GB1578570A - Stirrer for metallurgical melts - Google Patents

Description

(54) STIRRER FOR METALLURGICAL MELTS (71) I,JAN-ERIKijSTBERG, a Swedish Subject of Torps Säteri, 640 33 Bettna, Sweden, do hereby declare the invention for which I pray that a Patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following Statement: This invention relates to a mechanical stirrer for metallurgical melts.

Metallurgical melts are stirred in order to equalise differences in chemical composition and/or temperature. At the same time, or as a consequence of the stirring, other advantages may be obtained, for example acceleration of reaction rate, better efficiency or yield of a reaction, easier introduction of solid material into the melt, coagulation of suspended particles and degassing of the melt.

Various stirring means have been used for this purpose, including mechanical stirring devices. Of the latter devices, some are rotatable in the melt and are provided with internal channels, enabling the device to develop a pumping action within the melt.

The present invention relates to an improved rotary stirring device of this kind.

An early stirring device of the kind just referred to comprised a vertical pipe, open at its upper end but closed at its lower end.

Horizontal pipes were joined to the lower end of the vertical pipe, the outer ends of the horizontal pipes discharging into the melt. This device was rotated at high speed, about a vertical axis of symmetry of the device, and injected solid material into the melt by way of the vertical and horizontal pipes. The U.S. magazine "Foundry Trade Journal", 25th February 1940 edition, gives a report of such a pumping stirrer, which in addition to the just mentioned function allowed melt to enter into the lower horizontal pipes for the purpose of using the melt proper as a means of carrying the additives.

Another rotary stirring device is described in German Gebrauchsmuster No. 36 755/12 e and German Patent Specification No. 1,190,479. In this proposal the channels at the lower end of the stirrer had a larger pumping capacity than corresponding parts of the earlier stirrers mentioned above.

Fine-grained solid material to be injected into the melt was charged by way of a vertical pipe into the pumping centre, where liquid pressure was at its lowest, and the melt was believed to attract the solid material due to its lower pressure. While the earlier U.S. stirrers are reported to have given a lot of satisfaction, the German proposal seems to have been a failure. It is probable that the lower horizontal channels were to blame. They were shaped as distinct arms projecting into the melt and not only gave a pumping action by means of these channels but also produced a vigorous action by their protruding exterior surfaces.

Excessive rotation and splashing of the melt caused by the action of these arms was obviously harmful for pumping and for injection of solid material.

The primary aim of the present invention is to provide a rotary mechanical stirrer for metallurgical melts which has a high pumping capacity in terms of flow volume and which provides a very low pressure in the pumping centre when required, these objectives being obtained with a minimum rotation of the melt but with the development of powerful toroidal eddies in the melt which encourage the transportation of injected material and general homogenisation of the melt.

According to the invention, a mechanical stirrer for metallurgical melts comprises a stirrer body which is submersible in the melt and rotatable about a vertical axis which is an axis of symmetry of the body, a mixing chamber within the stirrer body, an inlet channel leading to the mixing chamber and having a length which is not more than twice its average internal transverse dimension, and a plurality of discharge channels from the mixing chamber which, in use of the stirrer, incline upwardly from the mixing chamber at an angle of at least 10 , each of said discharge channels having a length which is at least twice its average internal transverse dimension, and the mixing chamber being shaped so that its cross-sectional area in the region where, in the position of use of the stirrer, the lowest points of the discharge channels enter the mixing chamber is at least 4() per cetit greater than the cross-sectional area of said inlet channel.

The mixing chamber of a stirrer in accord ancc with the invention can be shaped to suit the intended purpose of the stirrer.

When it is intended that solid finc-grained material shall be added to the melt being stirred, the mixing chamber is designed to give a maximum mixing effect and a pipe for supplying the solid material to the mixing chamber may be equipped with devices adapted to inject the material in question into the mixing chamber. When the main purpose of the stirrer is to effect degassing of a melt, the mixing chamber may be shaped to provide adequate space for collecting the liberated gas.

