Liquid phase formation during reduction process

The formation of the liquid phase is very important since it enhances the reaction kinetics, because of the faster diffusion of cations and anions in the liquid than in the solid phase. The formation of liquid phase was investigated by using different mole ratios of lime and carbon and, different types of carbon (carbon black or graphite). From the experimental work carried out, the formation of the liquid phase appears to be as a result of; (i) melting of the mineral sulphide particles e.g. CuFeS2 melts at 1223 K

[171], so that a molten mineral sulphide phase reacts with CaO and C/CO gas and (ii) reaction between mineral sulphide particles and CaO, causing liquid matte phase (MO + MS) or eutectic melts as reported by Jha [32, 35, 172, 177, 178].

Chalcocite (Cu2S) is a very stable mineral sulphide with a melting temperature

of 1403 K but a liquid phase was observed during its reduction. In the earlier work by Jha, during the carbothermic reduction of Cu2S in the presence CaO, a liquid phase

belonging to the Cu-O-S system was suggested to be due to the binary eutectic at 1113 K, involving Cu2S and Cu2O [35, 172]. However, the Cu-O-S liquid phase may not

exist if the rate of reduction reaction is faster than the ion exchange reaction, because the Cu2O phase would be immediately reduced to Cu. In the present study, the Cu-Ca-

O-S liquid phase was only observed at molar ratios of MS:CaO:C(carbon black) = 1:2:1 and MS:CaO:C(graphite) = 1:2:2 because, the rate of reduction reaction is slower than that of the exchange reaction. In addition to equation 6.3a, metallisation can also occur via reaction 6.7 [35, 172]. In summary, there is a liquid phase in the reduction of Cu2S

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mineral particles but the presence of this liquid phase is greatly influenced by the mole ratio of carbon and / or its reactivity.

) ( 6

3Cu₂OCaS  CuCaOSO₂ g 6.7

Cenospheres were observed in the partially reacted samples at 1273 K as shown in figure 6.23. The formation of cenospheres was due to evolution of the S2 or SO2 gases

from the molten mineral sulphide particles. The mechanism of cenosphere formation has been explained very well by Li and Wu [179]. Elemental mapping confirmed that cenospheres were formed from the larger CuFeS2 mineral particles (> 30 µm) because,

more gas is evolved from the larger molten particles than the smaller particles [179]. By comparison, the cenospeheres were more common in the Baluba sample as this sample is dominated by the CuFeS2 which melts at 1223 K [171].

Figure 6.23 – Backscattered SEM images showing cenospheres in the partially reacted CuFeS2 particles, after reduction for 10 minutes at 1273 K, MS:CaO:C = 1:2:2 (a)

Nchanga and (b) Nkana. Argon flow rate = 0.6 litre min-1

Both the SEM and XRD analyses confirmed that the reduction of the FeS2 mineral

particles was the fastest compared to other mineral particles (CuFeS2, Cu5FeS4, Cu2S),

above 1173 K. The rapid reduction of the FeS2 mineral particles above 1173 K is due to

the presence of the liquid phase. As stated in chapter 2, liquid phases belonging to the Ca-Fe-O-S system have been observed by Jha, during carbothermic reduction of FeS [172]. The Ca-Fe-O-S liquid phases had compositions of 3FeO.3CaS and FeO.CaS [177]. However, the liquid phases belonging to the Ca-Fe-O-S system were not found in the partially reacted samples in the present study. The Ca-Fe-O-S liquid phase was not

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observed because, CaO was added at more than its stoichiometric mole ratio (MS:CaO = 1:2), in order to compensate for the reactions between CaO and gangue minerals. According to Rosenqvist [41], the FeO.CaS or 4FeO.3CaS phases can only be formed at higher mole ratio of FeS than CaO [41]. Furthermore, the absence of the FeO.CaS or 4FeO.3CaS liquid phases in the present study might be as a result of formation of the Ca2CuFeO3S liquid phase which was observed at the higher mole ratio of CaO than C or

in the absence of carbon (see section 6). On the other hand, a liquid phase belonging to the Fe-O-S system was found in the partially reacted FeS2 mineral particles. SEM-EDX

analysis showed that the Fe-O-S liquid phase had a composition of 56 wt. % Fe, 29 wt. % S and 9 wt. % O and this is slightly comparable with the eutectic composition of 68 wt. % Fe, 24 wt. % S and 8 wt. % O at about 1193 K [85].

A liquid phase belonging to the Cu-Fe-S-O system was determined during reduction of Cu-Fe-S type of mineral particles at equal mole ratios of CaO and C (MS:CaO:C = 1:2:2). The Cu-Fe-S-O liquid phases appear to form as a result of the reaction between the Fe-O phase at the periphery and the Cu-S phase at the centre. Two main observations were made during reduction of the Cu-Fe-O-S liquid phase:

i. The low oxygen, Cu-Fe-S-O liquid phase was found with the metallic iron spheres at higher reduction temperature (above 1173 K) as shown in figure 6.24(a).

ii. The oxygen rich Cu-Fe-S-O liquid phase was found with the metallic copper at lower reduction temperature (below 1173 K) as shown in figure 6.24(b).

Based on observations i and ii, it can be concluded that preferential metallisation of iron and copper from the Cu-Fe-O-S liquid phase, occurs at higher and lower reduction temperatures, respectively. From the thermodynamic prediction in figure 6.17, Fe co- exists with Cu2S at lower partial pressure of O2 gas whereas Cu co-exists with Fe-O and

in equilibrium with Cu2S, at higher partial pressure of O2 gas. Therefore, the reactivity

of carbon is high at higher temperatures e.g. 1273 K, such that the partial pressure of O2

gas decreases, leading to the preferential metallisation of Fe from the Cu-Fe-O-S system Nonetheless, the reactivity of carbon is low at lower reduction temperature, and hence the partial pressure of O2 gas remains high, and this is why metallic copper was found

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Figure 6.24 – Backscattered SEM images for the partially reduced samples at molar ratio of MS:CaO:C = 1:2:2; (a) 20 minutes at 1273 K and (b) 2 hours at 1073 K. Argon

flow rate = 0.6 litre min-1