Effect of Titanomagnetite Ironsand Coal Composite Hot Briquette on Softening-melting Performance of Mixed Burden under Simulated Blast Furnace Conditions

Abstract

Titanomagnetite ironsand coal composite hot briquette (ICHB) was proposed as a novel type of burden to enhance the incremental and high-efficiency utilization of ironsand in blast furnace. ICHB was prepared firstly under laboratory conditions, with a compressive strength higher than 3000 N. Then the national charging ratio of ICHB in the mixed burdens was explored to conduct softening-melting experiments with the simulated BF conditions. Finally the softening-melting-dripping mechanism of mixed burdens was discussed by thermodynamic calculations, SEM-EDS, and XRD detections in this work. It was showed that the softening-melting-dripping behavior and the permeability of mixed burdens could be improved obviously by an appropriate ICHB charging. With the increasing of ICHB charging ratio, the location of cohesive zone was shifted down gradually and its thickness was the narrowest at the ICHB charging ratio of 10%, which was beneficial to BF smelting. Meanwhile, the dripping ratio of mixed burden also achieved the maximum value of 66.71% when the ICHB charging ratio was 10%. However, the excessive ICHB charging would promote the precipitation of Ti(C,N) with a high melting point at the interface between metal and slag, which resulted in the deterioration of dripping and further worsening the gas permeability of mixed burdens. Comprehensively considering the softening-melting-dripping behavior and the permeability of mixed burden, and the precipitation of Ti(C,N), the recommended ICHB charging ratio was 10%.

1. Introduction

As a typical kind of titania-ferrous coexisting ore, titanomagnetite ironsand with rich Fe, Ti, V and other valuable elements is mainly made from of the volcanic lava.^1,2,3,4) It is widely distributed in the coast of Australia, New Zealand, Japan, and the Southeast Asia.^5,6) Recently, the production cost of steel has increased as the running out of the high-grade iron ore concentrate, and the demand for low-price and complicated iron ore resources, such as titanomagnetite ironsand, are growing rapidly.⁷⁾ Among titania-ferrous iron ores found throughout the world, the ironsand from New Zealand is in a large deposit and contains about 57 mass% of total iron, but its price is absolutely cheaper than that of the normal iron ores. Therefore, the efficient utilization of titanomagnetite ironsand plays a vital role in advancing the economic benefit for iron and steel industry.

The main processes of disposing ironsand include the blast furnace (BF) process and the non-blast furnace process. EHSV (The Evraz Highveld Steel & Vanadium) process applying in the few iron and steel plants in South Africa is a representative non-blast process. During this process, through controlling the addition of carbon, the iron and vanadium are selectively reduced while the left titania is dissolved in the slag by using submerged-arc furnace (SAF). The SAF has been replaced by open slag bath (OSB) in the recent years. Nevertheless, this non-blast furnace process is complex to operate and requires huge and serious energy consumption. At present, the BF process is the main and conventional route for smelting vanadium-bearing titanomagnetite including titanomagnetite ironsand in China.^{8,9,10,11,12)} Then the ironsand are usually prepared as ironsand sinter and ironsand pellet.^{13,14,15,16,17)} However, amounts of literatures indicated that the addition of titanomagnetite ironsand would worsen the metallurgical properties of sinter as well as pellet due to the following aspects: (1) Ironsand performs a high assimilation temperature and a low binding phase intensity, which leads to reducing the characters such as cold strength and reducibility of sinter and pellet;¹⁸⁾ (2) Owning to the spinel cubic structure, the high thermodynamic stability, and its high content of Ti, the titanomagnetite ironsand sinter generally shows a low strength and low reducibility.^19,20,21) Meanwhile, on account of its low specific surface area, coarse grain, dense structure, and smooth surface, the proportioning ratio of ironsand in sinter and pellet is seriously limited. Therefore, to realize the incremental and high-efficiency utilization of ironsand during BF process, it is necessary to develop a new type of ironsand-bearing materials for BF smelting.

