Alternative Reduction of Copper Matte in Reduction Process of Copper Slag

Abstract

In order to make full use of copper slag and solve the problem of its storage, the direct reduction of copper slag to antibacterial stainless steel was proposed. However, mixture of copper matte in the product reduces the value of the alloy. In this article, we studied the changes of Cu₂S and FeS in chemical reactions. The Fact-Sage software was used to calculate the ΔG of the reaction, the phase diagram of FeS–CaO–C system, Cu₂S–CaO–C system and FeS–Cu₂S–CaO–C system. Three reduction experiments have been done to verify the calculation results. Cu₂S and FeS in alloy can be alternatively reduced to elemental metal. In reduction of matte, the recovery of metal is 73.96%, and the removal rate of sulfur is 99.11%. In reduction of copper slag, the recovery of metal is 88.50%. In reduction of matte in metal, the removal rate of sulfur is 99.64%. The sulfur is with the presence of CaS in slag eventually.

1. Introduction

At present, almost 2.2 t copper slag are produced while 1 t copper produced.¹⁾ The cumulative stockpiling of copper slag has been up to 250 million tons. Plenty of solid wastes brought serious damages to environment.²⁾ With the continuous development of copper smelting technology, the smelting intensity and matte grade increasing, matte in copper slag also constantly improved, and copper in slag of rich oxygen bottom converter was up to 3–4%.³⁾ All copper slags contained a considerable amount of Cu (0.42–4.6%) and Fe existing in the form of copper matte close to or higher than ores, and in copper slag, there was also a lot of valuable metals such as nickel, cobalt, gold and silver resources.⁴⁾ As secondary resources with great energy value and economic value, slags had comparable or even better properties than their competitive materials,^5,6) and its recycling had been a hot research topic. The main use of copper slag is to recycle iron or copper inside.

Copper in slag was mainly Cu₂S and little Cu₂O because of the existence of FeS. The recovery body of copper slag was mainly copper matte, returned to smelting process of copper by means containing acid leaching, flotation recovery and depletion recovery.^3,7,8) Copper slag contains large amounts of fayalite, direct magnetic separation of iron recovery is limited. Recycling method for Fe is mainly roasting-magnetic separation recycling and recovery-magnetic separation recycling. Gyurov made copper slag oxidation decomposed under pure oxygen system, got different oxidation products at different temperatures and then recycled.⁹⁾ Kim used petroleum coke as the reducing agent, reduced copper slag under 1250°C, made reduction product broken-magnetic separation, and got the final magnetic products (iron, more than 65 wt.%).¹⁰⁾ Langberg and Tarasov use gas mixture of reducing agent gas, air or nitrogen to reduce Cu in copper slag.^11,12) Kawy and Ahmed using reflection furnace to achieve smelting and recycling of copper anode, the cathode and the residual.¹³⁾ Zhou X. used copper slag to produce weathering resistant steel.¹⁴⁾

By comparing contents of main component in the stainless steel and copper slag, it is known that we can make copper antibacterial stainless steel using copper slag. Considering the above factors, “Reduce copper slag after moderately depleted to smelt antibacterial stainless steel” is put forward. This process is that copper slag with high amounts of matte depleted, tailings reduced to smelt copper antibacterial stainless steel, matte back to copper process. It can achieve the purpose of depleted copper slag, and maximize the use of slag to obtain the highest value products.

In reduction process to obtain copper and iron, oxides will react with reducing agents to make single metal, slag isolated sulfide also become slag inclusion in metal, which makes value of metal reduced and goes against to steelmaking process. Therefore, the study about behaviors of the copper matte in reduction process is very important.

2. Methods and Materials

Generally, when copper sulfide removed, sulfide was oxidized to copper and iron oxide firstly, then reduced to metal, in which oxidation and reduction were repeated. Oxidation is a process that oxygen replaces sulfur, and reduction is a process that oxygen is reduced. The entire process is that sulfide finally becomes metals. Oxygen only played a substitute role. Under reducing conditions, an offer oxygen carriers and sulfur carrier can be found, copper matte can be alternatively reduced to metal. Calcium oxide is a traditional desulfurization agent, cheap and easy to get. If it is sulfur carrier, product is stable.

