Castable refractory material

Abstract

A castable refractory material for use in the manufacture of refractory products including fused silica, ceramic fiber, microsilica and a bonding material comprising colloidal silica.

Claims

1. A castable refractory material for use in the manufacture of refractory products, wherein the castable refractory material includes fused silica, ceramic fiber, microsilica and a bonding material comprising colloidal silica, wherein the castable refractory material comprises microsilica in a range of 2-15%, by weight.

2. The castable refractory material according to claim 1, comprising fused silica in a range of 30-90% by weight.

3. The castable refractory material according to claim 1, comprising ceramic fiber in a range of 5-45% by weight.

4. The castable refractory material according to claim 1, comprising colloidal silica in a range of 3-25%, by weight.

5. The castable refractory material according to claim 1, wherein the fused silica includes particle sizes in a range of 150 m to 3.5 m.

6. The castable refractory material according to claim 5, wherein the fused silica includes particles of mesh size 200 and particles of mesh size 325.

7. The castable refractory material according claim 1, wherein the ceramic fiber is a synthetic ceramic fibre.

8. The castable refractory material according to claim 1, wherein the ceramic fiber is an alkaline earth silicate fibre.

9. The castable refractory material according to claim 1, wherein the ceramic fiber is soluble in physiological fluids.

10. The castable refractory material according to claim 1, wherein the ceramic fiber is a chopped fibre having a fibre length in a range of 9-15 m.

11. The castable refractory material according to claim 1, further comprising a dispersing agent.

12. The castable refractory material according to claim 1, further comprising a non-wetting agent in a range of 0%-12% by weight.

13. A refractory product for use in aluminium processing, comprising a cast refractory material according to claim 1.

14. The refractory product according to claim 13, further comprising a component of a direct chill (DC) casting system.

15. The refractory product according to claim 13, wherein the product has a density in a range of 0.8-2.0 g/cm.sup.3.

16. A method of making a refractory product for use in aluminium processing, said method comprising providing a refractory material according to claim 1, casting the refractory material in a mold, and firing the cast product.

17. The method according to claim 16, further comprising machining the cast product.

18. The method according to claim 16, wherein the refractory material is casted using a freeze casting process.

19. The method according to claim 16, further comprising drying the cast product prior to firing.

Description

(1) Certain embodiments of the invention will now be described by way of example with reference to the accompanying drawings, wherein:

BRIEF DESCRIPTION OF THE DRAWINGS

(2) FIG. 1 is a plan view of a casting table for a DC casting system;

(3) FIG. 2 is a sectional view of a casting table assembly including a distribution device and other refractory components;

(4) FIG. 3 is an isometric view of a distribution device;

(5) FIG. 4 is a side view of the distribution device;

(6) FIG. 5 is an end view of the distribution device;

(7) FIG. 6 is a plan view showing the upper side of the distribution device;

(8) FIG. 7 is an isometric view of a thimble device;

(9) FIG. 8 is a plan view of the thimble device;

(10) FIG. 9 is a cross-section on line A-A of FIG. 8;

(11) FIG. 10 is an isometric view of a down-spout;

(12) FIG. 11 is a plan view of the down-spout;

(13) FIG. 12 is a cross-section on line A-A of FIG. 11;

(14) FIG. 13 is an isometric view of a trough;

(15) FIG. 14 is a plan view of the trough; and

(16) FIG. 15 is a cross-section on line A-A of FIG. 14.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

(17) The casting table 2 shown in FIG. 1 comprises a rectangular steel support table 4 and a distributor system 6 comprising a plurality of refractory distribution devices 8 for containing and distributing liquid metal to a plurality of casting sites beneath the table 4. This particular casting table 2 is suitable for use in a direct chill (DC) casting system for fabricating cylindrical billets from non-ferrous metals such as aluminium and alloys thereof.

(18) The present invention relates to refractory products used in aluminium processing and is distributor systems including but not limited to DC casting systems of the type shown in FIG. 1. These refractory products include such products as the distribution device 8 shown in FIGS. 1-6, the thimble 40 shown in FIGS. 7-9, the down-spout 50 shown in FIGS. 10-12 and the trough 52 shown in FIGS. 13-15, as well as other refractory products that may be used in aluminium processing and distributor systems. The present invention also relates to a castable refractory material for use in the manufacture of refractory products, and a method of making refractory products.

