Comparisons of Acid and Water Solubilities of Rice Straw Ash Together with Its Major Ash-Forming Elements at Different Ashing Temperatures: An Experimental Study

Abstract

Recycling utilization of straw ash as a fertilizer in farmland is expected to play an important role in the sustainable development of both agriculture and biomass energy. However, the ashing temperature and the aqueous solution characteristics may affect the recycling properties of the nutrients contained in the ash. The solubilities of both the ash and its elements can represent the above recycling properties. This paper presents a systematic experimental investigation on the acid solubilities of both rice straw ash and its major elements produced from combustion at 400–800 °C, and these findings are compared with the corresponding water solubilities obtained from the authors’ previous work. Meanwhile, the correlations of two solubilities with the ashing temperature were given based on the experimental data. Results show that the acid solubility of rice straw ash decreases linearly by approximately 76% as the ashing temperature increases from 400 to 800 °C, while it is significantly higher than the corresponding water solubility at different temperatures. The acid solubilities of K, P, Ca, Mg, and Na are higher than their water solubilities, whereas two solubilities of S and Cl have almost no dependence on the temperature and the acidity of solution. This study also reveals a strong negative linear relationship between the solubility of K and the temperature. The solubilities of other elements (P, S, Na, Ca, Mg, and Cl) with the temperature have quadratic curve or cubic curve relationships. Furthermore, it is recommended that the ashing temperature should be lower than 600 °C to avoid the loss of some nutrients in the straw ash.