XRD is an important tool for characterizing the mineralogy of bauxite. Bauxite is a mineral mixture of several Aluminum sources (mostly gibbsite, boehmite) and gangue minerals that are commercially non-valuable in Aluminum production (quartz, kaolinite, hematite, goethite, rutile, anatase, and other less common minor phases).
Red bauxite ore is transformed in the Bayer process to white alumina powder. This is the raw material for the electro-metallurgical extraction of Aluminum metal. The yield of the Bayer process is not simply related to the Aluminum grade in Bauxite that is determined by chemical analysis.
A complete mineralogical characterization is needed to predict the processing performance in the refinery, but also fine particle size and crystallinity may hamper the process. X-ray diffraction is sensitive to crystallite size, therefore different size fractions of the same mineral can separately be quantified using TOPAS.
Silent, undetected recovery losses are a major concern in modern mineral processing. Some Aluminum may hide in the crystal structure of the iron minerals hematite or goethite by an Al-for-Fe substitution. This part of Aluminum is lost because it is not accessible to the recovery process. Chemical analysis cannot separate the hidden from the accessible Aluminum. TOPAS, however, can directly relate the amount of hidden Aluminum to the peak positions and intensities in the XRD pattern.
Rietveld quantitative phase analysis (QPA) using DIFFRAC.TOPAS is based on the calculation of the full powder pattern from crystal structure information. Therefore, this method does not need calibration curves and no tube ageing needs consideration.
Aluminium and Iron atoms exhibit different sizes. Therefore, a mixture of both varies the volume of the host structure, the mineral Goethite in this case. This shifts the XRD signals and therefore easily can be correlated with the Aluminum concentration in Goethite using the algebraic expressions that are unique to the TOPAS software.