Comparação da sinergia entre os tratamentos ácido, térmico e físico na cinza da casca do arroz e na casca do arroz para extração de sílica

Abstract

Rice husk is an abundant agro-industrial residue generated on a large scale during grain processing, posing an environmental challenge due to its high volume and low biodegradability. Its conventional use as a biomass fuel produces rice husk ash (RHA), a material rich in silica (SiO₂). When properly processed, this ash presents significant potential for industrial applications, especially as a supplementary cementitious material (SCM) in high-performance concretes (UHPC) and other mineral-based products. High-purity silica derived from rice husk can also be employed in advanced technological sectors such as coatings, ceramics, semiconductors and lithium-ion batteries, thereby increasing the added value of this residue. This versatility enables partial replacement of Portland cement, contributing to reduced CO₂ emissions and enhanced durability of cement-based structures. Producing high-purity amorphous silica, however, remains challenging due to the need to remove metallic oxides (K₂O, Na₂O, CaO, MgO) and organic fractions (cellulose, hemicellulose, lignin), which promote silica crystallization and lower reactivity. In this study, combined physical (milling), chemical (acid leaching) and thermal routes (pre-pyrolysis and controlled calcination) were applied to both raw rice husk and industrial residual ash, XRF, XRD, R3, Chapelle and laser granulometry analyses revealed marked differences between the two processing pathways. Milled and acid-leached husk calcined at 700 °C produced a fully amorphous silica with high reactivity (950 mg CaO g⁻¹), even with an intermediate particle size (D50 = 30 μm). In contrast, residual ash milled and reheated at the same temperature, although finer (D50 = 8 μm) and chemically comparable, showed lower reactivity (650 mg CaO g⁻¹) due to its pre-existing crystallinity and the formation of K-SiO₂ phases during uncontrolled industrial combustion. Milling effectively broke vitrified agglomerates and exposed trapped carbon, improving reactivity relative to the untreated ash, but did not reverse crystallization. Overall, the route starting from raw husk proved superior by combining thermal control with efficient removal of impurities, while the residual-ash route—although simpler-generated silica of intermediate reactivity. These findings confirm that both rice husk and its residual ash can be transformed into viable sources of high-purity amorphous silica, aligned with sustainability goals and innovation in construction materials. Beyond advancing scientific understanding of agro-industrial residue valorization, this study demonstrates the practical applicability of the obtained silica in high-performance cementitious systems, contributing to reduced environmental impacts and improved durability in construction materials.