCl2 and FeC13 as stabilizer have been optimized.When pH value was 6.5?7.5 and 4.0?5.0,adding FeCl2 and FeCl3 with Fe/As molar ratio of 1,respectively,after 1 hour of reaction at room temperature,the target arsenic waste was maximally stabilized and the arsenic leaching concentration was lower than 2.5 mg/L.It was achieved to the requirements of safety landfill admission.It was helpful to the reaction of iron and arsenic wastes when CaO was added less 4%and 2%,respectively.The reduction of ferric salts dosage and the synergy of stabilization of arsenic waste were due to using MnO2 and activated carbon as oxidant and adsorbent.Arsenic species were studied by BCR continuous extraction experiment in this paper.Arsenic species in No.2 and 4 arsenic wastes were dominant with acid dissolved,oxidation and associated residuals,in No.1 and 3 arsenic wastes were associated residuals mainly.This indicated that the biological toxicity of some arsenic species in this waste was extremely strong and they could release into the environment along with the presence of acid rain.In addition,the migration of some exchangeable arsenic species was very strong and could be transformed and directly used by organisms,have strong potential harmfulness.The arsenic species was changed from easily soluble arsenic to difficult soluble arsenic after stabilizing,but the migration and transformation of arsenic was slightly different in FeC12 and FeC13.The arsenic species was changed from acid dissolved and reduction to associated residuals by FeC12,and acid dissolved to oxidation and associated residuals by FeC13.The morphology characteristics,crystallization condition and microstructure of the target arsenic waste were observed and studied through the RT-IR,XRD,SEM,STM and energy spectrum instrument in this study.And the stability mechanism of arsenic was investigated.The possible mechanism were consisted of:(1)the difficult soluble arsenic compounds of iron was generated directly by precipitation,coprecipitation with arsenic acid radical ions;(2)the arsenic acid precipitation of amorphous or secondary oxidation state of refractory minerals was generated by adsorbing the anion such as AsO43-to the solid-liquid interface by not obligate adsorption,then replacing to the bidentate ligand(chelation)or bridge ligand by obligate adsorption;(3)the adsorption of arsenic and the production of arsenic compounds of iron were accelerate to oxidant or adsorbent with oxidation of arsenic or iron salts.