Device for degassing molten metal

Abstract

A device for degassing molten metal comprising: a hollow shaft for a gas stream; a rotor (10) comprising roof and base sections (30) separated by a void and being connected to said shaft, the roof section (20) comprising an opening (60) for gas from the shaft; 3 to 8 supports (40), each radiating outwards and connected to the roof and base sections 20); a manifold section (50) for distributing gas through a mixing zone inlet (70); a mixing zone (90) contiguous to the mixing zone inlet (70), wherein the roof and base sections (20, 30) and adjacent supports (40) each extend radially outwards from the manifold section (50) to form the mixing zone (90); and a cut-out section (80) located between a rotational volume envelope of the rotor and the periphery of the rotor, wherein the cut-out section (80) is between 25% and 75% of said rotational volume.

Description

[0001] Device for degassing molten metal

[0002] Field of the Invention

[0003] The present invention relates to a device for degassing molten metal for first creating a vortex from which to deliver additives to the molten metal; and then forming and distributing fine bubbles of fluxing gases throughout the molten metal; molten metal processing systems comprising said device; and a process of treating molten metal using said device or system.

[0004] Background

[0005] Molten metal, such as aluminium, prior to casting, contains many impurities which, if not removed, cause high scrap loss in casting, or otherwise result in poor quality metal products. Typical undesirable impurities requiring removal include dissolved hydrogen, alkali or alkaline earth elements and undissolved non-metallic inclusions.

[0006] The injection of inert or reactive gas mixtures (i.e. fluxing gases) into molten aluminium is a commonly used technique for the removal of the above impurities. The rate at which these impurities are removed depends to a great extent on how the fluxing gas is injected into the molten metal. Optimum performance in this type of metal treatment process is achieved when fine gas bubbles are generated creating a large interfacial contact area for the metal treatment reactions to occur, and when these gas bubbles are distributed in a uniform fashion throughout the entire cross-sectional area available for metal flow.

[0007] Processes are known in which a rotating impeller is used to inject gas into a body of molten metal without the use of a filter bed. The function of the impellers used in these processes is to generate small gas bubbles, and to distribute them uniformly throughout the entire volume of metal to be treated, or to set up a metal flow pattern such that all of the metal to be treated passes through some portion of the rotating impeller. They also have some beneficial effect on metal cleanliness by removal of undissolved particulate impurities, or inclusions primarily by flotation. However, reliability of such processes for inclusion removal is variable, due to turbulence on the surface of the treated metal associated with the rotating impeller. Such turbulence tends to re-entrain the inclusions as well as floating dross.

[0008] In addition to the addition of gases, other treatment agents may be added to the molten melt to assist in grain refinement or modify the microstructure and the properties of the resulting cast metals and alloys. The metal treatment agents may be added into a vortex and readily dispersed throughout the melt. Any turbulence in the melt will lead to the introduction of air, and subsequently lead to the formation of oxides in the metal. Therefore, the vortex is only employed for a short part of the treatment cycle and once the mixing stage is complete, it is stopped (e.g. by application of a baffle plate). A vortex is created by the rotor blade pump liquid and, in doing so, generating vacuum cavities behind the blade that pressurise after the next blade arrives. An efficient rotor will create a vortex and disperse the treatments agents as quickly as possible in order to keep the turbulence in the melt to a minimum. Degassing and removal of the reaction products from the melt is then carried out. The use of the rotor to both create a vortex for efficient treatment agent addition and then for efficient distribution of fine bubbles creates inherent challenges in rotor designs, with the intense mixing required for vortex creation contrasting against the need for fine bubble distribution within the melt without turbulence which would lead to the detrimental re-entrainment of gas and impurities.

[0009] As the crucible or refractory lined container increases in size (over 800 mm diameter and up to 1.5tonne of aluminium, less frequent 3 tonne of aluminium and 10 tonne of aluminium in extreme cases), the vortex is more difficult to achieve. Rotor heads need to increase in diameter and height (and therefore weight, increasing the vibrations and cyclic shaft stress) as rotation speed are increased to obtain the desired vortex and therefore head erosion increases. All the above pushes towards redesign of the heads so that vortex generation is still fast and deep whereas weight and rotation speed increases are minimized.

[0010] As such, there is a growing need to a rotor which is able to efficiently form a vortex in a body of molten metal, and also then efficiently distribute fine bubbles of gas therein.

[0011] US8,281,964 partially addresses this need with the disclosure of a rotor in which molten metal is drawn into an opening in a base of the rotor and mixed with gases before being ejected outwards through radial passages. While this rotor design enables a generally acceptable gas distribution and vortex creation, there is still a demand for improved bifunctional rotor performance, particularly in terms of improved vortex creation efficiency.

[0012] As such, there is still scope for improved rotor designs and systems for treating molten metal, particularly for treatment in higher volume vessels.

[0013] Summary of the invention

[0014] In a first aspect of the present invention, there is provided a device for degassing molten metal comprising: a. A hollow shaft for carrying a gas stream (e.g. fluxing gas); b. a rotor comprising a roof section and a base section separated by a void, said rotor connected to said hollow shaft; c. in the range of 3 to 8 supports, each radiating outwards and connected to the roof section and the base section; d. a manifold section wherein the stream of gas from the hollow shaft enters through an opening in the roof section and said stream of gas is distributed radially through a mixing zone inlet; e. a mixing zone contiguous to the mixing zone inlet and radially offset from the manifold; the roof and base sections and adjacent supports each extend radially outwards from the manifold section to form the mixing zone forming at least part of the void, with peripheral edges of the roof, base sections and adjacent supports defining a mixing zone outlet; and f. a cut-out section located between a rotational volume envelope of the rotor and the periphery of the rotor, wherein the cut-out section represents in the range of between 25 and 75% of the rotational volume of the rotor.

[0015] In an alternative phrasing, the invention concerns a device for degassing molten metal comprising: a. a hollow shaft for carrying a gas stream; b. a rotor comprising a roof section and a base section separated by a void, wherein said rotor is connected to said hollow shaft, wherein the roof section comprises an opening for allowing entry of gas from the hollow shaft; c. in the range of 3 to 8 supports, each radiating outwards and connected to the roof section and the base section; d. a manifold section for distributing gas from the hollow shaft radially through a mixing zone inlet; e. a mixing zone contiguous to the mixing zone inlet and radially offset from the manifold, wherein the roof and base sections, and adjacent supports each extend radially outwards from the manifold section to form the mixing zone, wherein the mixing zone forms at least part of the void, wherein peripheral edges of the roof, base sections and adjacent supports define a mixing zone outlet; and f. a cut-out section located between a rotational volume envelope of the rotor and the periphery of the rotor, wherein the cut-out section is in the range of between 25% and 75% of the rotational volume of the rotor. Preferably, in use, the stream of gas from the manifold section (50) and a molten metal stream entering from the mixing zone outlet (100) to come into counter-current contact. The resultant metal and gas dispersion then radially discharges from the mixing zone (90) through the mixing zone outlet (100).

[0016] In some embodiments, the ratio of the minimum radial length (L1) between the inlet of the mixing zone (70) and the mixing zone outlet (100) and the minimum radial length (L2) between the inlet of the mixing zone (70) and the periphery of the rotor (L2) is in the range of >0.0 to 0.70. Whilst it is possible to have a negative ratio L1 :L2, these results when the inlet of the mixing zone feeds directly into the cut-out section, thereby bypassing the mixing zone. Further this arrangement also results in the supports not fully extending radially, reducing the rotor’s capacity to circumferentially pump the molten metal dispersion a hence reducing the vortex creation efficiency of the rotor.

