FIELD OF THE INVENTION
The present invention relates to channel inductors of
channel induction furnaces .
In particular, the present invention relates to
channel liners of channel inductors .
The present invention also relates to channel
inductor furnaces .
BACKGROUND TO THE INVENTION
Channel induction furnaces are used in industries for
melting a metal (which term includes metal alloys) and
maintaining the metal in a molten state. For example,
channel induction furnaces are used in galvanising and
foundry industries for melting Zn-containing alloys and
Al-containing alloys, including Al/Zn-containing alloys,
and maintaining the alloys in a molten state .
A known channel induction furnace comprises (a) a
steel shell, (b) a lining of a refractory material, such
as an aluminosilicate , internally of the shell, c ) a pot
for containing a bath of molten metal that is defined by
the refractory- lined shell, and (d) one or more than one
channel inductor for heating metal that is connected to
the shell and in fluid communication with the pot via a
throat that extends through the refractory-lined shell to
an inlet in the channel inductor.
The channel inductor comprises (i) a steel shell,
i i ) a lining of a refractory material, such as an
aluminosilicate, (iii) a channel defined by the
refractory-lined shell that forms a path for molten metal
to flow from the pot through the channel and back into the
pot, and iv ) an electromagnetic coil which generates an
electromagnetic field. At any given time during the
operation of a channel induction furnace, molten metal in
the channel of the channel inductor becomes a secondary
circuit of a transformer and is heated and kept molten by
currents induced by the electromagnetic field. The
channel inductor is a bolt-on assembly on the shell of a
channel induction rnace . The refractory material that
forms the lining is selected to accommodate a range of
specific mechanical requirements, thermal insulation
requirements, and resistance to chemical attack by molten
metal . These requirements are competing requirements to a
certain extent in the sense of needing different material
properties and hence the selection of the refractory
material tends to be a compromise .
Channel inductors have a limited life when exposed to
molten metals such as Zn-containing and Al-containing
alloys and typically f il in the following modes :
• Cracking of the refractory material, particularly along
central planes of channel inductors, during heat-up,
dry-out, or operation, and subsequent penetration of Zn
and/or Al metal or Zn vapours into the cracks which
extend the cracks , ultimately resulting in a metal leak
f om the channel inductors .
• Additionally, in the case of Al-containing alloys, by
reduction of Si0 2 in the refractory material by Al,
thereby forming AI2O3 and Si, with an associated
reduction in the volume of the refractory material and
penetration and/or spalling of the refractory material.
• Additionally, blocking due to adherence of corundum
growth within the channel, which is compounded by pieces
of altered refractory or dross from a pre-melt pot main
area entering the channel .
Typically, the life of channel inductors in Alcontaining
alloys is 6-24 months and is one of the main
reasons for metal coating line shut-downs .
The applicant is developing a new inductor having
greater reliability and, more particularly, less tendency
to fail due to cracking.
The new inductor is described in International
publication WO2011/120079 in the name of the applicant.
The channel inductor that is described and claimed in
the International publication comprises (a) a channel
liner that is formed from a refractory material that is
resistant to chemical attack by the molten metal in the
channel and is the only material of the channel inductor
that is in direct contact with the molten metal and (b) a
back-up liner that supports the channel liner and is
formed from a refractory material that is optimal for
thermal insulation material properties and mechanical
strength properties, such that the integrity of the
channel liner and is not compromised during heat-up, dryout,
or operation of the channel induction furnace.
A channel inductor made with a channel liner and a
back-up liner in accordance with the invention of the
International publication was found to have issues with
cracking when used on a manufacturing plant of the
applicant for coating steel strip with Zincalume® molten
metal .
The applicant has investigated the causes of the
cracks and the present invention was made in the
investigation .
The above discussion is not intended to be a
statement of the common general knowledge in Australia and
elsewhere .
SUMMARY OF THE INVENTION
The applicant carried out a post mortem on the
channel inductor made in accordance with the invention of
the International publication that was used on the
manufacturing plant of the applicant and made the
following findings, which are the basis of the present
invention .
1 .It is beneficial to select the refractory material of
the channel liner so that there is a chemical
reaction between the material and the molten metal in
the furnace that results in the channel liner
becoming more resistant to further penetration by
molten metal and resistant to blockages developing
within the channel. Typically, the chemical reaction
results in the formation of a denser phase in the
channel liner. Typically, the original material
includes silicon carbide blended with a corundum
mineral when the molten material is an Al/Zncontaining
alloy that contains sodium. Sodium may
act as a catalyst for the chemical reaction.
