The transition to hydrogen-based reduction processes within the iron and steelmaking industry will generate new types of slag compositions that require valorization routes. Using slags as supplementary cementitious materials (SCMs) addresses the carbon dioxide emissions of the cement industry since the SCM requires neither calcination nor clinkering. Conventionally, ironmaking slags from the blast furnace (BF) are recycled as SCMs, i.e., ground granulated BF slag (GGBS). Ideally, future slags from electric arc furnaces (EAFs) operating on hydrogen-based direct reduced iron should be valorized analogously. Since the hydrogen-based process route is not yet realized in an industrial scale, the literature lacks data to support this valorization route, and additionally, literature on scrap-based EAF slags is scarce. Therefore, the present study aimed to offer insights into the utilization of ore-based EAF slags as SCMs based on an industrial slag sample from an EAF operating on hot briquetted iron. The slag was remelted, modified, and water-granulated in laboratory scale, and its performance as an SCM was compared to water-granulated ladle slag and two commercial GGBS. The results showed promising reactivities measured using the R³ isothermal calorimeter-based testing protocol. Based on the comparison to GGBS, the study indicated that generating reactive and appropriate SCMs from EAF slags will partly be a challenge in balancing the crystallization of the MeO-type solid solution rich in magnesia and addressing the iron oxide content in the amorphous phase. Graphical Abstract

During the pyrometallurgical extraction of copper, a significant fraction of this metal is lost with discard slag, which decreases profits and overall copper recovery. These copper losses can be reduced by using a settling furnace, in which suspended droplets containing copper separate from slag under the influence of gravity. An industrial trial was conducted in a settling furnace to increase the knowledge of the effect of temperature and settling time on the copper content of slag, and thus enhance the settling process to increase copper recovery. Slag samples were collected from four sample points: the ingoing and outgoing slag stream, within the furnace during settling, and the granulated slag. The chemical composition of the slag samples was analyzed and compared between batches with different temperatures and settling times. The appearance of copper and its associated phases were analyzed using a scanning electron microscope with an energy-dispersive X-ray spectroscopy detector (SEM-EDS). The results indicated that the outgoing slag copper content increased with an increase in temperature, and it was also concluded to be influenced by the attachment of copper to spinels and gas bubbles. The results indicate that regulating the settling furnace temperature to a lower interval could increase copper recovery.

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 slag viscosity and copper droplet size distribution (CDSD) are critical parameters. Reduced copper losses suggest improved raw material efficiency, and the slag becomes a more environmentally safe byproduct. In this study, iron silicate slags from a smelting furnace are industrially CaO modified in a fuming furnace to contain a CaO content between 8 and 18 wt.%. The viscosity of slags with and without CaO modifications is evaluated in the temperature range from 1423 to 1723 K. The influences of the CaO modifications on the CDSD, slag matrix copper content, and total copper content were determined. The results show that the slag viscosity decreases with increasing CaO concentrations in the slag. In addition, the copper content decreases in the slag phase, and the CDSD shifts to contain relatively larger droplets. The effect of CaO slag modification reveals a linear relationship between the overall copper recovery and viscosity, which increased from 63 to 88% when the viscosity in the respective batch was 0.51 and 0.25 Pa·s. Graphical Abstract

Copper in discarded slag decreases the profits and copper recovery during the pyrometallurgical extraction processes. The copper losses to slag can be reduced by using a settling furnace, in which mechanically entrained copper droplets separate from the slag under the action of gravity. The settling rate of entrained droplets can be increased by modifying the slag composition and, thus, the slag properties, which are known to influence the settling rate. The knowledge of industrial CaO slag modification in a reduced iron silicate slag with a Fe/SiO₂ ratio close to unity is limited. An industrial trial was thus conducted in an electric settling furnace, where the slag had been pretreated in a fuming furnace, to investigate the effect of CaO slag modification on the final slag copper content. Slag samples were collected from the ingoing and outgoing slag and from within the furnace of batches modified with CaO up to about 16 wt %. The trial was evaluated by comparing the final slag copper content and the copper recovery in the settling furnace. The results indicate that the settling becomes more efficient with the CaO modification as the final slag copper content decreased with increasing CaO content.

Pyrometallurgical copper extraction generates between 2.2 and 3.0 tons of slag per ton of copper. Copper smelting slag usually contains between 1 and 2 wt pct Cu, resulting in a substantial amount. Effective slag cleaning is critical to enhance copper recovery, often achieved through settling where entrained droplets separate from the slag. Controlling slag viscosity is vital, directly influencing the settling rate and overall efficiency. The literature has limited data on the correlation between viscosity, ionic structure, and the molar ratio of (Al₂O₃ + SiO₂)/MO (MO represents basic oxides) of iron silicate melts. This study investigates how FeO/SiO₂ ratio (1.08 and 1.25 at pct/at pct), CaO content (1.2 and 12 at pct), and Al₂O₃ content (1.4 or 5 at pct) affect the viscosity and melt structure in a FeO–SiO₂–Al₂O₃–CaO–MgO–Cr₂O₃ system. The study was done by synthesizing the melts, followed by water granulation. The ionic structure was analyzed using Raman spectroscopy, and the viscosity was measured using a high-temperature rheometer. The findings reveal a correlation between melt structure and viscosity, where the viscosity increased when the slag was more polymerized, having a higher content of SiO₂ and Al₂O₃. The study offers insights for optimizing slag properties in industrial settings.

Viscosity is a critical parameter during metal extraction, influencing the settling of valuable droplets, slag handling, and mass transfer. The viscosity of oxide melts can be measured with high-temperature rheometers. During such measurements, interactions between the crucible and the oxide system can change the chemical composition, melt structure, and thus the viscosity. For increased reliability of viscosity measurements, the influence of crucible and spindle materials on viscosity must be studied. In this study, the viscosity of a synthetic FeO_x–SiO₂–Al₂O₃ melt was measured using the rotating spindle technique and three different crucible and spindle materials (Mo, Fe, and Ni) to determine the interactions with the melt and the influence on viscosity. The interaction was evaluated by comparing the post-experimental chemical analyses, the Fe/Fe²⁺/Fe³⁺ content, visual observations, and using scanning electron microscopy–energy dispersive spectroscopy (SEM–EDS). The results showed that all the crucibles partially dissolved into the melt, affecting the melt structure, composition, and, thus, the viscosities. The viscosity data obtained using the Mo setup interacted the least with the melt, was the most stable, and with the best reproducibility. Graphical Abstract