GB2365513A

GB2365513A - Refractory components for use in metal producing processes

Info

Publication number: GB2365513A
Application number: GB0019088A
Authority: GB
Inventors: Mark Vincent
Original assignee: Pyrotek Engineering Materials Ltd
Current assignee: Pyrotek Engineering Materials Ltd
Priority date: 2000-08-04
Filing date: 2000-08-04
Publication date: 2002-02-20
Also published as: US20020185794A1; CA2377897A1; AU3036301A; EP1218308A1; AU755199B2; GB0019088D0; JP2004505779A; WO2002012147A1

Abstract

A refractory component for use in a metal producing or refining process in which the component is at least partially immersed in molten metal, comprises a graphite member (2) and a refractory sleeve (10) that covers at least part of the graphite member (2). The sleeve (10) covers the portion of the graphite member that in use extends through the surface of the molten metal. A recess (12) is provided in the surface of the graphite member and the refractory sleeve (10) is located in the recess. The component may be a rotatable hollow shaft used in injecting gas into the molten metal. The spinning action of the shaft breaks up the gas stream emerging from the shaft into fine bubbles. The molten metal may be aluminium or aluminium alloy. The sleeve (10) may be of a ceramic material, for example, fused silica, alumina, silicon carbide, silicon nitride, silicon aluminium oxy-nitride, mullite, zircon or zirconia, or a combination thereof.

Description

2365513 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 nonferrous metals including aluminium and aluminium alloys, where the shaft is at least 10 partially immersed in the molten metal (the "melt").

Liquid aluminium, aluminium alloys and other non-ferrous metals contain inclusions, dissolved hydrogen and alkali metal 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 injected 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 stirrer, 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 NaCI, K2C13, MgC 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 10 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 15 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 par& 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 signifi-cant 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.

3 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 dimensi ons 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 5030Omm, preferably 100-1 50=. This ensures that the component is protected, even if the level of the liquid metal varies significantly.

4 Advantageously, the refractory sleeve is cast in situ in the recess provided in the surface of the graphite member, so ensuring a good fit. Preferably, the external surface of the refractory sleeve is substantially level with the external surface of the graphite member, to avoid causing turbulence.

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 2089', preferably approximately 60'. This locks the sleeve into the recess. The sleeve may have a thickness in the range 1-25mm, 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 1 mm. 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 30-20Omm, preferably approximately 75min, 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, amould is placed over the recess and a refractory material is injected into 5 the recess beneath the mould.

Advantageously, the refractory sleeve is fired on the graphite member.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fiaure 1 is an isometric drawing of a refractory component comprising a graphite shaft with 0 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, and Figure 4 is a sectional side view at a larger scale, showing part of the component shown in Fig. 3.

The refractory component shown in the drawings comprises a substantially cylindrical shaft 2 of solid graphite, having a length of approximately I m and a diameter of approximately 75nim. 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 8nun and a width of about 250mm, which extends around the circumference of the cylindrical shaft 2. The 25 circumferential walls 16 that define the upper and lower edges of the recess 12 are inclined 6 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 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 7rmn, leaving a I mm gap 20 between the sleeve 12 10 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:

Dimension Preferred value Range A Angle of recess walls 60 200-890 B Expansion gap Imm 0.5 - 5nun C Thickness of sleeve 7mm 1 - 25nim D Length of sleeve below metal level 150mm 50 - 30Omm E Length of sleeve above metal level 1 OOMM 50 - 30Omm F Diameter of shaft 75mm. 30 - 20Omm Length of shaft 1.3m 0.5 - 2m 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).

7 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 1 OOMM longer than the recess 12, to provide a SOmm 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 rid 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 3 80C, 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.

8 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 anaular 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 invention is applicable to degassing, gas-injection, flux-injection, chlorine-injection, stirring and treatment of liquid aluminium, its alloys and non-ferrous metals, where 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 15 graphite part is immersed into the liquid metal.

9

Claims

1 A refractory component for use in a metal producin'. 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 50-30Omm, preferably 100-150mm.

4. A refractory component according to any one ofthe 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 any one of the preceding claims, characterised in that the recess has a depth in the range 1-30mm, preferably 8mm.

