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Distribution of copper and iron components with hydrogen reduction of copper slag
Journal of Alloys and Compounds 824 (2020) 153910
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Distribution of copper and iron components with hydrogen reduction of copper slag Guorui Qu a, Yonggang Wei a, b, c, *, Bo Li a, b, Hua Wang a, Yindong Yang c, Alexander McLean c a b c
State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, 650093, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China Department of Materials Science and Engineering, University of Toronto, Toronto, ON, M5S3E4, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2019 Received in revised form 10 January 2020 Accepted 17 January 2020 Available online 23 January 2020
In this study, a 70% H2e30% N2 mixture was used to simulate waste cooking oil pyrolysis gas products for reduction of copper slag. The microstructure of slag before and after reduction was analyzed, and the distribution characteristics of copper and iron components during the reduction process were evaluated. The results show that there is always a sedimentation behavior of the matte in the slag during the cleaning process. The residual matte in the cleaned slag is mainly related to chemically dissolved copper. A clear layering was observed between slag and matte after the reduction process. These findings provide support for the recovery of matte from copper slag using waste cooking oil as a green reductant. © 2020 Elsevier B.V. All rights reserved.
Keywords: Copper slag Hydrogen reduction Components distribution Microstructural Image analysis
1. Introduction Copper slag is one of the important by-products of the copper pyrometallurgy process. The copper content in slag is usually higher than 0.5% and has significant recovery value [1]. Copper slag cleaning can be divided into the pyrometallurgical process and hydrometallurgical process. In contrast with the pyrometallurgical process, the hydrometallurgical process is inefficient, and the wastewater is difficult to handle [2]. Therefore, it is wise to use the pyrometallurgical process. The reduction of copper slag usually uses coal, coke, diesel and natural gas as reductants. However, as energy shortages and environmental pollution intensify, the metallurgical industry needs a clean and sustainable reductant to replace traditional reductants. Due to the pyrolysis characteristics of biomass at high temperature, biomass becomes an ideal substitute for traditional reductants. As a renewable resource, biomass is characterized by sustainability and extensive sources. In recent years, there has been an
* Corresponding author. State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, 650093, China. E-mail address: [email protected] (Y. Wei). https://doi.org/10.1016/j.jallcom.2020.153910 0925-8388/© 2020 Elsevier B.V. All rights reserved.
increasing amount of literature on the use of biomass as the reductant in the metallurgical field. Zuo et al. [3] conducted phase equilibrium calculations of reduction in copper slag by biomass and analyzed the feasibility of biomass as reductant from a thermodynamic viewpoint. Li et al. [4] used waste cooking oil as a reductant to reduce magnetic iron in copper smelting slag and the results showed that the magnetic iron content could be reduced to less than 2% and the reduction efficiency of magnetic iron was over 90% after a reduction time of 4 min. Kumar et al. [5] studied the production of iron with waste macadamia as a reductant and obtained higher reduction degree of iron oxide. Zhou et al. [6] studied walnut shell as a reductant to reduce waste copper slag. Their research showed that walnut shell reduction consisted of multistep reactions and produced an iron concentrate of 73.20 wt% with an iron recovery of 95.56% after reduction and magnetic separation. As a waste biomass resource, waste cooking oil has low carbon emissions and reuses the waste [7e9]. The waste cooking oil can reduce the magnetite content in copper slag to less than 3% with a reduction efficiency of more than 90% [4]. The main gas produced by the high-temperature pyrolysis of waste cooking oil is hydrogen, which accounts for more than 70%. The remainder consists of a small amount of CO、CO2 and CH4 and is an ideal substitute for traditional reductant [8].
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To date, several studies have investigated the cleaning of copper slag with hydrogen-based reductant. Liu et al. [10] studied the phase transition during hydrogen reduction of copper slag. The results indicate that the reduction products of the slag are metallic Fe and vitrified SiO2 at 900 C and reduction time of 6 h. Zhang et al. [11] used mixed COeCH4-Ar gas to reduce copper slag. The research results show that the reduction rate of copper slag is a first-order reaction and the apparent activation energy is 58.8 kJ/mol. Nakazato et al. [12] research showed that the rate constant of hydrogen to reduce 2FeO,SiO2 slag is 0.4 mol/(sm3atm), which is much higher than CO as reductant. However, few studies have been conducted on the characteristics and variation of phases within the slag. Based on the characteristics of waste cooking oil pyrolysis, this paper discusses the reduction process of copper slag by hydrogen. The microstructural variations in the slag after reduction were studied in detail, and the distribution of copper and iron components during the reduction process was investigated. 2. Experimental 2.1. Analysis of copper slag sample Fig. 1. XRD pattern of the copper slag sample.
