Quantitative phase analysis of lateritic bauxite with NIR-spectroscopy

• NIR-spectroscopy is a suitable method for quantifying bauxite over conveyors.
• Predictabilities of the major mineralogical phases are within 2.5 wt% (1σ).
• For measurements over conveyors crushed material is preferential.
• Influence of humidity on the spectra needs to be corrected in the models.

The applicability of near-infrared (NIR) spectroscopy as tool for continuous and direct quality control of bauxite material on conveyor belts at mining sites was assessed. Modal quantifications of the bauxite mineralogy were performed by X-ray powder diffractions (XRPD)/Rietveld-analysis on 200 aliquots derived from 4 charges from the same sample locality and compared to Near Infrared (NIR) spectra measured in reflectance mode. Based on the two methods multivariate calibration models from partial least squares (PLS) regressions were evolved. Laboratory scaled results show a predictability of the major mineralogical phases (gibbsite and kaolinite) within 2.5 wt% (1σ).In order to test the quality of the laboratory scaled calibrations and their applicability to mining procedures, additional measurements were performed using an industrial NIR-analyzer from ABB that was specifically developed for conveyor belts. Results show that for an industrial usage two crucial points have to be considered: (1) The modal mineralogy of individual chunks of material depends on the grain size. Specifically, larger chunks contain a higher modal content of gibbsite and often are coated by a kaolinite-rich shell. Therefore, modal mineralogy determined by NIR spectroscopy in reflectance mode is biased by the grain size. In order to yield unbiased measurements, the material must be crushed at least to cm-size. (2) Humidity has strong influence on the NIR-spectra by changing band intensities. Therefore, in order to get accurate predictions, absorption bands of water have to be taken into account as additional constituent in the calibration models.