The invention will now be described, by way of example. with refcrence to the accompanying drawings. in which Figure I is a schematic perspective view of one embodimcnt of a mechanical stirrer in accordance with the invention, the stirrer being shown in position for stirring a metallurgical melt in a ladle, Figure 2 is a sectional view, on an enlarged scale, of part of the stirrer of Figure 1, and Figure 3 is a sectional view of part of a modified form of the stirrcr of Figures 1 and 2.

The stirrer shown in Figures 1 and 2 comprises a body of refractory material, generally designated by the numeral 1, having a circular cylindrical surface 2, a lower, frusto-conical surface 3 and an upper, concavely-curved surface 4 which merges into another circular cylindrical surface 5, the axles of these various surfaces being aligned with one another and representing the vertical axis of the body 1 about which it is rotated when in use.

The surface 3 of the body 1 merges into the external circular cylindrical surface of a downwardly-open pipe which provides the inlet channel 6, of internal diameter Dl, of the stirrer body. Within the stirrer body the inlet channel 6 communicates with two or more discharge pumping channels 7. The axis of the channel 6 is aligned with the aforesaid vertical axis of the body 1, and the channels 7, which open at 8 into the surface 2 of the body 1, are symmetrically disposed around said axis. The channels 6 and 7 open into a mixing chamber 30 within the stirrer body 1.

An annular frusto-conical member 9 is embedded in the body 1, with its axis aligned with the aforesaid axis of the body 1, the member 9 being joined to a hollow drive shaft 10, which also has its axis aligned with the axis of the body 1. In the embodiment shown in Figure 1, in which the stirrer is being employed to stir a melt M in a ladle 11, the shaft 10 passes from the ladle through the lid 12 thereof and is supported for rotation in a bearing 13 mounted on a support 14 secured to the lid 12. The shaft 10, and with it the stirrer body 1, is rotated by an electric motor 15 secured to the support 14, this motor being arranged to drive a gear 16 which meshes with a gear 17 secured to the shaft 10.

The stirrer body 1 has a vertical, centrally disposed hole 18 therethrough which extends from the upper end of the body 1 down to the mixing chamber 30, the hole 18 being lined with a tube 19, of internal diameter D5, which projects from the upper end of the body 1 and passes up through the hollow shaft 10, the upper end of the tube being connected through a rotary coupling 20 to a hopper 21. A vertically disposed rod 22 is slidably supported (by means not shown) within the tube 19. At its lower end the rod 22 carries a plug 23 and at its upper end it is connected to a device 24 which is capable of raising and lowering and rotating the rod 22. Figure 2 shows the rod 22 in a raised position. When the rod 22 is lowered from this raised position, the plug 23 closes the lower end of the tube 19.The lower end of the tube 19 is spaced above the mixing chamber 30, and below the tube 19 the hole 18 has a portion 25 of frusto-conical shape, this portion 25 having a diameter D6 where it opens into the mixing chamber 30.

The tube 19 serves to introduce additive material into the mixing chamber 30 from the hopper 21 into which the material is charged. In order to encourage the feed material to rotate as it descends in the tube 19, the latter may be provided on its internal surface with inclined ribs or a helical rib (not shown). In an alternative arrangement, the plug 23 may be dispensed with and the lower end of the rod 22 provided with a rotatable screw conveyor 26, as shown in Figure 3 for the purpose of positively forcing the feed material into the mixing chamber. In use of the stirrcr, the hopper 21 is normally closed, and means (not shown) may be provided for controlling the pressure within the hopper.

The hopper may also be provided with further means (not shown) for discharging feed material from the hopper into the tube 19.

When the stirrer described above is rotated in the melt M, as shown in Figure 1, melt enters the channel 6, as indicated by the arrow A, and is discharged from the openings 8, as indicated by the arrows B.