[0017] Preferably, the ratio of L1 :L2 is at least 0.05 or at least 0.10 or at least 0.20 or at least 0.30. In general, the higher the ratio, the greater the 2D footprint of the mixing zone. Preferably, the L1 :L2 ratio is no more than 0.60 or no more than 0.50 or no more than 0.40 or no more than 0.38 or no more than 0.35. Too high a ratio may result in a lower capacity of the rotor to circumferentially move the molten metal dispersion to facilitate the formation of a vortex.

[0018] In some embodiments, the radial length from the mixing zone inlet to the mixing zone outlet increases as the mixing zone outlet approaches a support (e.g. Figure 9b). In other embodiments, the radial length from the mixing zone inlet to the mixing zone outlet is constant (e.g. the mixing zone inlet and mixing zone outlet share the same focus of circles of difference radius). In some embodiments, the cross-sectional area of the mixing zone outlet is larger than the cross-sectional area of the mixing zone inlet.

[0019] The device of the present invention is able to operate in an initial vortex formation mode to effectively form a vortex. This is achieved through the depth of the cut-out sections enabling a large volume of molten metal to be rotated in the same direction around the axis of the hollow shaft, thereby promoting the formation of a vortex, from which metal treatment agents can be effectively distributed within the mass of molten metal. Vortex formation becomes increasingly difficult as the volume (and internal diameter) of the vessels (e.g. crucibles) increase. Therefore, there is a growing demand for degassing devices which are able to efficiently create vortex in larger vessels, whilst also providing acceptable fluxing gas dispersion efficiency.

[0020] The device of the present invention is also able to create an efficient gas - molten metal mixing zone at the interface where the gas is being distributed radially outwards by the manifold section (50) and wherein the rotational action of the rotor blades (i.e. supports), formed by the plurality of supports (40), forces the molten metal into each of the mixing zone (90) to create a region of high shear forces where fine bubble formation and homogeneous distribution is promoted as the mixture is ejected out of the mixing zone into the body of molten metal.

[0021] By high shear forces, it is meant that forces generated by a stream of molten metal in a body moving in one direction colliding with a stream of gas moving in an opposing direction. For instance, when an impeller (i.e. support) of the rotor is rotated, the molten metal flows in the same direction as the impeller at a speed less than the speed of rotation of the impeller. However, both the rotor speed and the molten metal speed are usually not very different and hence the shear forces are relatively low compared to fluids colliding in opposing directions as what occurs within the mixing zone of the rotors of the present invention. The greater the difference in these two speeds of the molten metal and the fluxing gas, the greater the capability for dividing fluxing gas into fine bubbles by the shear force.

[0022] Whilst some conventional rotors inject the fluxing gases in between the rotor blades through small holes, the volume of high shear is relatively low as once the fluxing gas has been injected into the molten metal it is carried co-currently circumferentially around the central axis of the rotor. Through forming a mixing zone in which the molten metal can be mixed with the flux gas prior to exiting the mixing zone, mixing efficiency may be improved.

[0023] When the rotation of the impeller draws molten metal into the mixing zones, the colliding streams of molten metal and gas from the manifold creates a shear force which drives the formation of fine gas bubbles within a mixed molten metal phase which is then ejected into the bulk of the molten metal. It is this period of collision between the molten metal and gases which induces the greatest shear forces and has the greatest impact upon the efficiency and effectiveness of fine bubble formation and distribution within the molten metal.

[0024] The same rotational forces which drive the mass of molten metal in the same rotational direction to assist vortex formation (in vortex formation mode), also drive a portion of that molten metal mass into the high shear mixing zone in gas dispersion mode. The volume of molten metal and duration of time that the molten metal is within the mixing zone will determine the efficiency of fine bubble formation and distribution, which will thereby influence degassing efficiency.

[0025] In general, the larger the mixing zone, the better the degassing efficiency and the larger the cutout component the better the vortex creation efficiency. A consideration to this general concept is the need to sufficient mixing zone outlets and cut-out zone to promote the required high shear environment and flow patterns to promote mixing and vortex creating efficiencies. Rotors of between 3 and 8 supports provide such an environment.

[0026] The final rotor design may be a compromise between the mixing and vortex creating capacity of the rotor for the characteristics of the molten metal being processed and the geometry of the vessel holding the molten metal.

[0027] Mixing zone

[0028] The mixing zone is where a molten metal stream and a fluxing gas stream preferably first come in contact. This contact is counter-counter contact which preferably generates a high shear contact to generate fine bubbles dispersed within the molten metal.

[0029] The mixing zone forms at least part of the void between the roof section and base section. In some embodiments, the mixing zone consists of the void between the roof section and the base section. In other embodiments, the manifold section includes a portion of the void prior to the voids radiating laterally. In these embodiments, the diameter of the manifold section may broaden adjacent the opening in the roof section. The mixing zone inlets typically comprise a height lower that the height of the manifold section. The mixing zone height may be in the range of 20 mm to 100 mm; or 30 mm to 80 mm; or 40 mm to 60 mm.

[0030] As the mixing zone defines the space at which gas from the manifold is flowing radially outwards in a lateral direction; with the molten metal being drawn into the mixing zone from the opposing direction (i.e. through the mixing zone outlet), the mixing zone possesses a high shear environment which promotes the formation and dispersion of small gas bubbles within the molten metal. Each mixing zone preferably has one outlet. The outlet and inlet of each mixing zone preferably comprises a roof cut-out section and a base cut-out section and two support sections.

[0031] Each of the mixing zones is defined by the void between a roof section and a base section and adjacent radially extending supports. In some embodiments, each of the mixing zones is separated by adjacent supports. A mixing zone not defined by adjacent supports (e.g. in the form of a singular annular space with a plurality of outlets between the supports) may be more correctly defined as an enlarged manifold (See Comparative Example C-5). The enlarged manifold section comes at the expense of reducing the mixing zone area. It has been found that an enlarged manifold provides relatively poor mixing performance indicating that, in addition to mixing via the counter-current flow of fluids, the shearing action of the supports on the fluid and their restriction of flow contributes to increased turbulence and hence mixing efficiency of the rotor.

[0032] The roof section may taper in height from the opening (60) to the periphery of the supports. Alternatively, the roof may extend horizontally from the opening to maximise the rotational volume of the rotor.

[0033] In some embodiments, the mixing zone extends to the periphery of the rotational area. In other embodiments, the mixing zone only extends part of the way to the rotational area.

[0034] Preferably, the cross-sectional area of the mixing zone has a lower limit range of at least 4% or at least 5% or at least 8% or at least 10% or at least 12% or at least 15% and an upper limit of no more than 50% or no more than 40% or no more than 35% or no more than 30% of the rotational area of the rotor (TT*D2 / 4). The cross-sectional area of the mixing zone (taken from a mid-point in the height of the mixing zone) is measured from a top view of the rotor and represents an area in which a base section and roof section cover the inlet and outlet of the mixing zone. Similarly, the volume of the mixing zone may have a lower limit of at least 1% or at least 2% or at least 3% or at least 4% or at least 5% and an upper limit of no more than 25% or no more than 20% or no more than 15% of the total rotational volume.

