Effect of calcination temperature for magnesite on interaction of MgO-rich phases with boric acid

Magnesia (MgO), which can be obtained by calcination of natural magnesite, is one of the most effective known sorbents for borate in aqueous solutions. Here we examine the effect of calcination temperature on sorption of borate for MgO-rich phases produced by calcination of magnesite at 873–1373 K. Calcination at or above 1273 K produced a single magnesium oxide phase, whereas basic magnesium carbonates (mMgCO3·nMg(OH)2·xH2O) formed in association with magnesium oxide at or below 1073 K. Calcination temperature directly affected the efficiency of decarbonation of magnesium carbonate, and the solubility and basicity of magnesium oxide in the resultant calcined products. These factors (along with the boron concentration) essentially control the immobilization mechanism of borate on the calcined MgO-rich phases. After sorption of borate on products calcined at lower temperatures, different types of basic magnesium carbonates were formed that are less effective at immobilizing borate. At low borate concentrations, under saturation for magnesium borate hydrate (Mg7B4O13·7H2O), co-precipitation of borate with Mg(OH)2 predominates. However, as magnesium borate hydrate becomes supersaturated, both precipitation of Mg7B4O13·7H2O and co-precipitation with Mg(OH)2 contribute significantly to borate immobilization. Calcination temperature is a key practical factor affecting the borate sorption efficiency by changing the immobilization mechanism.