Calcination technology for manufacturing mineral fertilizer using CaO-enriched sewage sludge ash

Abstract

A calcination technology was developed to directly convert sewage sludge ash (SSA) to mineral fertilizer. SSA samples, having a wide range of P 2 O 5 content (15.6-36.0 mass%), were collected from ten different wastewater treatment plants. CaCO 3 was added to SSA samples to adjust their CaO content to 45 mass%. The CaO-enriched SSA samples were then heated in an electric furnace at temperatures of 1250 or 1300 ° C for 10 min. This could increase the citric acid solubility of P 2 O 5 to 80-99%, regardless of the P 2 O 5 content of SSA tested. In addition, approximately 75-97% of SiO 2 in SSA became soluble in 0.5 M HCl after being subjected to calcination. The high citric acid solubility of P 2 O 5 and HCl solubility of SiO 2 in calcined products were attributable to the generation of silicocarnotite (Ca 5 [(SiO 4 )(PO 4 )](PO 4 )) and gehlenite (Ca 2 Al 2 SiO 7 ), respectively. The levels of toxic heavy metals such as Cr, Ni, As, Cd, and Hg in the calcined product were below their regulation levels for calcined sludge fertilizer in Japan. The calcination of CaO-enriched SSA is potentially a useful option to increase the solubility of P 2 O 5 and SiO 2 , thereby improving their plant availability, regardless of the P 2 O 5 and SiO 2 contents of SSA.