Experimental Study on Oxygen-Enriched Sintering of Vanadium-Titanium Magnetite

Ph.D Student in College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China Lecturer in School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China Associate Professor in School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China Student in School of Metallurgy and Materials Engineering, Chongqing University of Science and Technology, Chongqing 401331, China College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China


Introduction
Vanadium-titanium magnetite is rich in vanadium and titanium.
ese two elements are important strategic resources mainly for steel, nonferrous metals, and chemical raw material production. e vanadium-titanium magnetite resources in China are mostly distributed in Panzhihua, Sichuan, and Chengde, Hebei. e reserves in Panzhihua account for approximately 618 million tons, which is approximately 95% of the total reserve in China and more than 35% of global reserves. Compared with the sintering of ordinary iron ore, vanadium-titanium magnetite has lower iron, FeO, and SiO 2 contents but higher TiO 2 , MgO, and Al 2 O 3 contents. ese characteristics are not favorable for sintering, resulting in low production, low grade, poor strength, low yield, and high solid energy consumption of the sintered ore [1,2]. Numerous studies have shown that few bonding-phase minerals are generated during the sintering of vanadium-titanium magnetite, but abundant perovskites are found in the sinter. ese phenomena lead to the deterioration of the sintered ore strength and decrease in yield. Moreover, the low-temperature reduction pulverability rate of the vanadium-titanium sintered ore RDI −3.15 is significantly higher than that of ordinary sintered ore [3][4][5][6][7].
Oxygen-enriched sintering refers to the addition of a certain amount of oxygen in the intake of air during sintering to facilitate the su cient burning of the sintering fuel and sintering process of the reaction. Using the ignition method of oxygen enrichment can improve the oxygen content in the ue gas and reduce the ignition temperature of the fuel. is process results in the full utilization of the solid fuel on the surface, which is bene cial in accelerating the combustion and heat transfer of the surface material solid in the ignition area. Consequently, the sintering productivity is improved, sintering is intensi ed, and fuel consumption is reduced [8][9][10]. Under high oxygen content, most of the liquid-phase reaction and mineral formation process may be completed at temperatures below 1400°C. e concentration of Fe 2+ in the liquid phase is low, and the concentration of CO in the exhaust gas is also reduced. When alkalinity >1.2-1.5, a certain amount of calcium ferrite is formed in the solid phase [8]. In addition, the liquid-phase components and crystallization of various minerals are also assimilated in the air according to certain laws. e drawbacks encountered during the sintering of vanadium-titanium magnetite are the lack of strength, low yield, more return mines, and high rate of low-temperature reduction pulverability. ese limitations increase the cost of production energy consumption [10]. Oxygen-enriched sintering can e ectively improve vertical sintering speed, sintering process, and sintered ore yield.
In this study, the e ects of di erent oxygen enrichment times, oxygen enrichment loads, ignition temperatures, and other factors on the industrial production of the vanadiumtitanium sintered ore are simulated using sinter pot experiments. Changes in performances, such as sintered ore drum strength, antiwear index, reductivity, low-temperature reduction pulverability index, and soft melting performance, are also studied and analyzed. is study provides a basis for using vanadium-titanium magnetite as a raw material in industrial productions and to obtain high-quality sintered ore with high yield.

Experimental Materials.
All sintered raw materials are obtained from a steel enterprise in southwest China. e main components of iron ore powder, sintered ux, and sintering return mines are listed in Table 1. e chemical components of coke are shown in Table 2.
e full size distribution of the mix ore powder and coke powder is provided, as shown in Figure 1. e sintering proportion scheme during the experiment is shown in Table 3. e basicity of the sintered ore (R CaO/SiO 2 ) is set to 2.7 because the sintering characteristics of oxygen-enriched sintering are mainly explored in this study.

