MIXING CHAMBER

Field of the Invention

This invention relates to a mixing chamber. Particularly, the invention relates to a mixing chamber that is suitable for use within a metal transfer vessel (such as a tundish or holding furnace) to enhance the mixing of alloys during a continuous casting process.

Background to the Invention

In the continuous casting steel-making process, molten steel is poured from a ladle into a large metal transfer vessel known as a tundish. A ladle shroud is employed to try to protect the liquid steel from re-oxidation when flowing from the ladle to the tundish. A so-called impact pad is positioned in the tundish beneath the ladle shroud. In addition to protecting the base of the tundish from erosion, the impact pad, depending on its specific geometry, may act to dampen the energy of the incoming stream of metal thereby suppressing turbulence within the tundish. One or more outlets are provided in the tundish to allow the molten steel to flow from the tundish into one or more respective moulds. A submerged entry nozzle or shroud is located between the tundish and each mould, and guides the molten steel flowing through it from the tundish to the mould. The molten steel cools and forms an outer skin as it leaves the moulds eventually forming a continuous length of solid metal.

It is known to use a series of weirs, baffles and dams set transversely along the length of the tundish to control the flow of steel therein. In particular, these flow control devices are employed to increase the residence time of the metal in the tundish and to promote upwardly directed flow to aid so-called inclusion removal, which is the mechanism whereby undesirable impurities such as metal oxides tend to float to the top of the liquid metal. These devices also help to reduce turbulence, which could draw impurities back down into the body of the liquid metal.

Traditionally, alloying of steel is performed in the ladle and/or furnace. Thus, all elements required for a particular composition have been introduced into the steel when it is in the ladle. The alloyed mixture is then flowed via the tundish into moulds, as

described above. When it is desired to form a different alloy the existing ladle must be exchanged for a fresh ladle in which the new alloy is introduced/mixed. Thus, any of the previous alloy mixture that is still in the ladle when the casting of that particular alloy is no longer required is redundant and will need to be disposed of. This often results in a large amount of waste.

A particular problem with leaded-steel alloys arises at least in part because of the very different relative densities of lead and steel. The lead has a tendency to sink or separate out from the steel. As a result, a significant amount of the lead tends to gather on the base of the tundish. This results in the alloy in the tundish, and consequently in the mould, not having a homogeneous distribution of lead.

The problems of alloying elements coming out of solution and/or improper mixing are encountered in the continuous casting of other materials such as aluminium alloys.

Thus, it is an aim of the present invention to provide an apparatus and method that can be used to enhance the mixing of alloys to maintain consistency of the alloy during continuous casting but without introducing undesirable turbulence within the metal transfer vessel. It is also an aim of the present invention to reduce the amount of waste incurred when forming alloys.

Summary of the Invention

According to a first aspect of the present invention there is provided a mixing chamber for use in a metal transfer vessel (for example, a tundish used in steel manufacture or a holding furnace used in aluminium manufacture), the mixing chamber comprising a base with a first series of wall components defining a first mixing zone and a second series of wall components, a second mixing zone being defined between at least one of said second series of wall components and at least one of said first series of wall components; a continuous flow path being provided from the exterior of the mixing chamber to said first mixing zone, from said first mixing zone to said second mixing zone, and from said second mixing zone to the exterior of the mixing chamber; the region of said flow path between said first mixing zone and said second mixing zone

being defined by a first cut-out in at least one of said first wall components and the region of said flow path between said second mixing zone and said exterior of the mixing chamber being defined by a second cut-out in at least one of said second wall components; wherein the first cut-out is spaced from the base and said first and second cut-outs are mutually spaced along the flow path within the second mixing zone such that in use the general directions of flow within the second mixing zone and between the first and second mixing zones are different.

According to a second aspect of the present invention there is provided a metal transfer vessel incorporating a mixing chamber according to the first aspect of the present invention.

