EP2747920B1

EP2747920B1 - Slag granulation device

Info

Publication number: EP2747920B1
Application number: EP12756123.1A
Authority: EP
Inventors: William Barry Featherstone
Original assignee: Primetals Technologies Ltd
Current assignee: Primetals Technologies Ltd
Priority date: 2011-08-26
Filing date: 2012-08-07
Publication date: 2017-03-15
Anticipated expiration: 2032-08-07
Also published as: IN2014DN00179A; WO2013029934A1; EP2747920A1; CN103764320A; WO2013029934A9; CN103764320B

Description

This invention relates to a dry slag granulation device, in particular for dry slag granulation using a rotary atomising granulator to obtain granulated glassy slag.
A method of granulation of molten slag using air jets to break up slag ejected from a rotating flat disk or cup is described in GB2148330 . Typically, for slag granulation, the flat disk or cup is made from some type of refractory ceramic material, such as high alumina refractory ceramic, or refractory with chromium additions.
However, it has been found that as slag flows rates increase, from a low slag flow type of typically between 1 and 100 Kg/min to high flow rates of about 3 to 6 tonnes per minute, as required in a typical blast furnace, then the flat refractory disk becomes inadequate, unable to cope with the increased flow rates, even if the diameter of the flat disk is increased and although atomisation can be improved by use of the refractory cup for higher flow rates, the edges of the cup rapidly become worn, as does any metal protective shell that is provided and the cup is ground down to a flat disk shape too.
Thus, as described in our co-pending patent application GB1204069.7 , a metal disk is provided for the granulator and a cooling system cools the metal disk. However, the structure of the rotary atomising granulator is such that the cooling system may not cool evenly across the metal disk, with the result that the centre of the disk receives the bulk of the hot slag, but has the least consistent cooling applied.
WO-89/00471 relates to a method for forming particles and fibres from normally solid materials, such as metals. RU-2274516 relates to devices for production of metal powders.
In accordance with the present invention, a dry slag rotary atomising granulator comprises a rotatable disk mounted on a support for rotation about an axis of rotation, wherein the disk comprises an annular metal disk having a first surface remote from the support and including a central opening; wherein the support comprises a hollow cylindrical structure coupled to the opening in the metal disk and a castable refractory material in the hollow cylindrical structure; and wherein the castable refractory material comprises slag.
The present invention addresses the problem of wear in a metal rotatable disk when operating at high slag flow rates by providing a central impact pad of refractory material to receive the molten slag.
Preferably, the disk further comprises terracing and a layer of solidified slag formed on the first surface.
Preferably, the metal comprises one of stainless steel, SG iron or mild steel.
The cylinder could be a punctuated cylinder comprising a plurality of columns, but preferably, the cylinder is open only at one end.
Preferably, the granulator further comprises a slag supply outlet for supplying slag onto the first surface of the disk, the outlet comprising one of a slag runner or tundish.
Preferably, the granulator further comprises a cooling system for supplying a coolant to the disk.
In one embodiment, the cooling system comprises coolant sprays directed at a second surface of the disk remote from the first surface.
The coolant sprays ,may comprise aerated liquid sprays, but preferably the sprays comprise water sprays.
Alternatively, the cooling system comprises a flowing coolant system.
Preferably, the granulator further comprises a housing, the housing comprising a cylinder coupled to an annular disk and mounted relative to the rotatable disk and support, such that the cylinder and annular disk form an annular passage for containing a flow of coolant between the housing and the rotatable disk and support.
Preferably, the granulator further comprises a sump for collecting coolant which has exited the annular passage at the circumference of the disk.
Any suitable coolant fluid may be used, but preferably, the coolant comprises water.
The disk may be substantially flat, but preferably, the first surface of the disk is concave.
Preferably, an extension of a chord between two points on the circumference of the disk, the chord passing through the axis of rotation, forms an acute angle with a tangent of the disk surface at the edge of the disk pointing towards the axis of rotation.
Preferably, the acute angle is between 30° and 50°.
More preferably, the acute angle is 40°.
An example of a slag granulation device according to the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 shows a first example of a slag granulation device, with a cup shaped disk;
Figure 2 shows an alternative to the device of Fig.1, with a flat disk;
Figure 3 shows a second example, with spray cooling;
Figure 4 shows a third example with flow cooling;
Figure 5 illustrates the second example, with terracing;
Figure 6 illustrates the third example, with terracing;
Figure 7 illustrates a first embodiment of a slag granulation device according to the present invention, applied to the example of Fig.5; and,
Figure 8 illustrates a second example of the slag granulation device of the present invention, applied to the example of Fig.6.

