Fluidized magnetization roasting of refractory siderite-containing iron ore via preoxidation–low-temperature reduction

1.   Introduction

Iron ore is the major raw material used by the steel industry. In 2021, global iron ore consumption reached approximate 2.3 billion t, with a huge market demand [1]. Low-grade refractory iron ore has become an important iron ore resource as the reserves of high-grade iron ore continue to decrease [2]. Magnetization roasting is one of the most effective refractory resource utilization methods for low-grade weakly magnetic iron ore, such as siderite (FeCO₃), hematite (Fe₂O₃), and limonite (Fe₂O₃·nH₂O) [3]. In this process, weakly magnetic iron minerals are converted into strongly magnetic magnetite (Fe₃O₄) through roasting. Then, iron minerals are separated from the gangue through weak magnetic separation to obtain iron concentrate products with high grades and recovery rates.

The magnetization roasting of hematite and limonite is a relatively simple weak reduction reaction, as shown by reaction (1). Numerous experimental results for the magnetization roasting of hematite and limonite ores have shown that compared with the traditional beneficiation technology, magnetization roasting can increase the iron grades and recovery rates of concentrates by approximate 3wt%–8wt% and 13%–43%, respectively [4].

Siderite is an iron-bearing carbonate mineral and is the most abundant in sedimentary iron formations on earth [5]. However, as illustrated by reaction (2), the magnetization roasting reaction of siderite is different from that of hematite. The specific reaction can be divided into two steps, which are shown as reactions (3) and (4) [6]. Theoretically, the reaction does not require a reducing agent and acts as self-magnetization roasting [3].

However, reports showing that weakly magnetic wüstite can appear during roasting in a neutral atmosphere exist. Zhang et al. [7] for Guangdong siderite showed that after roasting at 620°C for 10 min, the proportion of the wüstite phase can reach 18.5%. Luo et al. [8] found that Xinjiang siderite could transform into wüstite when the temperature exceeded 550°C. Ponomar et al. [6] demonstrated that Bakal siderite would generate wüstite at 600°C. Therefore, in natural mineral roasting, reaction (2) remains unstable. In addition, in natural low-grade refractory iron ores, siderite and hematite (or limonite) often coexist [9]. According to reactions (1) and (2), for achieving high magnetic conversion under neutral atmosphere roasting, the molar ratio of the siderite mineral to the hematite mineral in the mixed iron ore should not be less than 1:1 to balance consumption by hematite reduction under the assumption that CO is fully utilized. This requirement necessitates the consideration of magnetization reduction roasting for refractory hematite iron ore with low siderite content. However, weakly magnetic wüstite, such as the Gansu siderite ore roasted in CO reported by Zhu et al. [5], also appears in the reduction roasting of siderite. Therefore, further developing magnetization roasting with wide applicability for low-grade refractory siderite-containing iron ore and the ability to avoid wüstite generation during siderite roasting and is unlimited by the content ratio of siderite and hematite in ore remains necessary.

Three main types of equipment for industrial magnetization roasting are shaft furnace, rotary kiln, and fluidized bed. Among them, the fluidized bed has the advantages of full powder ore (0–3 mm) acceptance and high reaction efficiency at low temperature [10]. In industrial fluidized magnetization roasting systems, the device for preheating ore powder is arranged in front of the fluidized bed, and high-temperature oxidizing gas obtained after the combustion of reducing tail gas from the fluidized bed is used as the medium for gas–solid direct heat exchange [11–12]. We systematically studied and proposed the fluidized preoxidation–low-temperature reduction magnetization roasting process and industrial production conditions for low-grade iron ore containing refractory siderite and hematite. This process has wide applicability because it is unlimited by the proportion of siderite and hematite in iron ore and can avoid the roasting of siderite into wüstite. In particular, the behavior and path of siderite phase transformation during oxidation and reduction were investigated.

2.   Experimental

The system for the fluidized magnetization roasting experiment is shown in Fig. 1. The fluidized bed reactor and gas distribution plate were made of high-purity quartz. The gas flow into the fluidized bed was fixed at 3.5 L·min^–1. The reducing gas was composed of high-purity N₂, CO, and CO₂ gases (purity 99.99vol%). The oxidizing atmosphere was dry air.

