GB2624201A

GB2624201A - Degasser shaft

Info

Publication number: GB2624201A
Application number: GB2216738.1A
Authority: GB
Inventors: Crowley James; Anza Inigo; Braun Peter; Kawale Tarik; Kawale Nitin; Shao Vic
Original assignee: Molten Metal Systems GmbH
Current assignee: Molten Metal Systems GmbH
Priority date: 2022-11-10
Filing date: 2022-11-10
Publication date: 2024-05-15
Also published as: GB202216738D0

Abstract

A shaft for degassing molten metal comprises a first end connectable to a motor, a second end which is either removably or permanently connected to a rotor, and a passage through which gas can be fed from the first to the second end of the shaft. The shaft has its minimum diameter proximal the rotor and a first portion located proximal to or at the first end and comprising a tapered frustoconical shaped portion, which comprises 48-100 % of the total length of the shaft. Figure 1 shows a shaft which is tapered along all of its length. Figure 4 shows a shaft where the ends of the shaft are both cylindrical and are connected by a frustoconical portion. Figure 5 shows a shaft which comprises from its first end, a frustoconical portion tapering outwardly, a cylindrical portion and a frustoconical portion tapering inwardly to a cylindrical portion which is connected to the rotor. Figure 6 shows a shaft which comprises a series of cylindrical and inwardly tapering frustoconical portions from the first end to a final cylindrical portion and the second end.

Description

Degasser Shaft

Field of the Invention

The invention relates to degassing shafts; the use of the degassing shafts to degas molten metals and, in particular, molten aluminium and magnesium.

Background

In the degassing process, inert gasses are pumped into aluminum melts to remove hydrogen and prevent subsequent porosity in cast parts. In a rotary degassing method, an inert or chemically inactive gas is purged through a rotating shaft and rotor. The energy of the rotating shaft causes formation of a large number of fine bubbles providing a very high surface area to volume ratio. The large surface area promotes fast and effective diffusion of hydrogen into the gas bubbles resulting in equalizing activity of hydrogen in liquid and gaseous phases.

There are a variety of degassing procedures many of which require the shafts to rotate at high speeds which results in high stresses being placed upon the shafts. Graphite and carbon composite shafts have been favoured for these types of applications due to their low density, high stiffness and superior flexural strength compared to shafts made from ceramic composite materials. However, graphite shafts while having good mechanical properties for these applications, are prone to oxidative corrosion resulting in graphite shafts having to be replaced on a regular basis.

EP3180455, in the name of Pyrotek Inc., addressed this problem through the carbon composite element being impregnated with an oxidation resistant chemical, such as phosphate based oxidation retardants disclosed in US4,439,491. Despite these advances, there is still a need for improved degasser shafts with a longer working life, which is able to be used in high stress applications.

Summary of the invention

In a first aspect of the present invention there is provided a degasser shaft for treating a molten metal with a gas comprising: (a) a first end connectable to a motor and comprising a first end diameter; (b) a second end connectable to a rotor and comprising a second end diameter, or the second end comprises a rotor, wherein the second end is deemed to terminate where the degasser shaft comprises a second end diameter being the minimum shaft diameter proximal the rotor; and (c) an external diameter; and (d) a passage through which the gas travels from the first end to the second end, the passage having an internal diameter; wherein the degasser shaft has a first portion located proximal to or at the first end and comprising a frustoconical shaped portion, and optionally, a shaped portion comprising a degasser shaft cross sectional area greater than the minimum cross-sectional area of the degasser shaft proximal or at the second end, said first portion comprising between 48% and 100% of the total length of the degasser shaft.

The degasser shafts of the present invention have a long working life due to the shaft being both resistant to oxidative corrosion and premature mechanical failure. Whilst the innovative design features may be applied to any material suitable for use as a degasser shaft, the degasser shafts are particularly advantageous when made of a ceramic composite. It is counter-intuitive that the objective of an extended working life may be improved using a composite material having generally lower flexural strength than graphite-based compositions.

The frustoconical shaped portion may make up at least 50% or at least 60% or at least 70% or at least 80% or at least 90% or 100% of the total length of the first portion. The longer the frustoconical shaped portion, the lower the incident angle from the shaft axis, thereby avoiding localised stress concentrations, if and, when the frustoconical portion abuts an optional cylindrical portion of the degasser shaft.

The shaped portion may be a cylindrical portion or a polygonal prism or any other suitable shaped portion. In embodiments where the shaped portion comprises a polygonal prism (e.g. a hexagonal prism), the shaped portion may function, in cooperation with a fastening tool (e.g. spanner) to fasten the degasser shaft to the motor or rotor, or connection thereof. Preferably, the shaped portion does not include any acute angles (e.g. less than 90° or less than 600), so to avoid any stress concentration points within the shaft (e.g. a pentagon or hexagonal). The shaped portion is preferably symmetrical around a central axis of the degasser shaft. In some embodiments, the shaped portion comprises a degasser shaft cross sectional area at least 5% or at least 8% or at least 10% or at least 12% or at least 15% or at least 20% greater than the minimum cross-sectional area of the degasser shaft proximal or at the second end.

The first portion may comprise at least 50% or at least 52% or at least 55% or at least 58% or at least 60% or at least 70% or at least 80% or at least 90% or at least 95% or 100% of the total length of the degasser shaft. In some embodiments, the first portion may be no more than 90% or no more than 80% or no more than 70% or no more than 60% of the total length of the degasser shaft. A larger first portion may assist in minimising localised areas of concentrated stress during operation and increase shaft stiffness, whilst a smaller first portion may enable a reduction in shaft weight.

