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Issue
Metall. Res. Technol.
Volume 117, Number 4, 2020
Article Number 404
Number of page(s) 7
DOI https://doi.org/10.1051/metal/2020036
Published online 06 July 2020

© EDP Sciences, 2020

1 Introduction

Cobalt, a significant strategic metal, plays important roles in many fields, such as fine chemicals, batteries, and specific alloys [13]. According to Roskill’s report [4], the global demand for cobalt increased by 8% annually between 2010 and 2017, and attained 118 kt in 2017; this is mainly attributed to the rapid development of electric vehicle industry in this period. China, as currently the largest global consumer of cobalt, is seriously short of cobalt resources, and needs to import plenty of ores or smelting intermediates of cobalt, for example, the cobalt white alloy, from Africa [58].

The cobalt white alloy is produced by reduction smelting of the Cu-Co oxide ores in an electric furnace [9,10]. It mainly consists of Co, Cu, Fe and Si in the form of alloy, and is very difficult to dissolve due to its corrosion resistant structure [8,11]. Currently, the leaching of the cobalt white alloy is usually carried out industrially under high oxygen pressure using autoclaves, which results in high investment and operation costs [12].

Many researches have been carried out aiming at developing an effective process for the leaching of the cobalt white alloy in aqueous sulfuric acid at atmospheric pressure. In order to enhance the leaching, a lot of technologies for the intensification and activation have been attempted, to name only a few, such as the intensification by the addition of Clor F ions [8,13], the activation by using mechanical or electrochemical methods [1417]. Eventually, these technologies had inevitably failed due to the difficulty in the solid to liquid separation, which was caused by dissolution of the silicon, as well as the resulting formation of silica gel. Otherwise, the addition of Cl or F ions will also increase the difficulty in the waste water treatment. Against this background, the pre-desilication from the cobalt white alloy before acid leaching has been viewed as a very effective method to solve the problem, which not only destroys the corrosion-resistant structure of the alloy, but also makes the liquid to solid separation easy to be carried out. Two pre-desilication methods by low temperature roasting with limestone [18] or NaOH [19,20] respectively have been reported. Even though these two methods can improve the subsequent sulfuric acid leaching effectively, the roasting process under low temperature progresses slowly due to the kinetic constraint, as a result, resulting in these methods unserviceable.

In this work, a novel process for the extraction of Co and Cu from the cobalt white alloy containing high Si content, as shown in Figure 1, was investigated. The process contains three main steps: (1) the pre-desilication by oxidation smelting based on the MnO-MgO-SiO2 slag; (2) the powder making of the desilicated alloy melt by water atomization; (3) the atmospheric oxygen leaching of the desilicated alloy powders by aqueous sulfuric acid solution. Compared with the low temperature roasting methods mentioned above, the process is much faster in the desilication rate, undoubtedly, conducive to the improvement of the production efficiency. The microstructures of the cobalt white alloy and alloy powders attained by water-atomizing were analyzed in details. Additionally, the influences of sulfuric acid dosage, temperature and time on the leaching efficiencies of cobalt, copper and iron of the water-atomized alloy powders were investigated.

thumbnail Fig. 1

Flow chart of the extraction of Co and Cu from cobalt white alloy containing high silicon.

2 Experimental

2.1 Materials

The cobalt white alloy used in this work was imported from the Democratic Republic of Congo, the main chemical composition of which was as follows (wt.%): Co 39.89, Fe 40.30, Cu 7.80, Si 8.75, P 1.18 and S 0.11. The cobalt white alloy was granular and irregular in shape, and the particle size of the cobalt white alloy was less than 10.0 mm.