Fig. 1.  Schematic of the experimental setup.

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The experimental process is as follows. The fluidized bed was heated to the experimental temperature, and N₂ gas was introduced into the fluidized bed to purify the air in the reactor. After switching the reaction gas, 30 g of ore powders was added. After the set reaction time was reached, N₂ was switched on, and the fluidized bed reactor containing the roasted ore was removed from the furnace. Water was sprayed onto the surface of the fluidized bed reactor for rapid cooling to room temperature. Finally, the roasted ore sample was removed from the fluidized bed for subsequent analysis and weak magnetic separation.

The weak magnetic separation of roasted ore is as follows. First, the roasted ore was milled (QM-3SP2, China) to the target particle size. Then, a standard Davis magnetic separator tube (d = 50 mm, CXG-99, China) was used to perform wet weak magnetic separation.

The Fe and Fe²⁺ content of the sample was measured by using chemical titration (Chinese standards GB/T 6730.5-2007 and GB/T 6730.8-2016). The distribution of the iron-containing phase for the sample was analyzed through combined standard physical and chemical quantitative analyses [13]. The X-ray diffraction (XRD) instrument was a Rigaku SmartLab 9 kW model. The models of the scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) were JSM-7800 F (Prime) and Aztec X-MaxN50, respectively. The thermogravimetric (TG) measurement instrument model is NETZSCH STA 449F3. The magnetic curve measurement instrument model was Lakeshore 7404. The 2θ of XRD detection was measured in the narrow range of 29°–45° mainly for the analysis of the iron ore phase transformation in the roasting process, in which the strongest XRD peak of the main iron-bearing phase was included. The strongest peaks of magnetite Fe₃O₄, hematite Fe₂O₃, siderite FeCO₃, and wüstite FeO were 35.479°, 33.142°, 32.002°, and 42.008°, respectively. The narrow-range scanning of XRD can amplify and intensify diffraction peak information and is more accurate in the determination of the existence of trace phase content.

3.   Results and discussion

3.1   Materials characteristics

SX ore used in this work (originated from Shaanxi Province, China) is the low-grade siderite iron ore, which has siderite iron reserves of approximate 300 million t [14]. Both ore powders were dried in an oven at 100°C for 24 h to remove free water before the experiment. The particle size distributions of <45, <74, and <150 μm accounted for 9.69%, 19.71%, and 59.96% of SX ore, respectively. The chemical compositions and iron-containing phase composition distribution of SX ore are shown in Tables 1 and 2, respectively.

Table  1.  Chemical components of SX ore wt%

TFe	Fe2+	CaO	MgO	Al2O3	SiO2	P	S	Others
23.49	16.55	0.41	2.40	13.50	36.90	0.12	0.94	13.37

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Table  2.  Iron-containing phase composition and distribution of SX ore

Iron mineral phases	Content / wt%	Distribution /%
Fe in magnetite	1.14	4.85
Fe in hematite	6.18	26.31
Fe in siderite	14.85	63.22
Fe in ferric silicate	0.65	2.77
Fe in pyrite	0.67	2.85
Total	23.49	100

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Fig. 2(a) shows the XRD pattern of SX ore, which mainly contained siderite, hematite, quartz (SiO₂), and muscovite (K{Al₂[AlSi₃O₁₀](OH,F)₂}). Fig. 2(b) shows the TG curves of SX ore in air. Two weight loss peaks were present. From 224 to 305°C, a small amount of limonite in SX ore was decomposed into hematite (Fe₂O₃·nH₂O = Fe₂O₃ + nH₂O) with a weight loss of approximate 0.7wt%. Approximate 10wt% weight loss was observed at 440–610°C, which corresponded to the thermal decomposition of siderite into wüstite and the release of carbon dioxide (FeCO₃ = FeO + CO₂). Under air, the product wüstite underwent oxidation (4FeO + O₂ = 2Fe₂O₃) and weight gain. Given that the initial decomposition rate of siderite was slow and the oxidation reaction rate of wüstite was fast, the second peak of the weight loss curve increased at the initial decomposition stage.

Fig. 2.  XRD (a) and TG (b) analysis of SX ore.

