A Mineralogical Investigation of Sintering in Cu-Rich Polymetallic Concentrates During Roasting in Inert Atmosphere

A simple quantitative assessment of minerals bearing impurity elements As and Sb was performed based on numerous EDS measurements on epoxy grain mounts of the concentrates. A count of the number of times the EDS compositions conformed closely to a specific mineral was determined. More than 100 such EDS measurements were performed at randomly located spots in frames of 400× magnification. N_EDS, the number of counts of encountering a mineral per 100 spot measurements, is presented in Figure 5.

On the basis of N_EDS, occurrence of a minor mineral in a concentrate is regarded as ‘P-Present’ (N_EDS > 4), R-Rare’ (N_EDS ≤ 4) or ‘NP-Not Present’ (N_EDS = 0) as listed in Table IV.

Notably, Ttr is present in all the concentrates as the main Sb mineral. Similarly, Asy is the main As-bearing mineral in the four concentrates. It is however found only a few times during EDS measurements on Rockliden 8232 and Garpenberg concentrates as they are relatively low in As content (Table II).

Microscopic study in SEM revealed that there were two modes of particle agglomeration leading to the sintering of calcines in all the four concentrates during roasting experiments:

The evidence for the above listed mechanisms of sintering is presented here.

Compositional variation of the iss phase contributing to sintering due to solid-state assimilation in the calcined residues of isothermal roasting at different temperatures was determined from the numerous EDS measurements. Based on the compositional limits of the iss phase shown in Figure 7, all compositions conforming to 5 mol pct < Cu < 30 mol pct together with 20 mol pct < Fe < 45 mol pct and balance S + Zn were accepted as the iss phase. Figure 9 presents a box plot showing the variation in sulphur composition in mol pct for all the identified compositions of iss phase.

Clearly, the iss phase has S ≈ 50 mol pct at all temperatures (within ± 2 mol pct), indicating a sulphur to cation molar ratio of S/(Cu + Fe + Zn) ≈ 1. Therefore, the composition values of iss phase determined by EDS measurements can be plotted in a CuS-FeS-ZnS ternary diagram with minor errors. Accordingly, fixing S = 50 mol pct, the concentrations of the metallic elements (Cu, Fe, Zn) in the measured iss-phase compositions were slightly adjusted using Eq. [1], and their ternary plots shown in Figure 10 were generated.

Equilibrium phase boundaries of the iss phase from Kojima et al.[20] (shown in Figure 7) are superimposed on these ternary composition plots. Additionally, for each concentrate, two calculated iss compositions are plotted based on the assumed scenarios that because of solid-state assimilation the initial Ccp at high temperature picks up: (1) all Fe in the concentrate (green squares) and (2) all Fe + Zn in the concentrate (red squares). These compositions are calculated from the elemental composition data of the concentrates presented in Table II.

The measured compositions of Ccp at room temperature (shown as open circles in Figures 10(a) through (d)) in the concentrates are found to be close to the theoretical formula (50 mol pct CuS, 50 mol pct FeS) in all the four concentrates. The corresponding high temperature iss phase gains more Fe and Zn.

Since the system of reacting solid particles in the calcine is extremely heterogeneous, the measured iss-phase compositions vary significantly, noted from the scatter in compositions at any given temperature. Nonetheless, these compositions are found to strictly adhere to the saturation limits set by the phase boundary isotherms, with only a few outliers. For example, the measured compositions in calcines corresponding to the experiment at 700 °C for Rockliden S7574 concentrate are located along the 700 °C isotherm (Figure 10(a)).

Second, the measurements that are found to be near the saturation limits at the highest temperature of 700 °C are in the vicinity of the calculated composition of all Fe + Zn assimilations in the initial Ccp (red squares). This composition is widely distinct for the four concentrates. For instance, for Garpenberg the concentrate it is located at the lowest FeS content amongst all four concentrates (Figure 10(c)) because of its relatively low Fe wt pct (Table I). Similarly, for Maurliden concentrate, it is at the lowest ZnS content (Figure 10(d)).

Therefore, sintering by solid-state assimilation is driven by the tendency of the initial Cu-, Zn- and Fe-bearing sulphide minerals in the concentrate to combine and form a single high-temperature solid solution, the iss phase in the CuS-FeS-ZnS system.

Figures 11(a) through (d) presents the BSE images comprising the characteristic texture of liquid phase in the calcines obtained by isothermal roasting at 500 °C for the four concentrates. EDS measurements were performed at the indicated spots (repeated measurements of 5 times at each spot) to determine the elemental composition of these liquids.

It is evident in these microscopic images that all four concentrates experienced melt formations during roasting at 500 °C. The melts for Rockliden S7574 and Maurliden concentrates solidified in a single phase (Figures 11(a) and (d), respectively), whereas the melts for Rockliden 8232 and Garpenberg concentrates developed two-phase intergrowth textures on solidification (Figures 11(b) and (c), respectively).

Results of the EDS measurements at selected spots shown in Figure 11 to determine the elemental composition of these liquid-forming phases at 500 °C are listed in Table V.

