Improved Lime Method to Prepare High-Purity Magnesium Hydroxide and Light Magnesia from Bischofite

Abstract

Precipitation of magnesium hydroxide [Mg(OH)2] from bischofite (MgCl2·6H2O) using lime milk, i.e., the traditional lime milk method, features a low cost. However, insoluble impurities such as CaO in the lime limit the purity of the magnesium hydroxide product. For this reason, a new method has been developed, using saturated lime supernatant instead of lime milk. The insoluble impurities in lime cannot dissolve in saturated lime supernatant, thus improving the purity of the produced magnesium hydroxide. This improved lime method, which achieves high Mg(OH)2 purity of 99% with CaO content of 0.094%, offers better performance in comparison with the traditional lime milk method. Besides, the optimum processing parameters including pH, crystal seed amount, agitation speed, reaction temperature, MgCl2 concentration, reaction time, aging time, and polyacrylamide (PAM) addition are reported. High-purity light magnesia (MgO) is obtained from magnesium hydroxide by carbonation and calcination, with quality fulfilling the HG1-324-77 standard. This improved lime method represents a promising approach for production of high-purity magnesium hydroxide and light magnesia from bischofite.