Recovery And Utilization Of Valuable Metals From Leaching Residue In Zinc Metallurgy

The neutral residue with low efficiency of secondary utilization in zinc smelting is one of the largest industrial discharge sludge, which resulted in serious environment problems and security risks. While realizing the comprehensive utilization of the neutral residuals in zinc smelting to recovering the lead, zinc and cadmium metals will produce both economic and social benefits. Base on the researches of the neutral zinc residues in zinc smelting industrial sludge of Xiang xi, the study mainly included four parts.（1） The study of chloride salt leaching, the effect of leaching temperature, hydrochloric acid concentration, leaching time, concentration of sodium chloride, liquid to solid ratio and residue size on the leaching rate were systematically studied,and the optimum leaching conditions was obtained, that is leaching temperature 60 ℃, hydrochloric acid concentration 1.5mol/L, leaching time 120 min, concentration of sodium chloride 300g/L, liquid to solid ratio 15 m L/g and particle size 160 mesh. Under the optimum conditions, the leaching rates of Pb, Zn and Cd can reached 94.8%, 81.2% and 95.3%, respectively. The results of Orthogonal experiments results showed that the influence factors on lead leaching rate are in the order of sodium chloride concentration > liquid to solid ratio > hydrochloric acid concentration > temperature > leaching time.（2） The study of slurry electrolysis process: the effect of electrolysis temperature, electrolysis time, current density, concentration of CaCl₂ and Fe Cl3 concentration and stirring speed on the electrolytic efficiency of lead were investigated. According to the experimental results, the optimal electrolytic conditions was obtained. Under the optimal conditions of tank voltage 1.8V, current density 150A/m2, CaCl₂ concentration 10g/L, electrolysis temperature 60℃, electrolysis time 30 min, stirring speed 400r/min and gelatin concentration 0.2g/L, the purity of sponge lead reached 91.0%, the recovery rate reached 70.5%and the current efficiency could reached 53.1%.（3） The recovery of iron, zinc and cadmium process: the iron element in the electrolyte was recovered by adjusting p H value of the solution on the first step; Then the cadmium was displacement out by adding zinc powder into the filter liquor; Finally, the Zn O was obtained by calcining the Zn（OH）2 sediments at 700 ℃. Through experimental research and analysis, the recovery rate of Iron oxide products could reached 80.6% with a 95.8% purity of Fe₂O₃, which has met the national standard ofGB1863-89. While the recovery rate of Zinc oxide products could reached 70.6% with a 96.3% purity of Zn O, which has met the industry standard of HGT2572-2006. While the recovery rate of Magnesium oxide products could reached 86.2% with a 93.5% purity of Mg O, which has met the industry standard of the first grade of HGT2572-94.（4） The research on leaching toxicity of residue: Leaching toxicities of the neutral residue of zinc smelting and the residue of slurry electrolysis were analyzed according to the leaching toxicity identification standard（GB5058.3-2007） for hazardous wastes. The results indicates that the concentration of Cd in zinc leaching solution and slurry residue leaching solution are determined to be 9.83 and 7.35 mg/L, respectively, which all above the discharge standard. In order to decrease the toxicity of the slurry electrolysis residue, the mechanical ball milling process to immobilize cadmium was conducted. by adding 7% of zero valence iron powder and under the grinding conditions of ball material ratio 25:1, grinding time150 min,liquid to solid ratio1.5:1,the Cd concentration of slurry residue leaching solution was found to be 3.82 mg/L. Despite of the leaching toxicity of cadmium is still exceed the national standard, the toxicity of the residue is significantly decreased.