Study On Lead Electrolytic Refining Process In Methanesulfonic Acid System

Electrolytic refining is one of the important processes in lead smelting.The traditional lead electrolytic refining process represented by chloride,fluorosilicate and fluoroborate systems still does great harm to the environment,equipment and operators because of the high toxicity and erosion of chlorine produced by chloride and the hydrolysates of fluorosilicic acid and fluoroboric acid.Methanesulfonic acid is a kind of electrolyte with high solubility of lead ions,high stability,low saturated vapor pressure,environmental protection and no pollution,low corrosiveness of equipment and high electrical conductivity.this study explored the electrolytic refining of lead with methanesulfonic acid system.A rapid method for measuring the surface roughness of cathode refined lead was established based on image recognition technology.the effects of lead ion concentration,free acid concentration,current density,temperature and additives on cell voltage,energy consumption,current efficiency and surface roughness of refined lead in lead electrolytic refining process were studied.The electrochemical mechanism of the effect of different process parameters on lead electrolytic refining was studied by means of cyclic voltammetry,polarization curve and galvanostatic potential test,and the cathodic and anodic polarization curves were fitted and analyzed respectively.the kinetic parameters of electrochemical reaction under different electrolysis conditions were obtained.The surface roughness of cathodic deposited lead was quantitatively analyzed by two methods of cross-section roughness and surface roughness.The results show that although the section profile method can quickly calculate the section roughness,due to the edge effect of dendrite growth,the edge dendrite will block the morphology of the main area of the plate,so the measured roughness is much larger than the actual roughness.Based on the established gray and height mapping,the surface roughness algorithm converts digital photos into grayscale images with three-dimensional data,thus calculating the surface roughness and ensuring that the sampling area is consistent with the overall roughness.The reliability of the algorithm is verified by testing the surface roughness of refined lead deposited under different additive conditions.The process optimization experiments show that the current efficiency can be improved by increasing lead ion concentration,current density and electrolysis temperature to a certain extent.In a certain range,increasing the concentration of free acid in the electrolyte,increasing the temperature of the electrolyte and adding phosphoric acid are beneficial to reduce the cell voltage and energy consumption.The use of 2.0 g/L calcium lignosulfonate as an additive can greatly improve the surface morphology of the cathode,make it smooth and compact,and increase the current efficiency and cell voltage at the same time.The optimized process conditions of lead electrolytic refining in methanesulfonic acid system are as follows:lead ion concentration 0.5 mol/L,methanesulfonic acid concentration 1.0 mol/L,current density 220 A/m2,temperature 40℃ and electrode distance 40 mm.Under this condition,refined lead with a mass fraction of more than 99.99%was obtained,which met the national lead standard No.2,and the average cell voltage was 0.1633 V,current efficiency was 99.72%,and energy consumption was 42.36 kWh/t.The electrochemical results show that the lead electrolytic refining reaction in this system is a simple process of anodic lead oxidation dissolution and cathodic lead reduction deposition.Under the condition of electrolytic refining,the bipolar reaction is in the weak polarization region,which is controlled by electrochemistry and mass transfer.The exchange current density and conversion coefficient of electrolytic refining reaction were calculated by using the extended Bulter-Volmer equation,the effects of reaction conditions on mass transfer and electrochemical reaction of cathode and anode reaction were explored,and the influence mechanism of process parameters on the results of lead electrolytic refining was clarified.