Note: Descriptions are shown in the official language in which they were submitted.
959
METHOD AND APPARATUS FOR MAKING
HIGH PURITY AMORPHOUS SILICA FIBERS
This invention involves a method of making high
purity amorphous silica fibers having excellent resistance
to devitri~ication by leaching sodium silicate ~ype fibers,
and apparatus used in the leaching process.
Background of the Invention -~
~. .
Following the approval of the space shuttle space
program a need arose for an amorphous s;lica fiber having
excellent resistance to devitrification at temperatures up
to 2500F and for exposure times at this temperature of up
to 4 hours. It has long been recognized that fibers of
substantially pure silica can be produced by forming fibers
from a glass such as type E glass having soPtening and
melting characteristics suitable for convenient fiber
formation followed by leaching to remove substantially all
of the non-silica components from the fibers by emerging
the E glass fibers in an aqueous solution of sufficient
acidity to extract the acid soluble components. Improved
fibers of this general type are disclosed in United States
Patent 3,687,850, but these Pibers w~ll not meet the above
described str~ngent dev~triPlcation res~stance requ~rements
because oP the relatively high alumina contents oP 4% to ~
8X and the relatively high contents of other non-sllica ~ ;
mater~als such as calcium ox~de, barium oxide, magnesium
oxide, and boron oxide.
. .
It has also been known to make silica fibers from
glass compositions containing most soda and silica, for
~ .
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~, . . . . ., . - , . .. . ~, .
~a)4S959
l example, see Un1ted States Patent No's. 3,092,531 and 3,560,177.
The fibers descrlbed ln the latter patent were useful to
temperatures of only about 220QF because of a relatively
high impur~ty level. Attempts to make amorphous silica
fibers having excellent resistance to devitrif1cation at
temperatures as high as 2500F according to the process
disclosed in United States Patent 3,092,531, but modified
for treatment of loose fiber instead of fibrous mats, resulted
in fibers having a high degree of nonuniformity and thus a
very low percentage, e.g., 15 to 20%, acceptance level based
on devitrificat~on resistance at 2500F for 4 hours.
Several techniques of leachiny were developed in `
an attempt to make the desired silica fibers. In one technique
sodium silicate glass fibers made using ordinary glass sand
were placed into an open tank. An acid solution was then
added to the tank and the tank was heated with burners
directly beneath the tank to bring the solution up to the
proper temperature for leaching. Once the acid solution was
brought up to the desired temperature, it was maintained at
this temperature until the leaching process was completed
with the batch in the tank being stirred occasionally to
break up the fiber clumps and red~stribute the fibers within
the tank. After the desired silica content was obtained7
the ac~d solut~on was drained from the tank and a rins~ng
operation was commenced. Once the fibers had been rinsed to
the necessary degree, they were removed from the tank and
pressed into cakes. This pressing operation eliminated
about 80% of the water from the fibers. The cakes were then
placed in an oven and dried. Afterwards, the outside surfaces
1 30 of the cakes were trimmed off and the cakes were broken up
;~ to form fiber clumps for shipment. Fiber made according to
~ this process proved to be very non-uniform in impurity level
. .
.
~S959
1 and in devitrifaction resistance and was unacceptable for
the intended use.
In an attempt to improve uniformity a process was
developed in which the fiber was put into a perforated
basket residing in the upper half of a leach tank. Hot acid
was recirculated in the tank~ passing down through the fiber
and rising up through an inverted funnel whose stem extended
into the middle of the basket area. This method was unsuccessful
for several reasons. First, the perking action (acid recirculation) a
lo did not begln until the temperature of the acid solution
reached 14nF. Vp to this time~ sodium was being extracted
from the fiber, but remained in the immediate vicinity of
the fiber. If the slurry was not periodically moved, enough `sodium salt could accumulate to recombine with the f~ber and
form a cementitious mass of bonded filaments. These conglomerates
were often too large to leach thoroughly. Second, stirring
could alleviate the consolidation problem, but was difficult
to do well by hand because of the stiffness of the fiber and
costly to do by machine because of the interference of the
center stem. Third, once the unit began to perk, lids were
required for the tank to protect the tank operator. Stirring
was needed during perking because, although a perforated lid
was set on top of khe basket to distribute the acid uniform~ty
over the fiber, channelling inevitably occurred as the acid
filtered down through the fiber, causing some pockets of
fiber to go incompletely treated. The installation of lids
prevented even hand-stirring unless the entire unit was shut
down. Finally, the perk tank could not be operated in a
controllable or reproducable manner.