The production process of copper slag is an oxygen enrichment smelting process. The chemical composition of the copper slag is shown in Table 1. The results showed that copper slag contained 33.06 wt% Fe content, of which the Fe3O4 content was 12.9 wt%. The copper content in the slag is 17.82 wt%. As revealed from the XRD patterns (Fig. 1), the copper slag mainly consists of matte, fayalite and magnetite. Scanning electron microscopy (SEM) images of the slag are shown in Fig. 2. Four distinct phases in the slag were identified by EDS: the light gray phase is matte (Point 2); the gray phase is magnetite (Point 3); the dark gray phase is fayalite (Point 4). The matte phase in copper slag exists in two main forms: the elliptical shaped large particle agglomerates, and smaller particles with nearly spherical shape that are randomly dispersed. In addition, elemental copper particles (Point 1) were found within the large elliptical matte particles. In brief, the copper slag contains a large amount of matte and metallic copper. 2.2. Experimental method The apparatus for reduction experiments is shown in Fig. 3. Reduction experiments were performed in an electric tube furnace. An alumina crucible containing 300 g of copper slag was placed in the furnace, heated to 1250 C under a protective N2 atmosphere and held for 30 min after which the mixed gas, consisting of 70% H2e30%N2, was injected into the molten slag at a flow-rate of 0.2 L/ min for a period of 20 min. The temperature was then maintained at 1250 C under a nitrogen atmosphere for 30 min to permit matte droplets to settle in the slag. The reduction process can be divided into four stages (Fig. 4): melting of the slag (Stage I); hydrogen reduction (Stage II); settlement (Stage III); slow cooling (Stage IV). Samples were taken at the different stages and analyzed. Except for the sample in stage IV, the cooling methods for the other stages (stage I, II, III) are rapid cooling. The slag samples were subdivided into four parts based on their
location (Fig. 5): Zone (1) 2.5 cme2.0 cm from the bottom of the crucible, Zone (2) 2.0 cme1.5 cm from the bottom of the crucible, Zone (3) 1.5 cme1.0 cm from the bottom of the crucible, Zone (4) 1 cm from the bottom of the crucible. A representative sample of the slag was embedded in epoxy resin, ground and polished. The sample was analyzed by electron probe microanalysis, using the backscattered electron (BSE) microscopy mode. The obtained images were processed using Image Pro Plus 6.0. The composition of the different phases is analyzed using a fully quantitative electron probe microanalysisewavelength-dispersive (WDS) spectroscopy system. 2.3. Image analysis The BSE image is first spatially calibrated then analyzed based on Gray Scale Segmentation using Image Pro-plus software. With respect to the object to be evaluated, the area and average diameter are used to determine the phase volume fraction. 3. Results and discussion 3.1. Thermomechanical analysis During the copper slag cleaning process, the main chemical reaction occurs in stage II. The cooling process (stage IV) will also cause the phase change in the slag. Therefore, the phase composition of stage II and stage IV was calculated using Factsage software. The phase composition of the reduction (stage II) and slow cooling processes (stage IV) were calculated, and the results are shown in Fig. 6. During the reduction process (Fig. 6(a)), the spinel phase decreases with the increase of the gas phase. The indicates that the mixed gas has reduced Fe3O4, which is also the reason for the increase of the slag liquid phase. The phase composition of the
Table 1 Chemical analysis of the copper slag sample. Component
Cu
Fe (Total)
SiO2
S
Al2O3
CaO
MgO
Zn
Fe3O4
Content, Wt%
17.82
33.06
19.13
8.07
2.96
2.12
1.95
1.83
12.90
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Fig. 2. Microstructures and EDS analyses of the copper slag sample:(a) copper slag sample; (b) a magnified view of the marked area in (a).
Fig. 3. Schematic of laboratory equipment.
cooling process is shown in Fig. 6 (b). As can be seen from the figure, the slag liquid phase decreases rapidly with the decrease of temperature. The slag liquid phase disappears at 1100 C. As the temperature decreases, the olivine phase forms. At the same time, the spinel phase increases with decreasing temperature. 3.2. Microstructural analysis of the copper slag after reduction Fig. 7 shows the microstructures and EDS analyses of the copper
slag sample after slow cooling (Stage IV). The microstructure of Fig. 7(d) (zone 4) was noticeably different from the other zones. The gray area (Point 2) and light gray area (Point 1) can be identified as matte, but the composition of the phases is slightly different. For the dark areas (Point 3), the main elements can be identified as Zn, Fe and S. The observed phenomena suggest settling of matte drops after hydrogen reduction. There was no significant difference in the phases between Fig. 7 (a) and (b). Both show strip-like structures of fayalite, a small amount of magnetite phase and a small amount of
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Fig. 4. Slag reduction process.
round bright matte droplets randomly dispersed in the slag. These observations strongly suggest that hydrogen has contributed to the reduction of magnetite. It is worth noting that a large amount of matte phase was observed in Fig. 7(c). After reduction, the copper content in zone 3 was 1.02%, and the copper contents in zone 1 and 2 were 0.53% and 0.55%. The copper content in zone 3 is significantly higher than that in zone 1 and 2. This is attributed to the fact that the matte drops collide with each other during the reduction process and settle due to influence of gravity. However, because of the influence of slag viscosity, matte does not settle completely to the bottom of the crucible (zone 4). In addition, clear layering was observed between slag and matte after the reduction process. WDS mapping of the slagematte interface is shown in Fig. 8. In the BSE image, the upper part is slag; the middle part is the interaction layer; the lower part is matte. For the matte, the main elements can be identified as Cu, Fe and S. Interestingly, it was found that a small amount of oxygen was distributed in the matte, and this oxygen was combined with iron. This is attributed to the presence of magnetite. The magnetite was dissolved in the liquid matte and precipitated during slow cooling [13]. Within the interaction layer, the main phase is magnetite. Due to the presence of the interaction layer, matte-slag separation is hindered and the settling velocity of the matte droplets is decreased [14]. Closer inspection of Fig. 8 shows that the Fe content increases between the lower part of the interaction layer and the upper part of the matte. A representative BSE image from the matteeslag interaction layer is shown in Fig. 9 which is taken from a similar location as Fig. 8. The resulting compositions from EDS analysis are summarized in Table 2. The analysis on point 2 indicates that it predominantly consists of magnetite while analysis at point 1 indicates mainly ferrous oxide (FeO). A possible explanation for this might be that FeS can reduce magnetite to ferrous oxide (FeO). Surveys such as that conducted by Guo et al. [15] have shown that magnetite is reduced to ferrous oxide (FeO) by FeS. The reaction might occur as follows: 3Fe3O4þFeS ¼ 10FeO þ SO2(g)
(1)
DGq ¼ 85.43e0.053T (kJ/mol)
(2)
And DGq also can be shown as follows: Fig. 5. Schematic of crucible.
DGq ¼ -InKq
Fig. 6. Phase composition at different stages.
(3)
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Fig. 7. Microstructures and EDS analyses of the slag sample after reduction: (a) Zone(1); (b) Zone(2); (c) Zone(3): (d) Zone(4).
Fig. 8. WDS elemental mapping of the matteeslag interaction layer.
Where: DGq is the Gibbs free energy of the reaction (2); Kq is the equilibrium constant of the reaction (2); Kq can be expressed by that:
Kq ¼
a10 Fe O Pso2
(4)
a3Fe3 O4 aFeS
Where: aFe3O4 is the activity of Fe3O4 in molten slag; aFeO is the activity of FeO in molten slag; aFeS is the activity of FeS in molten slag; PSO2 is the partial pressure of SO2 in the smelting process. Assume that of SO2 partial pressure of 102 atm and temperature of 1250 C, the relationships between aFeS and aFe3O4 in the copper slag with different values of aFeO were calculated by Eq. (4). It can be seen from equation (4) that when aFeO is a constant value, the a(Fe3O4) decreases significantly with the increase of aFeS. The results show that magnetite is readily reduced to FeO. Zivkovic et al.
