Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
131~6
NEW METHOD OF OPERATING MINERAL WOOL CUPOLAS AND
USING SPENT ELECTROLYTIC ALUMINUM POT LINING
Background of the Invention
l. Field of the Invention
This invention relates generally to methods of
operating mineral wool cupolas, fuel stock for mineral
wool cupolas and using expended pot lining removed from
electrolytic reduction vessels in the aluminum
industry. More particularly, the invention relates to
the provision of a particularly advantageous charge for
mineral wool cupolas which provides an improved thermal
balance and/or prevents siliceous build-up ln the
cupola, virtually eliminating shutdowns of the cupola,
for removal of such build-up. Use of the charge also
increases the smoothness of operation and the quality
of the product as well as the yield. More particularly
still, the invention relates to the use in the charge
made to a mineral wool cupola of an effective amount of
a fluorine containing carbon fuel charge which may
comprise a sized charge of expended lining from
electrolytic reduction vessels in the aluminum
industry.
2. Description of the Prior Art
Mineral wool cupolas for the melting of silica
containing slag and rock material and other siliceous
materials to form a melt which can then be formed into
elongated fibers useful for insulation and the like
usually take the form of unlined, or so-called self
~; lining, steel cupolas which receive a charge at the top
and from which molten siliceous material is tapped from
the bot~om. After tapping, the molten siliceous
product is commonly either centrifugally or
1313 5 ~ 6
pneumatically attenuated into long fibers, while the
siliceous material is still molten. The resulting
fibers are useful for insulation and the like.
Mineral wool cupolas are customarily charged with
alternating layers of siliceous materials and coke.
The siliceous materials may comprise various
metallurgical furnace slags plus siliceous or silica
containing rock such as trap rock. The coke is ignited
in the lower portion of the cupola and is burned by air
and/or oxygen admitted through tuyeres in the walls
near the bottom of the cupola. The heat melts the
siliceous materials into a homogenous composition that
can be formed into fibers. Since the siliceous material
is not corrosive to the walls of the cupola, the walls
are customarily not lined, but are self lined by a thin
layer of the chilled melt particularly against the
water cooled walls of the hearth area. The bottom, or
hearth, of the cupola usually has a layer of loam or
clay material applied to it to protect the hearth from
the molten charge and particularly from attack by
residual molten metals which collect in the center of
the hearth and are periodically tapped from the bottom
of the furnace.
A fairly typical mineral wool cupola or shaft
25 furnace is shown in U.S. Patent 2,467,889 to I. Harter
et al. which, however, shows the entire shaft wall
enclosed in a water cooling jacket. Details of
operation are also described by Harter. A wide ranging
discussion of the operation of mineral wool production
processes including various charge materials or raw
materials may also be found in U.S. Patent 2,576,312 to
L.J. Minnick.
One serious difficulty encountered in mineral
wool cupolas is so-called "siliceous build-up" in the
hearth area of the cupola and particularly upon the
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surface of the hearth itself. Such build-up can
seriously interfere with the capacity and operation of
the cupola. The usual method of combating siliceous
build-up is to take the cupola out of operation
periodically, often as frequently as every week, and
mechanically remove the siliceous build-up. Other
potential solutions also have been tried. For example,
cupolas have been designed in which the hearth bottom
is vertically movable with respect to the remainder of
the cupola so that as siliceous build-up occurs upon
the hearth, the hearth is periodically lowered, usually
by a screw arrangement, so the essential capacity of
the hearth area remains at all times the same.
Operators of conventional cupolas eventually have to
"~rop the bottom" or remove the hearth, usually by
opening a door-llke arrangement at the bottom to remove
the entire hearth contents, in order to eliminate the
siliceous build-up. It is, of course, particularly
important that the effective height of the tuyeres from
the hearth not be drastically altered so that ignition
and burning of the coke in the charge remain at an
optimum height above the hearth. However, it will be
evident that the lowering of the hearth bottom is
inherently limited and after such limits are reached
the cupola must be taken out of operation and the
bottom doors dropped to dump the excess build-up of
deposits of siliceous material on the hearth. In some
cupolas, furthermore, particularly where the side walls
above the tuyeres are not vertical, but slant
outwardly, such as in bosh type cupolas, siliceous
build-up may occur on the side walls as well as in the
hearth, seriously interfering with the operation of the
cupola and even with dumping the siliceous build-up
from the hearth.