In an attempt to avoid the problems associated
with the above-mentioned perking process, loose sodium
-3-
~L~45~S9
sil1cate fibers, made using ordlnary glass sand, were placed in
a perforated basket resting in the upper half of a leaching
tank. The acid solut~on was added to the tank until it just
barely covered the fibers in the perforated basket. Burners ~!
below the tank were ignited to heat the acid solution to the
desired temperature and immediately circula~ion o~ the acid
solution from the bottom of the tank back to the top of the
perforated baske~ was begun. This was accomplished using a
pump and line external of the leaching tank. The acid solution
was added back to the basket of fibers using either a rotating
spray manifold or by dumping the acid solution onto the top of
a perforated lid allowing the acid solution to drain down
through holes in the lid into t'ne fibrous mass. As soon as the
acid solution reached the desired temperature the burners were
used only to maintain the des1red temperature during the
leaching period. Following the leaching period the fibers were
rinsed and processed in the same manner as described above.
While this process eliminated most of the problems inherent in
the above-described perking process, the resultant fibers
showed a high dearee of non-uniformity and only about 15-20% of
the lots of Fibers made according to this technique displayed
;i the necessary devitrification resistance at 2500F that was
, .
requ~red.
It has been discovered that when the average level of
alumina in the leached fihers dropped below about 0.16%, by
weight, the fiber displayed good devitrification resistance,
but when the alumina content was above about 0.20%, substantial ~:
devitrification nearly always occurred in the 2500F :`
test. It was also determined that the alumina content in the
prior art leached fibers varied considerably from levels below
0.16% to levels well above 0.2% within the same batch.
:
:.
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. .
: ; , . .
~5~S9
Since the alumina impurity was being introduced into
these fibers by the use of industrial grade glass sand in the
fiber manufactur;ng process, it was decided that the problem
could be solved by using pure silica in place of the glass
sand. Fibers were made using "CAB-0-SIL", a high purity silica
product sold by Geofrey L. Cabot, Inc., and leached according
to the just above-described process. While having a very low
alumina content of below about O.OZ5%, by weight, and displaying
satisfactory de~itri~ica~ion resistance, the very ~ine particle
lo size of the "CAB-0-SIL", and other high purity silica materials,
would require pre-treatment, e.g., pelletizing, prior to melting
for the production of the glass fibers to prevent excessive
dust problems and losses. Also, the relatively high cost of ~ ;
these pure silica materials compared with ordinary glass sand
would be a significant disadvantage in the manufacture of
silica fibers.
Brief Summary of the Invention
It has now been discovered that if the silica raw
mater;al used to make the precursor sodium silicate fibers is
properly selected based on the level of non-silica impurities
it contains, and if the Pibrous mass is periodically gently
stirred and flufFed up during the recirculating acid solution
leaching and the rinsing cycles silica ~ibers can be produced
having excellent resistance to dev~triP~cat~on and relatively
low shrinkage and distortion up to 2500F. Using the process
of the present invention, relatively low cost silica raw materials
can be used to produce a silica fiber having alumina contents
above about 0.025% and below about 0.2%. The titanium dioxide
content, which is thought to be significant to devitrification
resistance, is preferably lower than about 0.02~ and preferably
below about 0.01% in the final silica fiber. Silica fibers
: :
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.~, .. . . I ~
:~. - ,: , , , . ,
,
l()~S9S~
made according to the process of the present invent1On and in
the apparatus of the present invention are very uniform in
properties as evidenced by an lncreased acceptance level of 90-
95% of the lots or batches of f1ber produced, based upon a
devitrification resistance for 4 hours at 2500F of less than 5
wt. percent crystallinity, preferably less than 2 wt. percent,
and most preferably less than 1 wt. percent, as determined by
X-ray diffraction in comparison with a 100% cristobalite control
sample.
The apparatus of the present invention provides a
mechanical stirrer or agitator in addition to or in combination
with the means used to distribute the acid solution to the top
surface of the fibrous mass in the perforated basket described,
supra.
Brief Description of the Drawings
FIG. 1 is an elevational view of a digestor comprising
a leaching tank, acld solution recirculation system, and removable
stirrer with the leaching tank shown in cross-section.
