Note: Descriptions are shown in the official language in which they were submitted.
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ZIRCON AND MgO PREHEATABLE INSULATING
REFRACTORY I.INERS AND METHODS OF USE T~EREOF
Background of the Invention
In the metal casting industry, it i9 custo-
mary to employ metal casting vessels, such as tun-
dishes and ladles, etc., as means which se~ve to
transfer various molten metals. Because of the
corrosive nature of the liquid metals and the slags,
and to prevent heat loss and premature solidification
of the metals, the metal casting vessels are prevented
~rom contacting with such metals and/or slags by
~ining the vessels with heat~insulating refractory
boards. Additionally,: a trend in the industry is to
preheat these lined vessels to minimize heat loss from
the initial molten metals po~red through the vessels
: at the start of casting, and to remove, if possible,
all sources of hydrogen derived from, fo~ instance,
moisture~(H2O) and/or organic compounds embodied in
the refractory linings, which can be dissolved by and :~
: incor~orated into the llquid metaIs passing through
the lined ve sel5. In partlcular, when low hydrogen
~grades of~steel are being cast,:it is especially ~`~
desixabI~to~preheat:such refractory lined ~essels to
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remove all possible hydrogen source~ which can serve
only to contaminate the liquid metals.
In addition to driving o~f all source~ of
hydrogen, it is desirable to minimize the amount of
unstable oxides, such as silica, which are present in
the heat-insulating refractory boards. Th~se unstable
oxides, and in particular silica, can react with
various elements contained in the molten metals and
l~ad to the formation of oxide inclusions in the
liquid metal:,. For example, some of the undesirable
reactions of silica with various elements leading to
the formation of oxide impurities are as follows:
SiO2 ~ ~2Mn] 2MnO + [Sil
3SiO2 + [4Al] 2 3 [ ]
1~ SiO2 + [2Fe] 2FeO + [Si~
The MnO and FeO formed can .urther attack
the silica in the heat-insulating refractory linings
by forming low melting liquid oxide slags at metal
casting temperatures.
Unfortunately, the dilemma facing the
metal~making industry concerning the addition of
unstable oxides which act to lower sintering and
solidus temperatures versus the use of pure, stable
refractory oxides for refractoriness and molten metal
purity is extremely~difficul~ to overcome, especially
with~preheatable heat-insulating refractory ~oards
which must sinter and develop sufficient hot stxength
for castin9 at sub-casting~temperatures which can be
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sometimes as much as about l,000F~ lo~er than casting
temperatures.
Another problem p~esently associated with
the heat-insulating refractory linings ~or the metal
casting vessels involves shrinkage of such linings
upon heating. One solution within the metal-making
industry to this problem is to fire such linings at
temperatures higher than those that are expected
during use, so that shrinkage during use can be
avoided. Agc?in, since preheating can occur at
temperatures as low as about 1000F. below casting,
this presents a further problem with the current
preheatable boards.
In the case of cold tundish practice, i.e.,
the pouring of a molten metal into a tundish without
first preheating it, the temperature increases as the
molten metal enters th~ tundish and decomposes the
organic binder under reducing conditions forming
carbon bonds. The carbon ~onds hold the refractory
grain together giving the tundish lining the required
hot strength. As the carbon bonds are dissolved by
the molten metal and oxidized, sintering of the
refractory grain occurs over time. Thus, in cold
tundish practice, the organic binder decomposition
gives carbon bonds~allowing the use of more stsble
refraotory~oxides which sinter~more slowly and~at
hlgher temperatures, i.e., MgO and silica.
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Nevertheless, in casting, the temperature
associated with cold tundish linings increases quickly
to that of casting such that by the time the carbon
bonds are completely disintegrated, the linings are
still held together by the formation of ceramic bonds
resulting from the sintering of the refractory oxides.
Because preheating may sometimes last up to, for
example, 12 hours before casting actually beginq, the
linings utilized in cold tundish practice are unsuited
for preheating use. The problem basically is due to
oxidation of the carbon bonds within the linings at
preheat temperatures which are generally too low for
ceramic bonds to form resulting in usually soft and
weak linings which will collapse due to their own
weight or wash away as the molten metal enters the
vessels~
In the past, several attempts or approaches
without success have been made to overcome the prob-
lems presently associated with preheatable heat-
insulating refractory boards for metal casting
vessels. For example, large amounts of low-melting
glass formers, such as borax, have been incorporated
into the linings in an effort to stick the refraotory
grain together at preheat temperatures. Unfortu-
nately, the glassy or liquid bonds allow the preheated
linings to be deformed easily at preheat and casting
temperatures after the carbon bonds burn out.
Further, the~preheated lininqs generally fail to
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develop the requisite hot strength for casting when
preheated at preheat temperatures for extended periods
of time prior to the start of ca~ting. A9 a result,
it has generally been found that at both preheat and
S casting temperatures, the liners would collapse or
wash away.
An additional problem associated with the
use of low-melting glass formers is that they are
generally thermodynamically unstable to, for instance,
ferrous alloys. In the case of B2O3, it can be
reduced resulting in the incorporation of boron into
the molten metals, such as ferrous alloys, that can
alter the properties of the ferrous alloys as well as
produce oxide inclusions.
