Recovery of Potassium salts from bioindustry ashes

Sustainable fertilizer production is crucial for society, and ensuring food security remains a global challenge. Virgin mining still dominates the production of potassium fertilizers and is associated with economic, geopolitical and environmental constraints. Exploiting secondary sources, in particular those associated with waste-to-energy plants and biomass refining, is an attractive alternative and complement for fertilizer production.
This work investigates the recovery of potassium from biomass-derived ash streams. Seventeen ash samples from different industrial processes were systematically characterized with respect to elemental composition, mineralogy and leaching behaviour to identify suitable feedstocks. Poultry litter fly ash was identified as the most relevant material due to its high potassium content (>20 wt.%), primarily present in water-soluble forms.
A process based on aqueous leaching and crystallization was developed for poultry litter fly ash. Water leaching at a liquid-to-solid ratio of 2.75:1 w:w at 70 °C dissolved more than 90% of the potassium present. Subsequent cooling, followed by either evaporative and antisolvent crystallization produced potassium sulphate and glaserite, while remaining chlorides were separated based on solubility differences. The recovered sulphate salts exhibited high purity, with most samples containing less than 0.5 wt.% chloride. The process enables selective recovery of potassium salts without external inputs.
The results demonstrate that biomass-derived ashes can serve as secondary resources for potassium recovery and that the proposed process provides a feasible route for producing fertilizer salts.