High Pressure Acid Leaching of Co Matte Converted from Co Concentrate
2016 Volume 57 Issue 10 Pages 1753-1758
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2016 Volume 57 Issue 10 Pages 1753-1758
Cobalt concentrates are often leached with an acid such as sulfuric acid. An alternative leaching process of using matte is suggested. Cobalt concentrates are first converted to matte, enriching their major elements of Co, Cu and Fe, and the matte is treated by sulfuric acid pressurized leaching. The Co concentrate includes Co oxides as valuable minerals, whereas Co, Cu and Fe exist predominantly as sulfides in the matte. The effect of experimental factors such as oxygen partial pressure, sulfuric acid concentration and sulfur content in the matte on the leaching efficiency of Co is investigated. The presence of oxygen as an oxidizing agent improves the leaching efficiency of Co. Sulfuric acid concentration of 0.19 M is sufficient for the full dissolution of Co from the matte in oxygen atmosphere. A higher sulfur content in the matte enhances leaching rate of matte slightly. The leaching results are well matched with physicochemical characterization of Co concentrate, matte and residues. This alternative leaching process using Co matte could lead to a higher Co leaching efficiency than the direct leaching process using Co concentrate.
Cobalt has been used in sculpture, pigment and ornaments from the old days, and widely used as a component of alloy. Superalloys containing cobalt are suitable as materials for gas turbines and aircraft engines because they have high temperature stability, corrosion- and wear-resistant properties.1) In addition, cobalt is also used in secondary battery materials, catalysts, and magnets. The reserves of cobalt are estimated to be about 7.5 million tons worldwide.2) Most of cobalt concentrates are distributed in a few countries, and about 80% of the world's cobalt production volume is supplied from Congo and Zambia.3,4) Cobalt typically occurs in a compound form such as cobaltite (CoAsS), erythrite (Co3(AsO4)2∙8H2O), glaukodot ((Co,Fe)AsS), sphaero-cobaltite (CoCO3), skutterudite ((Co,Ni,Fe)As3), and carrollite (CuCo2S4) rather than native cobalt.2,5) As can be seen in the Co-containing compounds, cobalt concentrates usually include other metals such as copper and iron. In general, cobalt concentrates are leached with an acid such as sulfuric acid or hydrochloric acid. For example, the leaching behavior of laterite containing cobalt and Cu-Co ore in sulfuric acid was investigated.6,7)
Matte is molten metal sulfide phases typically formed during smelting of copper, nickel, and other base metals. Although matte is usually encountered in pyrometallurgy, there are some papers that have used matte for hydrometallurgical leaching experiments. For example, Rademan et al. and Park et al. investigated respectively the leaching behavior of Ni-Cu matte and Cu-Ni-Co-Fe matte,8,9) and Anand et al. leached Cu-Ni-Co matte obtained from copper converter slag.10) As aforementioned, cobalt concentrates contain not only Co but also various elements such as Fe, Cu, Al, Mg and Ni. If cobalt concentrates are treated directly with acids, these elements can be leached simultaneously with cobalt during a leaching process, which makes a following separation process more complicated. Therefore, we attempt an alternative leaching process where cobalt concentrates are first converted to matte, enriching their major elements of Co, Cu and Fe, followed by sulfuric acid pressurized leaching. The effect of experimental factors such as oxygen partial pressure, sulfuric acid concentration and amount of sulfur in matte on the leaching efficiency of Co, Cu and Fe is herein investigated.
The Co concentrate used in this study was obtained from Democratic Republic of the Congo. Matte was manufactured by adding sulfur to Co alloy in melting furnace at 1300℃ after removing slag generated from smelting reduction of Co concentrate to Co alloy as shown in Fig. 1. The sulfur content in the matte was varied by adding excessive amount of sulfur to the Co alloy. The sulfur content (18.1 mass%) in matte 1 was slightly higher than that (16.3 mass%) in matte 2. The resulting matte was crushed in a jaw crusher, and grounded in a ball mill. Scanning electron microscopy (SEM) images were obtained by using a 6380 spectrometer (JEOL Ltd.) combined with an energy dispersive spectroscope (EDS, JEM 2300, JEOL Ltd.) for powder morphology and chemical characterization. Particle size distribution of powders was obtained by using a master sizer 20000 (Malvern). The elemental and mineralogical composition of samples was analyzed using inductively coupled plasma (ICP: ICAP 6500, Thermo Fisher Scientific) and X-ray diffraction (XRD: X'pert MPD, Philips) respectively.
