Glass manufacturing process having boron and fluorine pollution abating features

Boron and/or fluorine values are reclaimed from a boron and/or fluorine laden gas stream emanating from a glass melter by means of a preheating bed of glass-forming batch agglomerates. The boron and/or fluorine values in such gases are first reacted with a boron and/or fluorine reactive material and the gases then conveyed into such a preheating bed to separate at least a portion of the reaction products.

TECHNICAL FIELD 
The present invention relates to the art of glass manufacturing. More 
particularly, this invention relates to glass manufacturing wherein glass 
forming batch ingredients in the form of agglomerates are preheated with 
flue gases emanating from a fossil fuel fired melter. Even yet more 
particularly, this invention relates to recovering boron and/or fluorine 
values from such gases by means of such agglomerates. 
BACKGROUND AND SUMMARY 
Glass, for many years, has been manufactured by a process wherein 
particulate, glass forming batch ingredients were dispensed into a glass 
melter, for example a fossil fuel fired melter, then vitrified and formed 
into articles of commerce. More recently the glass industry, like society 
as a whole, has become acutely aware of the shortages of energy and the 
need to improve the quality of our environment or atmosphere. 
Consequently, steps have been taken to provide for glass manufacturing 
processes which consume lesser amounts of energy, minimize pollution, and 
increase the throughput, or output, of the melting furnaces employed. One 
technique which has been developed to accomplish this involves combining 
glass batch ingredients and water into agglomerates, for example, pellets 
or briquettes, and then conveying hot effluent gases from above a pool of 
molten glass in a melter through a bed of such agglomerates so as to dry 
and preheat the agglomerates, and thereby recover otherwise wasted energy, 
and likewise to recover, in the bed of agglomerates, materials which 
otherwise could pollute the atmosphere. The individual preheated 
agglomerates are then fed to the melting furnace and melted. While such 
processes have improved the efficiency of melting, for example, by 
providing for higher throughputs per square foot of melter area, and have 
decreased the amount of wasted energy, such processes still need 
improvement in their pollution abating characteristics. 
With the foregoing in mind, it is the general object of the present 
invention to provide for a glass manufacturing process which makes 
efficient use of energy, has a high melter throughput, and which has 
improved pollution abating characteristics. More specifically, it is the 
object of this invention to improve the recovery of boron values and/or 
fluorine values carried by furnace effluent gases to thereby minimize 
atmospheric pollution. These values are then recycled into the melter and, 
consequently, once the process reaches equilibrium, an adjustment may be 
made to the batch composition to employ lesser amounts of glass making raw 
materials. 
Thus, in accordance with the present invention, there is provided an 
improvement in glass manufacturing processes of the type comprising 
combining glass batch ingredients, including a source of fluorine and/or 
boron, with water into agglomerates, conveying hot, boron and/or fluorine 
containing, effluent gases from above a pool of molten glass to a bed of 
agglomerates and passing such gases directly therethrough so as to preheat 
the agglomerates to an elevated temperature, for example a temperature in 
excess of at least about 500.degree. C., feeding the preheated 
agglomerates to a glass melting furnace and melting the agglomerates 
therein. The improvement essentially resides in introducing a fluorine and 
boron reactive material into the hot gases prior to passing the gases 
through the bed of agglomerates, reacting boron and fluorine values in the 
gases with said material at a temperature, for example, in excess of about 
500.degree. C. and then recovering the reaction product thereof in the bed 
upon passage of the gases therethrough. As used herein, the terms "boron" 
and "fluorine" generally comprehend any compounds existing in the glass 
manufacturing process which contain boron or fluorine and includes even 
the elemental forms. Preferred sources of boron and fluorine for a glass 
manufacturing batch as provided herein will be colemanite which has been 
treated to remove substantially all of its chemically bound water, i.e., 
calcined colemanite, and sodiumsilicofluoride. Exemplary of the forms in 
which boron and fluorine may exist when being conveyed in the flue gases 
from the melting furnace are the compounds H.sub.3 BO.sub.3, HBO.sub.2, 
HF, and BF.sub.3. 
