Comparison of thermochemical treatment of sewage sludge ash with sodium sulphate in laboratory-scale and pilot-scale experiments

H Herzel,C Adam,J Stemann,S Simon

doi:10.1007/s13762-021-03252-y

Abstract

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 CaNaPO4. 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 (Ca3-x(Mg,Fe)x(PO4)2) 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 CaNaPO4 was clearly absent in the products from the rotary kiln but instead a Mg-bearing phase (Ca,Mg)NaPO4 was formed. Laboratory-scale experiments confirmed (Ca,Mg)NaPO4 is an intermediate phase between whitlockite and CaNaPO4. 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.

Highlights

Phosphorus together with nitrogen and potassium is the most important fertilizer ingredient
The aim of the present study is to report on a process upscaling from laboratory-scale experiments in crucibles to pilot-scale experiments in an indirectly electrically heated rotary kiln
Process upscaling was explored at varying temperatures by comparison of continuous experiments at 5 kg/h in a rotary kiln with laboratory-scale crucible experiments of 16 g

Summary

Introduction

Phosphorus together with nitrogen and potassium is the most important fertilizer ingredient. Conventional phosphorus reserves that are low in impurities and accessible will likely become scarce in the future (Gilbert 2009). Europe has a phosphorus import dependency of 92% (EC 2013). Given the criticality of phosphorus, Editorial responsibility: Maryam Shabani. International Journal of Environmental Science and Technology mainly into the crystalline mineral phase whitlockite type Ca3-x(Mg,Fe)x(PO4) (Peplinski et al 2009). Whitlockite is poorly plant available (Nanzer et al 2014). (2) SSA may contain elevated mass fractions of heavy metals (Krüger et al 2014). Post-treatment is necessary in order to increase plant availability and to remove heavy metals

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