Phosphorus and zinc dissolution from thermally gasified piggery waste ash using sulphuric acid

Ash from thermally gasified piggery waste (GA) was treated with sulphuric acid (H2SO4) using two extraction methods. First different loads (0.39–0.98 kg H2SO4/kg ash) and concentrations (0.2–2 M) were used in 3 h extraction. Second, titration of 1:25 (w/w) ash:water suspension was conducted with 4 M H2SO4 to determine ash buffer capacity at nine pH steps from 12 to 0.1. Total P and zinc (Zn) dissolution was monitored. Optimal acid load and concentration to dissolve 94% P and 55% Zn from GA was 0.98 kg H2SO4/kg ash and 0.6 M, respectively, which corresponds to acid demand of 19.2 kg H2SO4/kg P recovered. High concentrations (2 M) did not improve P dissolution, but Zn was easier released. Ash buffer capacity was the highest at pH 4 and 0.1, first one due to dissolution of Ca, the second one due to autoprotolysis of water. Acid load had stronger effect on dissolution than concentration in the first method, however in the second; both factors had comparable effect.