Influence and role of potassium in the reduction of hematite with CO/CO2

Potassium is known for having a favourable role in the reduction of hematite, either pure or as ore and a detrimental action on the crystals’ mechanical properties. However, several different interpretations have been put forward by previous authors: softening of the gangue, modification of the sintering process, consequences of the alkali penetration into the oxide lattices. Hence new data are desirable and have been recorded, with pure hematite crystals as well as with an hematite ore and with two doping techniques: immersion in a K₂CO₃ solution, or introduction of potassium components in the reducing gas. Microstructure investigation shows that potassium favours porous magnetite growth rather than lamellar growth and concentrates in a sublayer of magnetite at the inner interface. In fully reduced crystals, potassium lies mainly in the core of the particle. When the interface has a so-called topochemical configuration, the shrinking core model provides the rate constants as a function of temperature. Arrhenius plots lead to the conclusion that potassium significantly lowers the activation energy. The proposed interpretation is based on the transient formation of KFe₁₁O¹⁷, which is revealed by 3 different observations. It may act as a nucleation catalyst, thanks to easy epitaxy with Fe₂O₃. This is consistent with the change to porous rather than lamellar domains when potassium is brought into play and with the increased crystal fracturation. Hematite ore is less sensitive to potassium because its silica gangue behaves as a trap for it, as shown by treatment with HF before reduction.     

Hematite single crystal reduction into magnetite with CO-CO2

In view of the striking discrepancies among previous authors as regards the transition between porous and lamellar magnetites, we have carried out a wide series of experiments with single crystals prepared by chemical vapor transport. In addition to the classical temperature and CO pct parameters, which varied over a large range (400 ≤T ≤ 1000 ≤C and 2 ≤ CO pct ≤ 50), we also investigated the influence of the crystal size and of the fractional weight change. Through observation of a great number of cross-sections of partially reduced crystals, we established that lamellar magnetite is favored by high temperature and low CO pct. This is explained by consideration of the conditions governing the competition between cation diffusion in the semi-coherent hematite-magnetite interface and chemical reaction rate. At low temperatures, the crystals are severely fractured, because hematite is not plastic enough, especially at a high CO₂ pct. The kinetic data are analyzed with the shrinking-core model, where the reaction interface is topochemical. The chemical rate constant thus obtained is ϕ = 69 exp(−8950/T), in mol(CO) · m⁻² · s⁻¹, for crystals in the range 50 to 150 μm andT varying from 500 to 900 °C. Bigger crystals yield a slightly higher preexponential term, confirming that porous diffusion does not rule the kinetics. The nucleation frequency has also been evaluated; it tends toward a kind of saturation at around 700 °C with a value of 10 to 10⁹ s⁻¹. The nuclei growth rate is in reasonable agreement with direct measurements. Formerly Graduate Student     

Hematite single crystal reduction into magnetite with CO-CO2

In view of the striking discrepancies among previous authors as regards the transition between porous and lamellar magnetites, we have carried out a wide series of experiments with single crystals prepared by chemical vapor transport. In addition to the classical temperature and CO pct parameters, which varied over a large range (400 ≤T ≤ 1000 ≤C and 2 ≤ CO pct ≤ 50), we also investigated the influence of the crystal size and of the fractional weight change. Through observation of a great number of cross-sections of partially reduced crystals, we established that lamellar magnetite is favored by high temperature and low CO pct. This is explained by consideration of the conditions governing the competition between cation diffusion in the semi-coherent hematite-magnetite interface and chemical reaction rate. At low temperatures, the crystals are severely fractured, because hematite is not plastic enough, especially at a high CO₂ pct. The kinetic data are analyzed with the shrinking-core model, where the reaction interface is topochemical. The chemical rate constant thus obtained is ϕ = 69 exp(−8950/T), in mol(CO) · m⁻² · s⁻¹, for crystals in the range 50 to 150 μm andT varying from 500 to 900 °C. Bigger crystals yield a slightly higher preexponential term, confirming that porous diffusion does not rule the kinetics. The nucleation frequency has also been evaluated; it tends toward a kind of saturation at around 700 °C with a value of 10 to 10⁹ s⁻¹. The nuclei growth rate is in reasonable agreement with direct measurements. Formerly Graduate Student     

