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 500º-1,300º 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 ~800º 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 1,145º C to a melting range of 1,238-1,264º 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.

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 dissolved in the wustite concentrates in the unmetallized part, which is why MgO-levels much higher than the average concentration could be expected locally. In this work the impact of the elevated MgO-content on the reduction at 1 000-1 300 degrees C was studied by SEM-EDS. The MgO content in the pellet was also varied by additions of a), highly reactive olivine b) unreactive olivine c) combined addition of reactive olivine and fine quartzite and d) combined addition of unreactive olivine and fine quartzite. Two cases of metallization were observed 1) a gradual reduction front with only moderate magnesium levels and 2) a sharp reduction front with strongly elevated magnesium levels before the metal front. The samples with added quartzite reduced a little better at 1 100 degrees C, compared to those with only olivine, but apart from that, reduction was not affected much by the additives in the range 1 000-1 200 degrees C. The greatest difference in reduction degree appeared at 1 300 degrees C where a metal skin formed in most samples, hindering further reduction. At this temperature, the sample with addition of only reactive olivine had superior reducibility due to a porous morphology of the iron being mantained throughout the experiment.

In this study the magnesium diffusion behaviour was studied in pellets with fine and course olivine, with and without additional fine quartzite (<20 mu m) after isothermal reduction at 1 000-1 300 degrees C. It was found that, by using a fine olivine (<38 mu m) the whole magnesium content of the olivine was dissolved evenly in the wustite and in the slag, already at 1 000 degrees C, in agreement with the equilibrium tie-lines of the FeO-MgO-SiO2 phase diagram. This lead the liquid slag to precipitate into fayalitic olivine and the Al, Na, K, Ca, P-content to enrich in remaining inclusions in the olivine. This crystallization did not occur in the sample with only bentonite addition, or in the sample with unreactive olivine at these temperatures. However, with further addition of fine quartzite, the slag of the sample with coarse olivine also crystallized. In the samples reduced at 1 000-1 100 degrees C, magnesium gradients could be detected in the slag phase around coarse olivine particles until entering the interaction volume of an interfering particle at around similar to 600 mu m, or occasionally at distances of more than 1 mm. For the coarse olivine the main rise in magnesium occurs at 1 200 degrees C when the olivine particle cores begin to dissolve. The dissolution of all magnesium of the 2.5% olivine addition during oxidation lead to 6.5% Mg in the crystallized slag phase. The increase in melting point resulting from this compared to fayalite with no magnesium is similar to 50 degrees C, according to thermodynamic calculations. .

In the present study, magnetite pellets with the additives olivine, calcite and quartzite were isothermally reduced in a tubular furnace to study the interaction between iron oxides and the additives. Exaggerated amounts of additives were used in order to enable analyses of phases that do not otherwise occur in sufficient amounts for X-ray diffraction and EDS-analyses. The reduction was set to yield either magnetite or wüstite in the temperature range 500-1300ºC. For olivine, reduction tests were also performed to allow metallization in the range 1000-1300ºC. The mineralogical phases which had formed were studied after oxidation as well as after reduction. The results showed that it was possible to identify, by X-ray diffraction, the main phases formed by the additives in all samples, after oxidation as well as reduction.In the olivine sample, the forsteritic olivine particles react partly during the oxidation pre-treatment 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 1150°C in reducing atmosphere. When the hematite in the sample is reduced, and when temperature is high enough to allow solid-state diffusion at ~800ºC, the magnesium of the magnesioferrite redistributes so that the magnesium concentration approaches the same level throughout the structure. For magnetite, this does not occur below 800°C. At 1000°C, this magnesium reacts further with the silica in the glassy slag phase, which crystallizes into fayalitic olivine. At this temperature the magnesium diffuses over distances more than 600µm from the olivine particles. From this point the binding media to resist the swelling tensions in the pellet is mainly solid fayalite. The metallization front concentrates the MgO in the remaining wustite which can lead to MgO levels of up to 10% locally. The melting point of the fayalite is raised from 1145ºC to a melting range of 1238-1264ºC due to the MgO-increase, as estimated based on the phase diagram tuned to the pellets tested. Much of the olivine which remained unaltered in the oxidation process will be encapsulated by iron before the magnesium begin to dissolve in reducing conditions, and therefore play no role in the reduction before final melting of the particles occur.The quartzite particles are not affected by the oxidation pre-treatment. The binding strength of quartzite pellets therefore comes from the sintering of quartzite particles to neighboring hematite as well as the glassy slag resulting from the acid gangue and the bentonite. Substantial reaction of the quartzite particles during reduction did not occur before 1000ºC even though the process has occurred to a very low extent already at 900ºC. Also the glassy slag crystallizes into fayalite in the presence of quartzite. From this point fayalite represents the binding media in the pellet. Pure fayalite melts already at 1177ºC and can at this temperature dissolve up to 76wt% FeO. This leads to early softening, which is one of the main concerns for the softening/melting properties of the pellet. In the pellets with calcite, CaO reacts with Fe2O3 during induration to form a low-melting calcium ferrite slag in the pellet that melts to react with silica in the pellets. If more calcium is added than what is required to react with the silica, calciumferrites becomes part of the binding mass together with the dicalciumsilicate. The calciumferrites forming in pellets with larger additions of calcite are weak to resist the tensions arising due to the low-temperature reduction of hematite and are associated with low temperature disintegration. As the reduction proceeds to wustite, the calcium from the ferrite dissolves in the wustite so that porous calciumwustite forms. The dicalciumsilicate remain stable during the entire reduction until reaction and melting of the phase begin at 1283ºC.

