Reduction of Iron Ore Pellets, Sinter, and Lump Ore under Simulated Blast Furnace Conditions

A blast furnace (BF) is the dominant process for making iron in the world. The BF is charged with metallurgical coke and iron burden materials including iron ore pellets, sinter, and lump ore. While descending in the BF the charge materials reduce. The iron‐bearing materials should reduce fast and remain in the solid form until as high a temperature as possible to ensure reaction contact with reducing gas and iron oxides. Herein, the reducibility of the iron ore pellet, sinter, and lump ore in the BF shaft are focused on. The experiments are conducted isothermally with a blast furnace simulator (BFS) high‐temperature furnace at four different temperatures (700, 800, 900, and 1000 °C) for 300 min. The experimental atmosphere consists of CO, CO2, H2, H2O, and N2 simulating the conditions in the BF shaft. It is found that lump ore has lowest reduction rate in all test conditions, and at lower temperatures iron ore pellets reduce faster than sinter, and this is reversed at higher temperatures. Furthermore, the reduction rate of sinter and iron ore pellets begins to resemble each other at higher temperatures.


Introduction
A blast furnace (BF) is the most common process for hot metal production in the world. The BF is charged with metallurgical coke and iron-bearing materials separately, which leads to a layered structure. The iron-bearing materials consist of iron ore pellets, lump ore, and sinter. When the charge material descends, it is reduced by CO and H 2 . In the ideal situation, iron-bearing materials reduce fast and remain in the solid form in as high temperatures as possible.
The reduction of iron oxides occurs step wise from hematite to magnetite, magnetite to wüstite, and wüstite to metallic iron. Reduction occurs with CO and H 2 as shown here. [1] From hematite to magnetite (around 500 C) (2) From magnetite to wüstite (between 600 and 900 C) From wüstite to metallic iron (between 900 and -1100 C) FeO þ COðgÞ ! Fe þ CO 2 ðgÞ FeO þ H 2 ðgÞ ! Fe þ H 2 OðgÞ (6) There have been studies about the lowtemperature reduction degradation characteristics of pellets, sinter, and lump ore, [2] as well as the softening, shrinking, and melting reduction behavior of all charge materials or just part of them. [3][4][5][6][7][8][9] Even though some research has considered all iron-bearing materials (sinter, iron ore pellets, and lump ore), the authors were not able to find researches made in temperatures which simulated the upper part of the BF shaft with a temperature range from 700 to 1000 C, with hydrogen and water vapor in the atmosphere and for all three material types. Water vapor is always present in both top and shaft gases. The amount depends on the type of injectant fuel and injection rate. [10] Sources for hydrogen in BF are coke, moisture in blast air, and moisture in the injectant. [11] This research work focuses on the reduction behavior of iron ore pellets, sinter, and lump ore in simulated BF shaft conditions where H 2 and H 2 O are present in typical CO─CO 2 ─N 2 atmospheres.

Experimental Section
Sinter, lump ore, and iron ore pellets were used in experiments. The composition for all three materials is shown in Table 1. The total iron content (Fe tot ) and the oxidation stage of the iron were measured with a titration method and the sulfur content with flame analysis. The contents of other components were measured with X-ray fluorescence (XRF).
The total mass of iron-bearing materials used in one experiment was around 100 g per material (iron ore pellets, sinter, DOI: 10.1002/srin.202000047 A blast furnace (BF) is the dominant process for making iron in the world. The BF is charged with metallurgical coke and iron burden materials including iron ore pellets, sinter, and lump ore. While descending in the BF the charge materials reduce. The iron-bearing materials should reduce fast and remain in the solid form until as high a temperature as possible to ensure reaction contact with reducing gas and iron oxides. Herein, the reducibility of the iron ore pellet, sinter, and lump ore in the BF shaft are focused on. The experiments are conducted isothermally with a blast furnace simulator (BFS) high-temperature furnace at four different temperatures (700, 800, 900, and 1000 C) for 300 min. The experimental atmosphere consists of CO, CO 2 , H 2 , H 2 O, and N 2 simulating the conditions in the BF shaft. It is found that lump ore has lowest reduction rate in all test conditions, and at lower temperatures iron ore pellets reduce faster than sinter, and this is reversed at higher temperatures. Furthermore, the reduction rate of sinter and iron ore pellets begins to resemble each other at higher temperatures. and lump ore). Prior to determination of the original weight of pellets, they were heat treated at 110 C overnight to remove any moisture. Particle size and sample amount differed being 30 pcs per 10-12.5 mm for pellets, 19 pcs per 12.5-16 mm for sinter, and 2 pcs per 25-32 mm for lump ore. These particle sizes were chosen so they correspond to sizes which are mostly used in a BF.
The experiments were made with blast furnace simulator (BFS), [12] as shown in Figure 1. The BFS is a tube furnace with an inner diameter of 95 mm that was used to carry out hightemperature tests either isothermally or nonisothermally. Gases available included N 2 , CO, CO 2 , and H 2 , which were supplied from gas containers, and H 2 O, which was generated in a water pump by evaporating a predetermined flow of water. BFS was used to study the reduction and swelling behavior of iron ore pellets [12][13][14][15][16] and cold-bonded briquettes, [17][18][19] as well as the gasification of metallurgical coke. [11] Gas utilization degree is determined for CO and H 2 gases via Equation (7) and (8).
where etaCO and etaH 2 are gas utilization degrees. The reducing conditions are shown in Bauer-Glaessner diagrams in Figure 2.
Isothermal tests were conducted in four different temperatures (700, 800, 900, and 1000 C). The sample was placed in the furnace at room temperature, and nitrogen was used until the test temperature was reached. Then the gas composition was changed to simulate BF conditions [19] and reduction lasted for 300 min. Cooling was also conducted in nitrogen atmosphere till 200 C to prevent reoxidation. Total gas flow rate was 15 l min À1 at normal temperature and pressure (NTP) conditions. The precise information of used atmospheres is shown in Table 2.

