Microwave Carbothermic Reduction of Oolitic Hematite

Abstract

The microwave reduction of coke-bearing oolitic hematite composite pellets followed by magnetic separation was investigated in this work to explore the feasibility of microwave heating on utilization of oolitic hematite. The reduction was conducted in a temperature controllable microwave oven at 1273, 1373, 1423, 1473 and 1523 K for 5–60 min. The effects of temperature and holding time on reduction were studied. The highest metallization of 95.42% was achieved at 1523 K in 10 min. Then the air cooling metallic pellets were crushed and grinded and subsequent separated via magnetic separation. The effect of magnetic field intensity on recovery and grade of iron powder were investigated. The iron powder with iron content, recovery and phosphorus content of 92.89%, 85.28% and 0.24% respectively was achieved at the magnetic field intensity of 0.04 Wb/m². This will be a potential way to realize the comprehensive utilization of oolitic hematite in China.

1. Introduction

Iron ore is important raw material for the production of iron and steel. The steel output reached 800 million tons in 2014, while China need to import a large number of iron ore in each year due to serious shortage of domestic supply. The iron ore imported to 953 million tons along with an extreme high external dependence of 84% in 2015. So research the utilization technology of refractory iron ore hold important strategic significance for sustainable development of national economy of China.

Ningxiang oolitic hematite is a unique type of sedimentary iron ore. The proven reserves of this type of iron ore is 3.72 billion tons accounting for ~11% of the iron ore resources in China. The iron content of this type of iron ore is ~20–45%, the quartz content is ~15–35%, and the phosphorus content is ~0.4–1.1% or higher.

Phosphorus is a pernicious element for steel which major come from iron ore. Phosphorus can increase the strength of steel, but will greatly reduce the plasticity and toughness and increase the cold crack sensitivity. The key here is to realize the efficient separation of iron and phosphorus before entering the steel smelting process. The reported comprehensive utilization methods include flotation,¹⁾ chemical beneficiation,^2,3,4) and reduction-magnetic separation.^{5,6,7,8,9,10,11,12)}

Microwave is a kind of electromagnetic wave, with frequency between 3×10⁸ Hz and 3×10¹¹ Hz. Microwave radiation can penetrate and propagate through many dielectric materials, generate electric fields that induce the motion of free or bound charges; the resistance to these motions due to inertial, elastic and frictional forces causes loser and, as a consequence, the rapid heating of the material. Microwave heating provides several advantages through savings in energy, space and time, and a reduction in the environmental impact of material processing. There are have lots of literatures referred to the microwave reduction of oxides ores, such as magnetite,¹³⁾ hematite,¹⁴⁾ vanadium titano-magnetite,¹⁵⁾ steel making slag and sludge,¹⁶⁾ nickeliferous silicate laterite,¹⁷⁾ malachite concentrate,¹⁸⁾ bauxite ore,¹⁹⁾ and so on.

In the present work, the reduction of coke-bearing oolitic hematite pellets were conducted at temperature of 1273–1523 K in a temperature controllable microwave cavity with the output power of 0–1.5 kW. The effect of temperature and holding time on metallization were investigated. The reduced product was magnetic separated in order to obtain iron powder. The field strength was studied. This work tries to explore the application possibility of microwave heating on utilization of oolitic hematite.

2. Materials and Procedures

The chemical component analysis of raw ore is shown in Table 1. The total iron content is 43.50 wt%. The hematite content is 60.33 wt% accounting for 96.34% of iron oxides. The major non-ferrous components include SiO₂, Al₂O₃ and CaO. The phosphorus content is 0.85%.

Table 1. Chemical component analysis of raw ore.

Component	TFe	FeO	Fe2O3	SiO2	Al2O3	CaO	MgO	MnO	P
Content, wt%	43.50	1.67	60.33	18.80	6.67	3.66	0.67	0.17	0.85

The XRD pattern of raw ore is shown in Fig. 1. The major mineral phases are hematite (Fe₂O₃), quartz (SiO₂), dolomite (CaMg(CO₃)₂), chamosite ((Fe, Mg)₃ (Fe²⁺, Fe³⁺)₃AlSi₃O₁₀(OH)₈), and ledikite (KAl₂[(Si, Al)₄O₁₀]•(OH)₂•nH₂O). Parts of hematite are eutectic with chamosite. Almost all collophanite are eutectic with dolomite, chamosite and ledikite.

Fig. 1.

XRD pattern of raw ore.

Figuers 2(1) and 2(2) show the BSE morphologies of hematite ooid with cores constitute of collophanite and chamosite. Different colors are representing the aggregation of hematite, chamosite and collophanite. The macroscopic single crystal granularity of hematite and chamosite particles are ~5–20 micrometer. The single or multilayer band structure inside ooid result in very difficult to realize the monomer dissociation of hematite particle, also does not favor the reduction reaction. The energy dispersive X-ray analysis shows that the chemical components of collophanite are 54.64% CaO, 42.79% P₂O₅, 1.74% FeO and 0.83% SiO₂, Seen in Fig. 2(3).

