Investigation of scale formation in the Bayer process

Aluminosilicate ‘scales’ of the general formula Na8[AlSiO4]6X2.nH2O, X= ½CO3, OH that form during the Bayer process have been investigated. The ‘scales’ are composed of two main aluminosilicate phases, cancrinite and an intergrowth of sodalite and cancrinite, termed “intermediate”. Synthetic scales were prepared from kaolin, sodium hydroxide and sodium carbonate under hydrothermal conditions at temperatures of 150 and 220°C. The deuterated cancrinite structure, Na8Al6Si6O24.(CO3)1.06 1.44D2O, crystallises in the space group P63. The main, one-dimensional channels were found to contain two sites for carbonate ions while complete water molecule positions were located in the 11-hedral cages. The “intermediate” consists of a disordered stacking of sodalite and cancrinite structural elements. The materials lose water between 100-250°C with a contraction in unit cell dimensions as the sodium ions redistribute within the structure. At high temperatures the lattices expand with the onset of decomposition for “intermediate” being 100°C lower (500°C) than for ordered cancrinite (600°C). Addition of calcium hydroxide to systems that yielded carbonate cancrinite or “intermediate” was found to result in the formation of these two aluminosilicate phases containing a proportion of calcium replacing sodium and also, at high calcium concentrations, a sodium calcium hydrogen silicate (NaCaHSiO3OH). NaCaHSiO3OH crystallises in the P21/n space group. Strong hydrogen bonds (2.534 Å) link the SiO3OH units. Calcium doped cancrinites and “intermediate” were shown to form over the composition range Na7Ca[AlSiO4]6.(CO3)1.5.nH2O –Na6Ca2[AlSiO4]6.(CO3)2.nH2O, with a slight contraction in unit cell dimensions. Attempts to utilise a series of organic sulphate/sulphonate additives as potential scale modifiers/reducers resulted in the formation of the normal polycrystalline aluminosilicate phases. Seven organic polymeric additives were subsequently employed as potential scale modifiers/reducers, resulting in the formation of spherical cancrinite aggregates. The degree of spherical aggregates formation was related to temperature, reaction time and sodium hydroxide concentration and is likely to have involved decomposition of the polymeric additive. The replacement of NaOH with KOH in synthetic sodium aluminosilicate yielding systems was studied. At low KOH levels, PXD and SEM analysis of the products revealed potassium incorporation into the cancrinite and “intermediate” structures over the composition range Na7.5K0.5[AlSiO4]6.(CO3)1.nH2O – Na6K2[AlSiO4]6.(CO3)1.5.nH2O. At higher KOH levels the formation of KA1SiO4.1.5H2O was observed at 135°C, whilst at higher temperatures KA1SiO4 forms. The three systems investigated in the laboratory as possible scale reducers/modifiers were employed in large scale 25 L reactions, using a cyclic system (flow loop). Ca(OH)2 addition proved the most effective, a 30% reduction in the amount of scale formed was observed.