Anion-exchange separation in hydrochloric acid solutions for the ultrahigh purification of cobalt
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INTRODUCTION
PURIFICATION of cobalt to the high or ultrahigh grades is conventionally required by such applications as superalloys for aircraft turbines and other specialty materials used for high coercive-force magnets, cutting tools, or surgical implants, to give a few examples. Its use as component in (Fe-Tb-Co-Cr) thin films, applied on magneto-optical discs, is rapidly increasing with the advent of high-fidelity recording technology. Ultrahigh-purity cobalt is also promising as a raw material for the cobalt silicide electrodes in ultralarge-scale integration electronics. Besides uses in the metallic or compound forms, investigation of intrinsic properties may also motivate the research of cobalt purification. Purification of cobalt by the conventional fine refining methods, applying differences in physical properties at high temperatures (e.g., melting in vacuum and zone refining), may not lead to the desired level of purity. This is demonstrated by the relatively low resulting residual resistivity ratios, which hardly exceed the value of 200.[1,2] Iron and nickel, heading the list of the most important impurities in cobalt (Fe, Ni, Cu, Zn, and Pb) are not removed efficiently by zone melting.[2] Standard aqueous chemical methods based on the differences in the solubility and stability of compounds in solutions can be combined into a feasible hydrometallurgical processing route, as shown in Figure 1. Elimination of impurities from the cobalt solution by precipitation in the sulfide or hydroxide forms, however, consists of many steps, TAMAS KEKESI, Associate Professor, is with the Department of Nonferrous Metallurgy, University of Miskolc, 3515 Miskolc, Hungary. MASAHITO UCHIKOSHI, Research Engineer, is with the Fine Materials Corporation, Tagajo, 985-0843 Japan. KOUJI MIMURA, Associate Professor, and MINORU ISSHIKI, Professor, are with the Institute for Advanced Materials Processing, Tohoku University, Sendai 980-8577, Japan. Manuscript submitted December 12, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
implying the addition of various reagents and requiring elaborate solid-liquid separations. Complicated purification processes are easily plagued by multiple sources of contamination and cumbersome operation. In the case of cobalt and a number of other transition metals, a practical alternative to conventional separations is offered by the relatively novel method of anion exchange.[3] The preferable medium is hydrochloric acid, as it straightforwardly eliminates any remaining traces of chlorine by the melting step, which is required to convert the extracted metal into a bulk form. Hydrochloric acid (chloride ion) is an ideal complexing agent capable of converting many hydrated ions (Mev⫹) into chloro-complex species: [MeCln]v⫺n
[1]
The anionic species are proper targets for sorption by strongly basic anion-exchange resins. The tendency toward stability[4,5,6] and anion-exchange sorption[3,7,8] of the chlorocomplex species implies great diversity for the elements, allowing highly selective separations in i
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