Other types of preheatable lining3 are those
made with the addition of about 5% to about 20% quartz
(silica) for the purpose of bonding with MgO. Unfortu-
nately, these preheatable boaxds have two serious
drawbacks~ First, the addition of quartz or other
silica forms utilized by these liners is sufficiently
high enough to cause formation of oxide inclusions by
reaction of the molten~metals with the linings. In
order to minimize liquid metal contamination, the
metal manufacturers specify that the quartz or free
silica levels should be as low as possible. 5econdly,
presence of finely divided quartz or free crystalline
silica ca~ become airborne when, or ~nstance, the
boards are removed from the vessels after use
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presenting health hazards to the metal manufacturers
and workers.
Examples of still other types of preheatable
linings are those which contain about 85~-90~
magnesite and about 5~ to about 10~ calcium flouride.
The calcium flouride is typical of a strong fluxing
agent which reacts with oxides to develop a liquid
bonding phase at prehea~ temperatures. ~hese linings,
like those utiliziny the low-melting glass formers,
develop a lic~uid bonding phase when the organic binder
is burnt out at, for instance, 1900F. and up (preheat
temperatures). The linings, unfortunately, are also
very soft and weak at such temperatures after the
organic binder is oxidized. Thus, as with the pre-
heatable linings containing low melting glass formers,
these preheatable linings fail to develop the suffi-
cient hot strength for casting when heated at preheat
temperatures for typical preheat periods of time.
In summary, previous attempts or approaches
have been made to develop suitable preheatable insu-
lating refractory liners. Heretofore no satisfactory
preheatable heat-insulating refractory liner has ~een
developed which can overcome the problems afore-
mentioned. Basically, the past preheatable liners
fall into two categories: thos:e in which quartz i~
added in unacceptable amounts to form a ceramic bond;
and~those in~which low me}ting materials are added to
develop a liquid phase at preheat temperature~ as an
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unsatisfactory attempt to protect the carbon bonds
from oxidation and to ~ond the refractory grain and
promote sintering.
In other words, all of the preheatable
S heat-insulating refractory liners provided hitherto
invariably necessarily lack some of the key fundament
al qualities required to develop sufficient hot
strength at preheatablP or sub-casting temperatures
for the typical range in which preheating times occur
Consequently, there are strong commercial needs for
preheatable heat-insulating refractory liners for
metal casting ves~els that can initiate the develop-
ment of hot strength at preheat temperatures, that can
withstand preheat temperatures for extended periods of
preheat time, that can withstand molten metal erosion
and corrosion, that will not experience substantial
shrinkage on use, and that has minimum amounts of free
silica and hydrogen content.
Summary of the Inventlon
In brief, the present invention seeks to
alleviate the above-mentioned problems and shortcom-
ings of the present state of the art through the
discovery of novel preheatable molded refractory
insulating liners and methods of use thereof for
linin~ metal casting vessels intended to contain, for
instance, ferrous alloys~, such as steel and in par~
ticular low hydrogen grade of steel. In a preferred
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embodiment, the present invention is directed to a
preheatable molded refractory insulating liner for a
casting vessel suitable for developing sufficient hot
strength at vessel preheat and metal casting tempera-
tures which ar~ in the range of about 1900F. to about
3000F. comprising a liner structure of predetermined
shape, the liner structure comprising a molded uniform
mixture containing a particulate refractory component
comprised of zircon and MgO reEractory grain and a
binder for the component to maintain the predetermined
shape at least prior to the preheat temperatures
wherein the zircon and MgO refractory grain are in
amounts proportioned in the liner structure to facili-
tate the formation of fosterite bonding which results
in increased hot strength at vessel preheat and metal
casting temperatures when such a liner structure is
heated for a sufficient period of time. Typically, in
the industry preheat times can be from about one
half-hour and extend to about twelve hours or more.
Preferably, the particulate reractory ~omponent
comprises a~out;75% to about 98.5% by weight of the
liner a grain mixture of zircon and MgO refractory
qrain being in a ratio from about 1:1.5 to about 1:24,
respectively. The MgO refractory grain may be derived
from, for instance, natural, seawater or brine
magnesite, periclase grain, or other suitable sources,
or mixtures thereof:plus, if any, incidental impuri-
tie~. The MgO refractory grain and the:zircon are
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the main essential constituenks responsible for the
development o the hot strength at the vessel preheat
and metal casting temperatures. At such temperatures,
it is believed that the MgO refractory grain and
zircon react as follows to form the fosterite bonds
and zirconia needed for hot strength and refrac-
torinQss:
2MgO + Zro2 SiO4 Mg2Si4 + Zr2
It is thought that the formation o zirconia and
fosterite enhances the desirable hot strength and
corrosion resistance to the molten metals, such as
ferrous alloys, and slag. It should be appreciated,
however, that the formation of fosterite and zirconia
is believed to occur over the entire range of vessel
preheat and metal casting temperatures which are on
the order of about 1900F. to about 3000F. More
particularly, the vessel preheat temperatures, for
instance, can range from, for example, about~1900F.
to about 2400F. whereas, in the case of ferrous
,
alloys, the metal casting temperatures are gsnerally
at about 2800F. or above.
