WO2006058347A2

WO2006058347A2 - Heat resistant bead

Info

Publication number: WO2006058347A2
Application number: PCT/ZA2005/000171
Authority: WO
Inventors: Alistair Allardyce Elrick
Original assignee: Alistair Allardyce Elrick
Priority date: 2004-11-25
Filing date: 2005-11-21
Publication date: 2006-06-01
Also published as: WO2006058347A3

Abstract

This invention relates to a heat resisting bead for use in the manufacture of heat insulating products and comprises a generally spherical carrier body which contains a suitable heat insulating gas, and a shell of a solid heat resisting material which totally encapsulates and is in contact with the outer surface of the carrier body. The carrier body is preferably an expanded polystyrene bead. The method of making the heat resisting beads includes the steps of coating a predetermined quantity of the expanded polystyrene beads with a suitable settable binder material and mixing the coated beads with a suitable dry particulate refractory material until the particles of the material are embedded in the binder material and the beads are dry and separate from each other.

Description

HEAT RESISTANT BEAD

FIELD OF THE INVENTION

[0001] This invention relates to a heat resistant bead (HRB) which is principally used in the manufacture of heat insulating products and to a method of manufacturing the HRB's.

BACKGROUND TO THE INVENTION

[0002] HRB microspheres, in which air is trapped, are used in the manufacture of thermal insulating products and particularly foundry riser sleeves which are employed in metal casting.

[0003] The air trapped in the microspheres is the principal thermal insulating component of the spheres.

[0004] The type of microspheres which are largely used in the manufacture of the riser sleeves are those which occur naturally in fly-ash which is generally "mined" from coal burning power station ash dumps. The microspheres then need to be expensively separated from waste material and size classified before shipment to their place of use.

[0005] The expense of these microspheres is further compounded by their scarcity and transportation costs as their currently preferred source is from ash dumps in Australia. The size of these microspheres is limited to between 500 and 300 microns, or less. SUMMARY OF THE INVENTION

[0006] A heat resisting bead according to the invention comprises:

a generally spherical carrier body which contains a suitable thermal insulation gas, and

a shell of a solid heat resistant material which totally encapsulates and is in contact with the outer surface of the carrier body.

[0007] In a currently preferred form of the invention the carrier body is an expanded polystyrene bead which will at least largely vaporise in the shell at a predetermined temperature.

[0008] The heat resistant bead shell may consist of a particulate refractory material which is set in a binder material and is resistant to heat degradation at temperatures significantly above the predetermined vaporisation temperature of the polystyrene bead.

[0009] The refractory material may be talcum powder, silica flour, zircon flour, mullite flour or the like in dependence on the use to which a product which is to be made from the HRB's of the invention is to be put.

[0010] The binder material of the shell may be sodium silicate, colloidal silica or the like.

[0011] A method of making a quantity of the heat resisting beads according to the invention may include the steps of: coating a predetermined quantity of expanded polystyrene beads with a suitable settable binder material, and

mixing the coated beads with a suitable dry particulate refractory material until the particles of the material are embedded in the binder material and the beads are dry and separate from each other.

[0012] The method may include the step of wetting the beads with a suitable surfactant prior to coating the beads with the settable binder material.

[0013] The method conveniently includes the step of causing the binder material of the beads to set. The binder material of the beads may be caused to set by heat drying the beads at a temperature well below the vaporisation temperature of the expanded polystyrene beads.

[0014] The invention may extend to a product which is made from a quantity of the heat resisting beads. The product could be a foundry riser sleeve which is employed in the casting of metal products.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] An embodiment of the HRB and the method of making it according to the invention are now described by way of non-limiting examples only with reference to the drawings in which :

Figure 1 is a half-sectioned HRB of the invention, Figure 2 is an enlarged small segment of the shell of the Figure 1 HRB as it would be in use,

Figure 3 is a schematic block diagram of the apparatus for use in the method of manufacturing the HRB's of Figures 1 and 2 according to the invention, and

Figure 4 is a half-sectioned isometric view from above of a riser sleeve which is made from the HRB's of the invention for use in the foundry casting of metal products.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Figure 1 shows the HRB 10 of the invention to be composed of a single low density expanded polystyrene bead 12 which is totally encapsulated in a shell 14 of relatively much harder heat resistant material.

