Fused periclase is finding increasing use for the production of refractories and powders, and also as an electrical insulating material in electrical engineering and certain other industries.The main criteria of the quality of this material are the magnesia concentration and the impurity oxide content. High demands are placed on the chemical composition of electricalengineering periclase in which the main-component concentration (MgO) for various grades (GOST 13236-73) is 95-98% and above. Metallurgists use powdered fused periclase containing 93-97% MgO. The behavior of refractories in electric-insulating materials in service is also determined by their fine structure and degree of crystal defectiveness, the mineral and spatial form of impurity distribution, and other factors.Fused periclase is produced from magnesite from the Satkinsk, Talsk and Kirgiteisk deposits, Kul'dursk brucites, and calcined magnesia from the production of Soviet and foreign chemical factories. Electrlcal-engineering periclase of high quality is obtained from chemically beneficiated magnesites [i, 2].The chemical composition and electrical properties of fused periclase (Table i) show that the quality of the product largely depends on the chemical composition of the materials ( Table 2). The chemical composition of brucite and magnesite may be improved by crushing before fusing to obtain lumps less than 60 mm in size, followed by screening off the grains below i0 mm in which the largest amounts of impurities are concentrated. The magnesia content is thereby raised by 1-1.5%.Experimental studies and practical work at Bogdanovich have established that the high purity of the periclase (with regard to chemical composition) is not always the factor determining the best electrical properties. For example, periclase fused from chemically pure magnesia corresponds to class-3 in terms of electrical resistance (GOST 13236-73). At the same time, periclase obtained from Satkinsk magnesite with a high content of Fe203 in some cases meets class-2 demands for electrical resistance.The original materials undergo complex physicochemical changes during melting. The growth of periclase crystals is accomplished in different sections of the block as a result of five basic processes (Fig. i, l-V). The block thus acquires a zonal structure [3]. Due to the irregular temperature field, disparate growth of crystals with thedifferent mechanisms and the action of gravitation as the periclase melts, there is marked differentiation in the melt's components, and the various zones of the block are enriched with magnesia, while the impurities accumulate in a skin, the central zone, and the melter dust. Most redistribution in the block is experienced by the calcium oxide and the silica, and the least by the iron and aluminum oxides [3].Trial industrial meltings with various materials showed that the block structure, the nature of the zones, and the amount of impurities in the periclase can be regulated through batch feed, melting rates, and cooling rates. For example, per...
Fused periclase is widely used in the manufacture of several types of refractory articles and also as an electrical-insulation material in tubular electric heaters (TEH). During the manufacture of the electric heaters, the metal tubes filled with fused periclase are compressed on roller machinery. As a result, there is a significant compaction of the powdered periclase, for example, from an apparent density of 2.30-2.35 to 2.90 g/cm 3. Therefore, it seemed important to study the microhardness of periclase and the possibility of lowering this parameter with the aim of improving the compression process and the pressing of the refractory articles.The mierohardness of minerals is associated with the strength of the crystal. As Lomonosov stated; "the harder the solid, the more strongly associated are its particles" [i]. In the classical crystallochemical works by GolVdshmidt [2] and Fersman [3], the hardness is associated with the strength of the crystal lattice of the mineral which depends on the cation and anion charges and the interatomic distances.A further development of crystallochemical theorywas made by A. S. Povarennykh [4,5] who suggested that as well as the valence (charge) of the ions and the interatomic distances, the coordination number, the packing density of the atoms, the strength of the ionic and covalent bonds, and the structure of the electron shell must also be taken into account.However, the dependences obtained on the basis of crysta!lochemical hypotheses only give approximate values for the hardness, for example, for periclase by comparison with other minerals.In fact, the hardness is a strength characteristic and can change significantly even for the same crystal depending on the plane chosen for the measurement, the presence of dislocations, and other factors [6]. One of the important factors affecting the strength of crystals and, consequently, their hardness is the interaction between dislocations [7]. The interaction between dislocations leads to an increase in the resistance to movement in the plane where the dislocations lie in proportion to the accumulation of defects and to the strengthening (increase in hardness) of the crystal.An important effect on the hardness of crystals is provided by defects and impurities. Their action is twofold: on the one hand, the defects in the structure help to lower the hardness and, on the other, they strengthen the crystal by preventing the free movements of the dislocations.The strengthening of the structure can be explained by the fact that when the dislocation passes a hindrance (structural defects, inclusion, impurity of atom of a different size) it is forced to bend round the hindrance (to form a loop) and then straighten out again (Fig. i). Such a process is accompanied by the elongation of the dislocation and the local distortion of the structure for which an additional expenditure of energy is required. As a result, the defective part of the crystal is characterized by a high resistance to deformation, i.e., by an increase in hardness.As in ot...