In a further feature of this invention, the
particulate refractory component may contain in
addLtion to~the~zircon and MgO refractory grain a
suitable refractory filler in acceptable amount , such
:as~olivine or~zirconia.
:: ~ Therefore,:the:new and vastly improved
~preheat-ble~liner structures~ provide means for
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developing effectively the increased hot strength
needed for casting molten metals at ~oth vessel
preheat and metal casting temperatures. Further, the
unique preheatable liners possess the nec~ssary hot
strength during the range of casting temperatures that
are experienced in the metal making industry and
especially the ferrous alloy making industries. In
effect, a feature of the present invention is to
provide preheatable molded refractory insulating
liners that possess corrosion-erosion resistance to
metal making environments which is greatly superior to
that of the common commercial preheatable refractories
used heretofore. Thus, the pr~sent invention provides
a solution to the art that has long sought suitable
liners for preheating and makes it now possible to
preheat casting vessels lined with the preheatable
refractory insulating liners of this invention for
extended periods of time prior to the start of cast-
ing. Further, it lS ~ound that extended preheating
advantageously enhances the development of the desired
increased hot strength in the liners of the present
lnvention.
Magnesite and zlrco~ refractory composltions
for fabricating refractory bricks havlng utility
incLdent to th~ glass and metal making industries have
been heretofore known in the art. Examples of refrac-
tory bricks formu1ated from ~uch compositions can be
found in U.S. Patent No. 3,303,032, ~.5. Patent No.
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3,192,059 and U.S. Patent No. 3,528,830~ It also has
been heretofore known, aq described in U.S. Patent No.
3,303,032, that mixtures of stabilized zirconia,
fosterite and periclase have re~ulted only a~ter
prolonged firing of such bricks at temperature3
generally far in excess of those encountered at low
vessel preheat temperatures.
In the literature, it has been reported that
the decomposition of zircon to produce zirconia and
amorphous silica generally occurs at a~out 1600C.
(2980F.). Zirconia silicate (Zircon). IN:
Ryshkewitch, E. ~Ed~; Oxide Ceramics. Academic Press
pp. 399~406 11960). In addition, it has been reported
tha~ basic su~stances such as MgO decompose zircon at
about 1240C. (2264F.~. W. Eitel, "Silica Melt
Equilibria", Rutgers University Pres~, New Brunswick,
N.J. p. 17 (1951). This agrees with R. F. Rea, J~: Am.
Ceramic Soc., 22, g5 (1929) who reported MgO to be
among several oxides which react with zircon to form
melting slag mixtures. Thus, the early literature and
U.S. Patents teach that zircon is decomposed by MgO at
abou~ 2264P~ and develops a fosterite matris only
aSter prolonged ~iring at temperatures generally far
in excess of the usual low vessel preheat temperature~
associated with the preheatable liners presently
availabIe. :
Recently, S . Yangyun and R. J. Brook~
Preparation and Strength of fosterite - Zirconia
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Ceramic Composites. Ceramics International~ 9(2):39-45
(1983) described the reaction of magnesium oxide (MgO)
with zircon (ZrO2 SiO2) as follows:
2MgO + ZrO2 SiO2 2MgO SiO2 ~ ZrO2. The authors
therein used a low temperature calcined reactive fine
grain MgO intimately mixed with finely milled zircon.
The reactants were milled together in a micronizer and
pressed into pellets~ They reported that about 10%
reaction was achieved at 1100C. (2012F.) in two
hours,
Notwithstanding the fact that such teachings
as to zircon and MgO refractory compositions have been
well known in the glass and metal making industries,
it has been heretofore unknown to utilize zircon and
lS MgO refractory compositions to form fosterite at low
vessel preheat temperatures. Further, it has been
heretofore unknown to utilize such compositons to
fabricate preheatable molded refractory insulating
liner structures for casting vessels. Moreover, it
has been surprisinglv dlscovered that such composi-
tions in the liners react to facilitate development of
effective hot strength at low vessel preheat tempera-
tures.
To improve the quality of the molten metals
including ferrous alloys, and especially low hydrogen
gradas of steel, during casting, the heat-insulating
refractories employed to line the casting vessels
which come into contact with the molten metals should
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be composed o the most stable oxides possible ~i.e.,
the stronger the chemical bonding of the oxides, the
higher the melting point). Unfortunately, this means
that temperatures greater than preheatable tempera-
tures which are generally from about 1900~F. to about
2400F. are required to sinter the refractory grain of
the stable oxides present in the linings. Thusly, a
balance or compromise has to be struck between the
refractory stable oxides and impurities utilized in
the present heat-insulating refractory boards. The
refractory stable oxides must have enough impurities,
but without sacriicing quality of the casting metals,
to develop a dense sinter surface at temperatures of
about 2800F. or higher. The types and amounts of
unstable impurities which act to lower temperatures
needed for sintering and solidus, however, must be
controlled to maintain the needed heat-insulating
refractory lining requirements and to minimize the
contamination of the casting metals by the unstable
oxides. The present invention, however, has remark-
ably overcome this arduous dilemma by providing a
unique blend of stable and unstable oxides which is
suitable for developing the necessary refractoriness
and hot strength at vessel preheat and casting
temperatures;while developing a dense sinter a~ about
2800F. without signlficantly contaminating the molten
metal , such as ferrous alIoys, with impurities during
c`asting.