According to a third aspect of the present invention there is provided, in a continuous casting process, a method of producing a consistent alloy composition comprising introducing the constituents of the alloy into a mixing chamber according to the first aspect of the present invention located in a metal transfer vessel, whereby said constituents are forced along a tortuous route within the mixing chamber such that they are mixed and the metal emerging from the mixing chamber has a consistent alloy composition.

As used herein "consistent alloy composition' ^" means that the alloy emerging from the casting process has substantially the same composition throughout the duration of the process. The alloy may be homogeneous, i.e. when the alloying components are in solution or heterogeneous when the alloying components are not in solution.

Thus, the method of the present invention promotes the mixing of alloy constituents within a mixing chamber such that a consistent alloy composition is flowed from the mixing chamber into the metal transfer vessel. The mixing is encouraged through the provision of a tortuous flow path that results in turbulent flow within the mixing chamber. However, the design of the mixing chamber is such that substantially non- turbulent flow exits the mixing chamber and so the flow within the metal transfer vessel itself is substantially non-turbulent, as desired. A particular advantage when mixing a

leaded-steel alloy in the mixing chamber of the present invention is that the induced mixing of the alloy close to the mould minimises the separation of the lead from the steel. Moreover, any lead (or other components) that do separate or drop out of solution are likely to be contained within the mixing chamber and not flowed into the metal transfer vessel or mould.

In a particular embodiment of the present invention, the metal transfer vessel is a tundish configured for use in the continuous casting of steel.

In an alternative embodiment of the present invention, the metal transfer vessel is a holding furnace configured for use in the continuous casting of aluminium.

For ease of reference, the metal transfer vessel will hereinafter be referred to as a tundish.

In one embodiment of the present invention, at least one of said first series of wall components has a cut-out extending downwardly from a top edge and optionally inwardly from a side edge, the region of the wall component below the cut-out defining a weir.

Optionally, the second cut-out is defined by a hole in at least one of said second series of wall components. A plurality of holes (e.g. 4) may be provided (for simplicity said holes being collectively referred to hereinafter as "the second cut-out"). The hole(s) may be angled in said second wall component so as to direct flow to exit said mixing chamber in an upward direction.

In a specific embodiment, two second mixing zones are provided, each second mixing zone being defined between a different pair of said first and second series of wall components each being provided with an associated first and second cut-out respectively thereby defining a branch in the flow path between the first mixing zone and the two second mixing zones.

It will be understood that the number of second mixing zones is unlimited, the only requirement being that for each second mixing zone first and second cut-outs must be provided in the respective wall components of the first and second series defining it so that there is a flow path into and out of each second mixing zone.

Said first series of wall components may be a plurality of discrete walls. Alternatively, said first series of wall components are integrally formed to define a single wall.

Said second series of wall components may be a plurality of discrete walls. Alternatively, said second series of wall components are integrally formed to define a single wall.

At least some of said first series of wall components may be integrally formed with at least some of said second series of wall components.

Said mixing chamber may be a discrete assembly for insertion into a tundish.

Alternatively, said mixing chamber is assembled within a tundish from a series of discrete components. In one embodiment, parts of the tundish may form parts of the mixing chamber. Particularly, the base of the tundish may constitute the base of the mixing chamber. Additionally, or alternatively, at least one wall of the tundish may constitute one wall component of the first and/or second series of wall components.

At least one drainage hole may be provided adjacent the base in at least one of the first wall components. This allows for normal (as opposed to emergency) drainage of the first mixing zone. The drainage hole(s) may permit drainage directly from the first mixing zone to the exterior of the mixing chamber. Alternatively, the drainage hole(s) permit(s) drainage from the first to the second mixing zone. In this embodiment, it is preferable that the second cut-out extends closer to the base than the first cut-out so that, after use, the level of material remaining in the entire mixing chamber settles to the level determined by the second cut-out as opposed to the level determined by the first cut-out. This permits more material to exit the mixing chamber and therefore results in less wasted material remaining in the mixing chamber. In addition, when the mixing

chamber is initially being filled with material, the drainage holes allow passage of some material into the second mixing zone while the level of material in the first mixing zone is increasing. This helps to equalise the pressure on both sides of the wall components between the first and second mixing zones and therefore reduces the stress on such wall components on initial delivery of material into the first mixing zone.