[0035] In some embodiments, the cross-sectional area of the mixing zone may be at least 20% or at least 22% or at least 24% or at least 26% or at least 28% or at least 30% or at least 32% or at least 34% or at least 36% or at least 38% or at least 40% of the rotational area of the rotor.

[0036] The volume of the mixing zone may be a least 2% or at least 3% or at least 4% or at least 5% or at least 6% or at least 7% or at least 8% of at least 9% or at least 10% of the total rotational volume. In some embodiments, the mixing zone is no more than 15% or no more than 12% or no more than 10% or no more than 8% or no more than 7% of the rotational volume of the rotor. The larger the cross-sectional surface area (or volume) of the mixing zone the greater the proportion of the molten metal is exposed to a high shear zone environment in which the molten metal collides with the gas flowing into the mixing zone from the manifold section of the rotor.

[0037] The height of the mixing zone is typically in the range of 20 mm to 100 mm depending upon the specific application of the device. The width of the mixing zone (as measured between the supports) is governed by the number of supports within the rotor, but is typically at least 20 mm and may extend to 100 mm or more, particularly towards the outlet end of the mixing zone.

[0038] The outlet of the mixing zone is sufficiently large to enable the molten metal to enter into the mixing zone before exiting with a load of dispersed fine bubbles of fluxing gas. The minimum transverse dimension (e.g. height or width) is preferably at least 28 mm or at least 30 mm or at least 35 mm or at least 40 mm. The maximum transverse dimension of the outlet is typically no more than 125 mm or no more than 100 mm or no more than 80 mm or no more than 60 mm. Too small a dimensions results in blocking of the outlets and difficulties cleaning the outlet during maintenance, whilst too large an outlet may result in a lowering of the shear rate between the fluxing gas and molten metal or an increase in the propensity of channelling of components reducing the mixing effectiveness.

[0039] In some embodiments, each mixing zone outlet (100) spans at least 50% of the perimeter between circumferential ends of an adjacent pair of supports (40).

[0040] Manifold section

[0041] The manifold section functions to distribute the fluxing gas from the hollow shaft to the mixing zone, which is laterally offset from the manifold. The manifold section is preferably contiguous to the mixing zone inlet. Under normal operation (using standard designs i.e. the manifold section is not expanded at the expense of the mixing zone section), little or no molten metal enters the manifold section with the pressurised flux gas preventing molten metal flowing through the mixing zone and into the manifold section.

[0042] The manifold section connectingly interfaces with the gas exiting the hollow shaft. The manifold section is typically defined by the base and roof sections and the plurality of supports which, in combination, provide a plurality of mixing zone inlets from which the manifold distributes the gas. Whilst the base of the rotor is typically solid, with no openings, in some embodiments the base may comprise a small central aperture which is an artifact of the manufacturing process. In some embodiments, the base comprises no opening. In other embodiments the base section comprises an opening which is not sufficiently large to enable molten metal to be drawn into the opening, during operation, and mix with the gases in the manifold section of the rotor. In other embodiments the base comprises an opening with a diameter of less than 28 mm or less than 25 mm or less than 20 mm and, as such, does not function as an inlet for molten metal to mix with the gas in the manifold section. Typically, the narrow size of the opening results in it being covered by dross during operation, thereby effectively closing the opening in use.

[0043] The manifold section typically comprises a circular cross-sectional area from the top view of the rotor, although there are no limitations to the manifold’s section shape. However, to maximise the cross-sectional area of the mixing zone, the effective diameter of the manifold may be no more than 100 mm or no more than 80 mm or no more than 60 mm or no more than 50 mm or no more than 40 mm. The diameter of the manifold is typically at least 10 mm or at least 20mm or at least 30mm. Smaller inlet diameter may increase the effective velocity of the fluxing gas entering the mixing zone and therefore increase the degree of high shear and therefore fine bubble formation. However, a minimum inlet diameter may be required for the practicality of cleaning the rotor.

[0044] The height of the manifold section between the roof and the base section is typically in the range of 20 mm to 200 mm depending upon the application. In some embodiments, the height of the manifold section is at least 25 mm or at least 30 mm or at least 35mm. The larger the height of the manifold the larger the potential height of the inlet opening to the mixing zone, which enlarges the interfacial contact area between the molten metal and gas thereby enhancing fine bubble formation. Further, a larger mixing zone inlet height more readily avoids blocking up with dross and is also more easily cleaned.

[0045] In some embodiments, the manifold section comprising a top component with an internal diameter (ID-T) and a bottom component with an internal diameter (ID-B). The top component is typically associated with the section immediately adjacent the connection with the hollow shaft. The bottom section is typically associated with the section immediately adjacent the mixing zone inlet(s). The ID-T of the manifold section may be the same, or greater or smaller than the ID-B.

[0046] The ratio of the diameter of the rotor to the height of the rotor may be in the range of 1.5 to 5.0 or 1.7 to 4.0. (e.g. 180mm / 105mm to 160mm / 65mm).

[0047] Between 3 and 8 supports connects the roof and base section and forms part of the manifold section and the mixing zone. A smaller number of supports results in mixing zones that are relatively larger and therefore a high shear environment cannot be as easily created to promote efficient mixing, with channels of fluxing gas or molten metal able to pass through the mixing zone without adequate mixing being achieved. Further, with fewer supports, the flow patterns of the material between the supports are not as conducive to promote a vortex.

[0048] Having a greater number of supports results in a greater proportion of the rotor comprising supports increasing weight and decreasing the potential sizes of the mixing zones and cut-out sections. In some embodiments, the rotor may comprise 3 or 4 or 5 or 6 or 7 or 8 supports. In a preferred embodiment, the rotor comprises in the range of 4 to 6 supports.

[0049] In some embodiments, the supports connect the roof and base sections adjacent the inlet of the mixing zone, with the supports separating a plurality of mixing zones having a plurality of mixing zone inlets. In other embodiments, the cut-out section at least partially extends between the radial supports, such that at least a portion of the supports distal to the manifold section are in the form of blades comprising no circumferentially adjacent roof or base section. Within these embodiments, the mixing zone typically represents a relatively low portion of the rotational area.

[0050] In other embodiments, the cut-out section extends between the radial supports are such that the supports have circumferentially adjacent roof or base section. Within these embodiments, the mixing zone typically represents a relatively high portion of the rotational area.

[0051] The supports typically radiate from the manifold section. Due to the radiating arrangement of the supports, the inlets of the mixing zone generally have a smaller cross-sectional area than the outlet of the mixing zone. The dimensions of the supports are generally dictated by mechanical and erosive performance considerations. In a preferred embodiment, the minimum cross- sectional thickness of the supports is less than 20 mm or less than 18 mm or less than 16 mm or less than 14 mm. The minimum support thickness is preferably at least 6mm or at least 8 mm or at least 10 mm or at least 12 mm or at least 14 mm to provide sufficient corrosive and erosive resistance to enable the rotor to have a sufficiently long service life.

[0052] To aid in reducing the thickness of the supports, the supports are preferably manufactured from a carbon ceramic composite material, which has superior erosive resistance compared to graphite rotors. The reduced thickness of the supports, enable the mixing zone are to be increased and / or enables the cut-out section to be increased thereby assisting in enhancing degassing efficiency and / or vortex creation efficiency.

[0053] In some embodiments, the average thickness of the supports is in the range of 10 mm to 20 mm or in the range of 11 mm to 18 mm or in the range of 12 mm to 16 mm. Preferably, the average thickness of the supports is at least 10 mm or at least 11 mm or at least 12 mm or at least 13 mm or at least 14 mm or at least 15 mm.

