Copper smelting slag is a significant potential resource for cobalt and copper. The recovery of copper and cobalt from copper slag could significantly augment the supply of these metals, which are essential to facilitating the transition to green energy while simultaneously addressing environmental concerns regarding slag disposal. However, the complex mineral composition of copper slag poses an enormous challenge. This study investigated the mineralogical and chemical characteristics of copper slag, which are vital for devising the most effective processing techniques. XRD and FESEM-EDS were employed to examine the morphologies of copper slag before and after the reduction process. The effects of borax and charcoal (carbocatalytic) reduction on phase transformation were evaluated. The XRD analysis revealed that the primary phases in the copper slag were Fe₂SiO₄ and Fe₃O₄. The FESEM-EDS analysis verified the presence of these phases and yielded supplementary details regarding metal embedment in the Fe₂SiO₄, Fe₃O₄, and Cu phases. The carbocatalytic reduction process expedited the transformation of copper slag microstructures from crystalline dendritic to amorphous and metallic phases. Finally, leaching experiments demonstrated the potential benefits of carbocatalytic reduction by yielding high extractions of Cu, Co, and Fe.

Copper and cobalt are essential metals for the transition to renewable energy and various aspects of modern life. Their extraction from primary sources, such as ores, requires metallurgical separation from unwanted host materials, generating a substantial amount of waste. One type of this waste is copper smelting slag, with 39 million tonnes produced and discarded globally each year. These large quantities of slag occupy significant land space, often near residential areas, and pose a potential hazard by releasing contaminants into the environment. However, they also contain residual amounts of valuable copper and cobalt. To address the challenge of mitigating the negative impacts of this waste and exploring the potential commercial opportunities of the contained critical minerals, this study reviews global smelting slag production over the past 25 years, as well as the composition and technical reprocessing options. It provides a summary of the chemical and mineralogical characteristics of copper slag from various research and an extensive overview of the processing strategies for metal recovery from copper slag, including flotation, pyrometallurgy, and hydrometallurgy. The study demonstrates that a significant amount of smelting slag has been produced with considerable variation and complexity, representing a major potential resource for cobalt and copper metals. The overview of production trends and reprocessing techniques shows that while some effective options exist or are emerging, further research is required to enhance the reprocessing of smelting slag waste to create economic value, improve energy efficiency in metal production, increase the supply of critical metals, and reduce environmental impacts.