2.2 Experimental

2.2.1 Desilication and powder making

The oxidation smelting for the desilication of the cobalt white alloy was performed by means of a medium-frequency induction furnace fitted with an oxygen-enriched air blowing system. The furnace lining was rammed with high-graded magnesia ramming material. During the smelting, oxygen enriched air was conducted into the top of the furnace hearth through an unsubmerged lance of steel. For the reason that the cobalt white alloy can not be melted directly in the medium-frequency induction furnace. 7.0 kg of iron ingots were firstly melted in the furnace to make a molten bath, and then 20.0 kg of the cobalt white alloy ingots were fed to the furnace; after the alloy melting at 1600–1650 °C, oxygen-enriched air of 43.57 vol.-% O2 was blown into the molten bath through the vertical lance with a flow rate of 7.0 m3/h. 2.0 kg of Mn3O4 and 3.0 kg of MgO were added into the furnace to make slag with SiO2 produced by the oxidation of silicon in the alloy melt. After the end of the desilication, 9.03 kg of the desilication slag was tapped from the bath by using an iron spoon. An alloy sample of about 20 g was taken by using a quartz tube before water atomization of the alloy melt. The obtained slag was grounded and dissolved by using mixed acids of hydrochloric-nitric-perchloric-hydrofluoric acids. The concentrations of SiO2 and Mn in the slag phase were determined by gravimetric and ammonium ferrous sulfate titration methods, respectively. The Fe, Co, and Cu concentrations in the slag were determined using atomic absorption spectroscopy (AAS). The alloy sample was characterized using scanning electron microscope (SEM) attached with energy-dispersive X-ray spectrometer (EDS).

The alloy melt after the desilication at 1600–1650 °C was atomized into powders by high pressure water of 24 MPa. After the water atomization, the desilicated alloy powders were collected and dried for microstructure and chemical analyses. Analyses of Mn, Fe, Co, and Cu in the powders were determined by chemical titration method after dissolving with aqua regia.

2.2.2 Atmospheric oxygen leaching by aqueous sulfuric acid solution

The details of the experimental setup and procedure have been published in our previous paper [21]. Based on the previous study [21], the stirring speed, O2 flow rate and liquid/solid ratio were set 1500 rpm, 0.15 L/min, and 10/1, respectively. Excess of sulfuric acid was calculated according to the stoichiometric acid addition by the consumption of Cu and Co varied from 1.1 to 1.4. The leaching temperature (65, 75, 85 and 95 °C) and leaching time (2.0, 4.0, 6.0 and 8.0 hours) were also investigated. The concentration of Fe in the residue was measured by chemical method, while that of Co and Cu was determined by AAS. The concentrations of Co and Cu in the leach liquor were determined by the chemical method.

3 Results and discussion

3.1 Microstructure of cobalt white alloy

The microstructure picture and the elemental maps for Cu, Co, Fe, Si, P and O of the white alloy are shown in Figure 2. From the BSE image, it can be seen that there were mainly three major phases in the alloy. According to the images of the elemental maps, it is found that the light areas mainly consisted of Cu, while the dark and grey areas shared the same component elements, such as, Co, Fe and Si. The result is similar to the findings of Feng [8] and Bai [22]. Furthermore, the content of Si in the dark areas was higher than that in the grey area, as seen in Table 1. The results also showed that the main phases in the alloy consisted of Fe-Co-Si, indicating that it is very difficult to leach directly [8].

thumbnail Fig. 2

The microstructure image and elemental maps of the cobalt white alloy.

Table 1

Chemical composition of Co-Fe-Si phases and Cu phase by EDS analysis.

3.2 Results of desilication and powder making

The relationship between the standard Gibbs free energy of formation of Cu2O, CoO, FeO, MnO, SiO2, MgO and temperature was calculated with HSC Chemistry 9.0, and the results are displayed in Figure 3. It reveals that MnO, MgO and SiO2 are thermodynamically more stable than FeO, CoO and Cu2O. Therefore, the silicon in the cobalt white alloy can be preferentially oxidized into SiO2, and ultimately enriched into slag phase together with MnO and MgO, while the Co, Cu, and Fe in the white alloy is retained in the alloy phase during the smelting process. Further, it can also be seen from Figure 3 that the distance between MnO and valuable metal oxide (CoO) is significantly larger than that of FeO and CoO. This means that the oxidation smelting using MnO-based (rather than FeO-based) slag is favorable to decrease the losses of valuable metals and control the end point.