.It is beneficial to select the material for the backup
liner to be capable of absorbing stresses due to
expansion and movement of the channel liner .
Typically, the materials selection also includes
selecting a material that is capable of resisting
cracking due to thermal stress throughout the
operating temperature range and also resistant to
some reaction with the alloy which may reach the back
up liner . Therefore , selection of material with the
appropriate sintering characteristics and resistance
to attack by the molten alloy is an important
consideration. Typically, the material may be a dry
vibratory material such as a Dri-Vibe® composite
materials produced and marketed by Allied Minerals
Products, nc , for example as described in European
patent 1603850 in the name of that company.
Typically, the Dri-Vibe® materials may be metal
fibre, which typically includes metal fibre
reinforced aluminosilicate refractory composite
materials, with the refractory material component of
the composite containing 60-95 wt.% alumina,
preferably 60-70 wt.% alumina and 20-35 wt.%
In broad terms , the present invention provides a
channel inductor of a channel induction furnace, the
channel inductor comprising (a) a channel liner and (b) a
back-up liner that supports the channel liner such that
the integrity of the channel liner is not compromised
during heat-up, dry-out, or operation of the channel
induction furnace .
In the context of item 1 above, the material of the
channel liner may be selected so that there is a chemical
reaction between the material and the molten metal in the
furnace that results in the channel liner becoming more
resistant to further penetration by molten metal and
resistant to the development of blockages due to corundum
growth within the channel . The material may be otherwise
as described in item 1 .
In the context of item 2 above, the material of the
back-up liner may be selected to be capable of absorbing
stresses due to expansion and movement of the channel
liner. The material may be otherwise as described in
item 2 .
The channel liner may be any suitable shape .
The channel liner may be an elongate unit with the
channel being in the shape of a single U ("single loop
inductor") . More particularly, the channel may comprise
two arms extending from a base of the channel, with a
molten metal inlet in an end of one arm of the channel and
a molten metal outlet in an end of the other arm of the
channel, whereby molten metal can flow through one arm to
the base and through the base to the other arm and along
the other arm.
The channel liner may be an elongate unit with the
channel being in the shape of a double . More
particularly, the channel may comprise three arms
extending from a base of the channel that interconnects
the arms, with a molten metal inlet in an end of a central
arm of the channel and molten metal outlets in the ends of
the outer arms of the channel , whereby molten metal can
flow through the inner arm to the base and outwardly
through the base to the outer arms and along the outer
arms .
The channel liner may have a top wall, with the inlet
and the outlet (s) formed in the top wall, and with the
mounting flange extending outwardly f om the top wall .
The channel liner may comprise a side wall that
extends from a perimeter of the top wall, with the
mounting flange extending outwardly from an upper edge of
the side wall. This arrangement defines a vestibule or a
forebay .
The present invention also provides a channel
inductor furnace that comprises :
(a) a steel shell,
a lining of a refractory material internally
the shell ,
(c) a pot for containing a pool of molten metal that
is defined by the refractory-lined shell, and
one or more than one of the above-described
channel inductor for heating a metal that is
connected to the shell and in fluid
communication with the pot via a throat that
extends through the shell and the refractory
lining to the inlet in the channel inductor .
The molten metal may be selected from the group
comprising Zn-containing alloys and Al-containing alloys,
including Al/Zn-containing alloys. These alloys are not
confined to Al and Zn and may include other elements such
as Ca.
DESCRIPTION OF THE DRAWINGS
The present invention is described further by way of
example with reference to the accompanying drawings, of
which:
Figure 1 is a vertical cross -section through one
embodiment of a channel inductor furnace in accordance
with the present invention that includes one embodiment of
a channel inductor in accordance with the present
invention ;
Figure 2 is a vertical cross-section through one
embodiment of a channel inductor in accordance with the
present invention;
Figure 3 is a g rap h of the temperatures of the
channel liner and the back-up liner over the first 50 days
of service of the channel inductor made in accordance with
the invention of the International publication that was
used on the manufacturing plant of the applicant. This
Figure also shows the flat inductance ratio trend which is
a measure of a lack of channel blockage in the inductor.
DESCRIPTION OF EMBODIMENTS OF THE INVENTION
Figures 1 and 2 are the Figures of the abovementioned
International publication of the applicant.
Figure 1 is a cross-section of the main components of
a channel inductor furnace 3 for pre-melting an Al/Zn
alloy for use in a metal coating line for steel strip. It
is noted that the present invention is not limited to this
end-use and may be used as part of any suitable channel
induction furnace and for any suitable end-use
application.
