2024 年 64 巻 14 号 p. 1999-2007
This research aimed to investigate the production of ferro coke with high strength by cold pressing without adding binder, to meet the requirements of the metallurgical industry and avoid the environmental pollution problems caused by the hot pressing process. The effects of pressure and proportion of iron ore powder on the preparation of ferro coke from a weakly caking coal and a strongly caking coal were studied. The results show that when no binder is added, the ferro coke made from weakly caking coal has the highest compressive strength of 5.13 KN, while the ferro coke made from strongly caking coal has the highest compressive strength of 4.93 KN.
The relationship between the strength of ferro coke and the rate of dissolution loss was studied, and it was found that weakly caking coal produced under high forming pressure had good performance.
Ferro coke is a special raw material for iron smelting made by mixing coking coal and iron ore and subjecting the mixture to high-temperature carbonization. Ferro coke has the dual functions of high reactivity coke and pre-reduced ore, which can reduce the temperature of the blast furnace thermal reserve zone and improve the smelting efficiency.1,2) It can be produced using low-grade coking coal and low-grade iron ore, reducing the demand for high-quality coking coal and iron ore.3,4)
The strength of coke affects the stability and smelting efficiency of the blast furnace. Low-strength coke is unfavorable for the permeability of gases and liquids in the blast furnace.5,6) Nippon Steel produced ferro coke with a compressive strength of 4.0 KN by applying a briquetting pressure of 3.5 t/cm to a mixture of 5% to 8% iron ore and coking coal.7,8) JFE Steel Corporation started using ferro coke in 2011 and conducted industrial tests at its Chiba Plant, indicating that the strength of ferro coke is suitable for industrial application.9) Bao et al. prepared ferro coke by cold pressing with a briquetting pressure of 29.4 KN/cm.10) Wang et al. produced ferro coke with a compressive strength of 4.8 KN by hot pressing under briquetting pressure of 50 MPa.11) Wang et al. found that when the iron ore content was increased from 15% to 20%, the compressive strength of ferro coke decreased from 3.49 KN to 2.99 KN.12) Zhang et al. demonstrated that when 15% of iron ore was added to coking coal, coke reactivity was increased from 42.5% to 81%, and the crushing strength was reduced from 89% to 79%.13)
The preparation of ferro coke usually involves the addition of organic binders to improve its strength, but the cost of binders limits the production of ferro coke.14) Also, most organic binders are flammable and toxic. Zhong et al. used coal tar pitch as a binder for ferro coke.15) Uchida et al. used HyperCoal (HPC) as a binder for ferro coke.16) While some organic binders can avoid the above problems, the cost will also increase.17) The hot pressing method can avoid the use of binders in ferro coke production, subjecting the raw materials to high temperatures during the pressing stage. However, the hot pressing method generates a lot of gas and dust, and the working environment is demanding.18)
In this study, ferro coke with high compressive strength was prepared without binder by applying high mechanical pressure.19,20,21) The effects of the properties of coal, forming pressure, and proportion of iron ore powder on the strength and thermal properties of ferro coke were studied. The structure of ferro coke, the pyrolysis process of raw materials, and the characteristics of the gasification reaction of ferro coke were analyzed.
The raw materials for ferro coke production were as follows: a weakly caking coal (GC), a strongly caking coal (FC), and an iron ore powder (PB). The basic properties of coal are shown in Table 1. The results of elemental analyses of coal and iron ore are shown in Tables 2 and 3, respectively.
| Coal samples | Mad/% | Aad/% | Vad/% | FCad/% | G | Y |
|---|---|---|---|---|---|---|
| GC | 1.70 | 8.64 | 33.73 | 55.93 | 63.5 | 17.00 |
| FC | 0.73 | 13.80 | 29.61 | 55.86 | 88 | 26.00 |
The subscript ad is the air-drying basis, M is the moisture; A is the ash; V is the volatile matter; FC is the fixed carbon mass fraction; G is the bonding index; and Y is the micellar layer index.