These jets of melt leaving the openings 8 rotate in the melt around the axis of rotation of the body 1, and thus bring the surrounding melt into rotation. This rotation of the melt generates toroidal eddies in the melt, as shown, the detailed shape of which will be influenced by the shapes of the surfaces 3 and 4 of the body 1. In each particular case, the shapes of these surfaces 3 and 4 can be chosen to contribute to the most efficient toroidal eddies required. In order to keep the rotation of the melt to a minimum, not only should the body 1 be symmetrical about its axis of rotation, but the friction between the body 1 and the melt should be as small as possible. The external surface of the body 1 should, therefore, be as small as possible, and no part of the body 1 should be made horizontal if this can be avoided.

The toroidal eddies produced in the melt M homogenise the melt rapidly. Moreover, they give rise to a vertical movement in the melt of material added through the tube 19 and discharged into the melt from the channels 7. These favourable effects of the toroidal eddies are obtained at a fairly low rotational velocity. A high rotational velocity is not desirable, as it may deform the melt surface and give rise to splashing. The geometry of the stirrer should therefore contribute to a desired compromise between pumping and rotation, as will now be discussed.

The stirrer, whether used as an injector or a degasser, should produce not only a sufficient flow volume but also a correct pressure distribution. In the mixing chamber 30 the pressure should be low, in some cases extremely low. This pressure, however, is not only a function of the stirrer itself but also of the pressure-distribution in the melt surrounding the stirrer. There are so many factors which influence the matter that the situation cannot be accounted for in a mathematical formula. However, not only the discharge from the mixing chamber 30 has a bearing on the pressure therein, but also the rate at which melt flows into the mixing chamber by way of the inlet channel 6. The latter should not have too small a cross-sectional area as this will adversely affect the pumping effect of the stirrer.On the other hand, it should not have too large a cross-sectional area, or the large inflow of melt will prevent the establishment of low pressure in the mixing chamber 30. Normally the channel 6 should have a diameter Dl such that the cross-sectional area of this channel is approximately the same as the total area of the discharge openings 8. If, however, the friction in the channel 6 is very small, it may be necessary to reduce the diameter Dl, in some cases drastically.

In cases where the discharge channels 7 for practical reasons can be curved, this should be done. Frequently, however, this gives rise to severe problems as regards injection of material into the melt and other factors contributing to a high pumping efficiency must be relied on, for example making the channels 7 long in relation to their cross-sectional area or a reduction in their transverse dimension from D3 at the mixing chamber 30 to D4 at the openings 8.

However, the dimensions of the stirrer cannot be allowed to become too large, as this gives rise to problems in charging additive material and also increases the friction between the stirrer and the melt.

These conflicting considerations are solved by inclining the discharge channels 7 upwardly as much as possible, the inclination being at least 10 to the horizontal. This introduces the disadvantage, from the point of view of the overall flow pattern in the melt, that the jets of melt issuing from the openings 8 have an upward component of motion, but on the other hand it encourages movement of the melt in a high position in the latter. With this upward inclination of the channels 7 the ratio of the length of each channel, between the locations where D3 and D4 are indicated, and the average transverse dimension of the channel, can be kept as low as 2:1 while still retaining sufficient acceleration of the melt.

The efficiency of the stirrer as a pump is, however, enhanced by a reduction in the cross-sectional area of the channels 7 in the direction towards the openings 8. Preferably, this reduction amounts to about 20 per cent from the location where D3 is indicated to the location where D4 is located when the stirrer is rotated at a speed of 60 R.P.M. and 25 per cent when the speed of rotation is 90 R.P.M. Normally, the channels 7 are of circular cross-section, but they may be elliptical with either the minor or major axis of the elliptical cross-section vertically disposed. Any changes in cross-sectional area of the channels 7 should be continuous, and if there are changes of direction in the channels there should be a smooth transition between channel parts having different directions.

