Statistical analyses of precious metal contents in waste incineration bottom ashes

Introduction

Municipal waste management is one of the major problems in developing economies. Between 7 and 10 billion tons of waste are produced annually worldwide, including about 2 billion tons of municipal waste¹. The amount of municipal waste generated in the European Union in 2021 was 236 million tons². This is 12% of the municipal waste generated worldwide. As a result of the perceived problem, the European Union authorities have obliged member states to radically change their approach to waste by introducing a five-step waste hierarchy. In addition to the main priorities in the hierarchy, which are waste prevention, analysis of the impact of various waste management methods on the natural environment, and preparation for reuse, there is increasing pressure on waste recycling^{3, 4, 5–6}. As a result of the implemented regulations, member countries are obliged to increase the share of waste recycling to at least 65% by 2035⁷. The current average recycling rate in European countries is 43% and its theoretical upper limit is estimated at 80%⁸. Therefore, despite many controversies, waste energy recovery (thermal processing of municipal waste) will play an important role in the circular economy system⁹. Undoubtedly, the main benefits of incinerating non-recyclable waste are both the weight reduction of approximately 75%¹⁰ and energy recovery in the form of electricity and heat in the cogeneration process¹¹. Increasing attention is also being paid to the recyclability of incineration residues. Incineration bottom ashes^12,13 account for around 25% of incineration waste. The most common recovery methods include the recovery of ferrous and non-ferrous metals commonly found in bottom ash^{13, 14–15}. The average share in bottom ash is 5–15% in the case of ferrous metals and 1.0–5.0% for non-ferrous metals. The fine fraction < 4 mm, although it constitutes a negligible share of the total metals found in the bottom ash and amounts to only about 0.3%, could be a significant component of waste processing revenues due to the presence of heavy metal fractions, including precious metals^{16, 17–18}, as shown in Fig. 1.

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Fig. 1

Estimated total economic value of precious metals contained in bottom ashes from municipal waste incineration (based on¹⁸