Reduction Behaviour of Olivine Iron Ore Pellets in the Experimental Blast Furnace

An experimental study was conducted to determine the reduction behaviour of olivine iron ore pellets and associated reduction mechanisms in the experimental blast furnace (EBF) located at Luleå. Two sets of EBF samples, namely slowly annealed excavated samples and rapidly quenched probe samples of olivine bearing iron ore pellets were examined in detail. Pellet samples were analysed using SEM, XRD and SIROQUANT analysis to quantitatively determine iron ore phase transformations during descent in the EBF. In the tested EBF campaign, up to 75% of reduction occurred at less than 1100°C, i.e. before the pellet reached the cohesive zone while rest of 25% reduction was completed when pellets reached a temperature of 1300°C and hence within the cohesive zone. The reduction degree of pellets was found to have a linear correlation with distance from the stock line of the EBF. This study showed that the presence of olivine did not have a significant effect on reduction degree for temperatures less than 1100°C in the upper zone of the EBF. However, olivine increased the reduction rate in the final stage of reduction for temperatures in excess of 1100°C in the cohesive zone, which was attributed to the formation of an increased amount of molten FeO containing slag within the pellet. This study is expected to make important contributions towards further improvements in the pellet design as well as the optimization of blast furnace operation and efficiency.     

Rate of slag reduction in a laboratory electric furnace—alternating vs direct current

The objective of this laboratory investigation was to measure the reduction kinetics of nickel smelting and converting slags using alternating current (AC) and direct current (DC). The two slags tested contained 34 and 51 pct total iron in the form of FeO and Fe₃O₄. Laboratory experiments were carried out between 1200 °C and 1450 °C, and the rate of reduction was measured based on the CO and CO₂ contents in the off-gas from the furnace. Upon application of power to a pair of electrodes immersed in the molten slag, the reduction rate increased rapidly. This increase is explained by an increase in the electrode tip temperature enhancing the rate of the Boudouard reaction. The rate of reduction of the converter slag containing 29 pct Fe₃O₄ was 2 to 3 times faster than the smelting slag. With DC, the reduction rates at the anode and cathode were basically identical to each other, while for the smelting slag with only 8 pct Fe₃O₄, the anode and cathode reduction rates were quite different. With increasing current or power density, the temperatures of the electrodes increase above that of the bulk slag.     

Phase Equilibria in the System “FeO”-CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO at Different CaO/SiO<Subscript>2</Subscript> Ratios

The “FeO”-containing slags play an important role in the operation of an ironmaking blast furnace (BF), in particular the primary slags such as the system “FeO”-CaO-SiO₂-Al₂O₃-2 mass pct MgO with CaO/SiO₂ weight ratios of 1.3, 1.5, and 1.8 saturated with metallic iron. To investigate the characteristics of such a slag system and its behavior in BF, the phase equilibria and liquidus temperatures in the slag system have been experimentally determined using the high-temperature equilibration and quenching technique followed by an electron probe X-ray microanalysis (EPMA). Isotherms between 1553 K and 1603 K (1280 °C and 1330 °C) were determined in the primary phase fields of dicalcium silicate, melilite, spinel, and monoxide [(Mg,Fe²⁺)O]. Pseudo-ternary phase diagrams of (CaO + SiO₂)-Al₂O₃-“FeO” with a fixed MgO concentration at 2 mass pct and at CaO/SiO₂ ratios of 1.3, 1.5, and 1.8 have been discussed, respectively, simplifying the complexity of the slag system for easy understanding and applying in BF operation. It was found that the liquidus temperatures increase in melilite and spinel primary phase fields, but decrease in dicalcium silicate and monoxide primary phase fields with increasing Al₂O₃/(CaO + SiO₂) ratio. In addition, the liquidus temperatures decrease with increasing “FeO” concentration in dicalcium silicate and melilite primary phase fields, while showing an increasing trend in the spinel and monoxide primary phase fields. The data resulted from this study can be used to improve and optimize currently available database of thermodynamic models used in FactSage.     