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3.2   Conventional direct reduction magnetization roasting

The industrial reducing gas used in magnetization roasting can be coal, reformed, and blast furnace gases, which include CO and H₂ as effective components and the oxidative components CO₂ and H₂O [15]. In accordance with the metallurgical reaction principle [16], an important index of reducing gas is the reduction potential R, which is defined as the ratio of the volume fraction of CO and H₂ to the volume fraction of CO, H₂, CO₂, and H₂O. The reduction potential of the above industrial reducing gas is usually approximate R0.6 [17]. Magnetization roasting has a fast reaction rate, and the utilization rate of CO and H₂ gas can exceed 50% [10], that is, the reduction potential of the tail gas drops below R0.3. We fixed the reducing gas composition to 50vol% N₂ + 22.5vol% CO + 27.5vol% CO₂ with the median reduction potential of R0.45.

Fig. 3 shows that 500 and 550°C were selected as the roasting temperature for the high decomposition efficiency of siderite. “DR0.45” means the direct reduction without preoxidized treatment and the reduction potential is 0.45. The complete disappearance of the hematite phase after 2.5 min at 500 and 550°C is indicative of fast reaction efficiency. The siderite phase can be completely decomposed after 7.5 min at 550°C. However, it continues to exist even after 30 min at 500°C. This situation demonstrates that the roasting temperature needs to be at least at the thermogravimetric peak to enable the fast decomposition of siderite. Notably, Fig. 3 shows that at 550°C, wüstite is generated, and its peak intensity gradually increases with the extension of reaction time. This increment is accompanied by a decrease in the peak intensity of magnetite. Wüstite is also the final reduction product at 500°C. It is present as a product phase rather than as a transition phase during reduction.

Fig. 3.  XRD patterns of SX ore at 500 and 550°C for different reduction times.

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Numerous experimental studies [18–21] have reported that natural primary hematite can only be reduced into magnetite at 500–550°C, and the reduced magnetite and primary magnetite remain in the stable magnetite state. Therefore, the wüstite products of SX ore can only originate from magnetite after the decomposition of siderite, which is further reduced due to its instability. As a weak magnetic iron oxide, wüstite reduces the iron recovery rate of the concentrate in weak magnetic separation [22]. Therefore, the conventional reduction magnetization roasting process for hematite is inapplicable to siderite minerals.

3.3   Preoxidation roasting

In consideration of the weak generation of magnetic wüstite in the reduction of siderite, the preoxidized treatment is used to transform siderite into hematite (reaction (5)), expecting further be reduced to the stable magnetite as the natural primary hematite performance.

Fig. 4 shows the phase transformation of SX ore during oxidation at 550 and 610°C. The O means oxidation. In contrast to siderite, which is slowly oxidized at 550°C, the siderite phase quickly disappears after 2.5 min at 610°C, which is the end temperature of the thermogravimetric decomposition peak. Two kinds of hematite are presented in the weakly magnetic α-Fe₂O₃ and the strongly magnetic γ-Fe₂O₃. Both of these hematite materials exist simultaneously throughout the whole oxidation process. During oxidation, primary hematite and limonite remain in the weakly magnetic α-Fe₂O₃ state [23]. When primary magnetite is oxidized at temperatures above 400°C, it is also oxidized into α-Fe₂O₃ [24]. Therefore, γ-Fe₂O₃ can originate only from siderite oxidation.

Fig. 4.  XRD patterns of SX ore during oxidation at 550°C (a) and 610°C (b).