Iron ore carbon composite hot briquette (CCB) is novel and special carbon-bearing agglomerate. It contains carbonaceous and iron ore powder adjoining closely.^22,23,24,25) As a new type of BF burden, CCB exhibits a great reducibility because it closely adjoins coal and iron ore, accelerating the carbon gasification and iron oxides reduction.^26,27,28) Besides, some literatures showed that the reduction of iron ore in CCB started at a lower temperature and the process was completed faster than that of pellet and sinter.²⁹⁾ According to our previous research results, adding the appropriate ratio of CCB in the BF burden is beneficial to improve the softening-melting behavior and enhance the gas permeability of packed bed during the melting process.³⁰⁾

Based on the abovementioned backgrounds, the CCB technology will be applied to process the ironsand and proposed in the present work. First of all, the titanomagnetite ironsand coal composite hot briquette (ICHB) was prepared under the laboratory conditions. Then, the different charging ratios of ICHB in the mixed burden were explored to conduct the softening-melting experiments with simulated BF conditions. Finally, the softening-melting-dripping mechanism of mixed burden with charging ICHB was analyzed by thermodynamic calculation, SEM-EDS and XRD methods. And then the rational ICHB charging ratio could be summarized and obtained.

2. Experimental

2.1. Materials

The main chemical compositions of raw materials were listed in Table 1. The main raw materials used in this study included sinter, pellet, lump ore, vanadium-titanium magnetite (VTM), and New Zealand ironsand. Notably, the sinter, pellet, lump ore, and VTM were obtained from an iron and steel plant in China. The proximate analysis of the coal used in the experiment was listed in Table 2. As seen, FC represented the fixed carbon content was 58.97%, A_ad, V_ad, and M_ad described as ash, volatiles, and moisture was 10.90%, 27.09%, and 3.04% respectively. The subscript “ad” indicated that the coal in which the moisture and air humidity are in equilibrium was taken as the benchmark.

Table 1. Main chemical composition of raw materials (wt/%).

Materials	TFe	FeO	CaO	SiO2	MgO	Al2O3	TiO2	V2O5
Sinter	54.66	10.42	11.32	5.33	2.12	2.14	1.92	0.12
Pellet	59.16	0.98	1.50	5.86	3.02	1.68	2.95	0.20
Lump ore	61.57	0.19	0.15	5.66	0.19	0.66	0.13	0.01
Ironsand	57.22	27.80	1.48	4.07	3.29	3.41	8.00	0.54
VTM	61.88	28.08	1.07	3.54	1.50	1.67	5.40	0.37

Table 2. Proximate analysis of the coal used in the experiment (wt/%).

FC	Aad	Vad	Mad
58.97	10.90	27.09	3.04

The size distribution and specific surface area of titanomagnetite ironsand and VTM were shown in Fig. 1. Compared with the specific surface area, the specific surface area of ironsand was much lower than that of VTM. The reduction of specific surface area could decrease the contact points among the particles, which resulted in the decrease of ICHB strength.³¹⁾ In this experiment, ironsand was pretreated by wet grinding for 1 hour to enhance the specific surface area, thereby increasing the strength of ICHB. After wet grinding, the specific surface area of ironsand increased from 523 cm²/g to 3258 cm²/g, and its size distribution was similar to that of VTM.

Fig. 1.

Size distribution and specific surface area of VTM and ironsand. (Online version in color.)

To prepare ICHB under laboratory conditions, the raw materials were uniformly mixed after drying in a drying oven firstly. Next, the mixtures were heated to 300°C in a temperature-controlled heating furnace. Then, the pre-heated mixtures were agglomerated under a hot briquetting pressure of 50 MPa. And this patterned briquettes would be given an heat treatment in a temperature-controlled heating furnace at 500°C for 4 hours. In this experiment, the compressive strength of ICHB after heat treatment could be up to 3196 N, when the ICHB comprised an ironsand ratio of 40%, a VTM ratio of 27.5%, and a coal ratio of 32.5%, and its main chemical composition was listed in Table 3.