Experimental Methods and Devices.
e e ects of different oxygen enrichment conditions on the sintering performance of vanadium-titanium magnetite are investigated by changing the factors, such as oxygen enrichment time, oxygen enrichment load, and sintering ignition temperature. e in uences of these factors on the technical indices and sintered ore quality during sintering are explored. e time      e oxygen enrichment load is based on the three kinds of loads of the sintering trolley, namely, 1000 (low load), 2000 (medium load), and 3000 m 3 /h (high load). e converted equivalent sinter pot area oxygen enrichment loads are 0.97, 1.93, and 2.90 m 3 /h, respectively. According to the fixed six-bellow oxygen enrichment system, the converted time of the sintering process in the sinter pot is approximately 9.6 min.
In the experiment for sintering oxygen-enriched ignition, increased oxygen enrichment will increase the theoretical sintering ignition temperature. In addition, the ignition fuel used in the laboratory sinter pot experiment is liquefied petroleum gas with a quite different calorific value and composition from those of blast furnace gas used by the sintering trolley. erefore, the influence of ignition temperature on the sintering process is also studied experimentally.
e specific experimental scheme of oxygen-enriched sintering of vanadium-titanium magnetite is shown in Table 4.
After the raw materials are prepared in accordance with the proportion scheme in Table 3, the materials are first artificially mixed evenly on an iron plate. en, the materials are put into a cylinder mixer (diameter × length of 600 mm × 800 mm) and mixed for a mixed granulation time of 5 min. An appropriate amount of water is sprayed during mixing to wet the mixture, and the final moisture content of the mixture is controlled at 8.0% ± 0.5%. en, the mixed raw materials are added into the sinter pot of Ø300 mm × 900 mm. e thickness of the control material layer is approximately 600 mm. Approximately 3 kg of the sintered ore is paved at the bottom of the sinter pot as the bottom material before that. e particle size of the sintered ore is 10∼16 mm. e excess oxygen during sintering is provided by an additional oxygen distribution device ( Figure 2). is device is equipped with a stainless steel cone-shaped oxygen supply cover and a bottom hole for oxygen. e holes are evenly distributed at an angle of 60°, and the oxygen supply channel is naturally formed. After charging, the exhaust fan is controlled, in which the ignition negative pressure is 8.0 kPa, ignition time is 2 min, and the sinter negative pressure is 14 kPa. After sintering is completed, the technical indices, mechanical performance, and metallurgical performance of the sintered ore are determined. e phase composition of the sintered ore is analyzed by a Zeiss Axioskop 40 polarizing microscope and LEICA Q500 image analyzer. e sintered ore drum strength is detected using GB 8209-87. e proportion of the amount in excess of 6.3 mm of the drum sample total mass is considered as the drum strength, and the proportion of the amount that is less than 0.5 mm of the drum sample total mass is the antiwear index. Reductivity is tested by experiments according to GB/T 13241-91. A weight loss method is used to determine the iron ore reductivity. e average of the double specimen is considered, and the absolute value of the range is not more than 5%. e low-temperature reduction pulverability (RDI >3. 15 ) is determined according to the GB/T13242-91 method. After the experiment is completed, the ratio of the mass of the particles with the size greater than 3.15 mm grade and the total mass of the drum sample after reduction is considered as the evaluation standard.
No uniform test for determining the softening property is available.
is property can be determined using the following parameters: sample size, 10-12.5 mm; stock column height, 40 mm; upper coke, 15 g; lower coke, 20 g; load, 1.0 kg/cm 2 ; and gas flow rate, 15 L/min. e reduction gas composition is CO/N 2 � 30/70. e heating rate when it is below 900°C is 10°C/min. However, when it is at 900°C, it is placed under constant temperature for 60 min. When it is above 900°C, the rate is 5°C/min. When it is below 400°C, N 2 is injected to protect the test sample. When it is at 400°C, reduction gas is provided. e temperature at which the rate of shrinkage of the stock column is 10% is considered as the start softening temperature (T a ), while that at which the rate of shrinkage is 40% is the softening end temperature (T s ). e softening temperature range is ΔT sa � T s − T a , and T m is the temperature at which the slag iron starts to drip. Table 5 shows the main technical indices of the sintering of vanadium-titanium magnetite at different oxygen enrichment times. Compared with the absence of oxygen enrichment, the sintered ore drum strength, antiwear properties, and yield improved after oxygen-enriched sintering.