Due to the detrimental build-up of highly
~3155a6
-- 4 --
siliceous fractions on the cupola hearth and
occasionally on the side walls it has in the past been
frequently necessary to limit the percentage of
siliceous material in the cupola so that longer
operating times can be obtained. However, the
limitation on the content of siliceous material in the
charge may not allow the production of the high silica
fibers which may be desired.
A further attempt to limit siliceous build-up
upon the hearth and side walls has involved the
charging of chemical agents such as fluorides and the
like to the cupola to cut, or decrease, the viscosity
of the siliceous material in an attempt to prevent
sticking and build-up on the walls and bottom of the
cupola. Such efforts have not been successful in
complete elimination of siliceous build-ups.
In addition to the above, since mineral wool is
both a very desirable and a relatively inexpensive
product due to the nature of the charge materials and
its end use, price competition is severe and it is very
important in order to be competitive to conduct an
efficient, economical operation. It is important,
therefore, that the best possible thermal operation be
attained. Usually coke is used in the cupola as a
fuel. Coke is largely carbon and therefore burns with
a high heat release and is sufficiently strong to
support the burden in a cupola. It is advantageous for
the major combustion to occur directly above the hearth
at the tuyere zone where the burden is melted and flows
into the hearth. Hot gases are evolved which rise
through the cupola preheating the burden and preparing
it for melting. Actual melting is detrimental to the
proper operation of a cupola if it occurs too high in
the cupola shaft. Coke, however, tends to be porous
due to the expelling of copious amounts of various
131~ cr)~6
gases during pyrolysis as the major step in its
manufacture. While the coke may be made denser by
closely confining the coal from which it is made during
pyrolysis to prevent the coke from expanding while it
is in a plastic state and hydrocarbons are escaping, a
minimum porosity still remains. Furthermore, the
denser metallurgical cokes made from select coking
coals are more expensive than other cokes, although the
denser structure results in better strength, ignition
and burning properties. The properties of coke for use
in a shaft furnace, whether a metallurgical furnace or
other melting furnaces such as siliceous mineral
melting furnaces, for example, mineral wool melting
furnaces, are therefore not completely satisfactory,
particularly with respect to density and ignition
properties. The moisture content of conventional coke
is also quite variable after storage prior to use.
Consequently, there has been a need for a better yet
more economical fuel for shaft furnaces such as mineral
wool cupolas.
The electrolytic reduction vessels used in the
aluminum industry are lined with carbon to protect the
structural shell from the molten bath and serve as an
electrical conductor (the cathode) during operation of
the reduction vessels. There are two principal types
of such carbon linings, or pot linings, namely prebaked
block linings and rammed carbon linings. Cryolite,
which contains high percentages of sodium, fluorine and
aluminum, is used as the electrolyte in the
electrolytic smelting process used in aluminum refining
operations. After the carbon lining of the smelting
pot has been used to such an extent that it is
considered worn out, it contains on the average about
60% carbon, about 12% sodium, about 13% fluorine, 10%
aluminum and 5% miscellaneous other material, largely
5 ~ 6
-- 6
residuals from the ash content of anthracite fines used
as a major constituent in the carbon cathode lining.
The cryolite in its liquid form invades the pores in
the carbon lining and is responsible for the sodium,
fluorine and aluminum found in the worn out or spent
pot linings.
Spent pot lining, or SPL, is essentially a waste
product which must be disposed of in some ecologically
satisfactory manner. Although the high fluorine
content can be leached from SPL fines in a separate
operation, SPL does not lend itself readily to use as a
fuel or the like because of such high fluorine content
which may have other detrimental effects. For example,
SPL has been tried in the past as a coke replacement in
foundry cupolas and the like. Such tests have met with
little success, largely because the fluorine has been
found to seriously attack the lining of the cupola and
also creates a cold slag condition unacceptable in
foundry operations. There has been a definite need,
therefore, for some method of disposal of large amounts
of spent pot lining (SPL) preferably as a feed stock
for a succeeding process.
3. Objects of the Invention
It is an object of the invention therefore to
provide a method of operating a mineral wool cupola in
which a novel fuel charge material is used which
provides superior cupola furnace performance.
It is a further object to provide a novel highly
graphitized fuel which performs more efficiently than
coke as a fuel.
It is a further object of the invention to
provide a fuel charge material for mineral wool cupolas
which inhibits siliceous build-up in such cupolas.
It is a still further object of the invention to
5 ~ ~ 1
provide a method of operating a mineral wool cupola in
which the use of a novel fuel charge material provides
both heat values and removes detrimental siliceous
build-up.