| Coal samples | N | C | H | S |
|---|---|---|---|---|
| GC | 1.40 | 75.65 | 5.10 | 0.35 |
| FC | 1.32 | 78.02 | 4.97 | 1.39 |
| Ore | SiO2 | CaO | MgO | Al2O3 | TiO2 | TFe | FeO |
|---|---|---|---|---|---|---|---|
| PB | 4.51 | 0.11 | 0.15 | 2.99 | 0.15 | 61.52 | 0.44 |
Coal was crushed to less than 1.0 mm and iron ore was crushed to less than 0.0749 mm. Then, coal and iron ore powder were mixed and pressed into cylinders with dimensions of Φ25 mm×20 mm, and the pressure was maintained for 10 s. The formulation and molding pressure of each sample are shown in Table 4.
| Samples | Coal: Ore/% | Stress/MPa | |
|---|---|---|---|
| GC | GC-1 | 90:10 | 200 |
| GC-2 | 90:10 | 50 | |
| GC-3 | 85:15 | 200 | |
| GC-4 | 85:15 | 50 | |
| GC-5 | 80:20 | 200 | |
| GC-6 | 80:20 | 50 | |
| GC-7 | 75:25 | 200 | |
| GC-8 | 75:25 | 50 | |
| FC | FC-1 | 90:10 | 200 |
| FC-2 | 90:10 | 50 | |
| FC-3 | 85:15 | 200 | |
| FC-4 | 85:15 | 50 | |
| FC-5 | 80:20 | 200 | |
| FC-6 | 80:20 | 50 | |
| FC-7 | 75:25 | 200 | |
| FC-8 | 75:25 | 50 | |
The device used for briquette carbonization is shown in Fig. 1. The chamber was heated from room temperature to 1100°C at a heating rate of 3°C/min and kept constant at 1100°C for 3 h.
(1) Pyrolysis of ferro coke materials
The sample shown in Table 4 was pressed into cylinders with dimensions of Φ10 mm × 10 mm. A thermal analyzer (NETZSCH STA 449 F3) was used for the TGA testing of samples. Briquette was heated to 1100°C at 3°C/min in 50 mL/min N2.
(2) Gasification of ferro coke
The gasification reaction of the pyrolyzed sample was performed with NETZSCH STA 449 F3. Ferro coke was heated to 1400°C at 3°C/min in 50 mL/min CO2.
2.5. Compressive Strength of Ferro CokeThe compressive strength of ferro coke was determined using a pellet ore pressure tester (Anshan Kexiang QTY-10000). The average of ten samples was used as the test result for that sample.
2.6. Reactivity and Strength after Reaction of Ferro CokeA vertical tube furnace (Fig. 2) was used to test the dissolution loss reactivity of ferro coke. The sample was heated from room temperature to 1100°C at a flow rate of 5 L/min of N2 atmosphere and then constant temperature at 1100°C for 10 minutes. At the end of the constant temperature period, N2 was switched to CO2 for the reaction, with CO2 flow rate of 5 L/min. Four reaction times were set: 20 min, 40 min, 60 min and 80 min. At the end of each reaction, CO2 was switched to N2, with N2 flow rate of 5 L/min. The coke was removed after cooling to room temperature, and the dissolution loss for each piece of ferro coke was calculated. A pellet ore pressure tester was used to test the compressive strength of the ferro coke.
XRD analysis was performed using a Rigaku Corporation D/MAX 2500PC X-ray diffractometer, with Cu Kα radiation (40 kV, 40 mA) as the X-ray source at a wavelength of 1.5418 Å. The sample was crushed to a particle size smaller than 0.074 mm. Samples were scanned over an angular range from 10° to 90° 2θ at a scan speed of 0.6° 2θ per min by using a step size of 0.02° 2θ.