Flowchart of converting process from Co concentrate to matte.
The sulfuric acid pressurized leaching of Co concentrate and matte was performed in an autoclave. Leaching conditions in this study were as follows: a temperature of 150℃, a total pressure of $20 \pm 1$ atm (N2, O2 or air), a pulp density of 0.1 (matte 35 g) and agitation at 200 rpm. The autoclave was first heated up to 150℃, and then, N2, O2 or air as purged into the autoclave. 0.19 M or 0.38 M sulfuric acid was used as a lixiviant. The reaction time for the experiments was 2 h, and samples were taken at different time intervals (0 min, 15 min, 30 min, 60 min, 90 min and 120 min). The samples were filtered using filter papers and subjected to chemical analysis using ICP for calculating leaching efficiency. Residues were dried for 4 h at 100℃ and characterized by XRD.
SEM images of Co concentrate and matte are shown in Fig. 2. Figure 2(a) shows particles of Co concentrate with irregular shapes and various sizes. Similar to the Co concentrate, matte in Fig. 2(b) consists of particles with irregular shapes and various sizes. Particle size distribution measurements are illustrated in Fig. 3. Although the Co concentrate and matte were crushed and ground in the same way, they indicated a slightly different particle size distribution. The particle size of Co concentrate is distributed between 0.5 µm and 500 µm with the most probable size being 60 µm. In the case of matte, particle size ranges between 0.7 µm and 600 µm with dominant particle sizes around 70 µm. Because the difference in particle size of the concentrate, the matte 1 and 2 is not large, the effect of particle size on the leaching of concentrate and matte would not be significant. If any, the Co concentrate would have an advantage in leaching efficiency over the matte.
SEM images of (a) Co concentrate and (b) matte 1.
The comparison of particle size distribution of Co concentration, matte 1 and matte 2.
The chemical analysis of Co concentrate and matte 1 is given in Table 1. The Co concentrate contains various elements such as ~8 mass% Co, ~19 mass% Cu, ~4.2 mass% Mg, ~3.1 mass% Fe and so on. Matte 1 has Co, Cu, and Fe as three major elements and the other minor elements less than 1 mass%. In comparing elemental composition of Co concentrate and matte 1, it is evident that Co, Cu and Fe are more concentrated in matte 1. XRD analysis results are presented in Fig. 4 where the XRD spectrum of Co concentrate contrasts sharply with that of matte 1. It is found that SiO2, Mg2Al3(Si3Al)O10(O)8, Al1.9CuMg4.1Si3.3, Cu2(CO3)(OH)2 and CoFe2O4 are main compounds in the Co concentrate while Co, Cu and Fe exist predominantly as sulfides (Cu5FeS4, Co4S3, Co9S8, Cu8S5 and FeS) in matte 1. Incidentally, the XRD pattern of matte 2 is virtually the same as that of matte 1, and is not shown for clarity in Fig. 4.
| Element | Cu | Co | Mg | Fe | Al | K | Ca | Mn | Ni | Cr |
|---|---|---|---|---|---|---|---|---|---|---|
| Co concentrate (mass%) | 18.5 | 7.65 | 4.18 | 3.12 | 2.76 | 0.58 | 0.45 | 0.43 | 0.037 | <0.002 |
| matte 1 (mass%) | 43.7 | 22.8 | <0.002 | 7.78 | 0.18 | 0.012 | 0.0064 | 0.023 | 0.14 | <0.002 |
XRD pattern of (a) matte 1 and (b) Co concentrate.
To investigate the impact of oxidizing agent (O2) on leaching efficiency of Co, Cu and Fe in the matte, we conducted leaching experiments in three types of atmosphere (N2, O2 or air) as shown in Fig. 5. In O2 atmosphere corresponding to 20 atm of O2 partial pressure, Co could be fully leached within 2 h. While Cu showed a low leaching efficiency, Fe was leached moderately in the early stages with an abrupt decline in leaching efficiency to an insignificant level. However, this leaching behavior turned totally different in N2 atmosphere, which manifests the role of partial pressure of oxygen. Negligible leaching efficiency for Co and Cu was observed, whereas Fe was steadily leached to 100% after 2 h. In pressurized air corresponding to 4 atm of O2 partial pressure, the leaching efficiency of Co, Cu and Fe lay in between that in O2 atmosphere and that in N2 atmosphere. Consequently, it is evident that the existence of oxygen as an oxidizing agent enhances the leaching efficiency of Co and Cu although the effect partial pressure of oxygen seems more remarkable for Co than for Cu.