When the first portion does not extend the entire length of the shaft, the remainder of the shaft, which is not part of the first portion, may comprise a cylindrical segment or polygonal segment defined by the second end diameter and extending from the first portion to the second end. The second end diameter is typically the minimum diameter (or effective diameter) of the shaft.

The first shaft end diameter is typically the maximum diameter of the shaft. In some embodiments, the first shaft end diameter defines a cylindrical segment of the shaft which extends from the end of the frustoconical shaped portion towards the first end of the shaft.

In other embodiments, the first portion comprises two or more frustoconical shaped sections which connect the first shaft end diameter to the second shaft end diameter. The frustoconical shaped sections may progressively decrease in diameter as they progress towards the second end of the shaft. Each of the frustoconical shaped sections may be separated by cylindrical sections. The benefit of this "step down" configuration is that the width (and hence stiffness) of sections of the shaft may be customised to counter localised stress regions along the degasser shaft under the conditions of operation.

In some embodiments, a cylindrical segment extends to the first end of the shaft.

A motor guard or housing associated with the motor may be such that the diameter of the first end is required to be smaller than the first shaft diameter to enable the first shaft end to fit within such auxiliary components of the degasser system. In these embodiments, the first end diameter may be connected to the first shaft diameter by a frustoconical shaped segment.

Connectors and seamless connections The first end of the shaft is connectable to a motor and second end of the shaft may be connectable to a rotor. The connection to the motor and/or rotor may be a male connector; a female connector; or any other suitable coupling mechanism In some embodiments, the first end is connectable to the motor by a female connector. The female connected may be a spiral threaded coupling means (e.g. a spiral threaded cavity). To reduce stress concentrations around the female connector, the female connector preferably comprises an arced bridging connection to the passage. The arc bridging connection preferably has a radius of at least 5 mm or at least 10 mm. The arc bridging connection may also be incorporated into a male connector.

When there is a seamless connection of the shaft to the rotor, the deemed second end has been taken to be a minimum diameter of the shaft proximal the rotor. This ensures that the second end point does not include any of the tapered enlargement of the apparatus towards the wider diameter of the rotor, which is typically disc shaped.

Degasser shaft dimensions The length of the degasser shaft is measured along the central axis from the first end to the second end. The length does not include extensions attributable to connector, such as male connectors. When the second end of the degassing shaft comprises a rotor (i.e. the rotor is integral to the shaft or seamlessly connected (e.g. a monolithic construction)), the second end is deemed to terminate where the minimum shaft diameter is located closest to the rotor.

Degasser shafts typically range in length from at least 250 mm to no more than 2500 mm and typically no more than 2200 mm or no more than 2000 mm. The length of the shaft may be determined by the requirements of the degassing system. However, the rotational stresses on degasser shafts significantly shorten their durability once the shaft length extends to 2000 mm and beyond at the specified rotational speeds.

Degasser shaft may be categorised into three categories as indicated in Table 1 below:

Table 1

Type Lengthn,in Lengthn-a. RPMmin RPM.

(mm) (mm) Short 250 600 500 1000 Medium 600 1500 300 700 Long 1500 2500 200 600 As indicated in Table 1, as the degasser shaft lengthens, the rotation speeds they operate under decreases. This is at least partially related to the additional stresses placed upon longer degasser shafts.

For long shafts, the maximum diameter (Da") of the shaft is typically between 125 mm to 200 mm. The minimum diameter (Dmin) of the shaft is typically between 60 mm to 150 mm. The difference between the maximum and minimum shaft diameter (Dm. -Dmin) is typically at least 20 mm or at least 25 mm or at least 30 mm or at least 35 mm or at least 40 mm or at least 45 mm or at least 50 mm or at least 55 mm or at least 60 mm. Dmax -Dmin is typically no more than 100 mm.

For medium shafts, the maximum diameter (Dr.) of the shaft is typically between 100 mm to 150 mm. The minimum diameter (D,m) of the shaft is typically between 60 mm to 120 mm. The difference between the maximum and minimum shaft diameter (Dn,a" -Dmin) is typically at least 20 mm or at least 25 mm or at least 30 mm or at least 35 mm or at least 40 mm or at least 45 mm or at least 50 mm. Dm. -Dmin is typically no more than 90 mm.

For short shafts, the maximum diameter (Dn,a") of the shaft is typically between 60 mm to 100 mm. The minimum diameter (Dmin) of the shaft is typically between 40 mm to 80 mm. The difference between the maximum and minimum shaft diameter (Dmax -Dmin) is typically at least 10 mm or at least 15 mm or at least 20 mm or at least 25 mm or at least 30 mm or at least 35 mm or at least 40 mm. Dn,a" -Dmin is typically no more than 60 mm.

In some embodiments, the diameter of the first end may be up to 100 mm or up to 50 mm smaller than Dmax* In some embodiments, the ratio of the minimum shaft external diameter to the minimum wall thickness (Dniini Wallmin) was at least 2.7 or at least 2.9 or at least 3.1 or at least 3.3. A higher ratio promoted a stiffer shaft.