Fig. 2.

BSE morphologies of hematite ooid (aggregation of hematite and gangues) with a core constitute of collophanite and chamosite ((1)- podiform core; (2)-ring core; (3)-energy dispersive X-ray analysis of collophanite).

The dry basis fix carbon, ash, volatile of coke used in this work is 70.80%, 27.80%, and 1.40 wt% respectively. Spherical composite pellets of about 10 mm diameter were prepared using a pelletizer from a mixture of oolitic hematite powder (~75 micrometer), coke powder, organic cementing agent and water. The surplus coefficient of coke is 1.5. The as-prepared composite pellets were dried to a constant weight in an air oven at temperature 118°C before the reduction.

The microwave oven (HT-III, KMUST/Kunming), with 1.5 KW maximum power at 2.45 GHz, and automatic temperature control via numerical adjust of output power was used in this work. The schematic diagram of microwave oven is shown in Fig. 3(1). The iron metallization of reduced sample as a target to measure the reduction extends of composite pellets were calculated by following equation:   

$${\eta }_{Fe}=\frac{MFe}{TFe}\times 100\%$$(1)

where MFe is the metallic iron content of reduced sample, wt%; TFe is the total iron content, wt%. The content of metallic iron, total iron and phosphorus content in sample are decided by chemical titration analysis according to China National Standards of GB 6730.5-2007, GB 6730.6-86 and GB 6730.19-86 respectively.

Fig. 3.

Influence of temperature and holding time on metallization (upper left: schematic diagram of microwave oven; upper middle: hematite powder before reduction; upper right: pellets after reduction).

3. Results and Discussion

3.1. Influences of Temperature and Holding Time on Reduction

The appearances of oolitic hematite powder and reduced samples are shown in Figs. 3(2) and 3(3). The influence of temperature and holding time are shown in Fig. 3(4). The sample temperature was raised from room temperature to appointed in ~5–10 min. It can be seen from Fig. 3(4) that the temperature has positive impact on reduction process. The metallization were significant increased with the increase of temperature. For example, the metallization of samples reduced at 1273, 1373, 1423, 1473 and 1523 K for 20 min are 8.68, 59.25, 77.01, 83.83 and 92.27% respectively.

The direct and indirect reduction mechanism of hematite is step by step from Fe₂O₃ to Fe₃O₄→FeO→Fe⁰,^20,21,22) as shown in Table 2. According to the equation of standard Gibbs free energy changes, the solid-state reactions (reactions ①–③) between hematite and coke are slowly while the gas-solid reactions (reactions ⑥–⑧) are quickly. The solid-state and gas-solid reactions are depending on the coke reactivity which related to the temperature. The solid-state reaction stage would be shorter by boost of temperature, and then the reactions rapidly enter to the accelerated stage under relative higher temperature. According to the equation of standard Gibbs free energy changes, the rate-limiting steps are reactions ③ and ⑧ for hybrid direct and indirect reduction of hematite. Relative high temperature is beneficial to reactions ③, ⑤, ⑧ and ⑨ and inhibits the reaction ④.

Table 2. Reaction mechanism of carbon-bearing hematite composite pellets.^21,22)

No.	Solid-state reactions at initial stage	No.	Gas-solid reactions at accelerated stage
①	3Fe2O3 (s) + C (s) = 2Fe3O4(s) + CO (g) ΔGmθ=124429-224.37(T/K),J・mol-1	⑥	3Fe2O3 (s) + CO (g) = 2Fe3O4(s) + CO2(g) ΔGmθ=-42121-53.37(T/K),J・mol-1
②	Fe3O4(s) + C (s) = 3FeO (s) + CO (g) ΔGmθ=196720-199.38(T/K),J・mol-1 (T>868.15 K)	⑦	Fe3O4(s) + CO (g) = 3FeO (s) + CO2(g) ΔGmθ=30170-29.38(T/K),J・mol-1 (T>868.15 K)
③	FeO (s) + C (s) = Fe (s) + CO (g) ΔGmθ=149600-150.36(T/K),J・mol-1 (T>868.15 K)	⑧	FeO (s) + CO (g) = Fe (s) + CO2(g) ΔGmθ=-16950+20.64(T/K),J・mol-1 (T>868.15 K)
④	2FeO (s) + SiO2 (s)=Fe2SiO4 (s) ΔGmθ=-36200+21.09(T/K),J・mol-1 (T=323.15–1518.15 K)	⑨	Fe2SiO4 (s) +2CO(g) = 2Fe(s)+ SiO2(s)+2CO2 (g) ΔGmθ=12510+10.54(T/K),J・mol-1
⑤	Fe2SiO4 (s) + 2C (s) = 2Fe (s) + SiO2(s)+2CO (g) ΔGmθ=353924-338.91(T/K),J・mol-1

The increase of holding time from 10 to 60 min is unfavorable to the reduction process. The main reasons include the accumulation of fayalite and the decrease of pellet porosity caused by soft melting compounds. The increase of oxygen potential in open system also will result in the re-oxidation of metallic iron. At the temperature of 1523 K, the metallization of samples were first increased from 76.34% to 95.42% then decreased to 92.27% when the holding times were extended from 5 min to 20 min. The reduction is very fast compared to the conventional heating, seen in Table 3. Further investigations on holding time from 10–20 min will be done in the future to explore the optimal conditions.

