The kinetics of gibbsite dissolution in NaOH

The dissolution of aluminum hydroxides from bauxite ores constitutes a key process in the aluminum industry. Generally, aluminum minerals in bauxites are present in the form of gibbsite and/or boehmite. Kaolinite, an aluminosilicate that is also present in bauxites, is a critical contaminant of the process. The understanding of the effect of the bauxite composition and of the process variables on the kinetics of dissolution can lead to the development of kinetic models and allow for subsequent process optimization. In this work, gibbsite dissolution in sodium hydroxide was studied in an isothermal batch reactor. Reaction temperature and the initial sodium hydroxide concentration were varied within the ranges 60 to 100 °C and 5 to 10 wt.% (1.2 to 2.5 mol/L), respectively, in accordance with a full 3-level factorial experimental design. For each experimental run, dissolved aluminum concentration was measured along the reaction time (up to 300 min). Kinetic models were developed and implemented in order to describe gibbsite dissolution, assuming that the chemical reaction was the rate-controlling step and that nonporous particles shrank along the time. Parameter estimation was performed with the Particle Swarm method coupled with a Gauss–Newton method. The first-order reversible reaction model was shown to provide good fits of the available experimental data. The estimated activation energy (110 kJ/mol) and heat of dissolution (26.4 kJ/mol) are in good agreement with other values reported in the literature. When conversions are far from equilibrium, it is shown that a simpler first-order irreversible model can also be used with confidence to represent the process.