Conversion of magnesium fluoride to magnesium hydroxide

The conversion of magnesium fluoride to magnesium hydroxide was studied as part of a process to bleed magnesium from zinc sulphate electrolyte. Most electrolytic zinc plants have to deal with accumulating magnesium in their process liquors as the natural magnesium bleed usually does not balance input from concentrates. The most common forced magnesium bleed produces large amounts of waste gypsum and could potentially be environmentally unfriendly. An alternative process was suggested and discussed, which involves magnesium fluoride precipitation from zinc electrolyte. The magnesium fluoride is converted to magnesium hydroxide in order to produce a saleable product. Regeneration of chemicals from the resulting sodium fluoride solution can be accomplished by means of an electro-dialysis process. Previous experiments indicated a relatively high residual fluoride content in the conversion product, which might limit the application possibilities of the magnesium hydroxide. From the experiments described in this paper it followed that it is not possible to reduce the residual fluoride content of the magnesium hydroxide product to <1 wt.% by optimising operating conditions such as leach concentration, reaction temperature, reaction time and wash procedures. From X-ray diffraction analysis, it was deduced that the residual fluoride was incorporated into the brucite crystal structure. Therefore, the sharp conversion of magnesium fluoride to magnesium hydroxide, derived from thermodynamic analysis, is in practice not entirely valid. It is possible to reduce the fluoride content of the product by burning the (fluoride containing) magnesium hydroxide to magnesium oxide (with <1 wt.% F) at temperatures of >1273 K. Thus a saleable product could be obtained.