The diagrams from both the long-term and the short-term experiments were processed to obtain reliable average parameter values for each 15 min of experiment (Table 2).

In the long-term experiments, increasing the speed of pelletizer rotation from 6 to 7 rpm was accompanied by an increase in the speed of sintering carriage travel on sintering machines A and B of 2.91 and 2.45%, respectively. The spare capacity for increasing sinter- ing carriage speed, expressed as a negative pressure in the exhauster manifold 1.93% lower at a pelletizer rotation speed of 7 rpm than at 6 rpm, was not utilized in sintering machine B. However, machine A operated under greater stress at a pelletizer rotation speed of 7 rpm in terms of negative pressure in the exhauster manifold and in terms of temperature in vacuum chamber No. 15.

In the short-term experiments, increasing the speed of pelletizer rotation from 6 to 7 rpm was accompanied by an increase of 3.06% in the speed of carriage travel. The spare capacity for increasing the speed of carriage travel, expressed as a 1.04% lower negative pressure in the exhauster manifold and a 1.45% higher combustion products temperature in vacuum chamber No. 15, was not utilized in these circumstances.

Thus increasing the speed of pelletizer rotation from 6 to 7 rpm increases sintering machine output and improves sinter quality. Assessing the effectiveness of pelletizer opera- tion in terms of the relative improvement in charge gas permeability as a result of pelletiz- ing treatment greatly facilitates determination of its optimum operating routine, and in- creases the production of sinter and improves its quality in the end.

DESULFURIZING PIG IRON WITH MAGNESIUM INGOTS

E. N. Skladanovskii, V. I. Machikin, A. G. Kuzub, M. Z. Levin, V. V. Stepanov, M. A. Zalevskii, G. A. Mashkov, and A. M. Spirin

UDC 669~162o267.642

In 1970-1971 the Donetsk Plant and the Donetsk Polytechnic Institute together developed a new method for the controlled addition of magnesium to pig iron [M. Z. Levin, V. I. Machikin, E. N. Skladanovskii, et al., Metallurg, No. 2, 10-12 (1973)] in which magnesium ingots sus- pended inside a hollow evaporator rod are lowered into the pig iron at a prescribed speed in the desulfurizing process. Air is fed into the evaporator to prevent magnesium vapors and iron from entering the rod.

A pig iron desulfurizing installation suitable for this method was designed and built (Fig. i), providing for simultaneous treatment of four hot metal transfer ladles and having rated capacity of up to i million tons of pig iron per year. It consists of two units, each with a small gantry along which a carriage travels with evaporators and lids to cover the ladles mounted on it.

The evaporators are positioned along the transverse axis of the carriage on arms at a distance equal to the sPacing of the transfer ladles, i.e.~ the evaporators on one carriage can process two transfer ladles simultaneously. This layout greatly reduces the mass of structural metal simplifies control, and facilitates servicing.

The installation service department is combined with the control point and comprises services premises and equipment for repairing the evaporators and preparing the magnesium, a furnace for drying the evaporators, racks, turntables, a pit for coating the evaporators, and so on. Monorails are laid along the gantry axis, connecting the installation with the service department, and TE-3-621 electric pulleys are fitted~

Donetsk Metallurgical Plant. Translated from Metallurg, No. 9, pp. 11-13, September, 1976.

This material is protected by copyright registered in the name o f Plenum Publishing Corporation, 227 West 1 7th Street, New York, iV. Y. 10011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, elec tronir mechanical, photocopying, microfilming, recording or otherwise, wi thout written permission o f the publisher. A copy o f this article is available f rom the publisher for $7.50.

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Fig. i. Installation for treating pig iron with magnesium: i) gantry; 2) evaporator carriage; 3) carriage for replacing evaporators; 4) preparation department; 5) transfer car; 6) control point; 7) switchboard; 8) furnace for d~ying evaporators; 9) turntable for evaporators; I0) electric telpher; ii) auxiliary track; 12) feeder rig.

The installation is equipped with an automatic process control system consisting of three units: i) automatic magnesium feed in accordance with a set program and monitoring of the amount of magnesium delivered; and 3) automatic process monitoring and amendment of the set program.