As described in our co-pending patent application GB1204069.7 , the example of Fig.1a shows a rotary atomising granulator 1 having a cup or dish shaped disk 2 mounted on a support 3 for rotation about an axis of rotation 6, the support being attached to a rotatable base 4. The disk 2 has sidewalls 10 and rotates within a shroud 5. The rotatable base 4 is connected to a variable speed drive shaft (not shown). The rotating disk 2 is typically formed in section as the segment of a circle. The radius of the circle varies depending on the diameter of the disk so that the edge of the disk is inclined at a preferred angle to the horizontal. The angle θ of the disk 2 at its outermost edge is preferably chosen such that an extension 9 of a chord 7 between two points A, B on the circumference of the disk passing through the axis of rotation 6 forms an angle of between 30 and 50 degrees to a tangent 8 of the disk surface at the edge of the disk pointing towards the axis of rotation, as illustrated in Fig.1b. The preferred edge angle is 40 degrees. The diameter of the dish is chosen dependent upon the design slag flow rate and a preferred speed of rotation of the disk, which is intended to avoid slag wool generation and to produce correctly sized slag droplets.
The disk is made of a metal, such as stainless steel, typically 310SS, or spheroidal graphite (SG) iron, or mild steel, although other suitable metals may be used. The metals need to be able to cope with the operating temperatures encountered in slag granulation and have good thermal conductivity. Fig.2 illustrates an alternative design, also with a metal disk, but in this case, rather than a cup shape, the disk is substantially flat, with sidewalls 10. As mentioned before, a flat disk does not cope with high flow rates as well as a cup or dish shaped disk, but the use of metal still allows an improvement over conventional ceramic refractory flat disks.
A further improvement is illustrated in Figs.3 and 4 that as well as using a suitable metal disk 2, the embodiments are provided with a cooling mechanism. The example of Fig.3 illustrates using cooling sprays and in Fig.4 using a flow of coolant. In both cases, the coolant is applied to the underside 18 of the disk 2, away from the surface 23 onto which the slag falls during operation. The examples show the disk 2 mounted on its support 3. This support may be a cylindrical support, concentric with the axis of rotation 6, attached to the base 4. However, in the embodiments where cooling by sprays is provided, the support 3 is typically a punctuated support to allow the coolant to reach the surface 18 of the disk nearer to the centre of the disk. In one example, a plurality of support columns, at least 3, preferably 4 are provided, spaced substantially equidistant from one another in contact with the disk surface 18. Thus the central part of the dish is also cooled by coolant sprays 13 as shown in Fig.3. Although, any suitable coolant could be used, water is preferred, as it is easily available, relatively inexpensive and does not require special storage conditions. In the example of Fig.3, the underside 18 of the dish is cooled by one or more water sprays 13. The rotation of the cup 2 and application of coolant ensures a good heat transfer coefficient (HTC) to keep the metal dish within its operating range of temperature.
The cooling effect of the water on the underside is enhanced by the velocity of rotation. The thermal conductivity of metals such as stainless steel, spheroidal graphite (SG) iron, mild steel, low carbon steel with a carbon content of less than 0.15%, or copper are such that applying cooling to the underside 18 causes a layer 22 of solidified slag to form on the upper surface 23. Operating temperature and thermal conductivity of the metal, in combination, influence the choice of material. Copper has a lower operating temperature, but conducts heat away more quickly, leading to a thicker layer of solidified slag forming on its upper surface, so still giving sufficient protection against wear caused by the impact of the hot slag. The solidified slag layer follows the outline of the metal disk. The thickness of the solidified slag layer is such that the shape of the disk is not changed, and particularly, the shape of the disk lip is not changed. The advantage of the formation of this layer 22 is that it protects the metal surface 23 which might otherwise suffer thermal shock caused by contact with each new molten slag flow 25 landing on the surface of the disk 2. This protective layer is illustrated in Figs.7 and 8. The slag 25 is supplied via an outlet of a slag runner or tundish (not shown) and falls substantially vertically through the outlet onto the centre of the rotating disk. Use of a tundish allows irregularities in the slag flow from the blast furnace to be evened out.
Typically, the top surface of the disk finishes at its outer edge with a 90 degree angle between that surface and the surface extending over the thickness of the dish material. The water from the water sprays is retained by the rotating side wall 10 of the disk and water thrown off the lower edge of the side wall is retained by the shroud 5 and returned to a sump (not shown). In an alternative example, partially illustrated in Fig.3, the sprays 13 may be air atomised water sprays provided with an air pipe 12 and a water supply pipe 13.