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The change trend of the contents of α-Fe₂O₃ and γ-Fe₂O₃ in the siderite oxidation product is calculated on the basis of the relative XRD peak intensity ratio [25]. The calculation formula is ω_γ${}_{{\text -}{\rm{F}}{{\rm{e}}_2}{{\rm{O}}_3}}$ = I_{γ${}_{{\rm{{\text -}F}}{{\rm{e}}_2}{{\rm{O}}_3}}$} / (I${}_{{\text{γ}} {\text -} {\rm{F}}{{\rm{e}}_2}{{\rm{O}}_3}}$ + I${}_{{\text α} {\text -} {\rm{F}}{{\rm{e}}_2}{{\rm{O}}_3}}$) × 100%, where I${}_{{\text{γ}}{\text -}{\rm{F}}{{\rm{e}}_2}{{\rm{O}}_3}}$ and I${}_{{\text{α}} {\text -} {\rm{F}}{{\rm{e}}_2}{{\rm{O}}_3}}$ are the strongest peak intensity of α-Fe₂O₃ and γ-Fe₂O₃, respectively. α-Fe₂O₃ includes primary hematite, which is the oxidation product of primary magnetite and the partial oxidation product of siderite. The relative contents of primary hematite and magnetite can be inferred from Table 2. The change trend of the relative content ratio of γ-Fe₂O₃ in the oxidation product of siderite is obtained by deducting the α-Fe₂O₃ content from the primary hematite and primary magnetite contents in the calculation formula. The results are shown in Fig. 5. The calculation range is 30 min at 550°C and ≥2.5 min at 610°C, at which time the siderite has been completely decomposed. The Fe²⁺ content of the oxidized samples in Fig. 5 is 1.75wt%, and the remaining ferrous iron is mainly gangue iron that cannot be oxidized. This result indicates that oxidation is almost complete. The amount of γ-Fe₂O₃ in the oxidation products decreases gradually with the extension of reaction time and the increase in temperature, indicating that γ-Fe₂O₃ is the only transition phase in siderite oxidation. The final equilibrium phase should be α-Fe₂O₃ if the oxidation temperature is high enough or the roasting time is long enough [26]. The proportion of γ-Fe₂O₃ decreases from 79.8% to 60.9% after oxidation roasting at 610°C for 2.5 and 30 min, which is still in the first half stage of the crystal structure transformation. In consideration of reaction efficiency, 610°C for 2.5 min is taken as the optimal preoxidation parameter for SX ore.

Fig. 5.  Relative content of γ-Fe₂O₃ in the siderite oxidation product Fe₂O₃.

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3.4   Reduction roasting after preoxidation

Although the preoxidation roasting of siderite can produce strongly magnetic γ-Fe₂O₃, its equilibrium phase is still weakly magnetic α-Fe₂O₃, and weakly magnetic α-Fe₂O₃ from siderite and primary hematite also exists. Further reducing and transforming all oxidation product phases into strongly magnetic magnetite is necessary. The SX ore sample preoxidized at 610°C for 2.5 min was taken as the reduction material. Firstly, similar to that in conventional fluidized reduction magnetization roasting [10], 550 and 500°C with the reducing gas R0.45 were selected for reduction roasting after preoxidation. The XRD patterns obtained during reduction roasting at 550 and 500°C are shown in Fig. 6. The preoxidized product Fe₂O₃ has completely disappeared after only 2.5 and 5 min of reduction roasting at 550 and 500°C, respectively. Notably, weakly magnetic wüstite begins to appear in addition to magnetite when α-Fe₂O₃ has disappeared completely after the initial stage of the reaction.

Fig. 6.  XRD patterns of SX ore during reduction at 550°C (a) and 500°C (b) after preoxidation.

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The contents of TFe and Fe²⁺ in the reduced ore determined through titration analysis are used to calculate the proportion of Fe in wüstite (wFe_wüstite) to total Fe from siderite (wFe_siderite). Fig. 6 shows that the calculation range for 550°C is 2.5–30 min and that for 500°C is 5–30 min, where α-Fe₂O₃ and γ-Fe₂O₃ have been reduced completely, leaving only wüstite and magnetite. Wüstite originates from siderite, and the sources of magnetite include siderite, primary magnetite, and primary hematite in raw ore. In the calculation, the magnetite from primary magnetite and primary hematite reduction are deducted in accordance with Table 2 to investigate separately the Fe element derived from siderite. Fig. 7 shows that with the increasing of temperature and extension of reduction time, the content of wüstite in the product significantly increases. This result indicates that in the preoxidized siderite, wüstite exists as a product phase during reduction at 500–550°C.

Fig. 7.  Proportion of Fe in wüstite to total Fe from siderite in SX ore after preoxidation.

