The effect of reaction conditions on the composition of desilication product (DSP) formed under simulated Bayer conditions

Abstract

NaCl, Na 2CO 3 and Na 2SO 4 impurities present in Bayer liquors have a detrimental effect on the efficiency of the Bayer process. However, it is possible to remove these impurities by incorporation of these salts into Bayer-sodalite or cancrinite (SOD or CAN, ideal formulae Na 8(Al 6Si 6O 24)X 2· yH 2O; X −=Cl −, OH −, 1/2 CO 3 2−, 1/2 SO 4 2−, Al(OH) 4 −). The Bayer-SOD or CAN is subsequently discarded with the red mud waste. This paper examines the composition of SOD or CAN formed after pre-desilication (100°C for 15 h) then digestion (150°C/30 min, 175°C/30 min, 250°C/10 min) in sodium aluminate liquors containing added Na 2SO 4 (0–36 g/l) and/or Na 2CO 3 (6–42 g/l) and/or NaCl (0–18 g/l). The results suggest that the magnitude of anion incorporation into Bayer-SOD under pseudo-Bayer conditions follows the trend: OH −≪Al(OH) 4 −<Cl −≤CO 3 2−≪SO 4 2− and that the overall SOD anion content can be approximated by: [0.5 mol Cl −/6SiO 2+0.5 mol Al(OH) 4 −/6SiO 2+moles CO 3 2−/6SiO 2+moles SO 4 2−/6SiO 2]≈1.15 [two standard deviations (2 σ)=0.25]. Incorporation of SO 4 2− into the SOD cages can best be modelled from the liquor Na 2SO 4 concentration by a Freundlich-type isotherm of the form (DSP molar SO 3/6ReSiO 2 ratio)= c 1[Na 2SO 4] c 2 , where c 1 and c 2 are constants. By contrast, the concentrations of Cl − or CO 3 2− incorporated into the SOD are quantitatively dependent on the concentrations of Na 2SO 4, Na 2CO 3 and NaCl present in the initial liquor. The concentration of Cl −, CO 3 2− or SO 4 2− in the SOD cages was observed to vary with digestion temperature. The mechanism responsible for this variation was studied. The results were found to be qualitatively consistent with a mechanism involving substantial anion incorporation during desilication at 100°C, subsequent dissolution of some of this original DSP and reprecipitation of a DSP with a composition approximately reflecting the liquor composition and temperature during digestion. The amount of reprecipitated DSP increases with temperature—experiments suggest a treatment at 150°C/30 min results in ≈10% of the SOD dissolving then reprecipitating, while treatments at 175°C/30 min or 250°C/10 min result in ≈17% and ≈50% of the SOD reprecipitating, respectively.