Experimental investigation of fluid dynamics effects on scale growth and suppression in the Bayer process

Scale formation on the process equipment is a major problem in the mineral industry because it leads to reduced plant efficiency and additional operational cost. Scale formation in the Bayer process equipment is a natural consequence of supersaturated solutions that are generated throughout the process and the costs involved in the de-scaling process may be as much as one-quarter of operational costs of an alumina refinery. The scale formation in the Bayer process mainly occurs from crystallisation of Bayer liquor which is not well understood yet. A series of systematic experiments were done using laboratory-made potassium nitrate (KNO3) aqueous solutions for safety reasons, since the real Bayer liquor requires high processing temperature and pressure and it has caustic property. The study provides a novel approach to elucidating the fluid dynamics effects on crystallisation scale growth and its suppression mechanism using for the first time a normal soluble salt to generate the crystallisation scale deposition in a newly fabricated lab-scale agitation tank that effectively replicates many industrial processes. Firstly, the impact of impeller agitation rate on the scale growth and its suppression was examined. Tests were conducted with three different size impellers (86, 114 and 160 mm) at varying rotational speeds ranging from 100 to 700 rpm using the KNO3 solutions of various supersaturation levels (4.5, 4.75 and 5.25 mol/dm3) to investigate the hydrodynamic effects on scale growth and suppression in the agitation tank. It was found that higher agitation rates suppressed the scale deposition on the agitation tank wall and lower agitation rates enhanced the scale deposition. The wall scale growth rate decreased asymptotically with time ranging from 58.06% to 6.79% and the corresponding bottom settled scale increased ranging from 4.19% to 80.2% depending on the agitation rate, impeller size, solution concentration and tank conditions. Secondly, the investigation of the impact of scale growth on heat transfer was conducted and observed that there was a significant variation of overall heat transfer coefficients (OHTC) and scaling thermal resistance (TR) coefficients due to crystallisation scale deposition. For a concentration of 4.50 mol/dm3, OHTC decreases asymptotically with time ranging from 75% to 38%, 73% to 23% and 72% to 2.6% for impeller diameters of 86, 114 and 160 mm respectively, due to crystallisation scale deposition on the wall of the tank with inserted baffles. For the unbaffled tank, OHTC decreases asymptotically with time ranging from 70% to 0.6% which depends on agitation rate and impeller size. The TR appreciably decreases with the increase of impeller agitation rate ranging from 159.37 to 0.57 cm2K/W. It is observed that the lesser scale deposition occurs in unbaffled condition compared to the baffled condition, due to the former creating a swirl flow condition that is conducive to augmentation of OHTC and reduction of TR. Finally, the effect of scale growth on different heat exchanger pipes material such as copper, aluminium, stainless steel, mild steel and polycarbonate (Cu, Al, SS316, MS, and Polycarbonate) during the convective heat transfer was investigated. The results show that crystallisation scale deposition increases with time and is augmented with an increase in thermal conductivity in the hierarchical order of copper (Cu) > aluminium (Al) > stainless steel (SS316) > mild steel (MS) > polycarbonate. The potential of a gum arabic additive to mitigate the crystalline deposition of normal soluble salt at convective heat transfer condition in the heat exchanger was also investigated, and a noticeable scale suppression was observed. The outcomes of this study offer a new body of knowledge in elucidating the scale growth characteristics and provide design guidelines for agitation tanks and selection of suitable impeller blades which attract less or no scale deposition under hydrodynamics conditions.