Quantification of hematite and goethite concentrations in kaolin using diffuse reflectance spectroscopy; a new approach to Kubelka-Munk theory

Kaolin ores are usually contaminated by some Fe-bearing minerals, the strong colors of which degrade the quality of the final product. A spectroscopic technique is sought to quantify the content of hematite and goethite, the main contaminant minerals in the kaolin from Capim River in Brazil, was the focus of this study. The total Fe content obtained through X-ray fluorescence showed a poor correlation with the brightness of kaolin, due to the inability to differentiate between the Fe contained in the (oxyhydr)oxides and the Fe present in the crystalline structure of kaolinite, especially when the Fe-bearing minerals occur in small quantities. Here, a new generic technique to quantify Fe (oxyhydr)oxides in kaolin, based on the Kubelka-Munk theory, is presented. A new interpretation of the theory was made that enables its use without the need to measure thin layers of material. The results with synthetic goethite and hematite were very positive (R2pred≈0.99) and experiments with contaminants from the mine are already underway.