It is a still further object of the invention to
provide a fuel for cupolas which chemically removes
siliceous build-up from said furnace.
It is a still further object of the invention to
provide a fuel charge for a mineral wool furnace which
provides an increased heat evolution and hotter slag
adjacent the tuyere zone.
It is a still further object of the invention to
provide a method of operating a mineral wool cupola
with an increased siliceous content which can be used
to produce higher silica content fibers.
It is a still further object of the invention to
provide a new method of operating a mineral wool cupola
using spent pot lining from aluminum operations.
It is a still further object of the invention to
provide a smoother, cleaner operation for a mineral
wool cupola and produce a more uniform melt with
superior fiberization characteristics.
It is a still further object of the invention to
provide a new and useful method of disposing of spent
pot lining from aluminum operations.
It is a still further object of the invention to
provide a combustible material which contains
sufficient amounts of fluorides adapted for release
low in the cupola hearth where they may react and
dissolve siliceous build-up.
It is a still further object of the invention to
provide a new method of operating a mineral wool cupola
with increased yield due to superior molten slag
consistency.
Other objects and advantages of the invention
13~ 5~
-- 8 --
will become cvident from consideration of the following
description of the invention and appended claims.
13~5~6
Brief Description of the Invention
The present inventor has discovered that
siliceous build-up in siliceous furnaces such as, in
particular, mineral wool cupolas can be both inhibited
and removed by adding to the furnace as one of the
charge materials a fuel material containing significant
amounts of fluorine. Such material can be formed by
treating graphite or other carbon with a fluorine
compound such as cryolite.
Alternatively spent pot lining from electrolytic
reduction vessels in the aluminum industry can be used.
The material is desirably sized from one-half (1/2) to
six (6) inches (1.27 to 15.54 cm) in diameter and
preferably in the size range or with the bulk of the
material in the size range of from about two to four or
somewhat less preferably two to five inches (5.08 to
lO.16 or 12.7 cm) in diameter. When provided in such
size range the fluorine is released by burning of the
carbon close to the hearth of the furnace, i.e.
adjacent to any siliceous build-up material, and has a
chance to react upon such material and enter into the
siliceous melt which can hold fluorine up to
approximately two percent of the weight of the bath.
The denseness of the carbon or graphite soaked in
cryolite or cryolitic type material also prevents early
release of the fluorine from the carbon so it has a
chance to react with the siliceous material contrary to
prior processes. The molten siliceous material is also
held in a more fluid state due to the influence of the
fluoride ions present and a greater heat evolution from
the tuyere zone during operation due to more efficient
ignition and burning of the denser graphitic carbon
lumps. Alternatively, a siliceous build-up inhibiting
or removing fuel briquette may be formed from the fines
of spent pot lining along with coke plant pitch which
131~
-- 10 --
acts as a binder together preferably with anthracite
fines and coke fines. In all cases the spent pot
lining may constitute up to about 70% of the fuel
carbon, plus or minus 10% and preferably plus or minus
S 5% without significant modification of the operation of
the usual cupola. Above about 75% to 80% spent pot
lining there may be excessive attack upon the hearth of
the cupola so that special precautions must be taken to
protect the hearth such as by forced cooling from
underneath when using 80% up to 100% of the special
fluorine containing fuel of the invention.
1~15~
Brief Description of the Drawing
Figure 1 shows a partially broken away schemat.ic
elevation of a mineral wool cupola showing the
alternating layers of fuel material, including the
S spent pot lining fuel of the invention, and siliceous
material in the form of slag and rock within the
cupola.
Figure 2 shows a fuel briquette formed of spent
pot lining fines and other material.
Figure 3 shows a plan view of an annular spray
head designed for cooling the bottom of the hearth when
more than about 80% of the fuel of the invention is
used in the cupola charge.
1315~
- 12 -
Description of the Preferred Embodiments
As indicated in the introduction, mineral wool
cupolas must be operated very efficiently in order to
stay within economic limits. The heating or operating
fuel in the form of coke is the most expensive of the
materials charged into a cupola. Coke is used as a
fuel because it has sufficient strength to support the
burden in the cupola. Coke, however, has certain
drawbacks as a fuel due to its porous nature which can
lead to variable moisture content and variable fuel
values. Coke tends also to react more or less with the
hot gases leaving the tuyere zone and thereby to be
partially consumed in an inefficient manner. A further
troublesome problem with mineral wool cupolas is the
frequent build-up of persistent solid deposits of
siliceous materials known as "siliceous build-up".
