Settling Mechanisms of Valuable Metal Droplets in Iron Silicate Slag

Abstract: A significant source of copper losses from pyrometallurgical copper extraction is attributed to dissolved and entrained copper in discarded slag. Entrained copper can be recovered via pyrometallurgical slag cleaning in a settling furnace where the droplets settle under gravity. Reduced copper losses suggest improved raw material efficiency, and the slag becomes a more environmentally safe byproduct as it contains less copper and its associated elements. The copper content in the discarded slag often equals or is higher than in the copper ore, implying that the copper slag is a valuable secondary resource for copper.The settling velocity of droplets and, thus, the metal recovery depends on the slag viscosity, copper droplet size distribution, dissolved copper content, and density difference between the slag phase and the copper droplets. The process parameters, temperature and settling time, theoretically affect the copper recovery, where an extended settling time means that droplets have a longer time to settle, and the temperature affects the viscosity, where a higher temperature means a lower viscosity and thus a higher settling rate. However, the temperature also affects the copper solubility; therefore, the overall effect of temperature on the copper recovery during industrial settling processes is unknown. Modifying the slag composition is another option to alter the viscosity and copper solubility. CaO has experimentally been shown to affect both factors in iron silicate slag positively and is thus a potential modifier for increased copper recovery in a settling process. However, there is a knowledge gap regarding the industrial CaO slag modification in a settling process and the effect on copper droplet size distribution, dissolved copper content, copper recovery, and the relationship between recovery and viscosity. Further research is necessary to bridge the knowledge gap and explore the potential benefits of CaO slag modification for improved copper recovery.Within the scope of this thesis, an industrial trial was conducted with the identified factors, temperature, settling time, and CaO slag modification to gain knowledge of the effect on the industrial settling process. The trial was evaluated by performing a slag characterization focusing on the appearance of copper and its associated elements and phases, copper droplet size distribution, slag matrix copper content, and investigating the slag copper content, copper recovery, and slag viscosity. The results showed that the copper droplets were mainly copper matte and speiss and were primarily associated with the slag phase and occasionally with a chromium-rich spinel and bubbles, which can hinder the settling. The results suggested that the viscosity and slag matrix copper content decreased when the slag was modified with CaO, and the copper droplet size distribution shifted to contain larger droplets. The CaO modification resulted in a higher copper recovery, revealing a linear relationship between the overall recovery and the viscosity.A crucible and spindle material with minimum interaction and influence on viscosity had to be identified to perform the viscosity measurement. Iron silicate slags are the dominating slag used during pyrometallurgical copper extraction of Cu-Fe-S concentrates. The dissolution of the crucible and spindle material into the melt is often analyzed and reported. However, the influence on viscosity is rarely investigated and reported. Three crucibles were studied, Mo, Ni, and Fe, concluding that Mo was preferable for viscosity measurements as it interacted the least with the melt and gave the most stable measurements with the highest reproducibility. Mo was thus the crucible choice for viscosity measurements of the industrial slag with and without CaO modifications. Based on the results from the industrial trial and the experimental evaluation, it was concluded that it is difficult to see an effect of regulating the temperature and settling time in the settling furnace process. It was, however, possible to increase the copper recovery by industrial CaO slag modification, which contributes to an increased raw material efficiency and, thus, a more sustainable pyrometallurgical copper extraction.