MgOとFe_2O_3との反応とそのマグネシアの焼結に対する影響

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The tablets consisting of chemically purest Mg(OH)_2 and 0〜1.5 wt per cent Fe_2O_3 were fired at 1000℃〜1550℃ to examine the appearance after the firing as well as the microstructure. The firing shrinkage and the bulk density began to increase markedly from 1000° to 1200℃ (Figs. 1〜4). At 1200℃ the coloured regions, which had developed by the diffusion of iron from Fe_2O_3 grains, began to spread abruptly with the remarkable growth of periclase crystals. From the results mentioned above, it may be inferred that the sintering of Mg(OH)_2 occurs in two successive stages; (1) up to 1200℃ the periclase crystals formed by the decomposition of Mg(OH)_2 come into intimate contact each other showing the remarkable shrinkage of the specimens, and (2) in the second stage during which the growth of periclase crystals takes place with the rapid diffusion of iron from Fe_2O_3 through the contact points between periclase grains. The mechanism of the reaction of Fe_2O_3 and MgO examined by poralization microscope and X-ray diffraction may be interpreted clearly with the aid of the phase diagram of the binary system MgO and Fe_2O_3 (B. Phillips, S. Somiya, and A. Muan. J. Am. Ceram. Soc., 44,169 (1961)). The reaction is a kind of the counter diffusion processes. MgO diffuses into Fe_2O_3 grains to form magnesioferrite. With the elevation of temperature, Fe_3O_4 content of the magnesioferrite becomes higher until it changes to very dark colour. On the other hand, Fe_2O_3 is reduced into ferrous state which duffuses into MgO to form a solid solution, magnesiowustite. With increasing temperature the content of ferrous iron increases up to the saturation limit with a result that the further introduction of iron oxide to the periclase phase gives rise to the formation of magnesioferrite. The acicular hematite crystals are exsoluted from the magnesioferrite during cooling, which distribute between the grains of magnesioferrite, as the solid solution limit of Fe_3O_4 decreases with decreasing temperature. Also magnesioferrite is exsoluted from magnesiowustite and spread throughout the periclase phase, as the solid solution limit of FeO in periclase decreases grately with decreasing temperature. The results of the experiments mentioned above suggest that the following conditions should be held in order to maintain the higher FeO content of magnesia clinker : (1) In order to enlarge the area of the solid solution limits of FeO in periclase, and of Fe3_O_4 in magnesioferrite, it is desirable that the firing temperature is kept as high as possible. (2) To maintain the conditions in higher temperature the clinkers should be cooled quickly. (3) To accelerate the reactions between Fe_2O_3 and MgO the grain size should be reduced as far as possible. In practice the refractories can not be cooled rapid enough to prevent the exsolution of FeO. If we consider only this phenomenon, it seems to be inferred that the magnesia clinker mineralized with Fe_2O_3 is not suitable for the fired refractories, and if the clinker mineralized with Fe_2O_3 is used, unburned refractories seem to be prefered rather than the fired ones. From the experimental results concerning the relation between the firing shrinkage and Fe_2O_3 contents, it is suggested that only a small amount of Fe_2O_3 would be sufficient for the mineralizer of magnesia clinker. During the heating up the growth of periclase grains proceeds gradually together with the diffusion of iron from Fe_2O_3 grams the oxygen releases by the reduction growth of periclase and concentrate in Fe_2O_3 grains. This tred is, not only important in interesting in connection with various phenomena occuring in the basic refractories used in, for example, open hearth furnaces.
社団法人日本セラミックス協会の論文
1962-12-01

MgOとFe_2O_3との反応とそのマグネシアの焼結に対する影響

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