Siliceous build-up, which occurs mostly in the hearth,
but also sometimes on the side walls of the cupola,
particularly where such side walls are slanted as in
the bosh type cupolas, seriously interferes with the
operation of a cupola. Such interference is due to the
reduction of the capacity of the cupola, by physically
taking up space particularly in the hearth where excess
build-up may leave little if any room for the molten
material prior to tapping.
The reduction in hearth capacity is particularly
serious with respect to molten metallics which normally
collect in the center of the bottom of the hearth and
are periodically tapped from the cupola hearth area,
usually on a four to eight hour schedule, to eliminate
such metallics. These metallics cannot become mixed
together with the molten siliceous material from which
mineral fiber is to be formed if such fiber is to be
satisfactory. The formation of siliceous build-up upon
the hearth also reduces the clearance between the
131~5~
hearth and the tuyere zone, or combustion zone, in the
furnace and disrupts the operation of the furnace by
changing the effective combustion level.
The present inventor has discovered that if spent
pot lining, commonly referred to as SPL, is substituted
for up to 100% of the normally used coke, not only is
the fuel efficiency increased, i.e. the cupola becomes
hotter for the same amount of charge, particularly in
the tuyere area, and the molten product, i.e. the
siliceous material which is melted to form fibers of
mineral wool, appears more fluid or less viscous as
well as more uniform, further improving the product and
the operation, but that siliceous build-up is also
decreased or inhibited and in fact with continued
operation of a cupola in which build-up has already
occurred, such build-up is over a period removed or cut
back. A major problem is thereby solved to such an
extent that virtually continuous operation of the
cupola or furnace can be obtained.
The present inventor has also found that the
elimination of the siliceous build-up problem is
largely due to the structure and chemistry of the SPL
in that the lining of electrolytic reduction vessels in
the aluminum industry is essentially graphitic carbon
which is relatively dense and that after having been
exposed to the cryolite electrolyte in the aluminum
reduction process the lining, i.e. after having
absorbed fairly large amounts of the electrolyte,
becomes more dense still. This causes the SPL to be
resistant to ignition and combustion so that the major
combustion occurs at the tuyere zone resulting in a
greater heat release in this zone and a superior
thermal operation of the cupola, essentially "keeping
the fire low" in the cupola. It is undesirable to have
the fire or combustion in the cupola extend to high
~ 3 ~
- 14 -
elevations since this results in coke being combusted
at lower temperature and in an inefficient manner. In
addition, less dense more reactive coke or other fuel
will react with hot ascending gases leaving the
combustion zone resulting in the partial solution of
the coke to produce gas which is richer than otherwise
in carbon monoxide, but of little value in the typical
mineral wool process. Such solution loss is therefore
wasteful and to be avoided and is, in fact, avoided by
the operation of the cupola with the novel SPL fuel of
the invention.
The present inventor has unexpectedly found
further that the low reactivity of the SPL has the
further advantage that the fluorides in the cryolite
electrolyte which has soaked into the graphitic SPL are
released upon combustion of the carbon or graphite low
in the cupola near the hearth and in the vicinity of
the siliceous build-up. This is contrary to past
experimental additions of flux type materials such as
fluorides and the like to the burden in the furnace or
cupola in an effort to decrease the siliceous build-up.
Such materials tend then to be carried from the cupola
in the melt rather than reacting with the siliceous
build-up material.
Figure 1 shows a partially broken away schematic
elevation of a mineral wool cupola 11 comprised of a
steel shell 13 which may be air cooled in the upper
portions and water cooled by a water jacket 15 in the
hearth area 17 and extending upwardly from the hearth
and the tuyere area 19. Meltables ,1, normally
comprising rock and slag, such as trap rock and blast
furnace and electric furnace slag from iron and steel
making operations, and solid fuel materials 23 either
SPL or coke mixed with SPL are fed into the furnace in
alternating layers, as shown in the Figure. A skip
131~
hoist arrangement 25 comprising an inclined track 27
and a skip hoist car 29 conveys the burden materials,
i.e. the fuel materials (coke and/or SPL) and
meltables, to the top of the cupola and dumps them into
the cupola, preferably through a periodically opening
bell arrangement 31 as shown. The materials from the
skip hoist will accumulate on the closure 33 of the
tubular bell structure until the cupola operator opens
the closure by operation of winch 35 which is connected
to and movably supports the closure 33 via a cable 37
passing over a pulley arrangement 39. When the closure
33 opens, the burden material slides into the furnace
after which the closure 33 may automatically return to
the closed position until burden material again
collects on its upper surface and the operator again
opens the closure 33. Other bell or charging
arrangements may, of course, be used.
