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

NaCl, Na2CO3 and Na2SO4 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(Al6Si6O24)X-2. yH(2)O; X-= Cl-, OH-, 1/2 CO32-, 1/2 SO42-, 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 degrees C for 15 h) then digestion (150 degrees C/30 min, 175 degrees C/30 min, 250 degrees C/10 min) in sodium aluminate liquors containing added Na2SO4 (0-36 g/l) and/or Na2CO3 (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- much less than Al(OH)(4)(-) < Cl- less than or equal to CO32- much less than SO42- 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 CO32-/6SiO(2) + moles SO42-/6SiO(2)] approximate to 1.15 [two standard deviations (2 sigma) = 0.25]. Incorporation of SO42- into the SOD cages can best be modelled from the liquor Na2SO4,, concentration by a Freundlich-type isotherm of the form (DSP molar SO3/6ReSiO(2) ratio) = c(1)[Na2SO4](c2), where c(1) and c(2) are constants. By contrast, the concentrations of Cl- or CO32- incorporated into the SOD are quantitatively dependent on the concentrations of Na2SO4, Na2CO3 and NaCl present in the initial liquor. The concentration of Cl-, CO32- or SO42- 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 degrees 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 degrees C/30 min results in approximate to 10% of the SOD dissolving then reprecipitating, while treatments at 175 degrees C/30 min or 250 degrees C/10 min result in approximate to 17% and approximate to 50% of the SOD reprecipitating, respectively. (C) 1998 Elsevier Science B.V. All rights reserved.