[0054] In some embodiments, the support tapers from the top to the bottom or vice-versa. For example, one end of the support may have a thickness of 12 mm and the other end may have a thickness of 7mm.

[0055] In some embodiments, at least a partial or full section of the supports taper from proximal the central axis to a distal position to the central axis (i,e, the height of the supports decrease as the supports radiate outwards). Alternatively, a partial or full section of the supports are maintained at a constant height. The height of the support may vary from 30 mm to the full height of the rotor (e.g up to 200 mm). The supports preferably extend from the mixing zone inlet and extend to the periphery of the rotational area. Supports which start at the mixing zone outlet and extend to the periphery of the rotational area have reduced.

[0056] Cut-out sections

[0057] The cut-out sections are preferably contiguous to the mixing zone outlet. The cut-out section is located between a rotational volume envelope of the rotor and the periphery of the rotor. For clarity, the cut-out section is not inclusive of the manifold section.

[0058] The cut-out sections (e.g. of the roof and base) represent the difference between the rotational area of the rotor with a diameter D and the top / bottom view profile of the rotor (i.e. the cut-out section are cut-outs removed from a circle of diameter D to result in the top view profile shape of the rotor. Each of the segments may be any shape and size. Preferably, each of the cut-out segments are the same shape and size. Preferably, the roof cut-out sections and the base cutout sections are the same size and location, such that from a top view of the rotor the roof and base cut-out sections are fully superimposed upon each other. Having at fully superimposed cut-out sections enables the mixing zone areas to be maximised.

[0059] For completeness it is noted that the void between the roof and the base is within the periphery. The periphery of the rotor may be defined as being bounded by the smallest geometric surface connecting the periphery of the roof and the periphery of the base.

[0060] In some embodiments, fluid flow modelling may determine that an increased region of high shear may be obtained with some degree if misalignment between roof and base cut-out sections (i.e. roof and base cut-out sections may be partially superimposed). In other embodiments, fluid flow modelling may indicate that an increased region of high shear may be obtained with full alignment between roof and base cut-out sections.

[0061] The % area of the cut-out section of the rotor will equal the minimum of the cut-out section in the roof and base sections. Therefore, if the roof had a 5% cut-out section relative to the rotational area and the base had a 8% cut-out section relative to the rotational area, then the rotor would be deemed to have a % cut-out area of 5%. If the base or roof section had no cutout portion then the cut-out section would be deemed to be 0%. If the cut-out portion had no supports, then the cut-out section would be also deemed to be 0%, as without the supports, the cut-out section would not have the capacity to rotationally move the fluid to form the vortex. The %volume of the cut-out section may be determined by multiplying the minimum % area of the cut-out section in the base or roof and multiplying this area by the average height of the roof and base sections adjacent the roof and base cut-out sections.

[0062] In one embodiment, there are the same number of roof cut-out section as base cut-out sections. In some embodiments, there are a different number of roof cut-out section to base cut-out sections. While a single cut-out (roof and base) is typically produced between adjacent supports, it is possible that several cut-outs may be produced between adjacent supports.

[0063] They cut-out sections may be symmetrical or asymmetrical. The cut-out sections may be part circular or parabolic or linear. Part circular cut-outs have the advantage of being easily machined into the rotor. A greater depth of the cut-out sections enable the rotors to move more molten metal between the supports, which function as rotor blades. The more molten metal which can be moved circumferentially, the more readily the rotor may form a vortex around the rotating hollow shaft.

[0064] In one embodiment, a cut-out section comprises a first length extending from the perimeter of the cut-out from a peripheral end of a support to a turning point of the cut-out closest to a central axis of the rotor; and a second length extending from the turning point and extending along the perimeter of the cut-out to a peripheral end of the adjacent support defining the cutout, said first length greater than said second length.

[0065] The rotor is preferably configured such that the second length of each of the cut-out sections is the leading edge of the rotor. This configuration has been found to enhance vortex creation.

[0066] Within this embodiment, the ratio of the first length to the second length is preferably in the range of >1 to 5.0 and more preferably in the range of >1.2 to 2.5.

[0067] In some embodiments, the ratio of the area of the cut-out section to the area of the mixing zone is 1.0 or greater or 1 .2 or greater of 1 .4 or greater or 1.6 or greater or 1 .8 or greater. The superior mixing efficiency of the counter-current flow regime in the mixing zone, enables a proportionately higher amount of the rotor to be devoted to improving vortex creation.

[0068] For the purposes of the present invention, the cross-sectional area of each component (including the supports, manifold section, mixing zone) is the cross-sectional area from a top cross-sectional view (“top view”) presenting an orthogonal plane from the hollow shaft.

[0069] In some embodiments, the cross-sectional area of the cut-out section comprises in the range of between 20 and 70% of the rotational area of the rotor. In some embodiments, the cross- sectional area of cut-out section comprises at least 25% or at least 28% or at least 30% or at least 33% or at least 35% or at least 38% or at least 40% or at least 42% or at least 45% or at least 48% or at least 50% or at least 52% or at least 53% or at least 54% or at least 55% or at least 56% or at least 57% or at least 58% or at least 59% of the rotational area of the rotor. An increase in the % area of the cut-out section enhances the vortex creation efficiency of the rotor.

[0070] In some embodiments, the volume of the cut-out section comprises in the range of between 26 and 70% of the rotational volume of the rotor. In some embodiments, the volume of cut-out section comprises at least 28% or at least 30% or at least 35% or at least 36% or at least 37% or at least 38% or at least 39% or at least 40% or at least 41 % or at least 42% or at least 43% or at least 44% or at least 45% of the rotational volume of the rotor.

[0071] In some embodiments, the cut-outs sections extend greater than 50% (i.e. a linear line extending from the centre of the rotor to the peripheral of the rotational area is deemed the radius of the rotor and the cut-out section extends greater than 50% of the distance along this line from the periphery of the rotational area towards the centre of the rotor.), or greater than 51% or greater than 52% or greater than 53% or greater than 54% or greater than 55% or greater than 56% or greater than 57% or greater than 58% of the radius of the rotor. The greater the cut-out depth the greater the capacity of the rotor to increase vortex creation efficiency.

[0072] In other embodiments, the cross-sectional area of the cut-out section and the mixing zone comprises in the range of between 50 and 90% of the rotational area of the rotor. In some embodiments, the cross-sectional area of cut-out section and mixing zone comprises at least 59% or at least 60% or at least 61% or at least 62% or at least 63% or at least 64% or at least 65% or at least 66% or at least 67% or at least 68% or at least 70% of the rotational area of the rotor.

[0073] In some embodiments, the volume of the cut-out section and the mixing zone comprises in the range of between 30 and 80% of the rotational volume of the rotor. In some embodiments, the volume of the cut-out section and mixing zone comprises at least 36% or at least 37% or at least 38% or at least 39% or at least 40% or at least 41% or at least 42% or at least 43% or at least 44% or at least 45% or at least 46% of the rotational volume of the rotor.

[0074] In some embodiments, the rotor support thickness is governed by stress analysis, such that the size of the supports may be reduced with the cut-out section and / or mixing zone correspondingly increased in size. An advantage of increasing the portion of the mixing zone and the cut-out section is that this also promotes a reduction in rotor weight as the reduction in support and roof / base thickness result in an increase of the functional size of the rotor devoted to mixing and vortex efficiency. The use of carbon ceramic composites in the construction of the rotors, enables volumetrically lower material to be used compared to graphite due to the superior erosion resistance of carbon ceramic composites.

