Magnetite Pellets Research Articles

Interaction between iron oxides and olivine in magnetite pellets during reduction at 500°–1,300° C

In this study, the interaction between magnetite and the additive olivine was studied after oxidation as well as after isothermal reduction at temperatures in the 500°–1,300° C range. In the olivine sample, the forsteritic olivine particles react partly during the oxidation pretreatment to form magnesioferrite and vitreous silica along the particle corona. This breakdown of the olivine particles during oxidation liberates magnesium from the particles, which do not react until temperatures of above 1,150° C in reducing atmosphere. When the hematite in the sample is reduced, and when the temperature is high enough to allow solid-state diffusion at ∼800° C, the magnesium of the magnesioferrite redistributes, so that the magnesium concentration approaches the same level throughout the iron oxide structure. For magnetite, this did not occur until 800° C. At 1,000° C, this magnesium reacts further with the silica in the glassy slag phase, which crystallizes into fayalitic olivine. At this temperature, the magnesium has diffused over distances of more than 600 µm from large olivine particles after 2 hrs reduction. From this point, the primary slag phase in the pellet, until melting, is solid fayalite. Upon reduction to metal, the metallization front concentrates the MgO in the remaining wustite, which can lead to MgO levels of up to 10 mole% locally. The melting point of the fayalite is raised from 1,145° C to a melting range of 1,238–1,264° C due to the MgO increase, as estimated based on phase diagrams, which were adapted to the pellets tested. Much of the olivine that remained unaltered in the oxidation process will be encapsulated by iron before the particles begin to dissolve in reducing conditions and, therefore, plays no role in the reduction before final melting of the particles occurs.

Non-isothermal reduction mechanism and kinetics of high chromium vanadium–titanium magnetite pellets

Non-isothermal reduction kinetics and mechanism of high chromium vanadium–titanium magnetite pellets were studied at 400–1100°C, simulating the lumpy zone of blast furnace conditions. The phase transformation behaviour of valuable elements including Fe, Cr, V and Ti and microstructural changes of reduced pellets were examined by means of X-ray diffraction (XRD) and SEM-EDX. The swelling of reduced pellets was highest at 900°C, while cold crushing strength was highest at 1100°C. Phase transformation behaviour of valuable elements in the lumpy zone is as follows: Fe2O3→Fe3O4→FeO→Fe; Fe2Ti3O9→Fe2TiO4→Fe5TiO8→FeTiO3; V2O3→VO; (Fe0·6Cr0·4)2O3→FeO·Cr2O3→Cr2O3. By analysing the non-isothermal reduction kinetics of high chromium vanadium–titanium magnetite pellets, based on the evaluation of reaction activation energy calculated according to Coats–Redfern method, gaseous internal diffusion through solid product layer and interfacial chemical reaction are most likely to be the main rate controlling steps in the reduction process.

Early Gaseous Oxygen Enrichment to Enhance Magnetite Pellet Oxidation

It is suggested that oxygen enrichment in the gas atmosphere, during continuous heating of magnetite pellets, can cause pellets to be oxidized throughout their volumes, eliminating unoxidized cores. The peculiarities of the oxidation kinetics of magnetite concentrate imply that such oxygen enrichment might be particularly effective at lower temperatures. This suggestion was tested by developing and testing a mixed-control model for pellet oxidation (to allow the sizes of unreacted cores to be predicted), and by experimentally testing the effects of oxygen enrichment at relatively low temperatures (“early oxygen enrichment”). The results confirmed that the extents (depth) of oxidation and pellet strength were both improved significantly by applying oxygen enrichment up to 873 K (600 °C), as part of a heating cycle up to 1073 K (800 °C).

Microstructure of Bentonite in Iron Ore Green Pellets

Sodium-activated calcium bentonite is used as a binder in iron ore pellets and is known to increase strength of both wet and dry iron ore green pellets. In this article, the microstructure of bentonite in magnetite pellets is revealed for the first time using scanning electron microscopy. The microstructure of bentonite in wet and dry iron ore pellets, as well as in distilled water, was imaged by various imaging techniques (e.g., imaging at low voltage with monochromatic and decelerated beam or low loss backscattered electrons) and cryogenic methods (i.e., high pressure freezing and plunge freezing in liquid ethane). In wet iron ore green pellets, clay tactoids (stacks of parallel primary clay platelets) were very well dispersed and formed a voluminous network occupying the space available between mineral particles. When the pellet was dried, bentonite was drawn to the contact points between the particles and formed solid bridges, which impart strength to the solid compact.