FIG. 2 is a cross-section of the stirring blade shown
in FIG. l as viewed along lines 2 2.
FIG. 3 is a modified version of the apparatus shown
ln FIG. 1 wherein khe stirring blade and rotating spray manifold
are combined,
FIG. 4 is a cross-section of the combined stirring
blade and spray manifold shown in FIG. 3 as viewed along lines
4w~,
FIG. 5 is another version of the apparatus of the
type disclosed in FIGS. 1 and 3.
Detailed Description and Preferred Embodiments
Precursor sodium silicate glass fibers for use in the
present invention are made using conventional glass fiberizing
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,
S959
1 techniques, such as the well known flame attenuation process.
The resultant fluffy fibrous mass which contains no binder is
collected on a moving permeable belt and should have properties
as set forth in Table I.
TABLE I
Suitable Most
Range Preferred Pre~erred
Average Fiber
Diameter (Microns) 0.2-5 1-2 1.4-1.7
Chemical Analysis
(Wt. Basis)
SiO2 72.6-76 74.35-74.65 74.4-74-6 ~ ?
Na20 23 Min. 23 . 5 Min. 24 . 2 Min ~ -
A123 0.16-.02 0.11-.025 0.07-.03
TiO2 0.01 Max. 0.008% Max. .005X Max.
K20 0.06 Max. 0.06 Max. 0.06 Max.
MgO 0.01 Max. 0.01 Max. 0.008 Max.
CaO 0.03 Max. 0.026 Max. 0.02 Max.
Fe203 0.03 Max. 0.028 Max. 0.025 Max.
Sum of
Other R203 0.01 Max. 0.01 Max. 0.01 Max.
Sum of
~ Other RO 0.01 Max. 0.01 Max. 0.01 Max.
Sum of
Other R02 0.01 Max. 0.01 Max. 0.01 Max.
; The mat-like fibrous mass is removed From the perforated
collection belt in the form o~ clusters or sheets having frayed
edges and are leached in apparatus shown in FIGS. 1-5.
Referring to FIG~ 1, the leaching apparatus includes
a cylindrical tank 22 which is made of stainless steel or
"MONEL", availabte from International Nickel Co., or other acid
resistant material, and is mounted on a support frame 24 above
a bank of burners 26 which heat the leaching solution 28 within
the tank 22. Where the tank is made from a material not suited '~ -
to heating in this manner, such as plastic or reinforced plastic,
~ .
~(14S95~
l a conventional submers~on heater (not shown) can be used in
place of the burners. A conventional control valve 30 is
provided to ~egulate the burners 26 to control the temperature
of the leaching solutlQn 28 within the tank.
A removable stainless steel or "MONEL" basket 32
having a permeable bottom wall 36, such as a perforated plate,
screen, etc., is set in the tank 22. The basket 32 is cylindrical
in shape and is supported by an annular ledge 34 which engages
a bottom wall 36 of the basket. The bottom wall 36 of the
basket ;s preferably a perforated plate provided with a plurality
of round apertures 38 having diameters about 1/8 to 1/4 inch
distributed throughout the bottom wall to permit the passage of
leaching solution therethrough while retaining the fibrous mass
in the basket. The basket 32 is preferably dip coated in
polyethylene for further protection from corrosion.
The tank 22 is provided with a drain 40 for draining
1eaching solution and rinse water from the tank, The bcttom ~ ~
portion of the tank need not be flat. The tank is also `
provided with a circulating system 42 for circulating and `~
recirculating the leaching solution through the fibrous mass
during the leaching cycle and including the initial start-up
of the process while the leaching solution is being brought
up to the desired temperature. The circulating system 42
includes a sta1nless steel outlet llne 44 provided with an
on-off valve 46. The outlet line 44 is connected to an
impellor type or tube type pump 48 which pumps the fluid
into a discharge line 50 for discharge into the top of the
tank above the basket 32. A by-pass line 52 with a regulatory
or on-off valve 54 is provided to direct fluid back into the
tank 22 at a point below the basket 32, if desired. A discharge ~-
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:~ . . . - : .
~3~a~9 5~3
line so is also provided w'th a reyulatory valve 56 to
regulate the amount of leaching solution discharged into ~he
top of the basket. By adjusting the regulatory valve 56 and
the valve 54 in the by-pass line 52, the flow rate of the
leaching solution through the basket 32 can be easily regula~ed.
