Influence of Direct Reduction Condition of Hematite Pellets with H2/CO on the Oxidation Behaviour of DRI in Air

Direct reduced iron (DRI) is the product of some commercial direct reduction (DR) of iron ore on base of natural gas. DRI tends to oxidize in air generally above 300 °C and then follows spontaneous combustion. To control the oxidation mechanism, several investigators have used different iron samples and methods. This paper gives the results of experimental work carried out for determination of DRI oxidation. The behaviour of DRI oxidation in air after isothermal reduction of hematite pellets with different size, temperature and H₂ / CO mixture is investigated.     

Reduction of Sn-Bearing Iron Concentrate with Mixed H<Subscript>2</Subscript>/CO Gas for Preparation of Sn-Enriched Direct Reduced Iron

The development of manufacturing technology of Sn-bearing stainless steel inspires a novel concept for using Sn-bearing complex iron ore via reduction with mixed H₂/CO gas to prepare Sn-enriched direct reduced iron (DRI). The thermodynamic analysis of the reduction process confirms the easy reduction of stannic oxide to metallic tin and the rigorous conditions for volatilizing SnO. Although the removal of tin is feasible by reduction of the pellet at 1223 K (950 °C) with mixed gas of 5 vol pct H₂, 28.5 vol pct CO, and 66.5 vol pct CO₂ (CO/(CO + CO₂) = 30 pct), it is necessary that the pellet be further reduced for preparing DRI. In contrast, maintaining Sn in the metallic pellet is demonstrated to be a promising way to effectively use the ore. It is indicated that only 5.5 pct of Sn is volatilized when the pellet is reduced at 1223 K (950 °C) for 30 minutes with the mixed gas of 50 vol pct H₂, 50 vol pct CO (CO/(CO + CO₂) = 100 pct). A metallic pellet (Sn-bearing DRI) with Sn content of 0.293 pct, Fe metallization of 93.5 pct, and total iron content of 88.2 pct is prepared as a raw material for producing Sn-bearing stainless steel. The reduced tin in the Sn-bearing DRI either combines with metallic iron to form Sn-Fe alloy or it remains intact.     

Reduction of Iron-Ore Pellets Using Different Gas Mixtures and Temperatures

Direct reduction of iron ore (DRI) is gaining an increased attention due to the growing need to decarbonize industrial processes. The current industrial DRI processes are performed using reformed natural gas, which results in CO₂ emission, although it is less than carbothermic reduction in the blast furnace. Carbon-free reduction may be realized through the utilization of green H₂ as a reducing agent, in place of natural gas. Herein, the effects of various gas mixtures and temperature on the reduction kinetics of the hematite iron-ore pellets are focused on in this work. Pellets are reduced at 700, 800, 850, and 900 °C in hydrogen and using various gas mixes at 850 °C. Morphology of the pellets is investigated with the help of scanning electron microscopy and mercury intrusion porosimetry. The effects of temperature and gas composition on the reduction kinetics and porosity of the pellets are discussed. A notable effect of reduction rate on the internal structure of the pellets is detected, slower reduction rate yielded bigger pores offsetting the gas composition. Higher temperature results in coarser pores and higher porosity. Increase of CO content in the gas mix also leads to bigger pore size.     

Behavior of direct reduced iron and hot briquetted iron in the upper blast furnace shaft: Part I. Fundamentals of kinetics and mechanism of oxidation

It is considered that the use of prereduced ferrous materials and sources of metallic iron such as direct reduced iron (DRI) or hot briquetted iron (HBI) improves the productivity of the blast furnace (BF). However, oxidation of DRI/HBI can occur in the upper zone of the BF, which may increase the content of the reducing gases but may not decrease the coke rate substantially. The behavior of DRI and HBI was investigated by measuring the rate of oxidation of the materials in CO₂ gas in a temperature range of 400 °C to 900 °C. In addition, the microstructure of “as-received” and oxidized materials was examined. The iron oxide phases formed due to oxidation were determined using X-ray diffraction (XRD) and a vibrating sample magnetometer. The results of isothermal experiments indicated that the kinetics of oxidation of metallic iron is slow at 400 °C. In DRI samples, the initial rate is controlled by the limited mixed control of chemical kinetics at the iron/iron oxide interface and pore mass transfer, whereas gas diffusion in pores is the rate governing step during the final stages of oxidation. The oxidation of wustite from iron is found to be faster than the oxidation of the former to magnetite. The structure of DRI after oxidation resembled a “reverse topochemical-oxide on the surface metal in the center” structure at 600 °C to 700 °C. The final iron oxide phase formed in DRI after oxidation was magnetite and not hematite. The oxidation of HBI was limited to the surface of the samples at lower temperatures; at 900 °C, moderate oxidation was observed and a topochemical iron oxide layer was formed.     