Reduction by CO-CO2 and cracking of iron ore sinters,part 1: kinetics

A synthetic sinter has been prepared, and compared to an industrial ore, in order to obtain a better understanding of the mechanism and circumstances of degradation by cracking, during the low-temperature reduction of hematite to magnetite, in the upper part of the blast furnace. In the present paper, some kinetic data on the reduction by CO₂ are reported. It appears that the Shrinking Core Model correctly applies in all conditions, even at low temperatures where there is significant cracking. The chemical rate constants and nucleation frequencies are derived and compared with our previous data obtained with natural isolated hematite single crystals. It is also confirmed that the reaction is first order versus carbon monoxide.     

Microstructural Changes and Kinetics of Reduction of Hematite to Magnetite in CO/CO2 Gas Atmospheres

Metallurgical and Materials Transactions B - The microstructures of porous magnetite formed on gaseous reduction of dense hematite have been examined using high-resolution scanning electron...     

Microstructural changes on the reduction of hematite to maanetite

The microstructures resulting from the reduction of hematite to magnetite have been examined for a wide range of temperatures and gas conditions. A transition in product morphology from plate or lath magnetite to porous magnetite was found to occur at a constant free energy difference between the reducing gas mixture and the hematite over a range of reaction temperatures. Direct observations of lath nucleation and growth are reported and show the self-accelerating effect of the Fe₂O₃ → Fe₃O₄ transformation. The limits of porous growth are discussed in terms of established theories of discontinuous precipitation and a mechanism for the formation of lath magnetite is proposed.     

Morphology of hematite to magnetite reduction zone

The reduction process of hematite to magnetite results in distinct changes in morphology of magnetite. These changes depend on structural properties of parent hematite and reduction conditions. The reduction experiments were performed in 3% CO and 97% CO₂ gas mixture at 450 and 850°C on selected crystals of natural hematite. The phase composition and morphological characteristics of the reduced layer were determined on the basis of microscopic analysis. Singular blasts or blastoidal colonies of magnetite were formed in 450°C on the boundaries of the hematite grains. They began to grow and joined the layer. Magnetite formed at 450°C is distinctly microporous. Cracks and desintegration of hematite grains appear together with reduction of hematite. At 850°C nucleation of the magnetite is quite different then at 450°C. The formation of singular magnetite lamellae or a palisade of crystallographically oriented magnetite lamellae were observed. Their growth results in the formation of the magnetite layer. Tunnel-shaped pores in magnetite layer show the same direction as lamellar front of reduction.     

Effects of Calcium and Chloride Ions in Iron Ore Reverse Cationic Flotation: Fundamental Studies

ABSTRACT

In this work, the simultaneous effects of Ca²⁺ and Cl^? ions in an aqueous solution at pH 10.5 on the flotation of quartz (the main impurity in itabiritic iron ore) and hematite by starch and amine was investigated. A strong depression in the flotation of both quartz and hematite conditioned with CaCl₂ was observed. This effect was higher for hematite than for quartz. Based on zeta potential measurements and the speciation diagram of calcium in aqueous solutions, the physical adsorption of Ca²⁺ on the surfaces of both minerals was inferred. The infrared spectrum of quartz conditioned with CaCl₂ at pH 10.5 was similar to its reagent-free reference spectrum. However, a new band at the wavenumber of 1465 cm^?1 was identified in the spectrum of hematite conditioned with CaCl₂; this band did not exist in its reference spectrum. This new band may indicate the chemical adsorption of Cl^? ions on the hematite surface. The complexation of Ca²⁺ by ethylenediaminetetraacetic acid enabled complete quartz recovery with amine. For hematite, recovery was partially restored, probably because of the positive chloro-complexes on the hydrated iron surfaces of hematite, which prevented the adsorption of aminium ions at these sites. Therefore, the selective inverse cationic flotation of itabiritic iron ore at pH 10.5 in water containing Ca²⁺ is possibly only after complexing them with EDTA.     