As shown in FIG. l, the discharge line 50 is
connected to a rotating spray arm 58. The spray arm 58 is
rotated by a conventional motor 60, which can even be driven ;~
by the flow of leach~ng solution passing therethrough, and ~-
the arm is provided with a plura~ity of nozzles 62 or apertures ~ `
which spray the leaching solution onto the upper surface of ~ ;
the fibrous mass within the basket to distribute the solution ;;
,~
over the fibers, to stir the fibers, to prevent compaction
of the Fibers, and to force the fibers down into the leaching
solution. While the apparatus as shown with only one spray
arm, a plurality of rotating or preferably fixed spray arms
can be utillzed so long as they provide a uniform application
of leaching solution over the ent;re top surface of the
Fibrous mass in the basket 32.
As an alternative to one or more spray arms, the
basket 32 can be provlded with a cover which fits over the
upper end of the basket, e.g., see element 128 in FIG. 5.
This cover is provided with a plurality oF apertures d~strlbuted
throughout the cover to effect distrlbution of the leaching
solution over the entire upper surface of the fibrous mass
in the basket 32. With this arrangement, the discharge line
50 would discharge leaching solution directly onto the upper
surface of the cover.
With either one or more spray arms9 or the perforated
cover, the discharge line 50 is provided with a conventional
flow meter 64 to determine the rate of flow of the leaching ~
' g _ :
.
~4~ 9 ;~
solution through the basket. The dlscharge line is also
provided with a conventional thermometer 66 and a pH probe 68
to monitor the tempera~ure of the solutlon being discharged
into the basket and the pH of the solution so that the 1each~ng
solution can be mainta1ned at the desired temperature and pH
value for the particular leaching operation. For safety purposes,
a removable cover ~not shown) can be placed over the top of
tank 22 to prevent any over spray of the leaching solution.
~ The rotatiny spray arm 58 and drive 60 can be rotated
; lo away from the top of the tank 32 by rotation of a rotating
connectQr Sl about pipe 50. This is done to provide access for
the stirring mechanism 72 to the fibrous mass.
The stirring mechanism 72 includes two stirring
blades 74 made o~ an acid resistant material like stainless `~steel. These stlrring blades preferably have a cross-section
of the shape shown in FIG. 2, but other convent~onally shaped
stirring blades could be used. The stirring blades 74 fit into
a blade holder 76 which in turn is attached to a rotatable
shaft 78. The shaft 78 passes through conventional bearings `
(not shown~ mounted on support arms 80 and 82. The support
arms 80 and 82 are mounted to a vertical support member 84.
This vertical support member 84 is in turn mounted on a rod 86
of a cyl1nder 88 mounted to the ~loor adjacent the leaching
tank. The rod 86 can be raised or lowered by pumping fluid to
~, and away from the cylinder 88 at points 90 and 92 in a well
known manner. Thus rod 86 can be made to move up and down in
the direction shown by the arrows in FIG. 1 to raise and lower
the stirrina blades 74 out of or into the fibrous mass 70 in
the basket 32. ;
Shaft 78 is caused to rotate by starting a motor
94 mounted on support arm 80 which is connected to a sprocket
':
~.
5~59
l or pulley (not shown) on shaft 78 in the vicinity of support
arm 82 b~ any suitable drive means, such as a chain or belt (not
shown).
In the operation of the apparatus as shown in FIG. 1
the basket 32 and the tank 22 are first rinsed out with deionized
water having an electrical resis~ivity of at least 200,000 ohm-
cm. The tank 22 is then filled with deioni7ed water to within
6 to 10 inches from the bottom of the basket 32. The valves 46 -
and 56 are opened and the pump 48 is started. Next, a sufficient
amount of reagent grade sulfuric acid or hydrochloric acid is
added to the tank 22 to produce the desired acid solution
concentration. Hydrochloric acid is preferred and an acid
solution concentration of about 19 l/2 gallons of reagent grade
HCl to 800 gallons of water and 120 lbs. of fiber. This produces
a pH of about 0.5. The acid solution is circulated through the
pump 48 and the rotating spray arm 58 for about 5 minutes to
insure proper mixing. The circulating flow through the flow
meters 64 is adjusted by valve 56 to provide an acid solution
flow rate of between lO and 20 gallons per minute. The pump 48
is then stopped and the rotating spray arm 58 and drive 60 are
rotated away from the top of the tank 22 by rotation of member
51 about pipe 50.
