Hazenite and K-struvite production: phosphorus and potassium recovery from biomass power plant bottom ash using extraction and crystallization processes

The present study examined the P and K recovery from biomass bottom ash of a power plant that uses agricultural and animal wastes as fuel. The extractability of the elements contained in the biomass bottom ash was investigated using various organic and inorganic acids. H₂SO₄ yielded the most extracted P, K, and Mg. The optimum experimental conditions were determined using the Box–Behnken experimental design at 0.6 M H₂SO₄, 25 °C, and 8/1 liquid/solid ratio. A crystallization process was applied to recover P, K, and Mg from the obtained extracted liquid under optimum conditions. At a Mg:K:P molar ratio of 1:1:1 and pH 9, removal efficiencies of PO₄-P, K, and Mg were 98.67%, 47.44%, and 96.81%, respectively. The most efficient removal of PO₄-P and K was at pH 9 and a Mg:K:P molar ratio of 1.8:1:1.5. The crystal structure, morphology, and chemical composition of the precipitate recovered under these conditions were determined using XRD and SEM–EDS analysis. The results showed that hazenite (Mg₂KNa(PO₄)₂·14H₂O) and K-struvite (MgKPO₄·6H₂O) crystals were present in the precipitate. The study revealed that P, K, and Mg in biomass bottom ash could be recovered in the forms of hazenite and K-struvite, which have fertilizer properties.