The Deposition of Calcium Oxalate and Amorphous Silica Scale under Dynamic Conditions which Simulate Sugar Mill Evaporators

摘要

Composite fouling of calcium oxalate monohydrate (COM) and amorphous silica (SiO_2) in sugar mill evaporators poses a major processing problem. The significance of COM-SiO_2 interactions during composite scale formation has been recognized previously and their effects on the fouling behaviour of both species have been demonstrated in batch tests. This work investigates the mechanisms of composite fouling of COM and SiO_2 in a dynamic system under subcooled flow-boiling and continuous evaporation. The experimental approach used in this study is novel. It simulates the operation cycle in the latter effects of sugar mill evaporators within one experimental run whilst maintaining a relatively simple process. The composite fouling behaviour of COM and SiO_2 has been tested in aqueous solutions with a range of COM/SiO_2 supersaturation ratios to determine the critical feed composition at which the maximum degree of composite fouling occurred. The synergistic effect of COM on composite fouling occurred at an intermediate concentration of COM (50 ppm) whereas antagonism was obtained at either low or high COM concentration (20 and 100 ppm). This may be due to changes in the magnitude of the interfacial energy barrier between the surface of the particles and the wall, and to the differences in the physical properties of the foulins species such as varticle size.