Note: Descriptions are shown in the official language in which they were submitted.
~Q1'~~~~
1.
Ceramic welding process and powder mixture for use in same
This invention relates to a ceramic welding process in which oxidising
gas and a mixture of refractory and fuel powders are projected against a
surface
and the fuel is burnt to generate sufficient heat that the refractory powder
becomes at least partially melted or softened and a cohesive refractory mass
is
progressively built up against that surface. The invention also relates to a
ceramic
welding powder mixture comprising refractory powder and fuel powder, for use
in
such a ceramic welding process.
Ceramic welding processes are useful for the manufacture of new
refractory bodies, for example bodies of rather complicated shapes, but in
current
to commercial practice, they are most often used for lining or repairing hot
refractory
structures such as furnaces or ovens of various kinds, and they enable eroded
areas
of the refractory structure (provided that those areas are accessible) to be
repaired
while the structure is substantially at its operating temperature and in some
cases
even while the structure is still operating. In any event, it is desirable for
there to
be no deliberate cooling of the refractory structure from its normal operating
temperature. The avoidance of such deliberate cooling tends to promote the
efficiency of the ceramic welding reactions, avoids further damage to the
structure
due to thermal stresses set up by such cooling and/or by subsequent reheating
to
operating temperature, and also helps to reduce furnace "down time".
o In ceramic welding repair processes, refractory powder, fuel powder
and oxidising gas are projected against the site to be repaired and the fuel
is burnt
so that the refractory powder becomes at least partially melted or softened
and a
refractory repair mass is progressively built up at the repair site. The fuel
used
typically consists of silicon and/or aluminium, though other materials such as
?5 magnesium and zirconium may also be used. The refractory powder may be
selected so that the chemical composition of the repair mass matches as
closely as
possible the composition of the refractory to be repaired, though it may be
varied,
for example so as to deposit a coating of a higher grade refractory on the
base
structure. In usual practice, the fuel and refractory powders are projected
from a
30 lance as a mixture in a stream of oxidising carrier gas.
Due to the intense heat generated on combustion of fuel powders at or
close to the surface to be repaired, that surface also becomes softened or
melted,
and as a result, the repair mass, which is itself largely fused together
becomes
r_ 201'622
2.
strongly adherent to the repaired wall, and a highly effective and durable
repair
results. Previous disclosures of ceramic welding repair techniques are to be
found
in British Patents No 1,330,894 and 2,110,200.
Hitherto, one of the most widespread uses of ceramic welding repair
processes has been for the refurbishment of coke ovens which are formed from
silica refractories. The standard ceramic welding powder most often used for
the
repair of silica refractories comprises silica together with silicon and
optionally
aluminium as fuel powder. Silica refractories are in fact the easiest to
repair by
ceramic welding at least in part because silica refractories are of relatively
low
to refractory grade, so that the temperatures (e.g. 1800°C or more)
reached in the
ceramic welding reaction zone easily allow the formation of an adherent
cohesive
repair mass, and the refractory grade requirements of that repair mass are
usually
no higher than those of the original silica refractory structure.
We have found, however, that certain problems arise when repairing
IS refractories of higher grades or in other cases when the refractory grade
requirements of the ceramic weld mass are particularly stringent. Examples of
high grade refractories are: chrome- magnesite, magnesite-alumina, alumina
chrome, magnesite-chrome, chrome, and magnesite refractories, high-alumina
refractories, and refractories containing a considerable proportion of
zirconium
Zo such as Corhart (Trade Mark) Zac (a fused alumina - zircon - zirconia
refractory).
For achieving the formation of a ceramic weld mass which has a refractory
grade
and/or composition approaching or matching those of such high grade
refractories, it is not always sufficient to use a standard ceramic welding
powder as
described above.
zs A particular problem that arises in the case of a ceramic welding repair
mass which is to be subjected to very high temperatures during its working
life is
the avoidance of a phase within the repair mass which has an insufficiently
high
softening or melting point. The cohesiveness of a repair mass containing such
a
phase is impaired at high temperatures and its resistance to corrosion at high
3o temperatures is also not as good as might be hoped for. In general, a
refractory
phase which is relatively less physically resistant to heat is also more
easily
attacked chemically at high temperatures.
It is an object of this invention to provide a ceramic welding process,
and a ceramic welding powder for use in such a process, which results in the
3s formation of a weld mass in which the appearance of such a low-grade
refractory
phase tends to be reduced, and may, in some embodiments of the invention, even
be avoided.
