GB2314326A

GB2314326A - Formation of a refractory repair mass

Info

Publication number: GB2314326A
Application number: GB9612927A
Authority: GB
Inventors: Bernard Somerhausen; Jean-Pierre Meynckens
Original assignee: Glaverbel Belgium SA
Current assignee: AGC Glass Europe SA
Priority date: 1996-06-20
Filing date: 1996-06-20
Publication date: 1997-12-24
Also published as: GB9612927D0

Abstract

A powder mixture for the repair of refractory concrete, for example as used for the lining of a blast furnace runner for molten iron, includes combustible particles and refractory particles for projection in an oxygen-containing gas stream against a worn or damaged concrete surface. The combustible particles react against the said surface in a highly exothermic manner with the projected oxygen and thereby release sufficient heat of combustion to form a repair mass. The powder mixture includes, alumina as a major part by weight; together with magnesia and either silicon carbide or zirconia.

Description

FORMATION OF A REFRACTORY REPAIR MASS The present invention relates to the formation of a refractory repair mass, and in particular to the formation of a refractory repair mass on a refractory concrete material, for example as used for the lining of a blast furnace runner for molten iron.

Refractory concrete typically contains alumina (Al2Q), often together with significant quantities of one or both of silicon carbide (SiC) and carbon. A representative refractory concrete includes for example (percentages by weight): alumina 80W; silica (SlO2) 15%, calcium oxide (CaO) 2.0t and various "impurities" such as titanium dioxide (TiO2) and ferric oxide (Fe2O3).

Refractory concrete is used for applications in which it is exposed to elevated temperatures, notably in the steel industry.

One of its main applications is in the runners (troughs) used to convey molten iron from a blast furnace. Each runner typically includes, leading away from the furnace, a main runner (also known as a main trough or iron trough) into which molten iron flows, followed by slag, when casting is initiated by opening a tap hole at the base of the furnace. The molten flow from the furnace is at a temperature of about 15500C. This flow forms into distinct layers of slag and molten iron, with the slag on top. The main runner leads to a slag skimmer which diverts the slag to slag ladles or a slag pit. The molten iron continues into an "iron runner", usually leading to iron ladles. The overall length of the runner from the blast furnace to the ladles is typically 10 to 15 m.

The blast furnace may have one or more tap holes and thus one or more such sets of runners.

In cross-section each runner comprises an insulating structure, a permanent lining and a wear lining. The wear lining, which is formed of refractory concrete, is the part of the runner which is exposed to direct contact with the molten iron and the slag.

A typical refractory concrete for this layer includes (percentages by weight): alumina 70t; SiC 15 ; SiO2 5t; carbon (graphite) 8.0%, CaO 0.5 and "impurities" such as titanium dioxide (TiO2) and ferric oxide (Fe203). The SiC and carbon both serve to improve the resistance of the concrete to corrosion by the slag.

There are typically four to ten casts per 24 hours, repairs being made between casts.

The present invention is concerned with ceramic welding repairs.

"Ceramic welding" is the term that has come to be used for a refractory welding procedure first claimed in our GB patent specification 1330894, in which a powder mixture of refractory oxide particles and combustible particles is projected in an oxygen-containing gas stream against the surface of a substrate material. The combustible particles, typically finely divided silicon and/or aluminium, serve as fuel which reacts against the target surface in a highly exothermic manner and releasing sufficient heat of combustion to form a coherent mass of refractory oxide. There have been many subsequent patent specifications on ceramic welding, including our later patents GB 2110200 and GB 2170191.

Ceramic welding is well suited to the in situ repair of worn or damaged refractory walls or linings of furnaces such as metallurgical furnaces, and also for coke ovens and glass furnaces. It is particularly well suited to the repair of a hot substrate surface, making repairs possible while the refractory remains at or near to its working temperature. Adjustment of the components and proportions of the powder mixture is desirable according to the specific type of refractory to be repaired and according to its specific means of use.

