REFRACTORY COMPONENTS

Abstract: A refractory component is provided for use in a metal producing or refining process in which the component is at least partially immersed in molten metal. The component including a graphite member (2) and a refractory sleeve (10) that covers at least part of the graphite member. A recess (12) is provided in the surface of the graphite member (2) and the refractory sleeve (10) is located in the recess.

Description: REFRACTORY COMPONENTS The present invention relates to refractory components for use in a metal producing or refining process in which the components are at least partially immersed in molten metal. In particular but not exclusively the invention relates to a graphite shaft for use in a process for producing or refining non-ferrous metals, such as aluminium and aluminium alloys. The invention also relates to a method of making such refractory components. Graphite shafts are used for various purposes in processes for producing and refining non- ferrous metals including aluminium and aluminium alloys, where the shaft is at least partially immersed in the molten metal (the "melt"). Liquid aluminium, aluminium alloys and other non-ferrous metals contain inclusions, dissolved hydrogen and alkali meted impurities. These are undesirable as they adversely affect the physical properties of the metals. Various methods are conventionally used to remove such impurities, one such method being rotary degassing. In this process, inert gas is inj ected into the liquid metal through a hollow graphite shaft, one end of which is immersed in the liquid metal, well below the surface. A rotor may be fixed to the end of the shaft, and the whole assembly is rotated, typically at 200-700 RPM. This increases the efficiency of the process. The spinning action of the rotor breaks up the gas stream emerging from the shaft into fine bubbles, increasing the surface area of the gas. The gas then rises through the metal, removing dissolved hydrogen and inclusions and carrying them to the surface of the melt. Additionally, chlorine can be added to the inert gas and injected through the shaft and rotor into the metal. The chlorine reacts with alkali metals in the metal and the resulting liquid impurity is removed by the bubbles. This chlorine may be added as a gas, or injected as a solid (in powdered or granulated form) or a liquid salt mixture. In some cases, the rotor acts as a stiirer, or is replaced by a stirrer, and the chlorine can then be added in a solid salt form to the surface of the metal, and is mixed into the metal by the stirring action. A flux may also be added, usually in the form of solid or liquid salts, for example NaCl, K2C13, MgCl etc. Graphite is used for these shafts because it is resistant to thermal shock, is not wetted by liquid aluminium, has low thermal expansion, is mechanically strong and tough even at elevated temperatures, is easy to machine and does not react with the liquid aluminium. However, it is well known that graphite oxidises at elevated temperatures. The shafts therefore gradually erode, particularly in the region where the shaft passes through the surface of the liquid metal, and have to be replaced periodically. Many methods exist for treating the graphite to reduce the rate of oxidation. Typically, the graphite used in these applications is treated by impregnating protective compounds into its surface. This allows it to be used for extended times in liquid, non-ferrous metals. However, the failure mechanism of these shafts is still usually due to erosion and oxidation of the graphite at the metal line. To avoid this problem, some suppliers have tried to produce these parts in ceramic materials such as fused silica, alumina, silicon carbide, silicon nitride, silicon aluminium oxy-nitride, Mullite, zircon, zirconia and combinations thereof. However, despite being harder (and therefore more abrasion resistant) and resistant to oxidation, these materials are usually lower in strength, more brittle and more expensive to manufacture than graphite. This causes problems when connecting the ceramic parts to the machines, which is normally done with a screw thread. It also causes premature breakage due to impact from cleaning tools or foreign objects in the metal. The net result is that there is not a significant increase in the lifetime of the ceramic part compared to the graphite part. It is also known to apply a ceramic surface to the graphite, in the area of attack (i.e. along the metal line). Many systems have been tried, including the use of ceramic coatings (applied by being sprayed, brushed, dipped etc), fibre and ceramic bandages, vacuum formed sleeves, and pre-formed cast sleeves loosely mounted on the shaft. Whilst some of these improve the lifetime of the shaft, there is not generally a large enough improvement to justify the additional cost of treatment. An improved method of treatment is therefore required. Graphite components (including hollow and solid shafts) are also used for other purposes in processes for producing and refining non-ferrous and ferrous metals. For example, in the production of non-ferrous metals these components may be used for removing hydrogen by injecting gases such as nitrogen or argon, for removing inclusions and alkali metals by ' injecting reactive gases such as chlorine or solid or liquid chlorine salt fluxes, or as part of a stirring assembly to aid mixing of the