1. Introduction
Copper losses to discarded slag are one limitation of the copper recovery in pyrometallurgical extraction of this metal. Copper is in the form of chemically dissolved and mechanically entrained copper in the molten slag phase [
1,
2]. The mechanically entrained copper droplets can be recovered via slag cleaning, where the entrained droplets separate from the slag under the action of gravity. At the Boliden Rönnskär smelter in the north of Sweden, one of the slag cleaning routes includes a fuming furnace followed by an electric settling furnace. The settling rate of the entrained copper-containing droplets is dependent on slag properties, which can be modified for increased settling rate, which would result in decreased copper losses to slag and enhance the recovery. Therefore, the investigation of industrial slag modification and the influence on copper losses is a task of high practical significance.
The slag in the settling furnace at the Boliden Rönnskär smelter originates from a sulfidic copper concentrate smelted in an electric smelting furnace (ESF) together with fluxes, slag returns, and secondary material. The generated slag is sent to a fuming furnace for zinc extraction by reduction and vaporization, described in more detail by Lotfian et al. [
3]. The remaining slag is tapped to the settling furnace, where entrained copper phases can separate from the slag under the action of gravity. The mechanically entrained copper is in the form of matte (copper-iron-sulfide) and speiss (rich in copper and also contains arsenic, antimony, tin, and nickel) [
4]. The settled copper is tapped from the settling furnace and can be recycled to the smelter. The slag remaining after a given settling time in the furnace is water granulated [
5] at a production rate of ~250,000 tons per year, and commonly used as construction material and for sandblasting, for example. The utilization of the granulated slag enhances the circular economy for the smelter and decreases the extent of landfilling and the use of natural resources.
Slag generated in the smelting stage during pyrometallurgical production of copper often has the main components iron oxide and silica (FeO and SiO
2), and smaller amounts of lime, alumina, and magnesia (CaO, Al
2O
3, and MgO) as well as some minor elements (antimony, lead, bismuth, tin, and arsenic) [
6,
7,
8,
9,
10]. The copper smelting slag is often referred to as fayalite slag (Fe
2SiO
4), and usually has a copper content of about 1–2 wt % [
6]. The chemical composition of the slag influences the slag properties, which in turn, influences the settling of mechanically entrained copper in the slag. These slag properties are viscosity, density, interfacial tension, and melting temperature [
11].
The natural separation between the copper-containing phases and the molten slag phase is due to the density difference between the phases. The settling rate of entrained droplets in a molten slag can be described according to Stokes law (Equation (1)), where increasing density difference between the phases or decreased viscosity results in an increased settling rate [
12]. The settling rate
(m s
−1) is a function of gravitational acceleration
g (m s
−2), the copper droplet diameter
d (m), the density of the copper droplet
(kg m
−3), the slag density
(kg m
−3) and the fluid dynamic viscosity
μ (kg m
−1s
−1).
A possible way to decrease the copper losses is to modify the slag to enhance the essential properties for an increased settling rate. In the literature, the effect of CaO on the slag properties and the slag copper content has mainly been studied in experimental trials and not in slag cleaning routes. The effect of CaO addition on the viscosity has been examined with similar results, decreasing with the CaO addition [
13,
14,
15,
16,
17]. The influence of CaO on the viscosity was most significant in the temperature range between 1200 and 1250 °C [
13]. The theory behind the decreased viscosity of the molten slag is that the metal oxide CaO breaks the silicate polyions into smaller structural units, which thus decreases the slag viscosity. The viscosity of slags with the Fe/SiO
2 (wt %/wt %) ratio above 1.2 decreased with increasing ratio, which was explained by the depolymerization of the three-dimensional silicate network in the slag [
13,
14]. Aluminate is another possible network forming cation in slag, which increases the degree of polymerization and thus the slag viscosity [
18]. In a study by Park et al., the viscosity in an iron silicate slag first decreased with the addition of Al
2O
3 up to 5 wt %. However, with higher additions, the slag viscosity increased. The effect was suggested to be related to the amphoteric behavior of Al
2O
3, acting as a basic oxide at lower additions and as an acidic oxide at higher additions. The effect could also have been due to the modification of the liquidus temperature [
19]. CaO, MgO, and Al
2O
3 are minor oxides occurring in the ESF slag at the Boliden Rönnskär smelter.
