1. Introduction
Copper losses to slag limit overall recovery and, thus, decrease the profit and efficiency of raw material usage during pyrometallurgical copper extraction. The copper in slag is either dissolved or mechanically entrained as matte or metallic droplets. Slag generated in a smelting furnace often has a copper content of 1–2 wt% [
1], which can be partly recovered by slag-cleaning operations. However, the copper content in discarded slag is still often higher than that of sulfidic copper ores. Thus, the investigation of copper losses to slag can help to reduce these losses and is, therefore, a task of high practical significance. Challenges during slag-cleaning operations involving a settling furnace include predicting the settling time of entrained copper and the optimal conditions for the maximum settling rate.
At the Boliden Rönnskär smelter in Sweden, sulfidic copper concentrates are either roasted and smelted in an electric smelting furnace (ESF) or smelted in an Outokumpu flash furnace. The ESF is also charged with secondary material, flux, and converter slag returns. The slag generated by the ESF is transferred to a zinc fuming furnace (ZFF), which is mainly used for the extraction of zinc through its reduction with pulverized coal in the presence of air [
2]. The fumed slag is tapped into an electric settling furnace where the entrained copper, in the form of matte and speiss, can separate under the action of gravity. Speiss is a metalloid containing copper, arsenic, antimony, tin, and nickel. Subsequently, the settled copper phases are tapped from the settling furnace and thus recovered.
Already, several factors have been suggested to influence the content of the entrained copper in slag, with most of the studies being experiments with synthetic slag with an Fe/SiO
2 (wt%/wt%) ratio that exceeds or equals 1.2. The copper content in slag is dependent on external factors such as tapping, charging, and turbulence caused by e.g., gas injections. Minto and Davenport claimed that copper entrainment is partly caused by the production of SO
2 bubbles, where matte can lay as a surface film on the bubble when it rises through the slag [
3]. The same phenomena, of copper attached to gas bubbles, have been observed in several studies [
4,
5,
6,
7]. In other words, matte can be carried through the matte–slag interface and become dispersed as matte droplets when the surface film ruptures.
The entrainment of copper can also be caused by the precipitation of copper due to decreased solubility in slag [
8]. Precipitated metallic copper becomes dispersed as fine droplets that take a long time to settle, which limits copper recovery. Dissolved copper can also be reduced into liquid metallic copper upon slag reduction, which could be a part of slag-cleaning. A drawback of the presence of metallic copper in slag is that it tends to retain impurities such as arsenic, antimony, and bismuth from the slag [
9,
10]. Dissolved copper could also precipitate upon slag quenching [
11,
12], which makes it complex to determine if the copper in solidified slag originated as dissolved or entrained copper in the molten slag.
Chemically dissolved copper associates with oxide (O
2−) or sulfide (S
2−) and is distributed within the slag matrix [
13]. According to Zhao et al., copper cations can dissolve in fayalite [
14], which is often the major phase of copper slag. Solubility depends on matte grade [
15,
16,
17,
18], oxygen partial pressure [
19,
20], slag composition [
21,
22], and temperature. Currently, the correlation between copper solubility and temperature is under debate. Mackey has stated that oxidic copper solubility in general increases with an increase in the partial pressure of oxygen, and temperature [
13]. In a thermodynamic assessment by Shishin et al., it was shown that the copper content in slag (Cu-Fe-O-S-Si system) is at its maximum at intermediate matte grades, then falls slightly and rises again when approaching 80 wt% copper in the matte. The curve of the copper content in the slag at different matte grades was shifted towards a higher slag copper content when the temperature was increased [
15].
However, Toguri and Santander investigated the behavior of a copper–gold alloy in a silica-saturated fayalite slag, suggesting that the copper solubility decreases with increasing temperatures [
23]. Zivkovic et al. claimed that the slag copper content increased with decreasing temperatures in a reverberatory furnace because of the concomitantly increasing slag viscosity and the long stratification time within the furnace [
6]. The lack of agreement on the correlation between copper solubility and temperature probably reflects the difficulty of determining the state of copper (i.e., chemically dissolved, mechanically entrained, or both) in slag. Moreover, temperature is known to affect settling behavior as it, e.g., affects viscosity [
24,
25,
26] and the appearance of solid phases.
Settling processes can also be hindered by the attachment of copper to solid phases, as suggested in experimental studies by Bellemans et al. and De Wilde et al. [
27,
28,
29,
30]. Copper attached to spinels has a lower density than the underlying copper phase and thus cannot settle completely [
29]. The copper spinel entity could instead accumulate at the bottom of the slag layer above the settled copper phases.
Sukhomlinov et al. studied chromium solubility in iron silicate slags, suggesting that a chromite spinel could precipitate when the chromium content exceeded the solubility [
31]. The chromite spinel phase is denser compared to the slag phase, which leads to the accumulation of chromium in the furnace. According to Lennartsson et al., the ESF at the Boliden Rönnskär smelter has a bottom buildup comprising of spinel, matte, olivine, and metalloids, where the dominating phase was identified as spinel [
32]. Spinels are thus a possible solid phase in the ESF slag, which could hinder copper from settling. Currently, existing literature lacks information on the settling of copper in ESF slags (with an Fe/SiO
2 ratio of close to equal) that is treated in a ZFF, and then tapped into a settling furnace. It is thereby of interest to investigate how the mentioned phenomena influence settling and slag copper content on an industrial scale.
Given the above, a project was initiated to obtain fundamental insights into the mechanism of settling in slag. An industrial trial was conducted in the settling furnace at the Boliden Rönnskär smelter to increase the knowledge of the slag system and to determine the effects of temperature and settling time on slag copper content. The slag was characterized to determine the appearance of copper and its associated phases. Increased settling rates result in higher copper recovery, higher profits, and the increased efficiency of raw material usage.