Copper slag was obtained from Jinchuan Company from Gansu province, China. The mineral phase of initial copper slag was analyzed by X-ray Diffractometry (XRD)with a scanning rate of 4°/min from range of 2θ = 10º–90º and the elements composition was tested by X-ray Fluorescence (XRF). The XRD results are shown in Figure 1, the main mineral phases of initial copper slag are fayalite, magnetite and silicon oxide. Fayalite is 2FeO·SiO₂ in copper slag composed of net-like tetrahedrons. The XRF analysis are as follows: 37.3% of Fe₂O₃, 12.9% of SiO₂, 12.2% of CaO, 6.6% of Al₂O₃, 12.1% of ZnO as well as some minor elements like K, Cu, Pb, and Co.

Coal is a kind of lignite from Fuxin City, Liaoning Province, China. The compositions are as follows: 35.1% of moisture, 25% of ash, 20.5% of volatile, and 19.4% of fixed carbon. Coal samples were dried at 105°C in a constant temperature oven for 2 h, then sieved into powder with a size of 0.15 mm. The coal powder was used as a reducer for the next experiments.

In order to verify the oxygen release and absorption characteristics of OC, experiments were conducted using thermo gravimetric (TG). It was turned out 1,000 (OX) and 1,000 (RE) were the most suitable oxygen carriers according to XRD analysis. With respect to the oxygen release experiment, 20 mg mixture of 1,000 (OX) and coal powder heated to 1,000°C at a heating rate of 10°C/min and kept 1 h under a N₂ flow of 30 ml/min. As for oxygen absorption test, 20 mg 1,000 (RE) heated to 1,000°C at a heating rate of 10°C/min in N₂ atmosphere, then N₂ was switched to air and kept 2 h. As a blank control experiment, 10 mg coal was heated under the same condition. In this experiment, the degree of mass reduction can reflect the degree of oxygen release capacity of OC.

As shown in Figure 2, fayalite was decomposed into main phases (Fe₃O₄, Fe₂O₃, and SiO₂) in the air (Gyurov et al., 2011), the intensity of Fe₂O₃ in the calcined sample gradually increased and Fe₃O₄ phase decreased as the calcined temperature increased from 900°C to 1,100°C. It indicates that the transformation rule of iron phase in copper slag during calcination process is “2FeO·SiO₂→Fe₃O₄→αFe₂O₃” shown in R1-R2 (Gorai et al., 2003). At a relatively low temperature (900°C), the calcined reaction R1 dominated and more fayalite was decomposed into Fe₃O₄ and Fe₂O₃ (Deng et al., 2019). At 900°C, the XRD pattern shows characteristic peaks of magnetite, accompanied by weak characteristic peaks of hematite. Most of fayalite (Fe²⁺) was decomposed into magnetite (Fe^8/3+), indicating that the oxygen carrier has a poor oxidability under this circumstance. With the increase of temperature, the proportions of the characteristic peaks of Fe₃O₄ and Fe₂O₃ at 1,000°C are almost same because most of the Fe₃O₄ has been oxidized to Fe₂O₃ as R2 describes. At 1,100°C, although the Fe₃O₄ phase becomes weaker, it did not completely disappear, revealing that the copper slag has not been completely oxidized to Fe³⁺ at this temperature. This phenomenon may be caused by the calcination temperature which is not enough to complete the conversion of magnetite, or the oxidation of fayalite still proceed at this temperature, in other words, R1 and R2 take place simultaneously.

In summary, the higher the calcination temperature, the higher the degree of oxidation of the copper slag. However, Scanning Electron Microscope (SEM) results were shown in Figure 3, which indicated the most severe sintering occurred at 1,100°C. Based on this, it is better to control the calcined temperature at 1,000°C to ensure OC has more lattice oxygen and to avoid serious sintering in chemical looping process. 2 FeO ⋅ SiO 2 + O 2 → Fe 3 O 4 + SiO 2 (R1) Fe 3 O 4 + O 2 → α Fe 2 O 3 (R2)

The calcined copper slag in reduction condition is shown in Figure 2. It was shown that the Fe^+8/3 in Fe₃O₄ phase gradually disappeared, while the Fe²⁺ existing in CaFeSi₂O₆ and Ca₃Si₃O₉ enhanced with the increasing of reduction temperature. The reduction processes are carried out step by step respectively: FeO·SiO₂→FeO→Fe, Fe₃O₄→FeO→Fe (Yang et al., 2011). During the reduction process, coal powder as a reductant reacted with various iron oxides in the copper slag to produce CO and further generate CO₂, and simultaneously yield wollastonite Ca₃Si₃O₉ and hedenbergite CaFeSi₂O₆ (Yang et al., 2011). The reduction of iron oxide with C is called direct reduction (R3-R4) and with CO is called indirect reduction (R5-R6) (Bafghi et al., 1993). In the gas phase, the conversion reaction of CO₂, CO, and C is a Boudouard reaction (R7) (Zuo et al., 2020a). 2 FeO ⋅ SiO 2 + 2 C = 2 Fe +   2 CO + SiO 2 (R3) Fe 3 O 4 + C =  3FeO + CO (R4) 2 FeO ⋅ SiO 2 + 2 CO = 2 Fe + 2 CO 2 + SiO 2 (R5) Fe 3 O 4 + CO =   3 FeO + CO 2 (R6) C + CO 2 =   2 CO (R7)

