Study On Recovery Of Heavy Metals From Fly Ash Using Leaching-Electrochemical Processes

There are three main methods of disposal of waste,namely landfill,composting and incineration.Landfill and compost have a large footprint and are prone to environmental pollution.Incineration has the advantages of reducing capacity,reducing microorganisms,and generating electricity;it has established waste incineration power plants everywhere.After the end of waste incineration,there are mainly two kinds of solid wastes,namely fly ash and slag;fly ash is enriched with various harmful metals and high in content;therefore,the existing incineration technology needs to increase the reasonable fly ash treatment process.The fly ash characteristics were first characterized by X-ray diffractometry?XRD?,X-ray fluorescence spectrometer?XRF?,and flame atomic absorption spectrometer?AA-7000?.The crystal compounds in the fly ash were detected by XRD as KCl,NaCl,CaSO₄,CaCO₃ and the like.The main elements in the fly ash detected by XRF are Cl,O,S,Ca,K,Na,Si,Zn,Cu,Pb,Cd,Mg,Ti and the like.The analysis results of heavy metals in fly ash by acetic acid buffer solution,total digestion and BCR continuous extraction are as follows:the elements with excessive toxic toxicity are Pb and Cd,and the most abundant elements are Zn,weak acid extraction of Pb,Cu,Zn and Cd.State has the largest proportion.Secondly,the effect of acid leaching reagent on the leaching rate of heavy metals in fly ash and the detoxification of residual ash were studied.The leaching rates of HCl,HNO₃ and H₂SO₄on Pb,Zn,Cu and Cd in fly ash were studied,and HCl was selected as the leaching agent.The leaching rates of HCl concentration,liquid-solid ratio,reaction time and rotation speed in Pb,Zn,Cu and Cd in fly ash were discussed.The experimental results show that when the liquid-solid ratio is 2(mL·g^-1)and the concentration of HCl is 5 The leaching rates of Pb,Zn,Cd and Cu in fly ash were 99.31,97.75,99.03 and 81.38%,respectively,when mol·L^-1,rotating speed was 300 r·min^-1 and reaction time was 20 min.468.10,2,268.80,78.12,347.78 mg·L^-1.After leaching with HCl,the residual solid was washed with water using a NaCl solution,and the residual solid after washing with water was analyzed using an acetic acid buffer solution method,and the concentrations of Pb and Cd were lower than those of GB 16889-2008.Then,Pb,Zn,Cu,and Cd in the leachate are separated by an extraction method;and Pb in the raffinate phase is recovered by electrodeposition.The extraction rates of Pb,Zn,Cu and Cd in the leachate were studied by TBP,P204 and N235,and TBP was selected as the extractant.The effects of volume fraction of TBP,O/A ratio and extraction time on the extraction rates of Pb,Zn,Cu and Cd were discussed.When the volume fraction of TBP was 90%,the ratio was?O/A?1:1.The extraction time is 20 min.The extraction rates of Zn,Cd,Cu and Pb in the leachate are 95.35,97.56,89.14 and 0.98%,thus separating Pb from Zn,Cd and Cu.The effect of voltage and pole spacing on Pb recovery was studied.When the pole spacing was 20 mm,the voltage was 1.2 V,and the electrolysis time was 3 h,the recovery of Pb was 97.04%.Finally,in order to recover Zn,Cd and Cu in the extracted phase,Zn,Cd and Cu in the extracted phase are back extracted by using a suitable stripping agent.The stripping rates of Zn,Cd and Cu in the extract phase were studied with different stripping agents and compared with?O/A?.When water was used as stripping agent,the ratio was?O/A?1:2,Cu,The stripping rates of Zn and Cd were 91.68,93.16 and 95.35%,respectively.The stripping phase contains a large amount of Cu²⁺and Fe³⁺ions,and in order to reduce the content of impurities in the product when the Zn is recovered by electrodeposition;therefore,Cu is first recovered and then Fe is removed.Cu was recovered by a displacement reaction;the amount of substitution of Fe was1.2 times and the time was 20 min,and the recovery of Cu was 98.46%.By removing Fe by using the concentration constant of different substances,studies have shown that when a small amount of H₂O₂ is added and the pH of the solution is adjusted to 4,the removal rate of Fe ions is 98.43%,and the removal rate of Zn and Cd is less than 10%.The Zn and Cd were extracted by electrodeposition.The recovery of Zn and Cd was investigated by voltage and pole spacing.When the voltage was 1.8 V,the pole spacing was 20 mm,and the electrolysis time was 60 min,the recovery of Cd of Zn was 97.3 and 96.2%.