In accordance with another feature of this invention, there is provided an 
improvement in glass manufacturing processes of the type comprising 
providing a gravitationally downwardly flowing vertical bed of glass batch 
agglomerates, said agglomerates containing a source of fluorine and/or 
boron, passing flue gases emanating from a combustion fired melting 
furnace directly through said vertical bed to preheat the agglomerates, 
said flue gases containing fluorine and/or boron values, and melting said 
preheated agglomerates in the combustion fired furnace. The improvement 
comprises decreasing the fluorine and/or boron content of the gases after 
passage through the bed by reacting the fluorine and/or boron values in 
the gases, prior to passage through said bed and while the gases generally 
have an absolute humidity substantially no higher than their absolute 
humidity upon leaving the furnace, with an alkaline earth metal oxide, or 
a precursor compound of such oxide. In so reacting the boron and/or 
fluorine, the reaction products are then recovered in the agglomerate bed 
and recycled into the melting furnace. In this way, the amount of boron in 
the flue gases after passage through the preheating bed can be decreased 
by as much as 30 or 35 percent or even more, and the amount of fluorine 
can be decreased by as much as 15 or 20 percent or even more.

DESCRIPTION 
Referring to FIG. 1, it will be seen that glass forming batch materials, 
and water, are converted into individual agglomerates, preferably pellets, 
on a rotating disc pelletizer. The free water content of the pellets will 
vary with different batches, but generally may be about 10-20 percent by 
weight (dry basis). While not shown, the pellets, if desired, may be 
subjected to a screening operation to select pellets of a nominal size of 
about 3/8 inch to 5/8 inch in diameter. These pellets are then transported 
by suitable means 2, such as a belt conveyor, to a feed hopper 4 and then, 
in turn, the pellets, through a spider-like feeding arrangement 6, are fed 
to a pellet heater 15 which maintains a vertical bed of pellets (not 
shown) therein. The individual pellets generally move gravitationally 
downwardly in the bed of the pellet heater and are discharged therefrom as 
hot, individual pellets and supplied by a duct member 7 to a batch charger 
which conveys them to a fossil fuel fired glass melting furnace. The 
combustion gases, or flue gases, which carry boron and/or fluorine values 
from the melting furnace are conveyed by suitable means 8, for example, a 
duct, to a recuperator 10 where they are indirectly cooled with air, for 
example, from a temperature of about 1427.degree. C. to a temperature in 
excess of about 500.degree. C. and more typically to a temperature of 
about 760.degree. C.-816.degree. C. The then indirectly heated air is 
supplied, through appropriate ducts 28, for use as combustion make-up air. 
The flue gases, after passage through recuperator 10, are then conveyed by 
suitable duct means 9 to the pellet heater where they flow in direct 
countercurrent contact with the pellets, to dry the pellets and preheat 
them to an elevated temperature. The flue gases leave the pellet heater 
through a suitable outlet designated 12 and if desired may be passed 
through a secondary pollution abating device 19. This device may take the 
form of a cyclone or bag collectors. Preferably, the flue gases will be 
supplied to the pellet heater by a manifold-type arrangement with 
entrances into the heater being on diametrically opposed sides of a lower 
frusto-conical portion 14 of the preheater 15. In accordance with sound 
engineering practices, the gases will be distributed generally uniformly 
into the heater preferably by employing an inverted "V" shaped member 16 
which generally spans frusto-conical portion 14. The manifold-type gas 
inlet under the inverted "V" shaped member 16 is generally exemplified in 
the drawings by 17. Disposed intermediate the outlet of recuperator 10 and 
the inlet of pellet preheater 15 is a chamber 13, in the nature of a slag 
box, through which the flue gases pass and which provides an opportunity 
to remove large particulate material being pneumatically conveyed in the 
flue gases from the furnace. 
Disposed intermediate chamber 13 and the exit of the flue gases from the 
recuperator there is schematically illustrated the preferred mode of 
practicing this present invention by adding a boron and/or fluorine 
reactive material into the flue gas stream. Since there is no cooling of 
the flue gases by evaporative contact with any water spray, the flue 
gases, at the point of introduction of the reactable material, will have 
substantially the same absolute humidity as that of the gases exiting from 
the furnace. The reactable material is introduced by a device 
schematically illustrated at 11 which device preferably is a volumetric 
screw feeder. In order to more uniformly disperse the material into the 
flue gas stream, an air injection system is desirable and it will also be 
found desirable to protect the barrel, in which the screw feeder rotates, 
by providing a jacket therefor in which an appropriate cooling medium is 
circulated. Water can be employed as such a cooling medium. 