Transforming Hematite into Magnetite Using Mechanochemical Approach as a Pretreatment of Oolitic Hematite

Potential transformation of oolitic hematite into magnetite by mixing iron powder using the mechanochemical method has been achieved and discussed in this paper. The phase transition of pure hematite in the preliminary test was identified by X-ray diffractometer (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) techniques. The experimental results have shown that the crystallographic planes of magnetite, (220), (311), (400), and (511) were observed clearly in the Fe/α-Fe₂O₃ mixture after milling for 15 h, indicating that α-Fe₂O₃ had been effectively transformed into Fe₃O₄. The diffraction peaks of magnetite were also observed at d = 0.29605 nm (2θ = 30.163°), 0.25226 nm (2θ = 35.559°), 0.24156 nm (2θ = 37.190°), and 0.20898 nm (2θ = 43.458°) after 13 h milling-time. It suggests that the oolitic hematite is transformed into magnetite successfully by mechanochemical processing. The processing might be applied potentially for the magnetic separation of oolitic hematite.     

Sintering characteristics and kinetics of acidic haematite ore pellets with and without mill scale addition

Haematite ore pellets require very high induration temperature (>1573?K) while, magnetite ore pellets require much lower temperature due to the oxidation of magnetite during induration. Mixing of some magnetite in haematite ore can improve the sintering property of pellets during induration. Mill scale is a waste material of steel plant which contains mainly FeO and Fe₃O₄. It can also be blended in haematite ore pellet mix which can enhance diffusion bonding and recrystallisation bonding and facilitate sintering at the lower temperature like magnetite ore. The extent of improvement in sintering property, sintering mechanism and its kinetics in the presence of mill scale is very imperative to study. In current study, the sintering characteristics of acidic iron ore pellet with 15% mill scale and without mill scale has been studied separately through microstructure observation, apparent porosity measurement and volume change. The volume changes due to heating at varying temperature and time has been measured by mercury displacement method and the data has been exploited for sintering kinetics study, wherein, extent of sintering α has a power relation with time. Several kinetics parameters such as time exponent (n), rate constant (k) and activation energies have been estimated for above two pellets and compared. While acidic pellet without mill scale requires 385?k?cal?mol^?1, acidic pellet with 15% mill scale requires only 310?k?cal?mol^?1 activation energy.     

Sintering Performance of Magnetite–Hematite–Goethite and Hematite–Goethite Iron Ore Blends and Microstructure of Products of Sintering

Parallel experimentation allowing comparison of magnetite–hematite–goethite inland and hematite–goethite coastal mill blends in terms of sintering performance is reported. Magnetite–hematite–goethite blend affords slightly lower productivity, tumble index, and yield than hematite–goethite blend. However, magnetite–hematite–goethite blend required 9.2 kg · t^?1 lower solid fuel rate than the hematite–goethite blend. The lower sintering temperature of the magnetite–hematite–goethite blend than that of the hematite–goethite blend contributed to higher reducibility and lower low temperature degradation under reduction. Its sinter product also contained lower proportions of columnar silico-ferrite of calcium and alumina, magnetite, and fayalite.     

Gaseous reduction of iron oxides: Part I. Reduction of hematite in hydrogen

The reduction of high-grade hematite ore in hydrogen has been investigated. There is an unusual temperature effect for small granules with a dip in the rate at about 700°C, similar to those reported by previous investigators for different types of iron oxides. The particlesize effect on the time of reduction suggests that there are three major limiting rate-controlling processes: i) uniform internal reduction, ii) limiting mixed control and iii) gas diffusion in porous iron layer. Processes (ii) and (iii) are special cases of a so-called topochemical mode of reduction associated with the formation of product layers. Unidirectional reduction experiments revealed the significant role played by gas diffusion in porous iron layer as a rate-controlling process. The effective H₂-H₂O diffusivity in porous iron derived from, the reduction data is found to decrease markedly with decreasing reduction temperature. This is consistent with the fracture surfaces of porous iron as viewed by scanning electron microscopy. The present interpretation of the rate of reduction of hematite ore is found to apply equally well to previously published data on the hydrogen-reduction of natural and synthetic hematite pellets.     

