Selective Flotation for the Recovery of Valuable Metals from Waelz Slag

This thesis investigates the potential of selective froth flotation as a method for recovering valuable metals from Waelz slag. Waelz slag is a by-product of the Waelz process, used in the treatment of electric arc furnace dust (EAFD). While the Waelz process efficiently recovers zinc in the form of Waelz oxide, significant amounts of valuable metal oxides and metal sulphides, such as iron oxides, zinc sulphide and oxide, and copper sulphide remain in the slag and are currently lost through landfilling or low-value applications in construction. As environmental concerns and resource efficiency requirements increase, alternative treatment methods for Waelz slag become more important. The objective of this work is to evaluate the separation efficiency of collectorless froth flotation as a baseline approach for separating zinc-enriched and sulfidic phases from iron-rich phases in the slag. In the experiments, samples from four different industrial locations were investigated. Preliminary to flotation, a characterisation of the slag samples was conducted, using SEM-EDX analysis combined with a conversion program and phase clustering techniques. Flotation experiments were performed under laboratory conditions using minimal reagents, limited to a frothing agent, in order to investigate the inherent floatability of the materials. The separated samples were analysed via SEM-EDX and a conversion program. Under consideration of zinc sulphide, sulphur, copper, and iron oxide, the separation efficiency was evaluated. Therefore, the results for the compound compositions before and after flotation were compared. The results demonstrate a general tendency for the enrichment of sulfidic and zinc-bearing phases in the flotation concentrates, particularly in the early stages of flotation, while iron oxides are partially retained in the tailings. However, the achieved enrichment factors remain modest, with zinc enrichment not exceeding product-to-feed ratios of 1.4 and iron depression ranging between 10¿25% in the concentrates in reference to the feed. The limited selectivity is attributed to the mineralogical characteristics of the slag, especially the low abundance and fine grain size of hydrophobic zinc phases. Overall, the study establishes a fundamental baseline for the flotation behavior of Waelz slag. The findings indicate that while collectorless flotation shows initial potential, further optimization, applying collectors, depressants, and pH control, is essential to achieve economically viable metal recovery. Future work should also address particle size modification of zinc sulphide and improved process design to enhance separation efficiency.