Summary of the phase analysis using SEM

In order to further analyse the mechanism of overall reduction reaction, shown in equation 1.5a, systematic analysis using SEM and EDX techniques were undertaken. In figure 6.9, the area analysed for the reaction product after 2.5 hours of reaction at 1073 K is compared with the elemental distribution map, obtained via the EDX spectroscopic technique. The elemental maps of Fe and O show a ring of iron oxide around the copper-sulphide rich phase, which explains that the ions of iron and copper diffuse towards the surface and centre, respectively, and this mechanism also takes place during oxidation/roasting of Cu-Fe sulphide minerals (CuFeS2 and Cu5FeS4) [14, 28, 85]. It is

caused by the higher stability of FeO than Cu2O [14] so that FeS is preferentially

oxidised. The brightest phase in image B of figure 6.9 is metallic copper. Part of copper was formed at the interface between the Cu2S and Fe3O4 phases and this is because of

the equilibrium between the Cu2S + Fe3O4 and Cu + Fe3O4 phase fields at

log10P(O2)(atm) < - 17.9, at 1073 (figure 6.10). It can be observed from image B in

figure 6.9 that all the smaller Cu2S particles (< 5 µm) were reduced to metallic copper

due to the fact that the smaller particles have larger reacting surface area. The area analysed in figure 6.9 is devoid of cobalt, since the Nchanga concentrates contain the least amount of cobalt (0.4 wt. % of Co).

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Figure 6.9 – Elemental mapping for Nchanga sample after reduction at 1073 K, molar ratio of MS:CaO:C = 1:1.5:1.5, (A) low magnification image, the analysed area is

highlighted by the red box and (B) is the analysed area under BSE imaging

Figure 6.10 – The Fe-Cu-O-S predominance area diagram at 1073 K in relation to the phases obtained in figure 6.9, computed using the Factsage software 6.1 [24]

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The SEM images for the Baluba sample after carbothermic reduction at 1073 K are presented in figures 6.11a and 6.11b. The FeS2 particles in figure 6.11a were completely

converted to Fe-O which was identified as FeO and Fe3O4 in the X-ray diffraction

patterns in figure 6.2a. On the other hand, metallisation of cobalt occurred at 1073 K as shown in figure 6.11b. As noted in figure 6.11b, Co co-exists with Cu2S and Fe3O4 and

this agrees very well with the thermodynamic prediction in figure 6.12. The cobalt peaks did not appear in the X-ray diffraction patterns because cobalt is present in small concentration (< 2 wt. %), which is below the limit for XRD detection. SEM analysis for the Nkana sample at 1073 K also showed that the Co-S particles were reduced to Co at 1073 K. From the SEM analysis, it can be concluded that preferential metallisation of cobalt occurred at 1073 K but the metallisation of copper and iron were limited by the exchange and reduction reactions, respectively. Therefore, there is very good agreement between the experimental results and the thermodynamic prediction in figure 2.3a where, the Gibbs energy change is more negative for metallisation of cobalt than for iron and copper.

Figure 6.11 – Backscattered SEM images of the Baluba sample, after reduction at 1073 K and molar ratio of MS:CaO:C = 1:2:2. Argon flow rate = 0.6 litre min-1

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Figure 6.12 – The Fe-Cu-O-S predominance area diagram at 1073 K in relation to the phases obtained in figure6.11, computed using the Factsage software 6.1 [24]

The elemental maps for the Nchanga and Nkana samples which were reacted for 2.5 hours at 1273 K for molar ratio of MS:CaO:C = 1:1.5:1.5, are shown in figures 6.13 and 6.14, respectively. Since the Nchanga sulphide concentrates have the highest copper content, the analysed area in image A of figure 6.13 contains a lot of copper particles which were reduced from the Cu2S mineral particles. The reduced copper particles have

various sizes and shapes which are similar to the mineral sulphide, in the as-received mineral concentrates. On the other hand, the Nkana concentrates are rich in iron such that the analysed area in figure 6.14 contains a lot of iron particles. Furthermore, the analysed area in figure 6.14 also contains significant amount of cobalt which is associated with iron as a result of high solid solubility between iron and cobalt. The shape of the reduced Fe or Fe-Co particles appear different from their respective mineral sulphides due to dominance of the liquid phase during reduction of (Co)FeS2

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Figure 6.13 – Elemental mapping for Nchanga sample after carbothermic reduction at 1273 K for molar ratio of MS:CaO:C = 1:2:2, (A) is the sample area analysed from

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Figure 6.14 – Elemental mapping for Nkana sample after reduction at 1273 K, molar ratio of MS:CaO:C = 1:1.5:1.5, the analysed area is highlighted by the yellow box in (A)

and (B) is the analysed area under BSE imaging. Argon flow rate = 0.6 litre min-1

SEM-EDX semi-quantitative analysis was carried out in order to determine the compositions locally, a summary of the composition from figure 6.15 is presented in table 6.1. It can be observed in table 6.1 that a number of metallic phases had about 97 wt. % Cu in the Nchanga sample meaning that they were reduced from the Cu2S

mineral. It is worth noting that the purity of the copper phase is similar to that of blister copper [8, 10, 15], produced after conventional smelting and converting processes at temperatures above 1473 K. Most metallic phases in the Nkana and Baluba reduced samples had about 96 wt. % Fe meaning that they were reduced from the FeS2 mineral

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particles. The purity of iron produced by this process is much higher than that of the pig iron which contain about 93 wt. % Fe, produced by smelting of iron oxide above 1773 K [105]. It can be observed in table 6.1 that a metallic phase with 92 wt. % Co was produced in the Nkana sample at 1173 K. The semi-quantitative SEM-EDX analysis in table 6.1 further shows that the Co-Fe alloys were produced during reduction of the mineral sulphide concentrates, as observed from the elemental maping in figure 6.14.

Figure 6.15 – Backscattered SEM images for the reduced samples, the compositions are given in table 6.1. Argon flow rate = 0.6 litre min-1

Table 6.1 – SEM-EDX semi-quantitative results (wt. %) obtained from figure 6.1

Cu Co Fe O

(a) - Nchanga at 1273 K – spectrum 1 97 - 1 2 (b) - Nchanga at 1273 K – spectrum 2 97 - 2 1 (c) - Nkana at 1273 K – spectrum 2 3 - 97

(d) - Nkana at 1273 K – spectrum 1 7 11 82 (e) - Nchanga at 1273 K – spectrum 2 4 11 85

(f) - Nkana at 1173 K – spectrum 1 3 92 3 2 Baluba at 1273 K (image not shown) 4 - 96

Baluba at 1273 K (image not shown) 22 72 2 3