Mineralogical and chemical analysis of secondary raw materials – challenges and pitfalls

Secondary raw materials (SRM) are becoming increasingly more important in ensuring the stability of critical metal supply. Like for natural raw materials, processing and metallurgical treatment demands a detailed and meticulous geometallurgical characterisation. Unlike municipal solid waste and Waste Electrical and Electronic Equipment (WEEE), ashes, slags, dusts and other industrial residues are "produced" centrally in large quantities. In light of these circumstances, the logistics of supply is simpler. This makes these types of SRM ideal candidates for the extraction of critical metals.
Precise and accurate chemical and mineralogical data, knowledge of distribution of valuable and deleterious elements in the single phases as well as information about homogeneity and grain size distribution of the minerals are crucial for the development of new extraction technologies. Gaining these essential information can be achieved by using SEM-based automated mineralogical analysis. However, the large particle size range, the dominance of very small grain sizes (< 5 µm) and the diversity of phases are challenging for the analysis. Furthermore, in contrast to natural materials the analysis of secondary materials faces the challenge of developing new methods for non-natural extreme combinations of elements and phases. To overcome these challenges we used complementary analytical methods, like XRD and optical microscopy. Two of them, High-Speed PIXE and the “MEGA” instrument are still in the stage of development. Both instruments deliver additional information about trace element distribution.
Nevertheless, in contrast to natural materials the analysis of secondary materials faces the challenge of developing new methods for non-natural extreme combinations of elements and phases. We present initial results of ash and slag samples, like residues of pyrometallurgical recycling of spent lead batteries.