GB1564927A - Bonds for refractory materials - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO BONDS FOR REFRACTORY MATERIALS (71) We, PICKFORD HOLLAND & COMPANY LIMITED, a British Company of 381 Fulwood Road, Sheffield, S10 3GB, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to bonds for refractory materials.

There are many applications where refractory materials are in direct contact with molten metal, ferrous and nonferrous, and which ideally should display resistance to thermal shock, resistance to slag attack, resistance to attack by the molten metal, and adequate high temperature strength. In this context it is known to incorporate graphite into the refractory material and which can modify the refractory material in a number of ways. For example graphite imparts the property of non wettability to molten oxides, e.g. slags, and hence refractory oxide-graphite composites have better resistance to slags than refractory oxides alone which are wetted by slags. Also graphite frequently significantly increases the thermal shock resistance of refractories into which it is incorporated.This is partly because the graphite provides incipient flaws which prevent crack propagation, thereby reducing thermal shock damage, and partly because it increases thermal conductivity thereby reducing temperature gradients and thermal stresses in a thermal shock situation. Enhanced thermal conductivity can be a virtue in its own right in certain applications, e.g. crucibles for metal smelting and bricks for blast furnace boshes. In cases such as these advantage is often taken of the anisotropic nature of graphite. Thus the thermal conductivity in the direction parallel to the flakes of graphite is very high whilst in the direction perpendicular to the flakes it is very low.

Hence during the manufacture of graphite containing refractories care is often taken to ensure that the flakes of graphite are aligned in a direction appropriate to the application.

Despite its advantages, graphite per se is disadvantageous when used as a refractory material. In particular, it is dissolved relatively easily by, e.g. molten iron. Most refractory oxides (e.g. alumina, magnesia and zirconia) have good resistance to attack by molten metal. Thus the combination of refractory oxide and graphite results in a refractory which is particularly resistant to molten metal and slags. However to gain the maximum advantage from such a composite it is essential that the graphite is not oxidised away rapidly. Traditionally it has been common to use a clay bond and form the required shape by a plastic or semiplastic method. This has certain advantages in that the alignment of the flakes of graphite can be controlled to some extent.

and, by entirely surrounding each flake of graphite by clay, the oxidation rate of the graphite is slowed. However the low refractoriness and poor chemical resistance of the clay limits the usefulness of this type of refractory particularly at high temperatures. Also the porosity tends to be high and the abrasion resistance low.

An alternative approach has been to use carbon as a bond. This has better chemical resistance and refractoriness than a clay bond but has the disadvantage that it can be oxidised which results in loss of strength and increased permeability to oxidising gases and attacking fluxes. It is common, therefore to employ glazes with such articles either by externally coating or by incorporating glaze forming additives into the refractory body itself. The carbon bond is derived from a carbonisable binder, e.g.

pitch or resin, which is mixed in liquid form with the graphite and the refractory grog, and, after forming the required shape, fired under reducing conditions to carbonise the binder. The carbon bond generally has much lower oxidation resistance than flake graphite.

One application where resistance to thermal shock and resistance to attack by synthetic mould fluxes is of particular value is in submerged entry nozzles. With submerged entry nozzles these properties are known to be satisfied by vitreous silica.

However, vitreous silica wears at an unacceptable rate when high manganese steels are involved. With alumina based materials, which cannot be used in an unmodified condition because of the relatively low thermal shock resistance of high alumina materials, the incorporation of graphite into the structure, particularly flaked graphite, not only considerably improves thermal shock resistance, but also improves the resistance of high alumina materials to both mould flux and molten metal attack.

However, one of the drawbacks of socalled graphite high alumina is that at the flux line, oxidation of the graphite can take place allowing the ingress of fluxes, leading to severe erosion at that point. It is also the case that adding graphite to high alumina introduces the problem of adequately bonding the material. The presence of graphite significantly restricts the extent to which a ceramic bond network can be formed by conventional sintering techniques, leading to the production of an unacceptably weak structure.

According to the present invention, a method for the production of a refractory artefact comprises admixing a first suitably graded component of refractory material, a second silicon powder component and a third powdered graphite component, and firing said mixture in an atmosphere of nitrogen whereby the first refractory component is bonded to the third graphite component by the product of the reaction between silicon and nitrogen.

According to a further feature of the invention, a fourth silica component can be provided in the mixture, and when the bond between the first refractory component and the third graphite component comprises the products of the reaction of silicon with nitrogen and silica.

The refractory component and the graphite component can be provided by socalled graphite high alumina, to which the silicon powder component would be added.

It is further preferred that the refractory component is in the range 30van to 90% by weight, graphite is in the range 5 /" to 60% by weight and silicon in the range 5% to 20% by weight.

