Development And Application Of Reductive Sulfonates As Electrolyte Additives For Tin Electrorefining Process

The large-scale exploitation and utilization of tin minerals leads to decreasing grade of Chinese tin ores and increasing supply pressure of tin minerals.Recovering metallic tin from secondary resources containing tin is an effective strategy to relieve the supply pressure of tin minerals.Due to the wide sources of tin-containing secondary resources and their complex composition,the impurities in crude tin obtained from tin-containing secondary resources is complicated and their contents fluctuates frequently.Consequently,traditional fire refining for above mentioned crude tin has the disadvantages of long process,high energy consumption,and high cost.Therefore,electrorefining process is generally adopted to remove impurities and purify the crude tin.During the tin electrorefining process,Sn（Ⅱ）tends to be oxidized and further hydrolyzed into tin glue,which would result in quick depletion of Sn（Ⅱ）,low purity of cathodic tin,and tin dendrites growth.For this reason,phenol sulfonic acid(10～20 g·L^-1)is usually added to the electrolyte.However,phenol sulfonic acid（PSA）is toxic and harmful,which would seriously deteriorate the labor environment.Hence,it is urgent to develop a green additives to replace PSA.The present work aims to reveal the structure-performance relationship of traditional tin electrorefining additives,design the characteristic structure of target additives for tin electrorefining,and then guide the selection of green additives from commercial chemical products.Compared with the traditional screening additive development mode,it has higher efficiency,which is the innovation of this paper.The main conclusions obtained in this work are listed as follows:（1）The effects of methylsulfonic acid（MSA）and ethylsulfonic acid（ESA）on Sn（Ⅱ）stability,Sn（Ⅱ）deposition behavior,cathodic tin morphology and current efficiency during tin electrorefining process were compared and analyzed,and the influence of carbon chain length on the function of alkyl sulfonic acid was analyzed.The results show that MSA can improve the stability of Sn（Ⅱ）,retard the deposition of Sn（Ⅱ）cathode,increase the polarization of Sn（Ⅱ）deposition,and improve the density and flatness of cathodic tin.However,the presence of MSA would reduce the current efficiency.Increasing carbon chain length of alkyl sulfonic acid has no obvious effect on the deposition kinetics of Sn（Ⅱ）.However,longer carbon chain aggravates hydrogen evolution at the cathode,reduces the density and flatness of the cathode tin,and decreases cathodic current efficiency.Therefore,the ideal alkyl sulfonic acid additive can not be obtained by simply optimizing the carbon chain length.（2）The effects of phenol sulfonic acid（PSA）and toluene sulfonic acid（TSA）on the tin electrorefining process were comparatively studied,and the influence of the phenolic hydroxyl group and sulfonic acid group on the tin electrorefining process was inferred.The results show that PSA can significantly increase the polarization of Sn（Ⅱ）deposition,and improve the flatness and density of cathodic tin.TSA also has the leveling function of the cathodic tin,but the effect is not outstanding.The phenolic hydroxyl group of PSA is reductive,which can consume the dissolved oxygen in the electrolyte,thereby improving the Sn（Ⅱ）stability.The sulfonic acid group can complex the Sn（Ⅱ）in the electrolyte during the electrorefining process,thereby improving the Sn（Ⅱ）stability.（3）In order to verify the effect of phenolic hydroxyl group,the effects of PSA and gallic acid（GA）on the tin electrorefining process were comparatively studied.The results show that GA has strong reducibility and can improve the stability and current efficiency of Sn（Ⅱ）.Although GA promotes the rapid deposition of Sn（Ⅱ）,it still has a certain leveling effect on the cathodic tin.However,the effect is far worse than that of PSA.Therefore,the phenolic hydroxyl group alone has a reducing effect and a certain leveling effect.The excellent leveling effect of PSA is the result of the synergistic effect of the phenolic hydroxyl group and the sulfonic acid group.（4）Based on the structure-performance relationship of PSA,the target additive should have both phenolic hydroxyl group and sulfonic acid group.However,most of the phenolic substances are harmful to the human body and the environment.Therefore,this work chooses taurine,which has both a sulfonic acid group and a reducing property,as a green additive for tin electrorefining.The effects of taurine and PSA on the tin electrorefining process were comparatively studied.The results show that taurine can inhibit the hydrolysis of Sn（Ⅱ）,significantly improve the stability of the electrolyte,and the effect is better than that of PSA.However,taurine has no obvious effect on the deposition kinetics of Sn（Ⅱ）,the leveling effect of cathode tin is not obvious.