2.1. Materials
Two different types of rice husk were used for this work, named agulhinha and cateto. Distilled water, solutions of citric acid (10%, w/v), acetic acid (10%, v/v), and phosphoric acid 0.2 mol L−1 were used in the chemical treatment process. Also, two available silicas were used to compare with the obtained silica from this study, Aldrich and Tixosil 333® (Rhodia).
2.2. Extraction of silica
For the extraction of silica from rice husk 25 g of each type of rice husk for 150 ml of each acid solution were used and submitted to high pressure (2 kgf cm−2) and temperature (150 °C) for a period of 1 h. Rice husk treated with distilled water was submitted to high pressure and temperatures 4 times. The aim of this procedure is to remove flux materials that are present in the rice husk, like alkaline and alkaline-earth oxides (Della et al., 2002). Then, rice husk were washed with distilled water, dried for 18 h at 60 °C. Unlike previous studies, cited in Section 1, in this study slighter acid solutions were used taking into account its characteristics to exhibit lower levels of corrosion damage to the reactor while generating less aggressive residue.
Finally, the dry rice husk were calcined under a 10 °C min−1 heat rate up to 650 °C, where it was submitted to a isotherm for 1 h to ensure the removal of the organic compounds according to previous TGA analysis.