Hot water washing of rice husk for ash removal: The effect of washing temperature, washing time and particle size

Abstract

Rice husk is a carbon neutral and renewable source of energy and a biomaterial precursor, abundantly available at no cost. High ash content of rice husk is problematic for both applications and thus, reduction of ash constituents can be seen as a viable and sustainable solution. The effect of temperature, particle size and washing time on total ash removal percentage as well as elemental removal was evaluated. The ash leaching behavior was analyzed by fitting to a pseudo-second order kinetic model. 88–95% of K2O, 60–83% of P2O5, 38–54% of Fe2O3, 34–62% of Al2O3, 30–46% TiO2 and 19–24% of SiO2 has removed through water washing. According to the (K + Na)/(Ca + Mg) and (Si + K + P)/(Ca + Mg) molar ratios, ash melting tendency is expected to reduce significantly during combustion of washed rice husk. Increase of Ca and Mg in washed rice husk due to use of city water is beneficial for the ash chemistry. Washing temperature of 50 °C and 15 min of washing time with 1–1.4 mm particle size is recommended for washing pretreatment of rice husk.