The ratio of the maximum external diameter to the minimum external diameter of the degasser shaft is in the range of 1.05:1 to 3.0:1 or in the range of 1.1:1 to 2.5:1 or in the range of 1.2:1 to 2.0:1.

The maximum shaft diameter is typically at the first end or around the melt line. The minimum shaft diameter is typically at the second end of the shaft.

In some embodiments, the degassing shaft comprises an incident angle between a frustoconical shaped portion and a cylindrical shaped portion (or a central axis) of no more than that 16° or no more than 15° or no more than 14° or no more than 13° or no more than 12° or no more than 10°. Higher incident angles may result in a concentration of bending stresses at this intersection resulting in potential mechanical failure. The incident angle is greater than 0° and typically at least 0.3° or at least 0.5°or at least 1.0° or at least 2.0° or at least 3.0° or at least 4.0° or at least 5.00 to enable the required changes in shaft diameter to be achieved to obtain stiffness and weight objectives.

The internal diameter of the shaft (i.e. passage) may range from 10 mm to 80 mm or 12 mm to 70 mm or 15 mm to 60 mm or 18 mm to 50 mm or 20 mm to 40 mm. The internal diameter may be constant along the length of the shaft or the internal diameter may vary with the internal diameter decreasing along the shaft from the first end to the second end. When the first or second end comprises a female connector, then the internal diameter is measured immediately adjacent the cavity housing the female connector.

The minimum shaft wall thickness is typically at least 12 mm or at least 15 mm or at least 20 mm or at least 25 mm or at least 30 mm or at least 35 mm or at least 40 mm. The minimum shaft wall thickness is typically located towards the second end of the degasser shaft and typically corresponds to the minimum external diameter of the degasser shaft. For long shafts the minimum wall thickness of typically in the range of 20 mm to 45 mm or 22 mm to 42 mm or 25 mm to 40 mm. For short shafts, minimum wall thickness of typically in the range of 10 mm to 17 mm or 11 mm to 16 mm or 12 mm to 15 mm.

The maximum shaft wall thickness is typically no more than 80 mm or no more than 70 mm or no more than 60 mm. The maximum shaft wall thickness is typically located towards the first end of the degasser shaft and typically corresponds one or about where the melt line of the degasser shaft is positioned (e.g. ± 50mm). The melt line is typically located between 40 mm and 500 mm from the first end of the degasser shaft depending upon the total length of the shaft and the degasser system configuration. When a female connection is used to couple the degasser shaft to the motor, the melt line is preferably located below the female connection and preferably at least 20 mm or at least 40 mm below the female connection. The melt line and female connection are both regions of potential weakness due to corrosive stress concentration. As such, these regions are preferably separated.

A preferred embodiment there is provided the degasser shaft comprises: a. a total length of between 250 and 2200 mm; b. an optional shaped portion extending from or proximal to the first end comprising a length in the range of 0 to SOO mm; c. a frustoconical segment, abutting the optional shaped portion, comprising a diameter which decreases as the frustoconical segment extends towards the second end, said frustoconical segment comprising an axial length in the range of 100 to 2200 mm; d. an optional cylindrical segment extending from the frustoconical segment to the second end; e. a first shaft end diameter in the range of 60 to 180 mm; f. a second shaft end diameter in the range of 45 to 140 mm; and g. a passage diameter in the range of 10 to 60 mm.

The degasser shaft preferably comprises ceramic composite.

In another preferred embodiment there is provided the degasser shaft comprises: a. a total length of between 250 and 500 mm; b. the first shaped portion extending from the first end comprising a length in the range of 50 to 100 mm; c. a frustoconical segment abutting the first shaped portion comprising a diameter which decreases as the frustoconical segment extends towards the second end, said frustoconical segment comprising an axial length in the range of 100 to 400 mm; d. an optional cylindrical segment extending from the frustoconical segment to the second end; e. a first shaft end diameter in the range of 60 to 90 mm; f. a second shaft end diameter in the range of 45 to 70 mm; and g. passage diameter in the range of 10 to 25 mm.

The degasser shaft preferably comprises ceramic composite.

A preferred embodiment there is provided the degasser shaft comprises: a. a total length of between 500 and 1500 mm; b. the first shaped portion extending from or proximal to the first end comprising a length in the range of 0 to 400 mm; c. a frustoconical segment abutting the optional first shaped portion comprising a diameter which decreases as the frustoconical segment extends towards the second end, said frustoconical segment comprising an axial length in the range of 500 to 1500 mm; d. an optional cylindrical segment extending from the frustoconical segment to the second end; e. a first shaft end diameter in the range of 100 to 150 mm; f. a second shaft end diameter in the range of 60 to 120 mm; and g. passage diameter in the range of 10 to 40 mm. The degasser shaft preferably comprises ceramic composite.

Another preferred embodiment there is provided a degasser shaft comprises: a. a total length of between 1500 and 2200 mm; b. the first shaped portion extending from or proximal to the first end comprising a length in the range of 200 to 500 mm; c. a first frustoconical segment abutting the first cylindrical segment comprising a diameter which decreases as the frustoconical segment extends towards the second end, said frustoconical segment comprising an axial length in the range of 400 to 2200 mm; d. an optional cylindrical segment extending from the frustoconical segment to the second end; e. a first shaft end diameter in the range of 90 to 180 mm; f. a second shaft end diameter in the range of 80 to 140 mm; and g. passage diameter in the range of 10 to 60 mm.