Our patent specification GB 2257136, which is directed to the repair of a refractory based on a silicon compound, describes the use of a powder mixture containing a major proportion of refractory oxide particles, plus silicon and an additive substance which during the formation of the refractory mass causes the incorporation of silica, formed by the combustion of the silicon particles, into a crystalline lattice. Our patent specification 2284415 is directed to the repair of oxide-based refractory body and employs a powder mixture comprises oxide particles, fuel particles selected from magnesium, aluminium and silicon and, additionally, up to 10% by weight of silicon carbide.

The aim of refractory concrete repairs of a blast furnace runner is to maintain the wear lining of the runner in good condition for as long as possible, especially in the case of furnaces which have only one tap hole. A repair is usually best made immediately after a cast, while the lining is still at a very high temperature.

The object of the invention is to provide for refractory concrete a durable repair which is resistant to thermal shock and to corrosion by such materials as slag.

According to the present invention there is provided a process for the repair of refractory concrete, in which process a powder mixture of combustible particles and refractory particles is projected in an oxygen-containing gas stream against the surface of the refractory concrete and the combustible particles react against the said surface in a highly exothermic manner with the projected oxygen, thereby releasing sufficient heat of combustion to form a repair mass, characterised in that the powder mixture includes alumina (Al203) as a major part by weight; together with magnesia (MgO) and either silicon carbide (SiC) or zirconia.

The invention also provides a powder mixture for the repair of refractory concrete, which mixture includes combustible particles and refractory particles for projection in an oxygencontaining gas stream against the refractory surface, where the combustible particles react against the said surface in a highly exothermic manner with the projected oxygen and thereby release sufficient heat of combustion to form a repair mass, characterised in that the powder mixture includes alumina (Al203) as a major part by weight; together with magnesia (MgO) and either silicon carbide (SiC) or zirconia.

The invention provides a repair mass which bonds securely and durably to the refractory concrete and is sufficiently similar to the concrete in terms of constituents and crystalline properties to have similar thermo-mechanical properties thereto, especially in regard to such important properties as expansion coefficient. It can be applied to all types of aluminous refractory concrete, whether or not these contain silicon carbide or carbon.

The temperature of the concrete to be repaired is preferably in the range 800 to 11000C. A base temperature in this range ensures that a repair mass having a high level of cohesion and a high level of adhesion to the base material is obtained within a short period.

The temperature of the runner surface tends to fall quickly into the said range, and indeed to below it, when the molten flow from the furnace ceases. Repairs are therefore often most conveniently initiated soon after the flow ceases, while the temperature remains in the preferred range. If desired the runner can be covered with an insulating layer, for example slabs of refractory material, to prolong a high temperature for the runner and thus to give more time following a cast in which to initiate the repair. Alternatively repairs can be made after the melt-induced temperature of the runner has fallen below the said range by reheating the surface, for example with a flame, to bring its temperature back to the required level.

The main refractory oxide components of the powder, i.e. the alumina and magnesia, are preferably present in particulate form with substantially no particles having a size greater than 5 mm, most preferably not greater than 2.5 mm. These size characteristics are chosen to facilitate smooth projection of the powder.

The preferred proportion of alumina in the powder is influenced by the alumina content of the concrete to be repaired, and allowance should be made in this regard for the additional alumina that will be created by oxidation of the aluminium fuel in the powder, although the total proportion of alumina in the repair mass need not be identical to that in the concrete.

The magnesia promotes both the formation of a crystalline bonding phase in the repair mass and a bonding between the crystalline phases in the repair mass and in the concrete. It appears to achieve this by forming a crystalline lattice which encloses the silica formed by the combustion. The presence of the crystalline phase assists in ensuring a firm and durable bond between the repair mass and the base concrete. The proportion of magnesia in the powder is preferably in the range 3 to 10% by weight, more preferably 4 to 6% by weight.

The combustible particles are preferably one or both of silicon and aluminium. These represent the solid fuel content of the powder, i.e. the combustible material that reacts exothermically with the oxygen. They preferably have an average particle size of less than 45 zm and a specific surface area between 3,500 and 6,000 cm2/g. The term "average particle size" designates a dimension such that 50% by weight of the particles have a smaller dimension than this average. The total amount of fuel should preferably be 5 to 15 by weight of the powder. These properties encourage the required degree of intense heat release at the repair point. When both silicon and aluminium are used each of them is preferably in the range 3 to 8% by weight.