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3.
According to this invention, there is provided a ceramic welding
process in which oxidising gas and a mixture of refractory and fuel powders
are
projected against a surface and the fuel is burnt to generate sufficient heat
that the
refractory powder becomes at least partially melted or softened and a cohesive
refractory mass is progressively built up against that surface, characterised
in that
the fuel powder is present in a proportion of not more than 15% by weight of
the
total mixture and comprises at least two metals selected from aluminium,
magnesium, chromium and zirconium, in that at least the major part by weight
of
the refractory powder consists of one or more of magnesia, alumina and chromic
to oxide, and in that the molar proportions of silica and calcium oxide
present in the
refractory powder (if any) satisfy the following expression:
[Si02] % ~ 0.2 + [Ca0] %.
The invention also provides a ceramic welding powder, being a mixture
of refractory and fuel powders, for use in a ceramic welding process in which
oxidising gas and the mixture of refractory and fuel powders are projected
against
a surface and the fuel is burnt to generate sufficient heat that the
refractory
powder becomes at least partially melted or softened and a cohesive refractory
mass is progressively built up against that surface, characterised in that the
fuel
powder is present in a proportion of r.ot rr:ore than 1Jc'c by weight of the
total
zo mixture and comprises at least two metals selected from aluminium,
magnesium,
chromium and zirconium, in that at least the major part by weight of the
refractory
powder consists of one or more of magnesia, alumina and chromic oxide. and in
that the molar proportions of silica and calcium oxide present in the
refractory
powder (if any) satisfy the following expression:
z5 [Si02] % < 0.2 + [Ca0] %.
The use of such a powder in such a process eves rise to a ceramic weld
mass which is highly resistant to molten materials such as molten metals and
metal
slajs, and molten glass. Such weld masses can have good resistance to
corrosive
liquids and gases at elevated temperatures such as are encountered, for
example.
3o in the working or manufacture of steel, copper, aluminium nickel and glass,
and in
crucibles or other c.'~emical reactors exposed to dame action. Such weld
masses
are also capable of adhering well to high-grade refractory base structures.
C
201 ~ fi22
3a.
As used herein, the expression "a major part" refers to a greater
percentage than any of the other refractories present.
The occasional loss of refractory grade in the ceramic weld mass formed
is often observed when using a welding powder containing appreciable
quantities of silica or silica-forming materials, and it may be attributable
to the
formation of a vitreous phase in the weld mass at the very high temperatures
which can be achieved during the ceramic welding reactions. Such a vitreous
phase
C
201'~f 22
4.
often has a relatively low melting point, and also it may be relatively easily
attacked by molten material such as molten metals, slag and molten glass, and
its
presence would thus impair the quality of the weld mass as a whole. Silica is
often
present in refractories whether as a deliberately added constituent or as an
s impurity. By adopting the present invention, we limit the allowable
proportion of
silica to an amount which tends to form a refractory weld mass in which such a
vitreous phase is greatly reduced or avoided and the refractory grade of the
weld
mass formed is improved.
The refractory grade of the weld mass formed is improved if, as is
to preferred, the molar proportions of silica and calcium oxide present in the
refractory powder (if any) satisfy the following expression: (Si02] % ~ [Ca0]
%.
This promotes avoidance of an acidic phase in the weld, and improves its
resistance to corrosion by molten glass or metallurgical slugs.
It is preferred that the refractory powder is substantially free from
IS silica. The adoption of this feature also militates against the formation
of any
silica-based vitreous phase in the weld mass formed.
Advantageously, the projected refractory powder substantially consists
of one or more of zirconia, magnesia, alumina and chromic oxide. Such
materials
are capable of forming very high grade refractory masses.
In accordance with the invention, the fuel powder comprises at least
two metals selected from aluminium, magnesium, chromium and zirconium. Such
fuels combust to give oxides which are of good refractory quality and which
are
either amphoteric (alumina and zirconia) or basic (magnesia or chromic oxide),
and accordingly such fuels will contribute to the formation of a refractory
mass
25 which is highly resistant to corrosion by molten glass or metallurgical
slugs. This
feature of the invention also allows considerable flexibility in the choice of
fuel
elements, and thus in the refractory oxide product afforded on combustion of
those elements, so that the composition of the eventual .refractory weld mass
formed can be varied if desired.
