The particle size-dependent distribution of chemical elements in mixed commercial waste and implications for enhancing SRF quality

Abstract

Mixed commercial waste (MCW) contains contaminants that are important for several waste treatment options, e.g. the production and utilization of SRF for which limit values exist. Because these contaminants can be linked to specific materials but also occur in certain particle size classes, this paper presents the particle size-dependent distribution of 30 elements (Ag, Al, As, Ba, Ca, Cd, Cl, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, Pd, Sb, Si, Sn, Sr, Ti, V, W, Zn) in nine particle size classes (<5 mm, 5–10 mm, 10–20 mm, 20–40 mm, 40–60 mm, 60–80 mm, 80–100 mm, 100–200 mm and 200–400 mm) of coarsely shredded mixed commercial waste collected in the area of Graz, Austria. For 27 elements a negative correlation between concentration and particle size was observed, i.e., increasing concentrations with decreasing particle size. The fine fraction <5 mm contained the highest concentrations and the most considerable total amount of most analytes (e.g., > 40% of Hg, Mn, Mo, Sn, W). The removal of this fraction therefore theoretically reduces concentrations [mg/kgDM] of the 27 negatively-correlated elements in the screen overflow by 2.1 to 38%, while Cd (+10%), Sb (+12%), and Cl concentrations (+17%) and the lower heating value (+16%) increase. Besides containing contaminants, the fine fraction <5 mm consisted of ~11% Fe, 12% Ca, 12% Si, and 1.5% Al, all of which are valuable raw materials for the cement industry. Fe was found to be present as oxides, primarily as magnetite and wuestite. Calcium was present as carbonates (calcite, dolomite), but also considerable amounts of calcium sulfates (bassanite, gypsum, anhydrite). Concludingly, while screening can significantly improve SRF quality with respect to heavy metals, valuable raw materials for the cement industry may be removed as well, potentially resulting in a conflict between environmental protection and resource utilization.