Sewage sludge ash to phosphate fertilizer by chlorination and thermal treatment: residence time requirements for heavy metal removal

Abstract

Heavy metal removal from sewage sludge ash can be performed by mixing the ash with environmentally compatible chlorides (e.g. CaCl₂ or MgCl₂) and water, pelletizing the mixture and treating the pellets in a rotary reactor at about 1000 °C. Thermogravimetry–mass spectroscopy, muffle oven tests (500–1150 °C) and investigations in a laboratory-scale rotary reactor (950–1050 °C, residence time 1–25 min) were carried out. In the rotary reactor, up to 97% of Cu, 95% Pb and 95% Zn can be removed at 1050 °C. As Cl release starts from 400 °C (obtained from thermogravimetry–mass spectrometry experiments), heavy metals are already removed partially within the heating period. This heavy metal removal can be described as being similar to a first-order rate law. To meet the limit values specified in the Austrian and German fertilizer ordinances, residence times of the order of minutes are sufficient at 950 °C.

Keywords:

• sewage sludge ash
• heavy metals
• thermal treatment
• rotary reactor
• kinetics

Acknowledgements

This research is part of the Vienna Centre of Excellence on Urban Mining which was co-funded by Zentrum für Innovation und Technologie GmbH (ZIT) of the City of Vienna and ASH DEC Umwelt AG, Vienna.

Notes

^†This work is dedicated to the 60th birthday of Prof. Mikko Hupa from Abo Akademi University, Turku, Finland.