[0075] In some embodiments, the cut-out section comprises a stepped configuration, wherein the cutout section comprises a section between the mixing zone outlet and the rotational area / volume and a section above the mixing zone roof and between the upwardly extended supports. A further cut-out section below the mixing zone base and between the downwardly extending supports may also exist. Mixing zone dimensions to obtain a desirable high shear environment may also result in the ability to maximise the cut-out volume by using one or both of the spaces above and below the mixing zone roof and base. For example, the roof section proximal the opening (ID-T) may vertically extend down the top section of the manifold and then radiate outwards (horizontally or at an angle thereto), whilst the supports extend vertically above the radiating roof, partially or fully to the height of the opening. A section of the support may taper down (i.e, decrease in height) towards the periphery of the rotational area. In other embodiments the support height may remain constant from proximal the opening to the periphery of the rotational area.

[0076] Manufacture

[0077] The shaft and rotor may be formed separately and connected together with a releasable connection means, such as a screw thread on each of the rotor and the shaft. Alternatively, the shaft and rotor may be formed into an integral one-piece hollow shaft and rotor.

[0078] A rotor may be machined from a solid block of material, with the manifold, mixing zone(s) and cut-out sections machined out of the block. Alternatively, the rotor may be also formed through moulding and pressing techniques (e.g. isostatic pressing), particularly if the rotor configuration is not easily obtainable by machining alone. In particular, rotor designs which seek to maximise the %volume of the cut-out section and mixing zone relative to the total volume of the rotational volume are most likely achieved through moulding techniques or 3D printing manufacturing techniques.

[0079] In one embodiment, the hollow shaft section is inclusive of at least a portion of the roof section of the rotor. This enables the remaining part of the rotor to be machined from a single block. Greater design flexibility may be achieved in configuring the supports and mixing zones, without the obstruction of the roof section. To facility this configuration, the rotor section may comprise a male connector to join to a female connector on the hollow shaft section.

[0080] The rotor may then undergo heat-treatment to produce the final product. Additional machining of the components may also be performed. The rotor may also be coated with any erosive and / or anti-dross forming coatings.

[0081] The rotor and shaft may be made from graphite or a carbon-ceramic composite material. In some embodiments, the rotor comprises or consists of a carbon-ceramic composite material. Carbon-ceramic composite materials have the advantage of being more corrosive and erosive resistant than graphite. As such, the supports and walls of the rotor may have reduced thickness to increase the volume of the mixing zone and cut-out sections to thereby improve the rotor’s bifunctional performance. An example of suitable carbon-ceramic composition material is the Roton™ range of carbon-ceramic composition materials available from Molten Metal System GmbH.

[0082] The carbon-ceramic composite may comprise:

[0083] 30 to 70 wt% of a ceramic matrix

[0084] 30 to 70 wt% of a carbon material; and

[0085] 0 to 15 wt% additives.

[0086] The ceramic matrix is preferably selected for its combination of thermal and mechanical properties and may be selected from the group consisting of silica; alumina; carbides of Si, Ti, W, Ta, Nb, Zr, Hf, V, Cr, Mo; silicon nitride; magnesia; zirconia; boron nitride; aluminium nitride; or combinations thereof.

[0087] The ceramic matrix may comprise a silicon carbide, e.g. a beta silicon carbide and / or alpha silicon carbide.

[0088] The carbon material is preferably graphite and / or carbonaceous material derivable from an organic binder used in the formation of the composite material.

[0089] Additives may include silicon metal, FeSi, aluminium, boron, alumina-silicate (e.g. clay), borax and / or boric acid.

[0090] Molten metal degassing system

[0091] In a second aspect of the present invention, there is provided a molten metal degassing system comprising a device according to the first aspect of the present invention, and a molten metal vessel. The molten metal vessel may comprise an internal diameter of at least 650 mm or at least 800 mm or at least 850 mm or at least 900 mm or at least 1000 mm or at least 1100 mm or at least 1200 mm or at least 1300 mm or at least 1400 mm or at least 1500 mm. The maximum internal diameter of the molten metal vessel may be influenced by a variety of technical and commercial considerations, but is expected to be no more than 2000 mm or no more than 1500 mm.

[0092] In some embodiments, the ratio of the diameter of the rotor RDto the internal diameter of the vessel VDis in the range of 0.18 to 0.37 (e.g. 180mm / 960mm to 260mm / 700mm). The rotors of the present invention are particularly advantageous at RDto VDof less than 0.30 or less than 0.27 or less than 0.25 or less than 0.24 or less than 0.23 or less than 0.22 (e.g. a 300 mm diameter rotor in a 1500 mm internal diameter vessel).

[0093] Process

[0094] In a third aspect of the present invention, there is provided a process of degassing molten metal using the molten metal degassing system according to the second aspect of the present invention, wherein a rotor of the device rotates at a speed of greater than 300 rpm or greater than 350 rpm or greater than 400 rpm or greater than 450 rpm or greater than 500 rpm during a vortex formation mode before rotating at a speed of in a degassing mode that is less than the speed in the vortex formation mode. Preferably, the degassing mode speed is at least 50 rpm or at least 75 rpm or at least 100 rpm or at least 125 rpm or at least 150 rpm slower than the vortex formation mode.

[0095] Due to the rotor’s superior overall vortex creation and degassing efficiency, the average speed of the rotor in both modes may be lower than the average speed of other rotors in the prior art. In one embodiment, the average speed of the rotor is no more than 500 rpm or no more than 475 rpm or no more than 450 rpm or no more than 425 rpm or no more 400 rpm. Alternatively, by maintaining speeds similar to rotors in operation in the prior art, a superior vortex and or mixing efficiency may be achieved.

[0096] In some embodiments, the average rotor speed (i.e. speed in vortex mode + speed in degassing mode divided 2) to obtain a vortex depth of at least 550 mm after 40 seconds and a fluxing gas occupancy of at least 80% v / v (under the conditions outlined in the experimental section) is no more than 600 rpm or no more than 580 rpm or no more than 570 rpm or no more than 565 or no more than 550 rpm.

[0097] In some embodiments, the average rotor speed (i.e. speed in vortex mode + speed in degassing mode divided 2) to obtain a vortex depth of at least 400 mm after 15 seconds and a residual O2level of no more than 20% v / v after 450 seconds (under the conditions outlined in the experimental section) is no more than 450 rpm or no more than 425 rpm or no more than 400 rpm or no more than 375 or no more than 350 rpm or no more than 325 rpm.

[0098] Rotor

[0099] In a third aspect of the present invention, there is provided a rotor as defined in the first aspect of the present invention.

[0100] Rotational area

[0101] The rotational area is defined as the “top view” area covered by a rotor as it rotates around a rotational axis in a two-dimensional plane. The rotational area may be defined as TT*D2 / 4, where D is the diameter of the rotor.

[0102] Rotational volume

[0103] The rotational volume is defined as the volume covered by a rotor as it rotates around a rotational axis in a two-dimensional plane. The rotational volume may be calculated by determining the 3D envelope created by the rotor rotating in a two-dimensional plane. The envelope typically comprises a cylindrical portion towards the base section and may comprise a frustoconical portion towards the roof section, where a rotor roof section tapers downwards from proximal the opening towards the peripheral edges of the roof section.