Fig. 9. Representative BSE image from the matteeslag interaction layer.
Table 2 EDS Analysis of the slagematte interaction layer. Measured Content of Element, Atom %
Point 1 Point 2
Possible phase
O
Al
Si
S
Ti
Fe
Cu
49.73 52.81
0.68 6.13
0.57 0.48
0.25
48.16 38.81
0.87
0.44
Zn
Pb
1.06
0.01
FeO Fe3O4
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G. Qu et al. / Journal of Alloys and Compounds 824 (2020) 153910
[16] explained the phenomenon using the ionic theory of slag structure. They believed that the O2 ions activity in the region is relatively large, which is beneficial for FeS to reduce Fe3O4. 3.3. Distribution of Cu and Fe components during reduction process Image Pro Plus can be used to measure the volume fraction of carbides precipitated in alloys, and percentage area fraction of concrete pores [17,18]. In this paper, Image Pro Plus 6.0 is used to measure the phase area percentage of the slag and the average diameter of the matte droplets. The area percentage of magnetite and matte in different reduction stages are presented in Fig. 10. According to Fig. 7(d), matte droplets will settle to zone (4), so zone (4) is not included in Fig. 10. Fig. 10(a) presents the summary statistics for the area percentage of magnetite in different reduction stages. At stage (I), the area percentage of magnetite in each zone is higher. By contrast, the area percentage of magnetite decreased significantly at stage (II) since magnetite was reduced by hydrogen. Since the purpose of stage (III) is the settling of matte droplets, the area percentage of magnetite shows no significant difference compared with stage (II). At stage (IV), zone (3) reveals that there has been a marked increase in the area percentage of magnetite. This phenomenon has been reported in previous studies [19,20]. Due to the wettability of the copper and copper compounds in the slag, the copper and copper compounds are adsorbed on the surface of the magnetite during the sedimentation process [21]. In addition to this aspect, slow cooling could promote the formation and crystal growth of magnetite in the synthetic slag [22]. Fig. 10(b) shows the summary statistics for the area percentage of matte in different reduction stages. At stage (I), the area percentage of matte in each zone is higher. At stage (II), due to a portion of the matte droplets settling to the bottom of the crucible, the area percentage of matte significantly decreases. The reduction of Fe3O4 by hydrogen reduces the viscosity of the slag and improves the settling conditions of the matte droplets. The relationship between slag viscosity and Fe3O4 content was calculated by using Factsage software (Fig. 11). As can be seen from the Figure, the slag viscosity decreases with the decrease of Fe3O4 content. The reason is that the decrease of solid fraction in slag [23]. At the same time, the process of injecting the gas mixture also acts to stir the slag, providing kinetic conditions that favour the collision and aggregation of the matte droplets. The general expression for the settling
Fig. 11. Relationship between slag viscosity and Fe3O4 content.
velocity of matte droplets can be shown as follows [15,24]: V ¼ 2gr2(rm-rs)/9m
(5)
where V is the matte droplet settling velocity (m/h), g is the gravitational acceleration (m/s2), r is the radius of the droplet (m), rm is the matte density (kg/m3), rs is the density of slag(kg/m3), m is the viscosity of the slag (Pa‧s). As can be seen from Eq. (5), with a decrease in viscosity of the slag, the settling velocity increases. As mentioned above, reduction of Fe3O4 leads to a decrease in viscosity of the slag [4,25] while gas agitation causes collision and accumulation of the matte droplets, thus increasing their radius and improving the settling velocity which enhances the effectiveness of matte separation from the slag. In stage (III), the area percentage of matte decreases in zone (2) and increases in zone (3) due to the droplets settling by gravity. In stage (IV) the area percentage of matte in each zone increased. A possible explanation for this might be that slow cooling could reduce copper solubility, which results in the precipitation of chemically dissolved copper [26]. In addition, the area percentage of matte significantly increased in zone (3) thus confirming the
Fig. 10. Area percentage of phases at different reduction stages: (a) magnetite; (b) matte.
G. Qu et al. / Journal of Alloys and Compounds 824 (2020) 153910
settling behavior of matte droplets during slow cooling. In order to understand the distribution of matte droplets in slag, a statistical analysis on the diameter of matte droplets was performed. Fig. 12 provides a summary of the results. At stage (I) (Fig. 12(a)), the matte grain diameter in zone (1) is small, mostly 2e4 mm. The number of droplets 2e4 mm in zone (2) decreases while the number with diameter 6e8 mm is significantly higher than that in zone (1). At the same time, there are a few droplets with diameter larger than 100 mm. There are still some small droplets (2e6 mm) in zone (3). In contrast with zone (1), droplets larger than 100 mm increased within zone (2) and zone (3). In summary, at stage (I), the matte droplets gradually increase from top to bottom in the slag, that is, the matte has begun to accumulate and settle during the melting process. At stage (II) (Fig. 12(b)), the number of matte droplets significantly diminished in each zone since gas agitation during the hydrogen reduction process causes matte droplets in the slag to accumulate and settle to the bottom of the crucible. In addition, the reduction of Fe3O4 by hydrogen reduces the slag viscosity, which facilitates the sedimentation process. The number of matte droplets in stage (III) is significantly higher than that in stage (II). This is attributed to a decrease in Fe3O4 content which previous studies indicate can cause precipitation of dissolved copper in the slag [26,27]. The total dissolved copper in slag is the sum of the sulphidic and oxidic contents. The expression for total dissolved copper is as follow: (%Cu)t¼(%Cu)O þ (%Cu)S
(6)
7
Where (%Cu)t is total dissolved copper, (%Cu)O is oxidic dissolved copper, (%Cu)S is sulphidic dissolved copper. Mackey [28] proposed a model for commercial slag as follow: (%Cu)S ¼ 0.00495[S]Sl[Cu]mt
(7)
where [S]Sl is sulphur content of slag, [Cu]mt is copper content of matte. As can be seen from Eq. (7), with a decrease in the sulphur content of the slag, the sulphidic copper content decreases. Previous research has established that a decrease in Fe3O4 content of the slag leads to a decrease in sulphur content [26]. Toguri and Santander have reported the following expression for oxidic copper dissolved in slag at 1300 C [29]: (%Cu)O ¼ 29.73aCuO0.5
(8)
The oxidation of copper is represented by: Cuþ1/4O2 ¼ CuO0.5
K¼
aCuo0:5 1=4
P O2 aCu
(9)
(10)
Here K is the equilibrium constant of the reaction expressed by equation (9), aCuO0.5 is CuO0.5 activity, aCu is Cu activity, PO2 is oxygen partial pressure.
