Note: Descriptions are shown in the official language in which they were submitted.
~fl'~~.3'~~
PROCESS ANI~ MIXTURE FOR FORMII\IG A COI=iERENT REFRACTORY
MASS ON A SURFACE
The present invention relates to a process for forming a coherent
refractory mass on a surface wherein is projected against this surface,
simultaneously with oxygen, a mixture of refractory particles and combustible
particles which react in an exothermic manner with the p~~ojected oxygen by
s releasing sufficient heat to form, under the action of the heat of
combustion, the
aforesaid refractory mass. the invention also relates to a mixture of
particles
intended for a process for forming a coherent refractory mass on a surface by
projecting the mixture and oxygen against the surface, the mixture comprising
refractory particles and particles of a combustible substance which are
capable of
ro reacting exothermically with the oxygen in order to release sufficient heat
to form,
under the heat of combustion, the aforesaid refractory mass.
If it is desired to form a refractory mass in situ on a surface, one can
choose between two known principal processes.
According to the first of these processes, sometimes called "ceramic
is welding", illustrated in Patent GB 1,330,894 (Glaverbel) and GB 2,170,191
(Glaverbel), a coherent refractory mass is formed on a surface by projecting
onto
the latter a mixture of refractory particles and combustible particles in the
presence
of oxygen. The combustible particles are particles whose composition and
granulometry are such that they react in an exothermic manner with the oxygen
ao while forming a refractory oxide and while releasing the necessary heat for
melting, at least superficially, the projected refractory particles. Aluminium
and
silicon are examples of such combustible substances. It is known that silicon,
properly speaking, must be classified as a half metal, but because silicon
behaves
like certain metals (it is capable of undergoing considerable exothermic
oxidation in
2s forming a refractory oxide), these combustible elements are called
combustible
metallic substances for reason of simplicity. In general it is recommended to
perform the projection of particles in the presence of a high concentration of
oxygen, for example, by using oxygen of commercial quality as a gas carrier.
In
this manner a coherent refractory mass is formed that adheres to the surface
onto
3o 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 substance being treated, and it can soften or
melt
2
the surface in such a way that a good bond is produced between the treated
surface and the newly formed refractory mass.
These known ceramic welding processes can be employed for forming a
refractory article, for example, a block having a particular shape, but they
are most
s widely used for forming coatings or for repairing bricks or walls and are
particularly
useful for repairing or reinforcing existing refractory structures, for
example, for
repairing walls or coating refractory equipment such as furnace walls in
glassmaking or coke furnaces.
This operation is generally performed when the refractory base is hot.
~o This makes it possible to repair eroded refractory surfaces while the
equipment
remains substantially at its working temperature and, in certain cases, even
while it
is operating.
The second known process for forming a refractory mass on a surface is
called "flame spraying process." It involves directing a flame to the site
where a
rs refractory mass is to be fornzed and spraying refractory powder across this
flame.
The flame is fed by a gaseous fuel or liquid or even coke powder. It is
apparent
that the efficient utilisation of this flame spraying technique requires
complete
combustion of the fuel in order to generate the hottest flame possible and to
attain
maximum efficiency. In general, the temperature of the flame obtained with a
av flame spraying process is not so high as that which may be obtained with a
ceramic welding technique, with the result that coherence of the formed
refractory
mass is not so good, and since the bond between the new refractory mass and
the
surface of the refractoxy base is formed at a lower temperature, this bond
w111 not
be so firm. Moreover, such a flame is less apt than a ceramic welding reaction
to
2a penetrate slag which might be present on the refractory surface being
treated.
The composition of the mixture used in a ceramic welding process is
generally chosen in such a way as to produce a repair mass which has a
chemical
composition similar or close to that of the basic refractory. This helps to
ensure
compatibility with arid adhesion to the new material and the base material on
3o which It is fornzed.
We have observed, however, that problems occur if it is desired to repair
certain types of refractory structures and this, even if a refractory mass of
a
chemical composition is formed which is similar to that of the basic
refractory
mass.
as For example, repairing refractory surface structures having a silicon
carbide base with the help of a mixture containing primarily carbon silicon
particles
3
and also particles of metallic combustible substances, such as aluminium and
silicon
particles, produces a refractory mass which does not always demonstrate
sufficient
adhesion to the base refractory.