Phase equilibria in the system Fe-Zn-O at intermediate conditions between metallic-iron saturation and air

The phase equilibria in the FeO-Fe₂O₃-ZnO system have been experimentally investigated at oxygen partial pressures between metallic iron saturation and air using a specially developed quenching technique, followed by electron probe X-ray microanalysis (EPMA) and then wet chemistry for determination of ferrous and ferric iron concentrations. Gas mixtures of H₂, N₂, and CO₂ or CO and CO₂ controlled the atmosphere in the furnace. The determined metal cation ratios in phases at equilibrium were used for the construction of the 1200 °C isothermal section of the Fe-Zn-O system. The univariant equilibria between the gas phase, spinel, wustite, and zincite was found to be close to pO₂=1 · 10⁻⁸ atm at 1200 °C. The ferric and ferrous iron concentrations in zincite and spinel at equilibrium were also determined at temperatures from 1200 °C to 1400 °C at pO₂ = 1·10⁻⁶ atm and at 1200 °C at pO₂ values ranging from 1 · 10⁻⁴ to 1 · 10⁻⁸ atm. Implications of the phase equilibria in the Fe-Zn-O system for the formation of the platelike zincite, especially important for the Imperial Smelting Process (ISP), are discussed.     

The nitrogen reaction between carbon saturated Iron and Na2O-SiO2 slag: Part II. Kinetics

The kinetics of the nitrogen reaction between carbon saturated iron and Na₂O-SiO₂ slags and between Na₂O-SiO₂ slags and an inert gas phase were investigated at 1200 °C. For the nitrogen transfer from the iron alloy to slag, the overall mass transfer coefficient of nitrogen was calculated to be 2.2 × 10⁻⁴ cm/sec. For nitrogen transfer from Na₂O-SiO₂ slag to argon gas, it is shown that the rate controlling process is mass transfer in the slag phase, and the mass transfer coefficient of nitrogen is 9 × 10⁻⁴ cm/sec. Experiments were also conducted to demonstrate nitrogen removal from hot metal by Na₂CO₃ treatment at 1200 °C. In these experiments, 325 grams of Na₂CO₃ was added to the 6.5 kg of Fe-C-N(-Si) alloy. When the metal contained silicon, nitrogen was transferred from the iron alloy to slag after the silicon was oxidized. When the iron alloy contains no silicon, nitrogen removal was faster. In both cases the nitrogen reversion occurred because of the decrease of slag volume and slag basicity. Furthermore, the presence of silicon in the metal retarded nitrogen reversion. is on leave of absence from the Department of Metallurgical Engineering and Materials Science, Faculty of Engineering, The University of Tokyo Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University     

An investigation of the critical influence of potassium on the reduction of wustite

An investigation of the influence of potassium on the reduction of wustite single crystals has been carried out at temperatures ranging from 600 to 1000 °C in 30 pct CO-10 pct CO₂-3 pct H₂-57 pct N₂, a gas composition close to that of the reserve zone of the blast furnace. Three doping techniques have been used in order to perform either an insertion of potassium in the wustite lattice, by annealing, by surface doping at room temperature, or a doping by a potassium loaded reducing gas during the reduction. Regardless of which doping technique is used, no potassium has been detected in the wustite lattice. All dopings affect only the surface of the particles before the reduction. Potassium markedly increases the rate of reduction of wustite. This is explained first by the change in the morphology of the iron produced. In addition, potassium also acts on the chemical stage of the reduction by changing the nucleation and the growth of the iron. Another important result is the following: when potassium is added to partly reduced wustite, it changes the iron properties in the course of a subsequent reduction (this iron becomes much more permeable to the gas, increasing the transfer of the reactants).     

Influence of magnesia on sintering characteristics of iron ore

Abstract

The MgO in blast furnace slag provides an optimum condition in terms of both good flowability and desulphurisation. The mode of its addition to the blast furnace changed from, initially, as raw flux in the form of dolomite, to via sinter, with the argument that raw flux demands energy for its decomposition inside the blast furnace. Thus, the decomposition reaction was diverted from the blast furnace to the sintering bed, and the energy source for decomposition was changed from costly blast furnace coke to a relatively cheap coke breeze. Now olivine/dunite/serpentine is being used as a source of MgO, where energy for decomposition is not required; this also provides a source of SiO₂, which eliminates need for the addition of quartzite. The effect of MgO on blast furnace slag is fairly well established, but its effect on sintering and sinter quality is unclear. Operating results of the sinter plants show that, with an increase of MgO, the sintering rate, the fuel rate, and sinter strength and reducibility deteriorate; however, high temperature properties such as the reduction degradation index and the softening-melting characteristics of the sinter improve. The present work attempts to establish this influence on the sintermaking process and sinter quality with the help of operating plant data.     