Fig. 12. Grain diameter of matte:(a) Stage (I); (b) Stage (II); (c) Stage (III); (d) Stage (IV).
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G. Qu et al. / Journal of Alloys and Compounds 824 (2020) 153910
Fig. 13. Schematic diagram for the reduction of copper slag.
Hence, aCuO0.5 can be represented as: aCuO0.5 ¼ K*aCu*P1/4 O2
(11)
Inserting Eq [11]. into Eq. [8], the oxidic dissolved copper is expressed by: (%Cu)O ¼ 29.73K*aCu*P1/4 O2
(12)
As can be seen from Eq. (12), the oxidic dissolved copper decreases with a decrease in oxygen partial pressure. Since a decrease in Fe3O4 content causes a decrease in oxygen partial pressure and consequently a decrease in chemically dissolved oxidic copper, this will result in the precipitation of oxidic copper from the slag. At stage (IV), the number of matte droplets significantly decreases. This is attributed to the high temperature during preliminary slow cooling, when the matte droplets have considerable opportunity for sedimentation. 3.4. Analysis of reduction process Based on the previous discussion, a schematic diagram of the reduction process is shown in Fig. 13. During the melting process (Stage I), the slag viscosity decreases with increasing temperature. The reason is that the temperature increase is conducive to improving the slag fluidity. However, due to the high Fe3O4 content in the slag, the viscosity of the slag is still large. At the same time, the matte begins to agglomerate and settle, but this behavior is hindered because of the high viscosity of the slag. At stage II, the reduction of Fe3O4 by hydrogen reduces the viscosity of the slag, thereby improving the sedimentation conditions for the matte. At the same time, hydrogen injection stirs the slag, promoting conditions for the collision and aggregation of the matte droplets. As a consequence, the matte and magnetite contents in the slag are significantly reduced. During the settlement period (stage III), the matte droplets settle downward and the number of droplets gradually increases from top to bottom of the slag. The viscosity of the slag remains unchanged in the stage. The reason is that there is no temperature change and chemical reaction in stage (III). At stage
(IV), during the early period of slow cooling when the temperature is still sufficiently high, the matte droplets continue to settle. The viscosity of slag increased with the decrease of temperature and the precipitation of Fe3O4. At the same time, the slag matte interface forms a clear boundary. There is a large amount of magnetite above the matte layer, and some of the magnetite reacts with matte to form FeO. The large amount of magnetite sedimentation above the matte layer contributes to increasing copper loss in the slag [20]. 4. Conclusions In the present study, the distribution characteristics of copper and iron components during reduction of copper slag by hydrogen were investigated. The amount of matte droplets in the slag is significantly reduced after hydrogen reduction. The reduction of Fe3O4 by hydrogen reduces the viscosity of the slag and improves the settling conditions of the matte droplets. At the same time, the process of injecting hydrogen stirs the slag, providing favorable conditions for the collision and aggregation of the matte droplets. During the sedimentation stage, the matte droplets in the slag are distributed in a gradient, increasing gradually from top to bottom. Eventually, after the reduction process, a small portion of the matte droplets remain in the slag, and a clear interaction layer was observed between the slag and matte phases. In this interaction layer, where the main phase is magnetite, a small portion of the magnetite will react with FeS in the underlying matte layer to form FeO. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Guorui Qu: Conceptualization, Methodology, Formal analysis, Investigation, Validation, Visualization, Writing - original draft, Data curation. Yonggang Wei: Writing - review & editing,
G. Qu et al. / Journal of Alloys and Compounds 824 (2020) 153910
Supervision, Resources, Project administration, Conceptualization. Bo Li: Conceptualization, Supervision, Resources. Hua Wang: Conceptualization, Supervision, Resources, Project administration. Yindong Yang: Conceptualization. Alexander McLean: Writing review & editing. Acknowledgments This work was supported by the National Natural Science Foundation of China (51664039 and 51974142); and the Analysis and Testing Foundation of Kunming University of Science and Technology (2017P20161102004,2018M20172228012). References [1] Bipra Gorai, R.K. Jana, Premchand, Characteristics and utilisation of copper slagda review, Resour. Conserv. Recycl. 39 (4) (2003) 299e313. [2] Huiting Shen, Eric Forssberg, An overview of recovery of metals from slags, Waste Manag. 23 (10) (2003) 933e949. [3] Zongliang Zuo, et al., Thermodynamic analysis of reduction in copper slag by biomass molding compound based on phase equilibrium calculating model, J. Therm. Anal. Calorim. 132 (2) (2018) 1277e1289. [4] Bo Li, et al., Smelting reduction and kinetics analysis of magnetic iron in copper slag using waste cooking oil, Sci. Rep. 7 (1) (2017) 2406. [5] Uttam Kumar, et al., Cleaner production of iron by using waste macadamia biomass as a carbon resource, J. Clean. Prod. 158 (2017) 218e224. [6] Shiwei Zhou, et al., Cleaner recycling of iron from waste copper slag by using walnut shell char as green reductant, J. Clean. Prod. 217 (2019) 423e431. [7] de Araújo, Carlos Daniel Mandolesi, et al., Biodiesel production from used cooking oil: a review, Renew. Sustain. Energy Rev. 27 (2013) 445e452. [8] S.N. Gebremariam, J.M. Marchetti, Economics of biodiesel production, Energy Convers. Manag. 168 (2018) 74e84. [9] Shiwei Zhou, et al., Reduction of copper smelting slag using waste cooking oil, J. Clean. Prod. 236 (2019) 117668. [10] H.L. Liu, et al., Phase transformation during the reduction process of coppercontaining slag with hydrogen, Chin. J. Process Eng. 2 (2012) 265e269. [11] Zhang Huaiwei, et al., Reduction of molten copper slags with mixed CO-CH4Ar gas, Metall. Mater. Trans. B 45 (2) (2014) 582e589. [12] Hideki Ono-Nakazato, et al., Gaseous reduction behavior of iron oxide in mineral phases and in CaO-SiO2-FeO slag powder, Miner. Process. Extr. Metall.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Distribution of copper and iron components with hydrogen reduction of copper slag Guorui Qu a, Yonggang Wei a, b, c, *, Bo Li a, b, Hua Wang a, Yindong Yang c, Alexander McLean c a b c
State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, 650093, China Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, 650093, China Department of Materials Science and Engineering, University of Toronto, Toronto, ON, M5S3E4, Canada
a r t i c l e i n f o
a b s t r a c t
Article history: Received 31 October 2019 Received in revised form 10 January 2020 Accepted 17 January 2020 Available online 23 January 2020
In this study, a 70% H2e30% N2 mixture was used to simulate waste cooking oil pyrolysis gas products for reduction of copper slag. The microstructure of slag before and after reduction was analyzed, and the distribution characteristics of copper and iron components during the reduction process were evaluated. The results show that there is always a sedimentation behavior of the matte in the slag during the cleaning process. The residual matte in the cleaned slag is mainly related to chemically dissolved copper. A clear layering was observed between slag and matte after the reduction process. These findings provide support for the recovery of matte from copper slag using waste cooking oil as a green reductant. © 2020 Elsevier B.V. All rights reserved.
