The optical micrographs for the as-received mineral sulphide concentrates are presented in figure 4.2. It is evident from figure 4.2 that copper sulphide (Cu2S, Cu8S5) is the main
mineral sulphide phase in the Nchanga concentrates because there are many bluish [169] particles under the reflected light microscope. The optical images in figure 4.2 clearly shows that CuFeS2 (chalcopyrite) is the main mineral sulphide phase in the Baluba
concentrates, as there are many brass yellow particles.
Figure 4.2 - Optical micrographs of the as-received mineral sulphides concentrates, under reflected light
The elemental maps for the as received Nchanga mineral sulphide concentrates are presented in figure 4.3. The elemental maps for the as received Nchanga sample agrees with the XRD (figure 4.2) results, as the main phases are Cu-S and SiO2. The SEM-
EDX point analysis of the Nchanga sulphide concentrates showed that Fe was dissolved in some Cu-S mineral particles. The presence of Fe in the Cu-S mineral particles is
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because the Fe2+ ions can substitute the Cu2+ ions in Cu9S5 (4Cu2S∙CuS) and Cu8S5
(3Cu2S∙2CuS) [165, 167], as mentioned above. The Nkana mineral sulphide
concentrates has the highest gangue content and it is for this reason that the concentrations of Al, K, Si, Mg, Ca are high, from the elemental maps in figure 4.4a. The analysed area in figure 4.4a contains TiO2 and ZrO2 because these compounds are
present in the mineral sulphide concentrates, although in very low concentrations. As noted in tables 3.2 and 4.1, the Nkana mineral sulphide concentrates has the highest Co content and hence an area containing cobalt was analyzed and the elemental maps are presented in figure 4.4b. The elemental maps in figure 4.4b clearly shows the presence of Cu-Co-S and Co-Fe-S phases, and they were analyzed as carrollite (CuCo2S4) and
cobaltian pyrite (Fe,Co)S2 minerals, respectively, by SEM-EDX analysis. It is worth
noting that both carrollite (CuCo2S4) and cobaltian pyrite (Fe,Co)S2 are stable minerals
at room temperature. The Baluba sulphide concentrates have the lowest gangue content and hence the analysed area in figure 4.5 contains less gangue minerals. It can be observed from the elemental maps in figures 4.3 – 4.5 that the mineral sulphide particles have various shapes and sizes and this is due to the fact that non-uniform shapes and sizes are produced during the grinding stage.
Figure 4.3 – Elemental mapping for the as received Nchanga mineral concentrates, (a) is the analysed area under backscattered electron imaging
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Figure 4.4a - Elemental mapping for the as received Nkana mineral concentrate, (a) is the analysed area under backscattered electron imaging
Figure 4.4b - Elemental mapping for the as received Nkana mineral concentrates, (a) is the analysed area under backscattered electron imaging
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Figure 4.5 – Elemental mapping for the as received Baluba mineral concentrates; (A) is the analysed area under backscattered electron imaging
4.1.4 Determination of moisture content
It was important to determine the moisture content in the mineral sulphide concentrates, as moisture may alter the reactions during the carbothermic reduction in the presence of CaO. The moisture content was estimated by heating the mineral sulphide concentrates at 413 K, under argon atmosphere. The plots of % weight loss against time are presented in figure 4.6 and it can be observed that the weight losses occurred, within 30 minutes, suggesting that it was mainly due to the loss of water. It can be noted from figure 4.6 that the Nchanga mineral sulphide concentrates had the highest moisture content whereas the Nkana mineral sulphide concentrates had the lowest moisture content.
4.1.5 Determination of volatile constituents
The weights of the volatile constituents in the mineral sulphide concentrates were estimated by heating the mineral sulphide concentrates at 973 K, under argon
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atmosphere and the plots of the % weight loss against time curves are presented in figure 4.6. The precise determination of the volatile constituents require heating of the mineral sulphide concentrates above 1173 K [170] but the challenge is that there is loss of sulphur at this temperature, resulting from the thermal decomposition of pyrite (FeS2)
and chalcopyrite (CuFeS2) minerals when heated above 773 K [78]. The weight loss
occurred in about 240 seconds (4 minutes) in the Nchanga sample. The sharp weight loss in the Nchanga sample was mostly due to the thermal decomposition of the gangue minerals because; (i) the concentrates are rich in the Cu2S and Cu5FeS4 minerals, which
are stable at 973 K and (ii) the thermal decomposition of the CuFeS2 and FeS2 minerals
is very slow at 973 K.
There was a sharp weight loss in the first 480 seconds (8 minutes), followed by a gradual weight loss in the Nkana and Baluba samples at 973 K (figure 4.6). The sharp weight loss is mainly due to the thermal decomposition of the gangue minerals whereas the gradual weight loss might be due to the thermal decomposition of the sulphide and gangue minerals. The phases obtained after heating the mineral sulphide concentrates at 973 K, were similar to the ones in the as-received mineral sulphide concentrates, implying that the thermal decomposition of the mineral sulphides was very low. In summary, the weight loss of the samples at 973 K is mainly contributed by the thermal decomposition of the gangue minerals.
Figure 4.6 – Plot of the % weight loss versus time for the mineral sulphide concentrates, heated under argon atmosphere at 413 K and 973 K in the TGA equipment. Argon flow
rate = 0.6 litre min-1
0 1000 2000 3000 4000 0 2 4 6 8 10 12 14 16 Nkana at 413K Baluba at 413K Nchanga at 413K % W eigh t loss (%) Time (seconds) Nchanga at 973K Baluba at 973K Nkana at 973K
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