The signal from a strain-gauge pressure sensor mounted on the evaporator is used as the control signal for automatic process monitoring. Investigations have shown that the amplitude of pressure fluctuations with the evaporators alters according to the gas consumption (speed of magnesium evaporation). If it exceeds the prescribed level, the amplitude of pressure fluctuations will also exceed the permitted value; this will serve as the signal to shut off the magnesium feed until the amplitude falls (Fig. 2). The magnesium feed was shut off manually with visual observation of the process (when there were strong vibrations in the ladle and spitting of iron). The amplitude of pressure fluctuations increases sharply on the oscillogram at these times.

The first unit of the installations was brought into operation in the middle of 1973. By the end of the year 65,000 tons of pig iron had been processed in it. In 1974, 181,200 tons were processed (up to 16 ladles on some shifts) in this unit, while construction of the second unit was being completed. Pig iron with a sulfur content of 0.035-0.080% was processed in the installation. The degree of desulfurizing was 50-80%, average magnesium consumption was 0.43 kg/ton, uptake of magnesium by sulfur was 40-75%, and the speed of entry of the magnesium was 60-130 g/sec.

A reduction of 0.1-0.3% in the pig iron carbon content and of 0o01-0.04% in the manganese content as a result of desulfurizing was also observed. An additional drop in the sulfur content averaging 0.004% occurs during transportation of the ladle of pig iron to the mixer after desulfurizing.

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Fig. 2. Oscillograms showing pressures within the evaporator when magnesium is added to pig iron: a) normal process course; b) with irregular evapora- tion of magnesium (t o is the time of operation of the magnesium feed mechanism electric motor, tp is the pause).

A - A B-B

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Fig. 3. Drying furnace for evaporators: I) metal shell; 2) coated evaporator; 3) flues; 4) suspension hook; 5) door; 6) gas main; 7) injector-type burner.

Bringing the installation into operation at the Plant has resulted in an increase in the proportion of category-I commercial pig iron and the technical specification for mixer iron has been modified (the sulfur content has been reduced to 0.040 instead of 0.045%); the amount of pig iron with a silicon content above 0.9% has also been reduced, because the blast- furnace personnel have been bringing up the furnaces from cold without overconsumption of coke.

The second unit in the installation has been in operation since the second half of 1975; in this unit the magnesium feeder drive operates on direct current, which permits smooth regulation of the magnesium feed rate in the 50-200-g/sec range. As a result the second unit in the installation operates without lids to check metal spitting and the uptake of magnesium has increased by 10-15%.

The work of individual subunits was improved in the course of assimilation of the instal- lation, in particular the lining and drying of the evaporators; the temperature of the hot metal in the ladle during processing was measured, as was the temperature in the evaporator chamber during entry of the magnesium ingot into the iron, the reverse passage of sulfur from the slag into the iron was investigated, and the technical and economic results of desulfuriz- ing were studied.

Iron notch plugging material is used to line the evaporators. The reinforced metal base of the evaporator is covered by runner lining material to a thickness of 70 mm, kept in the shop for 3-4 days, then placed in a drying furnace (Fig. 3) and kept there for 3-4 h at 250- 300~ During this period the pitch in the material is coked and gives the coating the neces- sary strength.

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TABLE !. Passage of Sulfur from Slag into Hot Metal in Mixer

Hot metal Tonnage Hot metal of metal temp. =C in mixer

Slag mass, kgAon L~ LS

Equilib- rium achieved,

Passage AS, %

Without desulfurizing 378 1335 6, 12 I i , 4 17,6 65,0 O, 003 With pa_nial desulfurizing 290 1340 4,50 12,5 28,5 44,0 -- Desulfurized 348 1282 17,4 10,5 41,0 25, 6 0,010

Before being used to feed in magnesium, the evaporator is "fired" by immersion in the hot metal 2-3 times for 10-30 sec at intervals of 2-3 min. An evaporator prepared in this way can withstand up to 120 immersions (each of up to 8 min), i=e~ it can process 7000 tons of pig iron. Work is in progress at the Plant on increasing the life of the evaporator linings.

Experimental studies of the effect of the desulfurizing process on the temperature of the hot metal established that at magnesium consumption rates up to 0.5 kg/ton the tempera- ture of the hot metal falls mainly as a result of natural heat losses and is determined by the total time for transportation to the installation and ladle processing.

