Photocausticization of spent Bayer liquor: a pilot-scale study

The kinetics of the photodegradation of Bayer liquor has been investigated in a semi-batch pilot plant annular photoreactor using commercial titania as the catalyst. The major toxic constituent of Bayer liquor is sodium oxalate, which is formed during alumina processing and its removal and possible rejuvenation of the caustic solution is vital to plant economics. Experiments carried out under UV irradiation (250–380 nm wavelength) over 2-h runs showed that initial solution pH, light intensity, catalyst loading and dilution ratio all have strong effects on the decomposition rate. The experimental conditions were kept such that the mass transport effects were negligible. The kinetic data suggested pseudo-first order kinetics with respect to total organic carbon (TOC) concentration. Time-on-stream increase in pH as the reaction proceeded indicated the causticization of Bayer liquor.