The removal of nickel from copper electrorefining bleed-off electrolyte

When high nickel-containing copper concentrates from South Africa started to comprise part of the feed to a Zambian copper smelter at Mufulira, the level of Ni in the copper anodes produced increased. The high nickel, copper anodes started to pose a problem at the electrorefining stage as they led to a progressive increase in the Ni tenor of the electrolyte. In order to produce high quality copper cathodes with less than 1 ppm Ni, it became necessary to bleed-off large volumes of foul electrolyte contaminated with nickel. The study reported in this paper was part of the effort aimed at devising a less costly method for the removal of nickel in the electrolyte. The investigation was carried out on a laboratory scale using contaminated electrolyte collected fr om the refinery. In the first of two methods considered, it was established that aqueous ammonia is able to precipitate nickel from copper electrorefining bleed-off electrolyte, forming precipitates with 10-13% Ni. A major drawback of this method, however, was found to be the cost of ammonia solution used for Ni precipitation relative to the value of acid retained in the electrolyte. The other method considered, involved partial electrolyte evaporation with a view of crystallizing nickel sulphate from samples of bleed-off electrolyte issuing from a liberator circuit. It has been demonstrated that evaporative crystallisation of nickel sulphate could be a very effective means of controlling nickel in the Mufulira tankhouse. At 66.5% and 80% electrolyte evaporation, 81% and 100% of the nickel was crystallized from foul electrolyte, respectively. Over 95% of the sulphuric acid in initial samples was retained in the purified electrolyte at a concentration of over 1000 gn. Initial estimates have indicated that the cost of evaporative crystallisation of nickel sulphate would be quite low compared to the value of sulphuric acid that would be present in the purified electrolyte. (C) 1997 Elsevier Science Ltd. All rights reserved.