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Review

Hydrogen-Based Direct Reduction of Iron Oxides: A Review on the Influence of Impurities

by
Ali Zakeri
1,*,
Kenneth S. Coley
2 and
Leili Tafaghodi
1
1
Department of Materials Science and Engineering, McMaster University, Hamilton, ON L8S 4L7, Canada
2
Department of Mechanical and Materials Engineering, Western University, London, ON N6A 3K7, Canada
*
Author to whom correspondence should be addressed.
Sustainability 2023, 15(17), 13047; https://doi.org/10.3390/su151713047
Submission received: 14 July 2023 / Revised: 9 August 2023 / Accepted: 22 August 2023 / Published: 30 August 2023
(This article belongs to the Section Sustainable Engineering and Science)
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Abstract

:
Greenhouse gas emissions are the primary root cause of anthropogenic climate change. The heterogeneity of industrial operations and the use of carbonaceous fossil fuels as raw materials makes it challenging to find effective solutions for reducing these emissions. The iron and steel industry is responsible for approximately 35% of all industrial CO2 emissions. This value is equivalent to 7–9% of the global CO2 emissions from all sectors. Using hydrogen (H2) as the alternative reducing agent has the potential for a significant reduction in CO2 emissions. Despite decades of research on H2-based reduction reactions, the reaction kinetics are still not well understood. One of the key influencing parameters on reduction kinetics is the effects of impurities in the iron ore, which needs to be unraveled for a better understanding of the reduction mechanisms. The present review paper aims to explore the single and combined effects of common impurities on the reduction behavior as well as the structural evolution of iron oxides.

1. Introduction

Every year, over 1 billion tons of iron, equivalent to 70% of global iron production, are produced via blast furnace operations, which involve the reduction of iron ores with carbon as a reducing agent [1]. Subsequently, a near eutectic Fe-C alloy is obtained and further converted to steel in the basic oxygen furnace (BOF). In a blast furnace, coke acts as a fuel while also providing the reducing agent, CO. Consequently, this operation results in significant carbon dioxide (CO2) emissions. The iron and steelmaking industry accounts for 7–9% of direct CO2 emissions each year, making it a major contributor to global warming [2,3]. In addition, according to future projection reports, a massive increase in these emissions would be the trend in the coming decades if no disruptive technologies are implemented [4]. Hence, it is imperative to implement breakthrough innovations in the iron and steel industry.
The energy consumption in iron and steel production has been cut by 60% over the past few decades, causing current production to operate close to its thermodynamic limits of 20 GJ/t crude steel. The industry’s potential for increased energy efficiency is limited to 15–20%; thus, merely increasing energy efficiency will not result in substantial reductions in CO2 emissions [5]. Moreover, despite advancements in blast furnace technology, the energy consumption and CO2 emissions associated with this process have nearly reached their limits; thus, the process cannot be relied upon for optimal CO2 sequestration [6]. Another source of emissions in this process stems from the necessity for lumpy input materials, which require a prior sintering or pelletizing step. The pelletizing process has certain benefits in terms of energy conservation and emission reduction when compared to the sintering process, even though the cost of producing pellets is slightly higher [7]. Moreover, enhancing the burden reactivity in the BF has been explored as a means to decrease the necessary amount of reducing agent in this process. By utilizing high reactivity coke and carbon–iron ore composites, the contact between carbonaceous material and iron ore is enhanced, leading to a decrease in the temperature of the thermal reserve zone. Consequently, this improves CO gas utilization efficiency, resulting in the decreased consumption of the reducing agents [8]. Due to the large environmental impacts of BF technology, alternative ironmaking technologies have been explored; two widely used processes are direct reduction (DR) and smelting reduction (SR).
The DR processes can be classified as either gas-based or coal-based, depending on the reducing agent. In typical gas-based DR, reactions occur in a shaft furnace, wherein reductants derived from natural gas are utilized. The main components of these reducing agents are H2 and CO; however, the precise composition could vary depending on the gas sources. All reduction steps in the DR route occur at temperatures less than the melting points of the solid charge; therefore, the reduction process occurs in a solid state. The MIDREX and HYL-ENRGIRON are two well-established industrial processes using vertical shaft furnaces for the direct reduction of iron pellets [9]. In addition to the standard shaft furnace technology, fluidized bed technology can also be used for direct reduction [10]. Direct-reduced iron (DRI) can be treated in the electric arc furnace (EAF) by itself or in combination with scrap to produce crude steel. That being said, using large amounts of scrap in this process can lead to a constant accumulation of tramp elements (such as Cu and Sn), which is detrimental to high-grade steel production. In addition, the scrap supply is not sufficient to fulfill the global demand; therefore, fresh DRI will always be needed [11]. An overview of the major processes involved in iron and steelmaking is shown in Figure 1.
While employing the typical gas-based DR-EAF route reaches a 50% reduction in CO2 emissions compared to the BF-BOF route [1], significant CO2 emissions continue to exist due to the presence of carbon in reducing gases. Currently, several engineering solutions are being investigated to reach lower levels of emissions. The first strategy is to increase scrap-EAF production. Since no oxide reduction is needed in this route, the CO2 emissions are significantly lower. However, scrap availability and quality are critical challenges with this approach. Second, iron ore electrolysis is considered an alternative to the direct production of iron [12]. The major limitations of industrial production using this technology are related to the high melting point of iron oxide and the aggressive liquid salt environment at high temperatures. Third, the replacement of fossil fuels with hydrogen, which is produced from sustainable sources, is currently the most promising option for carbon-neutral iron production. It should be noted that using H2 is not a viable option at present mostly due to its limited availability.
Recently, a few review papers have been published on hydrogen-based ironmaking, with a focus on process challenges and adaptability [6,13,14,15], H2 production technologies [16], reduction behavior and kinetics [17,18,19], and technical and geopolitical evaluations [20,21,22,23]. However, there exists no review paper with particular emphasis on the influence of impurities present in iron ore. The present manuscript provides a comprehensive overview of the fundamental principles underlying the utilization of hydrogen in the direct reduction of iron ore, thereby establishing a foundation for investigating the role of impurity oxides. In the following sections, the advantages and challenges of using H2 as the reductant are examined, and an overview of the most important projects launched in this area is introduced. Next, a critical review of the kinetics of iron ore reduction, with particular emphasis on the impact of various gangue materials on the reduction behavior, is presented.