Since changes in the hot metal temperature in the evaporator chamber during the control- led additionof magnesium may determine the stability of the magnesium melting and evapora- tion process and consequently the stability of the treatment, the temperature of the hot metal in the evaporator chamber during desulfurizing was measured. A platinunr-platinorhodium thermocouple (Fig. 4) protected by two steel tubes and by graphite and molybdenum tips was mounted in the lining of the rod. The thermocouple junction and the molybdenum tip were placed below the holes in the evaporator chamber at a distance of 1/3 the height from the lower edge of the evaporator to the level of the holes. The signal was recorded on photo- graphic film in the oscillograph at the same time as the pressure inside the evaporator, as an index of the true speed of magnesium evaporation~

The measurements showed that the amount of heat within the evaporator chamber does not limit the amount of magnesium fed in per unit of time, and the reduction in hot metal temper- ature when the speed of magnesium evaporation increases to the limit does not exceed 20-30~ This stability of temperature is evidence of sufficient heat supply from the remainder of the hot metal due to heat transfer, and mainly as a result of fresh batches of metal being mixed in due to fluctuations in pressure in the evaporator--hot metal--gas system.

Slag is not removed from the ladles or the mixer at the Plant; a study was therefore made of the possibility of a reverse transfer of sulfur from slag to hot metal in the mixer. If the hot metal is not desulfurized, the mixer usually contains from 3 to !I kg of slag per ton of hot metal with an average sulfur content of 0.74%; the average basicity (CaO/SiO2) of the slag is 0.7 and it contains 3.06% FeO. The actual coefficient of distribution of sulfur between hot metal and slag LS, calculated taking account of corrections for FeO content, was 17.6, which is above the equilibrium coefficient (L~) for these conditions (11.4). This

leads to a situation in which the hot metal sulfur content increases by 0.003% on average when the metal is in the mixer for 5-6 h (Table i).

If the mixer contained 60-70% hot metal which had been desulfurized in the ladle the values of L s and L ~ were 28.5 and 12.5 respectively, i.e.~ the passage of sulfur from slag into metal ought t~ be more substantial. To find the extent of this passage, mixer No~ 2 was filled only with metal which had been desulfurized, over a period of 5 days. During this period a total of 2204.5 tons of hot metal was processed, with an average initial sulfur content of 0.045%~ Metal with a sulfur content of 0.019% was discharged into the mixer. The average sulfur content of the slag during this period was 1.19%, with basicity 0.73 and 3.91% FeO; the L S and L~ values for these conditions were 41.0 and i0.5~ respectively.

The average sulfur content of the hot metal discharged from the mixer was 0.029%, i.e., 0.010% sulfur passed back from the slag into the metal (Table 2)~ This is much less than

o values obtained. might be expected on the basis of the L S and L S

Apparently the MgS which forms during desulfurizing and enters the slag partly dissolves in it; therefore not all the slag sulfur participates in the distribution between slag and hot metal. The reverse passage of sulfur is considerable in itself, and the adoption of

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Fig. 4. Evaporators with thermocouple to measure hot metal temperature inside evap- oration chamber: i) thermocouple; 2) guide tube; 3) evaporator lining; 4) graphite tip; 5) molybdenum tip; 6) assembly tube; 7) graphite nut.

TABLE 2. Sulfur Balance in Hot Metal in Desulfurizing and Subsequent Holding in Mixer

Item Tonnageof hoe " Sulfur content metat ~ I kg

Intake to installation Intake to mixer Discharge Passage from slag into hot

met~al (AS), %~

2204,5 0,0456 1004,2 2204,5 0,0192 423,5 2193,5 0,0292 639,4

0,010 213,4

devices to run off the slag will appreciably increase the efficiency of hot metal desulfuriz- ing.

Work is also in progress on the installation to inoculate the iron, so that it can be subsequently used to cast molds. Magnesium consumption in these circumstances is 1.6-2.0 kg/ ton of iron, the speed of magnesium addition being 100-140 g/see. Crushed FS-65 ferrosilicon is fed into the spout of the tilter when the metal is being passed from the transfer ladle to the pouring ladle. The case molds were kept in the boxes for 24 h, and the micorstruc- ture was studied in core samples taken from the molds' inner walls at a distance of 400 mm from the upper face at a depth of 35-40 mm.

The inoculated iron molds were used in the general process flow on separate stools. The average life of these molds was 92 fillings, as against a normal mold life of 57 fillings. This yields a saving of 55 rubles per ton of castings, with total expenditure on inoculation of up to 8 rubles/ton.

The saving resulting from desulfurizing pig iron in 1974 and 1975 was 1.94 and 2.2 rubles per ton of iron respectively, the expenditure on desulfurizing being 0.52 and 0.60 rubles/ton.

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