The degasser shaft preferably comprises ceramic composite.

The degasser shafts of the present invention are compatible with a range of degassing systems, including the SNIF Pyrotek degasser, STAS degasser, Novelis ALPUR degasser and Hertwich degasser systems. Advantageous, the shafts of the present invention not only provide the mechanical durable under varying rotational speeds, but the degasser shaft have superior anti-oxidative corrosive resistance to ensure a longer working life.

Ceramic composite The ceramic composite may comprise a composite comprising refractory particles within an inorganic (e.g. glass and/or mullite) or organic binder matrix (e.g. ceramic carbon composite). The refractory particles may comprise any suitable refractory materials with suitable mechanical strength; oxidative resistance; thermal shock and impact resistance Suitable particles may include carbides, including silicon carbide; nitrides, including silicon nitride; alumina, zirconia and aluminosilicates. The refractory particles are preferably crystalline or partially crystalline. In some embodiments, the ceramic composite may comprise inorganic fibres which may enhance the flexural strength of the composite.

The ceramic composite may be formed from clay or resin bonded ceramic composite precursor material. Ceramic composites formed from a clay bonded composite precursor may comprise refractory particles embedded in an aluminosilicate matrix. The aluminosilicate matrix may be a glassy and/or crystalline (e.g. mullite). An aluminosilicate matrix is defined as a matrix comprises at least 60 wt% or at least 70 wt% or at least 80 wt% of alumina + silica. Ceramic composites formed from a resin bonded ceramic composite precursor may comprise refractory particles embedded in a carbon matrix.

Examples of suitable ceramic composites are disclosed in W02022013523, which is disclosed therein by reference. The ceramic composites generally have superior oxidative corrosion resistance compared to carbon composites or graphite-based shafts. However, ceramic composites are not directly substitutable for graphite and carbon composite materials due to their higher density and reduced mechanical properties including lower stiffness and flexural strength.

The ceramics composites may be coated or impregnated using compositions and methods known to those in the art.

Composite properties The ceramic composite typically comprises a density of at least 1.90 g/cc or at least 2.0 g/cc or at least 2.1 g/cc or at least 2.2 g/cc. The upper limit is generally limited by the refractory materials used in the composite, but is generally less than 2.5g/cc. Due to the lower density of carbon, ceramic composites comprise a carbon matrix generally have a lower density than clay/glass bonded ceramic composites The stiffness of a material may be determined through its Young's modulus. The Young's modulus of the composite materials are typically significantly lower than graphite based materials, which comprise a Young's modulus in the order of 13 GPa. In contrast the Young's modulus of ceramic composites is generally between 0.5 and 10 GPa.

The tensile bending strength or flexural strength is an indication of a material ability to withstand bending stress prior to mechanical failure. The flexural strength of the ceramic composites may be at least 8 MPa or at least 10 MPa or at least 12 MPa or at least 14 MPa with an upper limit of no more than 30 MPa or no more than 25 M Pa or no more than 20 MPa. The flexural strength of ceramics composites may be increased to the upper end of the range through controlling particle size and porosity of the composite as known to those skilled in the art. The flexural strength of graphite materials is typically greater than that of ceramic composites, in the range of 18 MPa to 36 MPa.

Operation The natural frequency of the shaft is an important design consideration as when the rotational speed of the shaft approaches the natural frequency of the shaft, there is a significant increase in the amplitude of vibrations of the shaft. This increase in vibration places increased stresses upon the shaft, which may result in premature mechanical failure of the shaft. Therefore, it is desirable for the natural frequency of the shaft to be above the target operational rotational speed.

There are a number of factors which may influence the natural frequency of the degasser shaft, including: * the length of the shaft (shorter shaft = lower natural frequency) * the weight of the shaft (an increase in weight = lower natural frequency) * the stiffness of the shaft (increase in stiffness = increase the natural frequency) The stiffness of the shaft may be influenced by the design of the shaft including the cross-sectional area of the shaft, with wider diameter shaft being stiffer. However, an increase in shaft weight may result in a decrease in the natural frequency. The selection of materials may also significantly impact the natural frequency of the shaft, with stiffer materials having a higher natural frequency. The applicant has found that through a combination of design modification and material selection, not only can longer shafts (e.g. greater than 1.5 m) be produced with a high enough natural frequency to enable the shaft to operate the a target rotational speed, which is sufficient below the natural frequency such to avoid any amplification of vibrations. Further, this objective may be met with ceramic composite materials which enable the oxidative corrosion resistance of the shaft to be enhanced, particularly in comparison to graphite based shafts.

In a second aspect of the present invention, there is provided a process for degassing a molten metal melt comprising submerging a degassing apparatus into the molten metal melt, said degassing apparatus comprising a degassing shaft according to the first aspect of the present invention and wherein the rotational speed of the degassing shaft is below the natural frequency of the shaft.

The rotational speed of the degassing shaft is preferably at least 20 rpm or at least 40 rpm or at least 50 rpm below the natural frequency of the shaft.

In one embodiment, the shaft length is in the range of 1.5 to 2.2 metres or at least 1.8 to 2.2 metres of 1.9 to 2.1 metres and the natural frequency of the shaft is greater than 300 rpm or 350 rpm or 400 rpm.

Reference to diameter is inclusive of effective diameter when the cross-sectional area of the shaft is not circular. The effective diameter is deemed to be the diameter of a circle with the same cross-sectional area as the non-circular shape.

