Method of preparing colemanite-containing glass batch

A process is described for preparing colemanite-containing glass batch compositions suitable for the preparation of glass fibers in which the batch, prior to its introduction into the furnace, is pelletized by adding water in sufficient quantities to maintain an adequate balling action and thus provide pellets in a form such that preheating by direct contact with flue gases does not cause deterioration of the pellets.

BACKGROUND OF THE INVENTION 
Fiber glass batches, unlike soda lime-glass batches and other commercial 
batches utilized to make containers and flat glass, are fed to fiber glass 
melting furnaces in an extremely fine condition, i.e., almost all batch 
particles are less than 20 mesh, with the majority being less than 200 
mesh. Because of the fineness of the batch ingredients, dusting is 
encountered in fiber glass batch melting furnaces. In addition, fiber 
glass batches contain considerable quantities of boron-containing 
materials and other rather expensive ingredients, some of which are lost 
in the stack gases should dusting occur during feeding. Further, some of 
these batch materials volatilize into the stack gases as they are melted. 
By cohesively binding batch ingredients of the fineness normally 
encountered in a fiber glass batch, reduced dusting and volatilization of 
the batch ingredients and the concomitant reduction in the loss of 
expensive ingredients can be achieved. In addition, by providing fiber 
glass batch in pelletized form, advantage can be taken of the sensible 
heat contained in furnace flue gases to preheat fiber glass batch pellets 
prior to feeding them to the glass melting furnace. Further, the close 
contact between the particles within the pellets improves their heat 
transfer characteristics and thus results in faster melting, improved 
energy efficiency and reduced furnace wear. 
Considerable activity has taken place in recent years and particularly in 
relation to the preparation of soda-lime glass batches in which the batch 
ingredients have been pelletized for feed to glass melting furnaces. Thus, 
a recent U.S. Pat. No. 3,880,639 describes the utilization of an 
agglomerated soda-lime glass batch in which the pellets are preheated via 
direct heat exchangers prior to feeding them to a glass melting furnace. 
Activity has also occurred in the preparation of fiber glass batches in 
that glass batches have been prepared with various binding materials for 
the preparation of briquettes for feeding to glass melting furnaces. U.S. 
Pat. No. 2,976,162 describes a process of this nature. In other patent 
literature involved in the preparation of fiber glass type batches, 
special treatments have been applied to the glass batch to provide for 
prereaction of glass batch ingredients prior to feeding them to the glass 
melting furnace. A process of this character is described in U.S. Pat. No. 
3,001,881. Still further, the glass batch ingredients themselves have been 
carefully selected to provide boron-containing glass batch materials of 
specific character to help eliminate some of the foaming problems 
occurring during melting utilizing high boron-containing glass batches, 
such as are encountered in the fiber glass industry. A patent describing 
one such process is U.S. Pat. No. 3,287,095. 
THE PRESENT INVENTION 
In accordance with the present invention, boron-containing fiber glass 
batch pellets utilizing colemanite as a boron source are prepared by 
introducing the batch ingredients in appropriate proportions into a 
pelletizing zone, such as an inclined rotating disc pelletizer, as is 
shown in U.S. Pat. No. 3,914,364, which is incorporated herein by 
reference, and adding to the batch ingredients as they are rotated on the 
pelletizer sufficient water to agglomerate and support the continuous 
production of glass batch pellets of a desired size. The pellets may 
range, for example, in nominal diameter from about 0.125 to about 1.00 
inch (0.3175 to 2.54 centimeters) and preferably between about 0.375 and 
0.625 inch (0.9525 and 1.5875 centimeters). Sufficient water is added to 
bind the batch ingredients and provide pellets preferably containing 
approximately 5 to 20 percent by weight free water. Most preferably, the 
water is added to provide approximately 10 to 13 percent by weight free 
water. 
The batch ingredients may contain only colemanite as the source of B.sub.2 
O.sub.3. Optionally, up to about 75 percent or more of the colemanite, on 
a B.sub.2 O.sub.3 basis, may be substituted by boric acid, while adjusting 
for lost silica, calcia and alumina contained in the colemanite. The 
pellets after formation are dried at a temperature preferably from about 
220.degree. F. (104.4.degree. C.) or less to 770.degree. F. (410.degree. 
C.) to a free water content of preferably about 1 percent by weight or 
less, to produce hard, substantially non-dusting pellets. The hard, 
non-dusting pellets thus formed can be fed to a glass melting furnace and 
exposed to conditions in excess of 2700.degree. F. (1482.2.degree. C.) 
without any explosions of the pellets occurring.

