Leaching Kinetics of Bauxite in Hydrochloric Acid

The Bayer process is currently used to produce cell-grade alumina from bauxite. However, if the reactive silica content in the bauxite exceeds 7%, losses of caustic and aluminium as sodalite (2 Na₂0.2 Al₂O₃·3 SiO₂·2 H₂O) become economically unacceptable. As Australia possesses large tonnages of bauxite containing more than 7% reactive silica, a process for the production of cell-grade alumina horn these bauxites using hydrochloric acid leaching was developed by the authors.

The process consists of the following steps: calcination of bauxite, leaching of calcined bauxite in hydrochloric acid, filtration of residue, crystallization of A1C1₃.6 H₂O, decomposition of AlCl₃.6 H₂O crystals to produce A1₂O₃, regeneration of hydrochloric acid for recycle into the leaching step.

The kinetic studies of the dissolution showed that the extraction of aluminium is independent of the solid to leachant ratios studied. The acid concentration has a marked effect on leaching kinetics of bauxite and the leaching time decreases substantially as the concentration of acid increases. The order of the reaction is greater than one. The rate equation can be described by the following least squares line of best fit.

r = 4.1 × 10^?5 HCl^0.5 + 1.27 × 10^?5 HCl² where

r = the initial reaction rate, g Al extracted/sec.

Aluminium extraction greater than 90% can be achieved from bauxite in 4 h using 25% HCl while 30% acid is considered the maximum acid strength due to the crystallization of A1C1₃.6 H₂O from leach solutions.

The rate of leaching increases as the temperature of leaching is increased. An Arrhenius plot of the initial reaction rate (g Al extracted/sec) against the inverse absolute leaching temperature produced a straight line. The activation energy for the reaction was determined to be 83.3 kJ/mole with a standard deviation of 2.9 kJ/mole. The correlation between the initial reaction rate and the inverse absolute temperature can be described by the following regression equation:

r = 4.71×l0⁸e^83820/RT

The large activation energy value suggests that the reaction is chemically controlled rather than diffusion controlled. The additives such as NaCl and FeCI₃ do not effect the leaching rate.