Kinetics of Gibbsite Dissolution in Concentrated NaOH + NaNO ₃ and NaOH + NaNO ₂ Solutions at Elevated Temperatures: A Tale of Two Legends

The kinetics of gibbsite dissolution and precipitation in alkaline solutions are important to many industrial processes. The kinetics in pure NaOH solutions has been relatively well-known, especially in the Bayer process to extract aluminum from bauxite ores. However, in tank wastes such as those at Hanford and Savannah River Sites, the kinetics can be greatly complicated by the presence of high concentrations of nitrate and nitrite in addition to NaOH. Understanding the effects of nitrate and nitrite on gibbsite dissolution and precipitation is important for the optimization of a pretreatment process for the tank waste sludges and associated supernatants. In this study, we investigated the dissolution kinetics of gibbsite in concentrated NaOH + NaNO3 and NaOH + NaNO2 solutions. In our experiments, NaOH concentration ranges from 1.0 mol·dm–3 (i.e., mol/L) to 3.0 mol·dm–3, while NaNO3 and NaNO2 concentrations range from 1.0 mol·dm–3 to 5.0 mol·dm–3, covering typical chemical conditions observed in Hanford Tank Waste. A kinetic model was formulated and fitted to the experimental data to extract key thermodynamic and kinetic parameters for gibbsite dissolution. Our experimental results reveal that the dissolution rate constants of gibbsite in nitrite media are about 1 order of magnitude lower than those in corresponding nitrate media, indicating an inhibitory effect of nitrite on mineral dissolution. This inhibitory effect can possibly be attributed to the formation of a passivating layer of > Al-NO2– on the mineral surface (where “>” denotes surface complexes). In solution, nitrite, a Lewis base, forms an aqueous complex of Al(OH)3NO2– and thus increases the apparent solubility of gibbsite, which pushes the dissolution toward a far-from-equilibrium state, leading to etch pit development on the mineral surface. The results presented here have important implications for the pretreatment of tank waste sludge.