Keywords: Copper slag Hydrogen reduction Components distribution Microstructural Image analysis
1. Introduction Copper slag is one of the important by-products of the copper pyrometallurgy process. The copper content in slag is usually higher than 0.5% and has significant recovery value [1]. Copper slag cleaning can be divided into the pyrometallurgical process and hydrometallurgical process. In contrast with the pyrometallurgical process, the hydrometallurgical process is inefficient, and the wastewater is difficult to handle [2]. Therefore, it is wise to use the pyrometallurgical process. The reduction of copper slag usually uses coal, coke, diesel and natural gas as reductants. However, as energy shortages and environmental pollution intensify, the metallurgical industry needs a clean and sustainable reductant to replace traditional reductants. Due to the pyrolysis characteristics of biomass at high temperature, biomass becomes an ideal substitute for traditional reductants. As a renewable resource, biomass is characterized by sustainability and extensive sources. In recent years, there has been an
* Corresponding author. State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of Science and Technology, Kunming, 650093, China. E-mail address: [email protected] (Y. Wei). https://doi.org/10.1016/j.jallcom.2020.153910 0925-8388/© 2020 Elsevier B.V. All rights reserved.
increasing amount of literature on the use of biomass as the reductant in the metallurgical field. Zuo et al. [3] conducted phase equilibrium calculations of reduction in copper slag by biomass and analyzed the feasibility of biomass as reductant from a thermodynamic viewpoint. Li et al. [4] used waste cooking oil as a reductant to reduce magnetic iron in copper smelting slag and the results showed that the magnetic iron content could be reduced to less than 2% and the reduction efficiency of magnetic iron was over 90% after a reduction time of 4 min. Kumar et al. [5] studied the production of iron with waste macadamia as a reductant and obtained higher reduction degree of iron oxide. Zhou et al. [6] studied walnut shell as a reductant to reduce waste copper slag. Their research showed that walnut shell reduction consisted of multistep reactions and produced an iron concentrate of 73.20 wt% with an iron recovery of 95.56% after reduction and magnetic separation. As a waste biomass resource, waste cooking oil has low carbon emissions and reuses the waste [7e9]. The waste cooking oil can reduce the magnetite content in copper slag to less than 3% with a reduction efficiency of more than 90% [4]. The main gas produced by the high-temperature pyrolysis of waste cooking oil is hydrogen, which accounts for more than 70%. The remainder consists of a small amount of CO、CO2 and CH4 and is an ideal substitute for traditional reductant [8].
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To date, several studies have investigated the cleaning of copper slag with hydrogen-based reductant. Liu et al. [10] studied the phase transition during hydrogen reduction of copper slag. The results indicate that the reduction products of the slag are metallic Fe and vitrified SiO2 at 900 C and reduction time of 6 h. Zhang et al. [11] used mixed COeCH4-Ar gas to reduce copper slag. The research results show that the reduction rate of copper slag is a first-order reaction and the apparent activation energy is 58.8 kJ/mol. Nakazato et al. [12] research showed that the rate constant of hydrogen to reduce 2FeO,SiO2 slag is 0.4 mol/(sm3atm), which is much higher than CO as reductant. However, few studies have been conducted on the characteristics and variation of phases within the slag. Based on the characteristics of waste cooking oil pyrolysis, this paper discusses the reduction process of copper slag by hydrogen. The microstructural variations in the slag after reduction were studied in detail, and the distribution of copper and iron components during the reduction process was investigated. 2. Experimental 2.1. Analysis of copper slag sample Fig. 1. XRD pattern of the copper slag sample.
