Magnesium removal in the electrolytic zinc industry

Most electrolytic zinc plants have to deal with dissolved magnesium in their process liquors, as zinc sulphide concentrates contain small amounts of magnesium. Applied magnesium bleed methods are generally expensive and environmentally unfriendly. In this paper, a new approach is suggested and discussed in which magnesium fluoride is selectively precipitated from purified zinc sulphate solutions. The magnesium fluoride is contacted with sodium hydroxide solution and thus converted into magnesium hydroxide, which could be saleable product. It is shown that the resulting sodium fluoride solution can be electrolysed in a membrane cell consisting of a zinc anode and an inert cathode. This yields zinc fluoride and sodium hydroxide solutions, which both can be recycled to the magnesium removal process. Various experiments, which are discussed in this paper, confirm that the proposed process is possible.