For the purposes of the present invention, a frustoconical shape portion is inclusive of polygonal prisms comprising a decreasing cross-sectional area from one end to the other. (for example a segment of a pyramid would be considered to be a frustoconical shape portion. However, in some embodiments, the frustoconical shape portion does not comprise any flat surfaces.

Flexural strength and transverse bending strength and used interchangeably throughout the specification.

For the avoidance of doubt it should be noted that in the present specification the term "comprise" in relation to a composition is taken to have the meaning of include, contain, or embrace, and to permit other ingredients to be present. The terms "comprises" and "comprising" are to be understood in like manner. It should also be noted that no claim is made to any composition in which the sum of the components exceeds 100%.

Further it should be understood that usage in compositions of the names of oxides [e.g. alumina, and silica] does not imply that these materials are in a specific stoichiometric form, but refers to the composition of the composite expressing the relevant elements as oxides. It would be understood at the elements may also be present in non-oxides forms.

For the purposes of the present invention, a cylinder may also encompass prismatic polygons with a least 5 sides and preferably at least 7 or at least 8 sides.

For the purposes of the present invention, the length of the frustoconical portion is taken to be the axial length.

Brief Description of the Figures

Figure 1 is a schematic diagram of a degasser shaft design with connectable rotor according to the present invention.

Figure 2 is a cross-sectional view of the degasser shaft design of Figure 1 without the rotor and showing an exploded view of a section of the female connection.

Figure 3 is a schematic diagram of a degasser shaft design of Comparative Example 1 (CE-1).

Figure 4 is a schematic diagram of a degasser shaft design of Example 2. Figure 5 is a schematic diagram of degasser shaft design of Examples 5 & 6. Figure 6 is a schematic diagram of degasser shaft design of Example 3. Figure 7 is a photograph of the degasser shaft design of Figure 3 which has mechanically failed. Details Description of a Preferred Embodiment of the Invention The degasser shafts of the present invention are made using conventional techniques with the raw materials mixed, dried and packed into moulds before being pressed and sintered at high temperatures (e.g. >1000°C) for sufficient time to sinter or otherwise bind the ceramic composite together.

With reference to Figures 1 & 2, there is illustrated a schematic diagram of degasser shaft 10 and a cross-section thereof 110. The shaft has a first end 20 which is connectable to a motor. The connection is a female connection 50, which may comprise a spiral threaded section 50 adapted to receive a complementary male connection (not shown) of the motor or apparatus connected to the motor. To avoid a concentration of stress in the female connector during operation, the connector comprises a rounded transition segment 58 between the spiral threated section and the passage 100. The degasser shaft also has a second end 30 which may comprise a rotor 40 in a one piece construction in which the rotor is seamlessly connected to the shaft. Alternatively, as illustrated in Figure 2, the second end of the degasser shaft may comprise a male spiral threaded section forming a connector 70 which is connectable to the complementary spiral threaded female (not shown) connector of the rotor.

The first end 20 comprises the maximum diameter of the shaft (Dlst). The second end 30 comprises the minimum diameter of the shaft (D2,d).

The degassing shaft has an external diameter 80 and an internal diameter 90, which defines a passage 100 which extends from the first end 20 to the second end 30. The external diameter of the degasser shaft defines a frustum of a right circular cone, which extends the total length (Ltotal) of the degasser shaft (i.e. the length of the male connector is not included in the total length of the degasser shaft).

The internal diameter of the degasser tube may be defined by cylindrical portions and/or frustoconical portions. To increase the stiffness of the degasser shaft the external diameter may be increased. To avoid the associated increase in weight of the shaft, the internal diameter may also be increased in at least some portions of the shaft. The passage may be formed using one or more cylindrical or tapered (i.e. frustoconical) mandrels in cooperation with a mould defining the external diameter of the shaft.

In operation the degasser shaft is positioned within a vessel containing molten metal. Depending upon the setup of the degasser apparatus, the degasser shaft will be immersed in the molten metal up to what is termed the "melt line", the level of the shaft which comprises the interface between the molten level and the gaseous atmosphere above. The melt line 60 of the shaft is exposed to the most oxidative corrosive environment and, as such, it prone to mechanical failure at this point due to weakening of the degasser shaft, due to oxidative corrosion, at this point. To mitigate the risk of mechanical failure of the degasser shaft around the melt line, the wall thickness of the shaft may be larger in this section of the degasser shaft compared to other sections of the degasser shaft.

Figures 3 & 4 illustrates variations of the frustoconical shaped degasser shaft of Figures 1 & 2, with the first portion 310, 410 comprising a cylindrical shaped portion 320, 420 and a frustoconical shaped portion 330, 430. The frustoconical portion abuts a cylindrical section 340, 440 which extends to the second end of the degasser 450.

The degasser shaft design 300 of Figure 3 is a comparative example as the first portion 310 does not extend the required portion of the total length (Loth') of the degasser shaft. The consequences of this are that (i) the stiffness of the degasser shaft is lower due to the relatively smaller external diameter of the degasser shaft (SD) in the lower cylindrical section 340; and (ii) the acuter joining angle 350 between the frustoconical portion and the cylindrical portion 340 relative to the corresponding joining angle 460 in Figure 4. These differences are illustrated through the first portion in the degasser shaft of Figure 4 being longer than the degasser shaft in Figure 3 (P2> Pi) and the frustoconical portion in Figure 4 being longer than in Figure 3 (P2-C1 > P1-C1). The combination of these two design features may, depending upon the operating conditions, significantly reduce the concentration of stresses on sections of the degasses shaft and thereby reduce the frequency of mechanical failure in the shafts. Figure 7 illustrates the location of mechanical fail of the degasser shaft design similar to that of Figure 3, with the failure point being at and around the intersection of the frustoconical 710 and cylindrical portion 720.

