Recovery of Silicon and Potassium from Rice Straw through Thermal Conversion and Residue Leaching

Abstract Retaining more soluble silicon (Si) and potassium (K) in the residue after the thermal conversion of straw and returning the residue to farmland as fertilizer can promote the sustainable supply of Si and K in farmland and alleviate the disposal pressure of the residue. In this study, rice straw was thermally converted at 350–850 °C under N2/air/CO2 atmospheres and the residues were leached for seven days to recover Si and K. Factors influencing recovery were analyzed with characterization data and recovery indicators. The results show that phytoliths dissolution affected the co-release of Si and K from residues produced below 650 °C or at 750 °C under CO2 atmosphere. The Brunner−Emmet−Teller specific surface area of residues had logarithmic positive correlations with Si recovery rate above 750 °C and K recovery rate in oxidizing environments. The total amount and content of soluble K in the residue had linear positive correlations with Si recovery rate. Residues with composite silicates or potassium carbonate tended to have lower Si recovery rates. The total K in rice straw was mainly lost by converted from soluble K to kinetically-insoluble K (≥ 14.5%) and oxidized-gasified K (≥ 19.8%), compared with thermodynamically-insoluble KAlSi3O8 (≤ 9.4%) and inert-gasified KCl (≤ 12.7%), respectively. The current straw thermal conversion technology can be improved concerning the above influencing factors to generate residues with more soluble Si and K, and to achieve both energy utilization of straw and resource utilization of residues.