Study On Bench And Pilot-scale Stabilization Mechanism Of Arsenic In Realgar Tailings

Realgar tailings are low-grade realgar produced by arsenic trioxide smelting.Realgar tailings directly piled in open-air without any proper treatment,leading to significant release of As to the environment,and posing serious risks to the environment and human health.Stabilization is widely used for the treatment of As-bearing wastes.It could reduce As mobility,solubility and toxicity of As-bearing wastes,and reduce their environmental harm.However,the stabilization mechanism of As in realgar tailings was seldom studied,and the pilot-scale study of As stabilization in realgar tailings was never reported.In this study,realgar tailings from Yunnan province,China were employed as the research object,the properties of realgar tailings were studied,and then stabilization of As in realgar tailings at both bench and pilot-scale studies were investigated.In addition,short and long-term stabilization mechanisms of As in stabilized tailings were elucidated.The main conclusions are as follows:?1?Realgar tailings consisted of As,Si,Ca,Al,etc.,and the total content of As in realgar tailings was 140,700 mg kg-1.As leaching concentration of realgar tailings was 135 mg L–1,exceeded the standard limit of “Identification standards for hazardous wastes – Identification for extraction toxicity”?GB5085.3–2007?,therefore,realgar tailings were identified as hazardous wastes.Realgar?As^?₄S₄?and pharmacolite?CaHAsVO₄·2H₂O?were the main As-bearing minerals in realgar tailings.In realgar tailings,61.5% of As existed in As?V?and 38.5% of As existed in As???.?2?In bench-scale study,ferrous sulfate heptahydrate?FeSO₄?was selected as the optimal stabilizer for As stabilization in realgar.The optimal conditions were as follows: 6 mm crushing particle size,20% FeSO₄ addition,150 m L H₂O addition per kg tailings and 2 days of curing.As leaching concentration of tailings treated with optimal conditions was reduced to 2.09 mg L–1,below standard limit?2.5 mg L-1?of “Standard for pollution control on the security landfill site for hazardous wastes”?GB18598–2001?,and leaching concentrations of other heavy metals were also meet their standard limits,respectively.?3?The transformation from relative soluble As-bearing minerals?i.e.,realgar and pharmacolite?to insoluble Fe-As minerals?i.e.,scorodite and symplesite?under weakly acid and open air conditions was the main stabilization mechanism of As after treated with FeSO₄.To be specific,H⁺ and SO₄^2-generated by the oxidation and hydrolyzation of FeSO₄ lead to the release of H₂AsO₄^-from pharmacolite?As?V?-bearing minerals?,and As???in realgar was partly oxidized into H₂AsO₄^-.Then insoluble scorodite and symplesite minerals were formed due to the reactions of H₂AsO₄^-with Fe?OH?₃ and Fe²⁺.In addition,long-term stability of As was achieved after treated with FeSO4.With prolonging the curing time from 1 d to 365 d,As leaching concentration of treated tailings decreased from 5.77 mg L–1 to 1.41 mg L–1,and adsorbed-As and amorphous Fe-As precipitates gradually transformed to more stable Fe-As crystalline minerals.?4?A pilot-scale study of realgar stabilization treatment was conducted in Yunnan province,China.In pilot-scale study,1 ton realgar tailings were treated in one batch experiment,and 6 batch experiments were conducted.The total throughput of pilot-scale study was 6 tons realgar tailings.Samples in pilot-scale experiments were collected from 2 different sampling points and 3 different depths.The total As contents of these samples varied from 69,000 mg kg-1 to 172,800 mg kg-1,and As leaching concentrations varied from 13.1 mg L-1 to 78.7 mg L–1.Successful results were obtained after treated with industrial FeSO₄ in pilot-scale study.With 25% industrial-FeSO₄ addition,As leaching concentrations of 6 samples were all successfully lower than the standard limit of 2.5 mg L–1,and the estimated cost of stabilizers?20% industrial-FeSO₄ and 150 m L kg–1 tailings H₂O?was ?50.6 per ton tailings.