CMA

45 Most of the washing procedures with the sole aim to use MSWI ash as a raw material in clinker production are mostly carried out in laboratory, i.e., both the washing procedure and the production of cement clinker. A summary of existing washing methods available are shown in Table 8. The recent full-scale demonstration showed satisfying results with water extraction (Wang et al., 2010). In this study, the washing procedure was carried out at the cement plant along with drying and pulverisation. The material was fed into the kiln from a separate hopper in 1 % by weight of raw meal indicating a treatment capacity of 30 tonnes per day. No detrimental effects were observed during the 3 days trial regarding the plant emissions and the quality on the final cement. Co-processing of MSWI ash in cement kiln This article considers only MSWI ash (bottom ash and fly ash) as a raw ingredient for cement production and does not consider MSWI ash used directly as a cement replacement. In cement clinker production, the cement raw meal is normally a mixture of the finely ground raw materials; limestone, clay, bauxite, and quarts (feed stock). The raw materials are mixed in a proportion that gives a favorable chemical composition of the raw meal for the cement clinkering process. The final chemical composition of the cement clinker determines the composition of the raw meal. The major oxide composition of cement clinker is typically around 21-23 % SiO2, 63-66 % CaO, 3-7 % Al2O3 and 1-6 % Fe2O3. When MSWI ash residues are co-processed in cement kiln, the ash is partly replacing one or more of the feedstock raw materials. The substitution rate is determined by the chemical composition to comply with the oxide composition of the raw meal to obtain the oxide composition of the clinker. In addition, the concentrations of alkalis, chlorides and heavy metals will affect the final cement and need to be limited. Municipal solid waste incineration (MSWI) ash has an elemental composition suitable for cement production and is thus a potentially feasible substitute for traditional raw ingredients (Kyle et al., 2020; Viczek et al., 2020; Ashraf et al., 2019; Sarmiento et al., 2019). The incorporation of MSWI bottom or fly ash (or a combination of both) into cement production presents itself as an environmentally favourable management option, an integrated use that may serve to mitigate environmental issues (Clavier et al., 2019b; Serclérat et al., 2000; Wey et al., 2006). This practice reduces the requirement for aggregate mining and shipping and extends the useful life of landfills, among other environmental benefits (Ferraro et al., 2016). In addition, municipal waste incinerators who produce, and are thus required to dispose of, MSWI ash may receive financial benefits from reduced landfill tipping fees while the cement manufacturer may save on the cost of raw materials. Sarmiento et al. (2019) found that MSWI ash fly ash is favourable for cement manufacture, but that allowable fly ash incorporation is severely limited by chloride and alkali content; up to 67.8% fly ash could create a suitable clinker based on mineralogical composition alone, but the viable percentage drops to just 3.72% when constrained by alkalis, and further drops to 0.33% when constrained by chlorides and alkalis. Studies have reported that lime saturation factor decreases as more MSWI ash is added to a cement raw mix. Raw materials considerations MSWI fly ash has been reported as having 17% or higher chloride content while bottom ash is generally under 1% chloride; bottom ash and fly ash have similar alkali contents that constitute a considerable portion of their total composition. One study reports clinker created with up to 35% MSWI fly ash contains up to 2% chloride, exceeding the 0.1% permitted in European criteria for cement (Bogush et al., 2020). Some kiln operations utilizing alternative raw materials will install an exhaust gas bypass to “bleed” some of the chloride and alkali-laden gasses to avoid this issue. Excessive chloride content in concrete structures is commonly known to cause corrosion of steel reinforcement. Cement containing too much alkali is also prone to causing alkali-silica reaction in concrete containing reactive aggregates; this deleterious reaction leads to the premature failure of concrete structures. Researchers have also found that excess alkalis in a concrete system cause a porous microstructure and reductions in mechanical strength. Multiple studies using MSWI ash as a cement kiln feed have noted a pre treatment requirement to avoid issues with chloride and alkalis (Mao et al., 2020; Diliberto et al., 2018; Yan et al., 2018). It is important to consider that pre-treatment of MSWI ash for an industrial scale cement manufacturing operation would require considerable capital investment and infrastructure, as well to manage large quantities of contaminated wash water.