In a further example, shown in Fig. 4, the sprays may be replaced by a flowing fluid cooling system, again typically water cooling. In this system, the water is delivered in an annular passage 19 formed between a drive shaft 15 and a stationary pipe 14. One end 16 of the pipe is attached to a stationary dish 17 shaped with substantially similar contouring to the contours of the underside 18 of the granulating disk 2 and spaced from the underside 18 by a small amount. The water flows between the stationary dish 17 and the rotating disk 2 and discharges at the outer radius 20 into a cavity 21 below the stationary dish and is returned to the sump. This design means that there is no need for more complex sealing between the rotating dish 2 and the shroud 5. Although the rotating disk itself could be provided with internal water channels for cooling, this gives rise to problems with seals and distribution of the cooling channels, which are close together near the centre and more distributed at the edges leading to uneven cooling across the disk, so generally external sprays of flowing coolant systems are preferred.
Another problem, which may arise at high rotational speeds of operation using the water cooled design, is that the protective layer 22 of slag which forms on the upper surface 23 of the disk 2 slides away, resulting in the metal surface being subjected to thermal shock caused by contact with a new molten slag flow falling on unprotected metal. In order to address this issue and also to aid in the formation of the protective slag layer 22, the rotatable disk 2 may be provided with terracing 24 on its upper surface 23.
Examples of terracing are shown in Figs. 5 and 6 for the examples of Figs.3 and 4. A series of terraces 24 are formed in the upper surface 23 of the disk 2, for example by casting, machining, or pressing. The mechanism used is chosen according to the material properties, some being easier to machine or cast. Pressing and welding on terracing involves more process steps and potential issues with the integrity of the weld at typical operating temperatures make this option less preferred. The terracing 24 helps to ensure that the protective layer 22 of solidified slag is formed at the start of operation and remains in place and does not slip off during slag granulation. The number of terraces 24 is dependant upon the diameter of the disk 2 and optionally, each of the terraces is arranged to have equal areas in plan view, resulting in the terraces being closer together at larger radii where the forces tending to remove the slag layer are greater. Use of terracing aids in the formation and retention of a protective slag layer on the dish surface, which helps to reduce damage due to thermal shock and therefore leads to increased service life. Although not illustrated here, the terracing may be used on an otherwise substantially flat rotating disk, such as that shown in Fig.2, or the surface of the disk could be roughened, or provided with protruding tags to slow the first slag that impacts on the metal surface, enough that the cooling effect forms the protective layer.
The present invention provides a further improvement to the features described above and examples of embodiments of the invention will now be described. As can be seen in both Figs. 7 and 8, the support 3 is modified to comprise a cylinder 28, typically made of metal, which is connected, e.g. by welding to both the disk 2 and the rotating part 4. However, rather than a continuous concave surface of the disk, passing through the axis of rotation, the disk 2 is annular. In the centre 29 of the disk, there is no metal, leaving the cylinder 28 open where it joins the disk 2. Additionally, in the example of Fig.8, the disk is provided with terracing on the upper surface beyond the maximum radius of the cylinder. Up to the minimum radius of the terracing 24, i.e. within the radius of the cylinder, there is no metal. The cylinder 28 is filled with a castable refractory 26, comprising slag which forms a solid impact pad for the molten slag. Although this filling could be made separately and dropped into the cylinder, in which case, the supports may be punctuated or columnar as previously, a cylinder open only at the slag receiving surface has the advantage that molten material can be poured in and cooled, so that the solidified slag filler so formed replenishes the impact pad if it wears down. It is also more robust and easier to manufacture, so whether spray or flowing coolant are used, a cylinder closed by the rotatable base and only open where it joins the disk 2 is generally preferred.
By arranging for the refractory filler 29 to be fully contained within the cylinder 28, with a flat surface for contact with the slag, the surface being substantially flush with the surface of the annular disk, the design is very durable. Any maintenance, if required, amounts to simply refilling the cylinder 28 with castable refractory, such as ceramic refractory, solidified slag, or crushed filing of old slag, or other suitable material with insulating, rather than conducting, properties.
Particularly for high flow rates of hot slag, several tonnes per minute, replacing the centre 29 of the rotating disk with a replaceable refractory section has he advantages of dealing with the issue of poor heat transfer due to the difficulties of applying sufficient cooling in the centre of the metal disk, where the slag impacts, which may lead to excessive wear and therefore the requirement to replace or repair the dish. This is not a problem in other areas of the dish, where there is no direct contact with hot slag, so installing a replaceable centre section is an effective solution without losing the advantages of a metal rotatable disk with cooling and terracing.