A ridge may be provided on the base, transversely of the general direction of the flow path in the first mixing zone. This acts to churn the flow in an upward direction to promote mixing.

The or each drainage hole may be positioned further downstream in the flow path than the ridge such that the ridge acts to prevent flow from flowing directly through the drainage hole. This construction also retards the flow of undissolved or unmixed constituents of the alloy from flowing directly out of the first mixing zone.

The first and second wall components of the mixing chamber may be shorter in height than the sides of the tundish in which the mixing chamber is to be used. This is advantageous because it allows heat to dissipate in each section of the tundish during the pre-heat phase.

Optionally, the wall component defining an exterior wall of the mixing chamber is provided with a cut-out extending downwardly from its top edge. This allows for quick emptying of the contents in the case of an emergency. In use, this wall may be aligned with the tundish overflow chute for emergency draining.

The mixing chamber may further comprise a lid. The lid may be provided with an opening into the first mixing zone configured for receiving the lower end of a ladle shroud. In a particular embodiment, the lid is constituted by the tundish lid.

Optionally, the mixing chamber is provided with lifting hooks.

In a particularly preferred embodiment the mixing chamber is a box having a base and

upstanding therefrom first and second end walls and a pair of side walls; a pair of mutually spaced inner walls extending between the end walls, the inner walls being spaced from the side walls such that the first mixing zone is defined between said inner walls and two second mixing zones are defined respectively between each inner wall and an adjacent side wall, each inner wall having a cut-out extending downwardly from a top edge and inwardly from the second end wall and each side wall having a cut-out spaced from the base and adjacent the first end wall.

Optionally, said side walls and said inner walls are substantially parallel and are perpendicular to the end walls.

One or both of the end walls may be provided with vertical guide slots within which the inner walls are located.

A ridge may be provided in the base parallel to and adjacent but spaced from the second end wall. Optionally, the ridge extends across the full width of the first mixing zone and may extend across the full width between the side walls.

In a further embodiment of the first aspect of the invention, the mixing chamber includes a third mixing zone interposed between the second mixing zone and the exterior of the mixing chamber such that rather than the flow path passing directly from the second mixing zone to the exterior of the mixing chamber it passes from the second mixing zone through the third mixing zone and then to the exterior of the mixing chamber.

It will be understood that the mixing chamber of the present invention obviates the need for a separate impact pad in the tundish, with the benefits associated with impact pads being largely retained.

In relation to the third aspect of the present invention, at least some of the constituents of the alloy may be introduced into the mixing chamber as solid particles or wire. Alternatively, the constituents may be introduced in molten form.

In a first embodiment of the third aspect of the invention, all of the constituents of the alloy are pre-mixed before being introduced into the mixing chamber. In this embodiment, the constituents may be mixed in a ladle and/or furnace.

In a second embodiment of the third aspect of the invention, at least one of the constituents of the alloy is added into the mixing chamber separately from but concomitantly with the other constituent(s). Thus, in this embodiment, a basic alloy prepared in a ladle and/or furnace can be used to create different alloys in the tundish by introducing further constituents directly into the mixing chamber when disposed within the tundish. This results in less waste of the constituents in the ladle.

Optionally, in the second embodiment of the third aspect of the present invention, the alloy is a steel alloy and said at least one constituent of the alloy added separately into the mixing chamber is lead or calcium.

Alternatively, in the second embodiment of the third aspect of the present invention, the alloy is an aluminium alloy and said at least one constituent of the alloy added separately into the mixing chamber is silicon or magnesium.

Note that in the present invention the nature of the base metal and alloying components are not limited to those described above. Nor is the mechanism for the introduction of the alloying components particularly limited. Granular or other particulate solids can be metered from a hopper or screw fed. Wires can be introduced via a motorised (and metered) spindle and liquids can be dosed from any suitable metered container.