Reduction mechanisms of vanadium–titanomagnetite–non‐coking coal mixed pellet

Abstract

Experiments were carried out by testing the specimens of separate layers of iron and coal and single pellets thermogravimetrically in a nitrogen atmosphere to study the non‐isothermal reduction mechanisms of vanadium–titanomagnetite–non‐coking coal mixed pellets. The degree of reduction was measured by the weight loss. The E values of the pellet reduction were calculated based on the mass action law. It was found that with increasing temperature the reduction processes may be divided into four stages: reduction via CO and H₂ from volatiles at 400–650°C, reduction via H₂ and C generated by cracking of hydrocarbon at 650–850°C, direct reduction of carbon via gaseous intermediates at 850–1050°C and direct reduction of carbon above 1050°C.     

Reduction Behaviors of Pellets Under Different Reducing Potentials

Owing to the change of gas composition in top gas recycling-oxygen blast furnaces compared with traditional blast furnace, many attentions are attracted to the research on iron oxide reduction again. In order to study the influence of H₂ and CO on the reduction behavior of pellets, experiments were conducted with H₂-N₂, CO-N₂ or H₂-CO gas mixtures at 1173 K by measuring the mass loss, respectively. It was found that the reduction degree increased with increasing the ratio of H₂ or CO in the gas mixture, but the reduction with hydrogen was faster than that with carbon monoxide. The reduction degree could reach 96. 72% after 65 min for the reduction with 50% H₂ + 50% N₂, while it is only 53. 37% for the reduction with 50% CO+ 50% N₂. The addition of hydrogen to carbon monoxide will accelerate the reduction because the hydrogen molecules are more easily chemisorbed and reacted with iron oxide than carbon monoxide. A scanning electron microscope was used to characterize the structures of reduced samples. Dense structure of iron was obtained in the reduction with hydrogen while the structure of iron showed many small fragments for the reduction with carbon monoxide. At the later stage of reduction with the gas mixtures containing carbon monoxide, the reduction curves showed a descending trend because the rate of carbon deposition caused by the thermal decomposition of carbon monoxide was faster than the rate of oxygen loss. Compared with the reduction with CO-N₂ and H₂-CO gas mixtures, H₂ gas could enhance the carbon deposition while N₂ gas would reduce this phenomenon. The results of X-ray diffraction and chemical analysis demonstrated that the carbons are mainly in the form of cementite (Fc₃C) and graphite in reduced sample.     

Role of Heat Transfer in Early Stage Decarburization of DRI in Slag

The gas generation from reactions between direct reduced iron (DRI) pellets and steelmaking slags is known to take place in two stages; (1) the reaction of FeO and carbon within DRI, i.e., pellet internal reaction, followed by (2) the reduction of slag FeO with DRI carbon at the pellet?Cslag interface, if any carbon remains from the first step. To understand the controlling mechanism of the reaction between FeO and C inside DRI, the rate of the gas release and the temperature of pellets suspended in a slag-free atmosphere were quantified. The results were used to determine the apparent thermal conductivity of DRI that showed values of approximately 0.5 to 2 W.m^?1.K^?1 for a temperature range of 573?K to 1273?K (300?°C to 1000?°C). Furthermore, it was found that the experimental gas evolution rates are consistent with the values predicted by a heat?Ctransfer based model, confirming that the FeO-C reaction within pellet is controlled by the rate of heat transfer from the slag to the DRI pellet.     

The influence of carbon deposition on the reduction kinetics of commercial grade hematite pellets with CO,H2, and N2

Experimental measurements are reported on the rate at which commercial grade, low silica hematite pellets react with a gas mixture consisting of CO, H₂, and N₂ over the temperature range 500 °C to 1200 °C. Systems of this type are of considerable practical interest, both regarding the operation of direct reduction processes and ironmaking in the blast furnace. The results of the work may be summarized as follows: No carbon deposition was found when operating the system above 900 °C and in the absence of CO gas. When operating the system below 900 °C carbon deposition occurred, which in effect prevented the normal conversion from reaching completion. The maximum rate of carbon deposition was found to occur between 500 °C and 600 °C. In general hydrogen (in the presence of CO) tended to promote carbon deposition, while the presence of nitrogen appeared to retard the deposition process. When the reaction process was being carried out below 900 °C with CO + H₂ gas mixtures, the reduction process occurred simultaneously with carbon deposition. At lower temperatures, say around 500° to 600 °C, the deposition process dominated, while at the higher temperatures, and particularly at a high hydrogen content of the reactant gas, the reduction process was dominant. The structural examination of the partially reacted specimens has shown that the carbon deposited was found primarily in the form of elemental carbon rather than cementite. Furthermore, X-ray analysis of the free pellet surface has indicated that iron was present in the carbon deposit phase. The practical industrial implications of these findings are discussed in the paper.     