Formation of potassium slag in olivine fluxed blast furnace pellets

Abstract

Mineralogical evaluation of olivine pellets coated with kaolinite, taken from the LKAB experimental blast furnace, shows significant reactions with potassium. Sampling has revealed strong potassium deposition in pellets in the lower shaft close to the wall, but much less deposition towards the furnace centre. Iron reduction and the deformation of the pellets were enhanced in the zone of high alkali deposition. Thin sections of pellet samples were prepared to distinguish amorphous and crystalline slag phases for a better understanding of the formation of the potassium rich slag. Olivine breaks down to various extents to form a SiO₂–FeO–MgO–K₂O glass. The kaolinite coating shows strong reaction throughout the cross-section of the lower shaft to form kalsilite (KAlSiO₄) and K₂O rich glassy slag. Studies of thin sections of the slag products were shown to be very useful in separating amorphous phases such as the K₂O rich glass from the crystalline olivine rim.     

Influence of CaO-SiO2-Al2O3 Ternary Oxide System on the Reduction Behavior of Carbon Composite Pellet: Part I. Reaction Kinetics

The reduction behavior of composite pellets comprising of hematite, synthetic graphite, and several oxide binder systems was investigated in a laboratory-scale horizontal tube furnace. Three oxide binder systems using silica-rich, alumina-rich, and conventional blast furnace slag compositions were selected to examine the effect of oxide chemistry on the reduction behavior of pellets. Compositional differences in the CaO-SiO₂-Al₂O₃ ternary system were confirmed to influence the reactions occurring in composite pellets during the reduction of iron oxide. An in situ visualization approach was used to observe the oxide/iron/carbon interactions at high temperatures from 1623 K to 1773 K (1350 °C to 1500 °C). The off-gas composition was measured by means of an infrared analyzer to determine the pellet reaction rates. Changes in physical appearance during the in situ reaction experiments demonstrated a strong correlation between the oxide composition and internal reactions. Moreover, the mechanical properties of pellets were investigated by measuring compressive strength to understand the relationship between physical properties of pellets and the associated oxide binder systems selected for this study.     

Reduction of lead minerals by CO/CO2 gas mixtures: Application of the grain model

The grain model is shown to describe satisfactorily the reduction of lead oxide and lead calcium silicate pellets by CO/CO₂ gas mixtures. The reduction of lead calcium silicate, the principal lead bearing phase in the feed material to a commercial blast furnace, is initiated at about 750 °C. The activation energy is 161 kJ mol⁻¹. These values are much higher than values for the reduction of lead oxide. The lack of chemical reaction in the upper reaches of the commercial lead blast furnace is attributed to the low reactivity of the feed material. This is a consequence of the low porosity, large particle size of the feed material, and the presence of lead in the form of complex silicates.     

Recovery of titanium from titanium bearing blast furnace slag by sulphate melting

Recovery of titanium from titanium bearing blast furnace slag by ammonium sulphate and potassium sulphate melting method is carried out for the first time, which transforms the titanium bearing substances such as perovskite (CaTiO₃) into water soluble ones, such as TiOSO₄. The effect of reaction temperature on recovery efficiency of titanium was investigated through the experiment. The results showed that the recovery efficiency of titanium increased slightly with the increase in reaction temperature from 320 to 410°C. The conditions of titanium recovery from titanium bearing blast furnace slag are 410°C for 35 min, with the mass ratio of water quenched titanium bearing blast furnace slag, ammonium sulphate to potassium sulphate at 1∶8∶1. Under these conditions, the recovery efficiency of titanium was 94·7%, which was higher than ammonium sulphate melting method. Recovery efficiency of titanium was increased due to the presence of potassium sulphate. In addition, the residue was in the form of a solid powder, and the content of TiO₂ in residue was 3·20%.

Pour la première fois, on a effectué la récupération du titane à partir de laitier de haut-fourneau porteur de titane par la méthode de fusion au sulfate d’ammonium et au sulfate de potassium, qui transforme les substances porteuses de titane comme la pérovskite (CaTiO₃) en substance soluble dans l’eau, comme le TiOSO₄. On a examiné par expérimentation l’effet de la température de réaction sur le rendement de la récupération du titane. Les résultats ont montré que le rendement augmentait légèrement avec l’augmentation de la température de réaction de 320°C à 410°C. Les conditions de cette récupération du titane sont de 410°C pendant 35 min, avec le rapport de masse du laitier de haut-fourneau porteur de titane, trempé à l’eau, au sulfate d’ammonium et au sulfate de potassium, de 1:8:1. Sous ces conditions, le rendement était de 94·7%, ce qui était plus élevé que la méthode de fusion au sulfate d’ammonium. Le rendement augmentait grâce à la présence du sulfate de potassium. De plus, le résidu avait la forme d’une poudre solide et la teneur en TiO₂ de ce résidu était de 3·20%.     

