THE REMOVAL OF NICKEL FROM COPPER ELECTROREFINING BLEED-OFF ELECTROLYTE

When high nickel-containing copper concentrates from South Africa star ted to comprise part of the feed to a Zambian copper smelter at Mufuli ra, 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 el ectrolyte. 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 o ut on a laboratory scale using contaminated electrolyte collected fr o m the refinery. In the first of two methods considered, it was establi shed that aqueous ammonia is able to precipitate nickel from copper el ectrorefining 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 o f acid retained in the electrolyte. The other method considered, invol ved partial electrolyte evaporation with a view of crystallizing nicke l sulphate from samples of bleed-off electrolyte issuing from a libera tor circuit. It has been demonstrated that evaporative crystallisation of nickel sulphate could be a very effective means of controlling nic kel in the Mufulira tankhouse. At 66.5% and 80% electrolyte evaporatio n, 81% and 100% of the nickel was crystallized from foul electrolyte, respectively. Over 95% of the sulphuric acid in initial samples was re tained in the purified electrolyte at a concentration of over 1000 gn. Initial estimates have indicated that the cost of evaporative crystal lisation of nickel sulphate would be quite low compared to the value o f sulphuric acid that would be present in the purified electrolyte. (C ) 1997 Elsevier Science Ltd. All rights reserved.