Carbon deposition on iron surfaces in CO–CO2 atmosphere

Thermodynamic calculations and thermogravimetric (TG) analysis were performed in order to understand the mechanism of carbon deposition on the surfaces of iron particles during the reduction of iron ore in a CO–CO₂ atmosphere. The results of the thermodynamic equilibrium phase analysis indicate that the phases of the carbon deposition process can be predicted on the basis of the carbon potential, reaction temperature and gas pressure. The optimal thermodynamic conditions for carburisation are a low temperature (T?<?T_m) and a high carbon potential (α_c>1). TG analysis is performed in a gas mixture of 65 vol.-% CO and 35 vol.-% CO₂ at 650, 706 and 750°C. Cementite (Fe₃C) is generated as an intermediate product, which acts as a catalyst for carbon deposition. Carbon deposition is inhibited at high temperatures (T>791°C) owing to the high stability of Fe₃C. When the reaction temperature is higher than the thermodynamic limit for the formation of Fe₃C, carbon deposition cannot occur. A mechanism for carbon deposition is proposed based on the experimental results.     

Catalytic effect of iron on decomposition of carbon monoxide: II. Effect of additions of H2, H2O,CO2, SO2 and H2S

In this work the effect of additions of H₂, H₂O, CO₂, SO₂, and H₂S on the catalytic decomposition of CO by iron has been investigated at 400, 600, and 800°C and atmospheric pressure. The catalyst was porous iron formed by the reduction of hematite ore granules with hydrogen. The relative importance of carbon deposition by the reactions 2CO → C + CO₂ and H₂ + CO → C + H₂O was determined as a function of hydrogen concentration. It was found that even low concentrations of hydrogen greatly enhanced the rate of decomposition of CO, presumably by a catalytic action of adsorbed hydrogen on iron. The presence of water vapor had a dual effect. At low concentrations of hydrogen the rate of decomposition of CO increased with the addition of H₂O, apparently by a catalytic effect. At high concentrations of hydrogen, however, carbon deposition was retarded because of the effect of the reverse reaction H₂O + C → H₂ + CO. In CO-CO₂ mixtures the rate of carbon deposition decreased with increasing CO₂ content, because of the effect of the reverse reaction CO₂ + C → 2CO. The presence of traces of sulfur-bearing gaseous species, such as SO₂ and H₂S, retarded the decomposition of CO on iron and brought about the early cessation of carbon deposition. This strong effect may be due to the retardation of the decomposition of the intermediate product, cementite, and the formation of pyrrhotite on the surface of the iron catalyst.     

Effects of the Composition of Residual Materials from Steelwork on the Crushing Strength of DRI

The goal of this study is to investigate the effects of the composition of residual materials, such as the basicity and (C/O_red)_mol, as well as the reduction temperature and time, on the crushing strength of direct reduced iron (DRI) under the carbothermic reduction through the rotary hearth furnace (RHF) process. Laboratory‐ scale experiments were executed to examine the DRI quality in the RHF operation. It is observed that when the composition of residual materials varies or the reduction temperature and time decrease, DRI crushing strength becomes lower than the 60 kg/piece requirement for blast furnace feed stock. Both basicity and (C/O_red)_mol of the residual materials would affect the crushing strength of DRI. As B₂ increase, the crushing strength of DRI also rises, and the maximum value is when the basicity is 1.4. The result shows that as the value of (C/O_red)_mol rose above 1.20, the DRI strength declined.     

The reduction of iron oxides by volatiles in a rotary hearth furnace process: Part II. The reduction of iron oxide/carbon composites

The reduction of iron oxide/carbon composite pellets with hydrogen at 900 °C to 1000 °C was studied. Compared to hydrogen, the reduction by carbon was negligible at 900 °C and below. However, significant carbon oxidation of the iron oxide/graphite pellets by H₂O generated from the reduction of Fe₂O₃ by H₂ was observed. At higher temperatures, reduction by carbon complicates the overall reduction mechanism, with the iron oxide/graphite composite pellet found to be more reactive than the iron oxide/char composite pellet. From the scanning electron micrographs, partially reduced composite pellets showed a typical topochemical interface with an intermediate region between an oxygen-rich unreacted core and an iron-rich outer shell. To determine the possibility of reduction by volatiles, a layer of iron oxide powders was spread on top of a high volatile containing bituminous coal and heated inside a reactor using infra-red radiation. By separating the individual reactions involved for an iron oxide/coal mixture where a complex set of reactions occur simultaneously, it was possible to determine the sole effect of volatile reduction. It was found that the light reducing gases evolve initially and react with the iron oxide, with complex hydrocarbons evolving at the later stages. The volatiles caused about 20 to 50 pct reduction of the iron oxide.     

