Note: Descriptions are shown in the official language in which they were submitted.
X136660
A method and powder mixture for repairing oxide based
refractory bodies
The present invention relates to a method of repairing an oxide-
based refractory body by a ceramic welding process.
Oxides of silicon, zirconium, aluminium and magnesium are used
as industrial refractory oxides. In particular, oxides of aluminium and
magnesium
are currently used in the metallurgy industry, where they are chosen for their
resistance to high temperature, to erosion and to corrosion by materials such
as
molten metal, slag and dross.
Magnesium oxide-based refractory materials, otherwise known as
basic ref ractory materials, may form the lining of a ladle for the transport
of
molten steel. Such linings become abraded by the molten steel and the slag in
use. Erosion of the lining occurs particularly at the level of the liquid.
There is
thus a need to repair such oxide based refractory bodies from time to time.
It has been proposed to repair refractory bodies by use of a
"ceramic welding" technique. In this technique, the refractory body to be
repaired is maintained at an elevated temperature, and a powder mixture is
projected in the presence of oxygen, said powder mixture comprising particles
of a refractory material and fuel particles which react in an exothermic
manner
with the oxygen to form a refractory oxide. By this method a refractory mass
builds up and adheres to the refractory body at the repair site. The technique
of
ceramic welding is illustrated in British patent nos. GB 1,330,894 (Glaverbel)
and GB 2,170,191 (Glaverbel). The fuel particles are particles whose
composition and granulometry are such that they react in an exothermic
manner with the oxygen while forming a refractory oxide and while releasing
the necessary heat for melting, at least superficially, the projected
refractory
particles.
However, it has been found that when a powder mixture
consisting of oxide particles and fuel particles is used to repair an oxide-
based
refractory body, and in particular a refractory body based on high melting
point
oxides such as magnesium oxide and aluminium oxide, the resulting refractory
mass may be porous. If there is a significant apparent porosity, the repair
mass
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is not useful for certain applications, particularly if the repair mass is
subject to
erosion or to corrosion by molten materials.
It is therefore an object of the present invention to provide a
method of repairing oxide-based refractory bodies which enables a refractory
repair mass to be formed with acceptable porosity.
We have surprisingly discovered that, where the fuel particles are
selected from particles of magnesium, aluminium, silicon and mixtures thereof,
this objective may be achieved by the incorporation, in the powder mixture, of
a
specific quantity of silicon carbide. This is contrary to the generally
accepted
principle of matching the composition of the refractory repair mass with the
composition of the refractory material surface being repaired. Further,
silicon
carbide is seen as an inert material in this ceramic welding process and is
not
wetted by the liquid phase which is formed during the reaction. The effect of
the
silicon carbide on the porosity of the mass is therefore somewhat surprising.
l5 While not wishing to be bound by theory, we believe that the
additional silicon carbide particles conduct heat into the refractory repair
mass
and that in time prolonged exposure to high temperatures causes a
decomposition of the silicon carbide particles to generate elemental carbon,
which is known to provide the refractory repair mass with good slag corrosion
resistance.
Thus, according to a first aspect of the invention there is provided
a method of repairing an oxide-based ref ractory body by projecting a powder
mixture against a surface of said body at an elevated temperature and in the
presence of oxygen, said powder mixture comprising refractory oxide particles
and fuel particles which react in an exothermic manner with the oxygen to form
a ref ractory oxide, characterised in that the fuel particles are selected
from
magnesium, aluminium, silicon and mixtures thereof and in that the powder
mixture additionally contains up to 10°rb by weight of silicon carbide
particles.
The level of silicon carbide in said powder mixture is preferably at
least 1% by weight. If too much silicon carbide is included, we have found
that
the result may be that no repair mass at all is formed because the repair
material
flows away from the repair site. Without wishing to be bound by theory, one
might expect that this may be due to the retention of too much heat following
the repair process, leading to a low viscosity liquid phase. If too little
silicon
carbide is used, the benefits of the invention are no longer obtained to a
significant degree.
