Synthesis And Application Of The New Copper Electrorefining Additive Gelatin-acrylamide Copolymer

ABSTRACT:Adjusting additives is one of the most important ways to obtain high quality cathode copper during copper electrorefining. Thiourea is one of the most common additives, but thiourea is easy decomposition during the electrorefining, which results in elevated levels of sulfur in cathode copper. So, it is necessary to develop a new kind of additive to replace thiourea. In this study, we first extracted the gelatin from the chrome-containing leather waste, and then the gelatin-acrylamide copolymer was synthesized. At last the copolymer was applied in copper electrorefining, and some research on its mechanism of action was conducted.CaO hydrolysis method was used to extract the gelatin from the chrome-containing leather waste, the results show that the yield of gelatin first increased and then decreased with the dosages of CaO increase and the yield of gelatin increased with the reaction temperature increase; the liquid-solid ratio and reaction time do not have much effect on the yield of gelatin; besides the relative viscosity of the gelatin solution (6.67%, wt%) decreased with the dosages of CaO, reaction temperature, liquid-solid ratio and reaction time increase respectively. The optimum conditions for extracting gelatin are that the dosages of CaO is7%of the leather quality, reaction temperature is75℃, liquid-solid ratio is5:1and reaction time is2h. The yield of gelatin is49.5%, the relative viscosity of the gelatin solution is1.95, and the content of Cr in gelatin is2.07ppm under the optimum conditions.The results show that the PGAM can be prepared by free radical polymerization method using potassium persulfate as initiator and deionized water as solvent, besides the relative viscosity of PGAM solution (0.003%, wt%) first increased and then decreased with the reaction temperature and the initiator concentration increase respectively; and the relative viscosity of PGAM solution increased with the reaction time and the acrylamide concentration increase respectively. The optimum conditions are set as that the reaction temperature is55℃, the reaction time is1h and25minutes, the initiator concentration is0.0012 g/mL and acrylamide concentration is0.06g/mL, meanwhile the gelatin concentration is0.04g/mL. The relative viscosity of PGAM solution is1.7at the optimum conditions.The electrolytic experimental results show that the PGAM is an appropriate substitute for thiourea in copper electrorefining, and has significant flocculation effect on floating slimes to make them precipitated into anode slime. When the PGAM was used as an additive combined with gelatin, bone glue and chloride ion, high-pure cathode copper can be obtained after electrolysis for168h with a current density of235A/m2at65℃. Impurities of As, Sb and Bi in the cathode copper are0.000051%,0.000124%and0.000069%, respectively, which are much lower than that obtained by using the traditional additives thiourea, gelatin, bone glue and chloride ion. In addition, the contents of As, Sb and Bi in the anode slimes were5.12%,4.04%and1.01%respectively, while that in the anode slime obtained by using the traditional additives were3.25%,2.20%and1.95%respectively.The electrochemical studies show that the electrodepositing of copper is an irreversible and simple charge transfer reaction excluding pre-conversion process, and the exchange current density is2.69×10-2A/cm2. The results show that the PGAM has polarization effect on copper deposition, but increases the peak current density, besides it has a little influence on the exchange current density. When the PGAM was used with the gelatin and bone glue the polarization effect becomes weak and peak current density decreases. When the concentration of thiourea is low, thiourea shows a depolarization effect on copper deposition, and increases the peak current density. But when the concentration of thiourea increased to a certain concentration it shows a polarization effect, which make the reduction peak potential shifted negatively and decreases the peak current density. Besides the exchange current density increase when thiourea was used.