The applicability of the Kubelka–Munk model in GCC brightness prediction

Ground calcium carbonate (GCC) is one of the most commonly used pigments in paper. In order to study the colouring effect of selected minerals on a GCC slurry, a high-quality calcite concentrate was spiked with contaminating minerals prior to micronisation and subsequent spectrophotometric measurements. 14 different mineral contaminant concentrates were tested: pyrite, pyrrhotite, chalcopyrite, galena, sphalerite (light and dark type), magnetite, hematite, ilmenite, quartz, K-feldspar, phlogopite, hornblende and graphite. The results show that the pyrrhotite, chalcopyrite and graphite concentrates display the most detrimental reduction in brightness R457 of the GCC slurry. In addition to brightness, the colour space for each contaminant concentrate is evaluated by using the CIELAB parameters. Furthermore, by applying the Kubelka–Munk function, the results show that most of the contaminants tested follow the Kubelka–Munk model with good accuracy, allowing a prediction of the brightness in a mixture to be made. Hence, the Kubelka–Munk model can provide a useful method to predict the colour of mineral mixtures in the production of GCC slurries.