The production process of copper slag is an oxygen enrichment smelting process. The chemical composition of the copper slag is shown in Table 1. The results showed that copper slag contained 33.06 wt% Fe content, of which the Fe3O4 content was 12.9 wt%. The copper content in the slag is 17.82 wt%. As revealed from the XRD patterns (Fig. 1), the copper slag mainly consists of matte, fayalite and magnetite. Scanning electron microscopy (SEM) images of the slag are shown in Fig. 2. Four distinct phases in the slag were identified by EDS: the light gray phase is matte (Point 2); the gray phase is magnetite (Point 3); the dark gray phase is fayalite (Point 4). The matte phase in copper slag exists in two main forms: the elliptical shaped large particle agglomerates, and smaller particles with nearly spherical shape that are randomly dispersed. In addition, elemental copper particles (Point 1) were found within the large elliptical matte particles. In brief, the copper slag contains a large amount of matte and metallic copper. 2.2. Experimental method The apparatus for reduction experiments is shown in Fig. 3. Reduction experiments were performed in an electric tube furnace. An alumina crucible containing 300 g of copper slag was placed in the furnace, heated to 1250 C under a protective N2 atmosphere and held for 30 min after which the mixed gas, consisting of 70% H2e30%N2, was injected into the molten slag at a flow-rate of 0.2 L/ min for a period of 20 min. The temperature was then maintained at 1250 C under a nitrogen atmosphere for 30 min to permit matte droplets to settle in the slag. The reduction process can be divided into four stages (Fig. 4): melting of the slag (Stage I); hydrogen reduction (Stage II); settlement (Stage III); slow cooling (Stage IV). Samples were taken at the different stages and analyzed. Except for the sample in stage IV, the cooling methods for the other stages (stage I, II, III) are rapid cooling. The slag samples were subdivided into four parts based on their
location (Fig. 5): Zone (1) 2.5 cme2.0 cm from the bottom of the crucible, Zone (2) 2.0 cme1.5 cm from the bottom of the crucible, Zone (3) 1.5 cme1.0 cm from the bottom of the crucible, Zone (4) 1 cm from the bottom of the crucible. A representative sample of the slag was embedded in epoxy resin, ground and polished. The sample was analyzed by electron probe microanalysis, using the backscattered electron (BSE) microscopy mode. The obtained images were processed using Image Pro Plus 6.0. The composition of the different phases is analyzed using a fully quantitative electron probe microanalysisewavelength-dispersive (WDS) spectroscopy system. 2.3. Image analysis The BSE image is first spatially calibrated then analyzed based on Gray Scale Segmentation using Image Pro-plus software. With respect to the object to be evaluated, the area and average diameter are used to determine the phase volume fraction. 3. Results and discussion 3.1. Thermomechanical analysis During the copper slag cleaning process, the main chemical reaction occurs in stage II. The cooling process (stage IV) will also cause the phase change in the slag. Therefore, the phase composition of stage II and stage IV was calculated using Factsage software. The phase composition of the reduction (stage II) and slow cooling processes (stage IV) were calculated, and the results are shown in Fig. 6. During the reduction process (Fig. 6(a)), the spinel phase decreases with the increase of the gas phase. The indicates that the mixed gas has reduced Fe3O4, which is also the reason for the increase of the slag liquid phase. The phase composition of the
Table 1 Chemical analysis of the copper slag sample. Component
Cu
Fe (Total)
SiO2
S
Al2O3
CaO
MgO
Zn
Fe3O4
Content, Wt%
17.82
33.06
19.13
8.07
2.96
2.12
1.95
1.83
12.90
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Fig. 2. Microstructures and EDS analyses of the copper slag sample:(a) copper slag sample; (b) a magnified view of the marked area in (a).
Fig. 3. Schematic of laboratory equipment.
cooling process is shown in Fig. 6 (b). As can be seen from the figure, the slag liquid phase decreases rapidly with the decrease of temperature. The slag liquid phase disappears at 1100 C. As the temperature decreases, the olivine phase forms. At the same time, the spinel phase increases with decreasing temperature. 3.2. Microstructural analysis of the copper slag after reduction Fig. 7 shows the microstructures and EDS analyses of the copper
slag sample after slow cooling (Stage IV). The microstructure of Fig. 7(d) (zone 4) was noticeably different from the other zones. The gray area (Point 2) and light gray area (Point 1) can be identified as matte, but the composition of the phases is slightly different. For the dark areas (Point 3), the main elements can be identified as Zn, Fe and S. The observed phenomena suggest settling of matte drops after hydrogen reduction. There was no significant difference in the phases between Fig. 7 (a) and (b). Both show strip-like structures of fayalite, a small amount of magnetite phase and a small amount of
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Fig. 4. Slag reduction process.
round bright matte droplets randomly dispersed in the slag. These observations strongly suggest that hydrogen has contributed to the reduction of magnetite. It is worth noting that a large amount of matte phase was observed in Fig. 7(c). After reduction, the copper content in zone 3 was 1.02%, and the copper contents in zone 1 and 2 were 0.53% and 0.55%. The copper content in zone 3 is significantly higher than that in zone 1 and 2. This is attributed to the fact that the matte drops collide with each other during the reduction process and settle due to influence of gravity. However, because of the influence of slag viscosity, matte does not settle completely to the bottom of the crucible (zone 4). In addition, clear layering was observed between slag and matte after the reduction process. WDS mapping of the slagematte interface is shown in Fig. 8. In the BSE image, the upper part is slag; the middle part is the interaction layer; the lower part is matte. For the matte, the main elements can be identified as Cu, Fe and S. Interestingly, it was found that a small amount of oxygen was distributed in the matte, and this oxygen was combined with iron. This is attributed to the presence of magnetite. The magnetite was dissolved in the liquid matte and precipitated during slow cooling [13]. Within the interaction layer, the main phase is magnetite. Due to the presence of the interaction layer, matte-slag separation is hindered and the settling velocity of the matte droplets is decreased [14]. Closer inspection of Fig. 8 shows that the Fe content increases between the lower part of the interaction layer and the upper part of the matte. A representative BSE image from the matteeslag interaction layer is shown in Fig. 9 which is taken from a similar location as Fig. 8. The resulting compositions from EDS analysis are summarized in Table 2. The analysis on point 2 indicates that it predominantly consists of magnetite while analysis at point 1 indicates mainly ferrous oxide (FeO). A possible explanation for this might be that FeS can reduce magnetite to ferrous oxide (FeO). Surveys such as that conducted by Guo et al. [15] have shown that magnetite is reduced to ferrous oxide (FeO) by FeS. The reaction might occur as follows: 3Fe3O4þFeS ¼ 10FeO þ SO2(g)
(1)
DGq ¼ 85.43e0.053T (kJ/mol)
(2)
And DGq also can be shown as follows: Fig. 5. Schematic of crucible.
DGq ¼ -InKq
Fig. 6. Phase composition at different stages.
(3)
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Fig. 7. Microstructures and EDS analyses of the slag sample after reduction: (a) Zone(1); (b) Zone(2); (c) Zone(3): (d) Zone(4).
Fig. 8. WDS elemental mapping of the matteeslag interaction layer.
Where: DGq is the Gibbs free energy of the reaction (2); Kq is the equilibrium constant of the reaction (2); Kq can be expressed by that:
Kq ¼
a10 Fe O Pso2
(4)
a3Fe3 O4 aFeS
Where: aFe3O4 is the activity of Fe3O4 in molten slag; aFeO is the activity of FeO in molten slag; aFeS is the activity of FeS in molten slag; PSO2 is the partial pressure of SO2 in the smelting process. Assume that of SO2 partial pressure of 102 atm and temperature of 1250 C, the relationships between aFeS and aFe3O4 in the copper slag with different values of aFeO were calculated by Eq. (4). It can be seen from equation (4) that when aFeO is a constant value, the a(Fe3O4) decreases significantly with the increase of aFeS. The results show that magnetite is readily reduced to FeO. Zivkovic et al.