Mechanism of Strength Improvement of Magnetite Pellet by Adding Boron-bearing Iron Concentrate

The mechanism of improving compressive strength of magnetite pellet by adding boron-bearing iron concentrate was studied. Boron-bearing iron concentrate and magnetite were mixed, pelletized and roasted under different roasting conditions. Then, compressive strength of pellets was tested, and polished sections of the roasted pellets were analyzed from the perspective of mineralogy. Finally, the effects of different proportions, roasting temperatures and roasting time of boron-bearing iron concentrate on the compressive strength of magnetite pellets were investigated and explained.

The Influence of Gangue and Additives on the Divalent Iron Content of Magnetite Pellets

During the pyroconsolidation of magnetite pellets in an aerobic atmosphere, the mineral practically oxidizes in the beginning stages of the process. However, as the temperature increases, the hematite dissociates into magnetite. The temperature at which this phenomena occurs depends on the additives present and is significantly lower than that corresponding to pure magnetite. In this paper a series of magnetite pellets were formed, varying the alkalinity (CaO + MgO)/(SiO2 + Al2O3) from 0.4 to 1.9, and the MgO/CaO ratio from 0.12 to 1.3. The pellets were pyroconsolidated at temperatures ranging from 1050 to 1350°C. Pellet structure was determined and analyzed using electron microscopy. The influence of additives and gangue composition was studied at the hematite disassociation temperature.

Kinetics of Oxidation Reaction for Magnetite Pellets

An experiment for the oxidation process of single magnetite pellet and theoretical analysis based on modified unreacted core shrinking (MUCS) model were carried out, and the controlling mechanisms of the initial and developing reactions were examined, respectively. From the study of the initial reaction, it was found that the chemical reaction of surface is the controlling step of the overall reaction when the temperature is up to about 750 K, while the mass transfer through the gaseous boundary layer dominates the reaction rate when the temperature is above 750 K. As the reaction developing within the pellet, the mass transfer through the produced layer becomes the controlling step. In addition, the effects of reaction conditions (such as oxygen concentration, temperature) on the fractional oxidation of magnetite pellet were determined.

An investigation of the relationship between compressive strength and dust generation potential of magnetite pellets

Iron ore pellets should have sufficient mechanical strengths against degradation at all stages of pellet production in pelletizing plants. Besides the strength, pellets should have less dust emission during operation since the process efficiency and the pelletizing equipments are adversely affected by dust. Dust is also a problem for sintered (product) pellets since they abrade during transportation from pellet production site to the reduction facilities. Sufficient mechanical strength and low dust emission of pellets are necessary for better operation and handling of pellets. In this study, dust emission mechanism of sintered magnetite pellets produced with different binders was comparatively studied. The results showed that the dust is not produced by pellet breakdown for sintered pellets with sufficient strength. It was found that dust generation of sintered pellets is not directly dependent on the mechanical strength. One of the dust generation mechanisms of sintered pellets with sufficient strength is the roughness of pellet surfaces. The attrition and impact forces during transportation cause dust generation from pellet surfaces. The surface smoothness is more important since the pellets with high strength and rough surfaces produced more dust than those with smooth surfaces and low strength. Half of the fines generated due to pellet attrition or impact forces during handling of sintered pellets will become airborne and are considered as loss and operational/environmental problem. The percentage of particulate matter (PM10) which is significant in health risk lied between 30% and 40% by weight of airborne pellet dust.

The Effect of Calcined Colemanite Addition on the Mechanical Strength of Magnetite Pellets Produced with Organic Binders

Iron ore pellets must have sufficient mechanical strengths against degradation in all stages of pellet production. Low strength is also a problem for product pellets since they abrade during transportation to the reduction furnaces. The use of a binder is necessary to provide sufficient strength to the pellets and for better operation and handling of pellets. Bentonite is the standard binder in the industry; however, it is considered an impurity due to its acid oxide contents. Organic binders have been tested for many years as alternative binder to bentonite. They have been found to give sufficient wet pellet properties. However, they failed to provide sufficient strength to the preheated and fired pellets due to lack of slag bonding. It has been assumed that one possible effective method to improve the preheated and fired pellet strengths is addition of a slag-bonding constituent. In this study, calcined colemanite was added to the pellet feed to overcome the lower strength problem encountered with organic binder use. The strength of pellets produced with organic binders and calcined colemanite alone and in combination was comparatively studied against the strength of pellets made with standard bentonite binder in magnetite concentrate pelletizing. The results showed that addition of calcined colemanite into the pellet mixture improved the preheated and fired pellet strengths of pellets produced with organic binders.

Interaction between Iron Oxides and Olivine in Magnetite Pellets during Reduction to Wustite at Temperatures of 1000^|^ndash;1300^|^deg;C

Interaction between Iron Oxides and Olivine in Magnetite Pellets during Reduction to Wustite at Temperatures of 1000^|^ndash;1300^|^deg;C

Interaction between Iron Oxides and Olivine in Magnetite Pellets during Reduction to Femet at Temperatures of 1000–1300°C

The addition of MgO to iron ore pellets is known to beneficially influences many high temperature reduction properties such as reducibility and swelling. When the pellet is metallized, MgO dissolve ...