The skip hoist car 29 is drawn up the track 27 by
means of a cable 41 which passes over an idler sheave
43 and then to a winch arrangement 45 mounted on the
top of the cupola.
Air is blown into the cupola in the combustion
zone 19 through tuyeres 47 by centrifugal blast fans
49. A layer of loam and/or clay 51 is supported upon
the bottom of the hearth area 17 of the cupola to
insulate the bottom from the molten material within the
hearth and particularly from melted metallics such as
iron and various residual metallic components of the
slag material charged to the cupola. This metallic
material must be tapped from the center of the
generally disk shaped hearth periodically through tap
hole opening 53 provided in the supporting bottom 52 of
the cupola. A temporary tap hole is periodically
burned or otherwise opened through the loam layer 51
upon the hearth, which loam layer may be usually from
1 3 ~ 5 ~ ~ 6
- 16 -
six to eight inches deep, to release the metallics
collected in the center of the disk shaped hearth and
prevent them from becoming mixed with the siliceous
melt material. The temporary tap hole through the
refractory loan material is then plugged. The molten
siliceous material is continuously tapped through a
water cooled replaceable steel slag notch 54 and
carried by a slag or melt trough 55 onto a rotating or
spinning fiber forming disk 57 which is rotated by
motor 59. As the molten siliceous material falls onto
the spinning disk 57, the centrifugal force engendered
by the rotation causes the material to migrate up or
along the inside of the disk to the edge where droplets
are flung off and attenuated into fibers which
promptly solidify. An air blast or steam blast from a
blast pipe, not shown, positioned along the edge of the
disk may be used to accelerate the droplets which then
form tails or fibers as they undergo drag effects as
they penetrate the surrounding air. Alternatively the
blast or blow may be used alone to shred the melted
materials as they flow from the tap spout. In any
event, the method of fiberization of melt materials
forms no part of the present invention. The molten
siliceous material will, however, as a resu]t of the
invention be desirably of a higher silica content and
more uniform consistency and will consequently form
superior fibers.
The fibers may be collected from around the
fiberization device, in this case the spinning dish 57,
in any suitable manner. If more than 80~ synthetic or
natural SPL fuel material in accordance with this
invention is used as a part of the charge in the
cupola, it may be desirable to use additional forced
cooling of the hearth such as water sprays 71 from an
annular spray head 73 to cool the bottom of the hearth
l 3 1 ~
and prevent dissolution of the hearth, i.e. to prevent
the opposite of siliceous build-up. The amount of
cooling may be varied somewhat depending upon the
fluorine content of the fuel.
The annular spray head 73 which is shown in plan
view in Figure 3 is removably positioned about 6 to 12
inches below the center of the hearth about the tap
hole area 53. Cooling water flows to the annular spray
head 73 through supply pipe 75 from any suitable supply
source, not shown, at a pressure sufficient to provide
the water sprays 71 from orifices 72 of a height which
will impinge upon the bottom of the hearth, cooling the
hearth and making it less reactive so that the hearth
materials will not fail over a period making the cupola
in effect self-tapping so that the contents of the
cupola are unexpectedly released. The supply pipe 75
may be conveniently hung from U-shaped hangers 77 on
the bottom of the cupola structure or may be otherwise
supported so the pipe and spray head may be removed
when the metallics are to be tapped.
The spent pot lining material prior to being
charged to the cupola is sized between one-half inch
(1/2") (1.27 cm) and six (6) inches (15.54 cm) by
crushing and screening and will be preferably between
two (2) and four (4) inches (5.08 to 10.16 cm) or
somewhat less preferably five (5) inches (12.7 cm),
i.e. the SPL after crushing will pass through a four
inch (10.16 cm) or five inch (12.7 cm) screen and be
retained on a two inch (5.08 cm) screen. As indicated,
it is desirable to use 2 to 4 inch (5.08 to 10.16 cm)
pieces of SPL material as the fuel charge as this
generally provides the best fuel and burden support
efficiency. However, from a practical standpoint, the
attempt during preparation of the SPL for use to obtain
the best range may result in such a large rejection of
131~56
- 18 -
fines of 30~ or even more, that it may prove more
economical to use the technically less preferred range
of 2 to 5 inches (5.08 to 12.7 cm) for the bulk of the
material within the broad range of one half inch (1.27
cm) to six inches (15.54 cm) particulate size. Thus,
while it might be desirable to use the best range of
SPL material, the use Gf the less preferable range for
either all of the material, or more practical still,
for the bulk of the material, will be operationally
satisfactory both as to burden support and gas
permeability.
