Recovery of potassium and phosphorus from biomass-derived ash

Sustainable fertilizer production is crucial for society, and ensuring food security remains a global challenge. Virgin mining still dominates the production of potassium and phosphorus fertilizers. 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 study is an in-depth material characterization of seventeen types of ash from the incineration of nutrient-containing biostock. This provides a novel and extensive comparison of ash outputs from a variety of industrial processes. Ash samples after the incineration of poultry litter, fish litter, and from the Kraft process were studied. Total elemental analysis, XRD, TGA, SEM/EDS, leaching and leachate titration investigations were done to identify the most promising materials for recycling potassium and phosphorus. Poultry litter fly ash was identified as a promising secondary source of nutrients due to its phosphorus (3.0 wt%) and high potassium content (24.4 wt%). Efficient and selective separation of potassium from phosphorus was possible using water leaching at ambient temperature. This generated an alkaline solution with sodium, potassium, chlorides, sulphates ions, and very low heavy metal content; and a solid residue in which phosphorus was up-concentrated to 11.3 wt%. We briefly outline processes described in the literature to separate salts from the aqueous leachate and produce potassium fertilizers; and methods to process the phosphorus enriched solid. These offer more sustainable routes to produce fertilizers by exploiting secondary sources and allow for nutrient circularity in the industries that generate these materials. • Ash from biomass incineration can serve as a secondary source for nutrient recovery. • Fly ash derived from poultry litter incineration contains high potassium content (>20 wt%). • Potassium can be selectively leached with water, separating it from phosphorus and impurities. • Removal of potassium enriches the phosphorus content in the remaining solid (>10 wt%). • Crystallization and precipitation enable the recovery of both potassium and phosphorus.