Fact-Sage, one of the world’s largest database computing systems fully integrated in the field of chemical thermodynamics, was founded in 2001, and combined the FACT-Win/F*A*C*T and ChemSage/SOLGASMIX two thermochemical packages.¹⁵⁾ Fact-Sage database content is rich, computing capabilities is powerful, and more pluralistic equilibrium calculation, phase diagram, predominance area diagram, thermodynamic optimization, mapping processing are all carried out. Calculation about alternative reduction of Cu₂S and FeS was done by Fact-Sage software, and experiments were done to verify the results.

The chemical composition (wt.%) of copper slag is: FeO, 37.31; Fe₃O₄, 20.14; Cu, 4.48; Zn, 2.54; CaO, 1.63; Al₂O₃, 2.56; SiO₂, 23.98; MgO, 1.21; S, 1.38; Pb, 0.29; Mo, 0.06. Photograph of copper slag is shown in Fig. 1(a) and SEM image is shown in Fig. 2. It can be known that copper slag is mainly composed of copper matte, Fe₂SiO₄, Fe₃O₄ and some SiO₂. Reduction of copper slag is reduction of oxide and copper matte. In order to verify the matte be alternative reduced, three reduction experiments were performed in vertical resistance furnace, whose diagram is shown in Fig. 3.

Fig. 1.

Materials photograph: (a) copper slag; (b) copper matte.

Fig. 2.

Elemental distribution of copper slag.

Fig. 3.

Diagram of vertical resistance furnace. 1-electronic scales, 2-furnace frame, 3-synchronous motor, 4-torque sensor, 5-rotating rod, 6-furnace tube, 7-high temperature furnace, 8-crucible, 9-thermocouple, 10-bearing, 11-electromotor, 12-furnace body elevator mechanism, 13-control cabinet.

Three experiments are reduction of copper matte alone, reduction of copper matte in slag, and reduction of copper matte in alloy. Temperature is 1723 K. High-purity graphite powder (carbon content greater than 99.99%) was as reducing agent, and CaO (purity greater than 98.00%) was as the carrier of sulfur. The materials were mixed and placed in an alumina crucible (Φ48(40)•120 mm). Heating rate was 10 K/min before temperature 1473 K and 8 K/min to 1723 K. When reaching 1723 K, the temperature remained constant for 2 h. Cooling rate was 10 K/min. And when temperature reached 1073 K, product was with the furnace cooling to room temperature slowly. Argon was used as the shielding gas in the whole process.

Material of experiment 1 is copper matte (mainly FeS and Cu₂S), obtained from depletion of copper slag. Image of matte is shown in Fig. 1(b), and elemental composition (wt.%) is: Fe, 10.12; Cu, 63.81; Zn, 1.12; S, 22.75. Experiment 1 was done to verify alternative reduction of copper matte alone. 30 g copper matte, 14.33 g CaO and 5.12 g graphite powder were mixed and put in alumina crucible. Alloy got in experiment 1 is named alloy 1 in the following.

Material of experiment 2 is copper slag, whose diameter is between 0.30 mm to 0.71 mm. Experiment 2 was done to verify alternative reduction of copper matte in copper slag. 150 g copper slag, 38.18 g CaO and 23.33 g graphite powder were mixed and put in alumina crucible. Alloy got in experiment 2 is named alloy 2 in the following.

Material of experiment 3 is alloy 2. Experiment 3 was done to verify alternative reduction of copper matte in alloy. 60 g alloy containing copper matte, 10 g CaO and 5 g graphite powder were mixed and put in alumina crucible. Alloy got in experiment 3 is named alloy 3 in the following.

3. Results and Analysis

3.1 Thermodynamic Calculation

3.1.1. Thermodynamic Calculation of Fe–Ca–C–S–O System

In hot metal, CaO is one of the most basic desulfurization reagents, but there is a good effect when graphite is added in. ΔG and phase diagram calculated by FactSage.

