WO2004080915A1

WO2004080915A1 - Refractory cement castables

Info

Publication number: WO2004080915A1
Application number: PCT/GB2004/001010
Authority: WO
Inventors: Juma Kassim
Original assignee: Carbon Application Technology Limited
Priority date: 2003-03-11
Filing date: 2004-03-09
Publication date: 2004-09-23

Abstract

Refractory castables are used as linings, nozzles, containers and other structures in order to contain and guide molten materials such as steel during processing. Previous refractory cement castables have had relatively short operational lifes due to limitations with respect to shock resistance, corrosion and other factors. The present refractory castable incorporates significant proportions of carbon which is inert and therefore extends the operational life of the castable. The carbon is introduced through addition of a particular pitch up to a portion of 50% by weight such that when dried and thermally activated there is significant carbon bonding within the material in order to provide the desired operational performance.

Description

Refractory Cement Castables

The present invention relates to refractory cement castables and more particularly to the materials from which refractory castables are formed.

During various metal making operations including forming steel it is necessary during the metal melting process to provide linings, nozzles, containers and other structures which act to contain and guide the molten material during processing. Typically, the molten metals are steel, iron or non- ferrous metals.

Previously, refractory cement castables have been formed from a mixture of a white refractory grade material or grit and a refractory cement. The cement is calcium aluminate. The refractory grit includes alumina, silica and magnesia etc in order to provide refractory protection whilst the cement provides a hydraulic bonding mechanism.

Such previous refractory cement castables have suffered from the following problems:

a) Low thermal shock resistance necessitating pre-heating of the castable prior to use. b) Low slag corrosion resistance. c) Low resistance to molten metal penetration. d) Low strength at high temperatures necessitating bigger sized and thicker cross-section castables with resultant greater material usage and heating requirements. e) Refractory cement has limited chemical resistance to both slag and molten metal e.g. steel attack. In view of the above, previous refractory cement castables have had a relatively short operational life. Such a short operational life adds significantly to costs with regard to replacing the refractory cement castable in terms of the price of that castable as well as the down time for a furnace or other structure incorporating the castable during such replacement.

In order to improve refractory cement castables it has been proposed to introduce carbon or graphite into the castable. Carbon is inert and so will increase castable performance but direct incorporation will increase the water demand upon the cement in order to create the hydraulic bonding mechanism. There will also be increases in the castable setting time along with a reduction in , castable strength and integrity if directly introduced. Thus, an indirect approach is required.

Refractory cement castables have relatively low bond strength between the refractory material and the refractory cement binder which combine to form the castable. Greater bond strength will improve thermal shock resistance as well as resistance to slag penetration and corrosion. Previously carbon precursors have utilised solvent based distribution and combination systems. Inherently refractory cement castables are based upon use of a refractory cement which in turn is a water-based bonding and distribution system. Thus, previous attempts at providing a carbon containing refractory castable through a solvent based precursor have resulted in reduced strength, greater setting time for the castable and reduced structural integrity with regard to the castable. There has also been an increased retension of water within the refractory castable which as a result of the relatively high operating temperatures present at least with regard to some uses of these refractory castables can cause additional fissure and cracking problems due to thermal expansion of the retained water. Nevertheless carbon bonding should provide increased resistance to slag penetration and corrosion resistance. Previously in order to incorporate carbon within the refractory it has been known to form a refractory cement castable and then injection of a carbon carrying resin into the surfaces of the refractory cement casting. The then impregnated refractory casting is fired to pyrolise the resin to leave carbon bonding. Injection of resin is repeated until a desirable level of carbon is provided within the refractory cement castable. It will be understood that there is a, rule of diminishing returns in carbon content increase with each injection cycle and generally it is not possible to achieve greater than a three to five per cent by weight carbon content within the refractory castable. Ideally far greater carbon content would be desirable in order to increase carbon bonding and therefore strength along with corrosion resistance within the refractory castable. Also, the process of repeated injection and pyrolising is expensive and may lead to uneven carbon distribution

In accordance with the present invention there is provided a refractory cement castable material comprising a refractory cement and a particulate pitch appropriately mixed and wetable with water to form a¹ slurry, the material including up to 50% by weight pitch whereby when dried and thermally activated there is significant carbon bonding created within the material.

