FIELD
The present invention relates to apparatus from processing molten metal and, in
particular, pumps, degassers, flux injectors and submergence devices.
BACKGROUND
Molten aluminium has high affinity towards hydrogen. This typical characteristic of
aluminium results in hydrogen gas entrapment in casting during solidification.
Entrapped hydrogen forms micro porosity and blow holes, resulting in casting
rejections. Hence, the process of minimizing gas content from aluminium alloys
before casting (formally known as degassing) has at-most importance in an
aluminium foundry. Different techniques used for removal of gas content includes:
• purging tablet of chlorine or chlorine free base
• Purging inert gas (e.g. Ar, N2) through ceramic rod/pipe,
• purging inert gases through rotary degassing too
However, rotary degassing is the most popular choice today for all sizes of
aluminium foundries due to higher efficiency & reliability. Rotary degassing
efficiency has very high dependency on rotor design, which is the enabler of creating
smaller bubbles & distribution of the inert gas throughout the liquid metal.
The longevity of rotor and associated shafts are dependent upon the rotor design
and the material of construction. Rotor and shafts may be produced from the same
or different materials. The shaft and rotor material are often produced from graphite
due to its thermal shock resistance. However, graphite apparatus (e.g. rotor) is
susceptible to oxidation and erosion and, as such, may need be regularly replaced.
As such, the rotor is usually detachingly connected to the shaft using a mechanical
connection (e.g. male/female threaded connection joint).
While there is a large variety of rotor designs available, many of the degassing
systems have scope for further improvements in respect to improved longevity,
flexural strength and oxidation-resistance.
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SUMMARY OF THE INVENTION
In a first aspect of the present disclosure, there is provided a molten metal apparatus
comprising wollastonite fibres embedded within a ceramic matrix, wherein the
wollastonite fibres are bonded to the ceramic matrix by a glassy bonding phase
comprising a glass component comprising oxides of calcium, silicon and aluminium.
There is also provided a composite material comprising wollastonite fibres
embedded within a ceramic matrix, wherein the wollastonite fibres are bonded to the
ceramic matrix by a glassy bonding phase comprising a glass component comprising
at least 80 wt% of oxides of calcium, silicon and aluminium.
The combination of the wollastonite fibres and the glassy bonding phase enables a
strong bond between the fibres and the glassy bonding phase that results in an
apparatus which has excellent mechanical properties, including impact and flexural
strength, oxidation resistance, non-wetting and thermal resistance. In a preferred
embodiment, the wollastonite bondedly is embedded into the glassy bonding phase.
The glassy bonding phase is preferably partially derivable from the wollastonite fibre
and, as such, the wollastonite fibres merge into the glassy bonding phase, thereby
enabling the mechanical properties (e.g. flexural strength) of the wollastonite fibres
to impact throughout the adjoining glassy bonding phase and the composite material.
The composite material may also comprise a carbon compound. The carbon
compound may comprise graphite and/or a carbonised resin (e.g. amorphous
carbon).
The glassy bonding phase may comprise a glass component and a crystalline
component. The crystalline component comprises crystals which have been formed
in-situ during the formation of the composite material. These crystals are generally
less than 10 micron or less than 5 micron in diameter and are dispersed throughout
the glass component. In some embodiments, the glassy bonding phase further
comprises mullite. Preferably, the mullite is derivable and formed from a ceramic
matrix precursor material (e.g. clay). The mechanical properties of the mullite
advantageously combines with the glass component of the glassy bonding phase
and wollastonite fibres.
In one embodiment, the composite material comprises:
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0.5 to 20 wt% (or 2 wt% to 15 wt%) wollastonite fibres;
0.5 to 40 wt% (or 5 wt% to 30 wt%) glassy bonding phase;
0 to 50 wt% (or 10 wt% to 35 wt%) ceramic matrix;
0 to 50 wt% (or 5 wt% to 25 wt%) carbon material; and
0 to 15 wt% (or 1 wt% to 5 wt%) other additives (e.g. Si, Fe-Si, borax).
