US3313619A - Iron refining processes - Google Patents

Description

Ami M, 1%? G. DECAMPS wmw IRON REFINING PROCESSES Filed June 16, 1965 United States Patent 3,313,619 IRUN REFINHNG PROCESSES Georges Decamps, Thionville, Moselle, France, assignmto Societe Lorraine de Laminage Coutiuu Scllac, Paris, France Filed June 16, 1965, Ser. No. 464,471 Claims priority, application France, Mar. 24, 1961, 856,679 1 Claim. (CI. 75-60) This application is a continuation-in-part of my earlier filed, copending application, Ser. No. 182,543 of Mar. 26, 1962, now abandoned.

It is well-known to refine pig iron by placing same in a rotary furnace having its axis inclined with respect to the horizontal and by blowing oxygen into the furnace by means of a lance or tuyere arranged substantially coaxially with the furnace. The oxygen ensures combustion of the unwanted constituents and ensures conversion of the pig iron into steel. Moreover, the heat evolved from these reactions contributes to maintaining the bath in the molten state.

When oxygen is blown into the furnace, the jet intersects with the bath surfaces along an elliptical contour. Part of the gas penetrates the bath and ensures combustion of the impurities, as well as of the carbon, the latter forming carbon monoxide. The remaining oxygen is retained within the furnace to form an atmosphere therein, enabling combustion of the carbon monoxide. According to this method, the lower end of the lance is constantly kept above the surface of the bath and not within the bath.

If it is attempted to increase the refining rate and hence the production output rate of the plant by increasing the rate of oxygen delivery while leaving the remaining operating factors unaltered, it is found that the temperature of the discharged combustion gases rises, indicating that the carbon monoxide now burns in the discharge stack rather than in the furnace, Thus no benefit is derived from the heat evolved by carbon monoxide combustion and hence, in practice, the oxygen blast delivery rate is limited thereby limiting the production rate.

It has already been suggested in US. Patent No. 3,057,- 616, issued for Wohlfahrt et al., to modify the inclination of the tuyere, which inclination affects the order in which the carbon and phosphorus are oxidized, as well as the combustion of CO to CO and it moreover allows of avoiding uneven Wear of the refractory lining of the furnace. According to this process each angle of inclination of the lance corresponds to a well-defined position of the lower end of said lance or tuyere.

Such a process, however, does not allow of considerably increasing the ox gen delivery rate, and hence does not lead to an increase in the refining rate. It was thus ascertained that, when applying this process, the maximum velocity of the refining reactions is not attained.

It is an object of the present invention to obviate these inconveniences and to provide an improvement in pig iron refining processes of the above-specified type, which will make it possible to increase the oxygen delivery rate during the refining operation, while ensuring at the same time an optimum combustion of the carbon monoxide in the furnace during each refining step.

The process according to the invention for the refining of pig iron in a rotary furnace having its axis inclined, by blowing oxygen at a pressure of the order of 2 kilos per square centimeter into said furnace by means of a movable blast nozzle arranged in the vertical median plane of the furnace above the surface of the metal bath, is characterized in that the position of the blast nozzle is adjusted as a function of the oxygen delivery rate, so that in accordance with the requirements of the various refinice ing steps, each inclination of the lance corresponds to different lengths of penetration of the lance and different oxygen delivery rates, and in that it comprises the following operating steps: at the beginning of the refining operation, when the refining operation is fired and Si and Mn are eliminated from the bath, the lance is disposed to extend a maximum distance into the furnace, so that its front end is positioned just above the bath, substantially at the centre of the surface of the latter, and that the inclination of the said lance is the greatest possible one relative to the horizontal line, and the rate at which the oxygen is delivered is relatively low; then, after the elimination of Si and Mn, during the following decarbonization step, the lance is raised so that its angle of inclination is lesser and it is retracted to a position where its front end extends very little into the furnace. The oxygen delivery rate is increased to about 2.5 to 3 times its initial value; and subsequently, after the decarbonizing step, and during the following dephosphorization step, the extension of the lance into the furnace is slightly increased, as well as its inclination, and the oxygen delivery rate is decreased, but it still remains about twice as high as its initial value.

According to the process of the invention, the oxygen delivery rate is relatively low at the beginning of the refining operation (firing and elimination of Si and Mn), i.e. of the order of conventional oxygen delivery rates of known processes, the lance being sufliciently inclined to the horizontal and a very long portion of its entire length extending into the furnace, so that the distance between the front end of the lance and the bath surface is small and that its axis is situated approximately in the central portion of the bath.