The viscosity also correlates with the temperature, and in general, the viscosity in slag decreases with increasing temperature [
19,
20,
21]. According to Chen et al., the viscosities of iron silicate slags are more sensitive to changes in temperature at higher SiO
2 concentrations [
22]. The slag in the settling furnace at the Boliden Rönnskär smelter has a Fe/SiO
2 ratio of close to unity, indicating that the viscosity of the slag could be relatively high and sensitive to temperature changes.
The slag viscosity is also affected by solid phases, where the viscous behavior of liquid melts containing solid phases increases linearly with increasing content of solids [
23]. Sukhomlinow et al. studied the chromium solubility in Al
2O
3-CaO-FeO-SiO
2 slags as a function of oxygen partial pressure (P
O2) and the effect of Al
2O
3 and CaO. The results showed that the chromium solubility was ~0.15 wt % when the P
O2 ≈ 10
−5 and increased to 0.30 wt % when the P
O2 ≈ 10
−10. The addition of CaO and Al
2O
3 slightly increased the chromium solubility in the slag, independent of the oxygen partial pressure. At chromium contents above the chromium solubility, chromium precipitated as a ferrous chromite spinel with some dissolved alumina [
24]. The ferrous chromite spinel phase is denser compared to the slag phase, which leads to the accumulation of chromium in the furnace vessel. The increased viscosity due to solid phases is an additional reason for controlling the presence of solid phases in the liquid slag.
As mentioned, other slag properties that influence the settling are density and interfacial tension. In previous studies, a rapid separation and settling of matte droplets have been observed within the first 10–15 min due to the density difference between the copper and slag phases [
25,
26]. The separation rate then decreased as other physical properties, such as viscosity or surface tension, became rate-determining, and the matte phase needs to form big enough droplets to settle. Zhang et al. claimed that the density of iron-rich slags decreases with the addition of CaO [
15]. The surface tension of slag is also influenced by the CaO modification, increasing with increasing CaO content [
15,
27]. The tendency for coalescence should increase with increasing interfacial tension between the copper-containing phase and slag [
28], thus increasing the size of the copper droplets, which, in turn, increases the settling rate.
Another enhancement from the addition of CaO to iron silicate slag is the copper solubility in slag, which decreases with the addition [
29,
30]. The decreased solubility is explained by the acid-base theory of slags, where, e.g., Ca
2+ ions in CaO replace some of the Cu
+ ions within the silicate structure of the slag [
29]. The effect enhances the settling as more copper becomes entrained instead of dissolved in the slag.
In an earlier study in the settling furnace, the slag copper content was concluded to increase with increasing temperature within a given interval [
4], which could be due to increased copper solubility at higher temperatures [
10,
31,
32]. The final slag copper content was also suggested to be more strongly correlated to the temperature compared to the settling time in the furnace. The CaO modification of slag enhances the properties that improve the settling of copper-containing droplets in slag, without increasing the copper solubility. CaO is thus a possible additive to the industrial slag cleaning processes of pyrometallurgical copper extraction. In the literature, there is limited information about the effect of CaO modification on the final slag copper content in an industrial slag cleaning system where the slag is treated and modified in a fuming furnace prior to the settling process. In the present work, the term ‘slag’ refers to an iron silicate phase with minor oxides including e.g., Al
2O
3, CaO, and MgO. In addition, the slag contains inclusions of matte and speiss droplets and other oxide phases. A slag characterization and the copper-containing phases can be found in a previous study of the slag system without the modification of CaO [
4]. The aim of the present work was to examine if industrial CaO modification can decrease the final copper content of the slag, after treatment in a fuming furnace and then in an electric settling furnace.
