Sewage sludge thermochemical treatment with MgCl2: Preparation conditions impact on trace metal elements vaporization and phosphorus recovery

Phosphorus (P) plays an essential role in crops growth. Due to the overexploitation of non-renewable phosphate rock deposits and their uneven distribution worldwide, P has turned into a strategic resource for the modern agricultural system. Finding sustainable ways to secure P for fertilizer applications has become critical. Sewage sludge (SS) is a waste material containing P in high amounts. Among several options, SS thermochemical treatment with Cl-donors addition such as MgCl2, CaCl2 or NaCl allows to separate P from inorganics pollutants such as trace metal elements (TME). The additives goal is to help fix and form bioavailable P species in solid residues while promoting TME vaporization through the gas phase via chlorination mechanisms. This paper studies the impact of MgCl2 addition on TME vaporization and P speciation during SS direct calcination treatment at 1000 °C for 4 h. Four different preparation methods for the “SS + MgCl2” mixtures were compared, with a variation of either the SS (raw or crushed) or the MgCl2 form (powder or solution). The samples were characterized by ED-XRF, FTIR, XRD, TGA/DSC and the P speciation was obtained thanks to Williams-Saunders method. The use of 0.06 g MgCl2/g SS as a Cl-donor divides the TME contents in the solid residues by a factor of more than 10 for Zn and Pb, about 2 for As and Ni but less than 1.25 for Cu. Concerning P, Ca associated P species (mostly Mg3Ca3(PO4)4) are formed with MgCl2 addition. The results showed that the impact of different preparation methods on the obtained solid residues characteristics is limited.