Characterization and Processing of Iron Ore Slimes for Recovery of Iron Values

Intensive characterization studies of iron ore slime carried out by X-ray diffraction spectroscope (XRD), scanning electron microscope (SEM-EDS), field emission scanning electron microscope (FESEM), and quantitative mineralogical evaluation by scanning electron microscope (QEMSCAN) are discussed. In slimes, mineral phases like hematite, goethite, gibbsite, kaolinite, and quartz are present in a complex and intricate way. SEM-EDS and QEMSCAN studies indicate that significant amounts of aluminum are associated with both ochreous and vitreous goethite. Hematite and goethite phases are contaminated with some amount of alumina and silica. The liberation of hematite in the coarser fraction (+500 µm) is only 20.6% compared to 40% in the finer fraction (?500 µm) size. A flow sheet, comprising of hydrocyclone and magnetic separation techniques, has been developed to produce an iron concentrate containing ～63% Fe with 70.7% weight recovery from a feed sample containing 56.8% Fe, 5.1% SiO₂, and 6.4% Al₂O₃.     

Effect of TiO2‐content on Reduction of Iron Ore Agglomerates

The effect of titanium oxide on iron ore agglomerates is studied by the use of test sinter, test pellets and synthetic briquettes under laboratory conditions. Titanium favours secondary hematite rather than magnetite, which is the main phase in the sinter of Rautaruukki's Raahe plant. Additionally, the effects of sinter RDI and pellet LTD on the blast furnace process are evaluated using the test results of basket trials in LKAB's Experimental Blast Furnace. The effect of titanium in synthetic hematite is studied as hematite is reduced to magnetite in the RDI test. This occurrence causes deterioration in burden permeability. Synthetic titanium‐bearing iron oxides under controlled conditions are investigated at the University of Oulu. The effect of TiO₂, in solid solution in magnetite, on the magnetite to hematite oxidation is studied separately in order to simulate the final stage of the sintering process. In other experiments, hematite samples doped with various contents of TiO₂ are studied using thermogravimetry under a controlled gas atmosphere (CO/CO₂/H₂/N₂). The TiO₂ content of hematite has a clear effect on reduction degradation. Also increasing content of TiO₂ in solid solution in magnetite radically accelerates the oxidation rate. In the pilot tests, TiO₂ content has a similar negative effect on the reduction strength of both sinter and pellets     

Reduction by CO-CO2 and cracking of iron ore sinters,part 2: cracking

The cracking of iron ore sinters during their low-temperature reduction to magnetite, in the upper part of the blast furnace, is a considerable handicap for the operation of the furnace. However, in spite of numerous previous investigations, it is not fully understood. Dealing with two different sinters, a synthetic and an industrial one, their hematite was reduced to magnetite by CO-CO₂, with varied temperatures and gas compositions. Cracking is strongly favoured by low temperature and low CO partial pressure. Under such conditions, the cracks are initiated by localized nucleation and there is no need for extended stress along the hematite-magnetite interface. The crack propagation is independent of the hematite crystals orientation, and the glassy gangue grain boundaries easily transmit the stress.     

TG-DSC法对莱钢进口铁矿粉烧结性能的研究

采用TG-DSC法对进口铁矿粉添加生石灰的烧结性能进行了研究。实验结果表明,此法可以表征铁矿物烧结过程中发生的物理和化学变化,弄清了添加8．45％生石灰后巴西MBR粗粉、澳大利亚库利安诺宾粗粉和哈默斯利粗粉在烧结过程中产生液相的初始温度、主体矿物熔化温度,以及结晶与凝固温度。三者相比,库利安诺宾粗粉和巴西MBR粗粉比哈默斯利粗粉更有利于低温烧结。实验还发现,铁矿物在熔化和凝固中分别伴随着赤铁矿分解—磁铁矿氧化过程,铁酸半钙存在对磁铁矿氧化有抑制作用。     

Column Bioleaching of a Low-Grade Silicate Ore of Uranium

The bioleaching of a low-grade Indian uraninite ore (triuranium octoxide, U₃O₈: 0.024%), containing ferro-silicate and magnetite as the major phases, and hematite and pyrite in minor amounts, has been reported. Experiments were carried out in laboratory scale column reactors inoculated with enriched culture of Acidithiobacillus ferrooxidans isolated from the source mine water. The pH effect on uranium recovery was examined with the same amounts of ores in different columns. With the presence of 10.64% Fe in the ore as ferro-silicate, the higher uranium biorecovery of 58.9% was observed with increase in cell count from 6.4 × 10⁷ to 9.7 × 10⁸ cells/mL at pH 1.7 in 40 days as compared to the uranium recovery of 56.8% at pH 1.9 with a corresponding value of 9.4 × 10⁸ cells/mL for 2.5-kg ore in the column. The dissolution of uranium under chemical leaching conditions, however, recorded a lower value of 47.9% in 40 days at room temperature. Recoveries were similar with 6-kg ore when column leaching was carried out at pH 1.7. The bioleaching of uranium from the low-grade ore of Turamdih may be correlated with the iron(II) and iron(III) concentrations, and redox potential values.     