In order to assist in controlling the temperature of the flue gases in the 
pellet heater and those liberated therefrom, a heat exchanger positioned 
in the pellet heater may be employed. As generally illustrated in the 
drawings, the heat exchanger comprises an inlet manifold 22 to which is 
supplied a suitable heat transfer medium via duct 26 and disposed on the 
opposite side externally of the pellet hopper is an outlet manifold 24 
from which the heated heat transfer medium is removed by a duct 26'. In 
fluid communication with the two manifolds will be a plurality of duct 
members (not shown) which are generally located in the pellet bed. Further 
details with respect to the arrangement set forth in the drawings can be 
seen in U.S. Pat. No. 4,184,861. Additionally, it will generally be found 
desirable to dilute the flue gases passing through the pellet heater with 
ambient air. Such ambient air may be added at any convenient location. 
Generally, the overall process will be operated such that the temperature 
of the gases exiting preheater 15 in duct 12 will have a temperature of 
less than about 450.degree. F. (232.degree. C.), more desirably, less 
than about 400.degree. F. (204.degree. C.), and even more suitably, less 
than about 300.degree. F. (149.degree. C.). It will, of course, be readily 
apparent to those skilled in the art that the temperature should not be 
allowed to drop so low as to be below the dew point of the gases, as 
undesirable condensation will result. 
The above arrangement is ideally suited for manufacturing a wide variety of 
glasses, but is especially well adapted for the manufacture of boron 
and/or fluorine containing fiberizable textile glasses. Exemplary of these 
glasses are low alkaline metal oxide containing glasses, for example, 
glasses containing, if at all, less than about 3 percent by weight of 
alkaline metal oxides and more typically less than 1 percent by weight. 
Further exemplary of such glasses are the alkaline earth aluminosilicates 
where, for example, the cumulative amount of the alkaline earth oxides 
plus alumina plus silica is in excess of about 80 percent by weight, and 
quite commonly in excess of about 90 percent up to in some instances 
virtually 100 percent by weight. Specifically exemplary textile glasses 
are those commonly referred to in the art as E-glass which may be 
categorized as an alkaline earth aluminoborosilicate glass. The latter 
type glasses typically will comprise at least about 85 percent by weight 
and more commonly on the order of about 93-95 percent by weight of silica 
plus alumina plus alkaline earth metal oxides plus boric oxide. Such 
glasses also include fluorine and may include such other adjuvants as iron 
oxide, titanium dioxide, and strontium oxide. 
When the more common glasses are contemplated for the practice of this 
invention, for example, bottle glass and flat glass which typically 
contain alkaline metal oxides in excess of that noted above, it is 
preferred to dry and preheat the agglomerates in accordance with the 
teachings of co-pending U.S. application Ser. No. 095,870 now U.S. Pat. 
No. 4,248,615 and Ser. No. 095,871, now U.S. Pat. No. 4,248,616 both of 
which are hereby incorporated by reference. 
While the above, and the example which follows, show a preferred manner of 
practicing this invention is using pellets, it will be apparent that any 
agglomerate forms may be employed. Such agglomerates are composite, 
integral self-supporting masses consisting essentially of all the 
substantial batch materials and may take the form of balls, extrusions, 
discs, briquettes and the like. 
Best results will be obtained if preheater 15 is designed with certain 
criteria in mind. First of all, as exemplified in the drawing, the pellet 
preheater will include an upper cylindrical portion and a lower 
frusto-conical portion. Generally, considering the diameter of the 
cylindrical portion as D, the height of the cylindrical portion should be 
about 1-2 times D and the height of the frusto-conical portion should be 
about one and one-half times D. The inverted "V" shaped gas distributor 16 
should generally be located in the frusto-conical portion so that the area 
under the inverted "V" is about 50% of the total conical cross section at 
that position, with the height of the pellet bed being maintained at least 
about 0.3-0.5 times D above the flow distributor. Additionally, it will be 
desirable that the included angle between diametrically opposed sidewall 
portions of the lower frusto-conical portion 15 be approximately 30-45 
degrees, and preferably 35 or 36 degrees. Generally it is preferred that 
the superficial velocity of the flue gases through the cylindrical portion 
with a diameter D be between about 20 to 60 standard feet per minute. 
The preferred boron and/or fluorine reactable material will be an alkaline 
earth metal oxide, or a precursor to such oxide, e.g., a compound which 
when heated will decompose to the oxide. Such compounds may, for example, 
be the hydroxides or the carbonates. Most desirably, the material employed 
will be calcium oxide, calcium hydroxide, or calcium carbonate. Dolomitic 
materials, e.g., burnt dolomite, are also suitable. The amount of the 
reactable material which is introduced into the flue gases should at least 
be a stoichiometric amount and preferably a substantial excess. 