Reduction Behavior of Panzhihua Titanomagnetite Concentrates with Coal

The reduction behavior of the Panzhihua titanomagnetite concentrates (PTC) briquette with coal was investigated by temperature-programmed heating under argon atmosphere in a vertical tube electric furnace. The mass loss behavior of the PTC-coal mixture was checked by thermogravimetric analysis method in argon with a heating rate of 5 K (5 °C)/ min. It was found that there are five stages during the carbothermic reduction process of the PTC. The devolatilization of coal occurred in the first stage, and reductions of iron oxides mainly occurred in the second and third stages. The reduction rate of iron oxide in the third stage was much higher than that in the second stage because of the significant rate of carbon gasification reaction. The iron in the ilmenite was reduced in the fourth stage. In the final stage, the rutile was partially reduced to lower valence oxides. The phase transformation of the briquette reduced at different temperatures was investigated by X-ray diffraction (XRD). The main phases of sample reduced at 1173 K (900 °C) are metallic iron, ilmenite (FeTiO₃), and titanomagnetite (Fe_3–x Ti _x O₄). The traces of rutile (TiO₂) were observed at 1273 K (1000 °C). The iron carbide (Fe₃C) and ferrous-pseudobrookite (FeTi₂O₅) appeared at 1473 K (1200 °C). The titanium carbide was found in the sample reduced at 1623 K (1350 °C). The shrinkages of reduced briquettes, which increased with increase in the temperature, were found to depend greatly on the temperature. With increasing the reduction temperature to 1573 K (1300 °C), the iron nuggets were observed outside of the samples reduced. The nugget formation can indicate a new process of ironmaking with titanomagnetite similar to ITmk3 (Ironmaking Technology Mark 3).     

内配碳团块CO-CO2气氛下反应行为的研究

为了建立了内配碳团块CO CO2气氛下反应的数学模型并验证模型的正确性,根据还原动力学原理模拟计算内配碳团块在CO CO2气氛下的还原过程,对不同温度下的团块还原度、团块碳转化率的模拟值和实验值之间的比较证明了模型的可靠性。经过研究发现模型的反应包括了铁氧化物的逐级还原、碳的boudouard反应和金属铁的再氧化。在1473K和CO CO2气氛下对内配碳团块的反应进程的模拟分析表明,在早期阶段内配碳团块显示出自还原的特征;在团块达到其最大还原度时,团块内氧化铁还原和金属铁再氧化同时存在;在后期阶段,团块内主要是金属铁的再氧化反应。在1473K和CO CO2气氛下对影响内配碳团块反应行为的相关因素的模拟结果表明,改变铁矿粉反应性或还原剂气化性不能有效提高内配碳团块的最终还原度,但是减小孔隙率可以提高团块的最终还原度。     

The Reaction Behavior of Direct Reduced Iron (DRI) in Steelmaking Slags: Effect of DRI Carbon and Preheating Temperature

An experimental study was conducted to quantify the rate of direct reduced iron (DRI) decarburization in a steelmaking slag using the constant volume pressure increase technique. Experiments were conducted by dropping DRI pellets into molten slag at temperatures from 1773 K to 1873 K (1500 °C to 1600 °C). Subsequent experiments were carried out in which the DRI pellets were preheated while the slag temperature remained constant. The effect of the initial carbon content and the preheating temperature of the DRI on the reaction rate was investigated. The decarburization of DRI seems to comprise two stages, a reaction between the FeO and DRI followed by decarburization through the iron oxide of slag. Carbon has a significant effect on the kinetics of both stages, whereas the preheating temperature mainly influences the rate of decarburization between FeO and carbon inside the pellet.     

