Circular economy perspectives for municipal solid waste incineration bottom ash valorisation

Bottom ash is the residual solid waste collected in waste-to-energy plants, where it accounts for about 25% wt of incinerated municipal solid waste. The chemical composition and physical properties of bottom ash make it worth of recycling metals and inert material for construction purposes. In 2014, 88 M tonnes of mixed waste were treated in EU waste-to-energy plants, generating 18 Mt of bottom ash, made of 80–85% mineral fraction, 10–12% ferrous and non-ferrous metals and a residual amount of unburnt materials. State-of-the-art technologies in countries with a long incineration history are able to separate the different components (metals are recycled in smelters while the mineral fraction is used for construction purposes), but the optimisation of the recovery of metals (particularly non-ferrous ones) and of the mineral fraction needs further efforts.The H2020 project “BASH-TREAT - Optimization of bottom ash treatment for an improved recovery of valuable fractions”, won the call ERA-MIN2 “Research and Innovation Programme on Raw Materials to foster Circular Economy” (May 2018-May 2021), is coordinated by Technical University of Hamburg and involves DIATI Politecnico di Torino as partner. The main goal of BASH -TREAT is the optimization of a physico-mechanical process, implemented at full-scale on a mobile plant (fed by 50 t/h of bottom ash), for the recovery of metals and inerts. Three full-scale industrial tests, performed in municipal solid waste (MSW) incineration plants in Germany, Sweden and Italy are planned in the project. This thesis was developed in the framework of BASH-TREAT project; the experimental activity was performed partially at DIATI (Biorefinery and Circular Economy Lab) – Politecnico di Torino and partially at Technical University of Hamburg (Waste Resources Management Lab) , Germany. Samples of bottom ash collected from two different MSW incinerators, located in Bremen and Malmö, have been analysed using different analytical techniques (component and particle-size analyses, XRF spectroscopy, ICP-OES, leaching tests) in order to identify their composition and to assess the feasibility of different recovery possibilities. The characterisation of bottom ash samples considered different classes based on particle-size and material composition (scraps of glass, ceramics, metal oxides, other minerals and a fine fraction having dimensions below 2 mm), in order to identify a specific recovery perspective for each class. The main objects of the activity were: 1. to identify the recycling perspectives and the economic value of the different classes; 2. to investigate the distribution of hazardous substances within the classes. Considering the gathered results, the main findings of the thesis are as follows. The fraction identified as “metal oxides” could be considered as a concentrated mix of metals and critical raw materials, therefore its valorisation could be economically viable. In particular the focus has been set on the major components Fe and Al and on the most valuable trace components (Cu, Ni, Mo, Ag, Y, V, Sc, Co). The main limitations for the recycling of glass scraps in the glass industry seems the presence of metals (Fe, Cu, Zn, Mn and others) and P and Cl; however this contamination seems to be mainly due to the presence of a surface layer that could be removed. Ceramics and the residual mineral fraction could be nonetheless considered as recycled aggregates for construction purposes.