3. Results and Discussion
The compositions of the average ingoing slag with different CaO content are presented in
Table 2, where the CaO content increases in the following order C1, C2, C4, C3, and C5. The six analysis (two replicates of three samples) of the ingoing slag copper content was statistically evaluated using a box plot, and two values were excluded, one from C1 and one from C4. The box plot can be seen in
Figure 3a, where the two outliers are marked as grey dots. An average was calculated for the remaining values of the copper content, which is presented in
Figure 3b, together with the error bars, which equals ± one standard deviation (st.d). The average ingoing copper content is independent of the CaCO
3 addition as the copper content in the slag originates from the ESF slag. The average ingoing copper content was lowest in C1 and highest in C5, with the corresponding copper content of 1.2 wt % and 1.7 wt %.
Table 3 presents the composition of the (average) outgoing slag and the granulated slag. The Fe/SiO
2 ratio (wt %/wt %) was based on the compositions of the granulated slag (from
Table 3) and varied from 0.79–0.97, highest in C1 and lowest in C5. The average CaO content in the outgoing slag increased from 3.1 wt % in C1 to 17 wt % in C5. The CaO content of the outgoing slag differs from that of the ingoing slag, which was ascribed to a dilution effect when the slag remaining from the previous batch was mixed with new slag tapped into the settling furnace.
Figure 4a presents a box plot of the copper content in all samples of the outgoing slag in batch C1–C5. Two values are marked as outliers (grey dots) in the plot, these values were excluded from the calculated average copper content in the batches (presented in
Table 3).
Figure 4b presents the copper content in the outgoing and granulated slag plotted against the CaO content. The slag copper content decreased with increasing CaO content in the granulated slag from 0.77 wt % in C1 to 0.57 wt % in C4 and C5. For the outgoing slag, the copper content is highest in C1 (0.64 wt %) and lowest for C2, C4, and C5 (0.55 wt %). The settling temperature was about the same (1247–1252 °C,
Table 1) for all batches except for C4, which had a temperature of 1223 °C. No correlation was observed between the outgoing slag copper content and the temperature.
The ingoing copper content differed between the batches (
Figure 3), highest in C5 and lowest in C1. This means that the copper content was not diluted with the CaO modification. Higher ingoing slag copper content means that a larger quantity of copper needs to settle to achieve the same final slag copper content, compared to a batch with lower ingoing slag copper content.
Figure 5 presents the effect of the CaO content on the copper recovery in the settling furnace, which was calculated according to Equation (2). The copper recovery was lowest in C1, without CaO modification, and highest in C5, which had the highest content of CaO. The most significant decrease in slag copper content was when the CaO content increased from 3.1 wt % to 7.2 wt %, with the corresponding copper recovery of 42% and 59%, respectively (C1 and C2). Batch C5, had the highest recovery of 66%. A contributing factor to the increased recovery between C4 and C5 is the ingoing copper content, which was 1.6 wt % in C4 and 1.7 wt % in C5 (
Figure 3), the outgoing slag copper content was equal in the two batches.
Entrained copper droplets of the same size have equal settling velocity independent of the total slag copper content. This means that if a batch has a higher ingoing copper content, with droplet sizes within a range that they will have time to settle under the given settling time, a higher amount of copper will be distributed to the underlying copper phase. In this work, the size distribution of copper-containing phases is unknown and thus both the copper recovery and the final copper content in the slag must be evaluated to see the effect of slag modification. The final slag copper content in the outgoing and granulated slag and the copper recovery indicate that the settling of copper phases was more efficient with the CaO modification of the slag.
The content of Cu, CaO, and Cr
2O
3 in the vertical slag samples from batches C1–C5 are shown in
Table 4. The other slag components were about equal at all levels and similar to the outgoing and granulated slag (
Table 3). No slag sample was analyzed from the second-highest level in batch C2 (start of the settling) due to a too-small sample amount. The vertical slag sample from level 1 is the bottom-most slag layer, level 2 second-lowest, and level 4 or 5 corresponds to the second-highest slag level in the furnace during sampling.