The channel inductor furnace 3 shown in Figure 1
comprises a pot defined by an outer steel shell 27 and an
inner lining 29 of a refractory material, such as an
aluminosilicate . In use, the pot contains a bath (not
shown) of a molten Al/Zn alloy. The furnace 3 also
includes two channel inductors 31 that are connected to
opposite side walls of the steel shell 27 and are in fluid
communication with the bath via respective throats 33 . In
use, molten Al/Zn alloy flows from the bath and into and
through the channel inductors 31 and is heated by the
channel inductors 31 .
The drawing of the channel inductor 33 in Figure 2
a vertical cross -section in order to show the component;
of the inductor that are particularly relevant to the
present invention. In addition, in order to make these
components as clear as possible, the electromagnetic coil
of the inductor 33 is not included in the openings 1 in
the drawing.
The channel inductor 33 comprises :
(a) a channel liner, generally identified by the
numeral 5 ; and
(b) a channel liner support assembly that supports
the channel liner.
The channel liner 5 is a single piece elongate unit
that defines the above-mentioned openings 1 and a double
" shaped channel for molten Al/Zn alloy to flow through
the channel inductor . The channel comprises a base and
three parallel arms 9 extending from the base. The upper
end of the central arm of the channel is an inlet 15 for
molten Al/Zn alloy and the upper ends of the outer arms of
the channel are outlets 17 for molten Al/Zn alloy. The
base of the channel is defined by a base section 7 of the
channel liner 5 and the arms of the channel are defined by
upstanding sections 9 of the channel liner 5 . These
sections 7 , 9 are thin-walled, hollow sections. The
channel liner 5 has a top wall 11, and the inlet 15 and
the outlets 17 for molten Al/Zn alloy flow are formed in
the top wall 11 . The channel liner 5 also comprises a
side wall 21 that extends around the perimeter of the top
wall 11 and a flange 19 that extends outwardly from the
side wall 21 . The top wall 11 and the side wall 21 define
a vestibule or forebay. The flange 19 is provided to
mount the channel liner 5 to a refractory material lining
(not shown) that defines a pot throat (not shown) of a pot
(not shown) of the channel inductor furnace, whereby
direct contact between molten Al/Zn alloy and the channel
inductor is limited to contact with the channel liner 5
onl .
The channel liner support assembly comprises (a) an
outer steel shell 23 and (b) a back-up lining 25. The
back-up lining 25 is not shown specifically in Figure 2 in
order to simplify the drawing. As indicated by the
numeral 25 and the drawing line in Figure 2 , the back-up
lining material fills the space between the shell 23 and
the channel liner 5 .
The present invention relates to the materials
selection for the materials from which the channel liner 5
and the back-up lining 25 are made.
As indicated above, a channel inductor made in
accordance with the above-mentioned International
publication of the applicant was found to have issues with
cracking when used on a manufacturing plant of the
applicant.
The applicant carried out a post-mortem on the
inductor and key points that emerged from the post mortem
include the following points :
• There was significant reaction between the material
of the channel liner 5 and the molten metal .
• This reaction progressed rapidly due to the presence
of trace amounts of sodium in the molten metal that
acted as a catalyst.
• The level of sodium was measured to increase in the
material of the channel liner 5 .
• The reaction was predominately with the SiC aggregate
in the refractory material of the channel liner 5 .
• One advantage of the SiC is that it produces less
thermal stress in the composite structure on initial
heating due to its lower coefficient of expansion in
comparison to the normal high alumina material .
• As the SiC was consumed the coefficient of expansion
of the material of the channel liner 5 increased -
which may help in making a tighter structure at the
hot face of the channel liner.
The resultant hot face layer that developed in
service resisted corundum build up/growth in the bore
of the channel inductor.
It is important for a suitable back-up lining 25 to
be selected to support the channel liner 5 .
The applicant made the following findings .
It is beneficial to select the refractory material of
the channel liner so that there is a chemical
reaction between the material and the molten metal in
the furnace that results in the channel liner
becoming more resistant to further penetration by
molten metal and more resistant to channel blockage.
Typically, the chemical reaction results in the
formation of a denser phase in the channel liner.
Typically, the material includes a source of silicon
such as Silicon carbide when the molten material is
an Al/Zn-containing alloy that contains trace sodium.
Sodium may act as a catalyst for the chemical
reaction.
It is beneficial to select the material for the back¬
up liner to be capable of absorbing stresses due to
expansion and movement of the channel liner.
