Silica precipitation from rice husk ash leachate: optimization and kinetic analysis

The production of silica from husk waste material addresses two problems: the accumulation of husk waste in the rice milling industry and demand for silica. This study successfully developed a complete process optimization and kinetics analysis for silica precipitation from Na₂SiO₃ (rice husk ash leachate). The process of silica production from rice husk ash was conducted via alkaline leaching followed by acid precipitation. Silica precipitation was carried out in a flask equipped with a thermometer and a pH meter. Precipitation was carried out at a pH range of 5–9, a temperature range of 30–90 °C, and a precipitation duration of 0–120 min. 0.5 M H₂SO₄ was added slowly using a dosing pump until the required operating conditions were achieved. The operating conditions were analyzed to determine the optimum temperature, pH, and precipitation duration. Kinetics analysis was also conducted to obtain a kinetic expression describing the precipitation process. A silica precipitation recovery of 73% was obtained under operating conditions of 90 °C and pH 5 for 90 min. The precipitation process can be described by the second-order kinetic equation. The kinetics equation that describes this process is \(k = 241.12 \;{\text{min}}^{ - 1} - \frac{{21.62\frac{kJ}{{{\text{mol}}}}}}{{{\text{RT}}}}\) $k=241.12{\text{min}}^{-1}-\frac{21.62\frac{\mathit{kJ}}{\text{mol}}}{\text{RT}}$ . The obtained apparent activation energy is 26.62 kJ/mol.