Effects of Different Oxygen Enrichment Times on the Sintering Performance of Vanadium-Titanium Magnetite.
is phenomenon is due to the fact that oxygenenriched sintering increases the oxidizing atmosphere during sintering. Consequently, fuel combustion is more sufficient, resulting in the formation of the sintering liquid phase. us, favorable conditions for the production of more high-quality calcium ferrite bonding phases are provided, resulting in the improvement of aforementioned indices. In addition, oxygen-enriched sintering ensures full combustion of fuel to reduce the local reduction, combustion layer temperature, and thickness. is process also reduces the increase in the amount of liquid phases, which is conducive to agglomeration and mineralization of sintered ore powder, and can also improve the strength and yield of the sintered ore.
In addition, the vertical sintering rate and sintering utilization coefficient are improved after oxygen-enriched sintering, which are all higher than 22.57 mm/min and 1.58 t/(m 2 ·h). ese characteristics are caused by the more sufficient fuel combustion during oxygen-enriched sintering, resulting in increased combustion layer temperature.
is process accelerates the solid-phase reaction of the sintered material layer and the vertical sintering speed, such that the sintering utilization coefficient is improved.
e main components and FeO content of the sintered ore after different sintering oxygen enrichment times are shown in Table 6 and Figure 3. e main components of the sintered ore are slightly changed after oxygen enrichment time, but the FeO content in the finished sintered ore is reduced from 6.50% to 5.95%. ese characteristics are caused by the more adequate fuel combustion during oxygen-enriched sintering, which decreases the CO produced by incomplete combustion of coke.
us, oxygenenriched sintering reduces the local reduction reaction, resulting in lowered FeO content. In addition, iron ore powder used during sintering is mainly magnetite (Fe 3 O 4 ). After oxygen-enriched sintering, the oxidizing atmosphere increases during sintering, and some Fe 3 O 4 is oxidized into Fe 2 O 3 , further reducing the FeO in the sintered ore. Figure 4 shows the test results of the sintered ore reductivity and reduction pulverability for di erent sintering oxygen enrichment times. e reductivity of the sintered ore increases from 78.10% to 80.08%, while the reduction   pulverability decreases from 65.07% to 62.86% with the increase in sintering oxygen enrichment time. ese results are due to the decreased FeO content in the sintered ore caused by the prolonged sintering oxygen enrichment time. is process will reduce the phase production of perovskite, kirschsteinite, and other minerals with poor reductivity but increase hematite, calcium ferrite, and other minerals with good reductivity, thereby improving the reduction of the sintered ore [11][12][13]. However, the hematite in the sintered ore undergoes crystal transformation during reduction (Fe 2 O 3 -Fe 3 O 4 -FeO), leading to great internal stress. is process causes the pulverization of the sintered ore by mechanical force, resulting in poor reduction pulverability of the sintered ore [14,15]. During sintering, the time of oxygen enrichment is increased, and the hematite produced by reoxidation in the sintered ore increases. us, a great amount of hematite undergoes lattice transformation and pulverability during reduction, which reduces the RDI >3. 15 . Table 7 shows the test results of the softening performance of the sintered ore for di erent oxygen enrichment times. No remarkable changes are observed in T a and T s of the sintered ore with prolonged oxygen enrichment time. However, the softening interval decreases from 51°C to 39°C, and T m decreases by 33°C.

E ects of Di erent Oxygen Enrichment Loads on the Sintering Performance of Vanadium-Titanium Magnetite.
e main technical indices of the vanadium-titanium magnetite sintering process for di erent oxygen enrichment loads are shown in Table 8. With the increase in oxygen enrichment load, the drum strength of the sintered ore increases from 65.33% to 68.00%, antiwear index decreases from 5.27% to 4.80%, and yield increases from 82.29% to 83.39%. ese phenomena are due to the enhanced oxidizing atmosphere during sintering caused by the increased oxygen enrichment load.
is process results in more su cient combustion of coke in the sintering mixture, increased temperature of the combustion layer, and increased thickness. Consequently, the sintering liquid phase is formed and increased, and this phase is conducive to the agglomeration and mineralization of the sintered ore, thereby improving the strength and yield of the sintered ore. e vertical sintering rate and utilization coe cient increase rst and then decrease with increased oxygen enrichment load. is phenomenon is due to the more complete combustion process caused by oxygen-enriched sintering.
us, the solid-phase reaction of the sintered material layer and the vertical sintering speed are accelerated, thereby increasing the sintering utilization coe cient. However, the sintering temperature remarkably increases because of the very high oxygen content. is process results in the early formation of  the sintering liquid phase, leading to the increase in the volume of the liquid phase. Consequently, the sintered mixture gap becomes jammed, and this condition reduces the vertical sintering speed and sintering utilization coefficient. Table 9 shows the main components of the sintered ore with different oxygen enrichment loads. e main component of the sintered ore does not remarkably change with the increase in sintering oxygen enrichment load. However, the FeO content in the sintered ore decreases from 6.50% to 6.14%. e increase in oxygen enrichment load results in the sufficient combustion of coke in the sintered mixture to reduce the local reduction reaction and the amount of FeO produced. Increasing the oxygen enrichment load enhances the oxidizing atmosphere during sintering, and this process can oxidize part of the magnetite into hematite and reduce the FeO content in the sintered ore. Figure 5 shows the phase diagram of the finished sintered ore with different oxygen enrichment loads. e main components of the sintered ore are hematite, magnetite, calcium ferrite, calcium silicate, and a small amount of perovskite. Increasing the oxygen-enriched content of the sintered ore results in increased hematite content but decreased magnetite content in the sintered ore, but magnetite content decreases. After oxygen enrichment of the sintered ore, the sintering atmosphere shows a certain degree of oxidability. At the same time, the magnetite content in the sintered ore after the oxygen enrichment is decreased. Figure 5 shows the test results of RDI >3.15 of the sintered ore with different oxygen enrichment loads. RDI >3.15 decreases from 65.07% to 62.85% with the increase in sintering oxygen enrichment load (Figure 3). e greater the oxygen enrichment load is, the more obvious the decrease in RDI >3.15 is. Figure 6 shows that the sintering oxygen enrichment load increases, and the hematite content in the sintered ore increases. During reduction, a large amount of hematite changes into the crystal form, resulting in internal stress, and this process deteriorates the sintered ore RDI >3. 15 . Table 10 shows the test results of the softening performance of the sintered ore with different oxygen enrichment loads. T a and T s of the sintered ore show no obvious changes as the oxygen enrichment load increases. e softening interval is reduced by 9°C, and T m has declined.

