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

Abstract

UDC 621o365o22:666~During the introduction and long service of OKB-955N three-phase smeiting furnaces for production of electrical periclase at the Bogdanovich Refractory Plant satisfactory parameters of the bath (bath diameter d b = 2450 mm, electrode diameter d e = 400 mm, diameter of decomposition of the electrodes dde = 850-950 mm), the interval of charging, and the electrical conditions (voltage step IX, U2L = 70.8 V, current I2 = 6585 A) were selected [i, 2]. 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.As a result of the small difference between the melting and boiling points, significant overheating of molten magnesium oxide is impossible [6, pp. 198-199].In addition, small variations in temperature lead to sharp changes in its aggregate conditions and physicochemical properties.The intensity of heat dissipation and the rate of crystallization must be in agreement with the rates of degassing and migration of the impurities~ With a constant geometry of the fused bath the change in the volume of the molten material is, in practice, determined by the change in its depth (thickness).According to our data, a practical criterion of stabilization of the quality of the periclase blocks is a depth of the...

Intensification of the periclase electromelting process

“…

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.

…”