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In a further feature of the present inven-
tion, the binder compGnent may be derived from organic
and/or inorganic binders or mixtures thereof in the
range from about 1.5% to about 15% by weight. The
inorganic binder may, for instance, constitute low
melting fluxing compounds, such as boric acid.
Additionally, the preheatable liners may contain a
fibrous material component which may also be derived
from organic and/or inorganic materials in the range
from 0% to about 10% by weight. In keeping with the
invention, the preheatable liners may further contain
a thixotropic sub~tance, such as bentonite, in amounts
ranging from 0% to about 5~ by weight.
In still a further feature of the present
invention resides in providing preheatable molded
refractory insulating liner structures suitable for
lining casting vessels like hot tops, ladles,
tundishes, troughs, or pipes, etc. for conveylng
molten ferrous alloys. The preheatable liners, if
desired, can be molded into the form of a plurality of
predetermined shaped lnserts, such as tundish koards.
An especially desirable corrosion-erosion
resistant preheatable molded refractory insulating
liner according to~this lnventlon comprises by weight
about 80% to~about 95% a particulate reractory
component containing MgQ refractory grain and zircon
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wherein the MgO refractory grain and zircon are in a
ratio~of about 5:1 to~about~l8:1, respectively, about
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1~ to about 8% fibrous material, abou~ 1.5~ to about
10% binder, and about 0.5~ to about 5% a thixotropic
substance. As noted above, the MgO refractory grain
may be derived from, for instance, natural, seawater
or brine magnesite, periclase grain, or other suitable
sources, or mixtures thereof. The lower silica and
hydrogen content of the preheatable structures provide
a further feature for reasons recited above; that is,
the cast molten metals and particularly the ferrous
alloys are di.stinctly purer as a result of less
contamination presently experienced from high silica
and hydrogen content associated with the common
preheatable refractories available hitherto.
A preferred form of the present invention
possessing the high degree of hot strength developed
at vessel preheat and metal casting temperatures
comprises by weight about 10~ zircon and about 80% MgO
refractory grain plus incidental impurities, wherein
the MgO is preferably derived from dead burned
natural, seawater or brine magnesite, periclase grain
or mixtures thereof which may range from abou~ 80% to
about 98% MgO pur.ityO the balance being binder, and if
desired fibrous material or bentonite or mixtures
thereof. Keeping the silica and hydrogen contents low
in this preferred form, the same as aforesaid, will
also provide distinctly purer cast molten metals.
,
~ Other incidental impurities are merely those
of extremely minor contaminants which result from the
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ordinary impurity contents normally associated with different grades of
raw material sources for MgO, zircon, etc. The total amount of impurities
should be kept to a minimum, if possible, to reduce or avoid possible
detrimetal effects to the above-noted properties and structural character-
istics.
In still another feature the present invention is directed to
a method of casting using a casting vessel comprising the steps of providing
within the vessel a preheatable molded refactory insulating liner of this
invention heating the vessel to at least a vessel preheat temperature
to initiate the development of hot strength in the preheatable liner for
casting at metal casting temperatures, and introducing a molten metal,
such as a ferrous alloy, into the vessel.
In summary of the above, this invention provides a preheatable
molded refractory insulative liner structure, having insulating porosity
of predetermined shape prior to preheating, for temporarily lining a casting
vessel and for developing sufficient hot strength to maintain the integrity
of the structure at vessel preheat and metal casting temperatures. The
range of temperatures during its use as a liner in a casting vessel is
from about 1900F to about 3000F. The liner structure comprises a molded
uniform mixture which, prior to preheatlng,~has substantial insulating
porosity on the order of about 50% containing a particulate refractory
component, a binder and inorganic fibrous material. The refractory component
is present in the amount of aùout 75% to 98.5% by weight of the preheatable
liner structure and comprises a mixture of zircon and MgO refractory grain
in a ratio of about 1:1.5 to about 1:24, respectively. The b;nder for
the component and the inorganic fibrous materlal must be present in suffi-
cient amounts in order~to maintain~the predetermined shape of the insulating
- porosity at least prior to the p~reheat temperatures wherein the zircon
and MgO refractory grain are of a particle size in the preheatable llner
to facili:ate the formation of~fosterlte bonding. This results in lncreased
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hot strength with substantial maintenance of insulating porosity and shape
without substantial shrinkage at both the vessel preheat temperatures of
about 1900F to about 2400F and higher metal casting temperatures when the
liner structure is used as a liner in a casting vessel and heated for a
sufficient period oF time.
