Desilication Of Synthetic Bayer Liquor With Calcium Sulfate Dehydrate: Experiments And Kinetics

There are three main methods for alumina production industry:Bayer method, sintering method and hybrid method (both Bayer and sintering). Desilication process is a critical step during alumina production with Bayer method. Desilication agents used in current alumina industry are calcium oxide and calcium hydroxide. The research and development of new desilication agent has become an important subject as calcium oxide and calcium hydroxide are non-renewable resources and costly.Calcium sulfate dihydrate (CaSO4·2H2O), as a new desilication agent, has the ability of desilication. The desilication kinetics of synthetic Bayer liquor by use of calcium sulfate dihydrate (CaSO4·2H2O) was studied experimentally and modelled mathematically. Factors affecting desilication reaction, namely temperature, initial silica concentration, the concentration of NaOH and Al(OH)3, the particle size and dosage of CaSO4·2H2O, were investigated systematically. Temperature, initial silica concentration and the dosage of CaSO4·2H2O were found to be the main factors affecting desilication reaction. CaSO4·2H2O was proved to remove up to 95% silica from sodium aluminate solution under the optimum operation conditions. A surface integration kinetic model, assuming that the desilication process follows a pseudo-second-order kinetics, was developed and successfully model the experiment data of desilication process via CaSO4·2H2O at the range of operating conditions investigated. An activation energy of 52 kJ/mol was estimated for the desilication process with CaSO4·2H2O over the temperature of 80-100℃. The desilication reaction products (DSP) were analyzed by XRD and SEM, and found to be mainly Hauyne (3Na2O·2CaSO4-3Al203·6Si02) and Lazurite (3Na2O·CaSO4·0.5Na2SO4·3Al2O3·6SiO2). The final sulfate concentration in the liquor after desilication with CaSO4·2H2O was 0.021 g/L.