In addition, in order to extend the lifetime of the refractories containing MgO, and decrease the consumption of MnO, the smelting experiment should be conducted by the MnO-MgO-SiO2 slag with MgO-saturated and low MnO content. According to the phase diagram of the MnO-MgO-SiO2 system calculated by FactSage 6.3, as shown in Figure 4, the target slag system can be chosen near the line labeled AB when the smelting temperature is controlled at about 1600 °C. It should also be noted that the line AB is nearly parallel to the line SiO2-MnO, implying that the slag system can dissolve more SiO2 without increasing the liquidus temperature significantly. As discussed above, it is more feasible to remove silicon selectively from the cobalt white alloy by oxidation smelting based on the MnO-MgO-SiO2 slag system.

The experimental results of the desilication and powder making process, as shown in Table 2, indicated that desilication of the cobalt white alloy through oxidation smelting based on the MnOx-MgO-SiO2 slag was feasible. In the process, the silicon content in the alloy decreased from original 8.75% to final 1.20%, and in other words, 83.17% of silicon in the cobalt white alloy was removed into the slag, while only 4.67% of cobalt, 6.32% of iron and 3.53% of copper were lost in the slag.

Figure 5 shows the microstructure and elemental mapping for O of the desilicated alloy before water atomization by SEM-EDS analysis. It can be noted from Figure 5 that the desilicated alloy was presented as a homogeneous phase with quite low oxygen content.

Many studies have shown that Si and Mn in the iron alloy melt can be oxidized during water atomization process due to their high affinity for oxygen [2325]. The microstructure characterization of the water-atomized alloy powders by SEM-EDS analysis is presented in Figure 6. As seen from Figure 6a, the desilicated alloy powders were of typical morphologies of water-atomized powders, such as, teardrop, sphere or their aggregate; the sizes of the powders were in the range from several to over one hundred microns; the surfaces of the powders were in a very rough state. A powder particle surrounded by red lines in Figure 6b was amplified, and then EDS line scans for such elements as C, O, Si, P, Mn, Fe Co and Cu were carried out on it, as shown in Figure 6c. From Figure 6c, it can be seen that such elements as O, Si, P and Mn were enriched on the surface of the particle to a certain extent; this may be attributed to their migration to the surface and the oxidation in the water atomization process.

thumbnail Fig. 3

The relationship between the standard Gibbs free energy of formation of Cu2O, CoO, FeO, MnO, SiO2 and MgO and temperature.

thumbnail Fig. 4

Thermodynamic phase diagrams of the MnO-MgO-SiO2 system (line AB represents the liquidus line at 1600 °C; P represents the chemical composition of the slag obtained).

Table 2

Experimental results of the desilication and powder making process.

thumbnail Fig. 5

Microstructure and elemental mapping for O of the desilicated alloy before the water atomization (R-Resin).

thumbnail Fig. 6

Microstructure characterization of the water-atomized alloy powders by SEM-EDS analysis.

3.3 Atmospheric oxygen leaching of desilicated alloy powders in sulfuric acid medium

The overall leaching reaction is as follow: (1)

The Fe2+ is then oxidized into Fe2O3 and FeOOH at pH values of 3.0–4.0 according to the following equations [21]: (2) (3)

Based on the previous study [21], the effects of excess sulfuric acid, leaching temperature, and leaching time on the leaching efficiency of Co, Cu and Fe were investigated in the leaching experiments. Figure 7a presents the effect of the excess of sulfuric acid (a ratio of the actual sulfuric acid dosage to the theoretical dosage) on the leaching efficiency of Co, Cu and Fe at 95 °C for 6 hours. It is clear that a ratio of actual sulfuric acid dosage to its theoretical dosage of 1.3 was the optimum as the changes in the leaching efficiencies of all metals became insignificant when the sulfuric acid dosage was further increased.

The effect of leaching temperature on the leaching efficiency of Co, Cu and Fe is shown in Figure 7b at a fixed excess of sulfuric acid 1.3 and leaching time 6 h. It was found that leaching rates of all metals increased with increasing leaching temperature. When the leaching temperature reached 95 °C, the leaching efficiencies of Cu, Co and Fe were about 97.75%, 98.10%, and 17.37%, respectively.