Reduction of Iron Oxide Fines to Wustite with CO/CO<Subscript>2</Subscript> Gas of Low Reducing Potential

The reduction of iron oxide fines to wustite between 590 °C and 1000 °C with a CO–CO₂ gas mixture of low reducing potential was studied. The reduction kinetics and the dominating reaction mechanism varied with the temperature, extent of reduction, and type of iron oxide. Reduction from hematite to wustite proceeded in two consecutive reaction steps with magnetite as an intermediate oxide. The first reduction step (hematite to magnetite) was fast and controlled by external gas mass transfer independently of the oxide type and the temperature employed. The second reduction step (magnetite to wustite) was the overall reaction-controlling step, and the reduction mechanism varied with the temperature and the oxide type. Moderately porous oxide fines followed the uniform internal reaction for the temperature range studied. For highly porous oxides, the second reduction step was controlled by external gas mass transfer above 700 °C. Below that temperature, a mixed regime that involves external gas mass transfer and limited mixed control, which comprises pore diffusion and chemical reaction, takes place. The rate equations for this mixed control reaction mechanism were developed, and the limited mixed control rate constant (k_lm) was computed. For denser oxides under uniform internal reaction, the product of the rate constant and pore surface area (k·S) was calculated.     

Mössbauer study of the treatment of nickeliferous laterite ore in argon and hydrogen/water mixtures

The phase changes during the heat treatment of nickeliferous laterite ore from Sukinda, Orissa (India) have been studied by Mössbauer spectroscopy. Mösbauer spectra were recorded at room temperature for the untreated ore sample as well as for laterite samples treated at 200, 400, 600 and 800°C in argon and H₂:H₂O (60:40) mixtures. The data on isomer shift, quadrupole splitting and effective magnetic field have been utilised to identify hematite (α-Fe₂O₃, nonstoichiometric magnetite (Fe_3-zO₄, magnetite (Fe₃O₄), Fe₃Ni and α-Fe, in the samples. The results are discussed together with the observations from the X-ray diffraction and ESCA studies conducted on these samples.     

Rate of the reduction of the iron oxides in red mud by hydrogen and converted gas

The drying and gas reduction of the iron oxides in the red mud of bauxite processing are studied. It is shown that at most 25% of aluminum oxide are fixed by iron oxides in this red mud, and the other 75% are fixed by sodium aluminosilicates. A software package is developed to calculate the gas reduction of iron oxides, including those in mud. Small hematite samples fully transform into magnetite in hydrogen at a temperature below 300°C and a heating rate of 500 K/h, and complete reduction of magnetite to metallic iron takes place below 420°C. The densification of a thin red mud layer weakly affects the character and temperature range of magnetizing calcination, and the rate of reduction to iron decreases approximately twofold and reduction covers a high-temperature range (above 900°C). The substitution of a converted natural gas for hydrogen results in a certain delay in magnetite formation and an increase in the temperature of the end of reaction to 375°C. In the temperature range 450–550°C, the transformation of hematite into magnetite in red mud pellets 1 cm in diameter in a converted natural gas is 30–90 faster than the reduction of hematite to iron in hydrogen. The hematite-magnetite transformation rate in pellets is almost constant in the temperature range under study, and reduction occurs in a diffusion mode. At a temperature of ～500°C, the reaction layer thickness of pellets in a shaft process is calculated to be ～1 m at a converted-gas flow rate of 0.1 m³/(m² s) and ～2.5 m at a flow rate of 0.25 m³/(m² s). The specific capacity of 1 m² of the shaft cross section under these conditions is 240 and 600 t/day, respectively. The use of low-temperature gas reduction processes is promising for the development of an in situ optimum red mud utilization technology.