metal, by driving a rotor or stirrer . In the production of ferrous metals, graphite may be used for components such as submerged entry nozzles, injection lances and flow control systems, for injecting gases under the surface of the metal or controlling the flow of the metal. It is an object of the present invention to provide a graphite component for use in a metal producing or refining process, that mitigates at least some of the aforesaid problems. A further object of the present invention is to provide a method of making such a graphite component. According to the present invention there is provided a refractory component for use in a metal producing or refining process in which the component is at least partially immersed in molten metal, the component including a graphite member and a refractory sleeve that covers at least part of the graphite member, characterised in that a recess is provided in the surface of the graphite member and the refractory sleeve is located in the recess. The refractory sleeve protects the covered part of the graphite member from oxidation and erosion. Because the sleeve is located in the recess, it is mechanically fixed very securely to the graphite shaft, preventing liquid metal from penetrating between the sleeve and the graphite. The arrangement is also very strong, but does not affect the overall dimensions of the component. Advantageously, the refractory sleeve covers the region of the graphite member that in use passes through the surface of the molten metal, thereby protecting the graphite member in that most vulnerable region. Preferably, the refractory sleeve is of a length such that, in use, it extends above and below the surface of the molten metal by a distance in the range 50- 300mm, preferably 100- 150mm. This ensures that the component is protected, even if the level of the liquid metal varies significantly. Advantageously, the refractory sleeve is cast in situ in the recess providedϊn the surface of the graphite member, so ensuring a good fit. Advantageously, the external surface of the refractory sleeve is substantially level with the external surface of the graphite member, to avoid causing turbulence. Alternatively, the external surface of refractory sleeve may stand above the external surface of the graphite member. This allows the depth of the recess in the graphite member to be reduced without reducing the thickness of the sleeve, and is the preferred arrangement where a deep recess would compromise the strength of the graphite member. The recess may have a depth in the range 1-30mm, preferably 8mm. Preferably, the recess has circumferential walls that are inclined towards one another. The circumferential walls may be inclined relative to the surface of the graphite member at an angle in the range 20- 89 ° , preferably approximately 60 ° . This locks the sleeve into the recess. The sleeve may have a thickness in the range l-25mm, preferably about 7mm. Advantageously, the refractory component includes an expansion gasket between the refractory sleeve and the graphite member, to accommodate differential thermal expansion of the two components. The expansion gasket may have a thickness in the range 0.5-5mm, preferably 1mm. The expansion gasket may include a layer of ceramic paper. Advantageously, the refractory sleeve is made of a ceramic material, which may be fused silica, alumina, silicon carbide, silicon nitride, silicon aluminium oxy-nitride, Mullite, zircon or zirconia, or a combination thereof. Advantageously, the refractory component comprises a substantially cylindrical shaft, which may have a diameter in the range 30-200mm, preferably approximately 75mm, and a length in the range 0.5-2.0m, preferably approximately 1.0- 1.3m. Advantageously, the shaft is hollow. Advantageously, the refractory component is suitable for use in a process for producing or refining non-ferrous metals, in particular aluminium and aluminium alloys. According to another aspect of the invention there is provided a method of making a refractory component for use in a metal producing or refining process in which the component is at least partially immersed in molten metal, the refractory component including a graphite member and a refractory sleeve that covers at least part of the graphite member, characterised in that a recess is formed in the surface of the graphite member and the refractory sleeve is cast in the recess. Advantageously, the refractory sleeve is cast in situ in the recess. Preferably, the graphite member is shaped on a lathe, and the recess is formed on the surface of the graphite member during the shaping operation. Advantageously, a mould is placed over the recess and a refractory material is injected into the recess beneath the mould. Advantageously, the refractory sleeve is fired on the graphite member. Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an isometric drawing of a refractory component according to a first embodiment of the invention, comprising a graphite shaft with a ceramic sleeve, showing some hidden details; Figure 2 is a side view of the component, showing the level of the liquid metal during use; Figure 3 is a side section through the component on line A-A of Fig. 2; Figure 4 is a sectional side view at a larger scale, showing part of the component shown in Fig. 3; Figure 5 is an isometric drawing of a refractory component according to a second embodiment of the invention, comprising a graphite shaft with a ceramic sleeve, showing some hidden details; Figure 6 is a side view of the component shown in Fig. 5, showing the level of the liquid metal during use; Figure 7 is a side section through the component on line A-A of Fig. 5; Figure 8 is a sectional side view at a larger scale, showing part of the component shown in Fig. 7; Figure 9 is a plan view of a refractory component according to a third embodiment of the invention, comprising a pump block, showing some hidden details; 5 Figure 10 is a side section through the component on line B-B of Fig. 9; Figure 11 is a sectional side view at a larger scale, showing part of the component shown in Fig. 10; Figures 12 and 13 are plan views of the component, not showing any hidden details; Figure 14 is an end section on line A-A of Fig. 13, and 10 Figure 15 in an isometric view of the component, partially broken away. The refractory component shown in Figs. 1-4 comprises a substantially cylindrical shaft 2 of solid graphite, having a length of approximately lm and a diameter of approximately 75mm. At the lower end of the shaft 2 there is a portion 4 of reduced diameter that is provided with a screw thread 6 for fixing the shaft to a rotor. A threaded bore 8 is provided 15 at the upper end of the shaft for fixing the shaft to a rotary drive mechanism. A sleeve 10 of ceramic material is located in a recess 12 provided approximately in the middle of the shaft 2. The sleeve 10 covers the portion of the shaft that in use extends through the surface 14 of the liquid, metal. The recess 12 comprises a shallow slot having a depth of about 8mm and a width of about 20 250mm, which extends around the circumference of the cylindrical shaft 2. The circumferential walls 16 that define the upper and lower edges of the recess 12 are inclined towards one another, at an angle of about 60° to the external cylindrical surface of the graphite shaft. The sleeve 10 is made of a ceramic material, for example fused silica, alumina, silicon 25 carbide, silicon nitride, silicon aluminium oxy-nitride, Mullite, zircon or zirconia, or a combination thereof. It is formed by injecting the material in liquid or semi-solid form into the recess 12 then allowing it to cure. The sleeve therefore essentially fills the recess and takes on its shape. The upper and lower edges 18 of the sleeve 10 are therefore inclined outwards, mechanically locking the sleeve 10 to the shaft 2. The sleeve 12 has a thickness of about 7mm, leaving a 1mm gap 20 between the sleeve 12 and the shaft 2, which in use accommodates differential thermal expansion of the sleeve and the shaft. The gap 20 may be filled with an expansion gasket, for example a sheet of ceramic paper. The refractory component may of course have different dimensions, according to the purpose for which it is intended. Typically, however, the ranges for those dimensions will be approximately as follows: The refractory component may also take the form of a hollow shaft, for injecting gas into the liquid metal as part of a rotary degassing operation, similar principles of construction may be employed in other refractory components made substantially of graphite that come into contact with liquid metal (ferrous and non-ferrous). The shaft 2 shown in the accompanying drawing may be made as follows: 1. The graphite shaft 2 is shaped by machining a billet of solid graphite on a lathe. The cylindrical surface of the shaft, the reduced diameter end portion 4 and the recess 12 are all formed during this shaping operation. 2. A surface treatment may be applied to the graphite, to reduce the rate of oxidation, for example, the graphite may be treated by impregnating protective compounds into its surface. This step is conventional and will not be described in detail. 3. A mould slipped onto the shaft, covering the recess 12. This mould may, for example, consist of a slotted cylindrical nylon sleeve having an inside diameter matched to the outer diameter of the graphite shaft 2, and a length about 100mm longer than the recess 12, to provide a 50mm overlap at each end. 4. The ceramic material is injected in liquid form through the slot in the sleeve, until it completely fills the recess 10. After allowing the ceramic to become solid or semi- solid, any excess ceramic material is removed from the slot using a simple scraper tool. 5. When the ceramic has cured completely, the mould is rotated then removed, a nd final smoothing may be undertaken with a diamond file to ensure that the edges of the sleeve 10 are smooth and flush with the external cylindrical surface of the graphite shaft 2. 