Preferably, the refractory cement castable incorporates refractory particulates e.g. grit. Typically, these refractory particulates will comprise, by weight, thirty per cent course particles (3 to 6 mm), forty per cent medium particles (0.2 to 3mm) and thirty per cent fine particles (less than 0.2mm).

Preferably, ten to twenty per cent by weight of the material is particulate pitch. Normally, the particulate pitch is a high melting carbon bond precursor. Generally, the particulate pitch is a powder. Typically, the' particulate pitch is a high melting pitch (greater than 180°C) for example the particulate pitch is Carbores supplied by Rudger VFT AG of Castrop - Rauxel, Germany. Preferably the refractory cement is calcium aluminate cement.

Typically, when water is added, and generally proportionally from the base necessary value of water for the refractory for cement, there is a 0.5 to 1.5 per cent increase in the necessary volume of water to be added for each additional five per cent of particulate pitch added to the refractory castable material.

Possibly, reinforcement fibres are added to the refractory castable material. Possibly, these reinforcement fibres are inorganic fibres such as calcium silicate, alumina or glass fibres. Alternatively, these reinforcement fibres are organic fibres such as carbon fibres, viscose fibres, polypropylene or polyvinyl chloride (PVC). Normally, where used, the reinforcement fibre will comprise up to 5% by weight of the material.

Further in accordance with the present invention there is provided a method of forming a refractory cement castable, the method comprising mixing a refractory cement and a particulate pitch in a proportion by weight of up to 50% particulate pitch within an appropriate volume of water to form a slurry, placing that slurry in a mould to provide a castable shape, removing the castable shape from the mould, allowing that castable shape to dry and then when appropriately dried firing that castable shape at an appropriate temperature to develop carbon bonding within the refractory cement castable.

Generally, refractory particulates (grit) are also added to the refractory castable mix.

Generally, the slurry is held within the mould to form the castable shape for at least five minutes. Generally, the castable shape is dried at room temperature for approximately two hours or a pre-determined strength is achieved by the dried castable shape.

Normally, the dried castable shape is fired at a temperature in excess of 600°C in the absence of air (vacuum) or in an inert atmosphere or a non- oxidising atmosphere.

Alternatively, the dried castable shape is glazed with an anti-oxidising layer such that the castable shape is then fired in situ when used to form carbon bonding. Typical glazing layers are formed from borate glass frit, silicon carbide and/or silicon metal.

Ideally, the dried castable shape will be initially fired and then glazed.

Further in accordance with the present invention there is provided a refractory castable formed from a material or in accordance with a method as described above.

Embodiments of the present invention will now be described by way of example only.

Refractory cement castables in accordance with the present invention will generally be shaped in a mould. These moulds will provide the necessary castable shape for performance in use. Thus, for example, when used with regard to steel manufacture a castable in accordance with the present invention

< may be formed in the shape of a furnace lining, gutter path or groove, ladle pourer or tundish. Refractory cement castables in accordance with the present invention may also be used in the chemical industry as tank linings for protection against acidic or alkaline attack as well as other situations at elevated temperatures or in highly corrosive environments. The present invention provides a refractory cement castable material which combines the benefits of a refractory cement in terms of moulding and casting efficiency with an appropriate particulate pitch to create carbon bonding when thermally activated. The refractory cement castable material also incorporates refractory particles or particulates which may be ceramics or inorganic matter. The present invention allows the mixing of the refractory grit cement and the particulate pitch as a carbon pre-cursor into a water wetted slurry for casting in a mould.

Generally, the refractory cement and the particulate pitch are mixed in respective powdered forms. The grain sizes of the refractory cement and the particulate pitch are fine and approximately the same for even mixing. This mixing is achieved through appropriate agitation including use of stirrers and/or vibration. As indicated previously normally refractory grit particles are incorporated within the refractory cement castable. The proportion of refractory grit particles will depend upon the operational use of the refractory cement castable. Thus with regard to high temperature environments such as with respect to castables used in steel production relatively higher levels and proportions of refractory particles will be incorporated within the refractory castable material. However, with regard to linings provided for vessels in order to achieve acid or alkaline attack resistance the provision of refractory particles may be less important and therefore their proportion reduced for operational necessities but may still be included as a bulking or reinforcement for the castable material in order to achieve an appropriate costing and structural strength for the castable lining.