Figure 6a presents the effect of CaO modification on the copper content in vertical slag samples at the settling end. The copper content at levels 3 and 2 decreased with the CaO content up to ~11 wt % (from 0.73 wt % (C1) to 0.51 wt % (C4) at level 2). The copper content at level 2 in C3 deviates and is higher compared to the copper content in C4, which has approximately the same CaO content. The upper levels (level 3 and 4) of the vertical slag samples from C3 had a higher copper content at the settling start compared to C4, which affects the copper content in the lower levels. The copper content in the top layers influences the content in the lower levels as the entrained copper droplets descend through the slag until they reach the underlying copper phases. The copper content in the highest level analyzed (settling end) was lowest in C3, with a value of 0.56 wt % and a CaO content of 11 wt %. The decreasing copper content indicates that the settling of copper-containing phases becomes more efficient with the CaO modification up to ~11 wt %.
Figure 6b presents the average copper content (levels 5/4, 3, and 2) at the settling end together with the corresponding standard deviation, plotted against the average CaO content in each batch. The average copper content in the level samples shows the same trend as the copper content in the outgoing and granulated slag, which decreases with increasing CaO content. The average copper content in the level samples decreased from 0.74 wt % in C1 to 0.56 wt % in C4. The average copper content of each batch can be found in
Table 4.
The reviewed literature revealed that the CaO modification decreases the viscosity of the slag [
13,
14,
15,
16,
17]. A decreased viscosity increases the settling rate of entrained droplets according to Stokes law (Equation (1)) [
22]. This is consistent with the observations in the evaluated industrial trial, where the final slag copper content decreased and copper recovery increased with the CaO modification. In theory, a decreased slag viscosity enhances the settling rate of the mechanically entrained copper droplets as the internal friction of the slag decreases [
23]. In most of the literature, where the viscosity is investigated in iron silicate slags, the Fe/SiO
2 ratio is 1.2 or higher. Kaiura suggested that the viscosity decreased as the composition moved away from silica saturation, which was explained by the depolymerization of the three-dimensional silicate network [
13]. The slag from the ESF at the Boliden Rönnskär smelter has a Fe/SiO
2 ratio close to unity, meaning that the slag is rich in silica. The relatively low Fe/SiO
2 ratio in the slag in the settling furnace indicates that the viscosity is rather high without the modification with CaO. The effect on the copper content is not as clear at CaO contents above ~11 wt % in the vertical slag samples, which is consistent with the copper content in the outgoing and granulated slag. The decreasing effect of CaO on the copper content is due to changed slag properties or because of the ingoing copper content, which increased from C1 to C5.
Figure 7 presents the chromium oxide content for all vertical slag samples (settling end). The vertical slag level (
y-axis) represents the position in the slag layer, the highest level analyzed is level 4 or 5 and the bottom-most slag layer is level 1. The figure shows that the chromium content increases towards the bottom of the slag layer which contacts the bottom buildup and consists of a chromium spinel, which attaches to copper droplets [
4]. The high chromium oxide content in the bottom-most samples indicates that part of the bottom buildup is present in the sample. Level-1 samples were thus excluded from
Figure 6a,b due to the possible presence of the chromium spinel, which influences the copper content. The chromium oxide content in the outgoing slag was similar to the content at levels 2, 3, and level 5/4 in the vertical slag samples.
The correlation between the bottom buildup, in the form of a chromium-rich spinel, and the slag copper content was observed in an earlier study, where copper droplets were attached to the spinel at the bottom of the slag layer [
4]. The attachment of copper to a solid phase hinders the settling as the copper-solid entity has a lower density compared to the underlying copper phase, instead, it settles to the bottom slag layer where it accumulates. When the chromium content reaches above the chromium solubility in the slag chromium could precipitate as a spinel [
24]. Spinels are possible solids in the slag layer, which increases the slag viscosity [
23], and thus decreases the settling rate.
A drawback with the slag modification is the increased slag amount and cost of raw materials. However, if the slag modification does not affect the process in a negative way or the usage of the granulated slag, these disadvantages can be disregarded after the economic perspective is evaluated. The effect of the CaO modification on the slag properties influencing the settling in a slag system similar to the one in the present work (Fe/SiO2 ratio close to unity) is a task of significant importance. An investigation to find the optimum CaO content for decreased slag viscosity needs to be evaluated experimentally and can thereby be applied for enhanced settling rate and thus copper recovery.