Effect of titania on the characteristics of blast furnace slags

The objective of this study was to understand the effect of 1 – 2 % titania on the high-temperature properties of blast furnace slags containing high alumina and magnesium oxide. The viscosity and liquidus temperature of semi-synthetic blast furnace slags were measured using a viscometer and a hot stage microscope, respectively, and the data were used to develop a statistical model for predicting the liquidus temperature and viscosity of blast furnace slags. Samples of the titanium bearing accretions were collected during tearing-out of the hearths of Tata Steel's blown-out D and E blast furnaces. They were subjected to various physico-chemical analyses (e.g. chemical, XRD, optical microscopy, SEM and image analysis) in order to understand the mechanism of hearth protection. In the range of composition studied, the liquidus temperature is found to lie between 1365 to 1430°C and the viscosity, between 0.30 to 0.60 Pa · s. Increase in titania fluidizes the slag and also makes it easy-melting. The titanium bearing compounds in the slag show traces of TiC, TiN, Ti₂N, TiO, Ti₂O₃, Ti₃O₅ and Ti₅O₉. A mechanism of formation of these precipitates is proposed in this paper.     

The slag-metal equilibrium in tin smelting

An equilibrium study was undertaken to investigate the effect of the CaO/SiO₂ and Fe/SiO₂ ratios and the SnO and Al₂O₃ contents of slags on the distribution of Fe and Sn between slag and metal in tin smelting. The experiments were performed at 1200 °C by equilibrating Sn-Fe alloys with silicate slags under reducing conditions in closed crucibles. The slag and metal analyses were used to calculate the γ_SnO/γ_FeO ratio in the slags and a multiple-linear regression on these values indicated that, in the range of slag compositions investigated, γ_SnO/γ_FeO is a function only of the CaO/SiO₂ ratio. At 1200 °C, γ_SnO/γ_FeO varies from about 1.1 for CaO-free slags to 3.6 for slags in which the CaO/SiO₂ ratio is 1.0. In practical applications, the slag-metal equilibrium in tin smelting is usually discussed in terms of the variation of the distribution coefficient,k, with the Fe content of the metal, wherek is defined ask = [pct Sn]/[pct Fe] · (pct Fe)/(pct Sn). An equation fork was derived in terms of the atom fraction of iron in the metal, the γ_SnO/γ_FeO in the slag, and the temperature. This equation was used to construct graphs ofk as a function of the iron content over the slag compositions and at temperatures which cover the range of tin smelting practice.     

Metal loss to slag: Part II. oxidic dissolution of nickel in fayalite slag and thermodynamics of continuous converting of nickel-copper matte

FeO-Fe₂O₃-SiO₂ slags were equilibrated in pure nickel crucibles under a CO-CO₂ atmosphere. The Fe/SiO₂ ratios in the slag were fixed at 1.51 and 1.97 at temperatures of 1200 and 1300°C. The CO₂/CO ratios were varied up to values corresponding to magnetite saturation. On the basis of solubility data obtained, a computer model was developed to predict the solubilities of nickel and copper in slag during the continuous converting of nickel-copper matte. These are given as a function of five parameters: temperature, iron content and Cu/Ni ratio in the matte, partial pressure of SO₂ in the gas phase, and magnetite activity in the slag. The model is helpful in comprehending converting reactions and of practical applicability in optimizing the conventional as well as continuous converting processes. Noranda Research Centre, Pointe Claire, Quebec.     

Effect of mixing charge of highly reactive semicoke nut on the reaction of high Al2O3 ferrous burden in blast furnace

In order to improve the performance of blast furnace ironmaking when using the high Al₂O₃ iron ore, the technology of mixing charge of high reactive semicoke nut with ferrous burden was proposed and systematically investigated at laboratory scale in the present paper. The CO₂ gasification activation energy of semicoke ranged from 136.5 to 150.3?kJ?mol^?1, which was lower than that of the traditional coke. At the temperature of 1200°C, the reduction degree of ferrous burden increased a little with the addition of semicoke nut and the consumption ratio of semicoke was 14.0?wt-% under the simulated blast furnace ironmaking condition. The mixing charge of semicoke could obviously reduce the softening beginning temperature, increase the melting temperature and improve the permeability compared with the standard and traditional coke mixing charge samples. During the total softening and melting process, about 90?wt-% of the semicoke nut and 45?wt-% of the traditional coke nut were consumed, respectively. The experiment results indicated that the mixing charge of high reactive semicoke nut could obviously improve the performance of blast furnace ironmaking when using high Al₂O₃ ferrous burden and the semicoke was more effective than traditional coke.     