[0017] The shell 14, as is explained in more detail below with reference to Figure 3, consists predominantly of a particulate refractory material such as talcum powder, silica flour, zircon flour, mullite flour or the like which is embedded in a heat settable, initially liquid, binder material such as sodium silicate, colloidal silica or the like.

[0018] In this embodiment of the invention, the beads 10 are used in the manufacture of hot or cold insulating products and more particularly in heat insulating riser sleeves 16, as shown in Figure 4, which are employed in foundry metal casting.

[0019] In the casting process of the riser sleeve 16 the heat from molten metal which fills the sleeve 16 in the casting process causes the expanded polystyrene beads in the sleeve to vaporise at a temperature between 100⁰C and 200⁰C to leave the shell 14 hollow with the shell then, for practical purposes, only being filled with heat transfer resistant inert air which was initially contained in air pockets in the polystyrene beads 12.

[0020] Figure 2 shows a fragment of the shell after the expanded polystyrene bead has been vaporised and because of its enlargement, particles 18 of the refractory material which are embedded in the set binder material matrix.

[0021] As the temperature of the sleeve is caused to rise above the vaporisation temperature of the sacrificial beads 10 their shell binder material continues to harden without perceptible heat degradation even at the temperature of molten stainless steel which is about 1680⁰C.

[0022] As is mentioned in the preamble to this specification the maximum currently available size of fly-ash microspheres is about 500 microns in general diameter which means that the air space in the spheres, for the purpose of providing a heat insulating void, is incredibly small. With the HRB's of the invention, however, the size of the sphere is limited only by the size of the expanded polystyrene bead 12 which carries the large diameter HRB shell 14 and it is therefore possible to make the HRB's in a size range of between 0.5 and 5.0 mm and even larger, if required, as is explained below. This will result in an insulation product, such as the riser sleeve 16 of Figure 4, being manufactured using substantially less HRB's than is the case with the currently available microspheres and will provide, because of their large still air carrying capacity, a significantly improved heat insulation capability. [0023] The method of manufacturing the Figure 1 HRB is now described with reference to Figure 3 in which the apparatus of the method is shown to include: an expanded polystyrene bead, bin or hopper 20, a wetting material bin 22, a binder material bin 24, a particualted refractory material supply bin 26, a paddle mixer 28, a vibrating screen 30, an excess refractory material collector bin 32, and a drying oven 34.

[0024] In exercising the method of the invention, a predetermined quantity of expanded polystyrene beads of the required size is fed from the hopper 20 into the mixer -28. A predetermined quantity of the wetting agent, which in this example of the method of the invention is a suitable surfactant such as a liquid soap, is fed from the bin 22 into the mixer 28 and is mixed by the mixer until the beads are lightly coated with the wetting agent to minimise adherence of the beads to one another. Once the beads have been suitably wetted, a predetermined quantity of the binder material, which in this case is either sodium silicate or colloidal silica as required is added, from the bin 24 to the wetted beads in the mixer to completely coat each of the beads in the mixer. Should it be required to reduce the viscosity of the binder material, to prevent the coated beads from adhering to each other, this may be achieved by the addition of the correct amount of water to the surfactant in the bin 22 to achieve the required result. [0025] Once the beads have each been fully coated by the binder material the particulate or powdered refractory material, which may be any suitable material such as talcum powder, silica flour, zircon flour, mullite flour or the like in dependence on the use to which a product which is to be made from the HRB's is to be put, is added to the mixer 28 from the bin 24. It is important that a quantity of the powdered refractory material in excess of the material which is calculated to be needed in the formation of the HRB shells 14 is added to the mixer from the bin 26 to ensure the build-up of a complete shell 14 on each of the beads 12 while being mixed in the mixer. When the outer surfaces of the shells 14 on the beads 12 have reached a dry powdered state, in which the HRB's are now totally separated from each other, the entire mix from the mixer is fed to the vibrating screen 30 which mildly sieves the excess powder which was added to the mixer mix, and loose powder on the shells 14, into the excess refractory material bin 32 from where it is recycled back into the supply refractory material bin 26, as shown in the Figure 4 diagram.