The solid-state diffusion of magnesium through iron oxides is important in understanding the oxidation and reduction of iron ore pellets containing olivine. In this study, a diffusion couple was made of olivine (containing 50% MgO) and magnetite. Initially, experiments were conducted under oxidizing atmosphere at a temperature above 1,250°C. A new 100-micron thick layer of magnesioferrite and 11% Mg now appeared at the interface of the couple. In the oxidized iron oxide layer there was no detectable presence of Mg. Subsequently, the oxidized samples were subject to reduction atmosphere sufficient for formation of wustite. The layers of oxidized magnetite (i.e. hematite) and the magnesioferrite reduced to wustite. In the reduced samples, the magnesium level in the wustite by the interface was found to be 1.5 wt%. At distance of 700-1,000 μm from the olivine boundary, the magnesium levels went below detection limit. A diffusion coefficient was estimated from the data. The implication of these observations in iron ore pellets will be discussed.

In this work, the solid state diffusion of magnesium was studied in magnetite based pellets at temperatures between 500 and 1000°C. The samples were laboratory produced pellets with a largely exaggerated addition of olivine. The results showed that the olivine particles after oxidation had decomposed along the particle boundary and turned into magnesioferrite crystals and pyroxene/vitreous silica. Large patches of magnesioferrite rich in magnesium oxide were spread out among the haematites in the interior of the pellet. In the subsequent reduction, the haematite was converted to magnetite at 500°C. At temperatures of 800°C and above, the magnesium in the magnesioferrite diffused out to the secondarily formed magnetite and wü stite. During reduction at 600-700°C, cracks appeared along this boundary as the haematite transferred into secondary magnetite. Comparison to a commercial olivine pellet showed that the diffusion of magnesium followed the same pattern as in the laboratory pellets

Magnetite-based pellets with large amounts of the additives olivine, calcite and quartzite were isothermally reduced in a tubular furnace to study and describe the reaction behaviour of the additive minerals in the pellets. The reduction was thermodynamically set to yield wustite at three different temperatures: 900, 1000 and 1150 degrees C. The mineralogical phases that had formed before and after reduction were studied by Scanning electron microscope and X-ray diffraction. The pellets with the different additives were different already before reduction due to different reaction behaviour during induration: The results showed that it was possible to identify the main reactions during reduction for pellets with all three additives. All but the very small quartzite particles remained unreactive in reducing atmosphere until they began to form a fayalitic melt at 1000 degrees C. The calcium ferrites of the pellets with calcite reacted to form a porous calciowustite already at 900 degrees C. In the pellets with olivine, the magnesium, which had constrained into magnesioferrite pockets after induration, redistributed into the entire iron oxide structure at 900 degrees C and also reacted with silica at 1000 degrees C. The olivine core which had not reacted during induration did not appear to react in reducing conditions at temperatures of 1150 degrees C and below. These reaction mechanisms have indicated a potential to reduce the required amounts of additives in the pellets.

In the present study, magnetite pellets with large amounts of the additives olivine, calcite and quartzite were isothermally reduced in a tubular furnace to study the interaction between iron oxides and the additives. A first attempt at using exaggerated amounts of additives was made in order to enable analyses of phases that do not otherwise occur in sufficient amounts for Xray diffraction and EDS-analyses. The reduction was thermodynamically set to yield either magnetite or wüstite at three different temperatures, 900, 1000 and 1150°C. For olivine, reduction tests were also performed at 500, 600, 700 and 800°C. The mineralogical phases that had formed were studied after oxidation as well as after reduction. The results showed that it was possible to identify, by X-ray diffraction, the main phases formed by the additives in all samples, after oxidation as well as reduction.The quartzite particles were shown to have remained quite intact after the oxidation treatment, except for small particles in the presence of impurities that formed melts. During reduction the quartzite particles reacted with iron so that fayalitic melts were formed already at 1000°C. After reduction at 1150°C all quartzite had transformed into a fayalitic melt so that most of the small pores had disappeared through sintering or had been filled by fayalite.In the sample with calcium oxide the additive particles had reacted during the oxidation treatment and formed calcium ferrites and calcium silicates. Upon reduction, the ferrites that formed during oxidation reduce, so that a porous calciowüstite becomes the primary phase already at 900°C. Calcium silicates that were formed during oxidation also remain in the sample as silicates during reduction.The results showed that the olivine after oxidation had reacted along the particle boundary and turned into magnesioferrite crystals and pyroxene/vitreous silica. Magnesium is liberated when the olivine particle breaks down, and finally ends up as islands of magnesioferrite surrounded by hematite in the original magnetite particles. In the pellet core the magnesium has diffused relatively long distances so that the magnesioferrite islands are not just found close to-, but also further away from the olivine particles. Upon reduction, the hematite converts to magnetite already at 500°C and in the tests carried out at 500-700°C, cracks were observed along the hematitemagnesioferrite boundary. At 800°C, temperature is enough to allow slow diffusion of magnesium from the magnesioferrite to the surrounding magnetite or wüstite, and at 900°C the cracks around the magnesioferrite phase disappear. The Mg stored in the wüstite then reacts with the silica slag in the sample when it approaches its melting point at 1000°C. The magnesium level in the wüstite then approaches a background level which was found to be about 2% after reduction for 2 hours at 1150°C.