Fundamental And Technological Study On Clean And Efficient Recycling Of Lead-Bearing Polyelemental Dust

In view of the fact that secondary lead dusts have the properties of element diversities and complex occurrence relationships,resulting in difficulty of comprehensive recovery and safe disposition.This thesis proposed the fundamental and technology on efficient recycling of lead-bearing polyelemental dust,including selective desalination,acid oxidation leaching of desalination residue followed by step separation,and Na₂CO₃ alkali smelting of acid leaching residue.The purposes are salt removal,Pb,Cd and Zn recoveries,and As immobilization,to establish a technology prototype for efficient recovery and harmless disposal of Pb-bearing polyelemental dust.The main research conclusions in this thesis are summarized as follows:（1）The effects of four desalination processes with H₂O,Na₂CO₃,Na OH-H₂O₂ and Ca（OH）₂-H₂O₂ as a reagent were investigated.After water leaching at 80℃,93.04%K,77.51%Cl and 22.97%F were leached,KCd Cl₃,KCl and Cd Cl₂ were dissolved.Meanwhile,28.46%Cd,11.90%Zn and 34.03%As were dispersed into the leaching solution.After Na OH-H₂O₂ and Ca（OH）₂-H₂O₂ leaching at final p H of 7.5 and 5.97-10.23,about94%K and 78%Cl was removed,and Cd,Zn and As were reprecipitated as arsenates in the desalination residue,realizing selective removal of K and Cl.After Na₂CO₃ leaching at p H 8.5,the removal efficiencies of Cl and F were increased to 86.12%and 60.27%,benefiting from Cl and F removal of Pb Cl F,and Cd and Zn were reprecipitated in the form of carbonates,but it is difficult to remove Cl from Pb₅（As O₃）₃Cl in the four processes.Therefore,Na OH-H₂O₂ and Ca（OH）₂-H₂O₂ leaching have good selectivity,while Na₂CO₃ leaching has a better effect for Cl and F removal and is more economical.（2）Cd-As coprecipitation research were conducted,and the mechanism of selective desalination was identified in the alkaline oxidation leaching:In the Cd²⁺-As O₄^3--H₂O system,the sediment varied from Cd₅H₂（As O₄）₄ to Cd₃（As O₄）₂ and then Cd（OH）₂ in turn with p H value increasing.The experimental results show that square-shaped Cd₅H₂（As O₄）₄·4H₂O,lamellar Cd₃（As O₄）₂·4H₂O and flocculent amorphous Cd（OH）₂ precipitates were formed in the mixed solution of Cd and Zn with the increase of p H value.Co-precipitation of Cd and As was Cd₅H₂（As O₄）₄·4H₂O at initial n_Cd/n_As 1.25 and p H 5.5 or Cd₃（As O₄）₂·4H₂O at initial n_Cd/n_As 1.50 and p H 7.5.The precipitation ratios of both Cd and As were above 99%,and K and Cl were selectively removed.（3）Reaction behaviors of elements in Na₂CO₃ desalting process were investigated:In the Cd²⁺-As O₄^3--CO₃^2--H₂O system with initial total concentration of 0.09 mol/L Cd,0.11 mol/L As and 0.24 mol/L C,a single solid phase is Cd₅H₂（As O₄）₄ at 5.0≤p H≤5.8.With the increase of p H,a two-phase coexistence domain of Cd₅H₂（As O₄）₄ and Cd CO₃ appears,and then the solid phase is converted to Cd CO₃ and Cd（OH）₂ at 7.4≤p H≤12.4and above 12.4,respectively.The optimized leaching conditions were obtained through the desalting experiments:Na₂CO₃ concentration 25 g/L,temperature 80°C,liquid to solid ratio 6 m L/g and 60 min,94.08%K,86.49%Cl,67.65%F,82.74%Br and 78.79%I were removed.Na₂CO₃concentration and liquid to solid ratio significantly affected the removal of Cl and F.Pb,Cd and Zn are retained as carbonates into the desalting residue.（4）H₂SO₄-H₂O₂leaching of the desalination residue and As precipitation by hydrothermal process of Fe（III）salt were studied in order to Cd and Zn recovery and As immobilization:Under the optimum conditions of H₂SO₄ concentration 120 g/L,30%H₂O₂ 55 m L/100 g desalination residue,liquid-to-solid 6 m L/g,temperature 60℃and 2 h,the leaching efficiencies of Cd,Zn and As were 96.59%,95.86%and 84.42%.Based on the thermodynamic analysis of Cd²⁺-Zn²⁺-Fe³⁺-As O₄^3--SO₄^2--H₂O system,selective precipitation of ferric arsenate can be achieved at p H 1.0,88.98%As were immobilized by hydrothermal process as the stable Fe As O₄·2H₂O with a morphology of flower cluster,while the loss ratios of Cd and Zn were only 1.47%and 1.08%.Overall,after three-stage treatment of preneutralization-hydrothermal precipitation of As-neutralization precipitation of Fe,the total removal ratio of As was more than 99.99%.The process achieves the purpose of selective As separation.（5）A new process of zinc plate cemention-microcurrent（ZPC-MC）synergistic extraction of cadmium was proposed to efficiently separate Cd from the Cd-Zn purification solution.The optimal parameters of ZPC-MC process and the distribution laws of Cd and Zn were determined:Under the conditions of anode current density 1 m A/cm²,initial p H 2.5,temperature45℃and 6 h,the separation efficiency of Cd was more than 99%with a low residual Cd concentration of 1.12 mg/L,in which 97.55%Cd was cemented into the anode Cd sponge with a high purity of 95.65%,and the zinc plate consumption was only 1.08 times of the theoretical cemention value.Compared with the traditional zinc powder cemention,the process has some advantages of less zinc consumption,high purity of cadmium sponge and low residual cadmium concentration.（6）The Na₂CO₃ smelting process for Pb recovery from acid leaching residue was carried out,the reaction sequence of Pb SO₄ and element distribution rules in smelting process were determined.In the reducing atmosphere,Pb SO₄gradually follows the pathway of Pb SO4^C→^{/C O}Pb S^N→^{a 2 C O 3/C/C O}Pb O^C→^{/C O}Pb with reductant dosage increasing.Under the optimum conditions of 18%coke addition and n_Na2CO3/n_S=2.0 at 1050℃for 2 h,a crude Pb bullion with a purity of 97.73%was obtained,and the direct recovery ratio of Pb was 96.75%.The residual Pb in molten salt slag mainly existed as Pb S.Therefore,one step extraction of lead from acid leaching residue was realized by alkaline smeltingAfter the whole treatment process of Pb-bearing polyelemental dust,78.29%Cl and 94.56%K were separated into the desalination solution;91.42%Cd and 97.05%Pb were recovered in the form of anodic Cd sponge and crude lead,respectively;94.30%Zn remained in the cemented solution;74.60%As was solidified as Fe As O₄·2H₂O.The process realizes the purpose of efficient recovery of valuable components and disposition of harmful elements from Pb-bearing polyelemental dust,it can provide a practical solution of polyelemental dusts from secondary lead smelting industries.There are 98 figures,34 tables,and 203 references in this paper.