Refractories having a base of silicon carbide are used in certain
s metallurgy equipment, in particular, in blast furnaces in the iron industry
or in zinc
distillation columns. During the operation of this equipment, certain portions
of
the refractory structures may have a rather low minimum operating temperature,
for example, on the order of 700°C, and may additionally be subjected
to
significant variations of ambient temperature. It has been observed that the
~o refractory masses produced by known techniques on these parts of refractory
structures do not always demonstrate sufficient adhesion to the base
refractory
mass and, in certain cases, particularly when the repair is made on a block or
a
refractory wall whose temperature is low, the new refractory mass becomes
completely separated from the base refractory mass and detaches itself during
the
~s operation of the equipment.
Similar problems present themselves if one desires to repair refractory
structures having a high density silica base (so named in order to distinguish
them
from traditional silica refractories whose density is lower), used in certain
coke
furnaces; even though ane can form a refractory similar in chemical
composition
so to the base refractory mass, the new mass does not always sufficiently
adhere and
may even separate rapidly from the base refractory mass when the furnace is in
operation.
A process is known from International Patent Application WO
90/03848 (Willmet/Willard) for the repair of, for example, furnace linings,
2s wherein an inert carrier gas and particles of refractory oxide and
combustible
oxidisable material are delivered to a flame spraying apparatus, wherein high
pressure oxygen aspirates and accelerates the carrier gas/particle mixture.
Willard
applies this process to the repair of refractory blocks/bricks in the tuyere
line of a
copper smelting converter as well as to the repair of silicon carbide tray
columns.
,3o For example, one projects a mixture containing 79°r6 silicon
carbide, 16.25°~
silicon, 4°r6 aluminium and 0.75°~ magnesium through a double
venturi air oxygen
system to a silicon carbide tray column.
However, the use of magnesium metal powder in this process is
disadvantageous, at least in that since magnesium metal is relatively
volatile, there
as is a degree of uncertainty about the cornpositlon of the formed refractory
coating.
One of the objectives of the present invention is to solve these
2~~~3'~~
problems.
The present invention relates to a process for forming a coherent
refractory mass on a surface based on a silicon compound, wherein there is
projected against the surface, simultaneously with oxygen, a mixture
comprising
s refractory particles and combustible particles which react in an exothermic
manner
with the projected oxygen by releasing sufficient heat to form the refractory
mass,
under the action of heat of combustion, characterised in that the mixture
comprises: (i) combustible silicon particles; (ii) as a major proportion by
weight of
the mixture, refractory particles of one or a plurality of substances; and
(iiia)
ro additive particles of another substance which, during the formation of the
refractory mass, causes incorporation of silica, formed by combustion of the
silicon
particles, into a crystalline lattice, and/or (iiib) additive particles of a
non-metallic
compound which, during the formation of the refractory mass, generates said
another substance which causes the incorporation of silica, formed by the
is combustion of silicon particles, into a crystalline lattice.
The present invention also relates to a mixture of particles intended for
a process for forming a coherent refractory mass on a surface based on a
silicon
compound by projecting the mixture and oxygen against said surface, the
mixture
comprising refractory particles and combustible particles capable of reacting
zo exothermically with oxygen to release sufficient heat to form, under the
action of
the heat of combustion, said refractory mass, characterised in that the
mixture
comprises: (i) combustible silicon particles; (ii) as a major proportion by
weight of
the mixture, refractory particles of one or a plurality of substances; and
(iiia)
additive particles of another substance which, during the formation of the
2s refractory mass, causes incorporation of silica, formed by combustion of
the silicon
particles, into a crystalline lattice, and/or (iiib) additive particles of a
non-metallic
compound which, during the formation of the refractory mass, generates said
another substance which causes the incorporation of silica, formed by the
combustion of silicon particles, into a crystalline lattice.
3o Such a mixture and such a process are useful for forming high quality
refractory masses for the repair of surfaces based on a silicon compound, such
as,
for example, refractory structures of furnaces as well as for welding pieces
together. It is possible to obtain a refractory mass which demonstrates
excellent
adherence to the base refractory when the repaired surface undergoes repeated
3s variations of thermal conditions during the operation of the equipment
and/or
when the repair is made on a surface whose temperature is relatively low, such
as
CA 02071370 2002-11-14
between 600°C and 1000°C (for example, 700°C), although
the invention is applicable
to surfaces having a temperature outside this range.
The refractory masses produced according to the invention present
thermal expansion properties at the interface between the surface and the
formed
5 refractory mass which are different from those that would be obtained if the
starting
mixture did not contain any substance causing the incorporation into a
crystalline lattice
of silica formed by combustion of silicon. We believe that the advantages
obtained by
the invention are due, at least in part, to this difference at the interface
and that the
refractory masses obtained demonstrate thermal expansion properties at the
interface
which are well adapted to those of the refractory structures in question.
