Study On Copper/Lead Anode Slimes Treated By Alkaline Oxidative Leaching

As an important by-product in electrolytic refining process, copper/lead anode slime contains copper, lead, selenium, tellurium, arsenic, antimony, precious metals and so on. The complete anode slime treatment process mainly includes the four steps as follows:pretreatment, pyro-treatment, hydro-treatment and precious metal extraction. Although many processes about anode slime treatment have been researched, most of them are carried out in acidic medium and have the disadvantages of low metal recovery, serious environmental pollution, severe equipment corrosion and high treatment cost. In view of the national policies about industrial development programming, energy saving and environmental protection, the study is proposed.The purposes of the study are valuable metal separation, less environment pollution, improved metal recovery and precious metal re-enrichment. When the metal separation effects of direct alkaline leaching and acid leaching of anode slime are compared, the united process combining alkaline oxidation leaching with acid leaching was successfully applied in the copper/lead anode slime treatment. Arsenic and selenium are dissolved into the solution and effectively separated from the slime in the process of alkaline oxidative leaching, when some other metals are also oxidized. Copper, tellurium and bismuth can be separated in the acid leaching process and then precious metals are efficiently enriched. The main content of the study are as follows:(1) According to the results of the study on main impact factors and metal behaviors in alkaline oxidative leaching process, it is feasible to select oxygen (or air) and hydrogen peroxide as the oxidant. Using nozzles can promote material mixture and oxidation, and pressurized oxidation can improve the reaction temperature, pressure and oxygen partial pressure together. According to the E-pH of Me-H20, behaviors of main metals in copper/lead anode slime were studied in the process of alkaline oxidative leaching. Copper, lead, bismuth, antimony, gold and silver exist in the alkaline leaching residue in the form of oxide, oxysalt or elementary substance, while arsenic, selenium, tellurium, sulfur, silicon and fluorin exist in the alkaline solution in the form of oxygen-containing sodium salts. Anyway, effective separation of valuable metals can theoretically be achieved by alkaline oxidative leaching.(2) The united process combining alkaline oxidative leaching with sulfuric acid leaching is supposed to treat with copper anode slime, according to the metal separation effects of different oxidation methods in the KOH and NaOH medium. Arsenic and selenium are leached into the solution and some other metals are oxidated by alkaline oxidation leaching. Copper and tellurium in the alkaline leaching residue are leached in the sulfuric acid solution and then can be efficiently extracted.The optimum conditions of the alkaline oxidation leaching of copper anode slime are determined. When NaOH concentration is 2.0mol/L, temperature 200℃, oxygen partial pressure 0.7MPa, time 3h, ratio of liquid to solid 5:1, filling ratio is 0.8 and stirring speed 1000r/min, residue ratio 76%, the leaching rate of arsenic and selenium is above 99%, that of lead and antimony is about 3%, and copper, silver and tellurium cannot be leached. In the sulfuric acid leaching process of the alkaline leaching residue, the optimum conditions have been researched. When the sulfuric acid concentration is 2.7mol/L, temperature 85℃, ratio of liquid to solid 5:1, time 2h, air pressure 0.1～0.2MPa, stirring speed 300r/min, residue ratio 60%, the leaching rates of copper and tellurium are 97.65% and 77.53%, respectively, both tin and antimony less than 2%, silver and nickel are 8.95% and 5.85%, respectively.In the alkaline oxidative leaching process, PbSO4 in the anode slime can be transformed into Pb(OH)2, and then Pb(OH)2 is transformed into PbSO4 again in the sulfuric acid leaching process. However, there is no phase transform on BaSO4 in the above leaching processes.(4) The influences of different oxidation methods on lead anode slime alkaline leaching and then hydrochloric acid leaching were researched. As a result, the united process combining alkaline oxidation leaching by step with hydrochloric acid leaching is employed to treat with lead anode slime. In the alkaline leaching process arsenic is leached into the solution and meanwhile other metals are oxidized and left in the residue. Copper and bismuth are leached into the acidic solution and therefore they can be separated from other metals.The optimum conditions of alkaline oxidation leaching by step of lead anode slime are determined. NaOH concentration is 2.0mol/L, ratio of liquid to solid 5:1, temperature 80℃, and stirring speed 300r/min. At first, oxidation is carried out at 0.2MPa air pressure for 4h, and then hydrogen peroxide is added by 0.2 times the weight of lead anode slime and reaction continues for 3h. as a result, the residue ratio is 80%, the leaching rates of arsenic and lead are 92% and 5.0%, respectively. The optimum conditions of hydrochloric acid leaching to separate bismuth from the alkaline leaching residue are as follows:HCl concentration is 3.0 mol/L, ratio of liquid to solid 5:1, temperature 85℃, time 2h, and stirring speed 300r/min. under the above conditions, residue ratio is about 40%, the leaching rates of bismuth, copper lead and antimony are 96.35%, 95.50%,12.35% and 12.45%, respectively.(4) The processes of valuable metals recovery from the leaching solution are studied. The results show that Se can’t be reduced by Zn or Al powder. Ag and Te can be reduced by Cu powder, and both their reduction rates are above 99%. The process combining neutralization hydrolysis with sodium sulfate precipitation can together remove lead and antimony in the HCl leaching solution. The value from theoretical calculation and the experimental result is uniform. When the solution pH is controlled at 1 and SO42- concentration at 0.2mol/L, antimony and lead concentration can be decreased to 1.38g/L and 0.83g/L, respectively, and the leaching solution is effectively purified.All the results show that the alkaline oxidation leaching process of copper/lead anode slime is feasible. However, there are no any other literature about the process and purpose in the study. Therefore, the study is of a great practical value for promoting the reform of anode slime treatment process, especially for improving the recovery of rare scattered metals, preventing secondary pollution of arsenic and improving the recovery of precious metals.