Fig. 9. Representative BSE image from the matteeslag interaction layer.
Table 2 EDS Analysis of the slagematte interaction layer. Measured Content of Element, Atom %
Point 1 Point 2
Possible phase
O
Al
Si
S
Ti
Fe
Cu
49.73 52.81
0.68 6.13
0.57 0.48
0.25
48.16 38.81
0.87
0.44
Zn
Pb
1.06
0.01
FeO Fe3O4
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[16] explained the phenomenon using the ionic theory of slag structure. They believed that the O2 ions activity in the region is relatively large, which is beneficial for FeS to reduce Fe3O4. 3.3. Distribution of Cu and Fe components during reduction process Image Pro Plus can be used to measure the volume fraction of carbides precipitated in alloys, and percentage area fraction of concrete pores [17,18]. In this paper, Image Pro Plus 6.0 is used to measure the phase area percentage of the slag and the average diameter of the matte droplets. The area percentage of magnetite and matte in different reduction stages are presented in Fig. 10. According to Fig. 7(d), matte droplets will settle to zone (4), so zone (4) is not included in Fig. 10. Fig. 10(a) presents the summary statistics for the area percentage of magnetite in different reduction stages. At stage (I), the area percentage of magnetite in each zone is higher. By contrast, the area percentage of magnetite decreased significantly at stage (II) since magnetite was reduced by hydrogen. Since the purpose of stage (III) is the settling of matte droplets, the area percentage of magnetite shows no significant difference compared with stage (II). At stage (IV), zone (3) reveals that there has been a marked increase in the area percentage of magnetite. This phenomenon has been reported in previous studies [19,20]. Due to the wettability of the copper and copper compounds in the slag, the copper and copper compounds are adsorbed on the surface of the magnetite during the sedimentation process [21]. In addition to this aspect, slow cooling could promote the formation and crystal growth of magnetite in the synthetic slag [22]. Fig. 10(b) shows the summary statistics for the area percentage of matte in different reduction stages. At stage (I), the area percentage of matte in each zone is higher. At stage (II), due to a portion of the matte droplets settling to the bottom of the crucible, the area percentage of matte significantly decreases. The reduction of Fe3O4 by hydrogen reduces the viscosity of the slag and improves the settling conditions of the matte droplets. The relationship between slag viscosity and Fe3O4 content was calculated by using Factsage software (Fig. 11). As can be seen from the Figure, the slag viscosity decreases with the decrease of Fe3O4 content. The reason is that the decrease of solid fraction in slag [23]. At the same time, the process of injecting the gas mixture also acts to stir the slag, providing kinetic conditions that favour the collision and aggregation of the matte droplets. The general expression for the settling
Fig. 11. Relationship between slag viscosity and Fe3O4 content.
velocity of matte droplets can be shown as follows [15,24]: V ¼ 2gr2(rm-rs)/9m
(5)
where V is the matte droplet settling velocity (m/h), g is the gravitational acceleration (m/s2), r is the radius of the droplet (m), rm is the matte density (kg/m3), rs is the density of slag(kg/m3), m is the viscosity of the slag (Pa‧s). As can be seen from Eq. (5), with a decrease in viscosity of the slag, the settling velocity increases. As mentioned above, reduction of Fe3O4 leads to a decrease in viscosity of the slag [4,25] while gas agitation causes collision and accumulation of the matte droplets, thus increasing their radius and improving the settling velocity which enhances the effectiveness of matte separation from the slag. In stage (III), the area percentage of matte decreases in zone (2) and increases in zone (3) due to the droplets settling by gravity. In stage (IV) the area percentage of matte in each zone increased. A possible explanation for this might be that slow cooling could reduce copper solubility, which results in the precipitation of chemically dissolved copper [26]. In addition, the area percentage of matte significantly increased in zone (3) thus confirming the
Fig. 10. Area percentage of phases at different reduction stages: (a) magnetite; (b) matte.
G. Qu et al. / Journal of Alloys and Compounds 824 (2020) 153910
settling behavior of matte droplets during slow cooling. In order to understand the distribution of matte droplets in slag, a statistical analysis on the diameter of matte droplets was performed. Fig. 12 provides a summary of the results. At stage (I) (Fig. 12(a)), the matte grain diameter in zone (1) is small, mostly 2e4 mm. The number of droplets 2e4 mm in zone (2) decreases while the number with diameter 6e8 mm is significantly higher than that in zone (1). At the same time, there are a few droplets with diameter larger than 100 mm. There are still some small droplets (2e6 mm) in zone (3). In contrast with zone (1), droplets larger than 100 mm increased within zone (2) and zone (3). In summary, at stage (I), the matte droplets gradually increase from top to bottom in the slag, that is, the matte has begun to accumulate and settle during the melting process. At stage (II) (Fig. 12(b)), the number of matte droplets significantly diminished in each zone since gas agitation during the hydrogen reduction process causes matte droplets in the slag to accumulate and settle to the bottom of the crucible. In addition, the reduction of Fe3O4 by hydrogen reduces the slag viscosity, which facilitates the sedimentation process. The number of matte droplets in stage (III) is significantly higher than that in stage (II). This is attributed to a decrease in Fe3O4 content which previous studies indicate can cause precipitation of dissolved copper in the slag [26,27]. The total dissolved copper in slag is the sum of the sulphidic and oxidic contents. The expression for total dissolved copper is as follow: (%Cu)t¼(%Cu)O þ (%Cu)S
(6)
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Where (%Cu)t is total dissolved copper, (%Cu)O is oxidic dissolved copper, (%Cu)S is sulphidic dissolved copper. Mackey [28] proposed a model for commercial slag as follow: (%Cu)S ¼ 0.00495[S]Sl[Cu]mt
(7)
where [S]Sl is sulphur content of slag, [Cu]mt is copper content of matte. As can be seen from Eq. (7), with a decrease in the sulphur content of the slag, the sulphidic copper content decreases. Previous research has established that a decrease in Fe3O4 content of the slag leads to a decrease in sulphur content [26]. Toguri and Santander have reported the following expression for oxidic copper dissolved in slag at 1300 C [29]: (%Cu)O ¼ 29.73aCuO0.5
(8)
The oxidation of copper is represented by: Cuþ1/4O2 ¼ CuO0.5
K¼
aCuo0:5 1=4
P O2 aCu
(9)
(10)
Here K is the equilibrium constant of the reaction expressed by equation (9), aCuO0.5 is CuO0.5 activity, aCu is Cu activity, PO2 is oxygen partial pressure.