Wherein the sum of the abovementioned components is greater than 90 wt% of the
composite material.
Preferably, the sum of wollastonite fibres+ glass bonding phase+ ceramic matrix+
carbon material + other additives is greater than 95 wt% or greater than 99 wt% or
100 wt% of the composite material.
The wollastonite fibres preferably have an aspect ratio of at least 3:1, with the fibre
length preferably being at least 0.5 1-1m or at least or 10 1-1m or 50 1-1m at least or at
least 50 1-1m 1 00 1-1m or at least 500 1-1m or at least 1 0001-Jm.
The glassy bonding phase is preferably derivable from a clay and/or alumina
material and more preferably a clay material which is able to form mullite crystals
upon firing at a sufficient temperature for a sufficient period of time. The glass
component of the glassy bonding phase comprises CaO, AI203 and Si02.
Preferably, CaO + AI203 + Si02 > 80 wt% or greater than 90 wt% or greater than 95
wt% of the glass component. While not wanting to be bound by theory it is thought
that at least a portion of the CaO and/or Si02 in the glass component is derived from
wollastonite fibres (i.e. a portion of the wollastonite fibres are consumed in the
formation of the composite material). Components of the glassy bonding phase may
also contain other glass forming components in trace amounts, such as alkaline
earth metals and/or alkali metal (e.g. oxides of K, Na, Mg and Fe).
As the glassy bonding phase may be generated through the dissolution of calcium
from the wollastonite fibres, the concentration of calcium in the glassy bonding phase
may be higher immediately adjacent the wollastonite fibres, with the calcium
concentration decreasing with increasing distance from the wollastonite fibres.
The ceramic matrix is preferably selected for its combination of thermal and
mechanical properties and may be selected from the group consisting of silica;
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alumina; carbides of Si, Ti, W, Ta, Nb, Zr, Hf, V, Cr, Mo; silicon nitride; magnesia;
zirconia; boron nitride; aluminium nitride; or combinations thereof.
The ceramic matrix may comprise a silicon carbide, e.g. a beta silicon carbide and/or
alpha silicon carbide.
The carbon material is preferably graphite and/or carbonaceous material derivable
from an organic binder used in the formation of the composite material.
The glassy bonding phase may bond the wollastonite fibres, ceramic matrix and/or
graphite together.
The additives may comprise carbonised organic binder; carbon oxidation inhibitors or
precursors thereof. Additives may include silicon metal, FeSi, aluminium, boron,
alumina-silicate (e.g. clay), borax and/or boric acid. Additives preferably make up
between 1.0 to 15 wt% or between 2.0 and 10 wt% of the composite material.
In a preferred embodiment, the composite material comprises:
0.5 to 10 wt% wollastonite fibres;
0.5 to 30 wt% glassy bonding phase;
10 to 50 wt% silicon carbide;
5.0 to 50 wt% graphite; and
1 to 15 wt% other additives.
Preferably the sum of wollastonite + glassy bonding phase + silicon carbide +
graphite + other additives is at least 90 wt% of the composite material.
In another embodiment, the composite material comprises:
0.5 to 10 wt% wollastonite fibres;
0.5 to 30 wt% glassy bonding phase;
10 to 50 wt% silicon carbide;
5.0 to 50 wt% carbon compound; and
1 to 15 wt% other additives
wherein the carbon compound comprises graphite and amorphous carbon.
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The amorphous carbon may be derived from a resin binder used in the starting
formulation. The resin binder may be carbonised during the sintering process, with a
portion of the carbonised resin reacting with silicon metal to form a beta silicon
carbide. The beta silicon carbide is typically in the form of fibres. The beta silicon
carbide in combination with the glassy bonding phase function as an oxidation
inhibitor for the graphite, reducing ingress of oxygen which is able to react with the
graphite.