The invention also provides a method of casting with a vessel
which serves to transfer molten metals wherein the vessel is temporarily
lined with a preheatable molded refractory insulating liner structure having
insulating porosity of predetermined shape prior to preheating. This method
comprises the steps of providing in the vessel the preheatable molded
refractory insulating liner structure of predetermined shape, heating the
vessel and introducing a molten metal into the vessel. The liner structure
must be suitable for developing sufficient hot strength to maintain the
integrity of the structure at vessel preheat and metal casting temperatures
in~the range of about 1900F to about 3000F. The preheatable molded
refractory insulating liner structure comprises a molded mixture having a
substantial insulating porosity on the order of about 50% prior to preheating
which contains a particulate refractory component, a binder for the component
and inorganic fibrous material. The refractory component which is present
in the amount of about 75% to 98.5% by weight of the liner structure consists
of a mixture of zircon and MgO refractory grain in the ratio of about 1:1.5
to about 1.24, respectively. The binder and inorganic fibrous material must
be present in sufficient amounts in order to maintain the predetermined
shape of the insulating porosity at least prior to the preheat temperatures
wherein the zircon and MgO refractory grain are of a particle size in
the liner structure to facilitatethe formation of fosterite bonding.
This results in increased hot strength with substantial malntenance
of insulating porosity and shape~without substantial shrinkage~ at both
vessel preheat and higher metal casting temperatures when the preheatable
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liner structure is heated for a sufficient arnount of tiMe. The
vesse1 is then heated to at least a vessel preheat temperature of
about 1900 to about 2400F in order to initiate development of hot
strength in the preheatable liner structure for casting at metal
casting temperatures. Molten metal is then introduced into the
vessel with substantial maintenance oF insulating porosity and shape
of said liner structure without substantial shrinkage.
Thusly, it can be appreciated that the special features
and unique advantages of the preheatable molded refractory insulating
liners of this invention makes the same highly effective refractory
liners suitable for preheating for extended periods of time prior to
the start of molten metal castlng.
The above and other features and advantages of the invention,
including various novel details of construction and composition, will
now be more particularly described with reference in the detailed
description and pointed out in the claims. It will be
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understood that a composition for a preheatable molded
refractory insulating liner embodying the invention is
shown in the example by way of illustration only and
not as a limitation of the invention. The principles
and features of this invention may be employed in
various and numerous embodiments without departing
from the scope of the invention.
Detailed Description of the Invention
By way of illustrating and providing a
better appreciation of the present invention, the
following detailed description and example are given
concerning the preheatable molded refractory insulat-
ing liners of the invention and their properties or
characteristics.
In accordance with the present invention, it
is directed to providing a preheatable molded refrac-
tory insulating liner for lining a casting vessel
suitable for developing sufficient hot strength a~t
vessel preheat and metal casting temperatures which
are on the order of about 1900Fo to about 3000F.
when such a liner is heated for a sufficient period of
time. This is accompIished in the present instance by
: means~of a preheatable molded refractory insulating
liner comprising a liner structure of predetermined
`:
shapa, the liner structure comprising a uniform molded
mixture contaLning~a~particulate refractory component
¢omprised of zircon and MgO refractory grain,~and a
binder therefore. In addition/ the preheatable liner
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preferably contains a fibrous material and, i~
desired, minor amounts of bentonite or other suitable
thioxtropic substances.
In the specification, the term "hot
strength" refers to a liner having sufficient hot
strength to support itself during extended preheating
and to withstand erosion resulting from a molten metal
entering a vessel during casting. In other words, the
preheatable liners made and used in accordance with
the teachings of this invention unexpectedly and
advantageously generally do not soften or weaken,
collapse or wash away after being preheated by a
molten metal entering the vessel, as currently ex-
perienced with other prior art preheatable liners~
l~ AdditionaIly, the preheatable liners of the present
in~ention are less prone to contaminate the molten
metals because o~ their low free silica and low
hydrogen contents.
In a preferred embodiment, ths preheatable
` moldsd refractory insulating liners are formsd of by
wèight of about 75% to about 98.5% a particulate
refractory~component comprised of zircon and MgO
refractory grain being in a ratio from about 1:1.5 to
about l:24, respectively, about 1.5% to about 15%
binder, 0% to about 10% fibrous material~and ~ to
i
about 5~ a~thlxotropic substancs. Preferably, the ;~
zirco~ is~about 5%~to about 15~, and most preferably
sbout l0~ by weight of the liner. ~The zircon, also
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known as zirconium silicate IZrSiO4 or ZrO2 SiO2), and
sometimes known as hyacinth, jargon, etc., can be
derived from natural or synthetic products, or any
other suitable sources not inconsistent with the
teachings of this invention. It has been surprisingly
discovered that when zircon, and especially finely
comminuted zircon, or zirconium silicate, is intimate-
ly mixed with MgO refractory grain in sufficient
amounts and made into a molded refractory insulating
liner for a casting vessel, the zircon and MgO refrac-
tory grain unexpectedly react at vessel preheat
temperatures which are in the range of about 1900F.
to about 2400F. to develop sufficient hot strength as
a result of the formation of fosterite bonding when
such a liner is heated for a sufficient period of
time.~ To this end, it is not necessary, but highly
preferable, that the~entire amount of zircon, or
zirconium silicate, used be very finely comminuted.
