Purification of TiO2 from Leucoxene for the Synthesis of NIR-Reflective Cr-doped CaTiO3 Pigment

This work aimed to explore the possibility of producing NIR-reflective pigment from Thai leucoxene which is an abundant mineral consisting of similar to 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 degrees C and 60 min, yielded TiO2 content of 93.94 +/- 0.14 wt.%. The Cr-doped CaTiO3 pigments (CaCrxTi1-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 degrees 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.