Coating processes – Restoring or repairing
Reexamination Certificate
1998-08-03
2002-04-16
Bareford, Katherine A. (Department: 1762)
Coating processes
Restoring or repairing
C427S142000, C427S236000, C427S422000, C427S427000, C427S452000, C427S453000, C106S450000, C106S489000, C106S313000
Reexamination Certificate
active
06372288
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the formation of a refractory repair mass, and in particular to the formation of a refractory repair mass on the surface of an electrocast refractory material.
2. Description of the Related Art
Electrocast refractory materials are oxides of certain controlled compositions obtained by fusion at very high temperatures, usually in an electric furnace, and by casting the so-produced molten material into moulds. There are several families of electrocast refractory materials, including zirconia-containing materials such as alumina/zirconia/silica (AZS), of which one brand is available under the trade name Zac, alumina/zirconia/silica/chromium (AZSC) and spinels such as magnesia/alumina and chromic oxide/alumina.
Electrocast refractory materials find use in a number of specialist high temperature applications, for example as refractory blocks for those parts of furnaces which are subjected to severe high temperature operational conditions. These conditions are encountered at various points in the superstructure of a glass melting tank, with especially severe conditions being encountered at the “glass line” (also known as the “flux line”), that is to say at the upper surface of the molten glass.
In the vicinity of the glass line the refractory material of the tank is subjected to direct thermal contact with the hottest layer of liquid glass and immediately above that to thermal contact with the adjacent furnace atmosphere. The liquid and the adjacent gas thus each subject the glass line refractories to substantial but different stresses. As the level of the glass line rises and falls in the course of the production process the refractories in its vicinity undergo significant thermal cycling. In addition to variable thermal stresses occasioned by this cycling, mechanical stresses are imposed by the liquid glass flowing through the tank having a mechanical scouring action.
Despite the high quality of electrocast refractory materials and their excellent suitability for such duties they nevertheless undergo considerable erosion in use. There is accordingly an ongoing need for repairs of the said materials and a requirement for the formed repair itself to be resistant to the severe conditions. Long-term durability of the repair is especially important given that the furnace may be required to operate continuously for a period in excess of ten years.
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 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 for combustion with the oxygen, reacting against the target surface in a highly exothermic manner and releasing sufficient heat of combustion to form a coherent refractory mass. There have been many subsequent patent specifications on ceramic welding, including our later cases GB 2110200 and GB 2170191.
Ceramic welding can be employed for making discrete refractory blocks or for binding refractory pieces together but has mostly been employed for the in situ repair of worn or damaged refractory walls of furnaces such as coke ovens, glass furnaces and metallurgical furnaces. Ceramic welding is particularly well suited to the repair of a hot substrate surface, making repairs possible while the equipment remains substantially at its working temperature and if necessary while the furnace as a whole remains in operation.
It is a well-established practice in ceramic welding that the composition of the ceramic welding mixture be chosen to produce a refractory repair mass which has a chemical composition compatible with and preferably similar to that of the furnace constructional material. It has however been found that merely matching the chemical compositions of the refractory substrate material and the repair, mass may not be sufficient to ensure a durable repair. Even with chemical compatibility there can still be a problem in ensuring a strong and lasting bond between the repair mass and the worn or damaged refractory substrate. The problem tends to increase if the repaired surface is subjected to very high temperatures or to thermal cycling.
Thus attention must also be given to the physical compatibility of the repair mass and the refractory substrate, most particularly with regard to their respective degrees of thermal expansion, which is linked to their crystallinity. In our copending application GB-A-2257136, which relates to the repair of a surface based on a silicon compound, steps are taken to produce in the repair mass during its formation a crystalline lattice which resembles that of the base refractory material, with a view to avoiding the problem of the formed mass becoming separated and detached from the base refractory material. With such silicon-based repair surfaces it is especially important to avoid the formation of a vitreous phase in the repair mass.
SUMMARY OF THE INVENTION
Surprisingly it has now been found that in the case of electrocast materials a feature which is necessary to ensure physical compatibility between the repair surface and the, repair mass is the presence of a vitreous phase. As a result it has been found that high quality durable repairs can be effected at such hostile locations as the glass line in a glass melting tank by ensuring the presence of a vitreous phase in the repair mass.
Thus according to the present invention there is provided a process for the formation of a coherent refractory repair mass on a surface of electrocast refractory material, in which process a powder mixture of combustible particles and refractory particles is projected in an oxygen-containing gas stream against the refractory surface and 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 the repair mass, characterised in that the powder mixture includes at least one constituent which enhances the production of a vitreous phase in the repair mass.
The invention also provides a powder mixture for the formation of a coherent refractory repair mass on a surface of electrocast refractory material, 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 the repair mass, characterised in that the powder mixture includes at least one constituent which enhances the production of a vitreous phase in the repair mass.
The invention is especially well suited to the repair of an electrocast zirconiferous refractory material, employing a powder mixture which comprises zirconia-containing refractory particles.
According to the invention the powder mixture containing a constituent which enhances the production of a vitreous phase in the repair mass is applied directly to the surface of the electrocast refractory material to be repaired. The presence of a vitreous phase in the repair mass has been found to provide the benefits of improving both the adhesion and maintenance of adhesion of the repair mass to the electrocast refractory surface. The vitreous phase exists in the bonding phase of the mass and resembles the vitreous phase which exists in the refractory material beneath the surface.
A particular advantage is that the vitreous phase expands and contracts in the same way in both the repair mass and the substrate. Moreover in the case of zirconiferous electrocast refractory material the vitreous phase absorbs both the contraction of zirconia (ZrO
2
) which occurs with the allotropic transformation from the monoc
Meynckens Jean-Pierre
Mottet Leon-Philippe
Bareford Katherine A.
Glaverbel
Piper Marbury
Rudnick & Wolfe LLP
Schneider Jerold I.
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