For example, preferably about 95% of the zircon
particles should pass through a 325 mesh screen, and
more preferably substantially all should pass through
a 400 mesh screen. Most preferably, ~he zircon should
have an average particle si~ze of about 10 microns. In
particular, ground zircon sand and especially zircon
flour~, or instance, are suitable sources of highly
ocmminuted z~ircon to be employed pursuant to the
invention. The term zircon employed herein iS to be
` understood in each instance as referring ~o a~chemical
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combination of zirconium, silicon and oxygen designat-
able by the formula ZrSiO4 and irrespective of its
origin be it synthetic or natural.
In carrying out the invention, the MgO
refractory grain, also known as magnesium oxide or
magnesia, can be derived from any suitable sources and
especially from sources, such as natural, seawater or
brine magnesite, periclase grain, or any other suit-
able sources, or mixtures thereof. The magnesite or
periclase grain, however, preferably is of the type
commonly referred to as dead burned magnesite or dead
burned periclase. By "dead burned" magnesite or
periclase is meant magnesite or periclase fired to
high temperatures to produce a hydration resistant
grain consisting ess~ntially of well-sintered low
porosity periclase crystals and this grain structure
distinguishes it from the more reactive lower tempera-
ture calcined caustic magnesites. Nevertheless, it
should be understood that it is preferred that the MgO
2~ refractory grain content, whether derived from natu-
ral, seawater or~brine magnesite, periclase grain, or
other suitable sources, should be substantially pure.
By "substantially pure", it means containing at laast
about 80~ MgO by weight on the basis of an oxlde
analysis, with the remalnder, if any, being only minor
amounts of incidental impurities. Generally, the best
results are achieved when the particle size distri-
bution of the MgO refractory grain is not too coarse
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-21- ~3~5~0
or too fine. Preferably, such particles should be
sized so that no more than about 40~ are retained on a
50 mesh screen and no more than about 40% can pass
through a 325 mesh screen. More preferably, no more
than about 15% of the MgO refractory grain particles
should be retained on a 50 mesh screen and no more
than about 30% should pass through a 325 mesh screen~
In accordance with a preferred aspect of the
invention, it is surprisingly found that the use of a
blend of different magnesite sources for MgO refrac-
tory grain achieve the best results. As noted above,
the blend should be of sources for MgO refractory
grain having an MgO content of at least about 80%.
Additionally, sources for MgO refractory grain should
be selected to minimize silica and hydrogen content.
Further, sources of MgO refractory grain should be
selectPd on the basis of their low tendency to hydrate
due to their high dead burning temperatures and as a
result of their composition. For example, it is
astonishingly found that a blend of equal parts of
about 88~ MgO magnesite and about 95~ MgO magnesite is
optimal for minimizing silica and hydrogen content
while still achieving a strong sinter and hot
strength. The 95% MgO magnesite contains about 2~ to
about 3% SiO2 and the 88~ MgO magne~ite contains about
s
7~% to about 8%;SiO2. Thus, in a most preferred~form
of the present invention, the preheatable molded
refractory~insulating liners comprise by weight about
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10% zircon, about 4~% of an about 88% MgO magnesite
and about 40~ of an about 95~ MgO magnesite wherein
the zircon and MgO refractory grain are in a ratio of
about 1:8, respectively, about 1.5% to about 15~
s binder, 0% to about 10% fibrous material, and 0% to
about 5% a thixotropic substance.
In another feature of the present invention,
a suitable refractory filler may be added in accep-
table amounts to the particulate refractory component
which comprises zircon and MgO refractory grain.
Exemplary of such fillers are olivine and zirconia
wherein the olivine may be by weight of the liner from
0~ up to about 70% and the zirconia may be by weight
of the liner from 0% up to about 80%. When a refrac-
tory filler is added to the particulate refractory
component, however, it should be understood that such
a mixture will still be by weight of the preheatable
liner from about 75~ up to about 98.5~ as afore-
mentioned. It should further be understood that the
2~ zircon and M~O refractory grain are in the stated
ratios not inconsistent with the teachings of this
invention so that sufflcient hot strength is~developed
in the preheatable llners at vessel preheat tempera-
tures and metal aasting temperatures. It should
additionally be understood that ~he refractory fillerq
preferably should have a particle size approximating
~the size distribution of the MgO refractory grain.
The advantages to adding~a refractory filler to the
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-23- ~ ~ 3 ~
particulate refractory component include, for
instance, a reduction in manufacturing cost or to
improve the corrosion resistance o~ the liner. ~n
example of a preheatable refractory liner containing
olivine as a refractory filler has a composition
comprising by weight olivine about 62%, zircon about
4%, MgO refractory grain about 25%, binder about 1.5%
to about 15%, fibrous material 0% to about 10~ and a
thixotropic substance 0~ to about 5%. It can be noted
that in this exemplary composition the MgO refractory
grain and zircon ratio by weight is about 6:1,
respectively.