Compositions of the solidified single phase in Rockliden S7574 and Maurliden calcines presented in Table V suggest that the melt formation is due to partial melting of Gn. While S ≈ 50 mol pct, Pb is partly substituted by As, Sb, Fe and Cu. Gn in its pure form melts at 1117 °C. High concentrations of As and Sb indicate that impurity-bearing minerals had a major role in partial melting of Gn. As presented in Table IV and Figure 5, Rockliden S7574 and Maurliden concentrates have Asy as the main As-bearing mineral, which is a rare mineral in the Rockliden 8232 and Garpenberg concentrates. Tomkins et al.[25] highlighted Asy as a melt-forming precursor, since it forms a melt phase (As-S_melt) at a low temperature of 491 °C (1 bar) during the decomposition of Py to Po. They suggested that this liquid phase has a tendency to partially melt sulphides at temperatures much below their melting points, and in this process, a variety of elements, especially the low-melting chalcophile elements (LCME)[17] such as Pb, Zn, Sb, Ag and Au can join the As-S_melt phase. High As content along with other elements in the partially melted Gn found in the present study (Table V) affirms the vitality of Asy as a melt-forming precursor in polymetallic concentrates. Such melts are therefore expected to form in concentrates comprising Gn + Py + Asy (in addition to other minerals). Notably, this high As + Sb-bearing partially melted Gn solidifies into a single phase unlike the general polymetallic sulphide melts.

The two-phase intergrowths in Rockliden 8232 and Garpenberg calcines (Figures 11(b) and (c), respectively) are found to be compositionally close to a Gn-Ttr pseudobinary system (Spot 1 composition is close to the theoretical PbS, which has 50 mol pct Pb, 50 mol pct S; Spot 2 composition is close to (Cu,Ag,Fe,Zn)₁₂(Sb,As)₄S₁₃, which stoichiometrically has 41.4 mol pct Cu + Ag + Fe + Zn, 13.8 mol pct Sb + As, 44.8 mol pct S). There are additional elements such as Sb, Cu and Ag dissolved in the Gn melt fraction. A thermodynamic explanation for this intergrowth formation can be found in the work of Pruseth et al.[24] They studied the high-temperature-phase relations in a PbS-Cu₂S-Sb₂S₃ ternary system, which included sulfosalts such as bournonite (Bour) and meneghinite (Mene) relevant in the present study (Table IV). This ternary system also included Skn, which is known to be a product of thermal decomposition of tetrahedrite (Cu₁₂Sb₄S₁₃) between 520 °C and 530 °C[29,30] that congruently melts at 607.5 °C ± 3 °C.[29,31] In the PbS-Cu₂S-Sb₂S₃ ternary diagram shown in Figure 12 for 500 °C, the Sb-rich extreme of the Cu-rich melt (point P) is almost collinear with the PbS-Cu₃SbS₃ join. Consequently, Gn and Skn (originally Ttr) should form a eutectic melt very close to 500 °C.

Considering the compositional similarity of Skn to Cu₁₂Sb₄S₁₃ and the expected lowering of the melt formation temperature due to the presence of additional elements such as Ag, As, Fe and Zn in Ttr, the two-phase intergrowths in Rockliden 8232 and Garpenberg calcines apparently correspond to a eutectic melt formation on the Gn-Skn join near 500 °C.

Pruseth et al.[24] also reported the involvement of another sulfosalt mineral, Bour, in causing early melt formation. They found that an assemblage of Gn + Skn + Bour started to form the Cu-rich melt phase at 459 °C ± 1 °C. Therefore, the presence of other sulfosalts such as Bour and Mene in the four concentrates (Table IV) in addition to Ttr further augments their early melt formation tendencies.

Therefore, concentrates having Gn + Sb-sulphosalts but missing Asy will likely form a melt phase that solidifies into a two-phase intergrowth.

Figure 13(a) presents a macroscopic image of a sintered calcine obtained from non-isothermal roasting of Garpenberg concentrate up to 700 °C. In this case, the concentrate was subjected to slow heating and slow cooling, unlike during the isothermal roasting. It is evident that several nuggets of solidified melt are trapped in this sintered calcine. Notably, as presented earlier for the Garpenberg concentrate, only one such nugget was visible in the sintered calcine obtained by isothermal roasting at 700 °C (Figure 3). Clearly, the non-isothermal run resulted in an increase in the quantity of melt generation. A microstructure (BSE image) of the sintered calcine together with an entrapped nugget is presented in Figure 13(b).

There are two distinct intergrowth textures (one relatively coarser than the other) in the microstructure of the solidified nuggets, most clearly shown in the BSE image in Figure 13(c). This suggests the formation of two distinct sulphide melts. The phase that appears bright in the BSE image is common in both textures. Table VI lists the composition of the contributing phases, measured at the locations indicated in Figure 13(c).

The composition of the phase constituting the fine intergrowth, measured at Spot 3 in Figure 13(c), matches closely with the theoretical stoichiometry of Ttr-41.4 mol pct Cu + Ag + Fe + Zn, 13.8 mol pct Sb + As and 44.8 mol pct S. Similarly, the phase in the coarse intergrowth, measured at Spot 1, corresponds to the iss phase of the Cu-Zn-Fe-S system presented in Section III–C–3 of this article. Also, the common bright phase has a composition similar to Gn (measured at Spot 2).

Therefore, the two liquid phases coexisting at 700 °C in the calcine are: (1) a low-melting phase belonging to the Gn-Ttr join appearing as the fine intergrowth; (2) a low-melting phase belonging to the Gn-iss join appearing as the coarse intergrowth. Here again, similar to the findings from the isothermal experiments, Gn is found to be a common contributor to two different types of low-melting sulphide melts. This is in agreement with the observations made by Frost et al.[17] that the presence of Gn in a sulphide mineral assemblage lowers the melt formation temperature.

The thermodynamic basis for low-melting phase formation on the Gn-Ttr join has been discussed previously in this article in Section III–C–4. Regarding the coarse intergrowth between Gn and iss phase, a similar system of CuFeS₂-PbS was investigated by Craig et al.[22] They stated that melt formation on the CuFeS₂-PbS join can begin at 630 °C. This has relevance to the present study since the iss phase was found to originate from the transformation of Ccp mineral, as presented in Section III–C–3. Stevens et al.,[23] studying another closely related system of Gn-Py-Sph-Ccp assemblage, also observed the initiation of partial melting of sulphides near 700 °C.