As indicated the graphitic material of the SPL
is inherently denser and less reactive than normal coke
and is also impregnated or coated with a cryolitic slag
which further seals any pores in the carbon structure.
The material is thus quite unreactive as compared to
coke and, in fact, reacts only to a very small degree,
if at all, in the upper portions of the cupola, but
burns steadily and with a large evolution of heat in
the tuyere area of the cupola. As explained, this is
very advantageous as the SPL does not therefore
significantly react with any rising gases as does coke
with basically a wastage of heat values in a portion of
the cupola where such heat values are neither desired
nor effective and frequently where the reaction of the
coke causes physical degradation of the coke leading to
loss of permeability in the coke bed in the cupola with
decreased combustion efficiency. The fully retained
size of the SPL as it migrates down the cupola assists
in the maintenance of good charge permability in the
bottom or lower portions of the cupola where a
satisfactory permability is required for the passage of
the incoming combustion air and for outward flow of
molten liquids. Thus the SPL fuel of the invention is
relatively more likely to reach the hearth area of the
1 3 ~
-- 19 --
furnace where it can be optimally combusted in
essentially its original condition. As explained, this
is not only because the graphitic material is
inherently more dense than coke, but also because the
initial porosity of the carbon as used in the lining of
an electrolytic aluminum reduction vessel is invaded
while in use in such vessel by the cryolite slag phase
which essentially removes or eliminates such porosity.
Hot gases in the cupola cannot thcrefore react in depth
with the carbon, but react only on the surface of the
spent pot lining material. The fluorine released from
SPL is soluble in the melt up to about 2%. The
fluorine ion is substantially the same size as the
oxygen ion and is therefore readily replaceable in
siliceous material for oxygen with no distortion of the
space lattice and no decrease in physical properties of
the siliceous material up to the solution limit of
about 2%.
In calculating the cupola burden coke is
usually arbitrarily assigned a carbon value of 90~,
i.e. 90~ of the coke will combust to form CO or CO2,
and spent pot lining material may also be somewhat
arbitrarily assigned an average carbon value of 60%.
The substitution of SPL for coke on a carbon-equivalent
basis, therefore, results in the use of approximately
1.5 pounds of SPL to replace 1.0 pound of coke in the
normal charge. An effective burden charge for a cupola
has thus been found to be as shown in Table 1
below where case B is a preferred practice of the
invention.
TABLE 1
CUPOLA BURDEN PRACTICE - POUNDS PER CHARGE
Meltables (slags, rocks) 3000
Fuels
Case A Coke 400
1 3 1 ~ ~ ~ 6
- 20 -
Case B 50% Coke 200
50% SPL 300
Sizing the SPL too large results in a loss of
fuel efficiency. Sizing the SPL too small results in
excessive fines loss, as the minus l/2 inch (1.27 cm)
material and preferably also the minus l-inch (2.54 cm)
material is not used in the cupola. It has been found
that a jaw crusher can produce SPL si zed between 6
inches (15.54 cm) and 1 inch (2.54 cm) with only about
lO~ loss in fines. When this size SPL is substituted
for coke, as in Table 1, the cupola tends to become
hotter evidencing that the fuel efficiency of SPL can
exceed the fuel efficiency of coke on a
carbon-equivalent basis.
The slag phase contains sodium, aluminum and
fluorine, all of which can benefit the melting process
in the cupola. The sodium and aluminum cations enter
the melt and are found in the glass phase resulting
from the fiberization of the melt. Aluminum helps to
make longer and stronger fibers while the sodium is a
strong base which is a commonly used constituent in
commercial glass manufacture. The fluorine helps bring
silica into solution in the hearth.
