Refinement of the isomorphic substitutions in goethite and hematite by the Rietveld method, and relevance to bauxite characterisation and processing

• Al-for-Fe and OH−-for-O2− substitutions in hematite calculated by Rietveld method.
• Agrees with Mössbauer spectroscopy iron distribution in bauxite samples.
• Refined Al-for-Fe substitutions in analysed samples range from 4.9% to 13.8% (mol).
• OH−-for-O2− substitutions vary enormously, from 4.3% up to 46%.
• QPA for all minerals improved, tested with 13 Brazilian bauxite CRM’s.

Although bauxites usually have a quite simple mineralogy – gibbsite (+boehmite), quartz, kaolinite, hematite, goethite, anatase (+rutile) and minor or less common phases, fine particle size, low crystallinity and variable compositions of the iron minerals might render phase quantification difficult, as well as impairing bauxite processing. A reliable and complete characterisation is therefore necessary in order to predict processing performance and ensure compliance to plant specifications.X-ray diffraction is the most important single tool for bauxite characterisation, and the constrained refinement of the Al-for-Fe substitution in goethite during one-step phase quantification by fundamental parameters Rietveld method has been successfully used. The same method was developed to analyse the coupled Al-for-Fe and OH−-for-O2− substitutions in hematite. The method was tested against Mössbauer spectroscopy iron distribution on bauxite samples with a large compositional range, and on bauxite Certified Reference Materials from the main Brazilian mines, with improved results and widened range of conclusions that can be drawn related to bauxite processing.