Preferably, when employing an alkaline earth metal compound, that 
compound, on an oxide basis, will be added at such a rate that the weight 
ratio of the oxide to the total boron (B) and/or flourine (F) flowing in 
the gases coming from the recuperator will be at least 4 and more 
typically in substantial excess thereof, for example, about 5-10 times 
that ratio or even higher. Those skilled in the art will routinely adjust 
the ratio to obtain optimum results for any given operation. 
While the above sets forth the invention so as to enable those skilled in 
the art to make and use same, nonetheless further exemplification of the 
present invention follows. 
Pellets were manufactured having a water content of approximately 17 
percent by weight (dry basis) using the pelletization technique and 
control disclosed now U.S. Pat. No. 4,212,613. The specific batch employed 
contained about 12.5 percent by weight of calcined colemanite. 
Additionally, the batch included approximately 24.9 percent limestone, 
31.6 percent sand, 30.1 percent clay, and approximately 0.9 percent of 
sodiumsilicofluoride. The particle size of the batch employed, as measured 
by Microtrac analyzer, showed 100 percent of the particles being smaller 
than 176 microns, 81.2 percent smaller than 44 microns, 54.0 percent 
smaller than 16 microns, 30.5 percent smaller than 5.5 microns, and 12.6 
percent smaller than 2.8 microns, with the surface area of the batch being 
approximately 0.84 square meters per cubic centimeter and an average 
particle diameter of about 22.6 microns. The specific pellet hopper 
employed had a diameter of about 8-9 feet and the total height of pellets 
in the pellet preheater (from the bottom of the frusto-conical discharge 
portion to the top of the bed) was approximately 18 feet. Flue gases 
exiting from the recuperator had a temperature of about 
1400.degree.-1500.degree. F. and the temperature of the gases exiting from 
the pellet preheater was generally maintained at less than about 
400.degree. F. The pull rate on the furnace employed was approximately 
1,100 pounds per hour. 
In a controlled run, no reactive material was employed and, referring to 
Table 1, it will be seen that when no calcium oxide was added the amount 
of total boron in the gases exiting from the pellet hopper in duct 12 was 
about 0.495 pounds per hour. Similarly, since the flue gases also 
contained fluorine, the total amount of fluorine in the gases exiting from 
the pellet hopper was approximately 0.391 pounds per hour without the 
addition of any CaO. Table 1 then shows, in pounds per hour, the amount of 
boron and the amount of fluorine in the flue gases exiting from the hopper 
when CaO was added to the flue gas using a volumetric feed screw for rates 
of 10 and 20 pounds per hour, respectively. 
Thus, it will be seen from Table 1 that the total amount of boron was 
decreased by about 35.3 percent when using 10 pounds per hour of CaO 
addition and about 36.7 percent when employing 20 pounds per hour of CaO. 
Simultaneously with the boron reaction and removal in the bed, the total 
fluorine was decreased by about 21.7 percent at the 10 pound per hour 
addition and about 23.5 percent at the 20 pound per hour addition. Thus, 
it will be seen that a substantial simultaneous decrease in the amount of 
boron and fluorine emitted from the pellet hopper was attained by 
injecting calcium oxide into the flue gas stream which carried boron and 
fluorine values. 
TABLE 1 
______________________________________ 
Boron (#/hr) 
10#/hr. 20#/hr. Fluorine (#/hr) 
No CaO CaO CaO No CaO 10#/hr. 
20#/hr. 
______________________________________ 
0.495 0.320 -- 0.391 0.306 -- 
0.495 -- 0.313 0.391 -- 0.299 
______________________________________ 
As will be readily apparent from the foregoing, the boron and/or fluorine 
reactive material will desirably be added in solid particulate form. The 
size of the material employed desirably will be such that it can be 
pneumatically conveyed in the flue gas stream. Of course, therefore, the 
size will vary with different installations, depending upon the velocity 
of the flue gases. Generally, however, it is preferred that the particle 
size of the material employed be relatively small, e.g., about 80-95% of 
the material will be less than 325 mesh (U.S. Sieve). 
While the foregoing sets forth the present invention, it will, of course, 
be apparent that modifications may be made which pursuant to the Patent 
Statutes and Laws do not depart from the spirit and scope thereof.