Adaptation Study Of Microbial Induced Carbonate Precipitation For Solidification Of Cyanide Tailings

Cyanide tailings are hazardous solid wastes produced in the process of gold smelting.The long-term accumulation of cyanide tailings not only occupies a large amount of land resources and spreads with the wind,but also the heavy metals contained in them will spread and migrate under the effect of rainwater leaching and surface runoff,polluting the surrounding environment.In this paper,microbially induced carbonate precipitation(MICP)technology was used to solidify cyanide tailings to address the environmental pollution problem.In order to adapt to the experimental conditions of the field demonstration,this study investigated the effects of bacterial population,reaction solution concentration,reaction time,environmental conditions(p H and temperature),crystalline morphology of carbonate precipitation and grouting method on the curing of cyanide tailings by MICP technology.The reaction conditions of the MICP process were optimized.Finally,a pilot test and field demonstration were conducted,which led to the study of the adaptability of the MICP technology to cyanide tailings.(1)The optimal culture conditions and tolerance range of Bacillus pasteurii were determined by investigating the effects of different p H,temperature and culture medium on the growth of Bacillus pasteurii.The optimum incubation time for Bacillus pasteurii was 48 h.The optimum temperature for Bacillus pasteurii was 30?.The p H tolerance range of Bacillus pasteurii was 6-9.15 g/L molasses,5 g/L(NH₄)₂SO₄,0.25 g/L KH₂PO₄and 10 g/L Na Cl were used as the optimum culture medium for Bacillus pasteurii in large-scale engineering practice.When this bacterium was applied to the curing of cyanide tailings by MICP technology,0.8mol/L urea and 0.5 mol/L calcium source were selected as the best configuration conditions for the cementing solution.(2)Microbially induced carbonate precipitation experiments were completed by adding magnesium acetate,calcium chloride,calcium acetate,calcium sulfate,calcium nitrate,and calcium lactate to urea to form six different cements.X-ray diffraction(XRD)and Fourier infrared(FTIR)spectra showed that the mineralized products were composed of calcium or magnesium carbonate under different cements.Scanning electron microscopy(SEM)results showed that when the cementing solution was urea and calcium chloride,a large number of calcite crystals were produced on the surface of the mineralized products,while when the cementing solution was urea and calcium acetate,a large number of clustered crystals and spherical chalcocite with hexagonal structure could be found on the surface.(3)Experiments on microbial cementation of cyanide tailings were completed using the six different cementing solutions formed.The cementing properties were compared and analyzed by measuring the dry density,water absorption,calcium carbonate or magnesium carbonate content,compressive strength,heavy metal leaching content and heavy metal morphology of the cemented products.In comparison,calcium nitrate had the worst curing effect,with compressive strength of 0.35 MPa and calcium carbonate content of 7.6%;calcium acetate and calcium chloride had the best curing effect,with compressive strength of 0.75 MPa and 0.72 MPa and calcium carbonate content of more than 12%.After five curing treatments,the heavy metal morphology in the cyanide tailings cured samples was transformed from unstable to stable state,in which the carbonate bound fractions of Cu,Pb,Zn and Cr increased by 19.25%,12.11%,16.10%and 23.98%,respectively,on the cyanide tailings treated with urea and calcium chloride as the cementing solution for MICP.The carbonate bound fractions of Cu,Pb,Zn and Cr increased by 16.58%,8.76%,5.16%and 14.31%,respectively,in the cyanide tailings treated with MICP using urea and calcium acetate as the cementing solution.The leaching contents of Cu,Pb,Zn and Cr of cyanide tailings after curing treatment showed a decreasing trend.After curing the cyanide tailings five times with urea and calcium chloride as cements,the leaching amounts of Cu,Pb,Zn and Cr were reduced to 14.91 mg/kg,60.62 mg/kg,2.17 mg/kg and 0.21 mg/kg,respectively.After curing the cyanide tailings five times with urea and calcium acetate as cements,the leaching amounts of Cu,Pb,Zn and Cr were reduced to13.08 mg/kg,55.82 mg/kg,2.09 mg/kg and 0.31 mg/kg,respectively.(4)Reinforcement tests were conducted on cyanide tailings at different scales by using microbial slurry technology to investigate the feasibility of its application to large-scale cyanide tailings curing.The cyanide tailings before and after curing were collected on site for characterization and analysis,and the formation of calcium carbonate in the treated cyanide tailings was confirmed by SEM results.FTIR and XRD mapping analysis showed that CO₃^2-and related heavy metal mineral components were present in the products of the cured cyanide tailings,and the main substance was Ca CO₃.Meanwhile,the content of Ca CO₃in the taken samples was measured to be 3.4%.The distribution of heavy metal morphology was studied for cyanide tailings at 0,10,20 and 30 days.It was found that the carbonate bound state of Cu,Pb,Zn and Cr increased by 1.58%,8.40%,2.13%and 9.53%,respectively,in the cyanide tailings after curing.It indicated that in situ cyanide tailings slurry had some effect,but not ideal,which may be due to the restricted diffusion of microorganisms in cyanide tailings.