Chiang Mai Journal of Science

     This work aimed to explore the possibility of producing NIR-reflective pigment from Thai leucoxene which is an abundant mineral consisting of ~89 wt.% TiO₂, 6 wt.% SiO₂ and 5 wt.% other oxides such as Al₂O₃ and Fe₂O₃. The leucoxene was purified by selective leaching of undesired oxides with NaOH under hydrothermal condition. Effects of NaOH concentration, hydrothermal temperature and time were investigated. The results revealed that dissolutions of the SiO₂ and Al₂O₃ increased with an increased NaOH concentration, while that of the ZrO₂ and Fe₂O₃ remained the same, resulting in the increased TiO₂ concentration. The SiO₂ dissolution also increased with an increased temperature and time. The optimum leaching condition, i.e. 4 M NaOH, 180 °C and 60 min, yielded TiO₂ content of 93.94 ±0.14 wt.%. The Cr-doped CaTiO₃ pigments (CaCr_xTi_1-xO₃; x = 0, 0.025, 0.05, 0.75 and 0.1), synthesized from the purified leucoxene, CaO and Cr₂O₃ by solid-state reaction at 1100 °C, exhibited brown color hue and NIR reflectance in a range of 34.5−59.8% depending on the Cr content. At the Cr content of 0.05, the pigment exhibited decent brownish appearance and modest NIR reflectance of 50.7% which was close to that of the pigment synthesized from pure rutile. The glaze consisting of 10 wt.% pigment had a glossy light brown appearance, and possessed NIR reflectance of 75.0 %, indicating a promising application in a field of NIR-reflective coating.