Investigating Aluminum Tri-Hydroxide Production from Sodium Aluminate Solutions in the Pedersen Process.

This study investigates applying the principles of the long-discontinued Pedersen process as a possible route for producing metallurgical grade alumina from low-grade and secondary feed materials. The investigation focused on the hydrometallurgical steps in the process, namely leaching, desilication, and precipitation, and adapting it to valorize bauxite residue. The test material used was a calcium–aluminate slag made by the smelting-reduction of a mixture of bauxite residue (dewatered red mud) and a calcium-rich bauxite beneficiation by-product. Samples of the slag were leached in a 1 L jacketed glass reactor with Na₂CO₃ solution, varying 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, varying the amounts of CaO used. Finally, the desilicated leach solution was sparged with a CO₂ gas mixture, after which the precipitate was allowed to age in the solution. The carbonation and aging temperatures and times were varied. As much as 67% of the Al was leached from the slag. The desilication process successfully removed 88% of the Si. The precipitation process produced a product composed mostly of bayerite [Al(OH)₃], but some tests had considerable amounts of the unwanted phase dawsonite [NaAlCO₃(OH)₂]. The results indicated that the highest Al recovery was obtained using low concentrations of Na₂CO₃ solutions, and aluminum tri hydroxide is formed from these solutions at low temperatures at a fast rate compared to higher solution concentrations and temperatures. [ABSTRACT FROM AUTHOR]