Claims

A dry slag rotatory atomising granulator comprising a rotatable disk (2) mounted on a support for rotation about an axis of rotation, wherein the disk (2) comprises an annular metal disk having a first surface (23) remote from the support and including a central opening; wherein the support comprises a hollow cylindrical structure (28) coupled to the opening in the metal disk (2), which hollow cylindrical structure (28) is filled with a castable refractory material (26); and wherein the castable refractory material (26) comprises slag.
A granulator according to claim 1, wherein the disk (2) further comprises terracing (24) and a layer of solidified slag (22) formed on the first surface (23).
A granulator according to claim 1 or claim 2, wherein the metal comprises one of stainless steel, SG iron or mild steel.
A granulator according to any of claims 1 to 3, wherein the cylinder (28) is open only at one end.
A granulator according to any of claims 1 to 4, wherein the granulator further comprises a slag supply outlet (27) for supplying slag onto the first surface (23) of the disk (2), the outlet (27) comprising one of a slag runner or tundish.
A granulator according to any preceding claim, wherein the granulator further comprises a cooling system for supplying a coolant to the disk (2).
A granulator according to claim 6, wherein the cooling system comprises coolant sprays (13) directed at a second surface (18) of the disk (2) remote from the first surface (23).
A granulator according to claim 7, wherein the coolant sprays comprise water sprays.
A granulator according to claim 6, wherein the cooling system comprises a flowing coolant system.
A granulator according to claim 9, wherein the granulator further comprises a housing, the housing comprising a cylinder (14) coupled to an annular disk (17) and mounted relative to the rotatable disk (2) and support, such that the cylinder (14) and annular disk (17) form an annular passage (19) for containing a flow of coolant between the housing and the rotatable disk (2) and support.
A granulator according to any of claims 6 to 10, wherein the granulator further comprises a sump, for collecting coolant which has exited the annular passage (19) at the circumference of the disk (17).
A granulator according to any of claims 6 to 11, wherein the coolant comprises water.
A granulator according to any preceding claim, wherein the first surface (23) of the disk (2) is concave.
A granulator according to claim 13, wherein an extension of a chord between two points on the circumferential of the disk (2), the chord passing through the axis of rotation, forms an acute angle with a tangent of the disk surface (23) at the edge of the disk (2) pointing towards the axis of rotation, wherein the acute angle is between 30° and 50°.
A granulator according to claim 14, wherein the acute angle is 40°.