3o Advantageously, the fuel powder comprises aluminium together with
one or more of magnesium, chromium and zirconium. Aluminium has excellent
combustion characteristics for the purpose in view, and it is also relatively
easily
obtainable as a powder.
Preferably, no element constitutes more than 80% by weight of said
35 fuel powder. This has been found beneficial for allowing control of the
conditions
under which combustion takes place. Thus for example by the adoption of this
preferred feature, a main highly reactive fuel ingredient is limited to 80% of
the
241'7622
s.
total fuel and the remainder of the fuel, at least 20% by weight, may be made
up
of a fuel element which reacts more slowly to control the combustion rate.
Conversely, a main less active fuel ingredient may have its reaction rate
boosted by
the addition of at least 20% by weight of one or more fuels elements which
reacts
s more rapidly.
Advantageously, the fuel powder comprises an alloy containing at least
30% by weight of one metal selected from aluminium, magnesium, chromium and
zirconium, the balance of the alloy being made up from at least one element
other
than such selected metal which element is also oxidisable to form a refractory
oxide. The use of particles of an alloy as a fuel is particularly valuable in
regulating conditions under which combustion takes place.
The projected mixture of powders need not necessarily be wholly free
of silicon in order to avoid or reduce the formation of relatively low grade
acidic
or vitreous siliceous phases. In some circumstances silicon may be present in
the
fuel powder. Indeed, we have found that the use of silicon as a fuel
constituent
can have advantages in stabilizing the way in which the ceramic welding
reactions
proceed. In some preferred embodiments of the invention, therefore, silicon is
present in said fuel in the form of an alloy of silicon with at least one of
aluminium, magnesium, chromium and zirconium The use of silicon as an
zo alloying constituent can have a favourable effect on the rate at which the
combustion reactions take place during performance of the process of the
invention. For example silicon in an alloy with magnesium can have the effect
of
tempering the rate at which the highly active magnesium burns. Furthermore,
because an alloy is an intimate mixture of its constituents, intimacy of the
reaction
zs products is promoted, and this militates against the silicon giving rise to
any
distinct acidic or vitreous phase within the refractory weld mass formed.
In other preferred embodiments of the invention, again in order to
promote the avoidance of inducing a siliceous acidic or vitreous phase in the
weld
mass formed, it is preferred that the molar amount of silicon (if any) present
in the
3o mixture is not more than the molar amount (if any) of zirconium. By way of
example, the refractory powder could contain a proportion of zirconium
orthosilicate (zircon) which is a quite acceptable high-grade refractory
ingredient.
Alternatively, or in addition, the fuel powder could contain a proportion of
elemental silicon which might combine with zirconium in the mixture (whether
as
35 elemental zirconium or as zirconia) to form zircon, without inducing an
acidic
phase in the weld mass formed.
Thus in some such preferred embodiments of the invention, said fuel
i
6.
incorporates elemental silicon in the form of particles having an average
grain size
of less than 10 micrometres, preferably less than 5 micrometres, and the
mixture
includes zirconia particles having grain sizes below 150 micrometres, such
zirconia
particles being present in a molar amount which is at least equal to the molar
amount of elemental silicon in the mixture. We have found that malting use of
this optional feature of the invention promotes the formation of zircon
(zirconium
orthosilicate) in the weld mass formed as a result of the ceramic welding
reactions,
so that that mass is substantially free from silica as such, and the risk of
forming a
vitreous low-grade refractory phase is small. In this way, the advantages of
using
to silicon as fuel can be achieved without also incurring the disadvantages of
incorporating a possibly vitreous acidic phase of silica in the weld mass.
In other preferred embodiments of the invention, the projected fuel
powder is substantially free from silicon. The adoption of this feature will
avoid
the formation of any silica-based vitreous phase in the weld mass formed.
In some preferred embodiments of the invention, the projected fuel
powder comprises magnesium and aluminium. The oxidation of aluminium and
magnesium in suitable proportions can generate ample heat for the performance
of the process of the invention, and gives rise to the formation of refractory
oxides
which can be incorporated into a highly refractory weld mass.
zo Preferably, the projected fuel powder comprises, by weight, more
aluminium than magnesium, for example, aluminium may be present in the fuel in
a molar amount about twice that of the magnesium This promotes the formation
of spinet (magnesium aluminate) in the weld mass. Spinet is a very useful high-
grade refractory material.
