Ορυκτολογική μελέτη και ιδιότητες βασικών πυριμάχων και νέων μαγνησιοσπινελικών συνθέσεων που παρήχθησαν από μαγνησίτη της Β. Εύβοιας

Abstract

The present thesis deals with the systematic study of the chemical, mineralogical composition as well as the natural and technological properties of several Greek magnesia materials, derived from magnesite, in order to reach conclusions not only fοr the improvement of the final products but also for the development and manufacture of new ones which, in turn, will allow the greek basic refractories to remain at the forefront of the international developments . Specifically, have been studied: 1) Raw materials. The dead- burned magnesia grains examined are divided into two groups according to the mode of beneficiation of the raw magnesite. A). Dead –burned magnesias of E21A, E12E, NE3 were produced from natural microcrystalline magnesite by firing at temperatures in the range of 1800-2000 oC in rotary kilns. Prior to dead burning magnestite was beneficiated by conventional methods (optical sorting, heavy media, magnetic separation). B) Dead –burned magnesias of Magflot Super A and Magflot A1 were produced from natural microcrystalline magnesite beneficiated by the process of froth flotation, which ensures homogeneity and low silica. The obtained filter cake is calcined to caustic magnesia, which is subsequently briquetted and finally dead burned in shaft kiln at temperatures up to 2000 –2200 oC. Chinese fused magnesia has been also studied in order to be compared with greek materials. 2) Magnesia bricks produced by VIOMAGN; MbE21A from magnesia E21A and MbMagflot Super A from Magflot Super A. In addition, a magnesia chromite brick derived from raw materials of Magflot Super A and chromite from China was studied. 3) New spinel-based compositions and new magnesia-spinel refractory materials which have been produced for the needs of this study. The chemical composition of the materials examined here lie essentially in the MgO-CaO-SiO2 system, since they contain only trace amounts of Fe2O3 and Al2O3. Although their CaO/SiO2 ratios can be used to approximate their mineralogical compositions, detailed XRD, and optical and analytical SEM studies revealed significant deviations from the expected phase assemblages. Their microstructures vary widely in terms of proportions of direct MgO-MgO bonding, amounts and phase constitution of the siliceous bonding and size of the periclase crystals. The qualities of Magflot and Fused Chinese Magnesia contain secondary phases at lower levels compared to the qualities E21A, E12E and NE3. The chemical compositions of the qualities E21A, E12E and NE3 favored the formation of secondary calcium-silicate phases of low melting point. (such as C3MS2, CMS), which reduce the refractoriness of the materials. The solid solubility of CaO in periclase varies between crystals of the same sample. Iron was always found in solution to periclase as wustite. The new spinel-based compositions were produced in a pilot plant at Madoudi of Evia and contain alumina ranging from 50% to 70% and additives of either ZrSiO4 (0,5-2%) or chromite (2-5%). Increase in the amount of Al2O3 in the spinel compositions with ZrSiO4 and chromite lead to an increase in the bulk density as well as in the amount of spinel formed. The stabilizing additive of silica zircon favored the formation of a refractory phase with increased melting point, which controls the formation, the amount and the type of the other secondary phases. The Cr2O3 of spinel compositions which enters mainly the lattice, increases the density and acts as a core for the formation of the spinel. Furthermore, the direct diffusion bondings are stronger than those of the spinel crystals in the spinel compositions without chromite and increase the resistance of spinel against the chemical corrosion from fused metallurgic slags. The secondary calcium-aluminate and calcium- silicate phases, which were traced in the eighteen spinel-based compositions occur in low amount, and not all of them are equilibrium phases. From the new spinel-based compositions, six new magnesia-spinel refractory materials were produced. The compositions with 10 and 20% Al2O3 were derived from : 1 Magflot and spinel composition R551Z which contains 50% alumina-50% Magflot and silica zircon as additive. 2 Magflot and spinel composition R559z that contains 70% alumina – 30% Magflot and silica zircon as additive, 3 Magflot and spinel composition R573cr that contains 70% alumina – 30% Magflot and chromite as additive. The compositions of the magnesia spinel refractories produced are expected to show endurance in thermal shocks as well as in the corrosion from slags and friction, in order to have a wide application in steel, cement industry etc. Chemical, mineralogical analyses as well as estimation of bulk density, thermal coefficient factor and cold crushing strength parameter, of the new magnesia-spinel refractory materials, revealed that: a) the S-MAL37z-20 and S-MAL37cr-20 qualities which contain high density spinel compositions (R559z and R573cr respectively), and Al2O3 of about 20 (% wt) exhibit high endurance in rapid thermal changes and in corrosion, b) the S-MAL55z-10, S-MAL55z-20 and S-MAL55cr-10 qualities show high endurance in crushing.