The stirr~ng blades 74 are then lowered into the
basket 32 by manipulation of the cyllnder 88 until the
bottom of the blades 74 are about 1/2 inch above the surface
of the acid solution ln the basket 32. The stirrer blade
drive 94 is started, usually as the blades are being lowered,
which causes rotation of the shaft 78. The output speed of
the drive 94 is adjusted to produce a blade rotation of less
than 20 rpm and preferably less than about 17 rpm to prevent
breaking the fibers by too rapid a movement o~ the stirring
,
~ --11-- ,,
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~L~4~9 S 9
l blades. Next flbers, preferably in the form of sheets
between about 4 and 30 lnches square, preferably between
about 12 and 18 inches square, and l/4 to 1/2 inch thick are
added to the basket 32, preferably without contacting the
st;rrer blades until the f1bers are ~n the acid solution.
As the fibers are added the acid solution level is raised
gradually by introduc~ng deionized water between the basket
32 and the tank 22. The blades 74 are mainta;ned sl;ghtly
above the acid surface by manipulation of the cyl-inder 8B.
After all of the fiber for a particular batch has been added
to the basket 32 the acid solution level is raised to
within about 6 to 8 inches below the top r;m of the basket
32 by addition of deionized water.
Ne~t, the st;rrer blades are slowly lowered, wh;le
rotating, through the fibrous mass and held for, a short
t;me, e.g., about one minute, on the bottom of the basket.
The rotating blades 74 are then gradually ra;sed through the
fibrous mass to fluff up the fibrous mass to prevent channeling
and to allow ac;d solut;on to contact all of the f;bers.
The blades 74 are raised at a rate that w;ll take about l/2
to 10 minutes, preferably about 3 minutes, for the blades to
clear the top of the fibrous mass.
The stirrer blades 74 are then raised to the
position shown in FIG. 1 and the blade drive 94 is shut
down. The spray arm 58 is then swung back over the basket
32 and the valve 56 is opened sufficiently to provide an
acid solution flow rate through the flow meter 64 and the
rotating spray arm 58 of about 30-50 gallons per minute.
The acid solution is sprayed onto the top of the fibrous
mass from the nozzle 62, flows through the fluffed up fibrous
mass 70, and into the acid solution reservoir 28 in the
bottom of tank 22 in the manner shown by the arrows, and on
- 12 -
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~O~S~59
l into plpe 44 for recirculation by pump 48. The burners 26
are then lit and adjusted by valve 30 to raise the temperature
of the acid solution as determ~ned by the thermometer 66 to
a temperature in the range of about 180 to 205F, preferably
180-190F, within a suitable period of time, e.g., in less
than 8 hrs. and preferably within about 4 to 8 hours.
Every hour while the acid solution is heating, and
after it has reached the prescribed soaking temperature, the
fibrous mass 70 is fluffed up by the rotating stirring arms
74 in the same manner as described above. When the fiber is
relatively stiff and tends to float near the surface of the ;
acid solution, the stirring blades 74 are lowered while
rotating through the fibrous mass until they reach the
bottom of the basket. They are left rotating there for one
minute and then raised very slowly, e.g., over a three
minute period, up through the fibrous mass 70 and out of the
top of the basket 32. Later in the cycle when the fiber
becomes well broken up and tends to lay near the bottom of
the basket, the stirring blades 74 are raised and lowered at
least 5 times over a three minute period through the fibrous
mass 70, allowing the blades to rest on the bottom for about
30 seconds during one of the raising and lower~ng cycles.
Dur1ng the leach~ng cycle the ac~d solution should
i be maintained ~t a level sufficiently high to just cover the
top of the fibrous mass 70. This acid solution level is
ma~ntained by adding deionized water to the tank 22 as
required. After the acid solution has reached 180F the
above described procedure is continued for one half to about
nine hours, or until the sodium oxide level in the fiber is
below about .02 wt. percent. At this point the acid solution
is drained from the tank 22 and the fibrous mass 70 by
opening the valve in line 40.
