| Mercury and its compounds attract human's wide concerns due to its mobility,toxicity and bioaccumulation.In recent years,the European Union(EU)and United State Environmental Protection Agency(USEPA)had issued several bans and regulations on mercury production and emission.More and more mercury-bearing solid wastes were being strictly regulated,especially those mercury-bearing solid wastes arising from metallurgical and chemical industries.The accumulation of mercury-bearing solid wastes can increase the environment risk,however,fundamental studies on detoxification and purification of these mercury-bearing solid wastes are still lacking.Thiosulfate had been used in several hydrometallurgical leaching processes.The thermodynamic diagrams,including Eh-pH diagrams,speciation diagrams and oxidation state diagrams of Hg-H2O system,Hg-S-H2O stable and metastable system and Hg-Cl-H2O system,were demonstrated in the paper.Thermodynamic calculation indicates that mercury can react with thiosulfate to form soluble mercuric-thiosulfate complex.In this work,the feasibility of leaching mercuric compounds and mercury-bearing solid waste with thiosulfate solutions was studied.Leaching of mercuric oxide with simple thiosulfate solution was studied first.The conditions of mercuric oxide leaching with thiosulfate,including initial pH,thiosulfate concentration and leaching temperature,was investigated.The results indicate that mercury extraction can reach 99.93%when the solid to liquid ratio is 0.4 g:500 mL,initial pH is 7,thiosulfate concentration is 0.01 mol/L,leaching temperature is 323 K and leaching time is 0.5 h.The leaching kinetic study indicates that the apparent activated energy is 23.30 kJ/mol,and the overall reaction rate is controlled by both diffusion and chemical reaction.Thiosulfate leaching of mercurous chloride was also been discussed.To achieve appropriate parameter,the influence of initial pH to water leaching and influence of initial pH,thiosulfate concentration,temperature to thiosulfate leaching was investigated respectively.The results of thiosulfate leaching indicate when the ratio of solid to liquid is 0.1 g:500 mL,initial pH is 8,thiosulfate concentration is 0.01 mol/L and leaching temperature is 313 K and leaching time is 2 h,mercury extraction can reach 60.53%.The leaching kinetic study indicates that the apparent activated energy is 6.67 kJ/mol,and overall reaction rate is diffusion controlledThe cuprous-thiosulfate leaching mercuric sulfide was studied.The influences of leaching conditions,including initial pH,molar ratio of copper to thiosulfate([Cu]/[S2O32-]),and temperature,was investigated.The mercury extraction can reach as high as 92.37%after 7-h leaching under the optimized condition which is solid to liquid ratio of 0.1 g:500 mL,initial pH of 8.50,molar ratio of copper to thiosulfate of 1:10 and leaching temperature of 313 K.X-ray diffraction(XRD),Scanning electron microscope-energy dispersive spectroscopy(SEM-EDS)and X-ray photoelectron spectroscopy(XPS)were applied analyzing the leaching residues.The results show that copper in its thiosulfate complex when substituted by mercury via Lewis substitution and cuprous sulfide is formed on the surface of leaching residue.The kinetics of mercuric sulfide leaching with cuprous-thiosulfate solutions is controlled by the product layer diffusion whose apparent activated energy is 32 kJ/mol,and overall reaction rate are controlled by both diffusion and chemical reaction.The mercury recovery from mercuric-thiosulfate complex solution was discussed.The sulfide precipitation technology was tested for mercury recovery.The results show that the sulfide precipitation reaction is fast,and A maximum mercury recovery ratio of 100%can be achieved when the ratio of mercury to sulfur is 1:1.The mercury recovery with aging process is studied.The leaching conditions such as initial pH,mercury concentration and thiosulfate concentration is investigated.In ageing process,a maximum mercury recovery ratio of 54.22%can be achieved when the initial pH is 12,mercury concentration is 1000 mg/L,thiosulfate concentration is 0.05 mol/L,duration is 45 d and temperature is 298 K.The mercury recovery with ultraviolet photolysis technology was tested,and the leaching conditions,such as initial pH,mercury concentration and leaching temperature,was investigated.In ultraviolet photolysis,a maximum mercury recovery ratio of 87.94%can be achieved when the initial pH is 12,mercury concentration is 1000 mg/L,thiosulfate concentration is 0.05 mol/L,duration is 4 h and temperature is 318 K.To study its thermal stability,the measurements such as XRD and Differential scanning calorimetry-thermogravimetric analysis(DSC-TGA)were carried out to analyze precipitates of aging and ultraviolet radiation.The results of ultraviolet photolysis indicate that the precipitates are composed of cinnabar and metacinnabar.The results of ultraviolet photolysis precipitates indicated that the precipitates are composed of cinnabar,metacinnabar,mercurous sulfide,element sulfur and sulfite.The thiosulfate leaching mercury contaminated soil which is near a domestic chemical plant was tested.The results show mercury extraction of mercury bearing soil can reach 54.30%when pulp density is 1 g:200 mL,leaching temperature is 338 K,thiosulfate concentration is 0.02 mol/L,and duration is 3 h.According to the contrast experiments of soil sample and leaching residue mercury morphological analysis,the mercury content of acid extractable fraction,easily reducible fraction,oxidable fraction and residual fraction decreased by 17.20 mg/kg,32.20 mg/kg,160.60 mg/kg and 308 mg/kg respectively.The cuprous-thiosulfate leaching of the mercury tailing in a domestic mercury mine was tested.Mercury extraction of mercury tailing can reach 85.68%when pulp density is 3 g:100 mL,initial pH is 8,molar ratio of copper to mercury is 1:5,leaching temperature is 318 K,thiosulfate concentration is 0.10 mol/L,and time is 6 h.According to the contrast experiments of mercury tailing and leaching residue mercury morphological analysis,the mercury content of oxidable fraction and residual fraction decreased by 412 mg/kg and 671.29 mg/kg respectively. |
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