[0104] Rotational volume envelope

[0105] The rotational volume envelope is defined as the envelope which encompasses the rotational volume.

[0106] The rotational area and volume are preferably the sum of the individual components of the rotor (e.g. manifold + supports + mixing zone + roof section + base section + cut-out section).

[0107] For defining the rotational area, rotational volume and rotational volume envelope, the axis of rotation is the geometric axis running through the center of the base and the center of the roof. This coincides with the geometric axis running through the center of the hollow shaft.

[0108] Brief Description of the Drawings

[0109] Figures 1a and 1b are a perspective and side view respectively of a rotor (Example 7) of the present invention illustrating the rotational volume of the rotor.

[0110] Figure 1c is a top view of the rotor (Example 7) illustrating the rotational cross-sectional area of the rotor. Figures 2a and 2b are a perspective and a cross-sectional top view* respectively of a rotor (Example 4) of the present invention.

[0111] Figures 3a and 3b are a side view and a cross-sectional top view respectively of a rotor (Example 1) of the present invention.

[0112] Figures 4a and 4b are a side view and a cross-sectional top view respectively of a rotor (Example 2) of the present invention.

[0113] Figures 5a and 5b are a side view and a cross-sectional top view respectively of a rotor (Example 3) of the present invention.

[0114] Figures 6a and 6b are a side view and a cross-sectional top view respectively of a rotor (Example 5) of the present invention.

[0115] Figures 7a and 7b are a side view and a cross-sectional top view respectively of a rotor (Example 6) of the present invention.

[0116] Figures 8a and 8b are a side view and a cross-sectional top view respectively of a rotor (Example 7) of the present invention.

[0117] Figures 9a and 9b are a side view and a cross-sectional top view respectively of a rotor (Example 8) of the present invention.

[0118] Figures 10a and 10b are a cross-section top view and a perspective view of a rotor of the present invention.

[0119] Figures 11a-11c are a side, perspective, and a cross-sectional top views respectively of a rotor (C-1) of the prior art.

[0120] Figures 12a and 12b are a side view and a cross-sectional top view respectively of a rotor (C-2) of the prior art.

[0121] Figures 13a and 13b are a side view and a cross-sectional top view respectively of a rotor (C-3) of the prior art.

[0122] Figures 14a and 14b are a side view and a cross-sectional top view respectively of a rotor (C-4) of the prior art.

[0123] Figures 15a and 15b are a side view and a cross-sectional top view respectively of a rotor (C-5) of the prior art.

[0124] Figures 16a and 16b are cross-sectional side view of the vessels used as part of the molten metal system of the present invention.

[0125] 'All cross-sectional top views are taken from the mid-point of the height of the mixing zone. Detailed Description of a Preferred Embodiment

[0126] With reference to Figure 1a and 1b, there is illustrated the rotational volume envelope of the rotor (Example 6), which comprises a cylindrical section (A) and a frustoconical section (B), which comprises five cut-out sections (C1) illustrating the rotational volume of the cut-out section. Figure 1c illustrates the cross-sectional area of the rotor, which comprises five cut-out sections (C2) illustrating the rotational cross-sectional area of the cut-out section. The rotational volume may be calculated through calculating the volume of the cylindrical and frustoconical components and adding them together. Whilst there is a trend to increase the rotational volume of the rotor to meet the needs of larger processing vessels, the increasing dimensions and speeds create increasing mechanical and erosive stresses on the rotor. To avoid these stresses which may shorten the working life of the rotor, the use of more erosion resistant materials and innovative rotor designs may be used. The black shaded sections of Figures 1a an 1b extend from the rotational volume envelope of the rotor to the periphery of the rotor and represent the cut-out sections of the rotor which contribute to the vortex creating efficiency of the rotor. An objective of the present invention to increase the vortex creating efficiency of the rotor, whilst maintaining acceptable mixing efficiency.

[0127] The rotational area and rotational volume of the rotors and components thereof were determined using “Autodesk® Inventor® Professional 2023” 3D CAD software. The volume component of the cut-out section of Example 6, as indicated in Table 2, represents 59% of the total rotational volume, with the mixing zone representing 5% of the total rotational volume of the rotor.

[0128] With Reference to Figures 2a to 2b, there is illustrated a rotor (10) for degassing molten metal, including, but not limited to aluminium, magnesium, and steel. The rotor comprises a roof section (20) and a based section (30), which is separated by a void space, while five supports (40) radiate outwards from the central axis of the rotor and connect the roof and base sections (20, 30) together.

[0129] The fluxing gas is fed into an opening (60) in the roof section (20) which is contiguous to the manifold section (50). The roof section may taper in height from the opening (60) to the periphery of the supports. The base section typically has a flat external base. The internal section of the base and roof may contain contouring consistent with machining tools used to remove material in a one-piece block construction process. The manifold section is typically cylindrical or frustoconical in shape. The manifold (5) may have a diameter the same or different to that of the opening (60). In some embodiments, the diameter of the manifold is larger than the diameter of the opening (60). The fluxing gas is distributed from the manifold laterally through mixing zone inlets 70 into a mixing zone 90, which forms at least part of the void between the roof section (20) and the base section (30). In embodiments, where the opening and the manifold are of the same diameter, the mixing zone and the void may consist of the same space.

[0130] As indicated in the cross-sectional top view in Figure 2b, there are 5 mixing zones (90), each with an inlet (70) and an outlet (100). The diameter of the outlet (S2) is larger than the diameter of the inlet (S1). The larger outlet diameter facilitates the ingress of molten metal into the mixing zone when the rotor is in operation. In other embodiments the inlet and outlet diameter may be the same (e.g. Figure 3b, Example 1).

[0131] The mixing zone outlets (100) are at least partially defined by cut-out sections (80) which have been made from a rotational area of the rotor. The rotational area is defined as the area covered by a rotor as it rotates in a two-dimensional plane, hence the rotational area may be defined as TT*D2 / 4, where D is the diameter of the rotor. The boundary of the rotational area is illustrated by a broken line circle in Figure 2b, with a diameter of 220 mm.

[0132] It has been found that an increase in the volume (or cross-sectional area) of the mixing zone correlates with an increase in degassing efficiency as measured by the % volume distribution of fluxing gases within a molten metal vessel (Figures 16a & 16b) after a specific time. It has further been found that an increase in the cut-out sections (80) in the rotor correlates to an improvement in vortex formation as measured by the depth of a vortex within a molten metal vessel after a specific time.

[0133] In order to maximise the degassing and vortex formation performance, it is desirable to reduce the proportion of the cross-section area / volume occupied by the manifold and supports. In respect of the supports, the circumferential thickness (T1) of the periphery of the supports (40) may be reduced. Additionally, the maximum thickness of the supports may be reduced. In some embodiments, the thickness of the rotor is substantially uniform (e.g. Figures 10a and 10b, Example 9).

[0134] In some embodiments, the mixing zone may be increased by extending the roof and base of the rotor radially. The outlets of each radially extended mixing zone may be linear profile between adjacent supports, is indicated by the dotted line (200) in Figure 10a, to form a pentagon shaped mixing zone separated by five supports. Alternatively, the mixing zone may be extended radially outward the same distance (L1) to form an annular shaped (top view) mixing zone separated by five supports. The height of the roof may be aligned to the height of the mixing zone and extend radially in a horizontal plane, such that the supports are higher than the extended roof. In such a configuration, additional rotational flow is obtained from the supports, functioning as rotor blades above the mixing zone roof. This has the advantages of reducing the rotor weight and partially offsetting the lost in the rotational volume to the mixing zone compared to if the roof extended to the top of the rotor, such as in illustrated in Example 4 (Figure 2a).