The photograph of ferro coke sample is shown in Fig. 3 and the compressive strength results of the ferro coke samples are shown in Fig. 4. The compressive strength of GC ranges from 1.04 KN to 5.13 KN and that of FC ranges from 1.15 KN to 4.93 KN. The molding pressure has a great influence on the strength of the ferro coke prepared from GC. The compressive strength of ferro coke prepared from weakly caking coal can be significantly improved by using high molding pressure. High forming pressure improves the caking between coal particles and increases the strength of the coke. On the other hand, high forming pressure increases the resistance to the escape of coal pyrolysis gases, resulting in more defects in the coke formed from strongly caking coal.
The properties of coal determine the change trend of the strength of ferro coke. GC is a weakly caking coal, and the strength of GC ferro coke decreases with the increase in proportion of iron ore powder. FC is a strongly caking coal, and it is difficult to release the pyrolysis gas without or with only a small amount of iron ore powder, resulting in many structural defects in the FC ferro coke. With increase in the proportion of iron ore powder, the pyrolysis gas escapes smoothly and the structural defects decrease, so the strength of FC ferro coke increases.
3.2. Structure and Composition of Ferro Coke 3.2.1. Microstructural CharacteristicsCoke is a porous material, and the shape, size, number, distribution, and wall thickness of its pores are related to its strength and reactivity. The pore structure of ferro coke is shown in Fig. 5. The ferro coke samples GC-2 and FC-8 prepared under pressure of 50 MPa have large pore diameters and structural defects. The ferro coke samples GC-1 and FC-7 prepared under 200 MPa have small pore diameters and dense structures. FC-7 and FC-1 are both prepared under 200 MPa pressure, but FC-1 shows large cracks. FC is a strongly caking coal, and the amount of iron ore powder added is small. Hence, the escape of pyrolysis gas is hindered, and the intense escape of the final gas causes crack formation in FC-1.
SEM of ferro coke is shown in Fig. 6. GC-2 and FC-8 have loose matrices with layered structures, long cracks, and large holes. The matrices of GC-1 and FC-7 are dense with few defects. The dense structure is conducive to dispersing stress, reducing the formation and extension of cracks, and thereby improving the strength.
The matrix structure of GC-7 is relatively loose, which is due to the addition of a large amount of iron ore powder in the weakly caking coal, decreasing the amount of plastic mass. FC-1 has long cracks, likely caused by the concentrated escape of pyrolysis gas in strongly caking coal.
The EDS image of the sample is shown in Fig. 7. Reduced iron particles in iron ore powder adhere to the surface of the matrix and small iron particles fuse to form larger particles. This indicates that the iron ore powder is preliminarily reduced in the pyrolysis stage.
The XRD results of ferro coke are shown in Fig. 8. Fe peaks are detected in the pyrolyzed samples. Fe in iron ore powder is in the oxidized state. During the pyrolysis of raw materials, Fe2+ and Fe3+ are reduced by CO and H2 to form Fe.
The XRD patterns of GC-1 and FC-7 gasification reactions at different times are presented in Fig. 9. From 0 to 40 min, the Fe peak in the samples increases, while the FeO and Fe2O3 peaks decrease. From 40–80 min, the Fe peak decreases and the FeO peak increases. In ferro coke, there is conversion of Fe to FeO, which reacts with CO2 as follows:22)
| (1) |
| (2) |
| (3) |
The pyrolysis process of ferro coke raw materials is shown in Fig. 10. The addition of iron ore powder significantly reduces the rate of volatile gas production between 350°C and 500°C. In this temperature range, coal is actively pyrolyzed, while iron ore powder hardly changes. GC-7, FC-7, and FC-8 have high gas production peaks at 700–1000°C, caused by the reduction reaction of the high proportion of iron ore powder.
