In this work, the solid state diffusion of magnesium was studied in magnetite based pellets at temperatures between 500 and 1000uC. 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 500uC. At temperatures of 800uC and above, the magnesium in the magnesioferrite diffused out to the secondarily formed magnetite and wü stite. During reduction at 600-700uC, 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.

No abstract

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°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°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°C. The greatest difference in reduction degree appeared at 1 300°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 μm) after isothermal reduction at 1 000-1 300°C. It was found that, by using a fine olivine (<38 μm) the whole magnesium content of the olivine was dissolved evenly in the wustite and in the slag, already at 1 000°C, in agreement with the equilibrium tie-lines of the FeOMgO-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°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 ~600 μm, or occasionally at distances of more than 1 mm. For the coarse olivine the main rise in magnesium occurs at 1 200°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 ~50°C, according to thermodynamic calculations.

No abstract