In the Bayer Process, between 2-35% aluminium (as wt. Al2O3%) is lost with bauxite residue (BR). This is primarily due to the inefficient dissolution of aluminium-bearing minerals during caustic leaching or from the formation of desilication product (DSP) that also constrains the recycling of caustic soda from the circuit. If these lost alumina and caustic can be recovered from BR, numerous potential processing options become available which could lead to zero-waste BR valorisation. Whilst the pyrometallurgical routes (lime soda and lime-soda sintering) have been extensively discussed in the literature for recovering alumina and sodium from BR, understanding of the mineralogical phase formation during sintering and the relationship to the thermodynamic driving forces are poorly reported .This paper presents the investigation of sintering tests conducted on two separate residues with different characteristics; Greek BR and Australian Br, and mineral phases present both before and after sintering for two distinctly different BRs. These mineral phases were characterised and quantified using XRD and XDB software. The Factsage thermodynamic software package was used to analyse reaction equilibria with potential sintering chemical reaction pathways being described.

We acknowledge the funding by Rio Tinto and from European Community's Horizon 2020 Programme ([H2020/2014–2019]) under Grant Agreement no. 636876 (MSCAETN REDMUD, http://www.etn.redmud.org), as well as Aluminium of Greece for the support of this project. This publication reflects only the author's view, exempting the Community from any liability. Special thanks to Peter Hayes, William Hawker, Kelfin Hardiman, Warren Staker, Johannes Vind and Vicky Vasilliadou.