The refractory cement used in accordance with the present invention will typically be a calcium aluminate cement. Such cements are readily available and as indicated above used with respect to refractories for steel making operations but in themselves are susceptible to thermal shock deterioration, slag penetration and molten metal corrosion. The particular grade of refractory cement used will depend upon specific operational requirements for the castable formed from the refractory cement castable material in accordance with the present invention but as indicated above will generally be of a fine power nature to ensure appropriate mixing and bond forming.

Particulate pitch is incorporated within the refractory castable material in accordance with the present invention to act as a carbon pre-cursor in order to generate the carbon bonding desirable for increased strength. The particulate pitch will be a high melting (greater than 180°C) carbon bond precursor and as indicated above will be in a powder form of similar grain size to that of the refractory cement. The particulate pitch will generally be supplied dry. In such circumstances, powder mixing of the constituents of the refractory castable material must be conducted in appropriate conditions ^; to achieve uniform distribution and mixing without agglomeration, prior to wetting with water to form a stiff slurry which is normally cast subject to vibration to help settling. It will be understood that a stiff slurry is defined by the nature of refractory cement used, the castable shape and necessity for sufficient flow to form the shape but too sloppy a slurry will not form the shape in the mould.

Casting in accordance with the present invention will be performed by combining and mixing the constituents of the refractory castable material in dry form to achieve as even a distribution as possible. Water will then be added to the dry refractory castable material in order to create the slurry. Generally manufacturers of refractory cement in their product details will specify a desired proportion of water to refractory cement in order to achieve appropriate performance. In accordance with the present invention, generally that percentage of water to be added to the refractory cement will be increased by up to 1.5 but normally one weight per cent per whole volume of refractory castable material for each five per cent portion of refractory castable material in particulate pitch form. Thus, as in accordance with the present invention up to fifty per cent by weight of the refractory castable material may be particulate pitch it will be appreciated that significantly higher volumes of water will be added to the refractory material than would be typical with respect to the expected refractory cement mixing conditions. Generally between six and ten per cent water by weight will be added to the refractory cement castable material in order to form the slurry required for casting in accordance with the present invention.

The slurry form of refractory castable material in accordance with the present invention will be presented to an appropriate mould for the desired shape. An even distribution of the slurry within the mould will be achieved in accordance with known techniques of mould manipulation, vibration and slip venting possibly in a reduced pressure environment. Normally the slurry will be left within the mould for at least five minutes in order to settle and form a castable shape.

Once the castable shape has achieved sufficient consolidation to present a self reliant form, that is to say it will retain its shape without support from the mould, then the castable shape is removed from that mould. The castable shape is allowed to dry naturally or in a controlled manner to avoid too rapid evacuation or evaporation of water from the castable shape. Typically drying of the castable shape will be performed in an open situation at room temperature. However, drying may be performed in different conditions dependant upon the castable shape and specific requirements. For example drying may be conducted in a reduced pressure environment or with forced air and elevated or reduced temperatures dependant upon requirements and to avoid too rapid a release of the water content causing fissures and porosity within the castable shape. Typically, drying will take in the order of two hours.

Once appropriately dry the castable shape will be fired a temperature in order to create carbon bonding within the final refractory cement castable and so provide enhanced strength and slag/corrosion resistance. Typically, the dried refractory shape will have in the order of one per cent water content by weight prior to firing but negligible water content after firing particularly if the refractory castable is to be used with regard to high temperature situations such as those used in steel forming and processing. Normally, the dried castable will be heated progressively with between a % and 20°C increase in temperature per minute until the firing temperature is achieved. Normally, the firing of the dried castable shape will be at a firing temperature in excess of 600°C and will be performed for at least 30 minutes at that firing temperature for carbon bond creation but ramping progressively of the castable to that firing temperature may take several hours. In any event, super heating of retained water does not cause fragmentation and fracturing of the castable under such conditions.

During the firing of the refractory castable the particulate pitch is essentially pyrolised in a non-oxidising atmosphere to facilitate and create carbon bonding within the material. Such carbon bonding greatly increases the structural integrity of the refractory castable improving thermal shock resistance to rapid transient exposure to widely different temperatures and also reduces porosity within the material forming the refractory castable limiting slag and metal penetration into the castable with improved resistance to such degradation in use. The carbon bonding provides a secondary bonding system to supplement the hydraulic bonding system of the refractory cement. Thus, the castable is more thermally and chemically stable. The non-oxidising atmosphere is provided by an absence of air or a vacuum or an inert atmosphere.

