Potassium recovery from the fly ash from a grate boiler firing agro‐residues: Effects of unburnt carbon and calcination pretreatment

BACKGROUND: The biomass-fired power in China is mainly fueled by agro-residue, and is suffering the bottlenecks of feedstock supply, super heater corrosion and nutrient cycle. How to recycle the ash nutrient to farmland is crucial for ecological sustainability. This work investigated potassium recovery from the fly ash of a grate boiler firing agro-residue using leaching methods. RESULTS: The studied ash contains a massive amount of fixed carbon (158.92 mg g(-1)) and little volatile matter (39.44 mg g(-1)). The unburnt carbon strongly decreases the concentration (C-K) in the leachate and recovery percentage (RPK) of potassium nutrient by 17.6-24.1% and 9.0-18.8%, respectively. Calcination pretreatment in air at 500 degrees C for 30 min should be employed before leaching to remove the unburnt carbon. Mineralogy analysis shows that low-temperature pre-calcination (<= 575 degrees C) does not affect potassium solubility. Under the optimal leaching of calcined ash inwater with liquid-to-solid ratio 3 for 30 min at room temperature, CK in the leachate, dissolution percentage (DPK) and recovery percentage (RPK) of potassium element reached 17.21 g L-1, 82.0%, and 62.6%, respectively. CONCLUSIONS: The fly ash from grate boilers firing agro-residue is much better than other marginal potash sources. This work is helpful to improve the technology of agro-residue fired power. (c) 2016 Society of Chemical Industry