1. Introduction
Slags play a vital role in pyrometallurgical processes, such as in the production, recycling and refining of liquid metal. The chemistry and structure of a slag controls the removal of impurities from the liquid metal to the slag, and vice versa—the partitioning of the selected valuable elements to metal phase from the slag [
1,
2]. The slag chemistry also influences both the thermophysical and thermodynamic properties, such as density, viscosity, electrical conductivity, foaming index, thermal conductivity, surface tension, partition ratio, molar entropy, diffusivity and mixing free energy of silicates [
3,
4,
5]. Successful operations of metal productions are dependent on these properties. Therefore, a clear understanding of the relations between the slags’ chemistry, structure and their properties is vital for designing suitable slags for the appropriate process conditions.
Urban ores are the waste materials that are sources of many base metals and other valuable elements. Urban ores consist of electronic waste (E-waste), low grade copper tailings, industrial sludge, spent catalysts and other low-grade scraps in metallic and oxidized states. In the context of processing of urban ores (such as E-waste, which contains Au, Ag, Pt, In, Ru, etc.) through black copper smelting, understanding the inter-relation between the slag’s chemistry, structure and properties is important for maximizing the recovery of the valuable elements into the copper phase.
Metallurgical slags are composed of different oxide components that can be acidic (e.g., silicate), basic (e.g., calcia) or amphoteric (e.g., alumina, that can behave as both acid and base depending on the conditions). Generally, acidic oxides are network formers; their addition increases the number of bridging oxygen (O
0) in silicate melts. Basic oxides, on the other hand, are network breakers that produce free oxygen ions (O
2−) and non-bridging oxygens (O
−) at the expense of bridging oxygen (O
0) in the silicate melt. This can be expressed in Equation (1) [
6]:
Silicate units in the melt are formed by the polymerization of [SiO
4]
4− tetrahedrons. Virgo et al. [
7] identified different anionic structural units according to the number of non-bridging oxygens (NBO) present in the tetrahedral units from Raman and FTIR spectra deconvolution. They expressed different anionic units using Q
n notation, where n is the number of bridging oxygens. The Q
0 ([SiO
4]
4−), Q
1 ([Si
2O
7]
6−), Q
2 ([SiO
3]
2−), Q
3 ([Si
2O
5]
2−) and Q
4 (SiO
2) are, respectively, the monomer, dimer, chain/ring and 3D-network of silicate tetrahedrons formed by the increasing number of bridging oxygens at the corner of the silicate tetrahedral units. The term degree of polymerization (DOP), which corresponds to the fraction of highly polymerized structural unit Q
4 in the silicates, has been commonly used and is measured by the parameter Q
3/Q
2 [
4]. Another structural parameter usually used to represent the silicate structure is non-bridging oxygen per tetrahedral units (NBO/T), which is calculated from the relative abundance of different structural units, as shown in Equations (2) and (3):
There have been many studies carried out investigating the structure of silicate melts in metallurgical applications; however, the focus has been on slags with compositions relevant to the ironmaking and steelmaking applications, and there are no published studies that focus on copper-making slags.
Previous research suggests that printed circuit boards (PCBs) contain as much as 220 ppm of Pd [
8], which is equivalent to USD 13,747 per ton of circuit boards [
9]. As PCBs are rich in copper and other valuable metals, secondary copper smelters, including Boliden Ronnskar, Umicore, Dowa mining, L.S. Nikko and others, use PCBs as the raw materials in the process. The inclusion of PdO in the slag is expected during the smelting process, as PCBs contain Pd. A thorough study on the effect of process parameters and slag compositions on the partitioning of Pd in the process is necessary to ensure maximum recovery of the metals. Moreover, the effect of PdO on the structure of slag is also important, as the thermophysical and thermodynamic properties of the slags are related to slag structures [
10].
Traditionally, iron silicate (FeO
x-SiO
2) slags are used for copper smelting processes, but this slag system, if not properly controlled, has the problem associated with magnetite precipitation, which is not good for the operation [
11]. This slag system generally exhibits a high viscosity and possesses the risk of formation of slag foaming during the operation. A different slag system of calcium ferrite (CaO-FeO
x) was first introduced for continuous copper processing by Mitsubishi. This highly basic slag overcomes the limitation of the iron silicate slag, but it aggressively attacks the refractories, resulting in significant wear-off the furnace linings. Currently, FeO
x-CaO-SiO
2 (FCS) slags with typical compositions of 45–65 wt.% FeO
x, 3–10 wt.% CaO and 30–45 wt.% SiO
2 are used for industrial copper smelting, which has intermediary basicity to that of iron silicate and calcium ferrite slags and overcomes the limitation of these two slags [
12,
13]. Magnesia-chrome refractory lining is used as a wear protection layer in copper production furnaces. The presence of MgO in the FCS is considered to simulate a situation in the industrial copper processing where dissolution of the MgO-composite refractory into the slag occurs. Therefore, the study of the structure of FCS-MgO (FCSM) slags and its effect on the partitioning of the valuable elements is necessary.
In the current work, the structures of FCSM and FCS-MgO-Cu2O-PdO (FCSM-Cu2O-PdO) slags relevant to black copper smelting were studied. The effects of the slag composition, partial pressure of oxygen (pO2) and temperature on DOP of the FCS-based slags were investigated. Improved semi-empirical relationships between the process parameters (chemistry, temperature and partial pressure of oxygen) and DOP of the melt were developed, including correlations that include the partitioning ratio of Pd and Ge.