The range of pyrolysis active temperature of ferro coke raw materials is shown in Fig. 11. The pyrolysis of GC before 450°C is more active than that of FC, but it is not as active as FC between 450°C and 500°C. The higher molding pressure causes closer bonding of the coal and iron ore powder particles in GC-1. FC is more active at temperatures ranging from 450°C to 500°C, producing more plastic mass. The gas cannot escape easily from the plastic mass wrapped around it, resulting in the formation of more defects. With the addition of more iron ore powder, the gas escape of FC-7 decreases, the structural defects are reduced, and the strength increases.
Therefore, in the preparation of ferro coke, it is necessary to pay attention to the gas production of ferro coke raw materials between 450°C and 500°C.
3.3. Gasification of Ferro CokeThe gasification reaction of ferro coke is shown in Fig. 12. The weight of ferro coke increases at 600°C to 800°C, due to the oxidation of Fe by CO2. As shown in Table 5, GC-1 has a starting reaction temperature that is 95°C lower than that of GC, and the maximum reaction rate is increased by 130%.
| Samples | Tf/°C | Tm/°C | rm/(%/min) |
|---|---|---|---|
| GC | 781 | 1042 | 1.74 |
| GC-1 | 686 | 997 | 2.28 |
| FC | 840 | 1124 | 1.03 |
| FC-7 | 719 | 995 | 1.70 |
Tf is the Starting gasification temperature; Tm is the Maximum gasification rate temperature; rm is the Maximum gasification rate.
FC-7 has a starting reaction temperature that is 121°C lower than that of FC, and the maximum reaction rate is increased by 160%. The addition of iron ore powder produces molten phase inside the ferro coke, promotes mass transfer and direct reduction of iron ore powder, and reduces the activation energy of the reaction.
3.4. Dissolution Loss and Strength after Dissolution Loss of Ferro CokeThe relationship between the ferro coke weight loss with time in the vertical furnace is shown in Fig. 13. The compressive strength results of ferro coke after weight loss are shown in Figs. 14 and 15. When the weight loss rate is zero, the compressive strength of 200 MPa ferro coke is significantly higher than that of 50 MPa ferro coke. When the weight loss rate is about 20%, the compressive strength of 200 MPa ferro coke decreases slightly. When the weight loss rate of ferro coke reaches about 30%, the compressive strength of ferro coke decreases obviously.
In blast furnace, the weight loss rate of coke averages 20%–25%.23,24) In this range, the 200 MPa ferro coke has less reaction time and higher strength after reaction than the 50 MPa ferro coke. In comparison, GC-1 shows excellent performance.
This paper investigated the production of ferro coke with high strength by cold pressing without adding binder. The following conclusions were obtained from this work.
(1) Applying high pressure can significantly improve the compressive strength of the ferro coke produced. By applying higher mechanical pressure during molding, the gap between the weakly caking coal particles and the iron ore powder is reduced, resulting in better caking, which improves the strength of the ferro coke. For strong caking coal, high pressure molding can lower the deterioration of ferro coke structure caused by the escape of large amounts of gas.
(2) The properties of coal determine the change trend of the strength of ferro coke. GC is weakly caking coal, which cannot effectively cake the components. As the proportion of iron ore powder added increases, the strength of the GC ferro coke decreases. During FC pyrolysis, a large amount of gas is produced, and the gas release resistance is relatively high, which leads to severe defects in the ferro coke. Thus, as the proportion of iron ore powder increases, the strength of the FC ferro coke increases.
(3) Ferro coke has a lower reaction temperature and a faster reaction rate than traditional coke. GC-1 has a starting reaction temperature that is 95°C lower than that of GC, and the maximum reaction rate is increased by 130%. FC-7 has a starting reaction temperature that is 121°C lower than that of FC, and the maximum reaction rate is increased by 160%.
(4) At 20 minutes of dissolution, the strength of the ferro coke made at 50 MPa decreases significantly, while that made at 200 MPa maintains a high strength. At 40 minutes of dissolution, the ferro coke made at 50 MPa has no strength, while that made at 200 MPa has higher strength.
No potential conflicts of interest was reported by the authors.
