Analysis of the Effects of Slurry Phase Carbonation on the Leaching Behaviour of Different Types of Waste Incineration Bottom Ash

The mineral fraction of Bottom Ash (BA) from solid waste incineration presents a quite similar physical and chemical composition (in terms of major constituents) to that of some raw materials employed in construction, such as aggregates or fillers. However, BA typically presents a higher content of inorganic contaminants, which may be leached upon contact with water depending on different conditions (pH, humidity, redox state and weathering). For this reason even in Countries in which high reuse percentages, mainly in road construction, are reported (e.g.: Denmark, the Netherlands, France), utilization is typically restricted to specific conditions, hence it resembles more containment than reuse. These management practices need to be considerably improved in view of the EU’s circular economy strategy. Typically, for BA the elements whose leaching concentrations do not comply with utilization criteria under unrestricted conditions include Ba, Pb, Cu, Cr, Mo and Sb. In addition, eluate pH and chlorides release may also exceed limits. Since several inorganic constituents with different properties (salts, amphoteric metals and oxyanion forming elements) may be critical, a single treatment may not be sufficient. The treatments that have been tested on BA include washing 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. In addition it can allow to store CO₂ in solid form. Slurry phase carbonation with enhanced operating temperature and CO₂ pressure was recently shown to be a promising treatment for BA since it may also allow to reduce salts release.

The aim of this study was to analyze the effects of slurry phase accelerated carbonation performed under enhanced operating conditions as a treatment to improve the environmental behaviour of BA in view of their use in unrestricted construction applications. Different types of BA were tested including those from a grate furnace incineration plant and those from a fluidized bed one. The former slag is rapidly cooled in water, while the other undergoes a dry treatment process.

Slurry-phase carbonation tests were carried out on the different types of slag with the aim of identifying the combinations of operating conditions that allow to significantly improve the leaching behaviour of each type of slag. The variables that were tested are temperature, pressure, L/S and contact time. In addition, other CO₂ contents of the gas flow besides 100% are being tested to assess if flue gas (around 10%) or biogas (40-50%) may be directly suitable for the treatment, which could have an important impact on the sustainability of the treatment. The product was analyzed applying both batch compliance and pH dependence leaching tests in order to try to identify the mechanisms that cause the changes observed in the leaching behaviour of the slag. Furthermore, the increase in inorganic carbon content of the treatment product was also analyzed to assess the amount of CO₂ that can be sequestered in solid form by the process and if and how it is correlated to changes in the product’s leaching behaviour.