Possible reactions between FeS, CaO and graphite powder are given.   

$$\mathrm{F}\mathrm{e}\mathrm{S}+\mathrm{C}\mathrm{a}\mathrm{O}=\mathrm{C}\mathrm{a}\mathrm{S}+\mathrm{F}\mathrm{e}\mathrm{O}$$(1)

  

$$\mathrm{F}\mathrm{e}\mathrm{S}+1/2\mathrm{C}+\mathrm{C}\mathrm{a}\mathrm{O}=\mathrm{C}\mathrm{a}\mathrm{S}+1/2\mathrm{C}{\mathrm{O}}_2\left(\mathrm{g}\right)+\mathrm{F}\mathrm{e}$$(2)

  

$$\mathrm{F}\mathrm{e}\mathrm{S}+\mathrm{C}+\mathrm{C}\mathrm{a}\mathrm{O}=\mathrm{C}\mathrm{a}\mathrm{S}+\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)+\mathrm{F}\mathrm{e}$$(3)

  

$$\mathrm{F}\mathrm{e}\mathrm{S}+4/3\mathrm{C}+\mathrm{C}\mathrm{a}\mathrm{O}=\mathrm{C}\mathrm{a}\mathrm{S}+\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)+1/3\mathrm{F}{\mathrm{e}}_3\mathrm{C}$$(4)

  

$$\mathrm{F}\mathrm{e}\mathrm{S}+5/6\mathrm{C}+\mathrm{C}\mathrm{a}\mathrm{O}=\mathrm{C}\mathrm{a}\mathrm{S}+1/2\mathrm{C}{\mathrm{O}}_2\left(\mathrm{g}\right)+1/3\mathrm{F}{\mathrm{e}}_3\mathrm{C}$$(5)

Variation with temperature of reaction ΔG^Θ is shown in Fig. 4. when graphite powder is added, reactions between FeS and CaO occurr. In 1723 K, ΔG is lower when graphite is added in. Adding graphite iron will generate Fe₃C, which will inhibit the generation of FeS to some extent.

Fig. 4.

ΔG^Θ variation with temperature of reaction FeS and CaO.

To ensure the reduction effect, CaO is generally added in excess, in this condition, it can be seen from the Fig. 5, FeS is reduced. With the increasing of graphite, FeO is typically generated Fe, Fe₃C. There is Fe₃C diagram of CaS and stable region. Description with the increase of sulfur, the first step appears CaS.

Fig. 5.

Phase diagram of FeS–CaO–C under 1723 K.

3.1.2. Thermodynamic Calculation of Cu–Ca–C–S–O System

Reactions between Cu₂S, CaO and graphite powder are given.   

$$\mathrm{C}{\mathrm{u}}_2\mathrm{S}+\mathrm{C}+\mathrm{C}\mathrm{a}\mathrm{O}=\mathrm{C}\mathrm{a}\mathrm{S}+\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)+2\mathrm{C}\mathrm{u}$$(6)

  

$$\mathrm{C}{\mathrm{u}}_2\mathrm{S}+1/2\mathrm{C}+\mathrm{C}\mathrm{a}\mathrm{O}=\mathrm{C}\mathrm{a}\mathrm{S}+1/2\mathrm{C}{\mathrm{O}}_2\left(\mathrm{g}\right)+2\mathrm{C}\mathrm{u}$$(7)

  

$$\mathrm{C}{\mathrm{u}}_2\mathrm{S}+\mathrm{C}\mathrm{a}\mathrm{O}=\mathrm{C}\mathrm{a}\mathrm{S}+\mathrm{C}{\mathrm{u}}_2\mathrm{O}$$(8)

Variation with temperature of reaction ΔG^Θ is shown in Fig. 6. It is can be known that, in 1723 K, Cu₂S and CaO can’t react. After graphite added in, alternative reduction of Cu₂S can happen.

Fig. 6.

ΔG^Θ variation with temperature of reaction Cu₂S and CaO.

From Fig. 7(a), the marked area without Cu₂S is calcium oxide content greater than Cu₂S. That content of each substance changes with the segment AB as shown in Fig. 7(b). The carbon content is 30%, with the increase of calcium oxide content, Cu₂S is gradually decreased and all disappear in 35%. Content of each substance changes with the segment CD as shown in Fig. 7(c). Content of Cu₂S and CaO are same. With the increase of carbon content, Cu₂S is gradually decreased and all disappear on condition carbon content 30%.

Fig. 7.