Figure 7c illustrates the influences of leaching time on the leaching of Fe, Co, and Cu in the alloy powders at a fixed leaching temperature of 95 °C and excess of sulfuric acid of 1.3. As leaching time raised from 2 h to 8 h, the leaching ratio of Fe decreased from 67.12% to 16.37%, while the leaching ratios of Cu and Co increased from 37.59% and 87.14% to 97.78% and 97.99%, respectively. It was worth noting that the leaching ratio of Fe was higher than that of Cu at the beginning, which confirmed that the transformation of Fe in alloy powders consists of two steps: Fe is firstly dissolved to form Fe2+, and then the Fe2+ is oxidized into Fe2O3 or FeOOH by oxygen gas. According to the X-ray diffraction pattern of the leaching residue (seen in Fig. 8), the leaching residue was goethite (FeOOH).

thumbnail Fig. 7

Effect of the following experimental conditions on the leaching rate of metals contained in the alloy powders: (a) excess of sulfuric acid; (b) leaching temperature; (c) leaching time at T = 95 °C.

thumbnail Fig. 8

X-ray diffraction pattern of the leaching residue.

4 Conclusions

The extraction of Co and Cu from cobalt white alloy with high silicon content was performed using a novel process involving oxidative conversion for pre-desilication, production of alloy powders by water atomizing and atmospheric sulfuric acid leaching of the desilication alloy powders. The results showed that 83.64% of Si in the alloy was removed effectively by oxidizing it preferentially into the MnO-MgO-SiO2 slag. Moreover, the residual silicon in the alloy melt was enriched directly into the surfaces of the alloy powders in the water atomization process, which makes the alloy powders easy to leach. The atomizing alloy powders were leached with sulfuric acid under atmosphere, and in the process, O2 was blown at a certain flow rate into the leaching slurry. 97.99% for cobalt and 97.78% for copper were leached under the optimal conditions: stoichiometric excess of sulfuric acid of 1.3, temperature of 95 °C, leaching time of 8.0 hours, agitation speed of 1500 rpm, liquid/solid ratio of 10/1, and O2 flow rate of 0.15 L/min. The overall recovery in this process attained 93.41% for cobalt and 94.33% for copper, separately. The transformation of Fe in alloy powders involves two steps during the atmospheric sulfuric acid leaching: Fe is firstly dissolved to form FeSO4, and then the FeSO4 is oxidized into FeOOH by oxygen gas according to the XRD result of the leaching residue.

Acknowledgements

The authors would like to thank the financial support of the National Natural Science Foundation of China (Grant No. 51704038).

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Cite this article as: Guoxing Ren, Zhihong Liu, Bing Pan, Songwen Xiao, A novel process for cobalt and copper recovery from cobalt white alloy with high silicon, Metall. Res. Technol. 117, 404 (2020)

All Tables

Table 1

Chemical composition of Co-Fe-Si phases and Cu phase by EDS analysis.

Table 2

Experimental results of the desilication and powder making process.

All Figures

thumbnail Fig. 1

Flow chart of the extraction of Co and Cu from cobalt white alloy containing high silicon.

In the text
thumbnail Fig. 2

The microstructure image and elemental maps of the cobalt white alloy.

In the text
thumbnail Fig. 3

The relationship between the standard Gibbs free energy of formation of Cu2O, CoO, FeO, MnO, SiO2 and MgO and temperature.

In the text
thumbnail Fig. 4

Thermodynamic phase diagrams of the MnO-MgO-SiO2 system (line AB represents the liquidus line at 1600 °C; P represents the chemical composition of the slag obtained).

In the text
thumbnail Fig. 5

Microstructure and elemental mapping for O of the desilicated alloy before the water atomization (R-Resin).

In the text
thumbnail Fig. 6

Microstructure characterization of the water-atomized alloy powders by SEM-EDS analysis.

In the text
thumbnail Fig. 7

Effect of the following experimental conditions on the leaching rate of metals contained in the alloy powders: (a) excess of sulfuric acid; (b) leaching temperature; (c) leaching time at T = 95 °C.

In the text
thumbnail Fig. 8

X-ray diffraction pattern of the leaching residue.

In the text

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