Magnesium removal in the electrolytic zinc industry

Abstract

Electrolytic zinc plants need to take measures to control the magnesium content in their process liquors, because the natural magnesium bleed does not balance the input from concentrates. Presently used methods are environmentally unfriendly (due to the production of large amounts of waste gypsum) or expensive. Therefore, an alternative process route is explored in which magnesium is removed from zinc electrolyte by selective precipitation of magnesium fluoride (sellaite). As standard applications for bulk amounts of magnesium fluoride could not be identified, a method was developed to convert the magnesium fluoride into a useful product. Magnesium fluoride can be converted into magnesium hydroxide (brucite) by contacting it with sodium hydroxide. Magnesium hydroxide, if pure enough, has a wide range of (industrial) applications. With a modified electrodialysis reactor, the sodium fluoride solution obtained from the conversion process can be converted into reagents, i.e. zinc fluoride and sodium hydroxide, which are required for previous process steps. After magnesium precipitation, it is necessary to remove residual fluoride from the zinc sulphate solution. In addition, the magnesium hydroxide appeared to contain some fluoride, which could limit the application possibilities of magnesium hydroxide. However, it proved to be possible to calcine the fluoride-containing magnesium hydroxide at high temperature (> 1000 d.C.) to sufficiently pure periclase (magnesium oxide). A rough economic evaluation indicates that the process may compete with the conventional process if international dumping fees for the dumping of gypsum become similar to the Dutch dumping fee for gypsum disposal in C2-deponies.