The bonding/strengthening mechanism of colemanite added organic binders in iron ore pelletization

The addition of boron compounds into pellet mix is proposed as a potential solution to overcome the insufficient compressive strengths of preheated and fired pellets produced with organic binders. Colemanite was tested as an alternative binder to bentonite either alone or in combination with some organic binders on magnetite pellets. The performances of the tested binders on pellet qualities: balling, wet pellet moisture content, drop number, compressive strengths and microstructure of pellets have been compared with the performances of reference bentonite binders. The results of the tests showed that the qualities of pellets are insufficient when organic binders and colemanite are used as binders alone. The former failed to provide sufficient preheated and fired pellet strengths, the latter failed in terms of wet and dry pellet quality. However, good quality wet, dry, preheated and fired pellets could be produced with the combination of these two binders. In addition, stronger pellets could be produced at lower firing temperatures like 1373K thanks to addition of colemanite. The bonding mechanism of colemanite containing pellets was examined by scanning electron microscopy. It was found that the improved preheated and fired compressive strengths of colemanite-added pellets were due to the physical melting of colemanite at the contact point of magnetite grains during thermal treatment.

High-Temperature Oxidation Behavior of Vanadium, Titanium-Bearing Magnetite Pellet

By means of isothermal oxidation and chemical analysis, great importance was attached to the parameters that made effects on the oxidation degree of vanadium, titanium-bearing magnetite pellet in high-temperature processing (1073 — 1323 K). Based on the experimental data, oxidation kinetics of pellet was analyzed according to shrinking unreacted-core model subsequently. Experiment results display that the oxidation degree of pellet increases with increasing of oxidation time, oxidation temperature and oxygen content, as well as shrinking of pellet diameter. Under the condition of oxidation time 20 min, oxidation temperature 1 223 K, oxygen content 15%, and pellet diameter 12 mm, oxidation degree of pellet reaches 92.92%. The analysis of oxidation kinetics indicates that oxidation process of pellet is controlled by chemical reaction with activation energy 68. 64 kj/mol at a relatively lower temperature (1073 — 1173 K). Oxidation process of pellet is mixed-controlled by chemistry reaction and diffusion with activation energy 39. 66 kj/mol in the temperature range of 1173–1273 K. When oxidation temperature is higher than 1 273 K, the limited link of oxidation reaction is the diffusion control with the activation energy 20. 85 kj/mol. These results can serve as a reference to the production of vanadium, titanium-bearing magnetite pellet.

Oxidation of iron-ore pellets

The oxidation of magnetite in the roasting of iron-ore pellets is considered, with particular attention to the influence of various technological factors on the process. Laboratory data are used in optimizing the design and operational parameters of individual zones in conveyer roasting machines for the heat treatment of magnetite pellets.

Binding effects in hematite and magnetite concentrates

An industrial taconite facility, ‘Plant F’, processed both magnetite and hematite ores during the year. The concentrates were pelletized with a binder, bentonite. Plant personnel believed less bentonite was required to make hematite pellets. Thus, the authors intended to quantify in-plant observations through bench-scale pelletization tests. As-received magnetite and as-received hematite were pelletized and tested for wet-drop number and dry-crush strength. Hematite pellets exceeded industrial minimum wet-drop and dry-crush values of 5 drops and 22 N/pellet without bentonite addition, while magnetite pellets exceeded industrial minimum values at a bentonite dose of 6.6 kg/t (0.66%). It is known that finer particles increase pellet strength, so additional magnetite was ground to a similar particle size distribution as the as-received hematite. The ground magnetite was pelletized and tested for wet-drop number and dry-crush strength. Wet drop and dry crush values increased after grinding the magnetite concentrate. However, they were significantly less than hematite pellets at similar bentonite doses. Consequently, particle size effects were not the dominant cause for higher strengths in the hematite concentrate.