The shape of the mixing chamber 30 is important; in particular it must be large. Its cross-sectional area in the region where, in the position of use of the stirrer, the lowest points of the channels 7 enter the mixing chamber should be at least 40 per cent larger than the cross-sectional area of the inlet channel 6. This region of the mixing chamber, indicated by D2 in Figure 2, is preferably not circular. For example, in the case of the stirrer shown in the drawings, which has two diametrically disposed channels 7, the mixing chamber may have plane wall portions disposed parallel to the axes of the channels 7. This has the effect of causing acceleration of the melt from the mixing chamber right from the centre of rotation of the stirrer.The effective length of the discharge channels 7 then extends, so far as pumping action is concerned, virtually to the centre of the mixing chamber 30, and the above mentioned demands regarding the geometry of the channels 7 between the locations D3 and D4 can then be relaxed to some extent.

In cases where the speed of rotation of the stirrer is insufficient to control the location of the surface of the melt in the mixing chamber, it may be advisable to control the pressure in the pipe 19, or at least to control the charging of additives from the hopper 21 so that the pressure in the mixing chamber does not vary appreciably. For example, a layer of material to be added may be maintained above the melt surface in the mixing chamber. If this is not sufficient, the material may be introduced by means of a screw feed.

In cases where the stirrcr is used not for adding material but as a degassing device for the melt, the pipe 19 may be closed or omitted entirely. However, the mixing chamber should still have the enlarged portion 25 to incrcase the capacity of the mixing chamber to collect gas liberated from the melt.

Although the above description relates to employment of the stirrer in a melt contained in a ladle, it will be appreciated that the stirrer can be employed in other kinds of melt containers and furnaces.

WHAT I CLAIM IS: 1. A mechanical stirrer for metallurgical melts comprising a stirrer body which is submersible in the melt and rotatable about a vertical axis which is an axis of symmetry of the body, a mixing chamber within the stirrer body, an inlet channel leading to the mixing chamber and having a length which is not more than twice its average internal transverse dimension, and a plurality of discharge channels from the mixing chamber which. in use of the stirrer, incline upwardly from the mixing chamber at an angle of at least 10 , each of said discharge channels having a length which is at least twice its average internal transverse dimension, and the mixing chamber being shaped so that its cross-sectional area in the region where, in the position of use of the stirrer, the lowest points of the discharge channels enter the mixing chamber is at least 40 per cetit greater thin the cross-sectional area of said inlet channel.

2. A stirrer according to claim 1, in which the external surfaces of the stirrer body above and below the discharge channels are shaped to encourage the melt to flow in toroidal paths.

3. A stirrer according to claim 2, in which said external surfaces of the stirrer body are concavely-curved or frustoconical.

4. A stirrer according to any of the preceding claims, in which each discharge channel has a cross-sectional area where it communicates with the mixing chamber which is at least 20 per cent larger than at its discharge end.

5. A stirrer according to any of the preceding claims, comprising a channel through the stirrer body for delivering additive material for the melt to the mixing chamber.

6. A stirrer according to claim 5, in which said additive delivering channel has its longitudinal axis coaxial with the axis of rotation of the stirrer body.

7. A stirrer according to claim 5 or 6, in which said additive delivering channel is connected to a hopper for additive material.

8. A stirrer according to claim 7, in which the hopper is provided with a closure device.

9. A stirrer according to claim 8, comprising means for varying the pressure within the hopper.

10. A stirrer according to any of claims 5 to 9, comprising means for closing said additive delivering channel.

11. A stirrer according to any of claims 5 to 10, in which said additive delivering channel comprises means for positively feeding additive material into the mixing chamber.

12. A stirrer according to claim 11, in which said feeding means is a screw conveyor.

13. A stirrer according to claim 6, or any of claims 7 to 12 when dependent on claim 6, in which said additive delivering channel is enlarged where it enters the mixing chamber.