Table 3. Comparisons with the results of conventional heating.

Ref.	Reduction temperature, K	Reduction time, min	Metallization of reduced sample, %
This work	1523	10	95.4
Ref. 5)	1323	150	88.1
Ref. 6)	1323	100	94.1
Ref. 11)	1473	50	~93.0

The XRD patterns of reduced samples are shown in Fig. 4. The metallization of four samples from bottom to up are 59.25, 77.01, 83.83, 92.27 and 95.42% respectively. As shown in Fig. 4, according to the relationship between content and intensity of diffraction peaks, the major phases in the order of content are metallic iron, quartz, fayalite (Fe₂SiO₄), calcium aluminosilicate (CaAl₂Si₂O₈), wustite (FeO), pigeonite ((Ca, Mg, Fe)(Mg, Fe)Si₂O₆), calcium silicate (Ca₃Si₂O₇), and phosphates (phosphorus containing phase i.e. Ca₂P₂O₇, Al(PO₃)₃, SiP₂O₇, and Fe(PO₃)₃). It can be seen from Fig. 4 that the peak intensity of metallic iron, calcium aluminosilicate and calcium silicate were increased with the increase of reduction degree. While the peak intensity of quartz, fayalite, wustite and pigeonite were decreased with the increase of reduction degree. The major phosphorus containing phase is calcium phosphate, followed by phosphate of Al, Si, Fe. Small number of Fe–P alloys may generate due to the reduction of P₂O₅ at relative high temperature.

Fig. 4.

XRD patterns of reduced samples.

3.2. Influence of Magnetic Field Intensity on Magnetic Separation

The air cooling metallic pellets were crushed and grinded into ~75 micrometer particles. The metallic iron and gangues then separated via magnetic separation. The influence of magnetic field intensity on recovery and TFe content of iron powder are shown in Fig. 5. It can be seen from Fig. 5 that the iron powder grade were decreased from 92.89% to 84.02% when the magnetic field intensities were increased from 0.04 to 0.16 Wb/m². While the iron recoveries were increased from 85.28% to 92.13% correspondingly. The escape of weak magnetic minerals such as fayalite at relative high magnetic field intensity will result in the decrease of iron powder grade.

Fig. 5.

Influence of magnetic field intensity on recovery and TFe content of iron powder.

The XRD pattern of iron powder is shown in Fig. 6. The reduction conditions are 1523 K and 10 min. The magnetic field intensity is 0.04 Wb/m². The recovery and TFe content are 92.89% and 85.28%. It can be seen from Fig. 6 that the X-ray intensities of metallic iron are 25667 (110 hkl) and 4300 (200 hkl) cps. The fitted grain size is 30.1 nm. The X-ray intensities of calcium aluminosilicate (CaAl₂Si₂O₈) and eutectic of magnetite (Fe₃O₄), fayalite (Fe₂SiO₄) and phosphorus containing phases i.e. Al(PO₃)₃, Mg₃(PO₃)₂ and FeP are 3383 and 3500 cps. The non-ferrous impurities are caused by entrainment during dressing process. The ferrous impurities are result from reduction of P₂O₅ and not completely reduction of fayalite. The phosphorus content is 0.24% along with a ~71.8% dephosphorization rate. The iron powder achieved in this work can be used as feed material for electric steelmaking and powder metallurgy iron powder production.

Fig. 6.

XRD pattern of iron powder.

4. Conclusions

In summary, the present work tries to explore the feasibility of microwave heating on utilization of oolitic hematite. The coke-bearing oolitic hematite composite pellets were reduced in a temperature controllable microwave cavity at specified temperature for several minutes. The highest metallization of 95.42% was achieved at 1523 K in 10 min. Then the air cooling metallic pellets were crushed and grinded and subsequent separated via magnetic separation. The iron powder with iron content, recovery and phosphorus content of 92.89%, 85.28% and 0.24% respectively was achieved under the magnetic field intensity of 0.04 Wb/m². Further optimization may includes appropriately reduce the reduction temperature and extend the reduction time to avoid reduction of P₂O₅ and thoroughly reduce the fayalite, add some kind of dephosphorizer in composite pellets, and decrease the magnetic separation feed particle size, is expected to reduce the impurity content in iron powder. This will be a potential way to realize the comprehensive utilization of oolitic hematite in China.

Acknowledgments

This work was supported by National Natural Science Foundation of China (Grant No. 51304005, 51574134, 51574042 and 51304004).

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