Some Studies in Alumina Trihydrate Precipitation Kinetics

Abstract

From measurements of the growth rate of single gibbsite (alumina trihy-drate) crystals in pure NaAlO₂-NaOH solutions at 80°C, it was possible to derive a growth rate formula which fits the data. This equation is dl/dt = k[(A — A_∞/FC]², where dl/dt is the linear growth rate, k is the rate constant, A is the Al₂O₃ concentration, A_∞ is the final equilibrium Al₂O₃ concentration, and FC is the free caustic concentration. A similar equation may also be derived theoretically from the equilibria involving the formation of the bialuminate ion, Al₂(OH)₇ ^-, and the assumption that this ion loses OH” in a rate-determining step to form gibbsite. Starting with the equation for dl/dt, an equation relating the change in Al₂O₃ concentration with time, dA/dt, was derived for gibbsite-seeded precipitation of NaAlO₂-Na0H solutions. It was possible to numerically integrate this equation provided the surface area of the seed as a function of time or A were known. For seeded batch precipitations where the surface behavior was measured, the integrated equation correctly predicted the fall in Al₂O₃ concentration as a function of time for the temperature range 50 to 80°C with an appropriate choice of the temperature-dependent rate constant k. An apparent activation energy of 12.7 kcal was found.