MINERALOGICAL ANALYSIS OF BAYER MATERIALS - RIETVELD OR NOT RIETVELD

Quantitative Rietveld x-ray diffraction analysis of Bayer materials is fraught with complications that preclude its use as a "black box" tool. The advent of fast and powerful personal computers has made the method widely available. The Rietveld method was originally developed for refinement of crystal structures using neutron diffraction data, but has been extended to quantitative analysis and x-ray powder diffraction data. This has raised many new problems. Equipment suppliers and others were quick to promote the technique without recognising and acknowledging its limitations. Bayer materials such as bauxite and smelter grade alumina present many issues that must be carefully dealt with to ensure that sensible results are obtained. Careful selection of analytical conditions is essential. Analysis of bauxites requires absorption effects to be eliminated. Other complications include minerals prone to preferred orientation, and the presence of alumina substituted iron oxides for which good crystallographic data is not readily available. In smelter grade alumina the poorly crystalline intermediate aluminas suffer from extremely large peak widths, and a lack of good quality crystallographic data. Examples of many of these issues are presented. Currently the combined use of chemical analysis with XRD data is the method preferred by Alcoa for mineralogical analysis of Bayer materials.