Palladium Separation From AgNO₃/Pd（NO₃）₂Mixed Solution By AO-PAN Resin

During the silver electrolytic refining process, various impurities in anode plate can partly enter the electrolyte and the platinum group metals also gradually enrich in the electrolyte, thus affecting the quality of electrolytic silver. In order to make the deep purification of electrolyte to improve the quality of electrolytic silver and recovery the platinum metals, it’s important to separate the palladium effectively from the silver electrolyte. The traditional methods of palladium separation from silver electrolyte mainly includes Roasting-decomposition method, Precipita-tion method, Adsorption method and Solvent Extraction method, all of which can achieve the goal of separating palladium from silver electrolyte, but accompanying with some disadvantages of large consume of chemical reagents, incompletely separation and unfriendly environment. The Ion Exchange Adsorption method has a good separation performance, but all the resins under research have low adsorption capacity and poor selectivity. In view of this difficult situation, the study of new resin preparation and palladium separation by resin adsorption were proposed in this paper. The main research content and conclusion are as follows:Firstly, the Amidoxime Polyacrylonitrile chelating resin(AO-PAN) was prepared by chemical modification of the commercial polyacrylo-nitrile(PAN) resin with hydroxylamine hydrochloride. The factors which may affect the conversion ratio of cyanide group during the chemical modification were investigated and the correspondingly optimum condition was obtained as follows:The commercial polyacrylonitrile resin with the cross-linking degree of10%was firstly immersed with ethanol for24h, and then mixed uniformly with175Sg·L-1, pH=7.0hydroxylamine hydrochloride solution under the ratio of liquid to solid (ml·g-1)of80. This mixture was placed in80℃for chemical reaction,8hours later the AO-PAN chelating resin was prepared with the cyanide conversion ratio up to87%. Infrared spectroscopy was used to analyze the structure of PAN resin and AO-PAN chelating resin, and the result showed the essence of this chemical modification lies in that hydroxylamine molecular convert the cyanide group into tertiary amine group and hydroximino group.Secondly, the palladium adsorption property by the AO-PAN chelating resin was investigated with the optimum adsorption condition listing as follows:Pd(NO3)2feed solution with [HNO3]≈0.75mol·L-1, was mixed with AO-PAN resin under60℃for90min, the palladium adsorption capacity can reach more than80mg·g-1with a brown-yellow flocculent precipitation of metal palladium generated. Moreover, the adsorption process belonged to chemical reaction, Pd2+in solution can replace the position of-OH and complex with the N atom in amidoxime group of AO-PAN resin, and part of the loaded Pd2+was reduced as palladium metal while the corresponding amidoxime group was oxidized into nitro-group. The process of Palladium adsorption by AO-PAN resin was endothermic with the△H of17.62KJ·mol-1; it was a second-order reaction with the controlled-step of particle diffusion; and it complied with the Langmuir isotherm equation with the theoretical maximum adsorption capacity Qmax of87.42mg·g-1. Palladium loaded on resin, can be effectively desorbed by20g·L-1thiourea+1.0mol·L-1HNO3solution with desorption ratio of>95%.Then the silver adsorbed by AO-PAN resin under the optimum condition for palladium adsorption was studied. The result showed that the silver adsorption by AO-PAN resin was a physical process with the capacity of223.89mg-g-1, and the silver loaded on resin can be effectively eluted by20g·L-1thiourea+1.0mol·L-1HNO3solution or12mol·L-1HCl solution.Thirdly, this AO-PAN chelating resin was used to separate palladium from silver-palladium nitrate mixed solution. The experimental results showed that when AO-PAN resin was mixed with the mixed nitrate feed solution of [HNO3]≈0.8mol·L-1,[Pd]=500mg·L-1and [Ag]=50g·L-1，under the condition of60℃and120min for adsorption, nearly80%Pd and10%Ag can be adsorbed on the resin. In the raffinate,[Pd]≈75mg·L-1and [Ag]≈48.5g·L-1.Palladium and silver on the mixed-loaded resin can be separated again by two-step desorption method. In the first step,20g-L"1thiourea+1.0mol·L-1HNO3solution is used to elute most of palladium, under the condition of30℃and150min, around65%Pd and7%Ag can be eluted into the first desorption solution, in which the silver content is two times of the palladium content. In the second step,12mol·L-1HCl solution is employed to elute residual silver. When repeating this process twice, about25%Ag and8%residual Pd can be eluted into the second desorption solution with [Pd]≈15mg·L-1and [Ag]≈1.5g-L-Finally, palladium in the first desorption solution can be recovered efficiently by chemical reduction method. The adsorption solution was firstly heated to80℃for1h to decrease acidity and desulfurize, after filtration to remove elementary sulfur, the pH of this desorption solution was adjusted by ammonia aqueous to around6.0, then it was mixed with80%hydrazine hydrate according to the volumn ratio Vdesoption solution:VN2H4of2.5:1, under the condition of75℃and60min for reduction reaction, more than96%Pd and Ag can be reduced into metal. In the black alloy powder residue, the silver content is about2times of palladium content.The whole study indicates that it’s feasible to use AO-PAN resin prepared by chemical modification of polyacrylonitrile resin to separate palladium from silver electrolyte, which can achieve the goal of separating palladium and making deep purification of the electrolyte through the clean metallurgy method.