Table 3. Main chemical composition of ICHB (wt/%).

TFe	FeO	CaO	SiO2	MgO	Al2O3	TiO2	V2O5	FC
41.94	19.32	1.18	3.59	1.65	2.37	4.10	0.33	19.94

2.2. Experimental Devices and Procedure

In this study, an RSZ-03 type iron ore high-temperature load reduction dripping device was used to investigate the effects of ICHB charging ratio on the softening-melting behavior of mixed burden. The apparatus system was shown in Fig. 2(a). It was composed of a displacement meter, a thermocouple, an electric furnace, a reaction tube, a supporting tube, a gas pressure transmitter, and a simple cooling vessel. The reaction tube included a Φ75-mm (inner diameter) graphite crucible with the numerous Φ10-mm dripping holes at its bottom. 500 g of mixed burden was charged into the graphite crucible. During the softening-melting experiment, the thickness of the mixed burden and the gas pressure dropping were recorded automatically. The dried coke with a diameter of from 10 to 12.5 mm and a layer thickness of 30 mm and 15 mm was placed under and over the mixed burden respectively to guarantee the dripping of molten iron and slag as well as the reduction gas passing through the packed bed.

Fig. 2.

Softening-melting experiment system of mixed burden (a) experimental apparatus schematic diagram (b) conditions used in the softening-melting experiment (c) scheme of softening-melting experiment. (Online version in color.)

As for the heating and load system, when the temperature was lower than 900°C, the load was 1.8 kg·cm⁻². And it increased to 3.5 kg·cm⁻² when the temperature was higher than 900°C. As shown in Fig. 2(b), the gas flow was 3 L/min of N₂ in the temperature range below 400°C; 9 L/min of N₂, 3.9 L/min of CO, and 2.1 L/min of CO₂ in the temperature range from 400°C to 900°C; 10.5 L/min of N₂ and 4.5 L/min of CO in the temperature range from 900°C to the end. The heating rate was 10°C·min⁻¹ in the temperature range below 900°C; 3°C·min⁻¹ in the temperature range from 900°C to the 1020°C; and 5°C·min⁻¹ in the temperature range from 1020°C to the end.

According to the on-site BF operation conditions of the iron and steel plant, the proportion of sinter, pellet, and lump ore was 70%, 20%, and 10%, respectively. In this study, the proportions of sinter and lump ore were remained as constants, and the sum proportion of pellet and ICHB was 20%. In other words, the proportion of pellet would decrease simultaneously with the increasing ICHB charging ratio, as shown in Fig. 2(c). The particle size of sinter, pellet, and lump used in this work is all about 10–12.50 mm. And the ICHB was ellipsoid with an particle size of 19 mm×21 mm.

3. Results and Discussion

To describe the melting-dripping behavior of mixed burden, some physical qualities were defined: T₄, the softening starting temperature, the temperature at which shrinkage ratio of mixed burden reached 4%; T₄₀, the softening end temperature, the temperature at which shrinkage ratio of mixed burden reached 40%; T_S, the melting starting temperature, the temperature at which the differential pressure exhibited a substantial increase; and T_D, the dripping starting temperature, the temperature at which the pig iron started to drip from the graphite crucible. On the basis of these indices, two temperature intervals were defined: (T₄₀-T₄) represented the softening temperature interval and (T_D-T_S) represented the melting temperature interval.

3.1. Softening and Melting Behavior

The effects of ICHB charging ratio on the softening behavior of mixed burden were shown in Fig. 3(a). With the increasing of ICHB charging ratio, T₄ reduced from 1141°C to 1120°C, and T₄₀ increased from 1123°C to 1249°C. As a result, the softening temperature interval (T₄₀-T₄) increased from 92°C to 128°C gradually. As a kind of carbon-bearing agglomeration, ICHB performed an excellent self-reducibility in the low-temperature zone. Consequently, ICHB showed a rapider reduction rate compared with the traditional sinter and pellet, which promoted the shrinking of mixed burden, thereby decreasing the T₄ effectively. The increase of T₄₀ was mainly caused by the following two reasons. Firstly, the reducibility of mixed burden was improved owning to the charging of ICHB with a good reducibility, which would decrease the content of FeO in the slag and increased the liquidus temperature of slag. Secondly, the ICHB with good high-temperature strength acted as the skeleton in the softening mixed burden, which could retard the shrinking of mixed burden and resulted in the increasing of T₄₀.