According to a still further feature of the invention, a method for the production of a refractory artefact comprises adding silicon powder to a powdered graphite containing refractory mixture and firing at an elevated temperature in a nitrogen atmosphere which is continuously flowing. Firing should take place in the range at least 11000C to 1500"C, although firing tQ above 1500 C does not necessarily detract from the properties of the bond. Preferably firing is in the range 12500C to 13500C. Thus the refractory component suitably graded along with the graphite and silicon powder addition, can be worked in relatively conventional dry or semi-dry pressing techniques to produce the required shape.

The result of the method is that the silicon reacts with the nitrogen to form silicon nitride, and, if silica is present, silicon oxynitride. Thus, with silicon present at the level of 10 ", by weight the strength imparted by the silicon nitride (and oxynitride when present) is sufficiently high for the majority of applications even when the graphite content is high by conventional standards such as 40van to 50% of the batch. It is equally the case that silicon nitride and oxynitride have a higher refractoriness than conventional clay bonds, and thus silicon nitride (and oxynitride) bonded graphite refractories exhibit higher hot strengths and hot abrasion resistance than hitherto known graphite refractories.Also refractories bonded in accordance with the invention exhibit good oxidation resistance at elevated temperatures, e.g. up to at least 1400"C.

Typically, the graphite is natural flake graphite although other forms such as electrode graphite can be used. The refractories that can be utilised are so-called refractory grog, alumina or aluminosilicates; magnesia; zirconia; zirconium silicate; chrome-magnesia; silicon carbide; and carbon, e.g. in the form of coke, or combinations thereof.

The invention will now be described with reference to the following, non-limitive examples: Example 1 An example of the successful use of silicon nitride/oxynitride bonded submerged entry nozzle is as follows: 50 ,/ fused mullite -22 BSS 30% flake graphite (flake size approximately 0.5 mm to 1.0 mmx0.05 mm to 0.1 mm) 10% silicon mean particle size 5 micron 5% Micronised* alumina 5% ball clay (* Registered Trade Mark) To produce the nozzle, the above mixture was mixed in a pan mill, and a temporary binder added, such as sodium silicate, polyethylene glycol, or sodium carboxy methyl cellulose.The refractory aggregate was isostatically pressed to shape, and subsequently fired in a controlled atmosphere furnace into which nitrogen was passed at a rate sufficient to purge out oxygen to prevent oxidation of graphite above 4000C and to nitride silicon at the top temperature. The furnace temperature was raised to 1300"C and held at that temperature for 16 hours.The resultant nozzle had the following properties: Bulk density 2.35 gcm-3 Apparent porosity 17.5% Apparent solid density 2.85 gcm-3 True specific gravity 2.91% True porosity 19.3An Modulus of rupture at 20"C 9.2 MN/m2 Modulus of rupture at 1200"C 7.0 MN/m2 Permanent volume change (2 hours at 1600"C) -1.4% X-ray diffraction showed that all the silicon had reacted to form silicon nitride and silicon oxynitride.

The nozzle was placed in a tundish in parallel with a carbon bonded nozzle of the type widely used in the industry. Both nozzles were subjected to the same conditions, e.g. 1 hour pre-heat at bright red heat followed by 2 hours casting during which time 300 tonnes of steel (0.12 , C, 0.65 nMn aluminium killed) was passed through each nozzle from two ladles.

At the end of the cast the nozzles were examined and it was found that cut back at the flux line had been 11 mm for the carbon bonded nozzle and 9.5 mm for the silicon nitride/oxynitride bonded nozzle. The depth of complete decarburisation on the outside of the nozzle was 5 mm for the carbon bonded nozzle and 2 mm for the silicon nitride/oxynitride bonded nozzle. Wear on the bore of the carbon bonded nozzle was 2 mm but only 0.5 mm for the silicon nitride/oxynitride bonded nozzle. Hence, from an original wall thickness at the flux line of 20.5 mm only 7.5 mm remained for the carbon bonded nozzle after casting whereas 10.5 mm remained for the oxynitride bonded nozzle. The silicon nitride/oxynitride bonded nozzle could have passed at least a further 150 tonnes of steel whereas it is doubtful whether the carbon bonded nozzle could have done so without fracture at the flux line.

The average wear rates at the flux line for 6 nozzles of either type were as follows:- Carbon bonded Oxynitride bonded Wear rate, mean mm/hr 6.4 4.2 Wear rate range mm/hr 5.1-8.3 3.1-4.9 Example 2 An example of the successful use of silicon nitride/oxynitride bonded graphitehigh alumina ladle nozzles is as follows:- The composition was: Fused mullite-l/l6" +22BSS 45 /n Fused mullite-22BSS 25 Flake graphite 10 Silicon (mean particle size 5 microns) 10 Micronised calcined alumina 5 Ball clay 5 Mixing was as before. This time however, the nozzles were pressed in a steel die using a hydraulic press.After firing in nitrogen to 1250"C for 16 hours the bond consisted mainly of silicon oxynitride. Mean apparent porosity was 15.7%. The nozzles were installed in a ladle above a sliding gate assembly. Four to five ladles of steel, each containing 65 tonnes, were passed through each nozzle tried corresponding to a maximum through put of 325 tonnes. Even after passing this amount of steel the wear on the bore was such that over 43 of the original wall thickness of the nozzle remained even in the worst affected area.