Fig. 12. Grain diameter of matte:(a) Stage (I); (b) Stage (II); (c) Stage (III); (d) Stage (IV).
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Fig. 13. Schematic diagram for the reduction of copper slag.
Hence, aCuO0.5 can be represented as: aCuO0.5 ¼ K*aCu*P1/4 O2
(11)
Inserting Eq [11]. into Eq. [8], the oxidic dissolved copper is expressed by: (%Cu)O ¼ 29.73K*aCu*P1/4 O2
(12)
As can be seen from Eq. (12), the oxidic dissolved copper decreases with a decrease in oxygen partial pressure. Since a decrease in Fe3O4 content causes a decrease in oxygen partial pressure and consequently a decrease in chemically dissolved oxidic copper, this will result in the precipitation of oxidic copper from the slag. At stage (IV), the number of matte droplets significantly decreases. This is attributed to the high temperature during preliminary slow cooling, when the matte droplets have considerable opportunity for sedimentation. 3.4. Analysis of reduction process Based on the previous discussion, a schematic diagram of the reduction process is shown in Fig. 13. During the melting process (Stage I), the slag viscosity decreases with increasing temperature. The reason is that the temperature increase is conducive to improving the slag fluidity. However, due to the high Fe3O4 content in the slag, the viscosity of the slag is still large. At the same time, the matte begins to agglomerate and settle, but this behavior is hindered because of the high viscosity of the slag. At stage II, the reduction of Fe3O4 by hydrogen reduces the viscosity of the slag, thereby improving the sedimentation conditions for the matte. At the same time, hydrogen injection stirs the slag, promoting conditions for the collision and aggregation of the matte droplets. As a consequence, the matte and magnetite contents in the slag are significantly reduced. During the settlement period (stage III), the matte droplets settle downward and the number of droplets gradually increases from top to bottom of the slag. The viscosity of the slag remains unchanged in the stage. The reason is that there is no temperature change and chemical reaction in stage (III). At stage
(IV), during the early period of slow cooling when the temperature is still sufficiently high, the matte droplets continue to settle. The viscosity of slag increased with the decrease of temperature and the precipitation of Fe3O4. At the same time, the slag matte interface forms a clear boundary. There is a large amount of magnetite above the matte layer, and some of the magnetite reacts with matte to form FeO. The large amount of magnetite sedimentation above the matte layer contributes to increasing copper loss in the slag [20]. 4. Conclusions In the present study, the distribution characteristics of copper and iron components during reduction of copper slag by hydrogen were investigated. The amount of matte droplets in the slag is significantly reduced after hydrogen reduction. The reduction of Fe3O4 by hydrogen reduces the viscosity of the slag and improves the settling conditions of the matte droplets. At the same time, the process of injecting hydrogen stirs the slag, providing favorable conditions for the collision and aggregation of the matte droplets. During the sedimentation stage, the matte droplets in the slag are distributed in a gradient, increasing gradually from top to bottom. Eventually, after the reduction process, a small portion of the matte droplets remain in the slag, and a clear interaction layer was observed between the slag and matte phases. In this interaction layer, where the main phase is magnetite, a small portion of the magnetite will react with FeS in the underlying matte layer to form FeO. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Guorui Qu: Conceptualization, Methodology, Formal analysis, Investigation, Validation, Visualization, Writing - original draft, Data curation. Yonggang Wei: Writing - review & editing,
G. Qu et al. / Journal of Alloys and Compounds 824 (2020) 153910
Supervision, Resources, Project administration, Conceptualization. Bo Li: Conceptualization, Supervision, Resources. Hua Wang: Conceptualization, Supervision, Resources, Project administration. Yindong Yang: Conceptualization. Alexander McLean: Writing review & editing. Acknowledgments This work was supported by the National Natural Science Foundation of China (51664039 and 51974142); and the Analysis and Testing Foundation of Kunming University of Science and Technology (2017P20161102004,2018M20172228012). References [1] Bipra Gorai, R.K. Jana, Premchand, Characteristics and utilisation of copper slagda review, Resour. Conserv. Recycl. 39 (4) (2003) 299e313. [2] Huiting Shen, Eric Forssberg, An overview of recovery of metals from slags, Waste Manag. 23 (10) (2003) 933e949. [3] Zongliang Zuo, et al., Thermodynamic analysis of reduction in copper slag by biomass molding compound based on phase equilibrium calculating model, J. Therm. Anal. Calorim. 132 (2) (2018) 1277e1289. [4] Bo Li, et al., Smelting reduction and kinetics analysis of magnetic iron in copper slag using waste cooking oil, Sci. Rep. 7 (1) (2017) 2406. [5] Uttam Kumar, et al., Cleaner production of iron by using waste macadamia biomass as a carbon resource, J. Clean. Prod. 158 (2017) 218e224. [6] Shiwei Zhou, et al., Cleaner recycling of iron from waste copper slag by using walnut shell char as green reductant, J. Clean. Prod. 217 (2019) 423e431. [7] de Araújo, Carlos Daniel Mandolesi, et al., Biodiesel production from used cooking oil: a review, Renew. Sustain. Energy Rev. 27 (2013) 445e452. [8] S.N. Gebremariam, J.M. Marchetti, Economics of biodiesel production, Energy Convers. Manag. 168 (2018) 74e84. [9] Shiwei Zhou, et al., Reduction of copper smelting slag using waste cooking oil, J. Clean. Prod. 236 (2019) 117668. [10] H.L. Liu, et al., Phase transformation during the reduction process of coppercontaining slag with hydrogen, Chin. J. Process Eng. 2 (2012) 265e269. [11] Zhang Huaiwei, et al., Reduction of molten copper slags with mixed CO-CH4Ar gas, Metall. Mater. Trans. B 45 (2) (2014) 582e589. [12] Hideki Ono-Nakazato, et al., Gaseous reduction behavior of iron oxide in mineral phases and in CaO-SiO2-FeO slag powder, Miner. Process. Extr. Metall.
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