The combustible silicon particles (i) may be used as the only
combustible material or they may be mixed with particles of a further
combustible
material, such as aluminum. Thus, the mixture preferably further comprises
combustible aluminum particles. Aluminum particles may be rapidly oxidised
with a
1 S significant release of heat and form refractory oxides themselves. The
adaptation of
this characteristic thus favours the formation of high quality refractory
masses.
The expression "major proportion" as used herein refers to a proportion
of more than 50% of the overall total.
The mixtures according to the invention, preferably comprise not more
than 15% by weight silicon. This is important for limiting the amount of
unreacted
silicon which may remain in the formed refractory mass. We have found that the
presence of unreacted silicon in the formed refractory mass may detract from
the
qualities thereof.
The refractory particles (ii) may be present in an amount of at least 70%
by weight, most preferably at least 75% by weight, in order to obtain a
homogeneous
mass.
The additive particles (iiia) and/or (iiib) preferably make up the
remainder of the mixture and may comprise up to 25% by weight of the mixture,
preferably from 5 to 15% by weight.
The combustible particles (i) used in the mixture preferably have an
average particle size of less than SO~m.
CA 02071370 2002-11-14
Sa
The refractory particles (ii) preferably comprise substantially no
particles with a size greater than 4mm, most preferably not greater than 2.Smm
in order
to facilitate the formation of a regular jet of powder.
The additive particles (iiia) and/or (iiib) used in the mixture preferably
S have a particle size of less than or equal to SOO~,m. If particles which are
too large
are employed, there is a risk that they will not play an effective role.
Preferably,
2~'~~~'~~
6
these particles have a size of at least l0pm. If particles which are too small
are
employed, there is a risk that they will volatilise during the reaction.
Various substances are suitable for inducing, during the formation of the
refractory mass, the incorporation of silica, formed by the combustion of
silicon,
s into a crystalline lattice.
The aforesaid additive substance (iiia) causing the incorporation of silica,
formed by the combustion of silicon, into a crystalline lattice, is preferably
introduced into the mixture in the form of magnesia particles.
The presence of this compound into the mixture which is projected onto
to the refractory surface to be repaired helps to ensure the correct heat
resisting
properties of the refractory mass formed.
Moreover, the introduction of magnesia into the mixture permits the
formation of a refractory mass in which one portion at least of the silica
formed by
the combustion of silicon is incorporated into a crystalline lattice of the
forsterite
15 type. This also helps to ensure the correct heat resisting properties of
the
refractory mass formed.
If the mixture contains aluminium as well as magnesia, a refractory mass
may be formed in which one portion at least of the silica formed by the
combustion of silicon is incorporated into a crystalline lattice of the
forsterite
zo structure and/or into a crystalline lattice of the spinet structure and/or
into a
crystalline lattice of the cordierite structure.
The presence of a crystalline lattice of the cordierite structure in the
refractory mass formed helps to ensure excellent resistance to thermal shocks
of
this mass. The presence of a crystalline lattice of the forsterite structure
and/or
2s spinet structure, on the other hand, favourably influences the heat
resistance of the
formed refractory mass.
Other oxides such as calcium oxide or iron (11) oxide, may also be used
as the additive substance (iiia) causing the incorporation of silica, formed
by the
combustion of silicon, in a crystalline lattice.
3o A mixture of particles may be used which additionally or alternatively
comprises an additive substance or substances (iiib) whose composition is such
that, when the refractory mass is formed, it/they generate a substance causing
the
incorpovation of silica formed by the combustion of silicon, into a
crystalline lattice.
For example, peroxides such as calcium peroxide, nitrides, carbides may be
used.
as An oxide, for example calcium oxide, may be introduced in the form of
a compound, for example, in the case of calcium oxide, in the form of
wollastonite
ccao.sio2~.
The present invention is particularly useful for repairing refractories
having a silicon carbide base or refractories having a high density silica
base.
Consequently, it is preferred for ceramic welding to be earned out with the
help of
s a mixture whose major portion by weight comprises silicon carbide or silica,
respectively.