Leaching efficiency of (a) Co, (b) Cu and (c) Fe in O2, N2, and air atmosphere (matte 1, 20 atm, 0.19 M H2SO4, 150℃).
The leaching reaction in the matte with partial pressure of oxygen in acidic atmosphere can be expressed by the following eq. (1).8)
| \[2{\rm H}^{+} + {\rm MS} + 1/2{\rm O}_{2} \to {\rm M}^{2+} + {\rm S} + {\rm H}_{2}{\rm O}\] | (1) |
| \[{\rm FeS} + 2{\rm H}^{+} + 1/2{\rm O}_{2} \to {\rm Fe}^{2+} + {\rm S} + {\rm H}_{2}{\rm O}\] | (2) |
| \[2{\rm Fe}^{2+} + 2{\rm H}^{+} + 1/2{\rm O}_{2} \to 2{\rm F}{\rm e}^{3+} + {\rm H}_{2}{\rm O}\] | (3) |
| \[2{\rm Fe}^{3+} + 3{\rm H}_{2}{\rm O} \to {\rm Fe}_{2}{\rm O}_{3} + 6{\rm H}^{+}\] | (4) |
Regarding the low Cu leaching efficiency, it is an unexpected result considering the fact that Co and Cu usually show a similar leaching behavior in oxidative acid leaching condition with high leaching efficiency. However, there are a couple of reports where Cu leaching efficiency is low showing a sharp contrast with high Co leaching efficiency.11,12) Yin et al. speculated that unique phases of Co and Cu in concentrate caused different leaching behavior.11) Liu et al. explained the low Cu leaching efficiency and the high Co leaching efficiency by the replacement reaction between Cu ions and Co.12) Although Cu can be leached in early leaching stages, it could be possible for leached Cu ions to be reduced leading to the dissolution of Co. However, further study on this issue would be necessary.
3.3 Effect of sulfuric acid concentrationThe influence of H2SO4 concentration (0.19 M, 0.38 M) on the leaching efficiency of metals in matte 1 in O2 atmosphere is examined in Fig. 6. In the case of H2SO4 concentration of 0.19 M, it seemed sufficient for the full dissolution of Co from the matte in O2 atmosphere. The leaching efficiency of Cu increased from 13% to 18% as the H2SO4 concentration increased from 0.19 M to 0.38 M. Meanwhile, Fe was rarely leached in both 0.19 M and 0.38 M H2SO4. For comparison purposes, the Co leaching efficiency under a similar experimental condition was referred such as Amer's (~80% leaching efficiency with 4 M H2SO4 and 10 atm O2 at 110℃ in 2 h.13)) and Anand et al.'s (80% leaching efficiency with 0.34 M H2SO4 and 7 atm O2 at 150℃ in 2 h.10)). It appears that the higher partial O2 pressure (20 atm) in our leaching condition might play a crucial role in obtaining 100% Co leaching efficiency even with the lower concentration (0.19 M) of sulfuric acid.
Effect of H2SO4 concentration on the leaching efficiency of metals in the matte 1 in O2 atmosphere for 2 h.
As mentioned in the experimental procedure section, the sulfur content in the matte was varied such that matte 1 and matte 2 had sulfur content of 18.1 mass% and 16.3 mass% respectively. Figure 7 compares the leaching efficiency of Co, Cu and Fe depending on the sulfur content in the matte. Although both matte 1 and matte 2 reached 100% Co leaching efficiency in 2 h, leaching rate of matte 1 was faster than that of matte 2 leading to a slightly higher Co leaching efficiency of matte 1. Meanwhile, the ICP result showed that the Co content (22.8 mass% in matte 1 was higher than that (20.7 mass%) in matte 2 by the same ratio of sulfur content between matte 1 and matte 2 (18.1 mass% vs. 16.3 mass%). Considering both the ICP result and the XRD result that Co9S8 and Co4S3 were two dominant cobalt sulfide compounds, the total amount of Co sulfides was increased in matte 1 fixing the ratio of Co9S8 to Co4S3 in the matte. Therefore, it could be concluded that Co leaching efficiency can be enhanced as Co sulfides occupy a higher proportion of matte because matte 1 showed a higher Co leaching efficiency than matte 2. On the other hand, there was no substantial difference in the leaching efficiency of Cu and Fe depending on the sulfur content in the matte. There would be more pronounced differences in the leaching behavior of Co, Cu and Fe if the sulfur content could be more varied than the current sulfur content range (16.3 ~ 18.1 mass%). However, it was not possible to change the sulfur content in the matte dramatically because most of excessive input sulfur became vaporized during matte formation.