DETAILED DESCRIPTION OF THE INVENTION 
Typical "E" glass type boron-containing glass fiber forming compositions 
are illustrated in U.S. Pat. No. 2,334,961, which is incorporated herein 
by reference. These compositions comprise silica, clay, coal, boric acid, 
limestone, fluorspar, sodium sulfate and ammonium sulfate. In lieu of 
boric acid, colemanite may be used. The use of colemanite is described in 
U.S. Pat. No. 3,274,006. Colemanite has a chemical composition of Ca.sub.2 
B.sub.6 O.sub.11.5 H.sub.2 O. Optionally, boric acid may be substituted 
for up to about 75 percent or more of the colemanite on an equivalent 
B.sub.2 O.sub.3 basis while adjusting for lost silica, calcia and alumina 
from the colemanite. There is no caustic soda present in the compositions. 
It is extremely important in the preparation of glass batch pellets in 
accordance with the present invention that, if colemanite is used as the 
single source of B.sub.2 O.sub.3, temperatures above 770.degree. F. 
(410.degree. C.) should be avoided during the drying step. Attempts to dry 
these pellets above this temperature result in the disintegration of the 
pellets and their return to the powdery state. Thus, extreme care is taken 
to provide pellets by regulating the drying operation, such that the 
pellets are dried at temperatures not exceeding 770.degree. F. 
(410.degree. C.). 
When boric acid is substituted for up to about 75 percent or more of the 
colemanite, with proper adjustments being made for silica, calcia and 
alumina in composition, these temperature parameters must still be 
followed. Hard, non-dusting pellets can be produced at drying temperatures 
up to 770.degree. F. (410.degree. C.). 
When drying the pellets of the present invention, should the temperature of 
the pellets exceed 770.degree. F. (410.degree. C.), it has been found that 
the pellets crack and disintegrate. This problem, however, may be solved 
by pretreating the colemanite prior to its addition to the glass batch, 
and forming the glass batch into pellets. This pretreatment comprises 
heating the colemanite at a temperature above 770.degree. F. (410.degree. 
C.) for a sufficient period of time until substantially all of the 
chemically bound water in the colemanite is driven from this material. An 
equation for this reaction is shown below as equation (6). This water 
amounts to approximately 21 to 22 percent by weight. There is also an 
expansion of up to 33 percent by volume of the colemanite when the 
chemically bound water is driven off, which accounts for the cracking of 
untreated pellets when heated above this temperature. By employing this 
pretreatment to the colemanite prior to its introduction into the glass 
batch, pellets as heretofore described may be dried at any temperature 
from about 220.degree. F. or less (104.4.degree. C.) to the melting point 
of a given pellet, and preferably between about 220.degree. F. and 
1000.degree. F. (104.4.degree. C. and 537.8.degree. C.), without fear of 
cracking, to produce hard, non-dusting pellets. 
Glass batch ingredients prepared in accordance with the instant invention 
are believed to undergo several chemical reactions during their deposition 
on the pelletizing disc and while water in the quantity sufficient to 
produce the pellets is being added to the ingredients and during the 
drying of the pellets. The primary reactions involving the preparation of 
the pellets in accordance with the instant invention are believed to be as 
follows: 
##EQU1## 
In reaction (1) the boric acid and water react to dissolve the boric acid 
and then the boric acid is recrystallized as shown in the equation. The 
recrystallized boric acid is dehydrated during the drying step to drive 
water off, as is shown in equation (2). Some of the boric acid itself 
during the drying of the pellets reacts with the calcium carbonate present 
to form hydrated calcium pyroborate, carbon dioxide and water in 
accordance with equation (3). Boric acid also reacts with sodium sulfate 
present in the batch in accordance with equation (4) to form hydrated 
sodium tetraborate and sulfuric acid. The limestone and sulfuric acid may 
also react to form calcium sulfate, carbon dioxide and water in accordance 
with equation (5). 
Regardless of the reactions that take place, pellets produced in accordance 
with the practice of this invention thus provided to a glass melting 
furnace for the production of glass fibers are hard and considerably less 
dusty than the loose batch ingredients conventionally employed. Further, 
since the boric acid is an excellent fluxing agent, the wetting of the 
boric acid upon its intimate contact with all batch ingredients, in 
addition to causing reactions as indicated hereinabove in equations (1) 
through (5), provides for intimate contact of the boron contained in the 
batch with all of the other batch grains present. This assists in rapidly 
melting the silica and alumina constituents of the glass batch which, as 
will be readily understood, are the most difficult ingredients to 
dissolve. 