An increase in the oxygen delivery rate after the elimination of Si and Mn leads to an increase in the velocity of the jet. Thus the amount of oxygen which penetrates the bath and, as a consequence thereof, the amount of carbon monoxide formed, equally increase. Supposing the position of the lance is not modified, a lesser proportional amount of oxygen will remain above the bath and it might happen that at a given moment the amount of oxygen present is no longer sufficient to burn up all of the carbon monoxide; the temperature of the discharged combustion gases rises.

If, according to the invention, the lance is retracted simultaneously, so that its front end is in the vicinity of the aperture of the furnace, but still within said furnace, the elliptical intersection contour increases in area, and the resistance offered by the bath to the penetration of the oxygen is increased as well; moreover the increased length of the path of the jet through the furnace results in an increased retarding action by the fiood of gases issuing from the bath. Thus there will be less oxygen entering the bath, and matters can easily be so arranged that the ratio of the amount of oxygen penetrating the bath to the amount of oxygen available in the furnace will substantially remain constant. If necessary, the inclination angle of the lance may also be decreased; in fact, the amount of oxygen penetrating the bath decreases :as the jet axis approaches a condition more nearly tangent to the bath surface.

The above-specified method is only effective if the blast pressure is at the most of the order of 2 kilos per square centimeter, and the desired result cannot be obtained with a higher blast pressure, as would be the case in so-called vertical-lance furnace, wherein combustion of CO to CO is undesirable and is in fact avoided, with a view of avoiding undue wear of the furnace walls which are not cooled by the bath.

Tests were performed with a Kaldo rotary furnace having a capacity of metric tons, of the type described hereinafter and illustrated in the sole figure of the accompanying drawing.

The rotary furnace 6 comprises a metal shell with an internal lining '7 and is supported within a bearing ring 8 formed with axially spaced lateral annular bearing surfaces or ways 5. When the furnace is rotated, each bearing surface moves over a set of rollers 10 mounted in bearings 11 formed on a cradle 12 which is supported in bearings 13 by way of journals 14, so that the furnace is adapted to be inclined to different positions depending on the charge, the melt and the heat treatment. One of the rollers 10 is driven from a motor 15 through gearing 16, 17. Thrust bearings 18 pivoted on pivots 19 and engaging the annular ways 9 of the bearing ring 8 serve to prevent lateral shifting of the furnace in inclined condition. The furnace is provided with a single aperture 20 at the centre of an end wall thereof. The gas discharge stack 21 has its inlet spaced by a clearance of about 10 cm. from said aperture when the furnace is positioned in its heat treatment position, at an inclination of say 15 to 20. The stack is provided with a cooling jacket 22 having a water inlet 23 and outlet 24. Mounted across the stack is a lance 25 which is cooled with water delivered at 26 and discharged at 27. At the end of the lance there is positioned a pipe 28, intended for the supply of oxygen and connected to an oxygen source (not shown) through a flexible piping 29.

The lance 25 is slidably mounted on a pivotable support by means of a rack-bar mechanism, the rack-bar 36 being secured to the lance 25, and the pinions 37, driven in synchronism, are provided on support 35. The lance 25 is guided on the support 35 by means of two guiding stirrup members 38, secured to the support 35. Said support 35 is pivotably mounted on the discharge stack 21 by means of a horizontal shaft 39 and a forkjoint 40, fastened to the discharge stack 21. Means are provided (not shown), adapted to maintain the support 35 in a predetermined inclined position.

As shown in the drawing, the furnace may be inclined to a predetermined position for charging the slagging material as well as the iron ore, through a hopper 30, and to a different position for charging the molten pig iron from a ladle 31, and further, it may be set to a third position for discharging the slag or the refined iron into a movable ladle 32.

The molten pig iron is charged into the furnace 6 containing the slag from a previous refining run. Then oxygen is discharged into the furnace in a known manner through lance 25 while the furnace is being rotated.