The last component of the powder, i.e. the silicon carbide or zirconia, serves primarily to impart to the repair mass resistance against corrosion by the slag. In the case of silicon carbide it is preferably present in an amount of 5 to 15% by weight of the powder, more preferably 8 to 12%, and its maximum particle size is preferably less than 1 mm. In the case of zirconia it is preferably present in an amount of 6 to 10% by weight of the powder and its maximum particle size is preferably less than 200 Rm.

A convenient source of the zirconia is an alumina/zirconia alloy. A preferred chemical composition of the alloy is given by the eutectic composition of approximately 55% Awl203 and approximately 40% ZrO2. The alloy is preferably present in the range of 17 to 20% by weight of the powder mixture.

The invention is further illustrated in the following nonlimiting examples.

EXAMPLE 1 In this example the refractory for repair was a concrete having the following composition, by weight: alumina 70%; SiC 15g; SiO2 5e; carbon (graphite) 8%; CaO 0.5%; the balance being "impurities" including titanium dioxide (TiO2) and ferric oxide (foe203).

A powder mixture employed for repair of the said concrete comprised, by weight: tabular alumina 72t MgO 5% Si 7% Al 6W.

SiC 10%.

The Awl203 had a maximum particle size of 2 mm. The MgO had a maximum particle size of 0.5 mm. The silicon and aluminium had an average particle size below 45 ym and specific surface areas respectively between 2,500 and 8,000 cm2/g and 3,500 and 6,000 cm2/g. The silicon carbide was a fine grade, with a maximum particle size of 125 corm.

The temperature of the concrete surface at the start of the repair operation was 10500C. The powder mixture was projected against a damaged portion of the surface at a rate of 60 kg/h in a stream of pure oxygen to form a repair mass thereon.

The formed mass had a good visual appearance, showing excellent continuity with and adhesion to the rest of the concrete surface. It had a high resistance to corrosion by slag. Its structure was examined under a microscope which also revealed excellent continuity in crystalline structure between the repair mass and the base refractory.

In a variation of the above example, the fine-grained SiC was replaced by a coarser grade, with particle sizes in the range 0.5 to 1.0 mm. The formed mass had a similar corrosion resistance to that formed with the finer grade material.

EXAMPLE 2 The procedure of Example 1 was repeated on a further worn portion of the same concrete but employing in this instance a powder mixture comprising, by weight: tabular alumina 63% MgO 5% Si 7% Al 5% Al2O3/ZrO2 alloy 20% The alumina, MgO, silicon and aluminium in the powder had the same particle size properties as in Example 1.

The alloy component was an electrocast eutectic alloy comprising, by weight, 55% Awl203, 40 % ZrO2 and small quantities of TiO2, Fe2O3, HfO2 and Na2O. It had a maximum particle size of 200 clam.

The temperature of the concrete surface at the start of the repair operation was again 10500C and the powder mixture was again projected at a rate of 60 kg/h in pure oxygen.

The formed mass again had a good visual appearance, showed good continuity with and adhesion to the rest of the concrete surface and had a high resistance to thermal shock. Its structure was examined under a microscope which also revealed excellent continuity in crystalline structure between the repair mass and the base refractory.

In a variation of Example 2, the electrocast eutectic alloy was replaced by mineral zirconia (baddeleyite) as the source of zirconia. The baddeleyite had a maximum particle size of 200 clam. The proportion of baddeleyite was 10% of the powder and the proportion of alumina was correspondingly increased to 73%.

The formed mass was slightly less dense than that formed with the alloy.

Claims

1. A process for the repair of refractory concrete, in which process a powder mixture of combustible particles and refractory particles is projected in an oxygen-containing gas stream against the surface of the refractory concrete and the combustible particles react against the said surface in a highly exothermic manner with the projected oxygen, thereby releasing sufficient heat of combustion to form a repair mass, characterised in that the powder mixture includes alumina (awl203) as a major part by weight; together with magnesia (MgO) and either silicon carbide (SiC) or zirconia.

2. A process as claimed in claim 1, in which the temperature of the concrete to be repaired is in the range 800 to 11000C.