Brief Description of the Drawings

Particular embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 illustrates schematically a perspective view of a mixing chamber according to the present invention, with internal structure illustrated with dashed lines;

Figure 2 illustrates schematically a top plan view of the mixing chamber of Figure 1 : Figure 3 illustrates schematically a side elevation view of the mixing chamber of Figure 1. with internal structure illustrated with dashed lines:

Figure 4 illustrates schematically a side elevation view of an internal wall component of the mixing chamber of Figure 1 : and

Figure 5 illustrates schematically a perspective view of an alternative mixing chamber according to the present invention, with internal structure illustrated with dashed lines.

Detailed Description of Certain Embodiments With reference to Figures 1 through 3, there is illustrated a first embodiment of a mixing chamber 10 according to the present invention. The mixing chamber 10 is generally in the form of an open-topped box with a rectangular base 12, first and second opposed end walls 14. 16 and first and second opposed side walls 18. 20. Each of the end walls 14. 16 and side walls 18. 20 are slightly sloped to diverge away from the base 12. The base 12, end walls 14. 16 and side walls 18. 20 are moulded as a single article.

An inner (first) mixing zone 22 extends along the length of the chamber 10 between the first and second opposed end walls 14. 16. Two spaced apart and parallel baffles (inner walls) 28. 30 extend along the length of the chamber 10 between the two end walls 14. 16. The baffles 28. 30 are each slotted into respective pairs of channels 32a. 32b, 34a, 34b provided on the internal surfaces of the end walls 14. 16. The channels 32a. 32b. 34a. 34b each being defined by a pair of ridges 36a. 36b, 38a, 38b extending from the top of the end walls 14. 16 to the base 12. The spacing between the two baffles 28. 30 serves to define central portions 24, 26 of the two end walls 14, 16. Thus, the inner mixing zone 22 is defined by the central portions 24. 26 of the two end walls 14, 16 and the baffles 28. 30 which collectively constitute a series of first wall components.

As shown best in Figure 4, each of the baffles 28, 30 incorporates a first cut-out 40. 42 at an upper corner thereof. The part of the baffle 28. 30 below the first cut-out 40, 42 serves in use as a weir 44. The first cut-outs 40, 42 are substantially square in shape resulting in the baffles 28, 30 being generally 'L'-shaped. The baffles 28. 30 are arranged within the chamber 10 such that their respective cut-outs 40, 42 are disposed

towards the second end wall 16.

A drainage hole 52. 54 in the form of a triangular cut-out disposed at the lower comer below the first cut-out of each of the baffles 28. 30 is also provided. The drainage holes 52, 54 are relatively smail in cross-sectional area when compared to the first cut-outs 40, 42.

As can be seen from Figure 2, two outer (second) mixing zones 56, 58 are defined between the baffles 28, 30 and the respective side walls 18, 20. Each of the outer mixing zones 56, 58 extends along the length of the chamber 10 between respective side portions 60a, 60b, 62a, 62b of the first and second opposed end walls 14, 16. Thus, in this embodiment, the respective side portions 60a, 60b, 62a, 62b of the two end walls 14, 16 together with the respective side wall 18, 20 can be considered a series of second wall components which partly define the respective outer mixing zones 56, 58.

Each of the side walls 18, 20 includes a set of four circular holes 64, 66 collectively defining a second cut-out in each of the side walls 18, 20. Each set of holes 64, 66 is disposed towards the first end wall 14 such that in use metal flowing into the second mixing zones 56, 58 through the first cut-outs 40, 42 must flow at least along part of the length of the second mixing zones 56, 58 before exiting through the holes 64, 66. In this particular embodiment, each set of holes 64, 66 comprises two upper holes and two lower holes which are slightly vertically off-set. Each of the holes 64, 66 is angled through the respective side wall 18, 20 such that its exit point from the chamber 10 is further from the base 12 than its entry point within the chamber 10.