Effects of MnO on slag viscosity and wetting behaviour between slag and refractory

As more and more Mn bearing iron ores are used to decrease steel cost and deal with the problem of hearth deposition, slag regime change and hearth refractory erosion in blast furnace become more often. To address these problems, it is urgent to clarify the effects of MnO upon the ironmaking production. Herein, the viscosities of slags with different MnO contents were measured for the first time, and the influence mechanism of MnO was analysed by infrared spectrum. The wetting behaviours between slags with different MnO contents and alumina–carbon refractory were investigated. The results showed that meltability temperature and viscosity decrease simultaneously with the increasing MnO content from 0 to 2.0?wt-%. Infrared spectrum analysis also proved that the existence of Mn⁺, Ca²⁺ and Mg²⁺ makes the Si–O bonds peak moving towards high frequency and the asymmetry of Si–O bond increasing, leading to the decrease in viscosity decreasing. In addition, the characteristic temperatures for wetting reaction increased by ～40°C with the increasing MnO content from 0 to 3?wt-% (basicity?=?1.18). The characteristic temperatures decreased by nearly 50°C with the basicity of slag increasing from 1.0 to 1.3 (MnO?=?1?wt-%). Therefore, the increasing MnO content in slag accelerates the erosion rate of BF hearth lining and then decreases the campaign life of blast furnace.     

Viscosities and activities in lead-smelting slags

Viscosity measurements at high temperature were made using a concentric cylinder viscometer. These measurements were conducted on a typical lead blast furnace slag, and this master slag doped with various amounts of SiO2, A12O3, CaO, MgO, and/or ZnO. It was found that additions of A1₂O₂ and/or SiO₂ increased the viscosity of the master slag over the entire temperature range of interest (1150 °C to 1350 °C). Additions of the basic oxides CaO, MgO, and/or ZnO decreased the viscosity at high temperatures but raised the slag liquidus temperature. The majority of the measured viscosities are accurate within ± 10 pct. The viscosity data of this study, along with that of several similar studies from the literature, were correlated with the weight parameter (WP), a composition-dependent function similar to a basicity ratio. Analytical expressions were developed relating viscosity and the WP at several temperatures between 1150 °C and 1350 °C. Viscosities calculated using these expressions have estimated accuracies of ±1 poise (within certain temperature and composition limits). PbO and ZnO activity coefficient data from the literature were collected and correlated with composition-dependent functions. The calculated activity coefficients are considered accurate within ±0.3 for γ_pbo and ±0.5 for γzn within certain composition and temperature limits. Formerly St. Joe Minerals Corporation Fellow in Extractive Metallurgy, Department of Metallurgical Engineering, Colorado School of Mines     

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.     

Simulation of primary-slag melting behavior in the cohesive zone of a blast furnace, considering the effect of Al2O3, Fe t O, and basicity in the sinter ore

The alumina content in the iron ore imported to Japan is increasing year by year, and some problems in blast furnace operation, due to the use of the high-alumina-containing sinter, have already been reported. In order to clarify the mechanism of the harmful effect of alumina on the blast furnace operation, the behavior of the primary melt, which is formed in the sinter at the cohesive zone of the blast furnace, has been simulated by dripping slag through an iron or oxide funnel. The effects of basicity, Al₂O₃, and Fe _t O contents in the five slag systems on the dripping temperature and weight of slag remaining on the funnel have been discussed. It was found that the eutectic melt formed in the sinter would play an important role in the dripping behavior of the slag in the blast furnace through the fine poreosity of the reduced iron and ore particles. Al₂O₃ increased the weight of the slag remaining on the funnel, and its effect became very significant in the acidic and low-Fe _t O-containing slag. It was estimated that the increase of the weight of the slag remaining on the funnel by Al₂O₃ in the ore could result in a harmful effect on the permeability resistance and an indirect reduction rate of the sinter in the blast furnace.     

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.