3LS3~S 9 5~
1 ~fter the actd has been drained, the sides of the
tank and the basket containing the fibrous mass 70 are
rinsed down with delonized water and dralned from the kank
22. The drain valve 1n the line 40 is then closed and the
tank 22 is filled with deionized water to a level just
slightly above the fibrous ~ass 70. The fibrous mass 70 is
stirred for about 1~2 to 10 minutes, preferably about three
minutes, by raising the lowering the stirring blades 74 ~ ~
through the fibrous mass about 2-5 times, allowing the `
blades to remain on the bottom of the basket for about 30
seconds during one of the cycles. The stirring blade drive
94 is then shut down and the water is drained from the tank
22 by opening the valve in the line 40. The valve is closed
and the tank is refilled with deionized water. The pump 1s
started and the rinslng water is recirculated through the
fibrous mass for 30 minutes at a flow rate of about 30-50
gallons per minute. During the last port1On, l.e., 10
minutes, preferably the last three minutes, of this 30
minute rinse the fibers are aga;n stirred as described
2 o above.
Following this rinsing cycle, three rinse water
samples are taken at different locat1Ons 1n the tank and
the1r resit1v1ty 1s measured. If all three samples have
res1t1v1t1es above 150,000 ohm-cm and 1f the difference
between the highest and lowest readings is lO0,000 ohm-cm or
less, the f1ber 1s suffic1ently rinsed. If the rinse water
fa11s th1s res1st1vity test the tank is dra1ned and the last
described r1nsing cycle is repeated.
A periodic stirring and fluffing of the fibrous
mass during the leaching cycle and the rinsing operation
produces the uniformity required in the leached fibers. I
Using the techniques described above, combined with a more
- 14 -
. . . - .
1~595g
1 uniform sllica source resulted in an improvement in recovery
rate of devitrification resistant s;lica fibers from the
prior art 15-20% to over 90%.
After rinsing the f~bers, the fibrous mass is
removed from the basket 32, dried, and packaged for shlpment.
Wh~le the fibers can be dried by pressing them into cakes or
centrifuging and drying the cakes or fiber in an electric,
dielectric, or gas ~ired oven, drying of the loose fibrous
mass at temperatures in the range of 500-600F or higher i5
preferred. Lower temperatures can be used but at the expense
of some strength in the dried fiber. If maximum strength is
important to the intended end use lower temperature drying
; should not be used.
While ordinary glass sand may frequently have a
composition suitable for producing fibers having a composi~ion
meeting the 1imitations shown in Table I, the non--sllica
porkion of ordinary glass sands vary and will occasionally
be excessive for use in the present process. Thus, careful
selection based on frequent chemical analysis for non-silica
components is required to insure successful results when
; using ordinary glass sands. Two, high-silica, natural raw
materials have been found to be especially useful in the
present invention. Typical chemical compos1t~ons of these
two naturally occurring materials are shown in Table II.
TABLE II
; CYhPemical Anal,ysis (wt.%) Material A Material B
S~2 99.6-99.9 99 7-99.8
A1203 .04-.08 .05-.06
Fe23 .004-.005 .02-.035
Ti02 Trace .007-.008
CaO .001-.007 .01-.018
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i~LS959
MgO .001 .002~.004
LOI Not .08-.lO
Determined
The two materials descrlbed in Table II have
particle sizes simllar to ord1nary glass sand and thus can
be handled and melted in the same manner as when ord;nary
glass sand is used for the silica component in the glass
fibers. The cost of the materials described in Table II are
significantly less expensive than synthetic high purity
silica materials. Material B above compares favorably in
1 0 ~ ~ ~
price with ordinary glass sand and the price of mater;al A
above is less than one tenth that of the synthetic high
purity fumed silicas.
FIG. 3 illustrates a modification of the apparatus
shown in FIG. l in which the stirring blades and the spray
arm have been combined to eliminate the need to move the
spray arm away ~rom the top of the tank to permit stirring.
Like elements ~n the embod~ment shown in FIG. 3 to the
embodiment shown in FIG. l are identified by the same numerals.
In this embodiment the stirring blades 75 are hollow and
have a preferred cross section as shown in FIG. 4 as viewed
along lines 4-4 in FIG. 3. ~he bottom plates of the stirrlng
blades 75 are fitted w~th no~zles 62 through whlch the acid
solution and rins~ng water are sprayed onto the fibrous
mass. The acid solution ~s fed to the hollow stlrring
blades 75 via a hollow blade holder 76 and a hollow shaft
78. The acid solution return line 50 is connected to the
hollow shaft 78 by a flexible acid resistant hose 61 to
permit the stirring blades 75 to be moved into and out of
the basket without requiring the disconnection of the acid `~
solution recirculating system. Using the embodiment shown ~-
in FIG. 3, it is not necessary to discontinue recirculation
- 16 -
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. - ~ . . . . . . .