In carrying out the invention, the binder
component may be derived from any suitable binder or
mixtures of binders of those known in the refractory
making and allied industries including organlc and/or
inorganic binders. Typically, vessel preheating is
conducted at abo~t 1900 F. to about 2400 F. and more
typically between about 2000F. and 2300F. These
preheat conditions cause the organic binders incorpo-
rated withln~the liners to burn out, for instance,
starting at the hot ~ace and sometimes throughout the
entire board thickness, of course, depending upon
preheat time, temperature:and board thickness.
Nonetheless,~ up until the point of burnout, the
orga~lic~ binders serve to hold or bind the other
materials together and compri~es by weight of the
:
liner from::about 1.0%~to about:l0%. Sample~ of:
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organic binders suitable to be employed in the liner3
of the present invention include, but not limited to,
starches, cereals, natural or synthetic resins, such
as amino re~ins, phenolic resins or mixtures thereof.
More particularly, the phenol-formaldehyde and urea~
formaldehyde resins are best suited for use and most
preferably is the phenol-formaldehyde resin. It
should be appreciated that when the phenol-formalde-
hyde resin is employed, a catalyst such as hexamethyl-
enetetraamine, also known as HMTA, Hexa, methenamine,
hexamine, aminoform, etc., should be added in suf-
ficient amounts to polymerize the phenolformaldehyde
resin to bond the refractory grains for making a rigid
structuxe suitable for use as a liner.
In addition to providing binding support
prior to the burnout of organic binder, the inorganic
binder generally serves to stick or hold the particu-
late refractory component together during preheat
conditions particularly after the organic binder has
been con~umed or buxnt out. To this end, it is
be}ieved that the inorganic binder forms a glassy or
viscous phase under preheat conditions developlng
characteristics suitable for sticking or binding the
particulate refractory component together. In other
words, an inorganic binder can act as a temporary
binder characterized as a low melting fluxing material
which;aids in maintaining the particulate refractory
component together subsequent to organic burnou~ under
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preheat conditions and until the fosterite refractory
bonding develops. Examples of inorganic binders
suitable for use are boric acid, borax, colemanite,
etc., and preferably boric acid. Generally, boric
acid comprises by weight from about 0.5% to about 5%
of the structure. In addition, upon heating boric
acid is converted to B203 which advantageously pro-
motes sintering of the MgO refractory gra~n for
improving the hot strength of the preheatable liner.
In further keeping with the invention, as to
the fibrous materials, the following is preferred but
not limited thereto: inorganic fibrous materials such
as rockwool, slag wool, glass wool, refractory alumi-
num silicate fibers, and especially slag wool; and
organic fibrous materiais such as cellulosic materials
derived rom paper, paper wood, sawdust, wood meal,
synthetic organic fibers or the like, and particularly
paper. These fibrous materials ~enerally serve to
reinforce the preheatable liners so that the liners
are not damaged by any impact during the manu-
facturing, shipment and installation. Additionally,
the fibrous materials serve to prevent the particulate
refractory component from settling out of the slurry
and to control porosity and permeability of the liner.
Further, by the use of such a fibrous material, the
resulting preheatable liners can become a porous board
which has low bulk density, whereby the heat-
~ insulating~effect thereoi is improve~. As noted
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above, the fibrous material represents by weight o~
the liner from 0% to about 10% and preferably about
5~ .
As to the incorporation of a thixotropic
substance, it generally acts as a thickening agent or
forming aid during preparation of the desired shape
and generally comprises by weight of the liner from 0%
to about S~. Exemplary of thixotropic substances are
bentonite, methylcellulose, alginates, etc., and
especially bentonite. As to the bentonite, it is
preferred that the calcium bentonite is employed as
opposed to ~he sodium bentonite.
In accordance with the present invention,
the preheatable molded refractory insulating liner
structures are suitable for formlng linings~for
casting vessels, such as hot tops, ladles, tundishes,
troughs and pipes etc., which are intended to contain
molten ferrous alloy metals. The versatility of
these structures enable them to be shaped, for
` example, into the form of a plurality of predet;ermined
shaped inserts. Preferably, the preheatable~liner~ of
this invention are in~the ~orm of a plural~ty of
shaped boards employed in tundi~shes.
As~ conventional~in the art o refractory
insulating;liners, manufacture can be readily done,
for instance, by;vacuum forming or injection molding
methods whLch,;;for;example, comprise forming an
aqueous~sl~urry of aolids;compri~in~ a miX~ture con-
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-27- ~3~80
taining a particulate refractory component, a binder
therefore and, most preferably, a fibrous material
component. Bentonite or other suitable thixotropic
substances may also be employed as discussed above.