Preferably, the glassy bonding phase is partially derived from an alumina-silicate
clay and the wollastonite fibres. The glassy bonding phase preferably comprises
mullite. The mullite is preferably formed in situ during a firing/sintering step which
partially converts the clay, or other mullite precursor material, to mullite.
The glassy bonding phase may comprise mullite crystals and other alumina and/or
silica crystals disperse within a glass phase. The crystal size distribution has a dso of
typically less than 3 1-Jm. The proportion of oxides of aluminium and silica in the
glass component may be at least 60 wt%. The glassy bonding phase or glass
component thereof preferably comprises at least 60% or at least 70 wt% or at least
75 wt% or at least 80 wt% or at least 85 wt% or at least 90 wt% aluminium and
silicon in an oxide form. The glassy bonding phase preferably comprises a
refractory glass phase/component which has a high melting point (e.g. greater than
900°C or greater than 1 000°C), such that it maintains its mechanical strength at the
operating temperature of the molten metal apparatus (e.g. 700-800°C). Typically the
glass phase has less than 20 wt% or less than 15 wt% or less than 10 wt% or less
than 5 wt% alkali or alkaline earth metal oxides. The high alumina/silica content
combined with the low alkali and alkaline earth metal oxide content of the glass
phase, combined with the dispersion of hard ceramic particles therein, results in a
corrosive and erosion resistant refractory binder capable of providing long lasting
mechanical and oxidation inhibiting properties. Furthermore, the softening of the
glass phase during the sintering phase reduces the porosity of the composite
material further improving the material's mechanical properties.
The graphite content is preferably between 20 and 40 wt%. Higher amounts of
graphite may result in the apparatus being more susceptible to oxidation and
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erosion. Lower graphite levels may not have sufficient molten metal non-wetting
properties or shock resistant properties.
The silicon carbide content is preferably between 20 wt% and 40 wt% to provide the
desired mix of mechanical properties when combined with the other components of
the composite.
It has been found the above combination of materials provides an excellent balance
of resistance to thermal shock, mechanical strength, non-wetting, thermal resistance
(insulative) and oxidation resistance.
The apparatus may be selected from the group consisting of a pump, a degasser, a
flux injector and a scrap submergence device. In a preferred embodiment, the
apparatus is a shaft and/or rotor of a degasser. In a more preferred embodiment,
the apparatus is a one piece shaft and impeller of a degasser. The composite
material of the present disclosure has been found to have sufficient thermal shock
resistance; mechanical strength and oxidation resistance to withstand the extreme
environment of a single piece shaft and rotor. Shafts and rotors have conventionally
been constructed as separate pieces and mechanically joined, due to the need to
replace graphite based rotor which have shortened life spans due to oxidation of the
graphite material.
The unique properties of the composite material (improved impact and flexural
strength) enable a one piece shaft- rotor design to be created which eliminates the
shaft- impeller connection, which is often a mechanical type connection (i.e. the
shaft has a threaded end which screwed into a cavity in the rotor). The connection
point is prone to failure with stresses concentrating at the connection point, which
typically comprises a right or angle (e.g. 90°) joint.

Claims
PCT/GB2021/051719
1. A composite material comprising wollastonite fibres embedded within a
ceramic matrix, wherein the wollastonite fibres are bonded to the ceramic
matrix by a glassy bonding phase comprising a glass component comprising
at least 80 wt% of oxides of calcium, silicon and aluminium.
2. The composite material according to claim 1, wherein glassy bonding phase
further comprises mullite.
3. The composite material according to claim 2, wherein the mullite is in the form
of crystallite particles embedded into the glass component.
4. The composite material according to any one of claims 1 to 3, wherein the
wollastonite fibres are bondedly embedded into the glassy bonding phase.