The silicon carbide preferably has a small particle size, such as less
than 200 ~.m. By "particle size" as used herein, we mean that the material
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concerned has a particle size distribution such that at least 90% by weight of
particles conform to the given limits. "Average dimensiori', as used herein,
designates a dimension such that 50% by weight of the particles have a smaller
dimension than this average.
The refractory oxide particles may comprise at least one oxide of
which the refractory body is fornled. Thus, where the refractory oxide body is
an
aluminium oxide-containing body, the refractory oxide particles may comprise
particles of alumina. Where the refractory oxide body is a magnesium oxide
containing body, the refractory oxide particles may comprise particles of
magnesia.
Preferably, a major portion of said powder mixture is formed of
refractory oxide particles selected from magnesia, alumina and mixtures
thereof.
These are the oxides in the presence of which the exothermic reaction is most
lively, and therefore has a higher risk of resulting in a highly porous repair
mass.
Preferably, the oxide refractory particles have a size below 2.5 mm, with
substantially no particles having a size above 4 mm.
The fuel particles are selected from particles of magnesium,
aluminium, silicon and mixtures thereof. A mixture of aluminium and silicon is
particularly advantageous. The fuel particles used in the mixture preferably
have
an average dimension of less than 50 ~.m.
The repair operation is generally performed when the refractory
body is hot. This makes it possible to repair eroded refractory bodies while
the
equipment remains substantially at its working temperature.
The elevated temperature may be above 600°C as measured at
the surface of the refractory body to be repaired. At this temperature, the
fuel
particles will combust in the presence of oxygen to liberate a refractory
oxide
and to generate sufficient heat to cause the oxide particles, together with
the
combustion product of the fuel, to form into the refractory repair mass which
constitutes the repair.
The invention also provides, according to a second aspect thereof,
a powder mixture for the repair of oxide-based refractory bodies, said mixture
comprising:
- from 80% to 95% by weight of refractory particles comprising a refractory
oxide; and
- from 5% to 20% by weight of fuel particles which react in an exothermic
manner with the oxygen to form a refractory oxide,
characterised in that said fuel particles are selected from magnesium,
aluminium,
AAA
silicon and mixtures thereof and in that said refractory particles include up
to
10% by weight, based on total mixture, of silicon carbide particles.
In order to obtain a homogeneous repair mass, an amount of at
least 80°r6 by weight of refractory particles, including the oxide
particles, should
be present in the powder mixture.
In a preferred embodiment, the mixture comprises:
- from 80°r6 to 94°rb by weight of refractory oxide particles
selected from
particles of alumina, magnesia and mixtures thereof ;
- from 1°r6 to 5°rb by weight silicon carbide particles; and
- from 5°r6 to 15°r6 by weight of said fuel particles.
Preferably; the refractory particles in the powder mixture,
including the silicon carbide particles, have a size of at least l O ftxn. If
particles
which are too small are employed, there is a risk that they will be lost
during the
reaction.
~5 A useful technique for bringing the powder mixture against a
surface of the refractory body to be repaired, is to project the powder
mixture
together with an oxygen-containing gas. In general it is recommended to
perform the projection of particles in the presence of a high concentration of
oxygen, f or example, by using oxygen of commercial quality as a gas carrier.
In
this manner a repair mass is easily formed that adheres to the surface onto
which the particles are projected. Because of the very high temperatures that
the ceramic welding reaction can reach, it can penetrate slag which might be
present on the surface of the refractory body being treated, and it can soften
or
melt the surf ace in such a way that a good bond is produced between the
treated
surface and the newly formed refractory repair mass.
This process is conveniently carried out with the use of a lance. A
suitable lance for use in the process of the invention comprises one or more
outlets for the discharge of the powder stream, optionally together with one
or
more outlets for supplementary gas. 1=or repairs carried out in a hot
environment, the gas streams may be discharged from a lance which is cooled
by fluid circulating through it. Such cooling may easily be achieved by
providing
the lance with a water jacket. Such lances are suitable f or projecting powder
at
rates of 30 to 500 kg/h.