2. Hydrogen as a Reducing Agent in the Direct Reduction Process

Recently, research efforts aiming to replace carbon-based reductants with hydrogen have received a great deal of attention, as shown in Figure 2. This is because the iron and steelmaking industry has a significant decarbonization capacity by transitioning from coal to hydrogen. According to the simulation results for DR processes, CO2 emissions could be reduced by up to 91% when using green hydrogen compared to natural gas [24]. However, the type of hydrogen (green, blue, and grey) which is defined by the production route has a diverse range of carbon intensities. Accordingly, sustainably produced hydrogen via a low-carbon route is necessary to achieve a high level of decarbonization [13].
A comparison between hydrogen and carbon as reducing agents requires considering the availability of H2, which, unlike carbon, cannot be obtained from the geosphere in the form of coal or from the biosphere in the form of biomass and charcoal [22]. At present, approximately 96% of the entire H2 production comes from the use of fossil fuels (grey H2), with natural gas reforming being the main process. Consequently, the majority of the existing processes for H2 production are not carbon-free; however, blue and green H2 supplies show promising potential for deep-decarbonization objectives. Blue H2 is the product of steam-methane reforming (SMR) combined with CO2 sequestration (via carbon capture and storage; CCS). Green H2 is generated via water electrolysis powered by renewable electricity [23]. It is also worth mentioning that high production costs and challenges with transporting H2 are the current major obstacles to its widespread use in the iron and steel industry. These concerns must be taken into consideration when evaluating the viability of commercializing hydrogen-based ironmaking.
Employing hydrogen as an energy source and a reducing agent can be achieved through process modification or developing new techniques. In a blast furnace, the substitution of carbonaceous materials with H2 would be limited. In fact, H2 could possibly substitute a significant portion of the pulverized coal injected into the tuyeres. However, coke is essential in the metallurgical zone to provide metal and slag outflux, as well as in the cohesive zone to support adequate gas permeability. In this case, the typical estimate for the mitigation of CO2 emissions is 20% [25]. In addition, H2 injection into a blast furnace has shown limited benefits due to the endothermic nature of the reactions with H2 addition. In other words, more coke is needed to produce adequate energy since increased H2 contents will result in higher energy consumption [6,26]. Nevertheless, BF is a mature technology, and it will be challenging to replace its large capacity in the next few decades. Replacing BF with alternative methods becomes crucial in countries with a high demand for steel, like China, which had a production share of 51.1% in 2019 [27].
It is theoretically possible to envision replacing all of the CO with H2 using the DR shaft furnace technology. Full metallization is predicted based on the results of mathematical modeling performed for a shaft furnace operating with 100% H2 [28]. The existing widely used DR processes successfully employ H2-rich gas as the reductant; 55–58% and 84–86% H2 content in the MIDREX and HYL-ENRGIRON, respectively. Therefore, there is a huge development prospect in introducing pure H2 to the shaft furnace. A schematic of the direct reduction of iron ore using green H2 is shown in Figure 3. The following section reviews some of the current projects that aim to test the practicality of using pure H2 through pilot scale trials.
Aside from the emission-free reduction of iron oxides, using H2 offers several advantages. Owing to its smaller molecular size, H2 exhibits better permeability and diffusivity, hence resulting in higher reduction rates. This effect is noticeable in the case of relatively dense raw materials, like lumps and pellets, as opposed to sinter which has high porosity. In addition to the advantages mentioned earlier regarding H2 reduction, more efficient heat exchange and lower activation energy are additional factors that contribute to its faster kinetics in comparison to CO reduction [29].

3. Research and Development Programs

In recent years, several institutions have announced carbon mitigation strategies and published technology roadmaps focusing on the industry’s decarbonization pathways. In 2020, the International Energy Agency (IEA) identified four major technology areas to promote more sustainable steelmaking, through which their successful implementation can contribute around 39% of CO2 emissions savings. These processes include hydrogen-DR, bioenergy, direct electrification, and carbon capture, use, and storage (CCUS) [30]. Employing these technologies would be beneficial for reducing CO2; however, factors such as the long lifetime of iron and steel plants, price volatility, and lack of strategic financial incentives make it complicated to adopt these processes efficiently.

3.1. ULCOS Project

In 2004, a European project called Ultra Low CO2 Steelmaking (ULCOS) was initiated to investigate breakthrough technologies for the significant reduction of CO2 emissions. The end objective was to cut CO2 emissions per ton of crude steel by 50% [31]. In the hydrogen subproject, assessments were made on massive hydrogen production, operating DR shaft furnaces with 100% H2, and utilizing carbon-free DRI in the EAF [32]. They concluded that using carbon-free DRI in this route would not cause any issues, provided that a satisfactory foaming slag is achieved by blowing carbon fines and oxygen [9,33]. That being said, utilizing DRI with 1.5–3.5% C (not sourced from coal) presents the upsides of higher combustion efficiency and avoiding sulfur, ash, or volatiles, which are harmful to the steel quality [34].

3.2. HYBRIT Project

Hydrogen Breakthrough Ironmaking Technology (HYBRIT) is another project aiming at eliminating CO2 emissions from the steelmaking process. This project was launched in 2016, and the overall goal was to utilize fossil-free feedstock and energy in all stages of production, from the mine to the final product. The ironmaking route was established based on the DR shaft furnace technology, with H2 being the main reducing agent, which is generated via water electrolysis [35]. After the completion of pre-feasibility studies, the hydrogen-based DR process was demonstrated at a pilot scale in 2021 [13].

3.3. COURSE50 Project

The COURSE50 project in Japan is attempting to operate a blast furnace with a high concentration of H2. It was launched in 2008, and the ultimate target is a 30% reduction in CO2 emissions by 2030. The first approach to decrease CO2 emissions is associated with replacing about 10% of the direct carbon-based reduction with H2 reduction by injecting a gaseous reductant containing H2, which was produced from reforming the coke oven gas (COG) [36]. The additional 20% reduction comes from developing technologies to capture, separate, and recover CO2 from BF top gas [37].

4. The Kinetics of Iron Oxide Reduction with H2

Given the increasing attention to the DR processes, studying different aspects of the gas–solid reduction reactions is of significant practical importance. Throughout the last few decades, a considerable body of research has been carried out on this subject. As a result, a plethora of information on the kinetics and structural changes associated with reduction has been provided by these studies. That being said, the mechanisms that govern kinetics, especially with respect to the gangue content of the iron ore, have yet to be fully understood. Also, it has not been possible to develop a generalized model of reduction that would be applicable to all conditions [11,38].
Generally, the reduction of iron oxides to metallic iron does not occur directly; instead, a two-step (Fe2O3-Fe3O4-Fe) reduction at temperatures lower than 570 °C or a three-step (Fe2O3-Fe3O4-FeO-Fe) reduction at temperatures above 570 °C occurs simultaneously and sequentially. Wüstite (FeO) is a defected phase that exists over a wide range of stoichiometry and is only stable at T > 570 °C. It is chemically described as Fe(1−x)O, in which the iron lattice’s vacancies are represented by (1 − x) [39]. That being said, for simplicity, it is referred to as FeO hereafter.
A complex interplay of various physical (transport), chemical (phase transformations), and mechanical (stresses) phenomena define the underlying mechanisms controlling these reactions, which determine the overall reduction kinetics. The following steps take place throughout the reduction of a porous iron ore pellet [19,40]: mass transfer of the reducing gases (H2 and/or CO) from the bulk gas onto the pellet surface, diffusion to the reaction interface through pores, oxygen removal via a chemical reaction, reduction of iron oxides, formation of gaseous products (H2O and/or CO2), formation of an iron layer via nucleation and growth, and diffusion of gaseous products back to the external surface of the pellet; followed by mass transfer to the gas stream. Based on the experimental conditions, the rate-controlling step can vary; therefore, the reaction mechanism recorded under a specific set of conditions may not be applicable to another set of conditions. It is also possible to have more than one rate-controlling step due to the comparable impacts of multiple steps on the overall rate [41].
Numerous studies have examined the impact of various process parameters on the kinetics of the gas-based DR of iron oxides with H2, CO, CH4, and their mixtures. While ongoing research in this area progresses, integrating statistical analyses that consider kinetic parameters can be advantageous for optimizing the process response. This approach aims to maximize the reduction degree and metallization, offering potential benefits in selecting appropriate reduction conditions for iron ores with diverse compositions [42]. Besides process parameters, the raw material properties such as morphology, grain size, internal microstructure, and porosity have a significant effect on the reduction behavior. Thermogravimetric analysis and off-gas analysis techniques have been used for kinetics studies. In addition, characterizing the microstructural evolution and product morphologies is an efficient method to explain the relationships between mechanisms involved in the movement of the reduction front and reduction kinetics [43]. The previous findings diverge largely due to discrepancies in materials and experimental conditions. Table 1 provides a summary of relevant articles that examined reduction behavior, along with the main findings of those studies.

