High purity alumina synthesised from iron rich clay through a novel and selective hybrid ammonium alum process

Abstract

The demand for high purity alumina (HPA) has been steadily increasing due to its use in advanced electronics and battery applications. This paper presents an innovative synthesis pathway, which is one of a series of pathways investigated to obtain HPA from a local Australian clay source with minimal clay processing. The novelty in this work was centred around impurity control since the feed clay had a high proportion of iron phases (19%). Ammonium aluminium sulfate (NH4Al(SO4)2·12H2O) was used as an intermediate to both selectively recover aluminium from sulfuric acid leachates and as a transition to finer refinement of aluminium in a chloride system through crystallisation of aluminium chloride (AlCl3·6H2O). The aluminium chloride was decomposed at 800 °C and 1200 °C to yield alumina with a purity of >99.99% Al2O3. The HPA product was characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and was found to be comprised of transitional alumina phases including κ and χ-Al2O3 after calcination at 800 °C and α-Al2O3 after calcination at 1200 °C. The purity of the HPA was assessed using an acid digestion method followed by analysis with ICP-OES and ICP-MS.