Influence of PbO/CuO ratio on phase composition, microstructure, melt wettability and recyclability of copper slag

Abstract

An estimated 30 % (∼11 million metric ton/year) of copper slag is still landfilled globally. Fayalitic copper slags are well-characterized, and they can be recycled into multiple safe end-products. Conversely, the major type of copper slag produced in Europe is lead-rich and warrants a deep understanding of phases present in it, properties, adverse environmental implications, and potential recycling strategies. The objective of this study is to thoroughly investigate such industrial slags by XRF, XRD, Mössbauer spectroscopy, SEM-EDS, novel automated image analysis using Aphelion, hot stage microscopy (HSM) and assess prospects for its valorization. These industrial slags were found to contain 26.1–50.5 wt% PbO, 3.4–5.4 % As2O5 (both hazardous solid wastes), 5.7–42 % CuO (present partly as Cu2O, an irritant), 3.2–18.2 % Fe2O3, 14–15.9 % SiO2 and oxides of chromium, cobalt (both carcinogenic), zinc and nickel (both toxic to aquatic life).The limitation of Rietveld XRD for glass quantification was overcome by newly-developed automated SEM-image processing that revealed that all three slags consisted of a lead silicate glass (36.3–65.8 vol%). Strikingly, rarely published arsenic- and lead-based compounds like (Ca,Pb)4As2O9-x, (Ca,Cu)2PbO4-x and Pb3Ca2Si3O11 were found when slag's PbO/CuO was 0.6, and Ca3As2O8-Pb2SiO4 formed when PbO/CuO equaled 9. The challenge of toxicity and landfilling can be overcome as these slags have high potential to be used for recovery of copper, cobalt, and lead. Radioactive waste immobilization glasses, gamma-ray shielding concrete and glass-ceramic consumer products may also be prepared from them. HSM revealed that a higher PbO/CuO ratio results in lower melting points and smaller melt contact angle indicating such slag can be useful for manufacturing recycled ceramics but corrosive towards refractories in copper converter.