6. The ceramic is kept damp for about 48 hours, for example by wrapping in wet rags or spraying with water, to prevent it cracking, and it is then allowed to dry. 7. Finally, the ceramic material is fired on the graphite shaft in a kiln at a temperature of about 380C , to drive out any water remaining in the ceramic. In use, the shaft 2 is mounted so that the ceramic sleeve 10 covers the part of the graphite shaft that passes through the surface 14 of the liquid metal. The sleeve 10 prevents oxidation of the graphite and protects the shaft from erosion and abrasion. The useful lifetime of the component is therefore considerably increased. The advantages provided by the invention include the following: • Reduced oxidation of the graphite shaft at the metal line. Substantially unaffected strength of the graphite. Increased toughness and impact resistance of the graphite part. Easy machining of threads into the graphite. The original dimensions of the shaft are retained (therefore there is no change in the shaft's angular velocity at the metal line, which can cause turbulence). The 'balance' of the shaft is unaffected (which is important when spinning quickly). Ingress of aluminium behind the refractory protective layer is minimised. • The component is reliable and inexpensive to manufacture. The refractory component shown in Figs.5-8 is substantially similar in many respects to the component shown in Figs. 1 -4 and where appropriate similar reference numbers have been used. The component comprises a substantially cylindrical shaft 2 of solid graphite, having a diameter of approximately 40mm. The shaft is therefore considerably narrower than that shown in Figs. 1 -4. At the lower end of the shaft 2 there is a portion 4 of reduced diameter that is provided with a screw thread 6 for fixing the shaft to a rotor. A threaded bore 8 is provided at the upper end of the shaft for fixing the shaft to a rotary drive mechanism. A sleeve 10 of ceramic material is located in a recess 12 provided approximately in the middle of the shaft 2. The sleeve 10 covers the portion of the shaft that in use extends through the surface 14 of the liquid metal. The recess 12 comprises a shallow slot having a depth of about 3mm and a width of about 250mm, which extends around the circumference of the cylindrical shaft 2. The recess is therefore much shallower than that on the shaft shown in Figs. 1 -4, to avoid compromising the strength of the shaft. The circumferential walls 16 that define the upper and lower edges of the recess 12 are inclined towards one another, at an angle of about 60° to the external cylindrical surface of the graphite shaft. The sleeve 10 is made of a ceramic material as described above and is formed by placing a mould over the recess and injecting the material in liquid or semi-solid form into the void formed by the recess 12 and the mould. The sleeve therefore essentially fills the recess and takes on its shape. The upper and lower edges 18 of the sleeve 10 are therefore inclined outwards, mechanically locking the sleeve 10 to the shaft 2. The mould is shaped such that the outside diameter of the sleeve is greater than the outside diameter of the shaft. The external surface 22 of the sleeve therefore stands proud of the external surface of the shaft. This allows the sleeve to retain a thickness of about 7mm, which it needs for strength, although only 2-3mm of that thickness lies under the surface of the shaft. As in the previous embodiment, a gap 20 is provided between the sleeve 12 and the shaft 2, which accommodates differential thermal expansion of the sleeve and the shaft. The gap 5 20 may be filled with an expansion gasket, for example a sheet of ceramic paper. The refractory component may of course have different dimensions, according to the purpose for which it is intended. Typically, however, the ranges for those dimensions will be approximately as follows: The invention is applicable to degassing, gas-injection, flux-injection, chlorine-injection, stirring, moving and treatment of liquid aluminium, its alloys and non-ferrous metals, where 20 a graphite part is immersed into the liquid metal. The invention is also applicable to refractory components used in the production and refining of ferrous metals, where a graphite part is immersed into the liquid metal. An example of such a component is shown in Figs. 9-15, which depict a component of a pump (a pump block 24) that, in use, is partially immersed in the molten metal. The block 25 24 is made of graphite and is substantially cuboidal in shape, with a shallow groove 26 that extends along one face 28, parallel to the longitudinal axis of the block. In use, the block is held upright, with the longitudinal axis vertical, and with the bottom third of the block 24 is made of graphite and is substantially cuboidal in shape, with a shallow groove 26 that extends along one face 28, parallel to the longitudinal axis of the block. In use, the block is held upright, with the longitudinal axis vertical, and with the bottom third of the block immersed in molten metal. The block is therefore prone to erosion where it passes through the surface of the metal. To prevent erosion, the block 24 is provided with a ceramic sleeve 30 that extends around the block to protect the area subject to erosion. The sleeve extends a few centimetres above and below the metal line, to allow for variations in the depth of the metal. The sleeve 30 is located in a recess 32 provided in the block 24. The recess 32 comprises a shallow slot having a depth of about 3 mm that extends around the circumference of the block. The circumferential walls 34 that define the upper and lower edges of the recess 32 are inclined towards one another, at an angle of about 60° to the external surface of the graphite block. These walls serve to retain the sleeve in the recess. An expansion gap 36 is provided behind the sleeve. The ceramic materials used in the sleeve and the method of manufacturing are substantially as described above. The methods described herein may also be employed for manufacturing protective sleeves for other graphite components used in the aluminium industry, which are prone to erosion owing to contact with the molten metal.

