I. V. Tananaev Institute of Chemistry and Technology of Rare Elements and Mineral Raw Materials (ICTREMRM KSC RAS). The Kola Science Center of Russian Academy of Sciences.

O. S. Kazakova, Post-Graduate Student;
S. A. Kuznetsov, Head of Laboratory, e-mail: kuznet@chemy.kolasc.net.ru

Cobalt of high purity with low content of metallic impurities can be obtained by electrorefining in water solutions. However the concentration of interstitial impurities (H₂, N₂, O₂, C) especially of hydrogen after electrorefining of cobalt in water solution remain still significant that leads to the brittleness of metal. The goal of the present study is production of high purity cobalt not only on metallic impurities, but also on interstitial impurities. The electrorefining of cobalt in NaCl—KCl—K₂TiF₆ (20% (mas.)) melt has been investigated. It was shown that complexes of Ti (III) and Co (II) appeared in the melt due to reaction 2Ti (IV) + Co → 2Ti (III) + Co (II) and this reaction was entirely shifted to the right hand side. The limiting current density of discharge Co (II) to metal in NaCl—KCl—K₂TiF₆ (20% (mas.)) melt was determined by steady-state voltammetry. The electrorefining of cobalt was carried out in hermetic electrolyser under argon atmosphere. Initial cathodic current density was changed 0.3—0.7 A·cm^–2, the electrolysis temperature varied within 973–1073 K. Influence of cathodic current density on crystals size was studied. Behaviour of impurities during cobalt electrorefining was discussed and influence of cathodic current density and temperature on the cobalt crystal size was studied. It was shown that electrorefining led to the elimination of most of the interstitial impurities (H₂, N₂, O₂, C), with the result that the remaining impurity levels below 10 ppm impart high ductility to cobalt.

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