[0135] With a roof thickness of 15mm, the height of the top cut-out section may in the range of about 20 mm to 50 mm, depending upon the total height of the rotor.

[0136] Simulation modelling

[0137] A series of rotors were evaluated for their vortex creation performance and their gas dispersion performance through a computer gas-fluid dynamics model (Flow 3D™ software) using molten aluminium alloy (AISi7Mg0.3) and nitrogen gas. Each rotor had a diameter of 220 mm. Each rotor had a height of 95 mm, except for Example C-1 (Figures 11a & 11 b) and Example C-4 (Figures 14a & 14b), which had a height of 85 mm. The base thickness for Examples 1 to 8 is 15 mm. For Examples 1 to 7, the roof / supports tapers from a height of 95 mm to a height of 60 mm at the periphery of the rotational area. In Example 8, the roof / supports tapers from a height of 95 mm to a height of 70 mm at the periphery of the rotational area. The height (H) of the mixing zone is 30 mm for Example 1 to 7, and 55 mm of Example 8.

[0138] Degassing efficiency is determined by placing a rotor to a submerged depth of 700 mm (and a distance of 150 mm from the bottom) in a vessel having an internal diameter of 784 mm with a baffle (600mm long x 135 mm wide x 30mm thick) placed a distance of approximately 45 mm parallel to the shaft and after 15 seconds of degassing time at a rotation speed of 450 rpm, the % volume that the flux gas bubbles occupied was measured. The higher the % volume occupied the more efficient the degassing process.

[0139] Vortex formation efficiency is determined under the similar conditions, except for the absence of the baffle; and a rotor speed of 680 rpm, with the depth of the vortex from the surface measured after 40 seconds of the system being in vortex formation mode.

[0140] Water basin testing

[0141] Preparation of the Water Basin:

[0142] A cylindrical container with dimensions of 1000 mm in diameter and 1000 mm in height is filled with water up to a height of 700 mm. The primary oxygen meter is positioned 100 mm from the bottom of the container, while the secondary oxygen meter is located 500 mm from the bottom.

[0143] The water is initially saturated with O2until it reaches a concentration of 17 ppm. The degassing process with N2begins when the O2concentration drops to 17-15 ppm. This process is conducted with varying impeller designs and a rotational speed of 300 rpm. Data from the oxygen meter is recorded every 50 seconds until a total duration of 450 seconds is reached. The impeller design that can decrease O2levels the fastest is considered the most efficient design.

[0144] In a cylindrical container filled with water up to a height of 700 mm, a vortex is generated using different impeller design and a rotational speed of 450 rpm. The vortex progress is video recorded (camera is placed in fixed position) and later reviewed to determine the vortex progress, which is measured in seconds, in increments of 100 mm. The time that it took for a vortex depth of 400 mm to be achieved, with the rotor operating at 450 rpm was recorded in Table 2. The shorter the time, the more efficient the rotor at creating a vortex.

[0145] Results

[0146] As indicated in Tables 1 & 2, both the vortex formation and flux gas dispersion efficiency of the rotor designs of the present invention are superior to the comparative examples of the prior art.

[0147] The superior vortex formation properties of the rotor designs of the present invention may be attributable to the relatively large portion of the rotor volume which the cut-out sections account for, with the Examples comprising cut-out sections ranging from 29% to 64% of the total rotational volume of the rotor. Furthermore, despite a relatively lower proportion of mixing zone volume (and cross-sectional area), the rotors surprisingly also posessed more efficient mixing performance as indicated by the lower residual O2levels (Table 2). This is at least partially attributed to the creation of turbulent mixing between the molten metal and gases in a counterflow arrangement.

[0148] Comparative Example C-1 (Figures 11a & 11b) which a high mixing zone size is able to obtain excellent mixing efficiency, however the cut-out section is not able to transport a sufficient volume of the molten metal - fluxing gas dispersion to efficiently form a vortex. Indeed, accordingly to the cut-out section definition, C-1 contains no cut-out section, as only the roof section of the rotor contains a cut-out section. Thus, it was not unexpected that this rotor design had the worst vortex forming performance (Table 2).

[0149] Example C-3 (Figures 13a & 13b), despite comprising a similar mixing zone size as the rotor in Example 4, has significantly lower mixing efficiency. It is considered that having fewer, but larger mixing zones is detrimental to promoting a high shear zone as the larger mixing zone size enables channelling of some of the fluxing gas through the mixing zone without effective dispersing into the molten metal stream. Further, the reduced number of supports, which function as blades, reduces the further dispersal of the fluxing gas within the cut-out regions. Thus, there is a balance between the proportion of the rotor which is allocated to the mixing zone and the number of mixing zones to enable a sufficiently high shear environment. In Comparative Example C-4 (Figure 14a & 14b), there is a greater propensity for the fluxing gas and molten metal to flow co-currently in the lateral direction as both the gas and molten metal are under a centrifugal lateral force once in the central mixing zone. The extent of counter-current flow if thought to be relatively lower, as the lateral passages offer a path of lower resistance and lower shear rate. In contrast, the mixing zone of the present invention force the gas and molten metal to collide at opposing lateral directions (high shear countercurrent flow), with co-current flow only obtainable after the force of the fluxing gas stream drives the molten metal - gas dispersion back out of the mixing zone outlet against the flow of an incoming molten metal stream.

[0150] In Comparative Example C-5 (Figure 15a & 15b), the base manifold diameter (ID-B) has been increased to extend to almost the periphery of the rotor (200). The supports (210) which do exist are at the periphery of the rotor. The location of the supports is such that the rotor has no mixing zone. Rather than the roof and base sections (20, 30) and adjacent supports (40) each extending radially outwards to form the mixing zone (90), the space between adjacent supports is occupied by the cut-out section (80). The result is inferior vortex creation and mixing efficiency compared to the designs of the present invention.

[0151] Table 1 (dimensions in mm)

[0152] What is apparent from Table 2, is that an increase in the % of the cut-out area is correlated to an increase vortex efficiency in both the simulation modelling and the water basin testing. Likewise, an increase in the mixing zone is correlated with an increase in the mixing efficiency.

[0153] As illustrated in Examples 1 to 3 (symmetrical) and examples 6 & 7 (asymmetrical) cut-outs may be maximised in size through reducing the relative size (top down perspective) of the supports, manifold and sections and mixing zone. The maintenance of a sufficient proportion of a high shear mixing zone is expected to maintain an acceptable dispersion efficiency. From the results, it appears that the symmetrical rotors performed marginally better than the asymmetrical rotors from a vortex formation performance perspective.

[0154] Example 8 comprises a higher proportion of mixing zone and cut-out section compared to the rotor of Example 4. This is achieved through decreasing the proportion of support and manifold sections of the rotor. As a result, we can expect both improvements in the vortex and dispersion efficiency relative to Example 4. However, the experimental results indicates that while there was an improvement in vortex creation efficiency, the dispersion efficiency, as measured in the residual level of O2was detrimentally affected. This may be the result of the rotor design creating additional surface area in the degassing mode (300 rpm) resulting in an increase in oxygen re-absorption. This highlights the need to be able to balance the rotor designs to the specific application, including vessel size and configuration. Further, an optimal degassing rotor speed should be selected to minimise oxygen re-absorption.