2. Previous Studies on Structures of Copper-Making Slags
Kaur et al. [
14,
15] investigated the wear of a magnesia-chrome furnace lining by different copper smelting slags at high temperatures and showed that the FCS slag is less corrosive to the refractories. They also investigated the partition ratios of Ni, Pb and Sb between the slag and copper and reported similar values compared to that of calcium ferrite slag. A number of researchers recently investigated the thermodynamics and partition of various valuable elements between liquid copper and liquid slag in the context of processing of electronic waste through secondary copper smelting (black copper smelting); i.e., Bi [
16], Sn and In [
17,
18,
19,
20]; Ga [
20]; and Ge, Ta and Pd [
20,
21,
22]. It has been reported that there are only limited partition ratio data available in the open literature, particularly that are relevant to secondary copper (black copper) smelting operations [
23]. There is a limited work published in the open literature on the investigation of the interrelationships between composition, structure and properties of MgO-containing FCS slags relevant to primary and secondary copper-making.
In the case of ironmaking and steelmaking slags, Jiao et al. [
24] investigated the correlation between the electrical conductivity and slag composition at different temperatures in the SiO
2-CaO-MnO-MgO slags. They concluded that the addition of basic oxides, such as CaO, MgO, FeO and MnO, increases the electrical conductivity, whereas addition of acidic oxides, such as SiO
2 and P
2O
5, gives a decreasing effect. Lee et al. [
25] studied the effect of FeO and MgO on the viscosity of CaO-SiO
2-Al
2O
3 (10–13 wt.%), MgO (5–10 wt.%) and FeO (0–20 wt.%) slags. They explained the decrease of viscosity with increasing amount of FeO and MgO by the increasing of slag depolymerisation. Kim et al. [
26] observed a decreasing trend of a foaming index with addition of FeO (up to 20 wt.%) in the CaO-SiO
2-FeO-Al
2O
3 slag; however, addition of FeO beyond this did not decrease the foaming index any further. The effect of Al
2O
3 on foaming index was not clearly revealed in the study.
In the above-mentioned works, direct measurements of the structure of the slags were not carried out. In other study, McMillan [
27] prepared CaO-MgO-SiO
2 melts using solar furnace technique and measured the distribution of different silicate structural units in the melts. McMillan observed the decrease of polymerization of the melts as the silica content was decreased and reported that the Mg substitution of Ca reduced the Q
3/Q
2. Brandriss and Stebbins [
28] investigated the effect of temperature on the structure of slags CaO (25 wt.%), MgO (25 wt.%), SiO
2 (50 wt.%) and other silicate systems. They concluded that the increase of temperature changes the Q
n speciation and this substantially accounts for the difference of the total enthalpy of the glass and crystal. Cooney et al. [
29] also observed and reported a similar network breaking action of Mg
2+ and Ca
2+ ions. Sohn and Min [
30] collated the viscosity data of the CaO-SiO
2-Al
2O
3-MgO systems and showed that the addition of MgO reduces the viscosity of the melt. The general role of MgO is to break the silicate networks; however, in highly basic slag the behavior of MgO can deviate. Mg
2+ may behave as a charge-balancing ion if the slag contains a significant amount of Al
2O
3. The structure and viscosity of SiO
2-CaO-MgO-Al
2O
3 (9 wt.%) were studied by Gao et al. [
31]. They reported the transformation of the slag to a simpler structure with increasing basicity or MgO, and therefore, the viscosity and the activation energy of viscous flow also decreased.
Very limited studies have been carried out correlating the slag structure with the thermodynamic parameters in the relevant slag systems. Park [
4] investigated the relationship between sulfide capacity and structure of CaO-SiO
2-MnO and CaO-SiO
2-MgO slags and obtained a relationship between sulfide capacity and non-bridging oxygen. In that research, it was revealed that the excess free energy of oxides in slag (CaO, MgO, MnO) is strongly dependent on the abundance of silicon anionic units, which was also elucidated further with ionization potential (
Z/
r2) of the metal ions. In another study, Park et al. [
32] reported an increase of depolymerization of the CaF
2-CaO-SiO
2-FeO
x slag system with increasing FeO
x (0 to 21.5 wt.%). In this basic slag, FeO
x acted as a polymerizing agent of the silicate slag. They also reported an increase in depolymerization due to the reaction of hydrogen with bridged oxygen in acidic slag. Hydrogen produces free hydroxyls by reacting with free oxygens and increases the DOP in highly basic slags.
Recently, the author’s group studied the relation of slag structure with thermodynamics of Ge at black copper smelting conditions [
33]. A semi-empirical equation relating thermodynamics (partition ratio of Ge in slag and copper,
) with slag structure (Q
3/Q
2) and process parameters (temperature and partial pressure of oxygen) was developed. In the case of Pd, there are only limited studies which focus only on the Pd partition between slag and matte under primary copper production conditions (matte-slag system) [
34,
35,
36]; and no published works on the effect of slag structure on Pd-partition (
). Recently, a conference publication by the authors who wrote [
37] explained some observations of the effects of Fe/SiO
2 ratio and basicity on the structure of Pd and Ge-containing slags. In this current work, an improved deconvolution approach for analyzing the FTIR data was adopted using more complete and comprehensive data. This resulted in improved semi empirical equations that relate partition ratio of Ge and Pd in the slag and copper with the slag structure and process parameters.
In summary, there have been previous studies in the slag system FeOx-CaO-SiO2-MO, as described above. Nevertheless, there is still lack of information on the structures of FCS slags, particularly regarding slag composition and conditions applicable to secondary (black) copper smelting practice (high concentration of FeOx and SiO2, and at low oxygen potential) for recycling of valuable metals from urban ores, such as E-waste.