(19) A distribution device 8 according to one embodiment of the invention is shown in FIGS. 3-6. The distribution device 8 includes a refractory body 9, which is made of a cast refractory ceramic material and includes a base 12 and a peripheral wall 14 that extends upwards from the base 12. The base 12 and the peripheral wall 14 together define one section of the open-topped trough 10. The peripheral wall 14, which may be continuous or discontinuous, comprises two short end walls 16 and two longer side walls 18. Each side wall 18 includes a central section 20 and two ends sections 22. The innermost parts of the end sections 22 curve outwards and the central section 20 thus stands out beyond the plane of the end sections 22. A U-shaped channel 24 is formed in the central section 20, which extends downwards from the top edge of the peripheral wall 14 through approximately two thirds of the height of the distribution device.

(20) When a plurality of distribution devices 8 are mounted together on a casting table as shown in FIG. 1 the central section 20 of each side wall 18 abuts the central section of the side wall of an adjacent distribution device and the U-shaped channels 24 formed in the adjacent walls are aligned with one another forming the open-topped trough 10 that allows liquid metal to flow between the distribution devices 8.

(21) Two circular feed holes 26 are provided in the base 12 of the refractory body 9. In use, liquid metal can flow through these holes 26 to the casting sites defined by the table 2, so as to form billets. Although in this example the distribution device 8 has two feed holes 26, it may alternatively have more or fewer than two feed holes. The distribution device 8 is mounted on the support table 4 as shown in FIG. 2. A sheet of ceramic paper 38 is positioned between the distribution device 8 and the upper surface of the support table 4.

(22) Additional refractory components of the casting system are provided to guide the flow of liquid aluminium from the distribution device 8 through the table 4 during formation of a billet. These refractory components may include for example a cylindrical sleeve (called a thimble or scupper) 40 that fits within the circular feed hole 26 and extends through the base of the refractory body 9 and the thickness of the table 4, a circular transition plate (or T-plate or top ring) 42 that extends radially outwards from the lower end of the thimble 40 below the lower surface of the table 4, and a tubular cylindrical graphite casting ring (or casting mould) 44 that extends downwards from the outer periphery of the transition plate 42. A gasket may be provided within the cylindrical joint 46 between the cylindrical thimble 40 and the transition plate 42 to prevent liquid metal leaking through the joint.

(23) The thimble 40 is shown in more detail in FIGS. 7-9. The shape of this refractory component may be conventional, for example as described in U.S. Pat. No. 4,598,763.

(24) In an embodiment of the invention a refractory product, for example a distributor device 8 or a cylindrical thimble 40, is made from a new ceramic refractory composition that provides significant advantages over the refractory materials conventionally used for these components. An exemplary formulation suitable for manufacture of the thimble 40 is set out below.

(25) TABLE-US-00002 Material Description % by weight Fused silica 200 mesh 37.80% Fused Silica 325 mesh 10.10% Micro-Silica 971u 10.00% Fibre None RCF 26.00% Dispex Dispersing Agent 0.10% Colloidal Silica Bond 16.00% 100.00%

(26) In this example a soluble ceramic fibre material is used (this type of material is also sometimes referred to as a non-refractory ceramic fibre). Specifically, in this example the ceramic fibre material is Insulfrax chopped fibre material from Unifrax Ltd. This is an alkaline earth silicate wool, which has high temperature stability up to 1200 C, low thermal conductivity and excellent wet forming characteristics. It is soluble (non-durable) in physiological fluids, for example lung fluid. This helps to reduce or avoid the health risks associated with the use of non-soluble fibres, which can cause lung disease if inhaled. Alternatively, a non-soluble refractory ceramic fibre may be used, for example Fiberfrax from Unifrax Ltd, which is an alumino silicate wool fibre.

(27) The refractory composition also includes fused silica (a fully dense amorphous or non-crystalline form of silicon dioxide SiO.sub.2). The fused silica is provided in crushed form and in the example set out above it is supplied in two mesh sizes: 200 mesh (<75 m) and 325 mesh (<44 m). Different distributions of particle size may be used depending on the product to be manufactured from the refractory composition. In general, if the product to be manufactured from the refractory material is dimensionally relatively small, or if it must be manufactured to tight tolerances, or if the cast product is to be machined after casting (as in the case of a thimble), relatively small particles of fused silica will be used (e.g. 200 mesh and 325 mesh). On the other hand, if the product to be manufactured is dimensionally relatively large, or if it is not required to be manufactured to tight tolerances, or if the cast product does not need to be machined after casting, relatively large particles of fused silica can be used. For example, a trough that does not need to be machined may include fused silica with a range of particle sizes such as 4/10 mesh, 10/20 mesh, 20/50 mesh, 50/100 mesh, 200 mesh and 325 mesh.

