Technical feasibility of the use of rice husk ash modified by iron (III) and chromium (III) as ceramic pigment

Full text: The rice husk ash is agroindustrial residues that, due to large volume and inadequate destination, can cause contamination of the environmental systems of water and air, as well as pose risks to human health. As a solution, ashes are used as raw materials in the synthesis of high silica catalysts, cement matrix composites, soil-lime bricks, however, with increasing rice production, is necessary to give another destination for these residues. In this context, this work aims to study the technical feasibility of the use of the rice husk ash as a ceramic pigment from the impregnation of Fe³⁺ and Cr³⁺ chromophores, evaluating the stability and its color properties. Initially, the rice ash was sieved in a 200 mesh. Then, 15% Fe³⁺ and Cr³⁺ ions were incorporated in the material through the wetness impregnation method, dried in an oven and heat treated at temperatures of 600 °C or 800 °C. Then they were characterized by XRF, XRD, MEV with WDS and EDS mapping and colorimetry. The pigments were mixed to glossy transparent glazes in the ratio of 3% in weight, coated the substrates and sintered at 1050, 1100 or 1150 °C. The husk ash contains high silica content (> 80%), with elevation of iron oxide content and presence of chromium oxide in the samples modified by Fe³⁺ and Cr³⁺ ions, respectively. The X-ray diffractograms revealed the predominant presence of amorphous silica, with partial crystallization when thermally treated. The mapping detected the increase in Fe³⁺ and Cr³⁺ concentrations in the modified samples, as well as the homogeneous distribution of the chromophores. The incorporated pigments presented coloration of red and green hue, respectively. However, after incorporation into the glaze, Fe³⁺ modified pigments showed yellow distortion. No significant changes were observed in the glazes tonality with the variation of the sintering temperature. Therefore, the rice husk ash modified presented technical feasibility to be used as ceramic pigments. (author)