(a) phase diagram of Cu₂S–CaO–C system under 1723 K; (b) Equilibrium composition of Cu₂S–CaO–C system when composition changes along the segment AB; (c) Equilibrium composition of Cu₂S–CaO–C system when composition changes along the segment CD.

3.1.3. Thermodynamic Calculation of Cu–Fe–Ca–C–S–O System

It can be seen from Fig. 8, the marked parts are restored area of FeS and Cu₂S, under reducing conditions, CaO and C, respectively, greater than the mole fraction mole fraction of sulfur compounds, copper matte does not exist in the equilibrium state of substance.

Fig. 8.

Phase diagram: (a) FeS–Cu₂S–CaO system (when content of Carbon account for half); (b) FeS–Cu₂S–C system (when content of C account for half).

3.2 Results of Experiments

In experiment 1, matte is reduced and alloy 1 is obtained. Photograph of products is shown in Fig. 9(a). 16.95 g alloy and 27.32 g slag containing metal particles were obtained. Elemental composition (wt.%) of alloy 1 and slag are shown in Tables 1 and 2 respectively. To alloy 1, the recovery of metal is 73.96%, and the removal rate of sulfur is 99.11%. The content of metal elements in slag is high. From Fig. 9(a), it can be known some metal particles stayed in slag. Matte had been reduced but wrapped in the slag. The reduction rate of metal is higher than apparent recovery. XRD of slag containing metal parties is shown in Fig. 10. The diffraction peak of copper and calcium sulfide are shown. Iron is little and the diffraction peak not appears. It can be known matte was reduced. Throughout the experiment, calcium oxide is as the oxygen donor and sulfur carrier.

Fig. 9.

Products of experiments; (a) experiment 1; (b) experiment 2; (c) experiment 3.

Table 1. The chemical composition of alloy 1 (wt.%).

Element	Fe	Cu	S	Zn
Alloy 1	6.67	89.50	0.36	2.07

Table 2. The chemical composition of slag in experiment 1 and experiment 2 (wt.%).

Element	Fe	Cu	S	Al2O3	SiO2	TCa	CaO
Slag in experiment 1	6.95	10.85	24.85	4.12	1.78	38.26	–
Slag in experiment 2	5.87	1.35	1.08	10.23	36.62	–	38.64

Fig. 10.

XRD patterns of alternative reduction product of experiment 1.

In experiment 2, 66.53 g alloy 2 containing copper matte and 101.31 g slag were got. Image of alloy 2 and slag are shown in Fig. 9(b). Elemental composition (wt.%) of alloy 2 and slag are shown in Tables 3 and 2 respectively. Alumina crucible was eroded and partially dissolved in the slag. The main component of slag is calcium silicate. The rest are calcium aluminate, calcium sulfide, little residue of metal and matte. The recovery of metal is 88.50%, and the removal rate of sulfur is 58.86%. Sulfur in slag is with the presence of calcium sulfide. Reaction mechanism of experiment 2 is shown in Fig. 11(a). Fayalite reacts with CaO. calcium silicate is obtained, and then metallic iron is generated with reduction of ferrous oxide. Sulfur with bigger density than slag is in the lower part of crucible. The reaction between additives and sulfur is difficult. So many matte inclusions are in alloy 2. SEM image of alloy 2 is shown in Fig. 12(a). It can be known the base phase of alloy 2 is pearlite, and alloy contains a lot of matte inclusions. Line scan is detected. Matte inclusions containing matte and some copper. SEM image of slag in experiment 2 is shown in Fig. 12(b), and surface scan is detected. The oxygen content is higher than actual value in spot scan results. Calcium sulfide can be found in slag.

Table 3. The chemical composition of alloy 2 and alloy 3 (wt.%).

Element	Fe	Cu	S	C
Alloy 2	87.86	8.08	1.28	2.14
Alloy 3	87.40	7.88	0.005	4.57

Fig. 11.

Reaction mechanism of reduction experiment: (a)experiment 2; (b) experiment 3.

Fig. 12.

(a) Electron microscopy image and line scan results along the yellow line of alloy 2; (b) microscopy image and surface scan results of slag in experiment 2.