Fig. 3.

Effect of ICHB charging ratio on softening and melting behavior of mixed burden. (Online version in color.)

The effects of ICHB charging ratio on melting behavior of mixed burden are shown in Fig. 3(b). With the increase of ICHB charging ratio, T_S increased from 1258°C to 1280°C, and T_D increased from 1377°C to 1409°C. When the charging ratio was less than 10%, (T_D-T_S) reduced from 118°C to 110°C gradually with increasing of ICHB charging ratio. When the charging ratio exceeded 10%, (T_D-T_S) increased from 110°C to 129°C gradually with increasing ICHB charging ratio. Notably, (T_D-T_S) reached the minimum value 110°C when the ICHB charging ratio was 10%.

The increase of T_S could be attributed to the changes of the liquidus temperature for slag system. The phase diagram of CaO–SiO₂–MgO–Al₂O₃–TiO₂–O₂ with an oxygen partial pressure of 1.0×10⁻⁵ calculated by Factsage 7.2 software was shown in Fig. 4. When the charging ratio of ICHB in burden was 0%, the content of CaO was 41.86%, SiO₂ was 27.79%, and the MgO was 10.7%. Besides, the binary basicity of the burden was 1.50. The slag systems of the mixed burden with different ICHB charging ratio were all located in the perovskite (CaTiO₃) zone. However, it should be pointed out that the liquidus temperature of slag system increased gradually with increasing ICHB charging ratio. As a result, the T_S of mixed burden increased obviously. On the other hand, the increasing of T_D was mainly caused by the generation of high smelting point compounds. With increasing ICHB charging ratio, the reduction potential of mixed burden was strengthened, which would promote the reduction of TiO₂. Relevant explanations would be discussed in the following parts.

Fig. 4.

Phase diagram of CaO–SiO₂–MgO–Al₂O₃–TiO₂ with a P_O2 of 1.0×10⁻⁵. (Online version in color.)

The location and thickness of cohesive zone could be described as (T_D-T_S).³²⁾ The effects of ICHB charging ratio on (T_D-T_S) were shown in Fig. 3(b). As increased ICHB charging ratio, (T_D-T_S) shifted down gradually, and its thickness was narrowed firstly and then widened. When the ICHB charging ration was less than 10%, the amplification of T_S was more obvious than that of T_D. It means that the effect of ICHB on the slag of melting-dripping system (T_S) was more significate than that on the generation of Ti(C,N) with a relatively high melting point (T_D). And then T_D-T_S would be narrowed. While the ICHB charging ration was more than 10%, the above changing tendency was the opposite, and the amplification of T_D was more obvious than that of T_S on the contrary. It means that the reduction of TiO₂ was promoted by the increasing reduction potential from more ICHB charging, and the effect of ICHB on the generation of Ti(C,N) with a relatively high melting point (T_D) was more significate than that on the slag of melting-dripping system (T_S). And then T_D-T_S would be widened. The thickness reached the narrowest when the charging ratio was 10%. During the BF process, a narrower thickness and a lower location of cohesive zone were both beneficial to advance the BF operation. Therefore, in the view of the cohesive zone, the suitable charging ratio of ICHB was 10%.

3.2. Permeability

The permeability had a significant effect on the BF smooth operation. Therefore, to evaluate the influence of ICHB charging on the permeability quantitatively, a characteristic value (named S-value) was defined. The Eq. (1) was introduced to calculate the S-value in this study.   

$$S={\int }_{T_S}^{T_D}(P_m-\mathrm{\Delta }{P}_S)\cdot dT$$(1)

Where, P_m was the pressure drop at certain temperature between T_S and T_D, Pa; ΔP_S was the pressure drop at the melting start temperature T_S, Pa.