Claims: Claims 1. A refractory component for use in a metal producing or refining process in which the component is at least partially immersed in molten metal, the component including a graphite member and a refractory sleeve that covers at least part of the graphite member, characterised in that a recess is provided in the surface of the graphite member and the refractory sleeve is located in the recess. 2. A refractory component according to claim 1, characterised in that the refractory sleeve covers the region of the graphite member that in use passes through the surface of the molten metal. 3. A refractory component according to claim 2, characterised in that the refractory sleeve is of a length such that, in use, it extends above and below the surface of the molten metal by a distance in the range 50-300mm, preferably 100-150mm. 4. A refractory component according to any one of the preceding claims, characterised in that the refractory sleeve is cast in situ in the recess provided in the surface of the graphite member. 5. A refractory component according to any one of the preceding claims, characterised in that the external surface of the refractory sleeve is substantially level with the external surface of the graphite member. 6. A refractory component according to claim 5, characterised in that the recess has a depth in the range 1-30mm, preferably 8mm. 7. A refractory component according to any one of claims 1 to 4, characterised in that the external surface of the refractory sleeve is raised above the external surface of the graphite member. 8. A refractory component according to claim 7, characterised in that the recess has a depth in the range 1-30mm. preferably 3 mm. 9. A refractory component according to any one of the preceding claims, characterised in that the recess has circumferential walls that are inclined towards one another. 10. A refractory component according to claim 9, characterised in that the circumferential walls that are inclined relative to the surface of the graphite member at an angle in the range 20-89°, preferably approximately 60°. 11. A refractory component according to any one of the preceding claims, characterised in that the sleeve has a thickness in the range l-25mm, preferably 7mm. 12. A refractory component according to any one of the preceding claims, including an expansion gasket between the refractory sleeve and the graphite member. 13. A refractory component according to claim 12, characterised in that the expansion gasket has a thickness in the range 0.5-5mm, preferably 1mm. 14. A refractory component according to claim 12 or claim 13, characterised in that the expansion gasket includes a layer of ceramic paper. 15. A refractory component according to any one of the preceding claims, in which the refractory sleeve is made of a ceramic material. 16. A refractory component according to claim 15, in which the refractory sleeve is made of fused silica, alumina, silicon carbide, silicon nitride, silicon aluminium oxy- nitride, Mullite, zircon or zirconia, or a combination thereof. 17. A refractory component according to any one of the preceding claims, wherein the component comprises a substantially cylindrical shaft. 18. A refractory component according to claim 17, in which the shaft has a diameter in the range 30-200mm, preferably approximately 75mm. 19. A refractory component according to claim 17 or claim 18, in which the shaft has a length in the range 0.5-2.0m, preferably approximately 1.0-1.3m. 20. A refractory component according to any one of claims 17 to 19, in which the shaft is hollow. 21. A refractory component according to any one of the preceding claims, for use in a process for producing or refining non-ferrous metals. 22. A refractory component according to claim 21, for use in a process for producing or refining aluminium and aluminium alloys. 23. A method of making a refractory component for use in a metal producing or refining process in which the component is at least partially immersed in molten metal, the refractory component including a graphite member and a refractory sleeve that covers at least part of the graphite member, characterised in that a recess is formed in the surface of the graphite member and the refractory sleeve is cast in the recess. 24. A method according to claim 23 , in which the refractory sleeve is cast in situ in the recess. 25. A method according to claim 23 or claim 24, in which the graphite member is shaped on a lathe, and the recess is formed on the surface of the graphite member during the shaping operation. 26. A method according to any one of claims 23 to 25, in which a mould is placed over the recess and a refractory material is injected into the recess beneath the mould. 27. A method according to any one of claims 23 to 26, in which the refractory sleeve is fired on the graphite member.

Inventors: VINCENT MARK, VINCENT, MARK
Applicants: PYROTEK ENGINEERING MATERIALS, VINCENT MARK, PYROTEK ENGINEERING MATERIALS LIMITED, VINCENT, MARK
Application year: 2001
Main classification: C04B37
IPC codes: B22D1, C04B37, F27D1