In use, the mixing chamber 10 is positioned in a tundish below a ladle shroud (not shown) through which molten metal will flow from a ladle such that metal enters the mixing chamber 10 close to the first end wall 14 in the inner mixing zone 22. The flow hits the base 12 and is deflected off the first end wall 14 and baffles 28, 30 but is relatively free to flow towards the second end wall 16. Thus, the general direction of flow within the inner mixing zone 22 is from the first end wall 14 towards the second end wall 16. As molten steel is continuously entering the inner mixing zone 22 the

incoming stream generates turbulence and therefore mixing therein. As the drainage holes 52, 54 are relatively small the majority of the flow is contained within the inner mixing zone 22 until the level of steel exceeds the height of the weir and the flow begins to exit through the first cut-outs 40. 42 into the outer mixing zones 56, 58 although it will be understood that some initial flow through the drainage holes 52, 54 is advantageous in that the force of the steel acting on the baffles 28, 30 from within the inner mixing zone 22 is partially compensated for. The flow is then contained within the respective outer mixing zones 56, 58 where it is further mixed due to deflections from walls defining the second mixing zones 56, 58. When the flow reaches the height of the holes 64, 66 the flow exits the mixing chamber 10 in a generally upwardly direction due to the inclination of the holes 64, 66. The flow exiting the chamber 10 is substantially less turbulent than that within it. It will be appreciated that the general direction of flow within the outer mixing zones 56, 58 is substantially opposite to the direction of flow within the inner mixing zone 22.

The mixing chamber 10 can be used to create an alloy in the tundish. In this embodiment the arrangement of the mixing chamber 10 in the tundish and the pouring of a base alloy into the mixing chamber 10 is as described above. In addition, a delivery mechanism (such as a hopper, a metering device, a coil or a pouring vessel) containing a further constituent of the alloy (i.e. lead) is used to introduce the further constituent into the mixing chamber 10 at a predetermined rate. The alloying constituent is usually introduced into the mixing chamber in solid form (such as in the form of particles or wire) but it may also be introduced in other fonns (e.g. as molten material). The alloying constituent or quantity thereof added to the base alloy can be readily changed to allow a range of alloys to be prepared conveniently and efficiently.

A second embodiment of a mixing chamber 1 10 according to the present invention is illustrated in Figure 5 with like components to those described above identified with the prefix T. This embodiment includes all features of the first embodiment with the following differences. An upper edge of the first end wall 1 14 is provided with a cutout 200. More specifically, the upper edge curves gently downwardly from the two side walls 1 18, 120 and levels off in the vicinity of its central portion 124. This allows for

quick emptying of the chamber 1 10 in an emergency by tipping the mixing chamber 110 backwards so that metal can exit the mixing chamber 1 10 through cut-out 200.

A further difference is that a transverse ridge 202 extending between the opposed side walls 1 18, 120 is provided in the base 1 12 as an integral part thereof. The ridge 202 has a generally triangular cross-section with a rounded apex 204. Each of the baffles 128,

130 has a complementary shaped cut-out 206, 208 to accommodate the ridge 202. The ridge 202 is disposed towards the second end wall 1 16 but is spaced further from the second end wall 116 than the drainage holes 152, 154. The ridge 202 extends vertically as far as the top of the drainage holes 152. 154.

The mixing chamber 110 is used in substantially the same manner as described above but with the added advantage that the ridge 202 helps to block flow from exiting straight through the drainage holes 152. 154 and also helps to generate further turbulence within the inner and outer mixing zones 122, 156, 158.

Although the specific embodiments described relate to a mixing chamber 10, 1 10 with one inner (first) mixing zone 22, 122 and two outer (second) mixing zones 56, 58, 156, 158. any number of first and second mixing zones may be employed although normally only one inner mixing zone will be provided since there will normally only be a single incoming metal stream (disregarding any components to be mixed separately). In addition, the shape and form of the first and second cut-outs 40. 42, 64, 66. 140, 142, 164, 166 may be varied to achieve the desired effect.

It will be appreciated by persons skilled in the art that various modifications may be made to the above-described embodiments without departing from the scope of the present invention. For example, whilst the above discussion has been primarily concerned with the mixing of alloyed steel, the invention is equally applicable to use in any continuous casting process.