It goes without saying that the invention may also be useful for repairing
other types of refractories based on a silicon compound than the ones
previously
mentioned, such as normal silica bricks and silica-alumina bricks.
ro The substance or substances constituting the major portion by weight of
the mixture may correspond to the composition of the refractory one desires to
repair, or may be of a different substance. In the latter case, a refractory
mass is
formed which may have properties different from, and ideally improved over,
those of the refractory under repair, for example improved resistance to
abrasion
~s or improved refractar9ty.
The present invention will now be further illustrated in more detail with
the help of the following examples:
FJCAMPIdE I
A refractory mass is formed on a wall of the zinc distillation column.
ao This wall comprises bricks having a silicon carbide base. A mixture of
refractory
particles, particles of a combustible substance which are exothermically
oxidisable
by forming a refractory oxide, and of magnesia particles is projected onto
these
bricks. The temperature of the wall is 800°C. The mixture is projected
at the rate
of 60 kg/h into a stream of pure oxygen. The mixture has the following
2s composition:
SiC 79°r6 by weight
Si 8°,6
Al 50~
Mg0 8°r6
ao The silicon particles have a dimension below 45 Nm and a specific
surface area comprised between 2,500 and 8,000 cm2/g. The aluminium
particles have a dimension below 45 pm and a specific surface area comprised
between 3,500 and 6,000 cm2/g. The dimension of the silicon carbide particles
is less than 1.47 mm with 60°r6 by weight from 1 to 1.47 mm,
20°~ from 0.5 to 1
as mm, and 20°~ below 0.125 mm. 'The Mg0 particles have an average
dimension
of approximately 300 l.~m. "Average dimension' designates a dimension such
that
8
50% by weight of the particles have a smaller dimension than this average.
The wall which has been repaired in this manner is subjected to
significant variations of ambient temperature and it is observed that the new
refractory mass adheres durably to the support.
s The structure of the fom~ed mass is examined under the microscope.
Excellent continuity is observed between the new refractory mass aril the base
refractory mass. It is also observed that the silica formed by the combustion
of
silicon is incorporated into the crystalline lattices of forsterite,
cordierite and
aluminous spinet.
~o For purpose of comparison, a mixture not containing magnesia is
projected under the same conditions. The composition of this mixture is as
follows:
SiC 87% by weight
Si 12%
~s Al 1°r6
It is observed that the refractory mass formed separates rapidly from the
wall and detaches itself in solid blocks if the zinc distillation column
continues to
operate.
In a modification of this example, the mixture is used to repair the
x~ bottom of a coke furnace formed of normal silica bricks and silica-alumina
bricks.
One obtains a repair rnass having good resistance to abrasion which adheres
well
to the wall, even when subjected to significant thermal variations.
As a variation of Example 1 a mixture having the following composition
2s is used:
SiC 82°r6 by weight
Si 8°~
A1 5%
Mg0 5%
,~o The wall being repaired comprises bricks having a silicon carbide base
and it has a temperature of 700°C.
The refractory mass obtained also adheres durably to the wall.
SAMPLE 3
The object is to form a refractory mass on a wal! of a coke furnace
as comprising high density silica bricks. ~Nhile the apparent density of
traditional
silica bricks is on the order of 1.80, the apparent density of high density
bricks is
9
approximately 1.89. Such bricks have recently appeared on the refractory
material
market, presenting advantageous characteristics by comparison with traditional
silica bricks, notably with respect to their properties of gas permeability
and
thermal conductivity.
s The repair is performed on a wall whose temperature is approximately
750°C with the help of the following mixture:
Si02 80.5 °~ by weight
Si 11.1%
A1 1%
~o Mg0 7.4°~
The dimension of the Si02 particles i~ less than 2 mrn, with a maximum
of 30% by weight from 1 to 2 mm, and less than 15°r6 by weight below
100 lam.
The formed mass adheres duxably to the wall.
By contrast, projection under the same operating conditions, of a similar
is mixture but not containing magnesia, furnished a refractory mass which
easily
separates itself from the wall if the latter is subjected to various thermal
conditions
which are present when the furnace is in operation.
EXAMPLE 4
The object is to form a refractory rnass on a wall of a coke furnace
ao made of a refractory based on a silicon compound which is subject to
significant
variations of ambient temperature and of which the temperature does no; exceed
900°C. The repair is performed on a wall whose temperature is
approximately
750°C with the help of the following mixture:
Si02 : 80°~O by weight
2s CaO.Si02 (wollastonite) : 8°r6
Si : 8°~
Al : 4°r6
The average dimension of the wollastonite particles is about 3001am.
The metal particle size is as given in Example 1 and the silica particle sine
is as
.3o given in Example 3.
