Formation of lime-containing desilication product (DSP) in the Bayer process: factors influencing the laboratory modelling of DSP formation

Abstract

This paper examines the effect of certain reaction variables on the mineralogy of desilication product (DSP) formed during the laboratory modelling of the reaction of kaolin and lime in synthetic Bayer liquors. In particular, the effect of the desilication (100°C) to digestion (250°C) heating rate, kaolin source and liquor composition are discussed. At a low desilication to digestion heating rate (1.6°C/min, sample cooled once the temperature reaches 250°C) the DSP mineralogy is generally unaffected by the kaolin source used or by the form in which the reactants are added (e.g.: adding the CaO, extra gibbsite and kaolin to a sodium aluminate liquor at 90°C versus adding the CaO, gibbsite and kaolin to an aluminate-free liquor at room temperature). Hydrogarnets (HG, Ca 3Al 2(SiO 4) n(OH) (12-4n) with a silicon incorporation “n” ≈ 0.65 and a calcium-containing cancrinite (Ca-CAN) form under these conditions and the reaction pathway appears to reflect the thermodynamics of the system. By contrast, the mineralogy of the DSP formed at the higher heating rate (30°C/min, sample cooled once the temperature reaches 250°C) depends on the specific reaction conditions employed and more care must be given to the choice of initial modelling conditions. The reactant addition procedure still influences the DSP mineralogy when the time at 250°C is extended from 0 to 10 min. The implications of these results for the modelling of DSP formation using synthetic reaction mixtures are discussed. A possible mechanism for the formation of Ca-CAN is also discussed.