Due to the occurrence of zinc and lead ore deposits in dolomite rocks, the sphalerite concentrates obtained from these ores contain an admixture of dolomite. In practice, a substantial amount of magnesium included in zinc ores passes to the last production stage, i.e. zinc electrolysis. The magnesium present in electrolyte impairs electrical conductance and appears in the technical and economical indexes. This paper deals the attempts to remove magnesium removal from initial sphalerite concentrates by means of chemical flotation using spent electrolyte derived from zinc electrolysis. The authors attempt to substantiate the existing relationships, as well as to derermine the optimum conditions for the procedure suggested. The leaching efficiency of magnesium amounted to about 80%, and is dependent upon the stage of the leaching. Losses of zinc were below 2%, and the magnesium concentration in solution amounted to about 20%. These solution can produce magnesium and zinc, which will be presented in the following paper.