Most of the bore in fact showed negligible wear. Nozzles containing 20% graphite performed equally satisfactorily.

It will be understood that whilst mullite is referred to in the Examples, any alumina or aluminosilicate (greater than 42 /n alumina) material, sintered or fused can be used, and the nozzle formed to shape by any other known pressing technique.

Example 3 Blast furnace bosh/lower stack brick.

Here high thermal conductivity is required and hence the following composition is appropriate: 1015/'n silicon 20--40 flake graphite, 45700/n silicon carbide The raw materials are mixed with a temporary binder and pressed hydraulically in a steel die. The flakes of graphite align themselves perpendicular to the pressing direction and hence bricks can be made with a strongly directional thermal conductivity chosen to be high in the direction perpendicular to the blast furnace walls such that the maximum effect of any cooling processes are left.

Example 4 Sliding gate plates 10-15% silicon 5--20", flake graphite 65--850, alumina or high alumina grog.

Example 5 Crucibles for metal smelting 109, silicon 335%, flake graphite 45--50a;; silicon carbide 0-15% clay The invention also embraces bonds for graphite refractories when produced by the method of the invention, and refractory shapes when including a bond formed by the invention.

WHAT WE CLAIM IS: 1. A method for the production of a refractory artefact comprising admixing a first suitably graded component of refractory material, a second silicon powder component and a third powdered graphite component, and firing said mixture in an atmosphere of nitrogen whereby the first refractory component is bonded to the third graphite component by the product of the reaction between silicon and nitrogen.

2. A method as in Claim 1, wherein the mixture comprises a fourth silica component and when the bond between a first refractory component and the third graphite component comprises the products of the reaction of silicon with nitrogen and silica.

3. A method as in Claim 1 or Claim 2, wherein silicon is added in the range 5%, to 20% by weight based on the total weight of the mixture.

4. A method as in any of Claims 1 to 3, wherein firing of the mixture takes place in a nitrogen atmosphere which is continuously flowing.

5. A method as in any of Claims 1 to 4, wherein firing takes place at at least 1 1000C.

6. A method as in Claim 5, wherein firing takes place in the range at least 11000C to 15000 C.

7. A method as in Claim 5 or Claim 6, wherein firing takes place in the range 1200"C to 13500C.

8. A method as in any of Claims 1 to 7, wherein the mixture is worked in a dry or semi-dry pressing technique to produce the required shape.

9. A method as in any of Claims 1 to 8, wherein the graphite component is natural flake graphite.

10. A refractory artefact when produced by the method of any preceding claim.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (1)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   Example 4 Sliding gate plates

   10-15% silicon 5--20", flake graphite 65--850, alumina or high alumina grog.

   Example 5 Crucibles for metal smelting 109, silicon 335%, flake graphite 45--50a;; silicon carbide 0-15% clay The invention also embraces bonds for graphite refractories when produced by the method of the invention, and refractory shapes when including a bond formed by the invention.

   WHAT WE CLAIM IS: 1. A method for the production of a refractory artefact comprising admixing a first suitably graded component of refractory material, a second silicon powder component and a third powdered graphite component, and firing said mixture in an atmosphere of nitrogen whereby the first refractory component is bonded to the third graphite component by the product of the reaction between silicon and nitrogen.

   2. A method as in Claim 1, wherein the mixture comprises a fourth silica component and when the bond between a first refractory component and the third graphite component comprises the products of the reaction of silicon with nitrogen and silica.

   3. A method as in Claim 1 or Claim 2, wherein silicon is added in the range 5%, to 20% by weight based on the total weight of the mixture.

   4. A method as in any of Claims 1 to 3, wherein firing of the mixture takes place in a nitrogen atmosphere which is continuously flowing.

   5. A method as in any of Claims 1 to 4, wherein firing takes place at at least 1 1000C.

   6. A method as in Claim 5, wherein firing takes place in the range at least 11000C to 15000 C.

   7. A method as in Claim 5 or Claim 6, wherein firing takes place in the range 1200"C to 13500C.

   8. A method as in any of Claims 1 to 7, wherein the mixture is worked in a dry or semi-dry pressing technique to produce the required shape.

   9. A method as in any of Claims 1 to 8, wherein the graphite component is natural flake graphite.

   10. A refractory artefact when produced by the method of any preceding claim.