The radius R defining the tapered shoulder of the rotor 360, will dictate the deemed end of the second end of the shaft 370, when the rotor is seamlessly connected to the degasser shaft 300.

The design configuration of Figure 4 is able to maintain a similar, if not better, performance to the designs of Figures 1 & 2, as the melt line is positioned within the cylindrical portion 420, thereby maintaining the melt line at the maximum diameter of the degasser shaft. Whilst the length (1_,) of the cylindrical portion 420 is the same as the cylindrical portion in Figure 3, 320, the length (Lt) of the tapered portion 430 is significantly longer than the tapered portion in Figure 3, 330, thereby contributing to a higher stiffness.

Further a relatively small cylindrical section 440 adjacent the second end of the degasser shaft 450 has been shown not to significantly impede performance, which may be at least partially offset by the lower weight of this section compared to the frustoconical counterpart.

Additionally, the internal diameter of the passage at the first end ID1 through to the internal diameter of the second end ID2 may be widened to reduce the weight of the degasser shaft. This is particularly advantageous in longer degasser shafts where total weight and weight distribution of the degasser shaft can affect the natural frequency of the degasser shaft in operation. The internal diameter of the passage at the first end 101 may be measured immediately below any female connection cavity 470 which may exist. The passage 480 may be cylindrical, in which case the wall thickness of the degasser shaft is at a maximum at the melt line, which is positioned within the cylindrical portion 420, thereby adding the oxidative resistance to the shaft at the point which requires most protection. A shaft may also have a tapered passage to reduce total shaft weight, if required.

Further variations of designs within the scope of the present invention are provided in Figures 5 & 6. The degasser shaft design of Figure 5 differs from Figure 4 in that the cylindrical portion 420 is partially substituted by a frustoconical portion 525 tapering from the cylindrical portion to the first end of the degasser shaft. This results in the cylindrical section 520 being offset from the first end. This configuration may enable the first end of the degasser shaft to fit into existing degasser apparatus which may include safter guards and housing features as well as reducing the overall weight of the shaft. Further, the maximum diameter of the shaft may be maintained around the melt line at, or around, the cylindrical portion. It would be understood that degasser apparatus may be set up to accept conventional shafts which may having smaller external diameters.

Figure 6 illustrates a shaft 600 with a design variation which still comprises a first portion 610 comprising a cylindrical portion at the top of the shaft 620 but includes a cascading or step-down portion which includes a series of cylindrical portions of decreasing diameters 640, 660, 680 separating a series of frustoconical sections of decreasing diameters 630, 650, 670, 690.

The specific design features of the degasser shaft will depend upon the operating environment that the degasser shafts will be exposed to, as well as the properties of the corrosive resistant ceramic composites requires to prolong the operating life of the degasser shaft. Short shafts (e.g. less than 600 mm in length) generally operate at high rotations speeds and the shaft design focus is upon reduce stress concentrations due to the deflection of the shaft, particularly shafts with a relatively low stiffness (i.e. relatively low Young's modulus). Increasing the shaft diameter and minimise stress concentration points are often a design focus. For longer shafts the avoidance of excessive vibrations resulting in heightened stress levels of the shaft is often a focus with shaft weight and weight distribution a further design priority.

The scope of the present invention is not limited to the specific embodiments illustrated herein. The skilled artisan would be able to readily use the teachings herein to create numerous modifications and variations falling within the scope of this disclosure.

Examples

A number of shaft designs was assessed under stimulated conditions. It is noted that, for the purposes of the experiment, the end of the shaft for the short rotors were taken to be the maximum diameter of the rotor rather than the minimum diameter of the shaft proximal the rotor. Given the weight of this additional portion was similar, the results are still considered valid for comparative purposes.

Short degasser shafts The Short degasser shaft designs indicated in Table 2 are capable of being operated at between 700 to 1000 rpm and were compared in relation to their maximum stress levels and maximum displacement when placed under a 25N loading.

The shafts were made from a ceramic carbon composite at Molten Metal Systems GmbH comprising approximately 65wt% refractory particles (about 40 wt% SiC with the majority of the remainder comprising alumina and aluminosilicate particles) and approximately 35wt% of a carbon matrix and having a density of 2.2 gicc, a Young's modulus of 2.1 GPa and a flexural strength of 11 MPa.

The geometry of the different degasser shaft designs is provided in Table 2, with reference to Figures 3 & 4.