Direct Reduction Behaviors of Composite Binder Magnetite Pellets in Coal-based Grate-rotary Kiln Process

Directly reduced iron is an imperative burden for EAF (electric arc furnace) to make good quality and special steel. As an alternative of direct reduction processes, an innovative coal-based direct reduction of composite binder magnetite pellets in grate-rotary kiln has been developed at Central South University and put into operation. A study of the direct reduction behaviors of composite binder magnetite pellets was carried out in a simulating coal-based grate-rotary kiln process in this paper. Preheated pellets made of magnetite concentrate and composite binder and fired oxide pellets containing bentonite as binder were directly reduced using non-coking coal as reductant in a tube furnace, and their reduction behaviors have been demonstrated by measuring reducibility, variations of compress strength, porosity, phases changes and micro structure under XRD, SEM and optical microscopy. It is shown that preheated pellets possess much better reducibility than fired oxide pellets. Superior reducibility of preheated pellets should be ascribed to their higher effective diffusivity due to higher porosity. The compressive strength of preheated pellets climbs quickly after reducing for 30 min and achieve a high value at the end of reduction, leading to smooth and intact metallized pellets whereas the compressive strength of metalized pellets from reducing of fired pellets is much lower, more cracks and fractures being formed. The preheated pellets possess self-curing function because of the multiple functions of composite binder, eliminate cracks and fractures and keep pellets intact during reducing, depressing reduction degradation and preventing accretion in rotary kiln.

Preheating and roasting characteristics of hematite–magnetite (H–M) concentrate pellets

Regressive orthogonal design experiments have been carried out based on the analysis of the oxidation and induration behaviours of individual magnetite and hematite pellets. The research shows that the optimal pellet preheating time and temperature, and roasting time and temperature change with hematite/magnetite (H/M) ratio. Under the conditions of preheating temperature 900°C, preheating time 10 min, roasting temperature 1275°C and roasting time 15 min, the compression strength of preheated and roasted pellets is found to be respectively more than 444 N/pellet (N/P) and 3993 N/P when the magnetite content in the pellets exceeds 20%. The pellet strengths meet the requirements of rotary kilns and blast furnaces respectively.

Application of ported kiln technology to the Grate-Kiln Process at Minntac

In 2001, the Minntac Division of US-Steel retrofitted one of their grate-kiln iron ore pelletizing lines with a ported-kiln air-injection system (PKAIS), the first of its kind in the world for oxide pellets. The ported kiln system injects air under the bed of pellets in the rotary kiln, causing rapid oxidation of the magnetite pellets. This rapid oxidation allows for lower kiln temperatures, which reduces fuel demand; allows for less heat load on the annular cooler; and produces higher pellet quality. Both the process and the equipment are patented developments by Metso Minerals. This paper discusses the process, the key equipment and the benefits to both the pellet plant and the downstream blast furnace operations.

Oxidation of magnetite concentrate powders during storage and drying

Oxidation of magnetite pellet concentrates (from LKAB, Sweden) during drying and storage was studied using thermogravimetric analysis (TGA). The Fe 2+content of a standard LKAB pellet feed decreased by 0.2% during heat treatment at 105 °C for 2 days. The results indicate that magnetite concentrates of pelletizing fineness already start to oxidize to γ-hematite during drying at 105 °C, although 105 °C is recommended by ISO 7764 and ISO 3087 and given as the maximum allowable drying temperature in ISO 3082. The importance of drying time for the amount of magnetite oxidized should also be recognized, but is not mentioned in the ISO standards. The sensitivity of magnetite particles for oxidation during drying could not be predicted by measuring the BET surface area. A simple isothermal TGA run gives exact experimental data and is recommended as a standard procedure before choosing the drying temperature for magnetite concentrate samples. Oxidation during storage at room temperature was observed as well. The Fe 2+content in a standard Luossavaara-Kiirunavaara (LKAB) pellet feed decreased by 0.3% Fe 2+during 4 years of storage at room temperature. Oxidation during storage was completely hindered when the samples were stored in a freezer at −50 °C. Therefore, freezer storage is recommended for all magnetite reference materials. These materials are used over several years and are especially susceptible to oxidation because they are usually finely ground and high in Fe 2+content. The results also show that if the magnetite sample has partly oxidized to γ-hematite, the oxidation curve obtained by TGA will overestimate the Fe 2+content of the sample, if background correction is necessary.

Sintering of magnetite pellets under oxidizing, neutral and reducing atmospheres

Iron ore concentrate pellets are now sintered (induration heat hardened) under oxidizing conditions. Pellets produced from magnetite ore concentrate are completely oxidized during sintering to hematite pellets. These pellets then require a blastfurnace to reduce the hematite to metallic pig iron. Partially reduced pellets would save energy and fuel during transportation and further reduction. It has been stated in the literature that "the most important single factor responsible for the development of strength in fired magnetite pellets containing no additive is the oxidation of the magnetite to hematite and the subsequent recrystallization and grain growth of this constituent. " However, experiments shown in this paper reveal that oxidation is not necessary to produce high-strength pellets. Magnetite concentrate pellets developed similar strength when sintered at 1,350°C (2,460°F) under either oxidizing (air) or inert (argon) atmosphere; the pellets sintered under argon remained magnetite. In addition, magnetite concentrate pellets sintered under reducing (CO) atmosphere also remained as magnetite, but required induration temperature of approximately 150 °C (300 °F) less than pellets sintered under the other atmospheres.