In experiment 3, 60.80 g alloy 3 was got. Image of alloy 3 is shown in Fig. 9(c). Elemental composition (wt.%) of alloy 3 is shown in Table 3. The carburization is further exacerbated in reduction conditions, unreacted carbon and iron combine to form cementite and the carbon content in alloy increases. The removal rate of sulfur is 99.64%, indicating that matte mixed in metal is reduced. Reaction mechanism of experiment 2 is shown in Fig. 11(b). In molten metal, sulfur is with lower density than metal. CaO and graphite are added in, and sulfur is alternative reduced to metal. SEM image of alloy 3 is shown in Fig. 13(a). It can be known matte disappears, while copper-rich region appears. Nano-copper is evenly distributed over the pearlite. After the heat treatment, Nano-copper will become ε-Cu, antibacterial phase in copper-containing antibacterial stainless steel. SEM image of slag in experiment 3 is shown in Fig. 13(b). The main component of slag is calcium sulfide. Compared with slag 2, content of aluminum and silicon are less, and content of sulfur and calcium are higher. Alloy 3 with S 0.005% is far less than alloy 2, indicating CaO with a good desulfurization effect under reducing condition. Content of Graphite in alloy 3 is more than copper-containing iron used to smelt antibacterial stainless steel. Before application, decarburization process is necessary.

Fig. 13.

(a) Electron microscopy image and line scan results along the yellow line of alloy 3; (b) microscopy image and surface scan results of slag in experiment 3.

On reducing condition, the difficulty of reaction between sulfide and calcium oxide is reduced, indicating activity of oxide greatly increasing. Calcium oxide is as the oxygen donor and sulfur carrier. After adding graphitic carbon, reaction mechanism of sulfide that not react with calcium oxide in usual terms is shown in Fig. 14. When calcium oxide contacts with sulfide, calcium ions and sulfur ions will have a certain bond energy, which is insufficient to make calcium ions take away sulfur ions, but weaken the bond energy between calcium ions and oxygen ions. At the same time, metal ions in sulfide and oxygen ions also form a certain bond energy, further weakening the bond energy between calcium ions and oxygen ions. Under reducing condition, because the bond is weakened, carbon atoms break the balance and take away the oxygen atoms. Calcium ions and sulfur ions have a stronger ability to bind,¹⁶⁾ so metal and calcium sulfide are formed. It is the alternative reduction of sulfide. It can be assumed that other sulfides, such as ZnS, MnS, PbS, all can be alternative reduced under reducing condition.

Fig. 14.

Desulfurization mechanism of metal sulfide under reducing condition.

Oxidative desulfurization increases oxygen potential, while the reduction process is required to reduce the oxygen potential. In reduction condition, the desulfurization process can be included in the reduction process. In industrial operation, the reduction process takes two steps in order to have better desulfurization. The first reduction process is the oxide reduced to metal. The second process is reduction of metal sulfides. There is a slagging process in both processes. Changing the operation process can make desulfurization in reduction process achieved.

4. Conclusion

In this study, we propose a new method of copper slag utilization and solve the question matte mixed in metal. The thermodynamics calculations and experiments on alternative reduction of copper matte in reduction process were investigated. The following can be concluded as:

(1) Through thermodynamic calculation, at temperature 1723 K, for Fe–Ca–S–O system, FeS and CaO can react; for Fe–Ca–C–S–O system, FeS and CaO can react easily; for Cu–Ca–S–O system, Cu₂S and CaO can’t react with each other; for Cu–Ca–C–S–O system, Cu₂S and CaO can react. Adding of graphite make alternative reduction of copper matte implemented.

(2) In three experiments that reduction of copper matte alone, in slag and in alloy, the alternative reduction of copper matte was proved to be carried out. Sulfur in matte is taken away by calcium oxide. The products are alloy (copper and iron) and CaS. The desulfurization rate can be achieved 99.64%.

(3) Under reducing condition, the difficulty of reaction between sulfide and calcium oxide is reduced. Sulfide can be alternative reduced. The desulfurization process can be included in the reduction process. For oxidizing slag, reductive desulfurization is more suitable.

Acknowledgement

This research was supported by the Basic Research Universities Special Fund Operations (N140204013, N130102002, N130702001); the Education Department of Liaoning Province (LZ2014021).

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