Results and Discussion
The reduction degree was calculated from the experimental results using Equation (9). The reduction degree for sinter, lump ore, and iron ore pellet is presented as where m 1 is mass of the sample before reduction, m 2 is mass after reduction, w 1 is iron (II) oxide content as a percentage by mass in the sample prior to the test obtained from chemical analysis, and w 2 is total iron content as mass% in the sample prior to the test obtained from chemical analysis. Reduction degrees as a function of time in all studied temperatures (700, 800, 900, and 1000 C) for iron ore pellets, sinter, and lump ore are shown in Figure 3. It is shown that the lump ore has the slowest reduction rate at each studied temperature. Pellets were the fastest to reduce in 700 C, but already at 800 C, sinter bypasses it and has the highest rate of reduction. Sinter has the highest reduction degree at 900 and 1000 C.
Field-emission scanning electron microscopy (FESEM) analyses are shown in Table 3, and images for optical microscopy and FESEM are shown in Figure 4. It is shown in Figure 4 that sinter has the most porous structure. This structure enables gas flow to permeate the sinter throughout. This is one explanation why at higher temperatures sinter reduces fastest when compared with pellets and lump ore. In the lowest researched temperature (700 C), pellets reached the highest reduction degree during the whole experiment.
Lump ore is the slowest to reduce in all studied temperatures. This may be because the particle size is largest, and hence it has the lowest reaction surface area. Even when the mass of all experimented iron-bearing materials is around 100 g, the  number of samples for the lump ore is only 2, whereas for pellets it is 30 and for sinter 19. The choice for the test particle size was done such that they correspond to sizes which are mostly used in the BF. Combining this particle size information with structure information (Figure 4), it yields that the lump ore has the smallest reaction surface area with which the gases can have an influence with. One reason why sinter reduces faster in higher temperatures could be the following. When looking at the chemical composition of pellets, sinter, and lump ore (Table 1), it can be seen that the total amount of iron is over 50% in all materials. It was verified with FESEM that this iron comes from hematite and magnetite. This happens because of the characteristics of the sintering and pelletizing processes, as well as, in case of lump ore, wüstite not being present in nature. [20] This research focused only on the reduction behavior, but softening behavior materials in Table 1 show the highest divalent iron content for sinter, indicating that it has the largest proportion of magnetite. Even though magnetite has less oxygen to be removed by reduction, hematite reduces easily. This is because of the different crystal structures of the iron oxides. [20] The magnetite in sinter first reduces slowly, yielding a higher reduction rate in pellets at lower temperatures.
Gas compositions vary as a function of temperature. When looking at the composition from the perspective of reactive gases (carbon monoxide and hydrogen), the reductive gas amounts increase. The carbon monoxide amount increases from 27% to 42% and hydrogen increases from 2.8% to 3.7%. This change in temperature and gas composition simulates the descent of charge material inside the BF. In the upper part of the furnace, the temperature as well as the proportion of reducing gases is lower than that in the lower part of the BF. This probably is also one reason why sinter reduces fastest at the highest temperature. When looking at the Bauer-Glaessner diagrams in Figure 2, it is shown that the composition point shifts further from the phase border to the side of metallic iron when temperature increases.   Figure 3. Reduction degree as a function of time for iron ore pellet, sinter, and lump ore at 700, 800, 900, and 1000 C.    All in all, it is interesting to see that in lower temperatures, iron ore pellets are the ones to reduce fastest compared with sinter and lump ore. This order changes in higher temperatures so that sinter bypasses the iron ore pellets, whereas lump ore stays the last.
From the reduction point of view, the optimal relation between iron ore pellets, sinter, and lump ore in the BF is such that the amount of lump ore should be the least. Therefore, this already used procedure in practice can be confirmed.
As mentioned earlier, in addition to reduction behavior, the softening behavior of iron-bearing materials is important in an industrial BF. This research focused only on the reduction behavior but softening behavior has to be studied in the future to determine the differences in reduction degree and in temperatures between the iron burden materials in the cohesive zone.

Conclusions
This research focused on the conditions in the BF shaft and how different temperatures combined with simulated gas compositions from the BF affect the reduction behavior of iron-bearing materials (iron ore pellets, sinter, and lump ore) prior to reaching the cohesive zone. Isothermal tests were conducted in four different temperatures (700, 800, 900, and 1000 C). It was seen that in lower temperatures and the gas composition tied to those, iron ore pellets are the fastest to reduce, whereas sinter and lump ore follow behind. In higher temperatures, sinter bypasses the iron ore pellets, and lump ore stays the last in all cases. The reasons behind this reduction behavior are the following: 1) the lump ore is slowest to reduce in all temperatures because it has the smallest reaction surface area, and the structure is not porous and 2) compared with iron ore pellets and lump ore, sinter has the highest proportion of magnetite, which does not easily reduce as hematite at low temperatures.