The Role Of Arsenic In Copper Electrolyte Purification Mechanism And Conversion Of Arsenic Valence State

During copper electrorefining, when arsenic with suitable amount and valence is addedin the copper electrolyte, a part of the impurities of As, Sb and Bi can spontaneouslyprecipitate from the electrolyte to the anode slime, so as to achieve the electrolytepurification effect. But there is still a dispute about the self-purification mechanism ofthe copper electrolyte. So the purpose of this paper is to solve two key problems forcopper electrolyte purification by arsenic, the role of arsenic in copper electrolytepurification mechanism and conversion of arsenic valence state. This research wouldbe a theoretic foundation of the industrial application in self-purification of the copperelectrolyte.The electrolytes used in the experiments contained the basic components: As, Sb,Bi, Cu²⁺and H₂SO₄. The purification experiments were carried out by heating thesynthetic electrolytes to65°C under stirring for2h, the removal rates of impuritieswere calculated according to the content of impurities prior to and after thepurification of electrolytes. The influence of As（III）, As（V）, total arsenic concentrationand nAs（III）/nAs（V）on the removal rate of Sb and Bi was investigated by adding arsenicwith different valence and concentration in synthetic electrolytes. The structure ofsediment which was produced under different conditions was characterized by usingXRD, SEM, TEM, IR and other test methods in order to reveal the role of arsenic inthe mechanism of As, Sb and Bi impurities removal from copper electrolyte. Besides,the As in acid solution under still standing and stirring was determined and traced toreveal the As valence state translation way. Meanwhile, the redox rule of As in acidicsolution was studied by means of electrochemical test. Finally, the way of arsenicvalence transformation was proposed according to the electrochemical theory.The results show that the content, valence state and ratio of arsenic play animportant role in removing impurities of arsenic, antimony, bismuth in the electrolyte.As（III） has more significantly effect on removal of antimony, bismuth impurities from electrolytes than As（V）. When total arsenic concentration and nAs（III）/nAs（V）in thecopper electrolyte are fixed as4-10g/L and1:1, respectively, the impurity removaleffect will be the best.In the copper electrolyte, As（III） can react with Sb（III, V） and Bi to generatecrystalline （Sb, As）₂O₃，BiSbO₄and AsSbO₄and an amorphous phases containing As,Sb, Bi and O, which is composed of many fine spherical particles. Moreover,antimonate is an important product from copper electrolyte purification, and also thegeneration of antimonate largely explain why As（III） and Sb（V） have more obviouseffects on the removal of As, Sb and Bi from electrolytes. The crystals mixture ofantimonate （AsSbO₄, BiSbO₄and Sb₂O₄）, arsenate （SbAsO₄and BiAsO₄） and （Sb,As）₂O₃are identified in the precipitate, and the crystal phases vary with difference ofAs（III）/As（V） molar ratio. The impurities are effectively removed from copperelectrolytes owing to these precipitates.In copper electrolyte, there are two main influencing factors of the oxidation ofAs（III）. On the one hand, because the air （O₂） can effectively dissolve in copperelectrolytes and then oxidize arsenic from trivalent to pentavalent. On the other hand,it is possible for Sb（V） to oxidize arsenic from As（III） to As（V） because the differenceof the standard electrode potentials. Arsenic valence state can be regulated by addingthe right amount of SO₂into the electrolyte, the coprecipitation reactions amongarsenic, antimony and bismuth can be promoted by regulating nAs（III）/nAs（V）at1:1.Electrochemical testing results show there is no obvious difference before andafter adding the As（III） in the anodic process. But, the cathode process appears a newreduction peak at-0.1V to-0.2V, the peak potential happens to negative shift with theaddition amount of As（III） increasing. After adding the As（V）, oxidation peaks appearnear the0.02V in the anodic process, the oxidation peak current gradually increaseswith the addition amount of As（V） increasing. The reduction peaks appear near the0.18V in the cathode process, with the concentration of As（V） in the bottom ofelectrolyte increasing, reduction peak potential happens to negative shift in the cathodeprocess and the peak current increases and then decreases.