Effect of sulfur content in the matte on the leaching efficiency of (a) Co, (b) Cu and (c) Fe (20 atm Po2, 0.19 M H2SO4).
As mentioned in the introduction, the leaching process that uses converted metal sulfide matte is expected to have an advantage of simplifying a separation process for the isolation of Co from various kinds of metals. Further, the leaching process adopting matte has a competitive edge in Co leaching efficiency as confirmed in Fig. 8 where Co leaching efficiency is compared between Co concentrate and matte under identical leaching conditions (0.19 M H2SO4 and 20 atm O2 at 150℃ for 2 h). Overall, the leaching efficiency of Co, Cu and Fe in the matte is higher than that in the concentrate. In particular, a difference in Co leaching efficiency is substantial such that all Co in the matte can be leached out whereas ~30% of Co in the concentrate was leached. Considering the fact that the particle size of concentrate is similar to that of matte or even slightly favorable to leaching, it is evident that the alternative leaching process using Co matte guarantees a higher Co leaching efficiency than the direct leaching process using Co concentrate comprising mainly Co oxides.
Leaching efficiency of Co concentrate and matte 1 (20 atm PO2, 0.19 M H2SO4, 150℃ and 2 h).
SEM images of matte 1 and leach residues after leaching experiments (0.38 M H2SO4 and 20 atm O2/N2 at 150℃ for 2 h) are compared in Fig. 9. There is no significant difference between the initial matte and the leach residues in terms of maintaining irregular shapes and diverse sizes. However, it appears that the surface of leach residue particles was partially dissolved leading to a porous structure as expected from the active leaching behavior of Co species in O2 atmosphere and Fe species in N2 atmosphere.
SEM images of (a) matte 1 and leach residues after leaching (0.38 M H2SO4, 150℃, 2 h), (b) in O2 atmosphere and (c) in N2 atmosphere.
XRD patterns of matte 1 and leach residues in O2 and N2 atmosphere are shown in Fig. 10. As discussed in Fig. 4(b), Co, Cu and Fe exist in sulfide forms of Cu5FeS4, Co4S3, Co9S8, Cu8S5 and FeS. The peaks of Co4S3 and Co9S8 disappear in the XRD patterns of leach residues in O2 atmosphere, which corresponds to 100% Co leaching efficiency under the same leaching conditions. It is found that Fe2O3 was generated because Fe was oxidized from Fe2+ to Fe3+ by the partial pressure of oxygen as shown in Fig. 10(b). In the meantime, the peaks of Cu5FeS4 and FeS disappear in the XRD pattern of leach residues in N2 atmosphere, which is matched well with 100% Fe leaching efficiency in this occasion.
XRD patterns of (a) matte 1 and leach residues in (b) O2 and (c) N2 atmosphere.
Leaching experiments of matte containing Co, Cu and Fe converted from the Co concentrate were performed by sulfuric acid pressurized leaching. The effect of the following factors on leaching efficiency was evaluated: oxygen partial pressure, H2SO4 concentration, and sulfur content in the matte. The Co concentrate contains various elements such as ~8 mass% Co, ~19 mass% Cu, ~4.2 mass% Mg, ~3.1 mass% Fe and so on, whereas Co, Cu, and Fe are enriched in the matte with other minor elements less than 1 mass%. It is found that the Co concentrate includes Co oxides as valuable minerals while Co, Cu and Fe exist predominantly as sulfides in the matte. It is evident that partial pressure of oxygen enhances the leaching efficiency of Co and Cu. Fe was leached moderately in the early stages of leaching with an abrupt decline in leaching efficiency to an insignificant level. H2SO4 concentration of 0.19 M was sufficient for the full dissolution of Co from the matte in O2 atmosphere. A higher sulfur content in the matte appears to enhance the leaching rate of matte slightly. The leaching results are well matched with physicochemical characterization of Co concentrate, matte and residues. High pressure sulfuric acid leaching that uses converted metal sulfide matte instead of Co concentrate shows a higher Co leaching efficiency, and is expected to have advantages of simplifying a separation process for the isolation of Co.
This study was supported by the R&D Center for Valuable Recycling (Global-Top R&BD Program) of the Ministry of Environment.