4.1. The Properties of the Starting Material

The reducibility of iron ores is highly influenced by their composition and physical characteristics. In general, hematite shows better reducibility compared to magnetite. This is because, typically, magnetite has a nonporous structure that favors the development of dense iron shells, which limits the reduction progress via solid-state diffusion instead of faster gaseous diffusion [49]. Moreover, particle size is one of the influential characteristics of iron ores, which can affect the rate-limiting step as well as the overall reduction rate. The concentration gradient of the reducing gas in porous pellets is influenced by intergranular pore diffusion and the reactivity of the individual grains. The reduction rate increases in smaller pellets, since the diffusion of the reducing gas to the unreacted center of the pellets is facilitated through a shorter diffusion path [50]. This is particularly important during the initial stages of reduction, in which gas diffusion acts as the rate-limiting step. Additionally, smaller pellets benefit from the larger reaction sites during the later stages of reduction when the process is governed by both gaseous diffusion and interfacial chemical reaction mechanisms. The effect of gas diffusion through the pores can become marginal if the diffusion occurs fast enough, for instance, due to the presence of micro-cracks, and/or when the reduction temperature is low [51].
The impact of porosity on the reduction rate has been thoroughly investigated. In principle, gas permeability is facilitated in materials with higher porosity. This in turn leads to a higher reduction rate. It should be noted that the reduced pellets could acquire additional pores with different structures that can affect the reduction behavior. It is pertinent to add that additional free volume could be introduced into the reduced pellets owing to local cracking and delamination, which results in enhanced kinetics by enabling faster diffusion [11]. Pore sintering with time may result in a larger mean pore diameter but with a smaller pore surface area. This can affect the rate positively or negatively.
After the formation of the product phase during the heterogenous gas–solid reactions, the reducing gas must be transported across this phase to reach the reacting interface. Therefore, the characteristics of this phase, which are usually different from those of the starting material, should be taken into consideration in kinetic studies. The recrystallization and sintering of the product iron at temperatures above 650 °C have been found to slow down the reduction rate during the final stages [49,52]. This is due to the formation of a dense iron shell that traps the unreacted oxides. Consequently, further reaction occurs via the solid-state diffusion of oxygen ions to the outer surface [47]. Furthermore, studies have indicated that the presence of H2 + CO atmospheres leads to the formation of cementite between reduced iron grains, effectively impeding subsequent sintering and densification processes [53].

4.2. Experimental Conditions

The study of iron oxide reduction kinetics with either solid or gaseous reductants has been generally carried out in the temperature range of 400 to 1200 °C. Most of the investigations have been conducted under isothermal conditions. Furthermore, the majority of researchers have performed their experiments at a critical gas flow rate, above which mass transport of the gas to the pellet has no noticeable effect on the reduction rate. At extremely low flow rates, gas starvation occurs, wherein the reaction rate becomes directly proportional to the flow rate. As the flow rate increases to higher levels, though not yet reaching a sufficiently high range, mass transport to the pellet becomes the dominant controlling factor. By accurately adjusting the flow rate, it is possible to eliminate the influence of mass transport to the pellet as a significant controlling step. At this point, the reaction rate is mainly determined by the internal phenomena occurring within the pellet [54,55].
Considering the temperature dependence of potential rate-limiting steps, higher temperatures are expected to lead to faster kinetics. Moreover, in the case of H2-based reduction, increasing temperature enhances the reduction thermodynamics; therefore, both kinetics and thermodynamics benefit from higher temperatures [19]. Furthermore, mathematical modeling investigations have substantiated the positive correlation between temperature and reduction behavior [56,57].
The composition of the reducing gas is another influential parameter on the reduction rate. It is clear that CO and H2 have different properties; thus, it is expected to observe distinctive reduction behaviors due to the differences in the thermodynamics and kinetics of the reduction reactions. Increasing the H2 content in the gas mixture resulted in the complete reduction in shorter periods [58,59]. Moreover, the beneficial effect of increased H2 has been shown at a temperature in which both CO and H2 exhibit identical reduction potential [60]. This is mainly due to the faster diffusion behavior of H2 and implies the significance of diffusion rate (kinetics) over reduction driving force (thermodynamics).

4.3. The Modeling of Reduction Kinetics

Numerous models have been developed to explore the reduction process of iron oxide, both at the single pellet and macroscopic reactor scales. By utilizing these models, not only can complex reduction systems be effectively addressed, but also, the expenses associated with experimental studies can be minimized. The reduction reactions in pellets have been represented by various mathematical models in the literature, which can be classified into three groups: the one-interface shrinking core model, the three-interfaces shrinking core model, and the homogeneous model. The formulation and fitting of diverse mathematical models to experimental data have yielded two significant outcomes: firstly, identifying the most suitable conceptual model that describes the physics of reduction, and secondly, determining the rate-controlling regime during the reduction process [61].

5. The Role of Impurities on the Reduction of Iron Oxides

Iron ores contain a variety of impurities, including oxides of Si, Ca, Al, Mg, and Mn. The composition of iron ore itself and the beneficiation method determine the type and quantity of these remaining oxides. In addition, high-temperature processes such as sintering and pelletizing may result in the formation of dilute solid solutions of gangue material in iron oxide [62]. Figure 4 depicts the estimated iron ore reserves in the major iron-ore-producing countries for the year 2022. The data indicates that Australia possesses the largest reserves in terms of both unprocessed (crude) ore and iron content, with Brazil following, though with a slightly lower average iron grade.
Some studies have employed reagent-grade materials to either exclude the effects of impurities or to incorporate a specific amount of impurities. Additionally, reduction experiments have been performed in both direct (iron oxide to metallic iron), and stepwise (e.g., hematite to magnetite) fashion. The reported impurity contents of the materials examined in previous studies are summarized in Figure 5.
The BF-BOF route is less sensitive to the iron ore quality since most of the gangue materials are removed with the BF slag. On the other hand, the DR process occurs in the solid state; therefore, gangue materials stay in the DRI and result in a high slag burden in the EAF [72]. Generally, high-grade pellets with a total impurity content of less than 5% are required for the DR-EAF process. Historical trends show a change in the composition of DR pellets during the last few decades. For instance, a significant reduction in silica content over 40 years, from 1.43% to 0.8% and from 4.5% to 1%, is reported for the DR-grade pellets produced by LKAB and Rio Tinto, respectively. In this context, a practical matter to consider is weighing the benefits of reaching a lower impurity content against iron losses during the separation processes. To decrease the amount of gangue materials in an iron ore concentrate, various separation techniques such as magnetic separation, gravimetric separation, and flotation can be utilized individually or in combination [73].