The effect of hydrogen addition on the carbon-deposition behaviour during the reduction of pellets for the blast furnace process

With the application of large amount of pulverised coal injection into the blast furnace, the hydrogen content in the gas will increase, which accelerates the reduction of iron ore in lump zone of the blast furnace as well as carbon-deposition reaction. This study has investigated the effect of hydrogen addition on carbon-deposition reaction during the reduction of pellets through thermodynamic calculation and experiment. The results show that H₂ can promote the carbon-deposition reaction, while the increase of temperature and CO₂ can significantly inhibit it. The preference region of temperature for C formation is about 600°C. Moreover, the promotion effect of H₂ on the carbon-deposition reaction at 700°C is better than that at 600°C. The SEM observation results show that the generated carbon is mainly distributed on the surface of the pellet, and only a little carbon is located inside the pellet. The agglomerated carbon could be more easily formed due to the dramatic carbon-deposition reaction caused by the lower temperature or higher H₂ content. But, most of the carbon just exists as an individual particle at the lower carbon-deposition reaction rate. The results of SEM–EDS reveal that carbon deposited is primarily in the form of elemental carbon rather than in the form of cementite. The study also shows that with increasing reduction time, the rate of carbon-deposition increases, mainly due to the promotion effect of reduced iron during the reduction process of pellets.     

The rate of reduction of iron oxides by carbon

The rate of reduction of Fe₂O₃ and FeO by coconut charcoal, coal char and coke, in an inert atmosphere within the temperature range 900 to 1200°C was investigated. The effects of pressure, particle size, and the amount of carbon were determined. The results indicate that the reaction takes place by means of the gaseous intermediates CO and CO₂, and that the overall rate is controlled by the oxidation of the carbon by CO₂. The rates of reduction of FeO and Fe₂O₃ by CO are relatively fast, and the CO₂/CO ratio for the oxidation of carbon is determined by their equilibria. The reduction of Fe₂O₃ by carbon is accomplished in two stages, with FeO forming first. The reduction of Fe₂O₃ to FeO is faster than that of FeO to Fe because its CO₂/CO equilibrium ratio is higher and hence the rate of oxidation of carbon is faster. A direct comparison was made between the rate constants for the reduction of FeO by carbon and those for the oxidation of carbon in the appropriate CO₂-CO gas mixtures, and they are in good agreement. Apparently the iron formed by the reduction does not significantly catalyze the oxidation of carbon; whereas for the reduction of NiO by carbon, the Ni formed does catalyze the oxidation of carbon.     

Cost and Life Cycle Analysis for Deep CO2 Emissions Reduction for Steel Making: Direct Reduced Iron Technologies

Among heavy industrial sectors worldwide, the steel industry ranks first in carbon dioxide (CO₂) emissions. Technologies that produce direct reduced iron (DRI) enable the industry to reduce emissions or even approach net-zero CO₂ emissions for steel production. Herein, comprehensive cradle-to-gate (CTG) life cycle analysis (LCA) and techno-economic analysis (TEA) are used to evaluate the CO₂ emissions of three DRI technologies. Compared to the baseline of blast furnace and basic oxygen furnace (BF–BOF) technology for steel making, using natural gas (NG) to produce DRI has the potential to reduce CTG CO₂ emissions by 33%. When 83% or 100% renewable H₂ is used for DRI production, DRI technologies can potentially reduce CO₂ emissions by 57% and 67%, respectively, compared to baseline BF–BOF technology. However, the renewable H₂ application for DRI increases the levelized cost of steel (LCOS). When renewable natural gas (RNG) and clean electricity are used for steel production, the CTG CO₂ emissions of all the DRI technologies can potentially be reduced by more than 90% compared to the baseline BF–BOF technology, although the LCOS depends largely on the cost of RNG and clean electricity.     

Inhibiting the reoxidation of directly reduced iron by vapour phase inhibitors

The inhibition of the reoxidation of direct reduced iron was investigated by establishing different contents of vapour phase inhibitors in oxidising gas mixture He-O₂ at 150 and 200 °C. The raw materials were iron oxide powder and iron ore pellets which were first pre-reduced by H₂ at 500 °C and then reoxidised. Three stages of oxidation are identified, a rapid initial stage, an intermediate stage in which the rate of reoxidation decreases significantly, and a final stage, in which the rate of oxidation is very low. The length of each stage and the rate of reoxidation are related to temperature and to the status of raw materials. Four inhibitors were tested. Benzylamine, ammonium benzoate, and the commercial VCI cortec 101 exert a good inhibition on the reoxidation of iron. The inhibition by ammonia, however, is not significant. Presence of water vapour enhances the reoxidation. XPS analysis reveals amines on the surface when benzylamine is added in the oxidation gases. The ignition temperature of reduced iron decreases with the addition of inhibitors in the oxidation gas atmosphere. This unwanted effect probably is due to organic deposits on iron surface, initiating the burning of iron. The extent of reoxidation, however, is significantly diminished by the inhibitors, thus heating of the material is suppressed and the danger of ignition is decreased.