To further improve structural strength, reinforcement fibres may be incorporated within the refractory castable material. These reinforced fibres may be inorganic or organic. Inorganic fibres include calcium silicate, alumina and glass fibres as well as mica and were possible asbestos. Inorganic fibres include carbon fibre, viscose fibre, polypropylene and polyvinyl chloride (PVC). Typically, the reinforcement fibres will have a strand nature such that reinforcement is provided through entanglement and incorporation length. It will also be understood that inorganic fibres will also tend to he pyrolised in the firing process creating further carbon bonding within the refractory castable for improved strength and slag corrosion resistance.

Due to the reduced porosity of the refractory castable shape both in final form and as taken from the mould for drying it will be appreciated that such a drying process in accordance with the present invention will be slightly longer with the present refractory castable material in comparison with previous refractory cement castables. In short the lower porosity and number of fissures within the castable shape reduces the possible pathways for water released during the drying process. Nevertheless, refractory castables will generally have low material cross sections and therefore water migration will still occur with sufficient rapidity to allow convenient drying.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. A refractory cement castable material comprising a refractory cement and a particulate pitch appropriately mixed and wetable with water to form a slurry, the material including up to 50% by weight pitch whereby when dried and thermally activated there is significant carbon bonding created within the material.

2. A material as claimed in claim 1 wherein the refractory cement castable incorporates refractory particulates or grit.

3. A material as claimed in claim 2 wherein these refractory particulates will comprise, by weight, thirty per cent course particles (3 to 6 mm), forty per cent medium particles (0.2 to 3mm) and thirty per cent fine i particles (less than 0.2mm).

4. A material as claimed in any of claims 1 , 2 or 3 wherein ten to twenty per cent by weight of the material is particulate pitch.

5. A material as claimed in any preceding claim wherein the particulate pitch is a high melting carbon bond precursor with a melting point in excess of 180°C.

6. A material as claimed in any preceding claim wherein the particulate pitch is a powder.

7. A material as claimed in any preceding claim wherein the refractory cement is calcium aluminate cement.

8. A material as claimed in any preceding claim wherein when water is added, and generally proportionally from the base necessary value of water for the refractory for cement, there is a half to a one and a half per cent increase in the necessary volume of water to be added for each additional five per cent of particulate pitch included in the refractory castable material.

9. A material as claimed in any preceding claim wherein reinforcement fibres are added to the refractory cement castable material.

10. A material as claimed in claim 9 wherein these reinforcement fibres are inorganic fibres such as calcium silicate, alumina or glass fibres.

11. A material as claimed in claim 9 wherein these reinforcement fibres are organic fibres such as carbon fibres, viscose fibres, polypropylene or polyvinyl chloride (PVC).

12. A material as claimed in any of claims 9 to 11 wherein the material includes up to 5% by weight of reinforcement fibres.

13. A refractory castable material substantially as hereinbefore described.

14. A method of forming a refractory cement castable, the method comprising mixing a refractory cement and a particulate pitch in a proportion by weight of up to 50% particulate pitch within an appropriate volume of water to form a slurry, placing that slurry in a mould to provide a castable shape, removing the castable shape from the mould, allowing that castable shape to dry and then when appropriately dried firing that castable shape at an appropriate temperature to develop carbon bonding within the refractory cement castable.

15. A method as claimed in claim 14 wherein in the order of five to ten per cent water is added to the refractory cement and particulate pitch mix.

16. A method as claimed in claim 14 or claim 15 wherein refractory grit particulates are also added to the refractory cement castable mix.

17. A method as claimed in any of claims 14, 15 or 16 wherein the slurry is held within the mould to form the castable shape for at least five minutes.

18. A method as claimed in any of claims 14 to 17 wherein the castable shape is dried at room temperature until a pre-determined strength is achieved by the castable shape.

19. A method as claimed in any of claims 14 to 18 wherein the dried castable shape is fired at a temperature in excess of 600°C for at least 30 minutes in a non-oxidising atmosphere to create carbon bonding.

20. A method of forming a refractory cement castable substantially as hereinbefore described.

21. A refractory cement castable formed from a material as claimed in any of claims 1 to 13.

22. A refractory cement castable formed in accordance with a method as claimed in any of claims 14 to 21.

23. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.