[0026] If for any reason it is required that the thickness of the shells 14 on the beads needs to be increased the beads are fed from the oven back into the bead hopper 20 and the entire bead coating process described above is progressively repeated to increase the bead wall thickness to the required dimension.

[0027] Subsequent to the sieving process of the HRB's they are fed to the oven 34 where they are dried of moisture and their shell binder material is at least partially set at a temperature well below the vaporisation temperature of the polystyrene beads in them. Typically the heat drying temperature will lie in the range of 3O⁰C to 50⁰C. In the event that the binder material of the HRB's is sodium silicate their drying and setting time process may be accelerated by heat drying them in the oven 34 in a CO² enriched atmosphere.

[0028] The HRB's could, if required, be coloured, to make different size beads or products made from them distinguishable from each other, by adding a suitable colouring material to the binder material when it is fed from the bin 24 to the mixer.

[0029] The riser sleeves, such as the sleeve 16 of Figure 4, are made in the conventional manner by means of a core blower with the sleeve binder material being sodium silicate.

[0030] The first and most important advantage of the HRB's of this invention over the known fly-ash microspheres is the capability of being able to manufacture them relatively economically in a desired and predictable range of sizes. A second advantage is that the shells 14 are supported against wasteful crushing during normal handling, transportation and in the manufacture of the riser sleeves 16 by the polystyrene beads 12 which are only eventually vaporised in them during use of the sleeves in a metal casting process.

[0031] The invention is not limited to the precise details as herein described. For example, in place of finding use in the manufacture of insulating products the HRB's of the invention also find use in the hot topping of open sleeves in foundry casting. Additionally the HRB's, for whatever reason, could be made larger than the 5.0 mm mentioned above by moulding a number of polystyrene beads into a shell carrier ball of the required size. Still further the apparatus of Figure 3 could be of any equivalent type, for example, the mixer 28 and/or the vibrating screen 30 could be of any suitable horizontally rotating drum type.

Claims

1. A heat resisting bead for use in the manufacture of heat insulating products characterised in that :

the bead comprises a generally spherical carrier body which contains a suitable heat insulating gas, and

a shell of a solid heat resisting material which totally encapsulates and is in contact with the outer surface of the carrier body.

2. A heat resisting bead as claimed in claim 1 wherein the carrier body is an expanded polystyrene bead which will at least largely vaporise in the shell at a predetermined temperature.

3. A heat resisting bead as claimed in claim 2 wherein the bead shell consists of a particulate refractory material which is set in a binder material and is resistant to heat degradation at temperatures significantly above the predetermined vaporisation temperature of the polystyrene bead.

4. A heat resisting bead as claimed in claim 3 wherein the refractory material is selected from the following or a combination thereof; talcum powder, silica flour, zircon flour, and mullite flour.

5. A heat resisting bead as claimed in claim 3 wherein the binder material of the bead shell is selected from sodium silicate or colloidal silica.

6. A method of making a quantity of the heat resisting beads of any one of claims 2 to 5 including the steps of :

coating a predetermined quantity of expanded polystyrene beads with a suitable settable binder material, and

7. A method as claimed in either one of claims 6 or 7 including the step of wetting the beads with a suitable surfactant prior to coating the beads with the settable binder material.

8. A method as claimed in claim 6 including the step of causing the binder material of the beads to set.

9. A method as claimed in claim 8 wherein the binder material of the beads is caused to set by heat drying the beads at a temperature well below the vaporisation temperature of the expanded polystyrene beads.

10. A heat insulating product which is made from a quantity of the heat resisting beads of any one of claims 1 to 5.

11. A heat insulating product as claimed in claim 10 wherein the product is a foundry riser sleeve which is employed in the casting of metal products.