Effects of water, acid, or alkali washing on Waste-to-Energy (WTE) bottom ash, fly ash, and combined ash

The washing pretreatment can facilitate the beneficial uses of Waste-to-Energy (WTE) ash as construction materials. In this study, bottom ash, fly ash, and combined ash were washed in the lab-scale processes with water and solutions of nitric acid (0.1, 0.5, 1 M) and sodium hydroxide (0.1, 0.5, 1 M) to understand the physicochemical transformation of ash and determine the ash streams processing by the changes in mass, chemical composition, mineral composition, and leachability of the three separated fractions: (i) washed ash, as the product; (ii) filter cake, as by-product, containing washed-out dust and reprecipitates; and (iii) dissolved ash compounds in wastewater. The results showed that 50% of the fly ash remained in the washed product, while other fly ash compounds were dissolved (e.g., Ca, chloride salts) or collected as filter cake, indicating severe materials loss via washing. Washing of the bottom and combined ash reduced the amorphous phases to 20–75% and transformed them into more stable crystalline phases (e.g., quartz in washed products and calcite in filter cake). After NaOH washing of combined ash, hydrocalumite-group minerals precipitated in the filter cake by-product. Water washing decreases the Cl concentration in the bottom/combined ash, thus avoiding chloride corrosion when the ash is utilized in construction. Applying water washing for combined ash processing is an effective option when the single pretreatment stream is executed; otherwise, only washing bottom ash fractions for aggregate uses is associated with stabilizing fly ash before disposal or other applications when separate ashes management streams are required.