5. The composite material according to any one of the preceding claims,
wherein the composite material comprises:
0.5 to 20 wt% wollastonite fibres;
0.5 to 40 wt% glassy bonding phase;
0 to 50 wt% ceramic matrix;
0 to 50 wt% carbon material; and
0 to 15 wt% other additives,
wherein the sum of the abovementioned components is greater than 90 wt%
of the composite material.
6. The composite material of claim 5, wherein the carbon material comprises
graphite and/or amorphous carbon.
7. The composite material of claim 6, wherein the ceramic matrix comprises a
beta silicon carbide and/or alpha silicon carbide.
8. The composite material according to any one of the preceding claims, wherein
the glass component comprises greater than 90 wt% of oxides of calcium,
aluminium and silicon.
9. The composite material according to any one of the preceding claims, wherein
the proportion of oxides of aluminium and silica in the glass component is at
least 60 wt%.
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1 0. The composite material according to any one of the preceding claims, wherein
there is a gradient of calcium concentration across the glass component of the
glassy bonding phase.
11. The composite material according to any one of the preceding claims, wherein
the concentration of calcium is higher in the glass component proximal to the
wollastonite fibre relative to the glass component distal to the wollastonite
fibres.
12. The composite material according to any one claims 6 to 11, wherein the
glassy bonding phase bonds the wollastonite fibre, ceramic matrix and/or
graphite, if present, together.
13. The composite material according any one of the claims 5 to 1 0, wherein the
ceramic matrix comprises one or more of silica; alumina; carbides of Si, Ti, W,
Ta, Nb, Zr, Hf, V, Cr, Mo; silicon nitride; magnesia; zirconia; boron nitride; and
aluminium nitride.
14. The composite material according to any one of the preceding claims,
comprising:
0.5 to 10 wt% wollastonite fibres;
0.5 to 30 wt% glassy bonding phase;
20 to 50 wt% silicon carbide;
5.0 to 50 wt% graphite; and
1 to 15 wt% other additives,
wherein the sum of the abovementioned components is greater than 90 wt%
of the composite material.
15. An apparatus for processing molten metal comprising the composite material
as defined in any one of claims 1 to 14.
16. The apparatus of claim 15, wherein the apparatus is selected from the group
consisting of a pump, a degasser, a flux injector and a scrap submergence
device.
17. The apparatus according to claim 16, wherein the apparatus is a degasser
and a shaft and/or a rotor of the degasser comprises the composite material.
18. The apparatus according to claim 17, wherein the apparatus comprises a
shaft and a rotor, and wherein the shaft and the rotor are of one piece
construction.
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19. The apparatus according to claim 18, wherein the shaft gradually increases in
diameter proximal to the rotor.
20. The apparatus according to claim 19, wherein interface angle between the
shaft and rotor is at least 1 00 °.
21. A process for producing the composite material of any one of claims 1 to 14 or
the apparatus of any one of claims 15 to 20 comprising the steps of:
a. Providing a precursor composite mixture comprising wollastonite fibres,
a ceramic matrix and a glassy bonding phase or precursors thereof;
b. Depositing the mixture into a mould; and
c. Sintering the mixture at a temperature of at least 800°C for sufficient
time to partially transform the wollastonite fibres into the glassy
bonding phase.
22. The process according to claim 21, wherein the mixture further comprises
graphite and/or an organic resin.
23. The process according to claim 21 or 22, wherein the mixture is sintered at a
temperature of at least 1 000°C and between 10 and 90 wt% of the
wollastonite fibres are converted to the glassy bonding phase during the
sintering step.
24. The process according to claim 23, wherein the mixture is sintered for
sufficient time and at a sufficient temperature to form mullite within the glassy
bonding phase.
25. The process according to any one of claims 21 to 24, wherein the mixture is
iso-statically pressed prior to the sintering step.
26. Use of the composite material according to any one of claims 1 to 14 in the
processing of a molten metal.
27. Use according to claim 26, wherein the composite material is exposed to
temperatures less than 1 000°C.