Investigating the leaching, desilication and precipitation of aluminium tri-hydroxides from a bauxite residue-bauxite by-product slag

The ENSUREALinitiative is investigating the revival of the long discontinued Pedersen process for use in processing lower grades and different types of bauxite ores. The Pedersen process is a combined pyro and hydro-metallurgical process that has been proposed as an almost waste free alternative to the Bayer process for production of metallurgical grade alumina. This study evaluated the hydrometallurgical processing of slag which was composed of a mixture of bauxite residue (red mud) and a calcite-rich bauxite benefication by-product. The slag contained Al in the form of gehlenite and mayenite. The study investigated the leachability of the slag, the effectivness of a desilication step and the precipitation of aluminium tri-hydroxides. All three processes where investigated at lab-scale using a 1 L jacketed glass reactor. The results from early tests in this ongoing study have been promising. Samples of the slag (100 g), ground to -75 μm, were leached using a 1 L solution of 60 g/l Na2CO3at 90°C for 90 mins. As much as 65% of the Al was extracted with an indication that perhaps the amount could increase with longer leaching time. The desilication step was carried out by treating the leachate solution with CaO and mixing for 2 hrs at 70°C. This resulted in a small reduction of the Si content by 27%. The desilicated solution was sparged with a mixture of Ar and CO2 at a gas flowrate of 1.5 slpm and CO2partial pressure of 50%. This was done for 30 mins at a temperature of 25 °C. When precipitation commenced the gas flow was stopped. The solution was allowed to age for 3 days at 25°C while agitated at 200 rpm. The resulting precipitate was composed of the aluminium tri-hydroxides bayerite (85%), gibbsite (12%) and nordstrandite (3%)