Fused periclase is being increasingly used as an effective refractory and insulating material for electric-heating devices. Particularly heavy demands are placed on periclase used in the form of powdered electric-insulating material.The new GOST 13236-83 for powdered electrotechnical periclase specifies, for the highest grade powders, a MgO content of not less than 97%, t Fe203 not more than 0.08%, CaO not more than 0.7%; and for first class powders --MgO not less than 96%, Fe=03 not more than 0.12%, and CaO not more than 1.3% [i].In the USSR electrotechnical periclase powder is prepared mainly from natural raw material: brucites from the Kul'dursk, and magnesites from the Kirgiteisk deposits [2].However, even from the purest forms of this raw material, for example, Kirgiteisk magnesite with a content of 0.04-0.05% Fe203 and 97% MgO, powders of the highest grade, corresponding to GOST 13236-83, are not obtained. Electrotechnical periclase powders of the first class are not always obtained, even from adequately pure natural raw material. The preparation from natural raw material of tiles for gate valves used on steel ladles normally provides a single useage, which is inadequate. One cause of this is the increased content in the periclase of silica, and the unfavorable CaO:Si02 ratio.Significantly higher and more stable properties should be exhibited by fused periclase prepared from raw material with a high content of magnesium oxide (98.0-99.9%) and correspondingly a lower amount of impurity compounds. The most undesirable impurity ions and elements are Na +, K +, CI-, S ~-, [SO~] 2-, Fe 3+, B 3+, C, and certain others.We investigated this problem in order to obtain fused high-grade periclase and electrotechnical powders based on it, satisfying the requirements of the highest class as specified by GOST 13236-83.The periclase was prepared from raw material with a high MgO content obtained by different chemical methods, including the chemical beneficiation of magnesites. The following starting materials were used for melting [3]: chemical purity grade magnesium oxide; magnesium oxide obtained by the hydrolysis method from solutions of MgCI2 which is formed during the processing of carnallites; magnesium oxide obtained from bischofite, consisting of magnesium chloride hexahydrate (MgCI='6H20); magnesium oxide prepared by the ammonium sulfate method by reaction of a solution of magnesium sulfate MgSo~ with a concentrated aqueous solution of ammonia according to the reaction MgSO~ + 2 NH~OH § Mg(OH)2 + (NH~)2SO,, and then double staged calcination, as a result of which the Mg(OH) is changed into MgO; magnesium oxide prepared from caustic magnesite using the soda-sulfate method, by which magnesite is dissolved in sulfuric acid, the magnesium sulfate is precipitated with sodium carbonate, followed by thermal decomposition of the magnesium carbonate and calcination at 800-900~ magnesium oxide obtained from magnesite by the nitric acid method.Material from bischofite or chemically pure MgO contained more than 99% MgO; the ...
In connection with the new standard which has been developed for electrical-engineering periclase, the East Institute of Refractories in cooperation with the Bogdanovich Refractory Plant have analyzed the standard indices for electrical resistivity and the breakdown voltage Ubr laid down in COST 13236-73, "Periclase, electrical-engineering" which continues to be effective until July 1, 1983. The statistical analysis of the material indicated the need to make a detailed study of the electrical resistivity norms.In order to evaluate how far the norms of this Standard correspond to the data of firms abroad* we have made a comparative analysis and illustrate the results in Fig. 1. The results indicate that the standard indices for electroresistivity at 1000~ line up in a single category with those of the foreign companies while at 600 and 800~ the values of COST 13236-73 are lower.In order to establish a correspondence between the actual certification data of the material, we made a statistical analysis of 945 batches of periclase produced by the Bogdanovich Plant in 1979.The certification material for electrical resistance includes a determination of p at three temperatures (600, 800, and 1000~From the value of p, the class of material at each temperature is determined and in relation to the lowest of the three classes, the general class of the material is determined in relation to electrical resistance. Accordingly, all the material was distributed into classes as follows, no highest grade production; 123 batches of 1st class, or 13% of the total sample; 469 batches of 2nd class or 49.63%; and 353 batches of 3rd class of 37.35%. When making individual certification in relation to p at each temperature, the production was distributed differently (Table 1).The analysis was carried out as follows. As the basis, we chose one temperature and classified all the material on electrical resistivity at this temperature. We then established whether this class of material at the given temperature corresponds to the same class at the two other temperatures.
Periclase, which is produced by fusion of various natural and artificial products, the chemical base of which is magnesium oxide, is a dielectric widely used as an electrical insulator in various types of electric heating equipment.Depending upon a number of technological factors (raw material quality, fusion conditions, cooling of the fused product, subsequent working of it) the electrical resistance of periclase may vary within significant limits. One of the methods of increasing the electrical resistance of periclase is additions [i].It is known that magnesium oxide, including fused, has a mixed conductivity (ionic and p-or n-type electron). Under normal conditions, magnesium oxide possesses primarily ionic conductivity [2, 3]. However, with a change in temperature and oxygen partial pressure or the addition of additions the ratio between the ionic, electron, and hole conductivities may change.In the presence of impurities with a charge greater than that of Mg 2+, such as those containing tri-and tetravalent cations, compensation of the positively charged substitutional defects must occur either as the result of formation of doubly charged vacancies of magnesium V~g or as the result of defects in the oxygen sublattice. Apparently, cationic vacancies predominate. This may be the result of the fact that the escape energy of an oxygen ion is greater than that of magnesium ion [4].Electron and hole conductivity are related to deviation from stoichiometry with respect to oxygen. The concentration of these defects in magnesium oxide is normally comparatively low but in view of the high mobility of electrons and holes the electron and hole conductivity may under known conditions determine its electrical conductivity.To study the nature of the conductivity and the influence of additions perielase fused from brucite was used, and as additions the flaky silicates hydroaluminum silicate and magnesium silicate (talc). These additions are used for improvement of the sliding of the crystals in compression of the powder in production of thermoelectric heaters.The conductivity was studied on samples of polycrystalline periclase of three compositions, without additions and with the addition of talc or hydroaluminum silicate.The additions were added in a quantity of i%. The chemical compositions of the periclase and the additions are given in Table I. The different contents of oxides with Me 3+ and Me 4+ in the original raw material and the additions may make it possible to reveal the influence of their content on the conductivity of the periclase.The samples were prepared from powder of fused periclase with a grain size of less than 0.5 mm in the form of 20-mm-diameter tablets pressed under a pressure of 150 MPa. The samples were fired at 1200~ and to their faces was applied a platinum paste (electrodes). The sample was placed in a quartz measuring cell and the cell with the sample was placed in a furnace with silicon carbide heating elements [5].The measurements were made in de with the use of an E6-4A megaohmmeter at 600-120...
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