7. 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.

8. A refractory component according to claim 7, characterised in that the circumferential walls that are inclined relative to the surface ofthe graphite member at an angle in the range 20-89', preferably approximately 60', 9. A refractory component according to any one of the preceding claims, characterised in that the sleeve has a thickness in the range 1-25mm, preferably 7mm.

10. A refractory component according to any one of the preceding claims, including an expansion gasket between the refractory sleeve and the graphite member. 11. A refractory component according to claim 10, characterised in that the expansion gasket has a thickness in the range 0. 5-5mm, preferably Imm. 12. A refractory component according to claim 10 or claim 11, characterised in that the expansion gasket includes a layer of ceramic paper. 13. A refractory component according to any one of the preceding claims, in which the refractory sleeve is made of a ceramic material. 14. A refractory component according to claim 13, 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. 15. A refractory component according to any one of the preceding claims, wherein the component comprises a substantially cylindrical shaft. 16. A refractory component according to claim 15, in which the shaft has a diameter in the range 30-20Omm, preferably approximately 75mm. 17. A refractory component according to claim 15 or claim 16, in which the shaft has a length in the range 0.5-2. 0m, preferably approximately 1.0-1.3m. 18. A refractory component according to any one of claims 15 to 17, in which the shaft is hollow. 19. A refractory component according to any one of the preceding claims, for use in a process for producing or refining non-ferrous metals. 20. A refractory component according to claim 19, for use in a process for producing or refining aluminium and aluminium alloys. 21. 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 11 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.

22. A method according to claim 2 1, in which the refractory sleeve is cast in situ in the recess. 23. A method according to claim 21 or claim 22, 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. 24. A method according to any one of claims 21 to 23, in which a mould is placed over the recess and a refractory material is injected into the recess beneath the mould. 25. A method according to any one of claims 21 to 24, in which the refractory sleeve is fired on the graphite member.

Amendments to the claims have been filed as follows 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.

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 50-30Omm, preferably 100-150nim.

6. A refractory co mponent according to any one ofthe preceding claims, characterised in that the recess has a depth in the range P30mni, preferably 8nim.

8. A reftactory component according to claim 7, characterised in that the circumferential walls that are inclined relative to the surface ofthe graphite member at an angle in the range 20-89', preferably approximately 6T.

9. A refractory component according to any one of the preceding claims, characterised in that the sleeve has a thickness in the range 1-25mm, preferably 7mm.

10. A refractory component according to any one of the preceding claims, in which the refractory sleeve is made of a ceramic material.

11. A refractory component according to claim 10, 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

12. A reftactory component according to any one of the preceding claims, wherein the component comprises a substantially cylindrical shaft.

13. A refractory component according to claim 12, in which the shaft has a diameter in the range 30-20Omm, preferably approximately 75mm.

14. A refractory component according to claim 12 or claim 13, in which the shaft has a length in the range 0.5-2.0m, preferably approximately LO-1.3m.

15. A refractory component according to any one of claims 12 to 14, in which the shaft is hollow.

16. A refractory component according to any one of the preceding claims, for use in a process for producing or refining non-ferrous metals.

17. A refractory component according to claim 16, for use in a process for producing or refining aluminium and aluminium alloys.

18. A refiactory 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 an expansion gasket is included between the refractory sleeve and the graphite member.

19. A refractory component according to claim 18, characterised in that the expansion gasket has a thickness in the range 0.5-5mm, preferably lmm.

20. A refractory component according to claim 18 or claim 19, characterised in that the expansion gasket includes a layer of ceramic paper.

\1i<.

21. 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 reftactory component including a graphite m ember 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.

22. A method according to claim 2 1, in which the refractory sleeve is cast in situ in the recess.

23. A method according to claim 21 or claim 22, 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.

24. A method according to any one of claims 21 to 23, in which a mould is placed over the recess and a refractory material is injected into the recess beneath the mould.

25. A method according to any one of claims 21 to 24, in which the refractory sleeve is fired on the graphite member.

26. A refractory component substantially as described herein with reference to the accompanying drawings.