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As mentioned above, the reduction products of natural primary hematite (α-Fe₂O₃) are only magnetite after reduction roasting at 500–550°C without the further reduction of wüstite. For the preoxidized SX ore, γ-Fe₂O₃ exists in addition to α-Fe₂O₃. The crystal structure of α-Fe₂O₃ is that of corundum (α-Al₂O₃). However, γ-Fe₂O₃ is an inverse spinel with a cubic unit cell similar to Fe₃O₄ in the form of the defective spinel □_1/3Fe³⁺_8/3O₄, where □ represents vacancies at cation sites [27]. Therefore, in combination with the phase transformation characteristics of natural primary hematite (α-Fe₂O₃) and the difference in the γ-Fe₂O₃ crystal structure, it can be deduced from this result that the existence of γ-Fe₂O₃ leads to the formation of wüstite product phases. Generally, wüstite is unstable at temperatures below 570°C [28]. However, the experimental results of Romanov et al. [29] and Pineau et al. [30] showed that in the reduction of chemically pure artificial hematite by H₂, wüstite could exist at 425 and 450°C. Thus, as a special product phase of siderite oxidization, it is also a special phenomenon for the reduction of artificial γ-Fe₂O₃ to wüstite.

By using the same calculation method shown in Fig. 7, the proportions of Fe in wüstite produced via direct reduction for 7.5 and 30 min at 550°C (Fig. 3) to the total Fe in the siderite are found to be 46.3% and 64.8%, respectively. Fig. 7 illustrates that the corresponding values of the samples after preoxidization have decreased to 27.2% and 43.4%, respectively, because α-Fe₂O₃ produced by the preoxidation of siderite increases the amount of stable magnetite in the subsequent reduction process.

Although the preoxidation of siderite reduces the amount of wüstite generated at 500–550°C, the remaining weakly magnetic wüstite certainly attenuates the subsequent effect of weak magnetic separation. As shown in Fig. 6, an “operation window” without hematite and wüstite phases does not exist during reduction. The thermodynamic phase diagram of iron oxide reduction [28] indicates that low temperature is beneficial for avoiding the formation of wüstite. Therefore, at the fixed gas reduction potential of R0.45, the reduction temperature after preoxidation is further decreased to 450°C. The XRD results are shown in Fig. 8. Given the loose structure after siderite decomposition, the complete magnetization transformation time in reduction is only 5 min, which reflects good phase transformation efficiency at low temperature after preoxidation. When the reduction roasting time is extended to 30 min, only the magnetite phase exists in the reduced samples. This result shows that low-temperature reduction can successfully inhibit the generation of wüstite from γ-Fe₂O₃ and achieves the ultimate aim of magnetization roasting with a high magnetic phase conversion rate.

Fig. 8.  XRD patterns of SX ore during reduction at 450°C after preoxidation.

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In summary, the fluidized magnetization roasting of refractory siderite-containing iron ore via preoxidation–low-temperature reduction can be proposed in view of the generation of the weakly magnetic FeO through the conventional direct reduction magnetization roasting of siderite. Through the preoxidation modification of siderite followed by low-temperature reduction at 450°C, the generation of FeO is inhibited, and the complete transformation of siderite into strongly magnetic Fe₃O₄ can be realized. Considering that this process has a reduction step, it is not limited by the content of primary hematite and limonite in the ore. Meanwhile, the efficiency of the low-temperature reaction does not decrease because of fluidization and iron ore preoxidation modification. The phase transformation paths of siderite and hematite in the different magnetization roasting processes are shown in Fig. 9.

Fig. 9.  Phase transformation paths of siderite and hematite in different magnetization roasting processes.

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Notably, direct reduction magnetization roasting at 450°C is theoretically capable of avoiding wüstite generation. However, Fig. 3 shows that siderite remains difficult to decompose or that decomposition efficiency is extremely low at this temperature.

3.5   Backscattering electron microscopy and energy dispersive spectrometry of ore powder

The backscattering electron microscopy (BSE) image of the cross-section of SX raw ore powders is shown in Fig. 10. The energy dispersive spectrometer (EDS) element composition of the main phases is presented in Table 3 (without containing carbon element). The main phases in SX raw ore are siderite, hematite, muscovite, and quartz. In siderite, the main impurity elements are Mg and Ca, and the content of Mg is higher than that of Ca. FeCO₃ can form a complete isomorphic series with MgCO₃ and MnCO₃ and incomplete isomorphic series with CaCO₃ [31]. Primary hematite has high iron content and low impurity content, primary siderite particles have a dense structure (Fig. 10(b)).