In order to facilitate the formation of a regular jet of powder, the
refractory particles preferably comprise substantially no particles with a
size
greater than 4 mm, most preferably not greater than 2.5 mm.
The invention is particularly useful for the repair or maintenance
of molten-steel ladles because it can be carried out rapidly, at a high
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temperature, between ladle charges, while the refractory bodies which form
part
of such ladles are particularly affected by contact with molten metal and
slag.
The region which requires the greatest repair tends to be the line of the
liquid
surface.
5 The invention will now be further described in the following non-
limiting examples.
EXAMPLE 1
A refractory repair mass is formed on a wall of the magnesium
oxide-based lining of a molten steel ladle. A mixture of refractory particles
and
particles of a fuel is projected onto these bricks. The temperature of the
wall is
about 850°C. The mixture is projected at the rate of 150 kg/h into a
stream of
pure oxygen. The mixture has the following composition:
Mg0 87°~b by weight
SiC 5°ib
Si 4°i6
A1 4%
The Mg0 particles have a maximum dimension of approximately
2 mm. The silicon carbide particles have a particle size of 125 N.m, with an
average dimension of 57 ~.m. The silicon particles and the aluminium particles
have a maximum dimension below 45 ~.m.
FXAMpLE 1A (Comparative)
By way of comparison, the same repair was carned out in the
same manner as described in Example 1, but using a powder mixture of the
following composition:
Mg0 92% by weight
Si 4%
AI 4%
The apparent density and apparent porosity (i.e. open porosity) of
the refractory repair masses formed in Examples 1 and 1A were measured and
the results were as follows:
Density
Example No kg/dm3 Porosity (%)
1 2.9 about 8%
1A 2 - 2.4 about 20%
In a modification of Example 1, an aluminium oxide-containing
refractory may be repaired in a similar way, but where the magnesia particles
in
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the powder mixture are replaced by the same amount of alumina particles of the
same granulometry.
EXAMPLES 2 to 4
Refractory repair masses are formed on a wall of the magnesium
oxide-based lining of a molten steel ladle. Mixtures of refractory particles
and
particles of a fuel are projected onto these bricks. The temperature of the
wall is
about 850°C. The mixtures are projected at the rate of 60 kg/h into a
stream of
pure oxygen. The mixtures had the following compositions (by weight):
Example No: 2 3 4
Si 4% 4% 4%
Al 4% 4% 4%
SiC 2% 5% 10%
Mg0 90% 87% 82%
The Mg0 particles have a maximum dimension of approximately
2 mm. The silicon carbide particles have a particle size of 125 mm, with an
average dimension of 57 mm. The silicon particles and the aluminium particles
have a maximum dimension below 45 mm.
The apparent density and apparent porosity (i.e. open porosity) of
the refractory repair masses formed in Examples 2 to 4 were measured and the
results were as follows:
Density
Example No kg/dm3 Porosity (%)
2 2.6 14%
3 2.7 10°r6
4 2.9 8%
EXAMPLE 5
A ceramic welding powder comprises the following composition
(°r6 by weight):
Alumina 87%
Silicon carbide 5%
Aluminium 6%
Magnesium 2%
The alumina used was an electrocast alumina. The alumina had a
nominal maximum grain size of 700 mm, the silicon carbide had the same
granulometry as given in Example 1 above, the aluminium particles had a
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maximum dimension below 45 ~.m and the magnesium particles had a
maximum dimension of 75 ~.m.
The above powder mixture may be used as described in Example
1, to repair a Corhart (Trade Mark) Zac refractory block (composition:
alumina/zircon/zirconia) in a glass melting tank f umace beneath the working
surface level of the melt after the tank has been partially drained to give
access
to the repair site.
