Mineralogical investigations of Jamaican hematite-rich and goethite-rich bauxites using XRD and solid state 27Al and 31P MAS NMR spectroscopy

Abstract

Bauxite is the ore that is the world's primary source of aluminum. It is a significant natural resource for the Caribbean island of Jamaica. Large portions of Jamaica's future bauxite resources are goethitic and have significantly higher phosphorus concentrations than currently-processed hematite-rich ores. Depending on mineralogy, phosphate minerals may dissolve during alumina extraction and cause various process problems, consequently identifying them is critical. This study investigates phosphorus and alumina minerals in current and future Jamaican bauxites and extraction residues using powder X-ray diffraction (XRD) and solid state 27Al and 31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies. The use of powder XRD conclusively identified crandallite as the major P mineral in the goethitic bauxites but its presence in the hematite-rich ore was uncertain. Bauxite analysis using solid state 27Al MAS NMR, gave central peaks (δ = 6.60–8.60 ppm) and associated spinning sidebands that were consistent with the octahedral coordination of Al3+ in gibbsite however one high phosphorus ore had an additional peak at 50 ppm attributable to the tetrahedral coordination of Al3+ in an unidentified aluminosilicate mineral. Solid state 31P MAS NMR studies identified crandallite in both the goethitic and hematite-rich bauxites while peaks corresponding to chlorapatite, wavellite and an unspecified aluminophosphate mineral were observed in high P bauxite samples; phosphorus existed as apatite in the red mud residue. These results indicate that solid state 31P MAS NMR can give meaningful information on the P mineralogy of bauxite resources. The technique complements the use of powder XRD for investigating the P mineralogy of both hematite-rich and goethitic bauxites and can therefore assist in managing their exploration and predicting their processability.