Mechanism and process of recycling copper and cyanide from a hazardous cyanide waste slag

China's gold mining enterprises produce a large amount of cyanide slag containing copper ions and cyanide every year, which causes serious environmental pollution if deposited directly. Aiming at hazardous waste cyanide slag, a new process of "cyanide slag washing, copper ions precipitation in washing solution and gaseous membrane recovery of CN^-" was developed by combining theoretical analysis with experimental research, which realized the comprehensive utilization of copper ions and cyanide in the slag. Firstly, the washing test of cyanide waste slag was carried out. The results showed that when the pH of washing water was 3.0 and the water amount was 0.8 m³/t, the total cyanide concentration in the leaching toxicity of cyanide residue was less than the limit value of 5 mg·L^-1 stipulated by the national standard. Secondly, the thermodynamics and chemical analysis results show that Cu²⁺/Cu(CN)_n^(n-1)- in the washing solution can be removed by CuCN and CuS precipitation, and cyanide is converted into HCN and absorbed by NaOH solution to form NaCN. Further experimental studies showed that the precipitation rate of Cu²⁺/Cu(CN)_n^(n-1)- reached 98.99% under the condition of pH 3.5 and NaHS dosage of 200 mg·L^-1. The total cyanide removal rate was 99.02% when the flow rate of washing solution was 0.4 m³/h and the two-stage membrane was used. The feasibility and stability of the new method were verified by the cycle test, which can not only solve the problem of toxic and harmful substances leakage in the tailings dam but also maximize the utilization of mineral resources.