Distribution And Transformation Of Sulfur In Tailings Of Metal Sulfide Mine

A large number of metal sulfide minerals piled up in tailings impoundment,and the accelerated transformation of sulfur occurred when suffering invasion by wastewater from washing and selecting minerals,and atmospheric precipitation.Among them,the biological reduction and disproportionation of sulfur drive the transformation of oxided sulfur species(such as SO₄^2-or S⁰)to reduced sulfur species(such as S^2-),which is related to the migration and release of heavy metals in tailings impoundment,and has great potential to become the core of heavy metal pollution prevention and recovery technology.There is a significant isotopic fractionation effect of sulfur in biological reduction and disproportionation,but in acidic environment（p H<4）is difficult to capture.On the one hand,the direct product S（-Ⅱ）has a faster reoxidation rate and less accumulation under acidic conditions.On the other hand,although S（-Ⅱ）can slow down oxidation and accumulate after the formation of metal sulfides,such as Cu S(log K_sp is just more than Ag₂S and Hg S),Cu S is only partially dissolved in traditional extraction of acid volatile sulfide（AVS）.However,the failure in extracting Cu S-S effectively interferes with the subsequent extraction of Fe S₂-S from chromium reducible sulfur（CRS）,hindering the conduct of isotope methods.The traditional sequential extraction method of reduced sulfur species includes three processes:the process of extracting element sulfur（ES）by organic solvent;the process of extracting AVS by HCl solution;extraction of CRS from acidic Cr Cl₂ solution.In this study,the effective extraction of Cu S-S and the effective separation of Cu S-S and Fe S₂-S were achieved by improving the sequential extraction method of reduced sulfur species,which provided opportunity for the study of sulfur cycle process,especially biological reduction and disproportionation process,in the environment of metal sulfide mine tailings impoundment.On this basis,Dabaoshan,a large metal sulfide mine in South China,was taken as the research object to combine reduced sulfur species with different forms of sulfate（including:water soluble sulfate（WSS）,exchangeable sulfate（Ex S）,acid soluble sulfate（ASS））content distribution and sulfur isotopic composition analysis,to explore the spatial and temporal characteristics of sulfur distribution in tailings under different inventory conditions.At the same time,the formation process of Cu S and its environmental stability were explored from the perspective of chemical kinetics and coagulation kinetics by simulating the tailings impoundment environment of metal sulfide mine,so as to clarify the final trend of S（-Ⅱ）,the direct product of sulfur biological reduction and disproportionation.The main conclusions are as follows:（1）The oxidation of Cu（Ⅱ）from Cu S to S（-Ⅱ）is the main reason that Cu S cannot be effectively extracted during AVS.Therefore,this study selected an appropriate reducing agent.After exploring the effects of the amount of reducing agent added during AVS on the extraction rates of Cu S-S and Fe S₂-S respectively,the separation effect of the improved sequential extraction method on Cu S-S and Fe S₂-S was determined,and its shielding effect on Fe³⁺oxidation was tested.The experimental results showed that Cu S-S and Fe S₂-S could be separated in the form of AVS and CRS,respectively,by the difference in the reducibility of Sn Cl₂ and Cr Cl₂.Moreover,the addition of Sn Cl₂ can effectively prevent the oxidation of S（-Ⅱ）by Fe³⁺in the extraction process.When the sample contained 1 g jarosite(corresponding to the release of 6 mmol Fe³⁺),the mole ratio of Sn Cl₂ to total S was controlled at 115,90%of Cu S-S was extracted in the form of AVS,and 98%of Fe S₂-S was present in CRS.The realization of the separation of Cu S-S and Fe S₂-S creates conditions for the identification of sulfur isotope signals in different metal sulfides,and lays a foundation for the study of sulfur cycle in the tailings impoundment environment of metal sulfide mines.（2）Based on the improved sequential extraction method of reduced sulfur species,the source and distribution characteristics of reduced sulfur species in tailings under different storage conditions were verified by combining the content distribution and isotope composition analysis of reduced sulfur species.The experimental results show that the S isotopic composition of primary sulfide minerals was relatively homogeneous,and theδ³⁴S_CRS values of tailings with CRS content greater than 250 mg/kg are concentrated in the range of-0.9‰to0.1‰.For tailings with CRS content less than 100 mg/kg,theδ³⁴S_CRS obviously depleted,and the conversion of AVS to CRS is the main reason for the depletion ofδ³⁴S_CRS.The extent of elemental sulfur disproportionation is different at the three sample points of Caoduikeng tailings pond（CDK）,upstream of Tielong tailings impoundment（S0）and downstream of Tielong tailings impoundment（S1）,and theδ³⁴S_AVS values show different degrees of depletion.Δ³⁴S(AVS-S_initial)is-2.8‰,-17.0‰and-8.6‰,respectively(δ³⁴S_initial=-0.3‰,representing the average S isotopic composition of primary sulfide minerals).The positive correlation betweenΔ³⁴S(ES-S_initial)andΔ³⁴S(AVS-S_initial)indicate that S（-Ⅱ）can also be converted into metal di-sulfide after forming metal mono-sulfide.（3）The distribution characteristics of WSS,Ex S,ASS and their isotopic compositions in tailings under different storage conditions show that sulfate is the main form of sulfur in tailings,accounting for about 42%of the total sulfur content,and its existence and distribution are generally controlled by the oxidation and physical diffusion of sulfide minerals,and the formation process of secondary sulfate minerals.TheΔ³⁴S(WSS-S_initial)values of CDK,S0 and S1 are in the range of-1.3‰～1.0‰,indicating that the WSS is derived from the oxidation of primary sulfide minerals.For CDK tailings exposed to air for a long time,about 55%of sulfate exists in the form of WSS.Dissolution and migration lead to ³²S depleted in WSS,Δ³⁴S(WSS-S_initial)is 1.0‰.However,the oxidation degree of S0 and S1 tailings submerged in water for a long time is low,but metal mono-sulfide and di-sulfide are still oxidizing,Δ³⁴S(WSS-S_initial)is negative（S0 is-1.3‰,S1 is-1.2‰）.The sulfates in S0 and S1 mainly exist in the form of ASS,accounting for 47%and 68%of the total sulfate,respectively.Δ³⁴S（WSS-ASS）are 0.9‰and0.2‰,respectively,indicating that the secondary sulfate minerals in S1 are nearly saturated,and the balance between WSS and ASS is reached.The source and redistribution mechanism of sulfate in tailings impoundment of metal sulfide mine are clarified.（4）The formation process of Cu S and its environmental stability were elucidated from the perspective of chemical kinetics and coagulation kinetics by simulating the tailings impoundment environment of metal sulfide mine.The results show that the formation of Cu（Ⅱ）and S（-Ⅱ）in acidic environment involves redox reaction.The coagulation process of copper sulfide was obvious within 20 minutes,and the coagulation rate was related to S/Cu ratio,electrolyte concentration and p H value.When Cu（Ⅱ）is excessive,the condensation rate is slow,while when S（-Ⅱ）is excessive,the condensation rate is fast.When Na₂SO₄ is used as electrolyte,with the increase of electrolyte concentration,the positive charge on the surface of copper sulfide colloid is neutralized by SO₄^2-,and the coagulation is accelerated.When the concentration of Na₂SO₄ is constant,the higher the p H,the lower the density of the positive charge on the surface of the copper sulfide colloid,the faster the coagulation.The critical micelle concentrations of copper sulfide colloid were 220,200 and 180 mmol/L at p H=2,3 and4 with Na₂SO₄ as electrolyte,respectively,which verified the possibility of Cu S agglomerating in metal sulfide mine tailings environment.