Production technology and quality of fused periclase

Abstract

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...

Distribution of power in a three-phase electric furnace in the melting of periclase

“…Similar results were obtained at the Magnesite Combine [3][4][5]. However, the average hourly power required for melting on voltage step IX varies within the range of 550-750 kW, which is significantly lower than the installed power of the furnace transformers, which is 1200 kVA.The increasing requirements of the national economy for an increase in the quantity and an improvement in the quality of periclase may be satisfied by optimization of the melting conditions on the basis of a thorough quantitative study of the interrelationship of the parameters.The process of conversion of the magnesia raw material (brucite, magnesite) into periclase consists of successive endothermic stages of decomposition and fusion and an exothermic stage of crystallization of the molten material.An increase in the line voltage (to 85.0 and 105.2 V) significantly reduces the melting time, but sharply reduces the output of acceptable periclase [4].An attempt to increase the effectiveness of melting by strengthening the heat dissipa ~ tion (water cooling of the bath, increasing the diameter of decomposition of the electrodes~ increasing the diameter of the bath) is represented by an insufficient volume of experimenta~ data [3].To provide for the processes of migration of the impurity oxides and degassing of the molten material, which determine the quality of the periclase, a certain time, which is re o~ fated to the viscosity of the molten material and the amount of it, is required~ Therefore, an increase in the melting time with a periodic reduction in the operating current has a positive influence on the thickness of the zone of single crystals and the general structure of the block [2,3]. In turn, the viscosity, temperature, and quantity of the molten material depend upon the density of the electric power released to its resistance and also upon the intensity of heat dissipation.…”

Increasing the number of uses of the refractories in the slide gates of large steel-teeming ladles

“…The purest in chemical composition are dense uniform dull lumps of whitish-gray color and nodulized semitransparent light gray pebbles, which were selected for melting from a total quantity of brucite of 70 tons. The average composition of the brucite and of its best varieties taken for melting is shown in Table i~ The brucite was sorted on the conveyer belt in loading of the bunkers of electric arc furnaces after washing in open storage and sifting out of the most contaminated 10-0-mm fraction [3]. The yield of enriched brucite was 54% of the total quantity.…”

Intensification of the periclase electromelting process

“…The productivity of the furnace was increased by supplying an oxygen enriched blast to the furnace [20], melting in a furnace with a current-and heat-conducting hearth [21,22], increasing the diameter of decomposition of the electrodes [7,18], increasing the current by 10-20% above the nominal, and the introduction of rational electrical conditions of melting…”

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At the Magnesite Combine a method has been developed and introduced forelectromelting of brucite into a block [1][2][3][4] and of fired magnesite powder [5,6] and of periclase-spinellides [7][8][9] with obtaining of fused materials for new effective refractories [I, 6, 9, 10-15] in OKB-955N furnaces with a 1200 kVA ETMN 1600/10 transformer.During introduction of the method work was done in the direction both of improving the quality of the fused periclase and increasing furnace productivity.The quality of the periclase was improved by a periodic change in power delivered to the furnace during melting [2], a rational ratio of the powers on the higher and lower voltage steps of the transformer [16], an increase in purity of the original material [17,18], increasing the melting time [18], providing the conditions Pd > Psh reached at Zf < 5"10 .3 [19], changing the form of the bath shell, and increasing the intensity of heat dissipation from its surface [3,4].The productivity of the furnace was increased by supplying an oxygen enriched blast to the furnace [20], melting in a furnace with a current-and heat-conducting hearth [21,22], increasing the diameter of decomposition of the electrodes [7,18], increasing the current by 10-20% above the nominal, and the introduction of rational electrical conditions of melting

[4, 5].As the result of work done on an OKB-955N furnace with a 1200-kVA transformer with a diameter of decomposition of the electrodes of 1050 mm a maximum productivity in melting of magnesite, brucite, and fired magnesite powder of 258 [20], 260 [4], and 236 kg/h [6], respectively, was obtained.

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