Table 2 (measurements in mm CE-1 1 2 3 CE-2 First end diameter (D1) 79 72 68 70 63.5 First end cylinder length 75 75 75 75 105 Second end diameter (D2) 51 50 47 47 54.5 Wallmin 18.7 16.3 13.8 13.8 17.3 Dmmi WalInnin 2.73 3.07 3.41 3.41 3.15 Second end flange radius (R) 150 100 50 50 50 First end internal diameter (ID1) 15 20 20.5 20.5 21 Second end internal diameter (ID2) 13.3 17.4 19.5 19.5 20 portion length 115 191 230 204 120 Shaft-Total length (14 otal) 253 266 292 292 292 % Pt portion of Lam! 45 72 79 70 41 %trust. Portion of Pt portion 35 61 67 63+ 13 Weight 2.5 2.5 2.0 2.0 2.0 Incident angle 19.3 3.2 3.9 9.5 16.7 + the frustoconical portion includes the decreasing cylindrical segments smaller in diameter then the first end cylinder.

Table 3

Max Stress (MPa) Max Displ (mm) CE-1 0.70 0.34 1 0.28 0.23 2 0.35 0.32 3 0.35 0.32 CE-2 0.45 0.35 As indicated in Tables 2 & 3, designs 1 to 3 possessed the lowest maximum stress and lowest maximum displacements when subjected to a 25N loading. The comparative examples possessed both higher maximum stress and displacement levels and hence are more prone to mechanical failure. Despite these superior mechanical properties, the corrosion resistance of graphite is significantly lower than ceramic composite materials. While CE-1 and CE-2 possessed a relatively high 15t portion, the frustoconical component of this was relatively small resulting in relatively high incidental angles which may have contributed to the higher maximum stress levels obtained with these designs.

Long degasser shafts The ceramic composite used was similar in composition and properties to that of the short degasser shaft, although the Young's modulus was 3 GPa. The properties of the ceramic composites are provided in Table 4. The natural frequency of the degasser shaft was determined when the shaft was attached to a rotor. The weight of the rotor was 5.8 kg, except for sample CE-4 where the rotor weighed 4.5 kg. This was due to this rotor being made of graphite rather than the ceramic composite that the other rotors were made of. Frequency response analysis of the shafts and rotors, to determine the national frequency, was determined based upon a 1 kg lateral excitation at the base of the rotor, with 0% dampening applied.

As illustrated in Table 4, the conventional cylindrical shaft (CE-3) made from a ceramic composite had a low natural frequency which was not suited to operation at or above 220 rpm without being prone to amplified vibrations. In contrast, the natural frequency of a similarly dimensioned degasser shaft made from graphite was 550 rpm. The increase in the natural frequency may be attributable to the properties of graphite which is stiffer (e.g. high Young's modulus) and lighter (e.g. lower density).

The tapered degasser shaft (4) is able to reduce the weight of the degasser shaft and, despite the reduced diameter of the shaft at the second end, the natural frequency of the degasser shaft increases to 260 rpm. Further increases in natural frequency of the degasser shaft is achieves in Examples 5 & 6 through increasing the diameter of the degasser shaft at the first end, despite this resulting in an increase in the weight of the degasser shaft compared to Example 3. Examples 3 to 5 all had a lower degasser shaft weight compared to the ceramic composite degasser shaft of Comparative example 3 (CE-3). The wider diameters of Examples 3 to 5 were combined with a lower minimum wall thickness to achieve this lower degasser shaft weight.

Table 4 (measurements in mm) CE-3 4 5 6 CE-4 Ltotal 1945 1945 1945 1945 Dn,a" 125 125 150 152.5 Drr," 125 100 90 95 ID 26 25 25 40 Walln,,n 49.5 37.5 32.5 27.5 Dimwit Wall,. 2.53 2.67 2.77 3.45 1' portion length 0 1945 1575 1175 OD first end 125 125 125 125 Cylindrical start from first end 275 275 Cylindrical finish from first end 415 415 Cylindrical portion (mm) 0 140 140 % 1st portion of Lew 100% 81% 60% % frust. Portion of 1st portion 100% 91% 88% Density (g/cc) 2.2 2.2 2.2 2.2 1.7 Flexural strength (MPa) 11 11 11 11 15.2 Young's modulus (GPa) 3 3 3 3 13 Shaft weight (Kg) 48.3 38.5 44.6 45.0 37.4 Natural Frequency (rpm) 220 260 340 360 550 Corrosive resistance* 000 000 000 000 0 *000 > 00 > 0 in terms of corrosion resistance

