Mineralogical Evolution and Leaching Behaviour of a Heap of Bottom Ash as a Function of Time: Influence on Its Valorization

International audienceWaste to Energy Bottom Ash (WtE BA) in France is reused as a substitute for natural aggregates in building material for road construction. Before BA reuse, treatment operations are generally necessary such as the removal of ferrous and non-ferrous metals, ageing, and particle size separation. BA valorization depends on the leaching behavior of the main Mineral Trace Elements (MTE), which in turn depends on pH. It is well known that ageing influences the natural pH of BA, as the initial pH of fresh BA (around 12–13) can decrease to a pH of 8 in weathered BA. Ageing can be influenced by different parameters, for example the presence of ferrous metals because of their exothermic oxidation increases temperature in heap and the CO2 content of the gaseous phase in the porosity of BA. However, as the removal of ferrous metals is now increasingly efficient, BA are different according to total content and mineral content. And when WtE BA is stored in heaps, atmospheric exchange are limited, the ageing process is no longer as efficient. The main objective of this study was to measure the efficiency of the ageing process when BA is stored in heaps. The mineralogical evolution and the leaching behavior of BA were observed during 7 months. Two artificial ageing processes were carried out on the oldest sample. All the results showed that when the BA was stored in heaps, the ageing process was inefficient, with a low evolution of pH, whereas ‘artificial’ ageing processes reduced the pH. This pH decrease was accompanied by an increase in the leaching of sulfates and antimony, which is an unwanted side effect. Mineralogical characterization showed differences between all the samples, providing insight into the leaching of MTE. The presence of some mineral phases known for their affinity with certain MTE made it possible to confirm certain mobility/retention hypotheses