Fluorine as explained above also keeps hearths
from building up with silica residuals, a problem which
often can cause the cupola operator to drop the cupola
bottoms to remove the build-ups. Trials have shown the
addition of SPL to cupolas which already have built-up
hearths will flush out the build-up and allow longer
operating campaigns. Fluorine, like sodium and
aluminum, can become part of the melt and a stable
component in the mineral fiber since as pointed out
above it has the same ionic dimensions as oxygen and
tends to replace oxygen on a one to one basis in the
space lattice. Since SPL also usually contains only
13~5&
- 21 -
about 0.15% sulfur as compared to 0.7 to 1% for the
usual coke used in mineral cupolas with somewhat
detrimental results, the decreased sulfur burden is
also advantageous.
The unexpected effectiveness of SPL in reducing
siliceous build-up and preventing "high bottom" and
other difficulties in cupolas is, as indicated above, a
direct result of the fact that the fluorine is released
from the SPL low in the cupola in the combustion zone
near the build-up rather than the slag melting zone
higher up in the cupola from where it would be tapped
from the cupola with little possible contact with the
siliceous build-up. Such delayed release is believed
due to two major factors (a) the cryolite slag
generally impregnates the SPL to the extent that there
is no porous structure as in the case of coke so it is
only upon combustion of the carbon in the SPL that it
is released in the combustion zone, (b) the relative
inactivity of the SPL carbon due both to its dense
graphitic structure as well as to the fact that it is
both coated, and thoroughly impregnated with cryolitic
slag which in itself reduces the reactivity with
combustion oxygen or other oxidizing gases passing up
the cupola shaft. A lesser advantage is the
substantial lack of any substantial moisture content in
the SPL as it is produced and stored without the pickup
of any substantial moisture due to its impregnation
with the cryolitic material.
The present inventor has also discovered that
while it is very desirable to use spent pot lining for
his invention in those cases where it is particularly
desirable to decrease silicon build-up in a mineral
wool cupola and SPL, which is economical and usually
readily available, is not available, that cryolite or a
similar composition containing in particular fluorine
~ 3 ~
- 22 -
can be deliberately mixed with a carbon containing
material such as coal, graphite or the like, sized,
and/or briquetted and then used in the cupola as a part
of the burden, whereupon, when the fluorine is released
low in the shaft the siliceous build-up problem is
alleviated. Briquettes can also be formed of spent pot
lining fines and coke fines and bound together with
coke plant pitch to form briquettes which can be used
as all or a part of the fuel burden of a mineral wool
cupola or other shaft furnace and will have the desired
effect of reducing any siliceous build-up within the
furnace because the fluorides from such briquettes are
released low in the furnace shaft. A briquette thusly
made is shown in Figure 2. This particular briquette
has a top 61, a bottom 63 and fins 65 on the sides
resulting from the molding process by which it was
formed, but could be in almost any form. A desirable
composition would be approximately 40% coke fines, 45%
spent pot lining fines and 15% coke plant pitch.
It has been found that the use of the cryolitic
material impregnated carbon fuel of the present
invention not only removes silicious build-up and
results in a more efficient cupola operation by burning
low in the cupola or hearth, but that the use of the
fuel of the invention also results in a smoother cupola
operation and more homogeneous melt discharged from the
cupola, both as to temperature and composition. The
solution of residuals in the melt is better so that
such residuals do not form inhomogeneities in the melt
which can result in defects in the final mineral wool
fibers. Smooth operation becomes evident, for example,
in a smoother running slag notch which requires less
attention from the slag notch tender, one of the most
difficult jobs in cupola operation.
Spent pot lining, or SPL, is as indicated
131~6
- 23 -
above, derived from the electrolytic alumina reduction
vessels. The fundamental principal of the Hall-Heroult
process for the electrolytic dissociation of aluminina
is the solution of the alumina in a bath of aluminum
fluoride plus the fluorides of one or more metals more
electropositive than aluminum, e.g. sodium, potassium
or calcium. Both natural and artificial cryolite are
used as a base for making up the baths and the actual
bath composition may vary over a fairly wide range.
Pure cryolite, Na3AlF6, melts at 995 C and a eutectic
mixture 81.5 to 18.5 cryolite-alumina melts at 935 C.
The melting points may be reduced to as low as 700 C
by the addition of various salts. The specific gravity
of the bath must be less than that of liquid aluminum
if the aluminum is to be tapped off at the bottom of
the electrolytic cell. A typical bath may contain
66.8~ aluminum fluoride and 33.2% sodium fluoride.
A bath containing 59~ aluminum fluoride, 21% alumina
and 20% calcium fluoride has been popular. As can be
seen, therefore, the composition of the aluminium
reduction bath or cryolitic bath may vary fairly
widely.