5.1. The Effect of Individual Impurities on the Reduction of Iron Ore Using Hydrogen as a Reductant

  • SiO2
An increased reduction rate at a temperature range of 450–1000 °C was reported for sinters with 5–20% added SiO2. However, at temperatures above 800 °C, the final stages of reduction experienced lower rates due to the formation of fayalite [74]. The impact of SiO2 addition on the FeO–Fe transformation was examined by Bahgat et al. [75], using the 0.5 and 2% silica-doped wüstite compacts. The results of the reduction experiments at 900–1050 °C with H2, CO, and 50 H2–50 CO showed decreased reduction rates with increases in silica content. This was associated with the decrease in the porosity of the pellet because of the formation of iron silicates during the reduction. Further, Kim et al. [76] showed that the formation of a dense Fe2SiO4 phase significantly hindered the CO-reduction of FeO compacts. The final reduction degree at 1000 °C was only 18% with the addition of 30 wt% SiO2 at 1000 °C. On the other hand, a considerably higher reduction degree, i.e., 78%, was observed for the same samples when reduced with pure H2. It was proposed that the reduction mechanism under CO gas included interfacial chemical reaction (at first) and mixed control of the chemical reaction and gaseous mass transport. The reduction mechanism with H2 was similar, except for the solid-state diffusion of oxygen through the fayalite phase, which enhanced the reduction extent compared to CO-reduced samples. In addition, the SEM observations demonstrated that iron nuclei were exclusively observed on the surface of the Fe2SiO4 phase when using CO. In contrast, the H2-reduced samples revealed the presence of iron nuclei not only on the surface but also within the internal structure of the Fe2SiO4 phase. This implies a higher H2 permeability for the fayalite phase compared to CO. Despite exhibiting higher reducibility when using H2 compared to CO, the addition of SiO2 in the range of 0–30 wt% resulted in a decrease in the reduction rate of FeO compacts under both gases. A continuous decrease in compact porosity was observed with the addition of SiO2, consistent with the findings reported by Bahgat et al. [75].
The influence of silica addition in the range of 0.1–5 wt% on the reduction behavior of wüstite with H2 at 1000–1345 °C is reported in [77]. The only reduction rate enhancement due to silica addition was found to occur with small amounts (<0.5%) at temperatures below 1100 °C. The lower reduction rates at higher temperatures were related to the formation of a dense iron layer, which prevented the contact between FeO and reducing gas, causing reduction stagnation. In addition, the formation of a melt is promoted at higher temperatures as the solid solubility of silica in wüstite decreases (Figure 6a). It was suggested that the presence of a molten phase during the reduction could result in the peeling-off of the dense iron layer and the development of a fresh iron surface (Figure 6b).
  • Al2O3
Suzuki et al. investigated the influence of 5 wt% alumina added to green and indurated hematite compacts. Alumina was added in two distinct polymorphs, namely α-Al2O3 and γ-Al2O3 [78,79]. Prior to reduction, green compacts exhibited higher porosity compared to indurated compacts, which had negligible porosity. The addition of impurities did not significantly impact the porosity of green compacts. The SEM analysis of reduced samples revealed notable differences in the reduction behavior. Due to the higher porosity of green compacts, reduction was proposed to take place simultaneously both internally and on the outer surfaces. Conversely, indurated compacts became porous as reduction progressed. It was proposed that γ-Al2O3 reinforced the formation of micropores, thereby restraining the reduction. This effect was the result of to two mechanisms; (i) hindering the growth of the pore size, and (ii) the formation of surface cracks due to the differences in shrinkage between the dissolved γ-Al2O3 and compact matrix, by which the number of micropores increased at the near-surface sites. Due to the conflicting reports regarding the effect of pore diameter on the reducibility of iron ores, it is not possible to conclusively determine whether the formation of micropores enhances the reduction rate. While some studies have suggested that smaller pores are beneficial [80,81], others argue that larger pores are more advantageous [82,83]. That being said, oftentimes the formation of surface cracks is found to promote the reduction rate by facilitating the diffusion of reducing gas [84].
The reduction behavior of wüstite plates containing 0.5–5% Al2O3 was investigated at a temperature range of 670–930 °C [85]. The results showed that the addition of 0.5% alumina promoted the development of a dense iron layer on the FeO surface, which decreased the reduction rate. Further increasing the alumina content to 1% resulted in the formation of hercynite (FeO·Al2O3) precipitates, causing an increased reduction rate. As shown in Figure 7, the formation of hercynite is feasible at a wide temperature range, and the reduction proceeded preferentially along the hercynite precipitates. It was suggested that the disturbed atomic arrangement caused via generated strains at the FeO–hercynite interface facilitated the diffusion, and hence promoted the reduction rate of FeO. Moreover, samples with 5% alumina exhibited a slightly enhanced reduction rate at temperatures above 890 °C, whereas a slower reduction rate was recorded at temperatures up to 800 °C.
  • CaO
The beneficial impact of lime (CaO) addition on the reducibility of pure iron oxide briquettes with H2 was reported by Seth and Ross [96]. The specimens were prepared by mixing chemically pure reagents followed by briquetting and sintering at 1100, 1200, or 1300 °C. Comparing the time required for 80% reduction showed that the specimens containing lime were reduced faster than pure iron oxide. This effect was more pronounced in the case of specimens with a 2% lime addition, compared to 5 and 10% additions. Some studies have attributed the increased reducibility to the presence of calcium ferrite [97]; however, this conclusion was not consistent with the observations of Seth and Ross [96]. They found that, although increasing the sintering temperature promotes calcium ferrite formation, it did not cause increased reducibility in all the reduction temperatures. Moreover, the examination of the initial porosity of the briquettes revealed that the lime-containing samples that were sintered at temperatures above 1100 °C exhibited higher porosity. Since the porosity developed during reduction is reported to have a more significant effect on promoting the reduction behavior than the initial porosity [49,98], the change in volume during reduction was measured to investigate the influence of porosity. The volume change measurements of the partially reduced specimens showed a similar change in the volume of specimens with and without lime. Therefore, the cause of increased reducibility could not be associated with porosity increase during the reaction. Considering the microstructures of the reduced briquettes, it has been suggested that wüstite is unstable in the presence of CaO and decomposes according to reaction (1) [99]. With this in mind, it is clear that the reduction of wüstite differs in the case of pure iron oxide and iron oxide with lime additions. The former case is controlled by the H2 diffusion through the briquette or the Fe diffusion through FeO, whereas in the latter case, the decomposition of FeO (reaction 2) is the rate-controlling step, and since it is a chemical reaction requiring no H2 diffusion, it occurs faster.
2CaO + 3FeO → Fe + 2CaO·Fe2O3
2CaO·Fe2O3 + 3H2 → 2CaO + 2Fe + 3H2O