3. A process as claimed in claim 1 or claim 2, in which the refractory oxide components of the powder are present in particulate form with substantially no particles having a size greater than 5 mm, preferably not greater than 2.5 mm.

4. A process as claimed in any preceding claim, in which the powder mixture includes magnesia in an amount in the range 3 to 10% by weight.

5. A process as claimed in claim 4, in which the proportion of magnesia is in the range 4 to 6% by weight.

6. A process as claimed in any preceding claims, in which the powder mixture includes silicon carbide in an amount of 5 to 15% by weight.

7. A process as claimed in claim 6, in which the powder mixture includes silicon carbide in an amount of 8 to 12t by weight.

8. A process as claimed in claim 6 or claim 7, in which the powder mixture includes silicon carbide with a maximum particle size of less than 1 mm.

9. A process as claimed in any of claims 1 to 5, in which the powder mixture includes zirconia in an amount of 6 to 10% by weight.

10. A process as claimed in any of claims 1 to 5 or 9, in which the powder mixture includes zirconia in the form of an alumina/zirconia alloy.

11. A process as claimed in claim 10, in which the alumina/zirconia alloy is present in an amount of 17 to 20% by weight.

12. A process as claimed in claim 10 or claim 11, in which the alumina/zirconia alloy has a maximum particle size of less than 200 pm.

13. A process as claimed in any of claims 10 to 12, in which the alumina/zirconia alloy is a eutectic alloy containing approximately 55% Awl203 and approximately 40t ZrO2.

14. A process as claimed in any preceding claim, in which the combustible particles in the powder mixture are particles of one or both of silicon and aluminium.

15. A process as claimed in claim 14, in which the total amount of silicon and aluminium in the powder mixture is in the range 5 to 15% by weight.

16. A process as claimed in claim 14 or claim 15, in which the silicon and aluminium have an average particle size of less than 45 Zm.

17. A powder mixture for the repair of refractory concrete, which mixture includes combustible particles and refractory particles for projection in an oxygen-containing gas stream against the refractory surface, where the combustible particles react against the said surface in a highly exothermic manner with the projected oxygen and thereby release sufficient heat of combustion to form a repair mass, characterised in that the powder mixture includes alumina as a major part by weight, together with magnesia and either silicon carbide or zirconia.

18. A powder mixture as claimed in claim 17, in which the refractory oxide components are present in particulate form with substantially no particles having a size greater than 5 mm, preferably not greater than 2.5 mm.

19. A powder mixture as claimed in claim 17 or claim 18, which includes magnesia in an amount in the range 3 to 10% by weight.

20. A powder mixture as claimed in claim 19, in which the proportion of magnesia is in the range 4 to 6t by weight.

21. A powder mixture as claimed in any of claims 17 to 20, which includes silicon carbide in an amount of 5 to 15 by weight.

22. A powder mixture as claimed in claim 21, which includes silicon carbide in an amount of 8 to 12% by weight.

23. A powder mixture as claimed claim 21 or 22, which includes silicon carbide with an average particle size of less than 1 mm.

24. A powder mixture as claimed in any of claims 17 to 20, which includes zirconia in an amount of 6 to 10 by weight.

25. A powder mixture as claimed in any of claims 17 to 20 or 24, in which the includes zirconia in the form of an alumina/zirconia alloy.

26. A powder mixture as claimed in claim 25, in which the alumina/zirconia alloy is present in an amount of 17 to 20% by weight.

27. A powder mixture as claimed in claim 25 or 26, which the alumina/zirconia alloy has an average particle size of less than 200 clam.

28. A powder mixture as claimed in any of claims 25 to 27, in which the alumina/zirconia alloy is a eutectic alloy containing approximately 55% Al203 and approximately 40% ZrO2.

29. A powder mixture as claimed in any of claims 17 to 28, in which the combustible particles in the powder mixture are particles of one or both of silicon and aluminium.

30. A powder mixture as claimed in claim 29, in which the total amount of silicon and aluminium is in the range 5 to 15% by weight.

31. A powder mixture as claimed in claim 29 or claim 30, in which the silicon and aluminium have an average particle size of less than 45 ym.