Fig. 10.  BSE image of SX ore (a) and siderite particles (b).

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Table  3.  EDS element composition of SX ore wt%

Particles	O	Mg	Al	Si	K	Ca	Fe
Siderite (point #1)	45.69	3.46	0.00	0.06	0.00	0.16	50.63
Hematite (point #2)	29.97	0.10	0.06	0.26	0.00	0.00	69.60
Muscovite (point #3)	48.70	0.29	17.30	21.85	8.50	0.14	3.23
Quartz (point #4)	52.77	0.00	0.00	46.40	0.03	0.04	0.76

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The results of the BSE and EDS analyses of the cross-section of the ore reduced at 450°C for 5 min after oxidation at 610°C for 2.5 min are provided in Fig. 11 and Table 4. After preoxidation–low-temperature magnetization roasting, the primary siderite and hematite minerals in the ore have completely transformed into strong magnetite without wüstite. Comparison with Fig. 10 reveals that interface migration between phases does not exist because the fluidized roasting temperature is considerably lower than the soft melting temperature of each mineral in the ore. On the basis of this result, in combination with the EDS results in Table 3, the magnetite in the reduced ore can be divided into two categories: the magnetite with high magnesium content (point #1) reduced from siderite and the magnetite with low gangue impurities (point #2) reduced from primary hematite. Tables 3 and 4 demonstrate that during low-temperature fluidized magnetization roasting, the impurity element components of the main iron-containing phase do not migrate with the change in phase, showing “hereditary” characteristics. Muscovite and quartz also present compositional stability. After the reduction of oxidized siderite, obvious internal strip holes are formed (Fig. 11(b)), which can reduce the internal diffusion resistance of oxidized siderite particles during reduction and improve low-temperature reduction efficiency.

Fig. 11.  BSE image of reduced ore after oxidation (a) and reduced particle of oxidized siderite (b).

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Table  4.  EDS element composition of the reduced ore after oxidation wt%

Particles	O	Mg	Al	Si	K	Ca	Fe
Magnetite (point #1)	28.66	4.74	0.32	0.13	0.03	0.67	65.45
Magnetite (point #2)	28.37	0.34	0.00	0.48	0.05	0.08	70.68
Muscovite (point #3)	48.14	0.39	16.77	21.85	8.31	0.15	4.38
Quartz (point #4)	52.95	0.08	0.00	46.53	0.00	0.07	0.37

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3.6   Magnetic characteristics of the roasted ore

The magnetization curves of the raw SX ore and roasted samples are shown in Fig. 12. The raw SX ore has a certain magnetism because it contains magnetite. The magnetization curve of the preoxidized sample has slightly increased because the preoxidized sample contains strongly magnetic γ-Fe₂O₃. Then, the preoxidized sample is reduced by using reducing gas with R0.45 at 450°C for 5 min. All hematite has been converted into strongly magnetic magnetite, and the magnetic curve of the reduced sample reaches its highest position when the saturation magnetization of the reduced sample is considerably larger than that of raw ore and preoxidized samples. When the reduction time is extended to 30 min, the magnetic curve of the sample neither decreases nor increases significantly. This phenomenon indicates that magnetite remains stable in the ore subjected to low-temperature reduction after preoxidation, which is consistent with the phase results given in Fig. 8.

Fig. 12.  Magnetization curves of raw SX ore and roasted samples.

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3.7   Weak magnetic separation of roasted ore

For SX ore, under the experimental conditions in this work, the optimal preoxidation–low-temperature reduction magnetization roasting parameters are preoxidation at 610°C for 2.5 min by air, followed by reduction at 450°C for 5 min. In accordance with the mineral liberation state of SX ore particles, wet ball milling was used to grind the roasted SX ore to <30 μm (82%) before magnetic separation. The first stage of weak magnetic separation was carried out at 1100 Oe. The first-stage tailings were then subjected to the second stage of weak magnetic separation at 1300 Gs. The concentrates obtained through the first and second weak magnetic separation processes were merged to obtain the final concentrate product. In addition, the roasted ore subjected to conventional direct reduction magnetization roasting at 550°C for 7.5 min under the reduction potential of R0.45 was selected for the comparative test on weak magnetic separation.