In a follow-up study by Strangway and Ross [100], the reducibility of hematite concentrates with various calcium carbonate contents was examined. As shown in Figure 8, the hematite concentrate’s reducibility was greatly enhanced by the addition of calcium carbonate. Experiments at a temperature range of 600–1000 °C showed that, at low temperatures, reducibility increased with increasing CaCO3 content; however, higher CaCO3 content did not improve the reducibility at high temperatures. The optimal increase in reducibility occurred in the case of samples that exhibited the highest increase in porosity during reduction. This impact was prominent for the samples with low CaCO3 content being reduced at high temperatures. Overall, the benefits of CaCO3 addition were attributed to increasing the initial porosity, the dissociation of wüstite, and promoting porosity development during reduction. These benefits were true for both cases of calcium oxide and calcium carbonate additions.
The effect of CaO addition on the wüstite reduction has also been reported in [101]. The experiments with pure FeO at 700 °C showed the formation of a dense iron layer covering wüstite grains and hindering the gas diffusion. Meanwhile, increasing the reduction temperature above 700 °C led to the formation of porous iron. The specimens with added CaO developed iron with fine pores at the wüstite surface, through which the reducing gas could diffuse at any temperature range of 600 to 1000 °C. It was found that the beneficial effect of CaO on the reduction was lessened with increasing temperature, because of the decline in the number of pores. However, further temperature increases above the α-γ iron transformation caused improvement in the reduction rate due to the slower sintering of γ-iron than α-iron. The CaO addition had no effect on the apparent activation energy, indicating that the effect of CaO on the wüstite reduction was not catalytic. Overall, it was proposed that the acceleration of wüstite reduction by CaO was due to the porous structure of the reduced iron near the reaction interface.
In another study [62], the reduction of calciowustites, (Fe,Ca)1−yO, was investigated using CO/CO2 and H2/H2O gas mixtures. Overall, CaO addition was found to extend the range of gas compositions in both systems that resulted in the formation of a porous iron morphology, and hence achieved fast reduction. In the reduction of pure FeO with H2/H2O, merely 0.5% H2O was necessary to establish a dense iron layer. On the contrary, the formation of porous iron was observed on the calciowustites reduced in a H2/H2O atmosphere containing more than 20% H2O. A similar promotion of porous iron growth was seen in CO/CO2 gas mixtures. Although the exact mechanism by which the CaO promoted porous iron layer growth was not clear, it was obvious that CaO facilitated the breakdown of the iron layer. This is suggested to be enabled by the presence of Ca ions at the Fe/FeO interface, through which (i) the Fe/FeO interfacial energy was lowered, (ii) the adsorption of other calcium ions at the Fe/FeO interface was promoted, or (iii) the interfacial strain energy or mismatch was affected by altering the oxide’s lattice parameter. Moreover, observations of the product morphology revealed that at high reduction potentials, a porous iron layer was formed with a dispersion of fine CaO particles. On the other hand, a two-phase structure composed of porous iron and dicalcium ferrite (2CaO·Fe2O3) was produced under high oxygen potentials. Figure 9 shows the effect of CaO or CaCO3 on the reduction behavior of synthetic iron oxides by comparing the time required to reach 80% reduction for samples with various CaO or CaCO3 content that are reduced at different temperatures.
  • MgO
Additions of MgO-bearing materials have been made with the objective of improving both the strength and reducibility of the pellets. It has been reported that the firing of iron oxides containing MgO causes the diffusion of MgO in the Fe2O3 lattice, thereby forming magnesioferrite (MgO·Fe2O3) with a spinel structure [102]. The presence of this phase has been shown to improve the strength of pellets and extend the temperature range at which softening occurs [103]. The gaseous reduction of magnesioferrite results in the formation of magnesiowüstite (MgO·FeO) at the final stages. Harkki investigated the reduction kinetics of this phase with H2 at 940 °C and found that the addition of 1 mol% MgO had the maximum effect on promoting the reduction of wüstite. Increasing the MgO content to 5 mol% showed a negligible effect, which was attributed to the differences in iron morphology formed during the reduction [104].
The reduction behavior of FeO-2%MgO-annealed compacts was investigated at 800–1100 °C [105]. It was shown that the addition of MgO decreased the reduction rate compared to pure wüstite samples. This behavior was linked to the formation of the magnesio-wüstite (MgO·FeO) phase which decreased the porosity to half of the pure FeO samples. These results are in contrast with the findings of Pan et al. [106], which showed a positive effect on the reduction rate of Fe3O4 sinters at 800 °C with the addition of MgO up to 15%. According to the X-ray diffraction patterns obtained in this study, it was suggested that Mg2+ could diffuse into the Fe3O4 lattice and form a MgFe2O4 solid solution by replacing a portion of Fe2+, causing the structure to become porous. The higher porosity enhanced the diffusion of the reducing gas and increased the contact areas, which promoted the reduction process. A notable distinction between the two studies lies in the starting material utilized prior to reduction. The incorporation of MgO into magnetite appears to facilitate the formation of magnesio spinels, thereby promoting a porous structure. Conversely, the addition of MgO to wüstite compacts yields a dense structure due to the formation of magnesio-wüstite.
The retardation in reduction rate due to the development of the magnesio-wüstite phase was also reported by Zheng et al. [107]. In this study, the impact of combining MgO addition with prior partial oxidation was explored as an efficient approach to preventing the sticking of magnetite-based iron ore. This type of iron ore typically exhibits a dense structure that can result in challenging issues such as a strong sticking tendency and low reducibility in the fluidized bed process. A significant issue in fluidized bed reduction is the loss of fluidization, where reduced iron particles stick together. This creates challenges in process control. A hindering effect was observed during the final stage of reduction (FeO to Fe) upon the introduction of 0.5 wt% MgO into the charged iron ore. It is worth mentioning that, in this study, MgO was introduced in a different manner compared to being added to the initial iron ore, thus restricting its influence solely to the surface of the iron ore particles. Nonetheless, it impacted the reduction behavior, which was consistent with the findings of [105].
  • MnO2
Given that a high content of manganese oxide (MnO2) is only present in specific iron ore deposits, its effects on the reducibility of iron oxides are not widely studied. The reduction behavior of Fe2O3- 2–6 wt% MnO2 compacts at 800–1000 °C was examined to determine the effect of MnO2 [108]. The annealed compacts with added MnO2 showed the formation of the manganese ferrite (MnFe2O4) phase. The results revealed that the presence of MnFe2O4 impeded the rate at the beginning of the reduction, and this phase was only partially reduced to iron manganese oxide (FeO0.899MnO0.101) at later stages. Unlike in pure Fe2O3 samples, the reaction was incomplete in the MnO2-containing compacts even at the final reduction stages. The degree of reduction varied depending on the MnO2 addition, with more MnO2 resulting in a lower reduction rate.

