Recovery Of Multiple Heavy Metals From Pb Smelting Residues By Three Low-Molecular-Weight Organic Acids

At present,the resource utilization of smelting waste residue has become one of the environmental hotspots all over the world.Hydrometallurgy is a rapid and effective metal recovery technology.The cost of conventional strong acid extractants is high and there is a potential threat to the environment and human health.Therefore,screening new economical,green and efficient extractants is the key to the development and application of this technology.In this study,citric acid（CA）,malic acid（MA）and acetic acid（AA）were used to extract heavy metals from Pb smelting residue,and the recovery efficiency of these three reagents under different leaching conditions was explored and compared.Based on the double objectives of leaching efficiency and benefit index,the extraction process was optimized to determine the optimal extraction parameters of the two optimization objectives,and explore the extraction mechanisms.The heavy metals in the leachate were recovered by chemical precipitation and adsorption metods,and the effect and mechanism were evaluated.The main research results are as follows:（1）The concentration,p H and reaction time of three complexing/chelating agents have significant effects on the leaching efficiencies of heavy metals from the residue.As the concentration of CA and MA increased from 0.05 to 0.40 mol L^-1,the leaching efficiencies of the three heavy metals increased significantly（p<0.05）,and then tended to be stable.The difference was that the leaching efficiencies of Zn,Pb and Cd increased significantly before the AA concentration reached 1.0 mol L^-1（p<0.05）;the leaching efficiencies of the three heavy metals reached the highest in the p H range of 2.0 to 4.0.However,with the increase of p H value from 5.0 to 6.0,the leaching efficiencies of Zn,Pb and Cd decreased significantly（p<0.05）.Therefore,the acidic conditions are more conducive to the leaching of heavy metals from Pb smelting residue by these three organic acids;with the increase of reaction time,the leaching efficiencies of Zn,Pb and Cd increased significantly within 10-60 min,and then increased slowly or tended to be stable（p>0.05）.（2）Three low-molecular-weight organic acids,CA,MA and AA have good leaching efficiency for heavy metals in Pb smelting residue.When the concentration of CA is 1.0 mol L^-1,the leaching efficiencies of Zn and Pb in the residues were the highest,up to 95.66 and82.68%,respectively,while the leaching efficiency of Cd is 80.11%at the concentration of0.82 mol L^-1.When the concentration of MA was 1.0 mol L^-1,the leaching efficiencies of Pb and Cd were the highest,73.23 and 78.78%,respectively,while the leaching efficiency of Zn was 83.68%at MA concentration 0.73 mol L^-1.At AA concentration of 1.0 mol L^-1,the highest leaching efficiency of Cd was 82.01%,while at this concentration,the leaching effficiencies of Zn and Pb were 79.42%and 61.80%,respectively.（3）The three organic acids extract heavy metal ions from Pb smelting residue in different ways.Firstly,acid dissolution and dissociation could promote the dissolution of alkaline minerals and the dissociation of metal ions in the residue.The electrostatic adsorption is due to the aggregation of carboxyl ions,resulting in electrostatic attraction to metal ions.Intermolecular/intramolecular complexation/chelation makes it combine with heavy metal ions to form a stable cyclic chelate.For AA,due to the one-stage ionization equilibrium,it can only combine with metal ions to form unstable complexes in solution.（4）By saturated D-optimal design,the leaching mechanism of three organic acids,including main factor,main effect and interaction effect was explored.The optimal leaching conditions were obtained on the premise of double objectives and three influencing factors changing at the same time.Considering the leaching efficiency as the goal,the optimal leaching efficiencies of CA for Zn,Pb and Cd were 90.79,80.09,and 90.46%,MA for Zn,Pb and Cd were 85.05,76.00,and 90.02%,and AA for Zn,Pb and Cd were 82.60,64.50,and90.46%,respectively.When considering the benefit index as the target,the optimal benefit indexes of CA,MA and AA are 16.41,15.69,and 14.64 respectively.（5）Chemical precipitation and adsorption methods can completely remove heavy metals from leachates from CA,MA,and AA.The sulfurization reaction of lead and cadmium is higher than that of zinc.Therefore,the efficient separation of zinc and the other two heavy metals can be realized by controlling the dosage of sulfide.The strongly acidic 001×7 type styrene cation exchange resin has good adsorption effect on heavy metals in leachates from three organic acids,the adsorption rates of Pb,Zn and Cd can reach more than 90%when the dosage is 7.5 g,and has good adsorption performance in strong acidic environment.（6）CA,MA and AA extraction have a great influence on the morphological distribution of Zn,Pb and Cd in Pb smelting residue.The extraction efficiency of F2,F3 and F4 by these three reagents decreased by 74.71-95.56%,68.68-95.57%and 95.56-97.60%,respectively.The three organic acids reduced the carbonate bound and iron manganese oxide bound components of heavy metals Cd,Pb,and Zn significantly in Pb smelting residue,and reduced the environmental risk.（7）The leaching toxicity of Zn,Pb,and Cd from Pb smelting waste residue was greatly changed by three organic acids.After CA,MA,and AA extraction,the leaching toxicity of three heavy metals in waste residue was significantly reduced（p<0.05）,weakening the environmental risk of easily migrating components.After treatment with these three organic acids,the leaching toxicity values of Zn,Pb,and Cd are lower than the maximum limit of the identification standard for hazardous wastes.