During the brine evaporation it has been determined that 50% of the initial natural brine lithium remains in the residual brine [1]. This phenomenon has been attributed to the retention of lithium by the precipitated salts [1,2]. The residual brine is highly concentrated with magnesium. Therefore the lithium recovery from this kind of brine becomes more difficult. Indeed the magnesium precipitation as an oxalate or a carbonate decreases the brine magnesium concentration and makes easier the lithium extraction. The calcinations of the oxalate and the carbonate, respectively at 500degreesC and 900degreesC produces the magnesia (MgO) which is used as a refractory product. The present study shows that the magnesium rate, in synthetic brines, has been influenced by the following factors: a) the reagents ratio as a function of magnesium concentration; b) the reaction duration; and c) the reaction temperature. By this work we have proved that the lithium retention depends on the formed allotropic variety (cc or) of the magnesium oxalate. When magnesium had been precipitated as a carbonate, all the lithium remained in the solution when the molar ratio CO3/Mg was superior to 0.91.