Production of Aluminium Tri-hydroxides from Secondary Bauxite Materials

The study is currently evaluating the hydrometallurgical aspects of the Pedersen process as an alternative
route to the Bayer process for producing metallurgical grade alumina. The Pedersen process is
potentially applicable on lower grade ores and a wider variety of bauxite materials. The test material
used was slag which was obtained through the smelter-reduction of a mixture of bauxite residue (red
mud) and a calcite-rich bauxite beneficiation by-product. The process was evaluated at lab-scale using
a 1 L jacketed glass reactor.
Six experimental programmes were conducted each consisting of a leaching, desilication and precipitation
test. Samples of the slag were leached with Na2CO3 solution at 90 °C. The tests varied in
Na2CO3 concentration and leaching time. Additionally, different approaches to leaching involving
mechanical treatment of the leached slag and re-leaching using either fresh or recycled solution were
also explored. The desilication step was carried out by treating the leachate solution with powdered
CaO at 70°C varying the amounts used. Finally, the desilicated solution was sparged with a gas mixture
of equal parts Ar and CO2 after which the precipitate was allowed to age in the solution for a
period of time. The carbonation and aging temperatures and times were varied. As much as 73 % of
the Al has been leached from the slag. The desilication process has shown to remove 88 % of the Si.
The precipitation process produced a product composed entirely of the aluminium tri-hydroxides. It
was observed that the unwanted phase of dawsonite in the final product can perhaps be avoided by
using a solution of Na2CO3 with concentrations not more than 60 g/l. Furthermore, the carbonation
and aging temperatures should not exceed 30 °C.