Waste incineration is generally classified as an energy recovery strategy; nevertheless its role in terms of material recovery/recycling has been also recently recognized. In fact, currently in Europe there are several large scale (around 200000 t/y) plants that treat bottom ash (BA) from incineration of residual Municipal Solid Waste MSW or Solid Recovered Fuel SRF with the main aim of recovering high value fractions such as ferrous and non ferrous metals. The mineral fraction (over 80% of BA) also presents recycling potential, since it has a physical and chemical composition (in terms of major constituents) similar to that of raw materials employed in construction such as aggregates or fillers. However, BA typically presents a higher content of inorganic contaminants that may be leached upon contact with water. For this reason even in Countries for which high recycling rates are reported (e.g.: Denmark and the Netherlands), utilization is typically restricted to specific conditions and requires long term monitoring, hence it resembles more containment than recycling. These practices should be considerably improved in view of the EU’s circular economy strategy. An example of such an effort is the Netherland’s Green Deal Agreement of 2012, which establishes that after January 1st 2020 BA cannot be recycled in restricted applications anymore. This means that to comply with the more stringent requirements for utilization a substantial improvement in BA quality, with particular regard to leaching, is needed. Elements that may be critical with regard to the above mentioned criteria include Ba, Pb, Cu, Cr, Mo and Sb. In addition, eluate pH as well as chlorides release may also exceed limits. Since, several inorganic constituents with different properties may be critical, a single treatment may not be sufficient. In addition, the types of treatments required and their effects may depend on the feed waste treated in the incineration plant, the technology employed and the BA extraction method (wet or dry). The treatments that have been tested on BA include washing, additives mixing and accelerated carbonation. The latter has been shown to reduce eluate pH from 12 to 9 and decrease the leaching of Cu, Pb and Zn, besides storing CO2 in solid form. Slurry-phase carbonation with enhanced operating temperature and CO2 pressure was recently shown to be promising for BA since it may also allow to reduce salts release; however, it may lead to the mobilization of elements, such as Sb and Cr.
In this paper, based on the results of lab-scale tests, potential treatment trains to substantially improve the leaching behaviour of different types of waste incineration residues so to meet the requirements for unrestricted unbound use in construction are presented and discussed. Specifically, the following types of BA were tested: the mineral fraction of quenched BA from grate furnace incineration of SRF, dry extracted BA from a fluidized unit treating SRF and the mineral fraction of a large scale plant treating BA from residual MSW. Two types of accelerated carbonation treatments (slurry-phase and wet route) were tested, as well as methods for reducing the mobility of metalloids such as antimony. The influence of the treatments on leaching was assessed through compliance, but also pH dependence tests for identifying the mechanisms leading to changes in the release of constituents of environmental concern. For the most promising treatments a preliminary cost evalutation will also be provided.
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