The effects of ICHB charging ratio on the S-value of the mixed burden were shown in Fig. 5. When the ICHB charging ratio was less than 15%, the S-value decreased from 2254.72 KPa·°C to 1396.227 KPa·°C with the ICHB increasing charging ratio. It was indicated that a certain amount of ICHB charging ratio could improve the permeability of mixed burden. However, the S-value increased from 1396.227 KPa·°C to 1836.112 KPa·°C when the charging ratio was higher than 15%. It was evident that the permeability of mixed burden would worsen gradually when the charging ratio exceeded 15%.

Fig. 5.

Effect of ICHB charging ratio on the permeability of the mixed burden. (Online version in color.)

When the ICHB charging ratio was less than 10%, both P_m and T_D-T_S decreased, which resulted in the decreasing of S-value, as shown in Eq. (1). Furthermore, the improvement of permeability was correlated to the structure of mixed burden. Figure 6 showed the macro structure of mixed burden observed on the inner wall of the graphite crucible after softening-melting experiments. The residual ICHB samples could be observed in the mixed burden, as shown in Figs. 6(b) and 6(c). ICHB acted as the skeleton in the mixed burden and retarded the shrinking of molten iron and slag mixture in the cohesive zone, which improve the permeability of mixed burden. The couple reaction in the mixed burden was described as Fig. 6(d). As a carbon-bearing agglomeration, the coal in ICHB reacted with CO₂ to produce CO (carbon gasification) during the reduction process. The CO produced by carbon gasification would react with iron oxide to form the metallic iron and CO₂. This circle couple reaction could strengthen the reduction of mixed burden and reduce the consumption of reducing gas. Further increasing the ICHB charging ratio to 20%, the reduction of Ti-bearing oxides would be enhanced and the much formation of Ti(C,N) with high melting point would lead to the decreasing mobility of dripping substance. The dripping behavior could not proceed successfully, then the melt in the burden was becoming more and the permeability at cohesive zone in the blast furnace was deteriorated. The detailed generation mechanism of Ti(C,N) during this process would be discussed in the following part of the work.

Fig. 6.

Macro structure of mixed burden after softening–melting experiments. (Online version in color.)

3.3. Dripping Behavior

The effects of ICHB charging ratio on the dripping ratio of mixed burden were shown in Fig. 7. With the increase of ICHB charging ratio, the dripping ratio increased from 63.21% to 66.71% firstly and then decreased to 52.41%. It reached the maximum value of 66.71% when the charging ratio was 10%. There were two main explanations for this variation of dripping ratio: (1) ICHB was a kind of carbon-bearing agglomeration. The mixed burden contacted with ICHB closely during the reduction, which improved the carburization process of metallic iron. It was well known that the carburization of metallic iron would decrease the liquidus temperature of iron, leading to the generation of molten iron and the increasing dripping ratio.³³⁾ (2) As discussed above, the permeability of stock column was advanced by charging some ICHB. And the improvment of permeability would be favorable to increase the dripping ratio. (3) Further increasing ICHB charging ratio amounted to the generation of the compounds with high melting points, which increased the viscosity, deteriorated the fluidity of slag and finally decreased the dripping ratio.

Fig. 7.

Effect of ICHB charging ratio on the dropping ratio of mixed burden. (Online version in color.)

3.4. Thermodynamics of Ti(C,N) Generation

Figure 8 shows the thermodynamics calculation by Factsage 7.2 software of the possible reactions of Ti-bearing phases during reduction progress. As shown in Fig. 8, the reactants were mainly consisted of ilmenite and titanomagnetite. And the resultants mainly included titanium carbide and titanium oxide. According to the principle of minimum standard Gibbs free energy, those resultants could exist in the reduction process. And it could be deduced that the possible generation sequence of titanium bearing phases was from titanium oxide to titanium carbide.