Claims

Claims 1. A degasser shaft for treating a molten metal with a gas comprising: (a) a first end connectable to a motor and comprising a first end diameter; (b) a second end connectable to a rotor and comprising a second end diameter, or the second end comprises a rotor, wherein the second end is deemed to terminate where the degasser shaft comprises a second end diameter being the minimum shaft diameter proximal the rotor; (c) an external diameter; and (d) a passage through which the gas travels from the first end to the second end, the passage having an internal diameter; wherein the shaft has a first portion located proximal to or at the first end and comprising a frustoconical shaped portion, and optionally, a shaped portion comprising a degasser shaft cross sectional area greater than the minimum cross-sectional area of the degasser shaft proximal or at the second end, said first portion comprising between 48% and 100% of the total length of the degasser shaft.
2. The degasser shaft according to claim 1, wherein the first portion comprises a cylindrical shaped portion disposed between the frustoconical shaped portion and the first end of the shaft 3. The degasser shaft according to claim 1 or 2, wherein the frustoconical shaped portion comprises a first frustoconical segment and a second frustoconical segment, wherein the first frustoconical segment extends from the minimum shaft diameter to a maximum shaft diameter and a second frustoconical segment extends from a maximum shaft diameter to the first end of the shaft.4. The degasser shaft according to claim 3, wherein the cylindrical shaped portion is disposed between the first frustoconical segment and the second frustoconical segment.S. The degasser shaft according to claim 1, wherein the frustoconical shaped portion comprises two or more frustoconical shaped segments separated by a cylindrical portion comprising two or more cylindrical segments.6. The degasser shaft according to claim 5, wherein the two or more cylindrical segments have decreasing diameters as the first portion progresses towards the second end of the shaft.7. The degasser shaft according to claim 1, wherein said first portion comprises at least 50% of the total length of the degasser shaft.8. The degasser shaft according to any one of the preceding claims, wherein the degasser shaft comprises an incident angle between a frustoconical shaped portion and a cylindrical portion of greater than 0° and no more than 16°.9. The degasser shaft according to any one of the preceding claims, wherein the shaft further comprises a melt line, said melt line being located at the maximum diameter of the shaft.10. The degasser shaft according to any one of the preceding claims, wherein the shaft comprises a minimum wall thickness in the range of 10 to 80 mm.11. The degasser shaft according to any one of the preceding claims, wherein the shaft comprises a ceramic composite material.12. The degasser shaft according to any one of claim 11, wherein the density of the ceramic composite material is in the range of 1.9 g/cc to 2.5 g/cc.13. The degasser shaft according to any one of claims 11 or 12, where in the Young's modulus of the ceramic composite materials is in the range of 0.5 GPa to 10 GPa.14. The degasser shaft according to any one of claims 11 to 13, wherein the transverse bending strength (TBS) of the ceramic composite material is in the range of 5 MPa to 30 MPa.15. A degasser shaft according to any one of the preceding claims, wherein a length of the shaft is in the range of 200 mm to 2500 mm.16. The degasser shaft according to claim 15, wherein the length of the shaft is greater than 1500 mm.17. The degasser shaft according to any one of the preceding claims, wherein the degasser shaft comprises a ceramic composite and comprises: a. a total length of between 250 and 2200 mm; b. an optional shaped portion extending from or proximal to the first end comprising a length in the range of 0 to 500 mm; c. a frustoconical segment abutting the optional shaped portion comprising a diameter which decreases as the frustoconical segment extends towards the second end, said frustoconical segment comprising an axial length in the range of 100 to 2200 mm; d. an optional cylindrical segment extending from the frustoconical segment to the second end; e. a first shaft end diameter in the range of 60 to 180 mm; f. a second shaft end diameter in the range of 45 to 140 mm; and g. a passage diameter in the range of 10 to 60 mm.18. The degasser shaft according to any one of the preceding claims, wherein the degasser shaft comprises a ceramic composite and comprises: a. a total length of between 1500 and 2200 mm; b. the first shaped portion extending from or proximal to the first end comprising a length in the range of 200 to 500 mm; c. a first frustoconical segment abutting the first shaped portion comprising a diameter which decreases as the frustoconical segment extends towards the second end, said frustoconical segment comprising an axial length in the range of 400 to 2200 mm; d. an optional cylindrical segment extending from the frustoconical segment to the second end; e. a first shaft end diameter in the range of 90 to 180 mm; f. a second shaft end diameter in the range of 80 to 140 mm; and g. a passage diameter in the range of 10 to 60 mm.19. The degasser shaft according to claim 18, wherein the degasser shaft comprises a minimum shaft wall thickness in the range of 20 mm to 45 mm 20. The degasser shaft according to any one of the preceding claims, wherein the degasser shaft comprises a ceramic composite and comprises: a. a total length of between 500 and 1500 mm; b. an optional first shaped portion extending from or proximal to the first end comprising a length in the range of 0 to 400 mm; c. a frustoconical segment abutting the optional first shaped portion comprising a diameter which decreases as the frustoconical segment extends towards the second end, said frustoconical segment comprising an axial length in the range of 500 to 1500 mm; d. an optional cylindrical segment extending from the frustoconical segment to the second end; e. a first shaft end diameter in the range of 100 to 150 mm; f. a second shaft end diameter in the range of 60 to 120 mm; and g. a passage diameter in the range of 10 to 40 mm.21. The degasser shaft according to claim 20, further comprising a second frustoconical segment disposed from the first cylindrical segment and extending to the first end of the degasser shaft.22. The degasser shaft according to any one of the preceding claims, wherein the degasser shaft comprises a ceramic composite and comprises: a. a total length of between 250 and 500 mm; b. the shaped portion extending from the first end comprising a length in the range of 50 to 100 mm; c. a frustoconical segment abutting the first shaped portion comprising a diameter which decreases as the frustoconical segment extends towards the second end, said frustoconical segment comprising an axial length in the range of 100 to 400 mm; d. an optional cylindrical segment extending from the frustoconical segment to the second end; e. a first shaft end diameter in the range of 60 to 90 mm; f. a second shaft end diameter in the range of 45 to 70 mm; and g. a passage diameter in the range of 10 to 25 mm.23. The degasser shaft according to claim 22, comprising a minimum shaft wall thickness in the range of 10 mm to 17 mm.24. A process for degassing a molten metal melt comprising submerging a degassing apparatus into the molten metal melt, said degassing apparatus comprising a degassing shaft according to any one of the preceding claims and wherein the rotational speed of the degassing shaft is below the natural frequency of the shaft.25. Use of the degasser shafts according to any one of claims 1 to 23 for the processing of molten metal.