Removal of Chromate and Borate from Solution Using Calcination Products of Ca(OH)₂– and CaCO₃–Al(OH)₃ Mixtures

Abstract

Ca(OH)₂–Al(OH)₃ and CaCO₃–Al(OH)₃ mixtures with Ca/(Ca+Al) ratios in the range 0 to 1 were calcined in the temperature range 873–1673 K for studying the effect of the calcination temperature and Ca/(Ca+Al) ratio on the calcination products. XRD analysis showed that the mixtures with Ca/(Ca+Al) ratios in the range 0.68–0.83 and calcined around 1273 K gave lime (CaO) as the major product and mayenite (Ca₁₂Al₁₄O₃₃), Ca₉Al₆O₁₈, and Ca₅Al₆O₁₄ as the minor products. These calcination products indicated that the efficiencies of the removal of chromate and borate from the solution were high. Upon immersing these calcination products in chromate and borate solutions, Ca₄Al₂CrO₁₀·12H₂O and Ca₄Al₂(HBO₃)_0.5(OH)₁₃·6H₂O were formed, respectively; the Ca/(Ca+Al) molar ratio in these compounds was 0.68, which was close to that in the raw mixtures. A Ca/(Ca+Al) ratio in the range 0.68–0.83 resulted in the optimum removal of chromate and borate from the solution. The Ca/(Ca+Al) ratio was more effective than these crystalline components in these calcination products. Further, Ca(B(OH)₄)₂·2H₂O precipitated as a result of borate removal.