5.2. The Effect of the Presence of Multiple Impurities on the Reduction of Iron Ore Using Hydrogen as a Reductant

Typically, the impurities present in iron ores are oxides of less noble elements with low-oxide-formation free energies at the common reduction conditions; therefore, they tend to remain in their original state [109]. Investigations of hematite and magnetite ore powders containing gangue oxides showed that these oxides could not be reduced even at a harsh reduction condition (excessive H2 at 1400 °C), and they remained in the iron matrix as spherical inclusions [66]. Moreover, the reduction degree obtained from experimental results was higher than the theoretical values. In fact, the results of the thermodynamic analysis showed that FeO remains present in the iron ores at 900 °C and 1400 °C, regardless of whether there is an excess of H2 or not. Nevertheless, experimental results revealed that both hematite and magnetite can be completely reduced to Fe by employing a continuous injection of H2. This was attributed to the effect of impurities on promoting the reduction kinetics by accelerating the mass transfer due to the formation of liquid slag phases at high temperatures.
The reduction degradation index (RDI), which is an important property of iron ore sinters, is expressed via three indices: the percentage by mass of the +6.3 mm, −3.15 mm, and −0.5 mm size fractions of the sample [110]. Essentially, lower RDI values are favorable for minimizing fuel consumption at the sinter plant and maximizing the production rate of BF operation. It has been reported that increasing Al2O3 content in the sinters worsens the RDI [111]. Industrial BF reports show that the addition of a small amount of alumina (0.1%) into a sinter containing 10–10.5% CaO increased the RDI [112].
Investigations by Iguchi and Inouye shed light on the effect of different foreign oxides (CaO, Al2O3, and MgO) added to different starting pure iron oxides (hematite, magnetite, and wüstite) [113]. The kinetic analysis of the reduction reaction shows that the mixed control mechanism is applicable to all synthetic iron oxides with added CaO, and to hematite and magnetite with added Al2O3. Three rate-limiting steps are taken into account in the mixed control equation: (i) mass transfer in the gas phase, (ii) gas diffusion in the reduced layer, and (iii) interfacial chemical reaction. The reduction rate of wüstite containing alumina, all the synthetic iron oxides with MgO, and the synthetic iron oxides with no added impurity were controlled via oxygen diffusion through the iron shell. In another study by the same authors, the influence of foreign oxide addition on the pore radius distributions of the reduced specimens was examined [114]. It was found that the added oxides had a distinctive impact on the reduced iron when the gangue oxides were dissolved in the iron oxide and developed a complex oxide through precipitating in wüstite during the reduction. In other words, when the foreign oxide is added, the wüstite grains undergo a nontopochemical reduction while pellets are reduced topochemically, increasing the pore volume of the reduced specimens. Interestingly, the pore radius of the reduced iron varies with the type of the added oxide: it decreases with Al2O3 and increases with CaO. Table 2 summarizes the impact of various foreign oxides on the distribution of the pore radius in iron obtained via the reduction of Fe2O3 with H2 at 800–1000 °C.
The effect of CaO and SiO2 content on the reduction behavior of sintered hematite and magnetite pellets is reported in [69]. Some of the results are shown in Figure 10. It was found that adding CaO lowered the reduction rate of both hematite and magnetite pellets, while SiO2 addition did not exhibit much effect. Further investigations on the pellets containing CaO showed that hematite remained in the pellet core until about 90% of oxygen was removed. This effect of CaO on the reduction behavior was attributed to the formation of molten calcium ferrite, which in turn blocks the pores in the hematite. In addition, the estimated effective gas diffusivity decreased when high contents of CaO and SiO2 were added to the pellet.
The experimental method used for heating the sample to initiate the reduction process is also relevant to the effect of impurities; however, very few studies have investigated this subject. The effect of several impurities on the formation and morphology of iron nuclei during the reduction behavior of wüstite was investigated with respect to two initiation procedures of reduction [117]. Since the growth of iron nuclei could occur very fast during the reduction, the experiments were conducted at a relatively low temperature of 670 °C and for a duration of 64 s. The first procedure involved heating the sample to the reduction temperature in a vacuum followed by the introduction of hydrogen. The results showed that, at high flow rates of hydrogen, CaO and MgO (to a lesser degree) promoted the local formation of porous Fe nuclei; however, samples with SiO2 and Al2O3 additions did not exert any influence on the formation of porous Fe nuclei. In the second procedure, the sample was inserted into the reducing gas stream, which was already at the reduction temperature. When the second procedure was used, the surface areas of all samples were covered with porous Fe, and the effects of impurities were not clear.
The majority of the studies covered in this review were conducted on the macro- and/or micro-scale, which might not elucidate the actual rate-limiting processes occurring at the near-atomic scale. A study by Kim et al. [11] employed the atom probe tomography (APT) technique to investigate the distribution of impurities in as-received and reduced hematite pellets. The findings indicated that the initial iron ore had a uniform distribution, on the nano-scale, of the most common impurity elements, such as Na, Mg, Al, Ti, and V. The APT data from the reduced samples exhibited nano-sized oxide islands embedded within the reduced iron matrix. These islands consisted of species originating from the gangue materials, and since they could not be reduced, they could either keep their initial uniform distribution as oxide nanoparticles or be expelled from the reduction front into the remaining oxide phase. The former scenario is thermodynamically unfavorable because of the high interfacial energy of the metal oxide interface. However, these experimental observations suggest that the driving force for the reduction of iron was sufficient to overcome that particular barrier and thereby strand the gangue oxides as nano-particles. The Na appears to be an exceptional case since it was segregated at the reduction interface. Further research is required to understand how the reduction kinetics are affected by the buildup of gangue elements.