Because of the copious amounts of water utilized in
making the aqueous slurry, vacuum sources which are
well-known in this art for removing substantial
amounts of water are preferably employed. The raw
batch of materials are suitably proportioned to
provide the desired final mixture and preferably are
intimately premixed in the slurry form prior to vacuum
forming. After the preparation of a sufficient amoùnt
of a desired slurry, the material is usually poured
into preformed molds of the desired shape and sub-
jected to sufficient sub-atmospheric or vacuum condi-
tions to suck away a substantial amount of the liquid
in the slurry so that the formed shapes can be removed
from the mold and dried. The wet vacuum formed shapes
axe passed through conventional hot air dryers to
remove or evaporate virtually all the water and to
heat the entire structure thickness to a suitable
temperature for curing the organic and/or inorganic
binder. The thickness of the liner when making a
bo~ard may range, for instance~, from about 3/4 of an
inch to about 2 inches.
According therefore to a further feature of
the present invention, there is provided a method of
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casting using~a casting vessel which serves to
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~28-
transfer molten metals, such as ferrous alloys,
comprising the steps of providing in the casting
vessel a preheatable molded refractory insulating
liner structure of this invention, heating the casting
vessel to at least a vessel preheat temperature to
initiate the development of hot strength in the
preheatable liner structure for casting at metal
casting temperatures and introducing a molten metal,
such as a ferrous alloy, into the casting vessel.
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EXAMPLE
~he following represents two preferred
composi~ions A and B for manu~acturing a preheatable
molded refractory insulating liner in accordance with
this invention:
Composition Ingredient %
A Phenol-formaldehyde resin 2.30
Hexamethylenetetraamine 0.20
Paper 1.40
. Slag wool 3.30
Calcium bentonite 1O60
Magnesite: about 88~ MgO 40.60
Magnesite: about 95~ MgO 40.60
Zircon Flour 10.00
100.00
: Boric Acid 2.5% of dry batch weight.
Composition Inq~ t 3
B Phenol-formaldehyde ~resin 2.30
Hexamethylenetetraamine 0.20
Paper 1.40
~ Slag wool ~ 3.30
: ~ ~ Calcium bentonite 1.60
: Magnesite: about 88% MgO 24.50
; ~ Zircon:Flour 4.00
` ~ Olivine ~ 62
` - ~ 100 . 00
~ Boric A~id 1.0% of dry batch weight.
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TABLE I
Effect of Preheat and Contact with Molten Steel
on the Porosity of the Tundish Board Manufactured
with Composition A Disclosed in the Exam~le
SPECIFIC
PREHEAT POROSITY GRAVITY
Prior to preheat 50.95% 3~33 g/cc
After 1 hour 2100F Preheat 56.03~ 3.55 g/cc
SPECIFIC
AFTER CASTING POROSITY GRA~ITY
Sample - 6 inches under
metal line
1 heat - medium carbon steel,
medium manganese steel
Hot Face - (11/16" thick) 22.2% 3.44 g/cc
Cold Face - (3/4" thick) 54.3% 3.57 g/cc
Sample - 18 inches under
metal line
1 heat medium carbin steel,
1~50% mangane~e steel
Hot Face - (5i8" thick) 14.5% 3.37 g/cc
Cold Face - (3/4'i thick) 53.9% 3.51 g/cc
GUNNA3LE TUNDISH COATING
A:~ter 1 heat sample 34.6% 3.40 g/cc
uniform:throughout 1" :
thick coating
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Preheat increased the poro~ity by 10% by burning
out organic binders sintering and reacting to form
fosterite. This efect was also observed by the
increase in specific gravity when the low specific
gravity organic material was removed.
Table I shows that after contact with a
molten ferrous alloy, the preheatable refractory board
formed a dense (approximately 20% porosity), imperme-
able 0.5 inch thick layer in contact with the steel.
The porosity of the cold side of the board remained
high at about 55~. The dense hot face contained some
closed pores and some oxide contamination which
lowered the apparent specific gravity compared to the
cold face and the preheated specific gravities.
X-ray diffraction showed that zircon and MgO
refractory grain was consumed, and that fosterite and
cubic zirconia were formed at tundish preheat condi-
tion and both fosterite and cublc zirconia were found
present in the hot face and the cold face of the
tundish boards after casting ferrous alloys.
In use, the dense board hot face which
developed resisted and reduced erosion and steel
contamination. The high porosity on the back of the
board advanta~eously gave lower thermal conductivity
through the board, th~us, lower emperatures at the
pe~manent lining and~le s heat 105s from the ves3el
~resulted~.~ Such boardJ~had low ero~ion with high
manganese steel5, low hydrogen contribution to the
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steel and were preheated to about 2300F for up to
about seven hours without adversely affectin~ their
hot strength during casting. The gunnable coating
referred to in Table I had an intermediate (34%)
porosity through its entire thickness. It failed to
develop a dense layer at the hot face.
It was observed that the preheatable tundish
boards developed sufficient hot strength and remained
substantially rigid and intact during the vessel
preheat temperatures and metal casting temperatures.
The present invention may, of course, be
carried out in other specific ways than those herein
set forth without departing from the spirit and
essential characteristics of the invention. The
present embodiments are, therefore, to be considered
in all respects as illustrative and not restrictive
and any changes coming within the meaning and equiva-
lency range of the appended claims are to be embraced
therein.
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