The Multi-step Leaching Mechanism Of Cadmium,Zinc,Copper And Lead From Metallurgical Sludge And Separation Process Enhancement

With the development of industry and social economy,the demand for metals such as copper and zinc has been increasing significantly in various industries.A great deal of metallurgical sludge is produced in the production of various valuable metals.In general,the metallurgical sludge has a high content of heavy metals.If it's not used and placed randomly,it is a great waste of non-renewable resources,and the leakage of toxic metals in the stacking plant will pose a serious threat to the environment.Therefore,it is a very urgent task to recover valuable metals in metallurgical sludge as secondary resources and to make the remaining sludge harmless.As usual,a mixture of a plurality of heavy metals extracted from the sludge needs purifying by a complicated separation process to achieve the corresponding value.In this paper,a two-step leaching method is proposed for the stepwise leaching of heavy metals in sludge.According to the discrepancy in the chemical forms of heavy metals in the sludge,the heavy metals in the easily migrated forms were first leached,and then the more stable metals were leached.The purpose of preliminary separation was achieved by the large differences between the metals'fractional distributions.In this paper,the metallurgical sludge produced by a copper smelting plant,was used as research material.The two-step leaching method was used to leaching Cd,Zn,Cu and Pb in the metallurgical sludge.A solution containing Cd²⁺,Zn²⁺,and Cu²⁺without Pb²⁺and a solution containing Cu²⁺,Zn²⁺,Pb²⁺,and low concentration of Cd²⁺were obtained,respectively.Cd²⁺and Zn²⁺were leached in the first step and Cu²⁺and Pb²⁺were leached in the second step.The two leachates were treated by sodium sulfide to recover heavy metals in the forms of sulfides.After the first leachate was precipitated by sodium sulfide,a mother liquid of extremely rare Zn²⁺was obtained.To recover Zn²⁺,the phosphate-based adsorbent and the amine-based adsorbent were designed with the Chinese-specific agricultural waste ramie stalk as the raw material.The Zn²⁺in the mother liquor was enriched by adsorption,and the agricultural waste was reused as a secondary resource.In this paper,the comprehensive treatment of metallurgical sludge was carried out,which not only recovered valuable metals of sludge,and avoided waste of resources,but also plays an important role in protecting the environment,which was of great economic,environmental and social benefits.The main research contents and results are as follows:?1?The chemical forms of Cd,Zn,Cu and Pb in metallurgical sludge were analyzed by BCR sequential extraction procedure.79.87%of Cd existed in unstable weak acid extraction fractions?F1?and reducible fractions?F2?and 92.28%of Pb was in the most stable residual fractions.Therefore,a two-step leaching method was designed,in which the F1+F2 fractions were leached in the first step and F3+F4 fractions were leached in the second step.Using 0.7mol/L hydrochloric acid with a liquid-solid ratio of 10:1?mL:g?,most of the Cd and Zn were leached,and Cd,Zn were separated from Cu,Pb at the greatest extent at 240 min.The leaching efficiencies of 82.89%of Cd,55.73%of Zn,10.85%of Cu,and 0.25%of Pb were obtained.The total leaching efficiencies of Cd,Zn,Cu,and Pb by the two-step leaching method were up to 99.76,91.41,71.85,and 94.06%,respectively.The preliminary separation of Cd,Zn,Cu and Pb were achieved by the two-step leaching method.?2?By comparison,it was found that there was a certain correlation between the amount of heavy metals leached at 3 min and the content of F1+F2,using hydrochloric acid as the leaching agent.After the first leaching,the remaining heavy metals migrated between the various fractions,mainly from the more stable F3 and F4 to the unstable F1 and F2,indicating that HCl leaching activates heavy metals and enhances EDTA's leaching effect on metals,especially for Pb.The heavy metals remaining in the sludge after leaching were mainly in the residual fractions,which were of low bioavailability and were harmless to the environment.?3?The heavy metal in the leachate obtained above were recycled by a sodium sulfide precipitation.For the first step leachate,the heavy metal in the solution is in free ion form,and the content was below the discharge standard by adding enough sodium sulfide.For the second step leachate,since the heavy metal in the leachate is in the complexing state,the neutralization method has no effect on the precipitation of heavy metals,but a large amount of EDTA were obtained by increasing the pH of the leachate.It can greatly reduce the amount of sodium sulfide,but the removal rate of Zn was only 20.08%.The addition of PAC has a significant improvement effect on the removal rate of heavy metals,and the concentration of residual liquid Cd²⁺,Zn²⁺,Cu²⁺and Pb²⁺can reach the discharge standard of copper industry in China?GB25467-2010?.?4?Phosphate-based biomass adsorbent and amine-based biomass adsorbent were prepared by chemical modification of phosphate and amino group with ramie stalk which were used to adsorb the extremely dilute concentration of Zn²⁺in the mother liquor obtained after precipitation of the leachate.The method also achieved the reuse of waste biomass.Through mechanism research,it was found that the main adsorption site of the phosphorylated adsorbent to Zn²⁺was the phosphate group on the surface of the adsorbent.The adsorption of Zn²⁺by the aminated adsorbent combines with the coordination of the surface amine group with Zn²⁺to form a coordination complex.The adsorption selectivity of the amine group to Zn²⁺was better than that of the phosphorylated adsorbent.The adsorption of aminated ramie stalk also had the same adsorption affinity for Cu²⁺.The results showed that the adsorption behavior and mechanism of Cu²⁺were the same as those of Zn²⁺,but the adsorption capacity was slightly higher than Zn²⁺.?5?A large amount of Ca²⁺and Mg²⁺in the residual solution after precipitation interfere with the adsorption of Zn²⁺by the adsorbent,and the Ca²⁺and some Mg²⁺in the solution needed to be removed in advance by oxalic acid precipitation.By adsorption,elution and recycle,the mass ratio of Zn²⁺to Mg²⁺transformed from 0.136:1 in the mother liquor to2.297:1?phosphorylated?and 13.192:1?aminated?,indicating the phosphorylation and amination of the ramie stalk adsorbent in the mother liquor realized the separation,enrichment and recovery for Zn²⁺.Forthermore,the enriched concentration of the aminated adsorbent was higher,and the separation of Zn²⁺and Mg²⁺was more thorough.