Selective leaching and crystallization methods to produce potassium and phosphate fertilizers from poultry litter fly ash

Potassium and phosphate salts are essential in agriculture playing a vital role in the production of fertilizers, which are crucial for global food security. Their conventional production relies heavily on mining, a practice subjected to economic, geopolitical and environmental constraints. Consequently, alternative methods to recover potassium and phosphate salts from secondary sources are receiving increased attention. Agricultural waste is a promising material, especially after incineration, which significantly reduces its volume, concentrates the contained nutrients, and eliminates organic contaminants. Poultry litter is one such important source of potassium and phosphorus nutrients. Poultry litter fly ash containing 24.4 wt% potassium and 3.0 wt% phosphorus was used for selective production of potassium and phosphorus salts using a combination of water leaching, and cooling and evaporative crystallization. Optimal leaching at liquid to solid ratio 2.75 w:w, 70 °C for 30 min resulted in >90 % potassium and > 99 % sulphate dissolution; and produced a solid residue with >10 wt% phosphorus, present mainly as calcium phosphate. Cooling crystallization recovered 25 % of the aqueous potassium as high purity potassium sulphate (> 99 % purity). Further recovery of potassium as glaserite with very low chloride content (< 0.5 wt%) was possible using evaporative or antisolvent crystallization. Leftover aqueous chlorides were separated by exploiting the different solubilities of potassium and sodium chlorides at different temperatures. The crystal morphology of potassium sulphate was significantly influenced by cooling parameters, including endpoint temperature and agitation, while the morphology of glaserite varied depending on the crystallization method. • Poultry litter ash used as a secondary source for potassium and phosphate recovery. • Optimized leaching yielded above 90 % potassium recovery in <30 min. at 70 °C. • High-purity potassium sulphate (>99 %) obtained via cooling crystallization. • Glaserite with <0.5 wt% chloride produced by evaporative or antisolvent methods. • Potassium and sodium chloride separated by exploiting solubility differences with temperature. • Phosphorus recovered as calcium phosphate solid residue (11.3 wt%) with routes to final product.