Typically, the materials selection also includes
selecting a material that is capable of resisting
cracking due to thermal stress throughout the
operating temperature range and also resistant to
some reaction with the alloy which may reach the back
up liner. Therefore, selection of material with the
appropriate sintering characteristics and resistance
to attack by the molten alloy is important.
Typically, the material is a dry vibratory material
metal fibre, such as steel fibre, reinforced
aluminosilicate refractory composite materials, with
the refractory material component of the composite
containing 60-95 wt.% alumina, preferably 60-70 wt.%
alumina and 20-35 wt.% silica
Channel liner material selection
One observation in the post-mortem that was not
expected from an initial laboratory assessment of the
channel liner material prior to using the channel inductor
in the manufacturing plant was the level of apparent
reaction and densif ication of the channel liner material .
In laboratory testing of the channel liner material,
minimal reaction was observed. In the case of the channel
inductor used on the manufacturing plant, the majority of
the channel liner 5 was penetrated by Zincalume® molten
metal used on the plant to give a darker denser looking
appearance . In the few locations where there was no
penetration, the appearance of a cut face showed a porous
texture suffering from grain pluck-out during sectioning
as if the bond strength had been reduced.
The post-mortem indicated that there had been a
reduction in the Si02 content of the channel liner material
and an increase in sodium and zinc phases in the channel
liner material. This indicates that there was a migration
of Na and Zn vapour through the channel liner 5 , ahead of
the penetration of the Zincalume® molten metal, and these
sodium and zinc phases lead to a reduction of silicate
binding phase in the channel liner 5 which then aided
further penetration.
The chemical analysis results for the penetrated,
i.e. dense, zone of the channel liner 5 indicate a marked
increase in I O3 , ZnO, SiO and 0 . The component that
was significantly reduced was the SiC level . These changes
in the dense phase are also reinforced by the XRD
comparison. The XRD is semi-quantitative and must not be
considered as an accurate number but it is a good
indication of the species that are present in the
penetrated liner and their comparative levels.
Some of the penetration phase was still in a metallic
form with a combination of aluminium and aluminium silicon
alloy. This was also evident in a microscopic examination.
The presence of an Al/Sin alloy suggests there had been a
chemical reaction between the refractory silicate phases
of the channel liner 5 or reduction of the fine SiC in the
matrix of the channel liner 5 to provide a source of
silicon .
Both the XRD and chemical analysis indicate a
reduction in the percentage of SiC in the dense phase.
This may be due to an attack on the SiC or a dilution
effect from the penetration into the refractory or a
combination of both. There was some evidence that the
dilution effect is a factor. This evidence includes a
microscopic examination where the majority of larger SiC
grains appeared to be unaltered and the presence of
aluminium metal in the porosity of the refractory was an
additional mass that would dilute the percentage of the
original components. However, there was also some
indication of a reaction occurring with some glassy phase
surrounding some SiC grains on the outer surfaces. Also,
minor components of the channel liner material such as Ba,
Ti and Ca did not show a dilution effect in the altered
channel liner and this supports a view that there was some
reduction in the level of SiC through reaction.
In general , even though the channel liner 5 was
penetrated by Zincalume® molten metal it become a denser
SiC/Al203 containing composite with the reaction products
and penetration metal making this composite even more
compatible with the contact metal. One further
encouraging observation was the lack of corundum growth or
any other blockages in the channels and this suggests
there is potential for this channel liner 5 to provide
less problems with inductor blockages .
Figure 3 is a graph of the temperatures of the
channel liner 5 and the back-up lining 25 over the first
50 days of service of the channel inductor that as used on
the manufacturing plant. The increase in temperature in
the early stages after start-up indicates that the channel
liner material was being penetrated and there were
reactions with the liner material which ultimately
produced a more stable phase, which was relatively stable
thereafter .
Figure 3 also shows the conductance ratio was very
stable for this inductor. The conductance ratio is a
measure that indicates negligible channel blockage has
occurred .
Dri-Vibe® composite material test work
The applicant carried out test work on Dri-Vibe®
composite materials to evaluate the suitability of the
materials. The test work is described below.
1 .Introduction
The applicant tested three Dri-Vibe composite
materials by exposing sample cups made from the materials
to molten Zincalume® Al/Zn-containing alloy.
The three sample composite materials were supplied by
Allied Minerals; Product A , Product B and Product C .
s
Product A and Product B materials are both mullitebased,
metal fibre containing composite materials. Product
C material is a fused alumina-based, metal fibrecontaining
composite material .
2 .Sample Details :
• Product A : Two cups prepared by Allied, made by
Allied with a Matripump 80AC castable back-up.