[0155] It should be noted that a balance between manufacturability, durability; vortex and dispersion efficiency performance is required for commercial reasons. The use of carbon ceramic composites enables greater design flexibility due to the material’s greater erosion resistance and results in the ability to create designs with larger cut-out and mixing zone volumes relative to the total rotational volume.

[0156] Effect of mixing zone dimensions.

[0157] The only %O2level result (Table 2) with a level at 20% or higher was recorded for Example 8. The mixing zone dimensions on example 8 had the highest outlet dimension (S2). In addition, Examples 8 had an increased mixing zone height compared to the other examples. This highlights that a narrower mixing zone narrow promotes improved mixing, which is consistent with an increase in turbulence in the mixing zone, as indicated via the theory underlying the Reynold’s number.

[0158] While Comparative Example C-2 had the narrowest mixing zone dimension (35 mm), the oxygen removal efficiency of this rotor is still lower than all of the rotors under the present invention, highlighting that the base opening of C-2 has a negative impact on the oxygen removal efficiency, relative to the design of the present invention.

[0159] It will be understood that modifications and variations may be affected without departing from the spirit and scope of the novel concepts of the present invention. Table 2

Claims

CLAIMS1.- A device for degassing molten metal comprising: a. a hollow shaft for carrying a gas stream; b. a rotor (10) comprising a roof section (20) and a base section (30) separated by a void, wherein said rotor is connected to said hollow shaft, wherein the roof section (20) comprises an opening (60) for allowing entry of gas from the hollow shaft; c. in the range of 3 to 8 supports (40), each radiating outwards and connected to the roof section (20) and the base section (30); d. a manifold section (50) for distributing gas from the hollow shaft radially through a mixing zone inlet (70); e. a mixing zone (90) contiguous to the mixing zone inlet (70) and radially offset from the manifold, wherein the roof and base sections (20, 30) and adjacent supports (40) each extend radially outwards from the manifold section (50) to form the mixing zone (90), wherein the mixing zone forms at least part of the void, wherein peripheral edges of the roof, base sections (20, 30) and adjacent supports (40) define a mixing zone outlet (100); and f. a cut-out section (80) located between a rotational volume envelope of the rotor and the periphery of the rotor, wherein the cut-out section (80) is in the range of between 25% and 75% of the rotational volume of the rotor.2.- The device according to claim 1 , wherein the ratio of a minimum radial length (L1) between the inlet of the mixing zone (70) and the mixing zone outlet (100) and a minimum radial length (L2) between the inlet of the mixing zone (70) and the periphery of the rotor (R) is in the range of >0.0 to 0.38.3.- The device according to any one of the preceding claims, wherein the minimum transverse dimension of the mixing zone outlet (100) is at least 28 mm.4.- The device according to any one of the preceding claims, wherein the base section (30) comprises no opening or comprises an opening which is not sufficiently large to enable molten metal to be drawn into the opening, during operation, and mix with the gases in the manifold section of the rotor.5.- The device according to any one of the preceding claims, wherein the base section (30) comprises an opening with a diameter of less than 28 mm.6.- The device according to any one of the preceding claims, wherein a cross-sectional area of the mixing zone outlet (100) is larger than a cross-sectional area of the mixing zone inlet (70).7.- The device according to any one of the preceding claims, wherein the cut-out section (80) of the rotor (10) comprises at least 26% of the rotational volume of the rotor.8.- The device according to any one of the preceding claims, wherein a cross-sectional area of the mixing zone is at least 5% of a rotational area of the rotor.9.- The device according to any one of the preceding claims, wherein the volume of the mixing zone is in the range of between 1% and 25% of the of the rotational volume of the rotor.10.- The device according to claim 9, wherein the volume of the mixing zone is at least 3% of the rotational volume of the rotor.11.- The device according to any one of the preceding claims, wherein the sum of the volume of the mixing zone and the volume of the cut-out section is in the range of 30% and 80% of the rotational volume of the rotor.12.- The device according to any one of the preceding claims, wherein the sum of the volume of the mixing zone and the volume of the cut-out section is at least 36% of the rotational volume of the rotor.13.- The device according to any one of the preceding claims, wherein the sum of the cross- sectional area of the mixing zone and the cross-sectional area of the cut-out section is in the range of 50% and 90% of the rotational area of the rotor.14.- The device according to any one of the preceding claims, wherein the sum of the cross- sectional area of the mixing zone and the cross-sectional area of the cut-out section is at least 59% of the rotational area of the rotor.15.- The device according to any one of the preceding claims, wherein the ratio of the cross- sectional area of the cut-out section to the cross-sectional area of the mixing zone is 1 .0 or greater.16.- The device according to any one of the preceding claims, wherein the cross-sectional area of the manifold section represents no more than 10% of the rotational area of the rotor.17.- The device according to any one of the preceding claims, wherein each of the supports (40) comprises a circumferential thickness (T1) within the range of 8 mm to 20 mm.18.- The device according to any one of the preceding claims, wherein each mixing zone outlet (100) spans at least 50% of the perimeter between circumferential ends of an adjacent pair of supports (40).19.- The device according to any one of the preceding claims, wherein the cut-out section (80) comprises a first length extending from the perimeter of the cut-out section from a peripheral end of a support to a turning point of the cut-out closest to a central axis of the rotor; and a second length extending from the turning point and extending to a peripheral end of the adjacent support defining the cut-out, said first length greater than said second length.20.- The device according to any one of the preceding claims, wherein the cut-outs (80) extend over a distance greater than 50% of the radius of the rotor.21.- The device according to any one of the preceding claims, wherein cut-outs (80) extend greater than 55% of the radius of the rotor.22.- The device according to any one of the preceding claims, wherein the rotor (10) comprises or consists of a carbon ceramic composite material.23.- The device according to any one of the preceding claims, wherein each mixing zone inlet (70) forms part of a circumferential perimeter with a radius of no more than 50 mm.24.- A molten metal degassing system comprising a device according to any of the preceding claims and a vessel for molten metal.25.- A molten metal degassing system according to claim 24, wherein the vessel has an internal diameter in the range of 600 and 2000 mm.26.- A molten metal degassing system according to claim 25, wherein a ratio of a diameter of the rotor to the internal diameter of the vessel is in the range 0.18 to 0.30.27.- The molten metal degassing system according to claim 26, wherein the ratio of the diameter of the rotor (10) to the internal diameter of the vessel is less than 0.23.28.- The molten metal degassing system according to any one of claims 24 to 27, wherein the internal diameter of the vessel is at least 1000 mm.29.- A process of degassing molten metal using the device according to any one of claims 1 to 23 or the molten metal degassing system according to any one of claims 24 to 28.30.- A process according to claim 29, wherein the rotor rotates at a first speed of greater than 300 rpm in a vortex formation mode31.- A process according to claim 30, wherein after rotating in said vortex formation mode, the rotor rotates at a second speed in a degassing mode, wherein the second speed is at least 50 rpm less than the first speed.32.- The process according to claim 31 , wherein the average of the first speed and the second speed to obtain a vortex depth of at least 400 mm after 15 seconds and a residual O2level of no more than 20% v / v after 450 seconds is no more than 450 rpm for a 220 mm diameter rotor in a 1000 mm diameter vessel filled to a level of 700 mm with water with oxygen probes positioned 100 mm and 500 mm from the bottom of the vessel.33.- The process according to claim 32, wherein the average of the first speed and the second speed is no more than 500 rpm.

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