Preparation of ultra-high-purity copper by anion exchange
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I.
INTRODUCTION
IMPURITIES can significantly affect the main electrical and mechanical properties[1,2,3] of a metal. While purity requirements are increasing for traditional copper uses, the major interest in superpurity is generated by new applications,[4] such as a stabilizer for superconductors or as a replacement for gold in integrated circuit bonding. Moreover, the importance of determining the intrinsic physical properties of pure materials also drives the need for extreme purification. Available refining techniques,[4] mostly based on electrorefining in sulfuric or nitric acid solutions and subsequent melting under vacuum or in a floating zone, offer ultrahigh purity at the expense of complicated operations and low productivity. Therefore, new methods that offer higher efficiency and facilitate large-scale applications are in great demand. Due to the different stabilities of chlorocomplex ions and their different distributions between the solution and resin phases, anion exchange is capable of separating even chemically similar elements like Nb, Ta, and Pa,[5] and the divalent transition elements:[6] Al, Ga, In, and Tl;[7] Pd, Pt, and Ir;[7] Ti, V, and Fe;[7] Mo, W, and U;[8] and Ge and As.[9] Kraus et al. carried out these separations in HCl[6,7,9] or HCl-HF solutions.[5,8] Isshiki applied anion exchange for the purification of iron[10] and cobalt.[11] Residual resistivity ratio measurements indicated the highest reported levels of purity. A large number of positively charged aquoions, Mv+, except for the alkali, alkaline earth, rare earth metals, and the elements in the first two rows of the p-block of the periodic table, can form complex anions in HCl solutions. These species can be proper targets for absorption in strongly basic anion-exchange resins. There are a number of advantages in separating complex anions:
(1) noncomplexing elements are directly eliminated; (2) separation of normally similar elements is possible; (3) exchange conditions can be controlled through the concentration of the complexing agent; and (4) short columns and fast operations are possible.[12] Purification of copper by anion exchange is feasible in HCl solutions[13] since copper is capable of forming negatively charged complex species with chloride ions,[12,14] which is not so in other common acids.[15,16] Traces of HCl remaining in the extracted metal are readily eliminated by subsequent melting and annealing.[17]
II.
PRINCIPLES OF SEPARATION
A. Anion-Exchange Equilibrium Elements can exhibit strikingly different ion-exchange characteristics, indicated by the equilibrium distribution between the HCl solution and the strongly basic anion-exchange resin[12,13,14] phases. There are extreme cases of virtually negligible or very high values of the ion-exchange distribution coefficient over the entire HCl concentration range. Figure 1 summarizes the ion-exchange distribution functions—showing the dependence of the distribution coefficient on the concentration of the complexing agent— which were obtained for copper and some of
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