At the beginning of the refining process (firing and elimination of Si and Mn), the lance is disposed to extend 2 meters into the furnace and it is provided with a nozzle having a diameter of 140 millimeters. The lance is inclined to the horizontal through an angle of 27. Oxygen is emitted from the lance at a rate of 120 cubic meters per minute. Owing to the described arrangement of the lance, the amount of free oxygen within the furnace 6 is limited, but said amount is sufiicient for burning all of the CO issuing from the bath, without there being any excess of oxygen, for a large proportion of the amount of oxygen entering the bath is absorbed by silicon and manganese, contained in the bath, and only a small proportion combines with the carbon of the metal bath. After the elimination of Si and Mn, i.e. after a period of about 5 minutes, during which period the rate of oxygen delivery as well as the above-mentioned position of the lance were not changed, the lance 25 is raised and retracted, so that its projecting length in the furnace 6 is reduced to 0.75 meter and that its angle of inclination to the horizotnal is 21. By means of said angle of inclination, it is still possible to keep the extremity of the intersection contour of the oxygen jet on the metal bath, thus preventing same from impinging upon the bottom of the furnace 6. During the following decarbonization period, after the raising of the lance 25 and its retraction towards aperture 20 of furnace 6, oxygen is delivered at a rate of 300 cubic meters per minute. The raising of the lance 25, as well as its retraction towards aperture 29 of furnace 6 into a position where the angle of inclination of pipe 28 to the horizontal is 21, and its extension into furnace 6 is 0.75 meter, take place in accordance with the increase of the amount of CO issuing from the bath.

The retraction of lance 25 as far as the opening 20 of furnace 6, results in an increase of the intersection area of the oxygen jet and a decrease in the velocity of said jet impinging upon the bath surface. This leads to a decrease of the amount of oxygen penetrating the bath and the proportional amount of free oxygen available in the furnace 6 above the bath is correspondingly increased. A reduction of the angle of inclination of pipe 28 has a similar favourable effect, however, this measure alone is not sufficient to allow of a considerable increase in the oxygen delivery rate without prejudice to the balance of the ratio of oxygen amounts respectively injected into the bath and available within the furnace above the surface of the metal bath.

After said period of decarbonization, the rotational speed of furnace 6 is decreased, so as to promote dephosphorization of the metal bath. Part of the oxygen entering the bath is then absorbed by the iron to oxidize the slag, another portion is absorbed by the phosphorous and the remainder is converted into the CO which issues from the bath. In view of the fact that the amount of CO has decreased, it is preferable to slightly lower the lance 25 until it reaches an angle of inclination of 23 to the horizontal and to make it extend more into the furnace, so that its front end is at a distance of 1 meter from opening 20 of the furnace, as well as to slightly decrease the oxygen delivery rate to a value of 240 cubic meters per minute.

By following the refining process according to the invention, it is possible to increase the oxygen delivery rate, i.e., to considerably reduce the time required for refining the iron, without destroying the equilibrium of the ratio of oxygen amounts respectively penetrating the bath and available above the surface of the bath for the combustion of CO to CO and this during the various steps of the refining process.

Thus the oxygen blast periods are reduced from 54 seconds per ton output according to known processes, to about 20 seconds per ton, according to the process of the present invention.

What I claim is:

In a process for refining pig iron in a rotary furnace having an inclined axis by blowing oxygen into the furnace at a pressure not exceeding 2 kilos per square centimeter through an injecting lance which is arranged in the vertical medial plane of the furnace above the surface of the metal bathand which is movable both angularly to vary the angle of inclination of the lance and axially to vary the length of the lance projecting into the furnace; the steps of (A) initially, during the firing of the refining process and the elimination of Si and Mn from the bath, axially disposing said lance to project a large length thereof into the furnace and to position the end of the lance from which the oxygen issues at a short distance from the surface of the bath with the axis of the lance being directed substantially at the center of said surface, angularly disposing said lance at the maximum angle of inclination thereof with respect to the horizontal, and delivering oxygen through said lance at a relatively low initial rate; (B) after the elimination of Si and Mn, and during decarbonization, decreasing the angle of inclination of the lance and axially retracting the latter so that said end of the lance projects only slightly into the furnace and thus is spaced a large distance from said surface, and increasing the rate of delivery of oxy-- 5 gen through said lance to a value approximately References Cited by the Examiner 2.5 t0 3.0 times said initial rate; and UNITED STATES PATENTS (C) after the decarbonization, and during dephosphorization of the bath, increasing the projection of 2598393 5/1952 Kalllling et a1 75 52 the lance into the furnace and also increasing the 5 g z i i f angle of inclination of the lance to dispose the latter intermediate the positions thereof for the elimination of Si and Mn and for decarbonization, re- BENJAMIN HENKIN Pnmary Exammerspectively, and decreasing the rate of delivery of DAVID L RECK Examiner oxygen through said lance to a value which is ap- 10 proximately 2 times said initial rate.

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