Study of pre-treatments of fly ashes aimed to the reuse as construction materials: chemical and mineralogical characterization

The management of waste and its sustainable reuse is one of the most important concern in our society in recent years, together with the increasing need to find primary materials without resorting to new extraction of resources. In this context, the thermovalorization of municipal solid waste (MSW) is currently the method that is spreading and replacing landfill disposal; the thermal treatment allows to reduce the volumes significantly, producing energy and returning bottom ashes (BA) and fly ashes (FA) in the measure of 20% and 5% of the total waste respectively.
The MSW incineration BA are classified as non-hazardous waste and can be reused as a raw material after some physical-chemical treatments.
The FA, on the contrary, are classified as hazardous waste and according to current legislation, they are usually subjected to vitrification treatments and stored in dedicated landfills. The hazard is due to the high content of soluble salts (chlorides and sulfates) and heavy metals (mainly Zn and Pb). Therefore, for their possible reuse as construction materials (e.g. ceramic, cement, concrete aggregates) or base roads, a preliminary stabilization step is required which often requires the use of significant quantities of energy.
In the present work, low energy cost methods are considered to reduce the dangerousness of FA and consequently make them more easily treatable for their reintegration into the production cycles.
Among the methods, washing of FA with water is examined, to find the lowest L / S ratio in the reduction of salts and heavy metals, analyzing the dissolution kinetics and the mineralogical content of fly ash before and after each washing treatment.
For a better definition of the kinetics, the FA are previously submitted to particle size separation to understand in which fractions the most dangerous substances are concentrated.
Washing treatments can be useful to remove or reduce soluble salts, in particular chlorides, by using a different liquid / solid (L / S) ratio, in order to obtain a more suitable material for the solidification / stabilization treatments carried out by geopolymerization or in cement.
The eluates of washing are also taken into consideration to evaluate the recovery of elemental species of interest and the purification of the liquid phase with biochar.