Mechanistic Study On The Effect Of Several Impurity Ions On The Zinc Electrowinning Process

Zinc electrodeposition process is the most important process in hydrometallurgical zinc smelting,accounting for about 80%of the total energy consumption in zinc production.In recent years,China has vigorously advocated energy saving and consumption reduction,and makes claims for improving the electrodeposition process to reduce energy consumption.With the depletion of high-grade zinc ore reserves,a variety of impurity ions and additives enter the zinc sulfate leaching solution,such as tannic acid,fluoride ions,chloride ions and so on which are brought into the leaching process,have a negative impact to the energy saving and consumption reduction of the zinc electrodeposition process and zinc quality.Therefore,this thesis researches the effect of concentration of tannic acid(40-120 mg/L),chlorine ion(0-1000 mg/L)and fluorine ion(0-400 mg/L)to the dynamics of zinc electric product cathode and zinc product quality through electrochemical analysis,electric integrated experiment and test validation,the cathode zinc corrosion experiment,the law of zinc nucleation and so on.Furthermore,the effect law and degradation of additive gelatin(0-10 mg/L)in the electrodeposition solution system are further studied,and the removal mechanism of tannic acid in the leach solution by using Poly Aluminum Chloride(PAC)as coagulant is preliminarily explored.The main conclusions obtained from the above research are as follows:(1)With the increase of gelatin concentration,the NOP value gradually increases,and the polarization level of the cathode increases,which hinders the nucleation site of the electrodeposition interface;At the concentration of 0 mg/L gelatin,the overpotential of hydrogen evolution at 500 A/m~2is 0.923 V,and at the concentration of 10 mg/L at 500 A/m~2is 1.104 V,which inhibited the precipitation of hydrogen;After adding gelatin,the number of pores on the surface of cathode zinc decreased,resulting in fine grains,smooth and compact crystals,and better metallic luster.However,excessive gelatin also made the electrodeposited zinc brittle,hard,striped cracks,and difficult to peel off.Gelatin degrades during zinc electrodeposition,and part of gelatin degrades into small amino acid molecules and loses its cathodic leveling.(2)When tannin concentration is 0 mg/L,NOP value is 111m V,and increases gradually with the increase of tannin concentration.When tannin concentration is 120mg/L,NOP value is 172 m V,and the mass transfer resistance on the electrode surface increases from 300?/cm~2to 470?/cm~2,which is not conducive to the precipitation of zinc.At 500 A/m~2,the cathode current efficiency and energy consumption are91.97%and 2669.43(k W)/t·Zn at 0 mg/L,and 82.12%and 3120.43(k W)/t·Zn at120 mg/L.The quality of cathode zinc becomes worse,the surface of zinc sheet loses metallic luster,becomes thin and fragile,and is difficult to peel off,and the grains pile up irregularly.(3)The conditions for the removal of tannic acid by Poly Aluminum Chloride coagulation are as follows:When the dosage of PAC is 100 mg/L,p H value of the solution is 4.5,temperature is 35?,stirring time is 10 min and stirring intensity is100 r/min,the removal rate of tannic acid is above 90%,and the retention rate of zinc is above 93%;The mechanism of removal of tannic acid by Poly Aluminum Chloride is the joint action of adsorption neutralization mechanism and bridging mechanism.After the removal of tannic acid,the nucleation overpotential of the electrolyte to the cathode decreases,and the polarization degree weakens.(4)The higher the concentration of chloride ions,the more obvious the corrosion level on the surface of aluminum cathode,the more active sites,the lower the mass transfer resistance,and the value of nucleation overpotential gradually decreased with the increase of chloride ion concentration.When not adding chlorine ion,NOP value is 105 m V,hydrogen evolution overpotential is 1.047 V at 500 A/m~2,cathode current efficiency is 91.97%,the energy consumption is 2669.43(k W)/t·Zn.When chloride ion concentration is 1000 mg/L,the NOP value is 83 m V,hydrogen evolution overpotential is 1.008 V,current efficiency decreases to 85.22%,the energy consumption increases to 2872.21(k W)/t·Zn,surface porosity of the obtained zinc product increases and uneven surface appears,the back of zinc plate has the“burning board”phenomenon.(5)When fluoride ion concentration is 400 mg/L,the surface of the electrode is seriously corroded,the corrosion pit is honeycombed,and the NOP value decreases from the original value 105 m V to 62 m V.The hydrogen evolution potential decreased from 1.047 V to 0.926 V,which promotes the precipitation of hydrogen.The energy consumption at 500 A/m~2increases by 36.75(k W)/t·Zn,the pores on the surface of zinc deposition increases,the crystal grains are deposited irregularities instead of hexagonal layers,and the surface of zinc grains is corroded by hydrofluoric acid in electrolyte.