Sedimentation behaviour of a ferro-metal slag for the conceptual design of a CCD process

Abstract

The need for sustainable waste management and the rising global demand for raw materials have necessitated the recovery of valuable metals from metallurgical by-products such as ferrous metal oxide slag. These slags contain substantial amounts of the base metal from which they originate and trace quantities of valuable metals. The effective use of hydrometallurgical processes, such as leaching, depends not only on the leach recovery but also on the solid-liquid separation and washing steps that follow. These steps must be efficient to maximise lixiviant recovery and reduce loss due to entrainment. Challenges in settling the leach residue arise from slurry properties such as solution viscosity and surface chemistry. Furthermore, in a continuous countercurrent decantation (CCD) process, sedimentation occurs under continuously changing conditions, including particle size distribution, solution viscosity, and solid-liquid ratio. To design a CCD system, the influence of these conditions on the solids settling flux must be understood and quantified. This project explores the sedimentation behaviour of a ferrous metal oxide slag during various simultaneous settling and leaching stages for the conceptual design of a CCD demonstration process. Key objectives included fully characterising the ferrous metal oxide slag, identifying an appropriate flocculant and dosage to improve settling rate, simulating the settling conditions of a CCD and conceptually sizing a CCD demonstration process. The ferrous metal oxide slag was first characterised, using XRD to identify the crystal structure, SEM-EDX to analyse surface elemental composition and morphology, XRF and microwave digestion combined with ICP-OES to assess the bulk elemental composition, and laser diffraction to measure particle size distribution. Then, a flocculant study was conducted to determine the flocculant type and dosage needed to improve settling of the leached slurry. Finally, a laboratory-scale simulation of the CCD process was performed, involving a primary leaching step followed by a series of interconnected batch settling tests designed to mimic the conditions during a continuous washing process. The lixiviant at each stage was analysed to determine the recovery of the target metals. The results identified an effective flocculant capable of functioning in harsh acidic conditions. The simulation demonstrated that the three-stage CCD performed well as a washing step, achieving 84% washing recovery at a solid-liquid ratio of 10 wt% and 98% at 5 wt%. However, as a secondary leaching process, the CCD showed limited ability to recover additional target metals. Analysis revealed that this step removed residual surface materials and leached considerable amounts of non-target elements, including silicon, aluminium, and calcium. As a secondary leaching stage, the CCD only recovered an additional 3.62% at a solid-liquid ratio of 10 wt% and 1.35% at 5 wt%. The settling fluxes determined in the laboratory-scale simulation of the CCD process were successfully used to size a CCD demonstration process.