• Product B : Two cups prepared by Allied, made by
Allied with a Matripump 80AC castable back-up.
• Product C : One cup prepared by Allied, made by Allied
with a Matripump 80AC castable back-up.
3 .Testwork :
The samples were dried overnight.
The Zincalume metal alloy were cut to length and at
least 5 cut sections were placed into each cup.
All of the cups were placed into the furnace .
The furnace was fired @ 5°C/minute to 600°C, then @
2°C/min to 830°C, and then hold for 168 hours.
• The furnace was allowed to cool and the samples were
removed.
• The 5 cup samples were cut in half.
• Photographs of the cut faces were taken and evaluated
- one half with the metal in place and the other half
with the metal removed
Table 1 : Summary of reactions in the cups .
4 .Discussion
On the basis of the tests , the Product B is not
suitable for use as a back-up liner material in a channel
inductor as it was heavily penetrated by Zincalume® metal .
Product C showed no reaction and would be suitable as
a back-up liner 25 from a penetration resistance
perspective. The higher alumina level in this material
would give the material a higher thermal conductivity and
so higher heat transfer to the coil area. This material
also has a tighter texture and greater strength as it was
supplied.
Product A performed well in the contact tests with
both the Zincalume® metal It was also more friable in
nature at the end of the test and is therefore likely to
resist cracking from thermal stress and absorb stresses
due to the expansion and movement of the channel liner .
Because the material is mullite-based rather than aluminabased,
it has a lower thermal conductivity than the
Product C material and so will help to reduce the transfer
of heat to the coil zones of channel inductors .
Many modifications may be made to the embodiment of
the present invention described above without departing
from the spirit and scope of the invention.
By way of example, the present invention is not
confined to the particular shape of the channel inductor 3
shown in the drawing.
By way of further example, the present invention is
not confined to a double " channel liner 5 and, by way
of example, also extends to single "U" channel liners 5 .
By way of further example, the present invention is
not confined to a channel liner 5 that is formed as a
single piece unit.
By way of a further example, present invention may be
used as is or modified slightly for alloys that may
contain other key elements such as magnesium

CLAIMS
1 . A channel inductor of a channel induction furnace ,
the channel inductor comprising (a) a channel liner and
(b) a back-up liner that supports the channel liner such
that the integrity of the channel liner is not compromised
during heat-up, dry-out, or operation of the channel
induction furnace .
2 . The channel inductor defined in claim 1 wherein the
material of the channel liner is selected so that there is
a chemical reaction between the material and the molten
metal in the furnace that results in the channel liner
becoming more resistant to penetration by molten metal and
more resistant to the development of channel blockages.
3 . The channel inductor defined in claim 2 wherein the
chemical reaction results in the formation of a denser
phase in the channel liner.
. The channel inductor defined in claim 2 or claim 3
wherein the material of the channel liner includes a
source of silicon such as silicon carbide when the molten
material is an Al/Zn-containing alloy that contains
sodium .
5 . The channel inductor defined in any one of the
preceding claims wherein the material of the back-up liner
is selected to be capable of absorbing stresses due to
expansion and movement of the channel liner.
6 . The channel inductor defined in claim 5 wherein the
materials selection for the back-up liner also includes
selecting a material that is capable of resisting cracking
due to thermal stress.
7. The channel inductor defined in claim 5 or claim 6
wherein the material of the back-up liner is a dry
vibratory material .
8. The channel inductor defined in any one of claims 5
to 7 wherein the material of the back-up lining is an
aluminosilicate refractory composite material which may or
may not have metal fibre, such as steel fibre,
reinforcing .
. The channel inductor defined in claim 8 wherein the
refractory material component of the composite contains
60-95 wt.% alumina.
10. The channel inductor defined in claim 8 wherein the
refractory material component of the composite contains
60-70 wt.% alumina and 20-35 wt.% silica.
11 . The channel inductor defined in any one of the
preceding claims wherein the channel liner is an elongate
unit with the channel being in the shape of a single
("single loop inductor") .
12 . The channel inductor defined in any one of claims 1
to 10 wherein the channel liner is an elongate unit with
the channel being in the shape of a double .
13. A channel inductor furnace that comprises:
(a) a steel shell,
(b) a lining of a refractory material internally of
the shell ,
(c) a pot for containing a pool of molten metal that
is defined by the refractory-lined shell, and
one or more than one of the channel inductor for
heating a metal that is defined in any one of
the preceding claims and connected to the shell
and in fluid communication with the pot via a
throat that extends through the shell and the
refractory lining to the inlet in the channel
inductor .