(28) The refractory composition also includes microsilica (also called silica fume, which is an amorphous spherical form of silicon dioxide SiO.sub.2, comprising an ultrafine powder consisting of particles smaller than 1 m and having an average particle diameter of 150 nm). The microsilica is used in undensified form, having a bulk density of about 250-350 kg/m.sup.3. For example, in the above embodiment the microsilica comprises grade 971u from Elkem AS. The refractory composition also includes colloidal silica (a suspension of amorphous silica particles in water, with particle sizes typically in the range 3 to 100 nm). In the example above we use Nalco 1140 from Nalco Chemical Company, which has a particle size of 15 nm and a silica content of 40%.

(29) The refractory composition also includes a dispersing agent. In this embodiment we use a polyacrylate dispersing agent (Dispex from BASF).

(30) The above materials are mixed and the refractory mixture is then introduced into a mould. The mould is vibrated as the mixture is introduced to help the mixture flow easily throughout the mould.

(31) The product is preferably moulded using a freeze casting technique. After freezing, the moulded part is removed from the mould and placed in a drying oven set at a temperature between 40 C and 140 C, according to the design and mass of the part. After drying, the product is fired in a kiln, typically at a temperature in the range 700 C to 1550 C.

(32) Finally, if required, the cast product can be machined to the required shape/dimensions. Typically machining is carried out on a CNC lathe so each part produced has repeatable accuracy. The cast product machines very well and to a very high accuracy, particularly if the fused silica has a small particle size (e.g. 200 mesh and smaller). However, for products that do not need to be manufactured to very tight tolerances, it may be possible to achieve the required accuracy through the casting process: it may then be unnecessary to machine the components. In that case, larger particles of fused silica may be included, for example with a particle size up to 6 mm.

(33) The cylindrical thimble 40 and/or other refractory components can be made from the new ceramic refractory composition described above, which in this embodiment includes fused silica, preferably having a particle size in the range 150 m to 3.5 m, non-refractory ceramic fibre (non RC fibre), preferably in the form of milled fibre, microsilica (e.g. Elkem 971U), and colloidal silica as a bonding material (e.g. Nalco 1140 grade). In this embodiment the material also includes a polyacrylate dispersing agent (for example Dispex from BASF).

(34) More generally the new ceramic refractory composition includes components in the following ranges (by weight): Fused silica: 30-90%, preferably 40-60% Microsilica: 2-15%, preferably 8-10% Ceramic fibre: 5-45%, preferably 10-40% Colloidal silica: 3-25%, preferably 12-20%

(35) The ceramic fibre is preferably soluble (non-durable) in physiological fluids (this type of fibre is sometimes called a non-refractory ceramic fibre): for example it may be an alkaline earth silicate fibre. However it may alternatively be a non-soluble refractory ceramic fibre, for example an alumino silicate wool fibre.

(36) In tests we have found that cylindrical thimbles made from this new refractory material are capable of producing more than 800 casts without failing. By comparison, thimbles made of a conventional cement-bonded fused silica refractory as described above will generally only operate for about 500 casts.

(37) The distribution device 8 can be manufactured from the same family of raw materials as stated, i.e. fused silica, microsilica, non-RCF fibre, dispersing agent and colloidal silica. However the actual grades of fused silica can change depending upon the product design and needs. All products made from this family of materials are typically freeze cast.

(38) Optionally, the refractory composition can if required include non-wetting additive (or anti-corrosion additive) such as Barium Sulphate and/or Calcium Fluoride. For example, we have used Barium Sulphate and Calcium Fluoride, both introduced at 3.5 wt % of the final body.

(39) The new refractory material described herein may also be useful for other components that come into contact with liquid aluminium in an aluminium production or refining process, including for example a down-spout 50 and a transfer trough 52, examples of such products being shown in FIGS. 10-12 and 13-15 respectively. The down-spout 50 consists of a substantially cylindrical tube with an axial bore 54 and an external wall 56 with a cylindrical lower portion 58 and a flared upper portion 60. The down-spout 50 is of conventional shape and is used for transferring liquid metal from a trough into a mould.

(40) The transfer trough 52 shown in FIGS. 13-15 comprises a base 62 and two parallel side walls 64, which together define an open-topped trough 66 for directing a flow of liquid metal. The trough is of conventional shape and may be made by a casting process using the new refractory material described herein.