The carbon cathode used in the aluminum
process may be prebaked blocks or rammed and baked in
place. It is important in both cases for the carbon
material to have a very low ash content. Consequently
the material of construction for such cathodes are
usually petroleum coke, anthracite fines of low ash
value and coke plant pitch which are pressed into
shape and baked resulting in a very dense graphitic
electrode which is effectively made more dense by
prolonged soaking during use in the cr-yolitic bath
material.
Furthermore, as will be recognized from the
above discussion, while a cryolitic soaked coke
131~6
- 24 -
material fGrms the basic fuel charge of the invention,
and this is most commonly provided by use of SPL, a
synthetic carbonaceous fuel charge may also be made for
the practice of the process of the present invention,
which synthetic carbonaceous fuel need not be the best
material for use in an aluminum reduction furnace, but
which is especially compounded for use in a mineral
wool cupola. Consequently, while as noted earlier, the
typical fuel charge material of the invention may
10 contain about 60% carbon, about 12% sodium, about 13%
fluorine, 10% aluminum and 5% miscellaneous materials,
materials suitable for the practice of this invention
may contain plus or minus 10% of these various
materials, but preferably plus or minus 5~. In other
words, the carbon content can vary within the range of
50% to 70% carbon, but preferably between 55% to 65~
carbon, fluorine may be between about 8% to 18%, but
preferably about 10% to 15%, aluminum between 5% to
15%, but preferably 7% to 13% and sodium may be between
20 7% and 18%, but preferably 9% to 15% with other lesser
materials, mostly potassium or calcium, making up any
remainder
It will be recognized from the foregoing
disclosure and discussion that the present invention
encompasses and provides all of the following:
(a) a new and advantageous method of
operating mineral wool cupolas,
(b~ a new and efficient method of disposing of
spent pot lining without any detrimental
environmental effects,
(c) a method for decreasing siliceous build-up
in mineral wool cupolas,
(d) a new and advantageous fuel for mineral
~ wool cupolas which is both more efficient
and more economical than present fuels,
131~6 f
(e) a new fuel product which aids in
decreasing and eliminating siliceous
build-up in cupolas.
In an example of the invention a mineral wool
cupola can be charged over an extended period with
consecutive charge burdens comprising:
200 lbs. per charge of coke
300 lbs. per charge of spent pot lining
800 lbs. of blast furnace slag
700 lbs. of electric furnace slag
700 lbs. of so-called phos slag, i.e.
electrofurnace phosphorus
reduction slag
800 lbs. of trap rock
Over an extended period a very desirable
mineral wool fiber is produced with a more uniform
temperature of the melts and no siliceous build-up in
the cupola.
Without SPL the normal charge for such cupola
20 would be:
400 lbs. coke
800 lbs. blast furnace slag
700 lbs. electric furnace slag
700 lbs. phos slag
800 lbs. trap rock
Over a period the melt from such charge can run
cooler in spite of the fact the carbon to meltables
ratio is actually higher than with SPL due to about 100
pounds of cryolite slag in the 300 pounds of SPL used
in the prior example. Periodic removal of siliceous
build-up is also necessary. The market value of coke
is, furthermore, about twice the cost of an equivalent
amount of SPL on an equivalent carbon basis.
It was previously thought by the inventor that
no more than 50% of the charge of a cupola furnace
131~5~6
- 26 -
should be SPL or a simulation of SPL. However, it has
now been found that up to about 80% of the carbonaceous
fuel charge can be SPL or SPL type material in normal
practice and that up to 100~ of the charge can be SPL
if special precautions are taken such as, for example,
forced cooling of the hearth bottom as explained above.
In the foregoing description and the following
claims, the following terms shall have the meaning
indicated as follows:
"Cryolytic material" means material which may
comprise natural cryolite or general simulations of
cryolite including very high percentages of fluoride
such as in particular aluminum fluoride, sodium
fluorides, calcium fluoride and the like.
"Carrier substance" means a particulate
substance adapted to hold a relatively large amount of
cryolytic material or the like on the surface and in
porosities, intersticies and the like in such material.
"Bulk of the material" means about 80% or more
of the material plus or minus about fifteen percent.
While the present invention has been described
at some length and with some particularity with respect
to several embodiments, it is not intended that it be
limited to any such particulars, but is to be construed
broadly with reference to the appended claims so as to
provide the broadest poc.sible interpretation of such
claims in view of the prior art and thereby to
effectively encompass the intended scope of the
invention.