~5 Advantageously, magnesium is incorporated in the projected fuel
powder in the form of magnesium/aluminium alloy. The use of a powdered alloy
of these metals rather than a mixture of powders further promotes the
formation
of spinet rather than the separate oxides as a result of the ceramic welding
reactions. The composition of the alloy may be varied, or additions of
3o supplementary aluminium or magnesium may be made in order to regulate the
relative proportions of aluminium and magnesium in the fuel powder as desired.
In other preferred embodiments of the invention, the projected fuel
powder comprises chromium and aluminium. Such fuel powders are useful for
the formation of high-chrome refractory weld masses, and advantageously, such
35 projected fuel powder comprises, by weight, more chromium than aluminium.
Preferably, at least 60% and in some embodiments at least 90% by
weight of the projected fuel powder has a grain size below 50 micrometres.
This
~~~.~s
7.
promotes rapid and effective combustion of the fuel powder for the formation
of a
cohesive refractory weld mass.
The process of the invention is particularly beneficial when applied for
the treatment of refractories which are themselves of a basic rather than an
acidic
character, and accordingly, it is preferred that the process be used for the
repair of
a structure built of basic refractory material.
Various specific ceramic welding powders according to the invention
will now be described by way of example only.
Example 1
to A ceramic welding powder comprises, by weight, the following:
Magnesia 82 % Mg/Al Alloy 5 %
Zirconia 10 A1 grains 3
The magnesia used had grain sizes up to 2 mm. The zirconia had grain
sizes below 150 micrometres. The Mg/Al alloy contained a nominal 30% by
weight magnesium and 70% aluminium, with grain sizes below 100 micrometres
and an average grain size of about 42 micrometres, and the aluminium was in
the
form of grains having a nominal ma~cimum size of 45 micrometres.
The magnesia used had a purity of 99% by weight. It contained 0.8%
calcium oxide by weight, and 0.05% silica by weight. The molar ratio of Si02
to
Zo Ca0 in the magnesia was therefore 1:17.4.
Another magnesia composition suitable for use has a purity of 98% by
weight. It contains 0.6% calcium oxide by weight, and 0.5% silica by weight.
The
molar ratio of Si02 to Ca0 in this magnesia composition is therefore 1:1.28.
Such a powder may be projected at a rate of 1 to 2 tonnes per hour
s5 from a lance well known per se in the ceramic welding art using oxygen as
carrier
gas for the repair of a steel converter formed from basic magnesia refractory,
the
repair site being at a temperature of 1400°C immediately before such
projection.
Example 2
A ceramic welding powder comprises, by weight, the follawing:
3o Magnesia 82 % Al grains 3 %
Zirconia 10 Al flake 3.5
Mg grains 1.5
The magnesia, zirconia and aluminium grains had grain sizes as given
in Example 1. The composition of the magnesia was one of those given in
35 Example 1. The magnesium had a nominal maximum size of about 75
micrometres and an average grain size of less than 45 micrometres. The
aluminium flake had a specific surface (measured by Griffin permeametry) of
over
201'~62~
s.
7000 cm2/g.
Such a powder may be projected as described in Example 1 for the
repair of a steel converter formed from magnesia-chrome refractory, the repair
site being at a temperature of 1400°C immediately before such
projection.
Example 3
A ceramic welding powder comprises, by weight, the following:
Chromic oxide 82 % Mg/Al Alloy 5
Zirconia 10 Al grains 3
The chromic oxide had grain sizes of up to 2 mm. The other materials
to were as specified in Example 1.
The chromic oxide was substantially free from silica, the merest traces
being found on analysis.
Such a powder may be projected at a rate of 150 to 200 kg/h from a
lance well known per se in the ceramic welding art using oxygen as carrier
gas, for
IS the repair of a copper converter formed from magnesia-chrome refractory,
the
repair site being at a temperature of 1100°C immediately before such
projection.
Example 4
A ceramic welding powder comprises, by weight, the following:
Chromic oxide 82 % Al grains 3
zo Zirconia 10 Al flake 3.5
lVlg grains 1.5
The chromic oxide was as specified in Example 3. The other materials
were as specified in Example 2.
