| The acidic heavy metal wastewater containing metal chloride has characteristic of enormous wastewater yield,high concentration of heavy metal and strong toxicity and pollution.Though this wastewater treated by membrane electrodeposition,which can recycle heavy metal ions and inhibit producing chlorine gas at the same time,still chloride reaction happen again due to lower chloride blocking of cation exchange membrane.The consequence is formation of secondary pollution and affecting metal recovery rate.Whether chlorine reaction depends on anode performance.In order to replace chlorine evolution reaction with oxygen evolution reaction,the excellent oxygen evolution electrocatalytic performance of anode is the key.Therefore,it is very important to seek an anode material with low oxygen overpotential,strong corrosion resistance and long service life.Ti/IrO2+MnO2 composite anode and Ti/IrO2+MnO2 with interlayer were prepared by coating-thermal decomposition.Field emission scanning electron microscope(FESEM),X-ray diffraction analyzer(XRD),energy spectrometer(EDS)were used to analysis anode surface morphology,phase structure,element type and content;At the same time oxygen evolution electro-catalytic property stability and durability were studied by means of linear scanning polarization,cyclic voltammetry,ac impedance spectroscopy and life testing of electrolytic electrochemical transient or steady state testing technology.The influence of active component matching,calcination temperature,metal oxide active loading and middle layer on physical chemical properties of anode and electrode life were systematically discussed,in order to obtain optimum technological parameters.Then anode quasi-steady Tafel curve was used to deeply study oxygen mechanism of Ti/IrO2+MnO2 in acidic solution,determine rate controlling step and establish oxygen reaction kinetics model.Finally,the experiment that heavy metal wastewater containing metal chloride treated by double-membrane three-compartment electrowinning was implemented,comparing homemade anode with traditional anode on the aspet of cell voltage,current efficiency,cobalt deposit quantity,energy consumption,chlorine suppression,quality of recovery of cobalt and corrosion resistance.Based on the above contents,the following conclusions are drawn:With the change of iridium/manganese mole ratio,the morphologys of anodes have characteristics of different quantity,width,depth of surface cracks and grains size.The elements of coating are mainly composed of Ir,Mn and O.The variation tendencies of these elements conform to the trend of nominal composition.However,the testing Ir/Mn mole ratio deviates from nominal ratio.Iridium content more than 30 mol%,there are IrO2 diffraction peaks and no MnO2 diffraction peak due to solid dissolution of IrO2 and MnO2.With the increase of iridium/manganese mole ratio,the open circuit potential,oxygen evolution current density,voltammetric charge capacity and accelerated life increase gradually and then decrease rapidly.The values of above four parameters for Ti/IrO2(0.7)+MnO2(0.3)anode are the highest,indicating that its oxygen evolution electrochemical catalysis is the best,and electrode life is the longest.With the increase of calcining temperature,the number of surface cracks of anode decreas gradually,and the grains grow continually,finally grains agglomeration and accumulation.The distribution of Ir,Mn and O elements are different with oxide coatings area.The crystallinity of complex oxide is very poor.The open circuit potential,oxygen evolution current density,accelerated life and voltammetric charge capacity increase and then decrease as a function of calcination temperature.When temperature is 400℃,the first three value are the maximum.while 350℃,the latter value is the maximum.Comprehensively consider,400℃is chose.As the oxide load increases,titanium substrate is covered with oxide coating continuously.The corrosion pits and craquelure of surface are filled and repaired by precursor solution,smooth and compact structure on the surface.The bright grains gradually precipitate out and grow larger.Excessive load,anode surface has the characteristic of defects and broken morphology.EDS show that the load has a small influence on Ir and Mn content,but the elements of different typical regions are very different.With the increase of load,the oxygen evolution current density and voltammetric charge capacity increase gradually and then decrease sharply.When oxide load is 10g/m2,the above two value reach the maximum.More than 10 g/m2,the increase amplitude of life is small.There are many factors to affect electrochemical surface structures of anode including iridium/manganese mole ratio,calcination temperature and load.The results show that the"internal" electrochemical active surface area of all anods are greater than the "external"electrochemical active surface area,but expect for anode prepared at 300℃ and 350℃because of coating morphology.The AC impedance spectrascopy shows that electrolyte resistance does not depend on the preparation parameters,and all of coating resistances are small.The oxygen evolution reaction resistance is the smallest respectively with iridium/manganese mole ratio of 7:3,temperature of 350℃,load of 10g/m2,which indicates that oxygen evolution reaction is very easy to occur.The dispersion index is also used to evaluate the roughness of surface topography.Adding antimony tin oxide into Ti/IrO2+MnO2 anode as interlayer,the results show that the middle layer may improve accelerated life of anode significantly and prolong the service life of electrode greatly without reducing oxygen evolution electrocatalytic activity,which is related to surface morphology.SEM image shows that the surface of anode with tin antimony oxide intermediate layer has a small number of cracks,smoothness and compactness,which indirectly indicates that the adhesion and bonding strength between titanium substrate and surface coating are good.EDS shows that tin antimony oxide intermediate layer does not change the elements types and proportions of original anode.The oxygen reaction processes of Ti/Ir02+MnO2 anode in acidic system are as follow:MOx + H2O→ MOx(·OH)ads*+H+ +e-(a)MOx(·OH)ads*→MOx(·OH)(b)MOx(·OH)→ MOx+1 + H+ +e-(c)MOx+1→MOx+1/2O2↑(d)In the low potential region,the formula(b)is the rate determining step;in the high potential region,the formula(a)rate determining step.In the Tafel linear region,Hydrogen ion does not directly participate in the reaction,and the average apparent activation energy of the oxygen reaction is 21.98 kJ/mol.Cobalt was recycled by electrodeposition in the double-membrane three-compartment cell with taking Ti/SnO2+Sb2O3/IrO2+MnO2、PbAg、Ti/PbO2 as anode respectively.The results show that compared with other anodes,the cell voltage and energy consumption of Ti/SnO2+Sb2O3/IrO2+MnO2 are the lowest;and the cobalt deposition mass of that is the most.The current efficiency of that is the highest(95%);the effect of inhibiting chlorine gas of that is the best(96.5%);the impurities of cobalt are the minimum,conforming to Co9998 brand.The pilot test of double-membrane three-compartment electrowinning suggests that the concentration of chlorine gas in the workshop was less than 1 mg/m3,which is consistent with the national standard of industrial hygiene in the workshop.In addition,hydrochloric acid is regenerated electrochemically in the middle compartment,which can be reused for other stages.This technology can produce considerable economic and environmental benefits. |
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