• Simon, Sebastian
• Herzel, Hannes
• Adam, Christian
• Stemann, J.

Abstract

<jats:title>Abstract</jats:title><jats:p>There is an ongoing debate on European scale concerning the criticality of phosphorus. In Switzerland and Germany, phosphorus recovery from phosphorus-rich waste streams will become obligatory. Sewage sludge ash is rich in phosphorus and may become an important secondary feedstock. Thermochemical treatment of sewage sludge ash with sodium sulphate under reducing conditions was shown to remove heavy metals from the solid product and produce the fully plant available crystalline phase CaNaPO<jats:sub>4</jats:sub>. Pilot-scale experiments in a rotary kiln were carried out at temperatures between 750 and 1000 °C and were compared to laboratory-scale experiments with crucibles. Process upscaling was successfully demonstrated but a series of differences were noticed: In comparison to laboratory-scale, solubility of phosphorus in samples from pilot-scale experiments was lower at all chosen treatment temperatures because of shorter retention time and incomplete decomposition of sodium sulphate. X-ray diffraction analysis revealed remaining phase fractions of whitlockite (Ca<jats:sub>3-x</jats:sub>(Mg,Fe)<jats:sub>x</jats:sub>(PO<jats:sub>4</jats:sub>)<jats:sub>2</jats:sub>) and sodium sulphate from the starting materials in products and thus indicated incomplete reaction. In contrast to the results of laboratory-scale experiments, the crystalline phase CaNaPO<jats:sub>4</jats:sub> was clearly absent in the products from the rotary kiln but instead a Mg-bearing phase (Ca,Mg)NaPO<jats:sub>4</jats:sub> was formed. Laboratory-scale experiments confirmed (Ca,Mg)NaPO<jats:sub>4</jats:sub> is an intermediate phase between whitlockite and CaNaPO<jats:sub>4</jats:sub>. However, both crystalline phases are characterized by high plant availability. It was shown that heavy metal removal increased at higher temperatures whereas solubility and thus plant availability of phosphorus already reached its maxima at temperatures of 950 °C in pilot-scale and 875 °C in laboratory-scale experiments.</jats:p>

Topics

• impedance spectroscopy
• x-ray diffraction
• experiment
• crystalline phase
• Sodium
• decomposition
• Phosphorus