Such a powder may be projected at a rate of 150 to 200 kg/h from a
zs lance well known per se in the ceramic welding art using oxygen as carrier
gas, for
the repair of a steel degassing nozzle formed from magnesia-chrome refractory,
the repair site being at a temperature of 1100°C immediately before
such
projection.
In a variant of this Example, the magnesium was replaced by zirconium
3o having a mean grain size of about 10 to 15 micrometres, taking all desired
precautions having regard to the well known high reactivity of zirconium.
Example 5
A ceramic welding powder comprises, by weight, the following:
Chromic oxide 90 % Cr 8
35 Al flake 2
The chromium was in the form of grains having a nominal maximum
grain size of about 100 micrometres and an average grain size of between 25
and
. _ 201'622
9.
30 micrometres. The chromic oxide was as specified in Example 3. The
aluminium flake had a specific surface (measured by Griffin permeametry) of
over
7000 cm2/g.
Such a powder may be projected at a rate of 40 kg/h from a lance well
s known per se in the ceramic welding art using oxygen as carrier gas, for the
repair
of Corhart (Trade Mark) Zac (fused alumina - zircon - zirconia) refractory
blocks
located at the level of the surface of the melt in a glass-melting furnace,
the repair
site being at a temperature of 1500°C to 1600°C immediately
before such
projection.
to The powder is equally suitable for the repair of a chrome refractory
(that is, a refractory containing more than 25% chromic oxide and less than
25%
magnesia) again located at the level of the surface of the melt in a glass-
melting
furnace.
Example 6
Is A ceramic welding powder comprises, by weight, the following:
Magnesia 72 % A1 grains 3 %
Zirconia 10 Mg/Al Alloy 5
Carbon 10
The carbon was coke having an average diameter of about 1.25 mm.
zo The other materials were as specified in Example 1. Such a powder may be
projected as described in Example 1 for the repair of a steel converter formed
from a magnesia-carbon refractory.
Example 7
A ceramic welding powder comprises, by weight, the following:
Zs Magnesia 82 % Si 2 %
Zirconia 10 Mg 4
Al flake 2
The silicon was in the form of Grains having an average grain size of 4
micrometres. The zirconia had a nominal maximum grain size of 150
3o micrometres. The other materials were as specified in previous Examples.
Such
a powder may be projected at a rate of 150 kg/h for the repair of a magnesia
basic
refractory steel ladle.
Example 8
A ceramic welding powder comprises, by weight, the following:
3s Alumina 92 % Mg ? %
Al grains 6
The alumina used was an electrocast alumina containing, by weight,
201'~6~~
10.
99.6% A1203. It contained 0.05% CaO, and 0.02% Si02. The molar ratio of Si02
to Ca0 in this alumina is therefore 1:2.68
The alumina had a nominal maximum grain size of 700 micrometres
and the aluminium and magnesium had grain sizes as specified in Example 2.
Such a powder may be used as described in Example 5 for the repair of Corhart
(Trade Mark) Zac refractory blocks in a glass melting tank furnace beneath the
working surface level of the melt after the tank has been partially drained to
give
access to the repair site.
In a variant of this Example, the electrocast alumina was replaced by
to tabular alumina.
The tabular alumina used had a nominal maximum grain size of 2 mm,
and contained, by weight, 99.5% A1203. It contained 0.073 mol% CaO, and 0.085
mol% SiO2. The molar ratio of Si02 to Ca0 in this alumina was accordingly
1:0.86, though it does clearly satisfy the expression: [Si02] % S 0.2 + [Ca0]
%.
IS Example 9
A ceramic welding powder comprises, by weight, the following:
Magnesia 80 % Mg/Si Alloy 5 %
Zirconia 10 Mg/Al Alloy 5
The magnesium/silicon alloy contained equal weight proportions of
zo the two elements and had an average grain size of about 40 micrometres. The
other materials were as specified in Example 1. Such a powder may be projected
as described in Example 1 for the repair of a refractory wall formed from a
basic
magnesia refractory.
Examples 10 to 16
zs In variants of Examples 1 to 4, 6, 7 and 9, the zirconia was replaced by
tabular alumina as described in Example 8. '
In variants of Examples 1, 3, 6, 9, 10, 12, 14 and 16, the alloy containing
30% magnesium and 70% aluminium had a maximum grain size of not more than
75 micrometres and an average grain size of less than 45 micrometres. In yet
3o further variants, the alloy contained equal weights of magnesium and
aluminium.