In this study, the Reaction Equations part of the HSC6.3 simulation software was used to confirm the initial reduction reaction temperature based on Gibbs free energy calculation. It was found that the initial temperature for R3, R4, and R6 are approximately 800, 700, and 790°C. If temperature higher than 800°C, these reactions occur spontaneously, moreover, the reduction reactions of C are easier than the reduction reactions of CO for iron oxide in solid copper slag. However, The Gibbs free energy for reduction reactions of CO and 2FeO·SiO₂ can’t take place spontaneously. In summary, the reduction temperature of C and copper slag should be set above 800°C. The above reactions are all endothermic reactions, which demonstrates that improving the temperature will promote the reduction reaction. Therefore, the copper slag calcined at 1,000°C has higher reduction degree and is more suitable as an oxygen carrier to a certain extent. This simulation conclusion is consistent with the results shown in the XRD plot.

The XRD results Figure 2 show that in addition to the three main phases of 2FeO·SiO₂, Fe₃O₄, and SiO₂, there is a weaker CaFeSi₂O₆ characteristic peak at a reduction temperature of 800°C. Since the CaFeSi₂O₆ is a silicate composed of Ca²⁺, Fe²⁺ and Si₂O₆, Fe²⁺ has relatively strong reducibility. To a certain extent, the appearance of characteristic peak of CaFeSi₂O₆ indicates the improvement of reduction degree. When the reduction temperature increased to 900 °C, the diffraction peaks of 2FeO·SiO₂ and Fe₃O₄ gradually weakened and decomposed into the characteristic peaks of FeO and Fe, there was simultaneously a large amount of 2FeO·SiO₂ in the oxygen carrier. Until the temperature rose to 1,000°C, almost all the iron phase was converted to iron metal.

At 1,000°C, the oxygen carrier has the deepest reduction degree and excellent reduction performance, which provides theoretical references for the operating characteristics and paraments of OC.

To further test the release and absorption oxygen performance of the calcined copper slag, two experiments were conducted by TG method:

1) The oxygen release characteristics of 1,000 (OX) was tested in a N₂ atmosphere.

The TG curve of 1,000 (OX) oxygen release is shown in Figure 4. There are two obvious weight loss stages, 25–600 and 600–1,000°C. The stage 1 is mainly caused by the pyrolysis of coal powder. In the stage 2, pyrolysis of coal powder is basically completed and the reduction reaction between the coal powder and 1,000 (OX) lead the mass loss. It was calculated that the mass loss caused by the oxygen release process accounts for 70.57% of the total loss. The curve of oxygen release as shown in Figure 3, the surface lattice oxygen of OC was captured by C element in the pulverized coal to produce CO/CO₂, meanwhile the oxygen vacancies generated by the outer lattice oxygen enforce the inner lattice oxygen move to the surface quickly to provide more lattice oxygen and to achieve the purpose of releasing oxygen (Zeng et al., 2012). Thus, the degree of mass loss reflects the reduction degree of oxygen release performance. In general, this curve has two vividly mass loss region, the result is consistent with Zuo (Zuo et al., 2021b). In first region, pyrolysis reactions of coal take place; The second region is the oxygen release reaction of OC, taking place at 600–1,000°C, which is the main reaction stage causes the loss of mass. Moreover, it is calculated that the mass loss caused by the oxygen release process accounts for 70.57% of the total loss. From this aspect, the granulated copper slag after calcination has a good oxygen release performance.

Figure 4 shows the oxygen absorption process of 1,000 (RE). The oxygen absorption is an isothermal process. OC was heated in an inert gas N₂ before 120 min, then N₂ was switched to air and kept 2 h to accomplish oxygen absorption. In other words, the actual oxygen absorption process took place between 120 and 240 min, so the x-axis of TG curve starts from 120. The longer the oxidation process, the heavier the OC, eventually the mass of OC increased by 4.2%. This phenomenon indicates the OC after redox calcination has an ability to absorb oxygen.

In summary, granulated copper slag as a precursor makes full use of waste heat resources. OC calcinated at 1,000°C has the best redox reactivity and has a fine ability of oxygen release and absorption. This study provided a reference for the operating temperature of OC in the chemical looping technology, as well as offered a new idea for the comprehensive utilization of metal oxide-containing metallurgical slag.