The pellets of the present invention may be preheated prior to their 
addition to the glass melting furnace such as, for example, by passing 
them through the flue gases of the furnace or passing the gases through a 
bed of the pellets. In addition to preheating the pellets and thus 
reducing the amount of furnace input energy needed to melt them, this 
passage of flue gases through a bed of pellets may cause a reduction of 
air pollution from the flue gases by removing via a filtering action at 
least part of the harmful materials, such as F.sub.2 and B.sub.2 O.sub.3, 
from the flue gases. Utilizing hot flue gases of temperatures typically in 
the range of about 800.degree. F. to 2850.degree. F. (426.7.degree. C. to 
1565.6.degree. C.) pellets can be preheated to temperatures of about 
200.degree. F. to 1500.degree. F. (93.3.degree. C. to 815.6.degree. C.) to 
recover sensible heat and assist in reducing the amount of fuel needed to 
melt the pellets fed to the furnace. 
EXAMPLE I 
An "E" type fiber forming glass batch comprising: 
______________________________________ 
Component Percent by Weight 
______________________________________ 
Silica 30.758 
Clay 27.986 
Limestone 20.922 
Coal 0.108 
Fluorspar 2.454 
Ammonium Sulfate 0.237 
Sodium Sulfate 1.041 
Colemanite 16.494 
______________________________________ 
was combined on a disc pelletizer with sufficient water to produce pellets 
containing about 12 percent by weight free water. The pellets were dried 
at temperatures of approximately 490.degree. F. to 525.degree. F. 
(254.4.degree. C. to 273.9.degree. C.) for approximately 5 minutes. The 
resulting pellets produced were rigid, however, most could be crushed by 
hand pressure. 
EXAMPLE II 
An "E" glass type fiber forming glass batch comprising: 
______________________________________ 
Component Percent by Weight 
______________________________________ 
Silica 31.040 
Clay 28.227 
Limestone 21.698 
Boric Acid 3.566 
Colemanite 11.853 
Fluorspar 2.411 
Sodium Sulfate 0.854 
Ammonium Sulfate 0.251 
Coal 0.100 
______________________________________ 
was combined into pellets. The colemanite used was treated at 1000.degree. 
F. (537.8.degree. C.) for approximately two hours to remove its chemically 
bound water prior to its addition to the batch. This composition 
represented a 30 percent boric acid substitution for colemanite, on a 
B.sub.2 O.sub.3 basis. The batch was pelletized and dried in the same 
manner as in Example I. The resulting pellets were hard and non-dusting 
and had good mechanical strength. 
EXAMPLE III 
An "E" glass type fiber forming glass batch comprising: 
______________________________________ 
Component Percent by Weight 
______________________________________ 
Silica 30.302 
Clay 27.632 
Limestone 24.320 
Colemanite 8.206 
Boric Acid 5.882 
Fluorspar 2.373 
Sodium Sulfate 0.939 
Ammonium Sulfate 0.247 
Coal 0.099 
______________________________________ 
was combined into pellets. In this example, the colemanite was not 
pretreated prior to its addition to the batch. This batch represented a 50 
percent boric acid substitution for colemanite on a B.sub.2 O.sub.3 basis. 
The batch was pelletized and dried as in Example I. The resulting pellets 
were hard and non-dusting and possessed good mechanical strength. 
EXAMPLE IV 
An "E" glass type fiber forming glass batch comprising: 
______________________________________ 
Component Percent by Weight 
______________________________________ 
Silica 30.137 
Clay 27.397 
Limestone 26.174 
Colemanite 4.061 
Boric Acid 8.659 
Fluorspar 2.348 
Sodium Sulate 0.881 
Ammonium Sulfate 0.245 
Coal 0.098 
______________________________________ 
was combined into pellets. The colemanite was not pretreated prior to its 
addition to the batch. This batch composition represented a 75 percent 
substitution of boric acid for the colemanite, on a B.sub.2 O.sub.3 basis. 
The batch materials were pelletized and dried as in Example I. The 
resulting pellets were hard and non-dusting and possessed good mechanical 
strength. 
While the present invention has been described with reference to specific 
embodiments thereof, it is not intended to be so limited thereby, except 
as set forth in the accompanying claims.