The narrow range XRD spectra of the tailings after the two magnetization roasting processes are shown in Fig. 13. Given that weakly magnetic wüstite is generated in conventional direct reduction, a large amount of wüstite is lost to the tailings due to its weakly magnetic properties. The tailings do not contain wüstite because preoxidation–low-temperature reduction magnetization roasting can avoid the formation of weakly magnetic wüstite. This result indicates the advantage of high magnetic conversion rates.

Fig. 13.  XRD patterns of tailings after different magnetization roasting processes.

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The weak magnetic separation results of SX ore subjected to preoxidation–low-temperature reduction and conventional direct reduction magnetization roasting are shown in Table 5. The concentrate grades of the two processes are relatively close and can reach the commercial iron concentrate grade of ~62wt%. The main nonferrous component contents of iron concentrate are CaO 0.33wt%, MgO 3.22wt%, Al₂O₃ 1.05wt%, SiO₂ 3.93wt%, P 0.06wt%, and S 0.85wt%. The iron recovery rate is calculated as ε = γ × (β/α), where γ is the iron concentrate yield after magnetic separation, wt%; β is the iron content of iron concentrate, wt%; and α is the iron content of roasted ore, wt%. Consistent with the results provided in Fig. 12, the iron recovery rate of conventional direct reduction is only 54.03% due to the production of a large amount of weakly magnetic wüstite. Considering that preoxidation–low-temperature reduction magnetization roasting avoids the formation of wüstite, its iron recovery rate is significantly improved, reaching the good level of 88.36%, and finally realizing the efficient magnetization roasting utilization of low-grade refractory siderite-containing iron ore.

Table  5.  Weak magnetic separation results from the two magnetization roasting processes

Magnetization roasting process	TFe of concentrate / wt%	Yield of concentrate / wt%	TFe of tailings / wt%	Yield of tailings / wt%	Iron recovery rate / %
Conventional direct reduction	62.4	26.25	18.9	73.75	54.03
Preoxidation–low-temperature reduction	62.0	40.67	5.6	59.33	88.36

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4.   Conclusions

(1) Under the direct reduction magnetization roasting conditions of 500 and 550°C with a gas reduction potential of 0.45, siderite generates weakly magnetic wüstite, thus reducing the magnetite conversion and iron recovery rates of the subsequent weak magnetic separation process.

(2) Weakly magnetic α-Fe₂O₃ and strongly magnetic γ-Fe₂O₃ are produced and coexist during siderite oxidation roasting at 550 and 610°C with oxidation time ≤30 min. With the increase in oxidation temperature and oxidation time, γ-Fe₂O₃ transforms into α-Fe₂O₃.

(3) Preoxidized siderite mineral still produces weakly magnetic wüstite during reduction at 550 and 500°C under the gas reduction potential of 0.45 due to the further reduction of the unstable magnetite produced by γ-Fe₂O₃. When the reduction temperature is reduced to 450°C, wüstite is absent, realizing the stable magnetite transformation with a high magnetic conversion rate.

(4) For SX ore, the optimal preoxidation–low-temperature reduction magnetization roasting parameters are preoxidation at 610°C for 2.5 min in air, followed by reduction at 450°C for 5 min under the gas reduction potential of 0.45. The TFe concentrate grade of 62.0wt% and the iron recovery rate of 88.36% can be obtained for roasted SX ore through weak magnetic separation. Compared with that of conventional direct reduction magnetization roasting at 550°C for 7.5 min, the iron recovery rate of the proposed process has greatly improved by 34.33%. The proposed fluidized magnetization roasting process can realize the efficient magnetization roasting utilization of low-grade refractory siderite-containing iron ore and is unlimited by the siderite and hematite proportions of iron ore.

Acknowledgements

The authors are grateful for the financial support from the National Natural Science Foundation of China (Nos. 51974287 and 21736010) and Innovation Academy for Green Manufacture, Chinese Academy of Sciences (No. IAGM-2019-A11).

Conflict of Interest

The authors declare that they have no conflict of interest.