14. A mechanical stirrer for metallurgical melts constructed and arranged substantially as herein described with reference to, and as illustrated in, Figures 1 and 2, or Figures 1 and 2 as modified by Figure 3, of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (14)

**WARNING** start of CLMS field may overlap end of DESC **. the stirrer shown in the drawings, which has two diametrically disposed channels 7, the mixing chamber may have plane wall portions disposed parallel to the axes of the channels 7. This has the effect of causing acceleration of the melt from the mixing chamber right from the centre of rotation of the stirrer. The effective length of the discharge channels 7 then extends, so far as pumping action is concerned, virtually to the centre of the mixing chamber 30, and the above mentioned demands regarding the geometry of the channels 7 between the locations D3 and D4 can then be relaxed to some extent. In cases where the speed of rotation of the stirrer is insufficient to control the location of the surface of the melt in the mixing chamber, it may be advisable to control the pressure in the pipe 19, or at least to control the charging of additives from the hopper 21 so that the pressure in the mixing chamber does not vary appreciably. For example, a layer of material to be added may be maintained above the melt surface in the mixing chamber. If this is not sufficient, the material may be introduced by means of a screw feed. In cases where the stirrcr is used not for adding material but as a degassing device for the melt, the pipe 19 may be closed or omitted entirely. However, the mixing chamber should still have the enlarged portion 25 to incrcase the capacity of the mixing chamber to collect gas liberated from the melt. Although the above description relates to employment of the stirrer in a melt contained in a ladle, it will be appreciated that the stirrer can be employed in other kinds of melt containers and furnaces. WHAT I CLAIM IS:

1. A mechanical stirrer for metallurgical melts comprising a stirrer body which is submersible in the melt and rotatable about a vertical axis which is an axis of symmetry of the body, a mixing chamber within the stirrer body, an inlet channel leading to the mixing chamber and having a length which is not more than twice its average internal transverse dimension, and a plurality of discharge channels from the mixing chamber which. in use of the stirrer, incline upwardly from the mixing chamber at an angle of at least 10 , each of said discharge channels having a length which is at least twice its average internal transverse dimension, and the mixing chamber being shaped so that its cross-sectional area in the region where, in the position of use of the stirrer, the lowest points of the discharge channels enter the mixing chamber is at least 40 per cetit greater thin the cross-sectional area of said inlet channel.

2. A stirrer according to claim 1, in which the external surfaces of the stirrer body above and below the discharge channels are shaped to encourage the melt to flow in toroidal paths.

3. A stirrer according to claim 2, in which said external surfaces of the stirrer body are concavely-curved or frustoconical.

4. A stirrer according to any of the preceding claims, in which each discharge channel has a cross-sectional area where it communicates with the mixing chamber which is at least 20 per cent larger than at its discharge end.

5. A stirrer according to any of the preceding claims, comprising a channel through the stirrer body for delivering additive material for the melt to the mixing chamber.

6. A stirrer according to claim 5, in which said additive delivering channel has its longitudinal axis coaxial with the axis of rotation of the stirrer body.

7. A stirrer according to claim 5 or 6, in which said additive delivering channel is connected to a hopper for additive material.

8. A stirrer according to claim 7, in which the hopper is provided with a closure device.

9. A stirrer according to claim 8, comprising means for varying the pressure within the hopper.

10. A stirrer according to any of claims 5 to 9, comprising means for closing said additive delivering channel.

11. A stirrer according to any of claims 5 to 10, in which said additive delivering channel comprises means for positively feeding additive material into the mixing chamber.

12. A stirrer according to claim 11, in which said feeding means is a screw conveyor.

13. A stirrer according to claim 6, or any of claims 7 to 12 when dependent on claim 6, in which said additive delivering channel is enlarged where it enters the mixing chamber.

14. A mechanical stirrer for metallurgical melts constructed and arranged substantially as herein described with reference to, and as illustrated in, Figures 1 and 2, or Figures 1 and 2 as modified by Figure 3, of the accompanying drawings.