• NPK fertilizer;
  • P fertilizer;
  • phosphorus recycling;
  • sewage sludge ash;
  • urban mining


In the present paper we show how P and NPK fertilizers can be prepared from thermochemically treated sewage sludge ashes (SSA) and which chemical reactions occur during these post-treatment steps. The SSA used for this investigation was treated thermochemically at a temperature of 1,000°C in a rotary kiln after the addition of calcium chloride hydrate or magnesium chloride hydrate to remove heavy metals and to increase the bioavailability of phosphorus (hereafter the products of thermochemical treatment are referred to as SSA-Ca and SSA-Mg). The major P-bearing mineral phase in SSA-Ca was chlorapatite (Ca5(PO4)3Cl), whereas in SSA-Mg the major P-bearing mineral phases were farringtonite (Mg3(PO4)2) and chlorapatite. Transformations of the mineral phases were observed when the P content of the products was adjusted by the addition of phosphoric acid (PA). The products of the reaction of SSA-Ca with PA were brushite (dicalcium phosphate dihydrate) and monocalcium phosphate monohydrate (MCPM). Further reaction products were found after the addition of PA to SSA-Mg: newberyite (MgHPO4·3H2O) and garyansellite ((Mg,Fe)3(PO4)2(OH)1.5·1.5H2O)). In addition, NPK fertilizers were prepared from SSA-Ca by mixing with molten NH4NO3 at approximately 180°C. No reaction occurred with the P-bearing mineral phase chlorapatite. When SSA-Ca was spiked with MCPM (to adjust the P content) before mixing it with molten NH4NO3, water-soluble NH4H2PO4 (monoammonium phosphate) was detected as a reaction product.