A kinetic investigation of gibbsite precipitation using in situ time resolved energy dispersive X-ray diffraction
The precipitation of gibbsite Al(OH)3 from a supersaturated caustic aluminate solution is the rate determining step in the production of alumina (Al2O3) by the Bayer process. Seeded secondary nucleation and ordered growth experiments of gibbsite from sodium, potassium and deuterated sodium aluminate solutions, at 60 and 80°C, have been investigated for the first time using the in situ energy dispersive X-ray diffraction (EDXRD) technique. Gibbsite was the only phase precipitated from sodium and potassium aluminate solutions under all experimental conditions investigated. A mixture of gibbsite and bayerite was precipitated from deuterated sodium aluminate solution at 60°C while gibbsite was the only phase observed at 80°C. Bayerite persisted over the duration of the experiment, suggesting that bayerite and gibbsite precipitated independently and that bayerite did not undergo polymorphic transformation to gibbsite. Kinetic calculations using the Cardew model indicate that seeded precipitation at 60°C is a nucleation controlled reaction with nucleation occurring more readily in potassium aluminate solutions than in corresponding sodium aluminate solutions. Calculations of the growth rate for experiments at 80°C suggest that growth occurred preferentially over nucleation. These calculations confirm that suitable experimental conditions were chosen for the promotion of either nucleation or ordered growth over the other growth processes from caustic aluminate solutions.