Characteristics And Mechanism Of The Bioleaching Of Thespent Zn-Mn Batteries At High Pulp Density And Its Resource Utilization

Bioleaching technology as a green,environmentally friendly and economical method has gained great attention for the resource utilization of spent batteries.However,the bioleaching system generally support only a 1%pulp density or even lower duo to very high content of alkaline substances and organic adhesive in the spent batteries which were toxic to the stains.For commercial application of bioleaching,the pulp density was a very important parameter which greatly affected the operational cost and subsequent separation.Consequently,the study of characteristics and mechanism of the bioleaching at high pulp density and resource utilization of spent Zn-Mn batteries are particularly important.The characterization of Zn and Mn in the spent Zn-Mn batteries was firstly analysed,and experimental results had shown that,they were present as ZnMn₂O₄?MnO₂?ZnO?Zn₅?OH?₂Cl₂?H₂O?and Zn₅?NH₃?₂Cl₂.In addition,anode materials for spent alkaline batteries contained ZnO should be removed and then combined its cathode material and Zn-C batteries well before treating the spent Zn-Mn batteries by using bioleaching.And then,Acidithiobacillus thiooxidans and Leptospirillum ferriphilumand were used for recovering Zn/Mn from the spent Zn-Mn batteries electrode materials at 10%pulp density and the effects of the Zn/Mn dissolution in bioleaching process were examine.The results showed as follows:the adding energy substrate could promote the strain growth and enhanced the dissolution efficiency,but as energy substrate increased it would reduce oxygen transfer and caused high shear and friction,endangering the growth of microorganisms;the proper temperature promoted the enzymatic activities,which was helpful to the dissolution of Zn/Mn;the lower initial pH value was beneficial to the dissolution of Zn/Mn,but this would lead to longer culture time;the addition of exogenous-acid in bioleaching process could significantly increase the dissolution of Zn/Mn due to maintaining active and quantity of strains;the smaller particle size of spent Zn-Mn batteries materials increased it and the strains contacted area to promote Zn/Mn dissolution;not only agitation speed of water bath could promote the growth of the stains,but also enhanced the mass transfer;the surface hydrophilicity of materials were improved by additon of nonionics;however,the excess dose would inhibit the activity of the strains and this was unfavourable for bioleaching.Moreover,based on single-factor experiments,the seven factors were optimally screened by using P-B and RSM with CCD.The experimental results and its statistical analysis showed that energy substrate,the addition of exogenous-acid and the incubation temperature were three key factors influence on the dissolution of Zn-Mn.The optimal process parameters including pulp density were determined and subsequently more than50%dissolution rate for Zn/Mn were obtained after 13 days leaching.In addition,the analysis of variance?ANOVA?results showed that exogenous acid to adjust pH and pulp density control were the main factors.Dissolution kinetics studies showed that the bioleaching behavior of Zn was best described by the chemical reaction-controlled model but Mn had the the diffusion-controlled model.The release of Zn/Mn from ZnMn₂O₄ was attained by Fe³⁺as an oxidant which was generated through a series of biological and chemical reactions based on a cycle reaction of Fe²⁺/Fe³⁺.Addition of extracellular polymeric substances?EPS?of the mixed bacteria can significantly improve the dissolution efficiency of Zn/Mn.Excitation-emission matrix spectroscopy?EEM?and fluorescence regional integration?FRI?was used to evaluate the relationships between recovey of Zn/Mn and EPS produced from the mixed bacteria during bioleaching process.It was found that Zn/Mn dissolution and normalized excitation-emission area volumes(?_T,n)had notable positive correlation.Calculations of the batteries sample's contact angle and surface energy demonstrated that,in the leaching process,EPS played the role of adsorption and oxidation batteries materials to promote it and the stains contacted each other,and this was conducive to the spreading of leachate and strengthened a cycle reaction of Fe²⁺/Fe³⁺.Hence we inferred that there was three the dissolution mechanisms of spent Zn-Mn batteries materials as follow:?i?direct leaching:the mixed bacteria firstly affixed themself to the surface of battery materials by EPS,and then dissovled Zn/Mn from battery materials attacked by the chelate Fe³⁺/Fe²⁺in EPS;?ii?indirect leaching:a small portion of Zn/Mn were released by microbial metabolites through biological acid or redox reaction;and?iii?cooperative leaching:the dissolution of acid soluble Zn/Mn could be quickly completed under indirect chemical leaching;however,there was a slow process to extract refractory Zn/Mn from the spent Zn-Mn batteries in the presence of EPS.In all bioleaching process,direct leaching was simultaneous with indirect one.Ferric ion played a great important role in the bioleaching process.The release efficiency were obtained 85.1%for Zn and 83.2%for Mn addition of 5 g/L Fe³⁺at 5%pulp density respectively,which were about four times higher than that of the pure-bioleacing system.Additionally,electrochemical studies were carried out by using a spent Zn-Mn batteries-carbon paste working electrode in different concentration of Fe³⁺ions bioleaching systems.It was found out that open circuit potential and Tafel polarization curves illustrated that additional Fe³⁺increased the corrosion current density,which was favorable to Zn/Mn extraction.Moreover,electrochemical impedance spectroscopy?EIS?had the similar feature in different solution,which was well-fitted with an equivalent circuit.The results of EIS suggested that Zn/Mn dissolution were controlled by the mass/charge transfer resistance,and Fe³⁺could reduced their effects in the present of the mixed bacteria's EPS;meanwhile,the formation of jarosite had produced corrosion layer or passivation layer and further inhibited the dissolution of Zn/Mn.Catalytic bioleaching performance of Zn/Mn from the spent Zn-Mn batteries by 4metallic ions was studied.The tested 4 metallic ions exhibited different catalytic activity towards extraction of Zn/Mn.Both Co²⁺and Ni²⁺had no enhancing effect on release of Zn and Mn,Ag⁺even adversely affected dissolution of Zn/Mn by bioleaching.Of the tested four metallic ions,only Cu²⁺improved mobilization of Zn/Mn from the spent batteries.When Cu²⁺dose increased from 0 to 0.8 g/L,the maximum dissolution efficiency elevated from 47.7%to 62.5%for Zn and from 30.9%to 62.4%for Mn,respectively.Meanwhile,the bioleaching course was then cut to 9 days.SEM and dynamic analysis showed that Cu²⁺catalysis boosted bioleaching of resistant hetaerolite through forming a possible intermediate CuMn₂O₄ which was subject to be attacked by Fe³⁺based on a cycle of Fe³⁺/Fe²⁺.Dissolution kinetics of Cu²⁺catalysis studies showed that both of Zn/Mn were best described by the chemical reaction-controlled model,and Mn also depended on the diffusion-controlled and product layer diffusion-controlled.Finally,the series of Mn-Zn ferritet soft materials were prepared by using hydrothermal synthesis method through bioleaching meadia as a precursor under controlling different Mn,Zn and Fe molar ratios,and its technological system and parameters were founded on middling experiment.Research indicated that the sample of Mn_0.6Zn_0.4Fe₂O₄ realized the optimum performance:the particle size distribution,dispersion and magnetic propertiesl,and it had good magnetic separation property,thermal stability and high acid-basic resistance.So,it is expected large-scale production for the resource utilization of spent Zn-Mn batteries in the future.