:, . ,..... , : , .. . ': . ~, , ' i
3L~D~j9 ~
1 of the acid solution during stirring. Note that in this
embodiment the spray arm drive 60 used in the embodiment
shown in FIG. 1 ~s not required s~nce the stirring blade
drîve 94 also serves as a spray arm drive. By using a
variable speed drive for khe drive 94 different rotational
speeds can be prov1ded, if desired, for the stirring function
and for the spraying function. Otherwise, the embodiment
disclosed in FIG. 3 is operated in the same manner as ~hat
described for the embodiment shown in FIG. 1.
A still further embodiment of the apparatus of the
present invention is illustrated in FIG. 5. In this embodiment
a plurality of stirring blades 96 are located at three
different levels in a perforated basket 98 setting in a tank
100. Baffles 102 are located on the interior o~ the basket
and inclined at 45 to cause the fibrous mass striking the
baffles to move upward along the outer walls of the basket.
A sufficient number of the baffles 102 are placed around the
inner periphery of the baske~ to produce a fluffing action
when the stirring blades 96 are rotated. The bottom and
middle sets of the stirring blades are angled to direct the
fibrous mass upward whereas the top stirring blades are
angled to direct the fibrous mass downward as the stirring
blades are rotated. The stirring blades 96 are attached ko
a drive shaft 106 by blade holders 104. The drive shaft 106
is rotated by a motor 110 and a variable reducer 108. The
tank 100 has fixed covers 112 to prevent over spray or
splash~ng of the acid solution. An outlet 113, a valve 114,
a return line 116, a pump 118, a flow meter 120, a thermometer
122, and a pH meter 124 are identical to their counterparts
in the other embodiments. An acid resistant flexible line
126 completes the recirculation system. A perforated plate
128 is illustrated in this embodiment for distributing the
- 1 7 -
- , . . . , ~,
5959
1 acid solution to -the fibrous mass in a uniform manner, but
it ls to be understood that thls perforated plate could be
replaced by one or more rotating spray arms, or by a plurality
of spray heads ln a fixed position to unlFormly cover the
top of the fibrous mass. In this embodiment the cover 112
and the entire stirrer assembly can either be lifted out of ~ -
the tank 100 and the basket 98 conventional means (not
shown), or the tank 100 and the basket 98 can be lowered
away from a fixed stirring blade assembly and cover 112 by a
lo conventional means (not shown). Drain line 127 and valve
128 allow the tank to be drained of acid solution or rinse
water in the manner above described. A conventional submersion
heater (not shown) provides the necessary heat for the
leaching solution.
The operation of the apparatus of this embodiment
is the same as the operation of the apparatus of the other
embodiments except that the stirring blades 96 are merely
turned on for a few minutes, e.g., about three minutes, each
hour during the leaching cycle and during the rinsing of the
fibers. This embodiment does offer the advantage that the
acid solution and rinse water recirculation can continue
from above the top of the fibrous mass during the entire
leaching and rinsing process, including the time during
which the st~rring blades are in operation.
Fibers made according to the present invention
typically have compositions as shown in Table III, in percent
by weight:
TABLE III
Chemical Analysis Suitable Preferred
, ~ ,
SiO2 99.6 min. 99.7 min.
A1203 0.20 max. 0.025 - 0.16
Na20~K20 0.02 max. 0.01 max.
- 18 -
~ ' '' ~,.
,
~S~59
1 CaO + MgO 0.04 max. 0.03 max.
TiO2 0.01 max. 0.007 max.
Most preferably the alumina content of the finished
fibers is within a range of 0~03 - 0.11 wt. percent.
In describing the invent;on certaln embodiments
have been used to illustrate the invention and the practice
thereof. However, the invention is not limited to these
specific embodiments as other embodiments and modiflcations
within the spirit of the invention will readily occur to
those skilled in the art on reading this specification. The
invention is thus not intended to be limited to the specific
embodiments disclosed, but instead is to be limited only by
the claims appended hereto.
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