Effects of Different Ignition Temperatures on the Sintering
Performance of Vanadium-Titanium Magnetite. Table 11 shows the technical indices in the sinter pot experiment with different ignition temperatures. e sintering drum strength and yield increase with the sintering ignition temperature, but the vertical sintering speed and sintering utilization coefficient gradually decrease. ese results may have been caused by the insufficient supply of heat at low ignition temperature. Consequently, the sintered mixture cannot form a good melt such that the sintered raw material cannot be well formed into blocks and results in low strength. At high ignition temperature, the sintering material layer temperature is high and can form more liquid phases to better bond the ore powder. us, the strength of the sintered ore is increased. However, the increase in the amount of liquid phase is not conducive to the gas permeability of the sintered material layer, and the sintering rate is lowered, resulting in a decrease in the sintering utilization coefficient. Figure 7 shows the test results of the reductivity and RDI >3.15 of the finished sintered ore at different ignition temperatures. No evident changes are found in the reduction of the sintered ore with the increase in ignition temperature, but RDI >3.15 increases first and then decreases. When the ignition temperature is 1150°C, the sintering RDI >3.15 reaches the maximum value of 66.29%. is phenomenon may have been caused by the insufficient heat at low ignition temperature, resulting in the nonformation of a good melt from the sintered mixture. e sintered ore has remarkable assimilation degree and low strength. An appropriate increase in ignition temperature is conducive to the formation of the sintered liquid phase, increasing the sintered ore strength. Too high sintering temperature overburns the sintered ore, resulting in a high amount of the vitreous material. Cracking easily occurs during low-temperature  Journal of Chemistry reduction, leading to the deterioration of sintered ore RDI >3. 15 . Table 12 shows the soft melting performance of the sintered ore at di erent ignition temperatures. e sintered ore T a , softening interval, and T m do not change signi cantly. Only T s slightly increases, indicating that the increase in ignition temperature has little e ect on sintered ore soft melting performance.

Conclusion
(1) e technical indices, such as yield, vertical sintering speed, utilization coe cient, and drum strength, gradually increase with the increase in oxygen enrichment time during sintering. However, the FeO content in the sintered ore is gradually reduced. e increase in oxygen enrichment time is bene cial in improving the reducibility of the sintered ore, but this process will deteriorate the RDI >3.15 of the sintered ore. Prolonging the oxygen enrichment time has no signi cant e ect on the soft melting performance of the sintered ore. (2) Increasing the oxygen enrichment load during sintering results in increased drum strength from 68.33% to 68.00%, decreased antiwear index from 5.27% to 4.80%, and increased yield from 82.29% to  83.39%. However, the vertical sintering speed and utilization coe cient are reduced. (3) Increasing the oxygen enrichment load during sintering will increase the hematite content but reduce the magnetite content in the sintered ore. is process results in the gradual decrease of the RDI >3. 15 of the sintered ore. However, no signi cant e ect on soft melting performance of the sintered ore is found. (4) Increasing the ignition temperature is bene cial in improving the strength and yield of the sintered ore.    However, too high ignition temperature will reduce the vertical sintering speed and utilization coefficient. e RDI >3.15 of the sintered ore first increases and then decreases when the ignition temperature is increased. No evident changes are observed in the reductivity and soft melting performance.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that they have no conflicts of interest.