5.3. The Effect of Impurities on the Reduction of Iron Ore Using Carbon Monoxide as a Reductant

Reduction experiments of iron ores with different contents of SiO2 and Al2O3 using CO have been reported in [118]. In silica-rich ores (4–8 wt%), the amount and formation rate of 2FeO·SiO2 had a significant impact on the overall reduction rate. At 1100 °C, slower reduction was measured in the case of silica-rich iron ores in which the SiO2 particles were distributed inside or at the edge of the hematite phase. On the other hand, silica-rich iron ores with large SiO2 particles that were distributed separately from hematite, developed a smaller amount of 2FeO·SiO2 phase and reduced faster. At 1200 °C, the silica-lean iron ores (0.5–0.7 wt%) showed complete reduction; however, the reduction of silica-rich ores stopped at 20% due to the formation of molten 2FeO·SiO2. The formation rate of the 2FeO·SiO2 phase was found to be influenced by both the quantity and distribution of silica. The formation of this phase was the most rapid when the silica content was high and evenly distributed throughout the ore. In these particular conditions, the microstructural analysis of the reduced samples revealed a dark layer surrounding the FeO phase, which is believed to be molten during the reduction at 1200 °C. The formation of this molten layer is due to the relatively low eutectic temperature of 2FeO·SiO2 and FeO, which is 1175 °C.
The influence of Al2O3 on the degradation and reduction behavior of magnetite briquettes under CO/CO2 atmosphere was studied by Paananen et al. [119]. The results showed that the presence of alumina had an enhancing effect on the reduction rate. This was attributed to the cracks seen in the structure of alumina-containing specimens. Alumina addition made the magnetite briquettes prone to cracking and swelling (Figure 11). This was suggested to be the consequence of the presence of Al3+ cations in magnetite. Briefly, Al3+ cations diffused away from the growing FeO due to low solubility (about 0.5 wt% at 900 °C) and accumulated in the magnetite. As reduction proceeded, the Al content in the magnetite increased to a point where the composition of the phase approached that of hercynite (FeO·Al2O3). Eventually, the volumetric difference between wüstite and hercynite led to the cracking and fracturing of the structure. Furthermore, considering the results of the reports using hydrogen, it seems that the presence of hercynite promotes the reduction regardless of the choice of the reductant.
Typically, a volumetric swelling of about 20% is acceptable during the direct reduction of the pellets. Higher volume expansion may lead to the disintegration of pellets, which in turn causes low gas permeability. Wang and Sohn [71] examined the way in which swelling and iron whisker formation were affected by the presence of CaO (2–5%) and SiO2 (4–8%). The reduction was carried out in CO atmosphere at 900 °C. Increasing CaO lowered swelling and long whisker growth, while increasing SiO2 changed the whisker nucleation sites from a scattered to a clustered arrangement. The SiO2 content did not affect the rate of reduction; however, increasing CaO content resulted in higher reduction rates at low SiO2 content. The results of the swelling behavior and the effect of silica addition on the reduction rate are not consistent with that of Wang et al. [120]. They found that SiO2 contents up to 7% were unfavorable for the reduction with 90CO/10CO2 gas mixture. Moreover, the addition of 3% CaO significantly increased the reduction swelling index (RSI) by more than 190%. This phenomenon was related to the increased size of the internal pores of pellets with the addition of CaO, as well as the development of slim and long whiskers of metallic iron, both of which resulted in a loose microstructure, hence, increasing the RSI of pellets. It is pertinent to mention that, besides different experimental apparatuses used in the two studies, the former study used iron ore concentrate whereas the latter study used pure reagents. Furthermore, the presence of fayalite, which was observed only in the latter study, had an adverse impact on the reducibility of the samples. This outcome could be attributed to the variations in the starting materials, with the use of pure Fe2O3 potentially facilitating the formation of fayalite.
The effects of adding MgO to synthetic Fe2O3 on reduction behavior with CO at 1173–1473 K were studied by El-Geassy [121]. The findings indicate that the initial reduction rate of hematite compacts (up to 25% extent) was minimally impacted by the presence of MgO. Further, MgO showed different effects on the intermediate reduction stages, in the range of 25–85%, depending on its concentration. While 0.5% MgO slowed down the reduction because of magnesio-ferrite formation, 1% MgO accelerated the reduction because of higher porosity and lattice disorder in the iron oxide. Higher concentrations of MgO (>1%) reduced this effect. At the final stages, MgO hindered the reduction of Fe2O3 because of magnesio-wüstite formation, which is a hard-to-reduce phase.

6. Considerations for Future Work

The findings of the previous investigations on the kinetics and reaction mechanisms of the direct reduction of iron ore are of significant impact in both research and industrial contexts. A few key factors that should be considered when utilizing the results of the previous research are listed below. Taking them into account facilitates consolidating the information gathered from different resources.
  • The experimental setup for reduction experiments may introduce certain inherent challenges including measurement inaccuracies caused by instability in the mass measurements, the incubation time required for changing the furnace atmosphere from inert to reducing, and temperature stabilization.
  • The reduction of iron oxides occurs through a series of successive reactions. However, most of the previous investigations examined the sample micro-structure after almost complete reduction. Therefore, the impact of impurities on each step is unclear.
  • The differences in the characteristics of the starting material cause variations in the reduction behavior. These differences include the mineralogy of iron oxides, the utilization of either industrial or synthetic samples, and variations in the pelletizing and sintering procedures leading to diverse shapes and porosity fractions.