Fig. 8.

Thermodynamic calculation of the possible reactions of Ti-bearing phases during reduction progress. (Online version in color.)

3.5. SEM-EDS Analysis of Residual Iron and Slag

The residual iron and slag taken out from the graphite crucible after softening-melting experiments was analyzed by SEM-EDS methods to investigate the effect mechanism of ICHB on the mixed burden during the softening-melting process. Figure 9 showed the BSE images of the residual slag with different ICHB charging ratio. In Fig. 9(a), the sample was without ICHB charging. According to the EDS results of point 1–3, it could be derived that the bright phase was molten iron phase and the dark phase was slag phase. Also, it could be observed that the iron phase showed a discrete mix of slag phase, which was mainly caused by the relative bad gas permeability of packed bed without ICHB charging. As shown in Fig. 9(b), the iron phase was separated from the slag phase apparently when the ICHB charging ratio was 10%. This was due to the fact that the ICHB acted as the skeleton in the mixed burden and guaranteed the gas permeability. Besides, the sufficient carbon in ICHB could promote iron carburization process, which accelerated the liquid iron appearing and aggregated at a relatively lower temperature. However, ICHB was also a carbon-bearing agglomeration with an excellent reducibility.

Fig. 9.

SME images of residual iron and slag with different ICHB charging ratio. (Online version in color.)

The excessive ICHB charging would aggravate the reduction of Ti-oxides and the generation of carbonitride. Figure 9(c) showed the BSE image of the residual iron and slag when ICHB charging ratio was 20%. It could be seen that some grey phases existed at the interface between the bright phase (iron) and dark phase (slag). According to EDS analysis result of point 6, it could be concluded that the grey phase was mainly titanium carbonitride (Ti(C,N)). To emphasize this phenomenon, the mapping scanning of the residual slag and iron with the ICHB charging ratio of 20% was carried out, as seen in Fig. 10. It was clear that Ti(C,N) mainly generated at the interface between iron phase and slag phase. The Ti(C,N) with a high melting point of 2700°C would increase the viscosity and deteriorate the fluidity of slag, which resulted in the worsening of the dripping behavior and gas permeability of mixed burden.³⁴⁾ Fully considering the softening-melting-dripping behavior, gas permeability, and the precipitation of Ti(C,N), the rational ICHB charging ratio in the mixed burden during the BF process was 10%.

Fig. 10.

BSE image and EDS mapping of mixed burden with 20% ICHB. (Online version in color.)

4. Conclusions

(1) Appropriate ICHB could improve the softening-melting-dripping behavior of the mixed burden. With the increasing of ICHB charging ratio, the location of cohesive zone shifted down gradually and its thickness was narrowed firstly at the ICHB charging ratio of 10% and then been widened, which was beneficial to BF smelting. The dripping ratio of mixed burden increased first and achieved maximum value when the ICHB charging ratio was 10%;

(2) The permeability of mixed burden could be improved by appropriate ICHB. As increased ICHB charging ratio, the S-value decreased first and then increased. It reached the minimum value of 1396.227 KPa·°C when the ICHB charging ratio was 15%.

(3) The excessive ICHB could intensify the reduction of titanium-bearing oxides and aggravate the precipitation of Ti(C,N) at the interface between the iron and slag, which would worsen the dripping behavior and the gas permeability of mixed burden.

(4) Comprehensively considering the softening-melting-dripping behavior and the permeability of mixed burden, the recommended ICHB charging ratio was 10%. Through this way, ICHB technology would realize the incremental and high-efficiently utilization of titanomagnetite ironsand in BF process compared with the traditional BF burdens.

Acknowledgements

The authors are especially grateful to National Natural Science Foundation of China (No. 51904063), Fundamental Research Funds for the Central Universities (No. N2025023, No. N172503016, No. N172502005, No. N172506011) and China Postdoctoral Science Foundation (No. 2018M640259) Xingliao Talent Plan (No. XLYC1902118).

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