7. Concluding Remarks

A switch from carbon to hydrogen is a substantial transition towards lower CO2 emissions in ironmaking. However, achieving full utilization of H2 in ironmaking processes poses significant challenges, as iron oxide reduction is influenced by various parameters and mechanisms. The process complexity is dependent on the kinetic and thermodynamic limitations, the reduction parameters, and the input material. Despite numerous experimental studies on the reduction behavior of iron oxides, the effects of iron ore impurities have not been comprehensively researched. This work provided an analysis of the effects of typical impurities on the reduction kinetics and structural changes of iron oxides. The type and amount of impurities in iron oxides have diverse impacts on the reduction behavior, which is strongly dependent on the experimental conditions. As a result, a consolidated picture of the impurity effects on the reduction kinetics requires further investigations. That being said, the information gathered in this review serves as a technical guide for researchers and companies to realize the influence of ore composition and reducing conditions on the direct reduction process.
In summary, the addition of SiO2 may lead to the formation of iron silicate (fayalite) during reduction. The presence of fayalite has a detrimental effect on the reduction rate, particularly during the later stages of reduction (wüstite to iron). This negative impact is more pronounced when the reductant is CO as opposed to H2.
Most of the reviewed studies reported a beneficial effect of CaO on the reduction rate. Some mechanisms by which CaO accelerates reduction include increasing the initial porosity, dissociation of wüstite, and promoting a porous structure of the reduced iron layer.
The positive impact of Al2O3 addition on the reduction rate can be attributed to the formation of hercynite precipitates. It is proposed that these precipitates facilitate gas diffusion by disrupting the atomic arrangement within the iron oxide structure.
The effect of MgO on the reduction of iron oxides can be either beneficial or deleterious, depending on the phases that form during reduction. The incorporation of MgO into magnetite appears to facilitate the formation of magnesio spinels, thereby promoting a porous structure. However, when MgO is added to wüstite compacts, a dense structure is formed due to the formation of magnesio-wüstite.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Iron- and steelmaking processes.
Figure 1. Iron- and steelmaking processes.
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Figure 2. Number of publications per year on the hydrogen-based reduction of iron oxides (data collected from WoS database).
Figure 2. Number of publications per year on the hydrogen-based reduction of iron oxides (data collected from WoS database).
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Figure 3. H2-driven ironmaking process.
Figure 3. H2-driven ironmaking process.
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Figure 4. Estimated iron ore reserves in the main iron-ore-producing countries in 2022 (data collected from U.S. Geological Survey [63]).
Figure 4. Estimated iron ore reserves in the main iron-ore-producing countries in 2022 (data collected from U.S. Geological Survey [63]).
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Figure 5. Typical gangue content present in the iron ores (data gathered from [11,64,65,66,67,68,69,70,71]).
Figure 5. Typical gangue content present in the iron ores (data gathered from [11,64,65,66,67,68,69,70,71]).
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Figure 6. (a) A section of an FeO-SiO2 phase diagram, and (b) cross-sections of partially reduced samples at 1300 °C (white: reduced iron, light gray: FeO, dark gray: fayalite, black: void); scale bar is 0.3 mm, reprinted with permission from ref. [77].
Figure 6. (a) A section of an FeO-SiO2 phase diagram, and (b) cross-sections of partially reduced samples at 1300 °C (white: reduced iron, light gray: FeO, dark gray: fayalite, black: void); scale bar is 0.3 mm, reprinted with permission from ref. [77].
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Figure 7. (a) Optimized FeO-Al2O3 phase diagram in equilibrium with iron [86,87,88,89,90,91,92,93,94], reprinted with permission from [95]. (b) Partially reduced FeO-5% Al2O3 sample at 890 °C (white: Fe, gray: FeO, H: FeO·Al2O3), reprinted with permission from ref. [85].
Figure 7. (a) Optimized FeO-Al2O3 phase diagram in equilibrium with iron [86,87,88,89,90,91,92,93,94], reprinted with permission from [95]. (b) Partially reduced FeO-5% Al2O3 sample at 890 °C (white: Fe, gray: FeO, H: FeO·Al2O3), reprinted with permission from ref. [85].
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Figure 8. Effect of calcium carbonate on the time required for 80% reduction at various temperatures; reprinted with permission from ref. [100].
Figure 8. Effect of calcium carbonate on the time required for 80% reduction at various temperatures; reprinted with permission from ref. [100].
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Figure 9. Time required to reach 80% reduction degree for CaO versus CaCO3 content [76,96,100].
Figure 9. Time required to reach 80% reduction degree for CaO versus CaCO3 content [76,96,100].
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Figure 10. The effect of CaO and SiO2 additions on the reduction rate at 900 °C; (a) hematite; and (b) magnetite pellets; reprinted with permission from Ref. [69].
Figure 10. The effect of CaO and SiO2 additions on the reduction rate at 900 °C; (a) hematite; and (b) magnetite pellets; reprinted with permission from Ref. [69].
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Figure 11. (a–c) top and (d) side photographs of porous magnetite briquettes with different alumina contents after reduction at 950 °C with 90CO/10CO2 gas mixture; reprinted with permission from Ref. [119].
Figure 11. (a–c) top and (d) side photographs of porous magnetite briquettes with different alumina contents after reduction at 950 °C with 90CO/10CO2 gas mixture; reprinted with permission from Ref. [119].
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Table 1. An overview of reported investigations on the reduction behavior of iron oxides.
Table 1. An overview of reported investigations on the reduction behavior of iron oxides.
ReferenceReducing Agent(s)Temperature (°C)Raw MaterialsMain Findings
[38]H2600–900Commercial hematite pelletsSignificant increase in the reduction rate was observed with increasing temperature.
Different incubation times were observed depending on the unsteady conditions of gas composition and/or temperature before the reduction reaction begins.
Heat transfer in the pellet was found to be the rate-controlling step.
[44]H2800–850Industrial hematite pellets, Pure hematite pelletsHigh density of pure hematite pellets (porosity < 20%) limited the mass transfer of reducing gas; thereby changing the reduction mechanism.
A layer-by-layer reduction mechanism was noted for the dense pure hematite pellets. whereas this mechanism was not followed in the reduction of industrial pellets.
The presence of layers with different phases in industrial pellets (observed by SEM) suggests that the reaction’s progress was controlled by both the internal diffusion of H2 and chemical reactions within the layers.
[45]H2-CO700–950Industrial hematite pelletsExamining the partially reduced samples confirmed the simultaneous progress of reduction reactions and cementite formation.
The reaction temperature and gas atmosphere were the major contributing factors to cementite formation.
The activity of carbon determines the carburizing process, and it decreases as the CO2 concentration increases. However, the higher CO2 levels enhanced the formation of cementite, indicating that the cementite formation kinetics were affected by CO2 as well.
The highest level of cementite (24–30%) was achieved when there was an absence of CO2 in the reducing gas at 850 °C. The introduction of 1% CO2 led to a substantial decrease in the Fe3C content (8%). However, subsequent increase in the CO2 concentration to 3 and 10% exhibited an increase in the quantity of cementite to 15 and 22%, respectively. It appears that having CO2 in the gas mix suppresses C deposition and hence promotes Fe3C formation.
At lower reaction temperatures, noticeable carbon deposition was observed on the pellets, which hindered the gas diffusion into the pellet.
[29]H2800–1100Commercial iron ore fines (<100 µm)Higher temperatures led to an increased reduction rate at the initial and final stages of reduction.
As the reduction proceeded, the apparent activation energy (Ea) increased. Consequently, it was concluded that gaseous diffusion and interfacial chemical reaction were the rate-controlling steps in the initial stages, whereas solid diffusion and interfacial chemical reaction controlled the later stages. The conclusions regarding the rate-controlling steps were based on the relationship between Ea and the reduction mechanism, as defined in [46].
[47]H2900–1100Magnetite single crystalsIrrespective of the temperature, the highest reduction rates were observed at the early stages, followed by a decrease until the end of reduction.
The low reduction rates at 900 and 950 °C caused longer reaction times, which resulted in the formation of a dense metallic layer around wüstite grains. This in turn hindered full reduction.
[48]H2-CO
Pure H2
Pure CO		750–900Analytical grade hematiteIncreasing the H2 content of the reducing gas resulted in much shorter reduction times.
The mean pore diameter of the specimens increased as the reduction proceeded. This was more pronounced in pure and CO-rich gas mixtures. That being said, producing a loose and permeable structure did not lead to an increase in the effective diffusivity when compared to H2-rich gas mixtures. This was related to the higher molecular weight of the CO-rich gas mixtures.
Table 2. Effect of foreign oxides on the distribution of pore radius in reduced iron.
Table 2. Effect of foreign oxides on the distribution of pore radius in reduced iron.
Foreign OxideSolubility in Iron OxidesEffect
Al2O3 Highly soluble in hematite and magnetite but almost insoluble in wüstite [114].Leads to the formation of micro-pores, and as the quantity of foreign oxide increases, the pore radius decreases [114].
CaOLimited solubility in wüstite
Limited solubility in hematite [114] and slightly greater solubility in magnetite (~1:3) was reported in [115].		Results in the formation of macro-pores, and as the quantity of foreign oxide increases, the pore radius increases [114].
MgOForms continuous solid solution with wüstite and does not exhibit limited solubility [114].
Insignificant solubility in hematite was reported in [116].		Minimal influence on the pore radius [114].
SiO2Almost insoluble in any iron oxides [114].No effect [114].
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Zakeri, A.; Coley, K.S.; Tafaghodi, L. Hydrogen-Based Direct Reduction of Iron Oxides: A Review on the Influence of Impurities. Sustainability 2023, 15, 13047. https://doi.org/10.3390/su151713047

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Zakeri A, Coley KS, Tafaghodi L. Hydrogen-Based Direct Reduction of Iron Oxides: A Review on the Influence of Impurities. Sustainability. 2023; 15(17):13047. https://doi.org/10.3390/su151713047

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Zakeri, Ali, Kenneth S. Coley, and Leili Tafaghodi. 2023. "Hydrogen-Based Direct Reduction of Iron Oxides: A Review on the Influence of Impurities" Sustainability 15, no. 17: 13047. https://doi.org/10.3390/su151713047

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