CA1054164A - Producing pre-treated glass batch materials - Google Patents

Abstract

PRODUCING PRE-TREATED GLASS BATCH MATERIALS
ABSTRACT OF THE DISCLOSURE
A process for producing pre-treated glass batch materials in pelletized form is disclosed. The process involves obtaining commercially available sand, sodium carbonate (soda ash), sodium sulfate (salt cake) and other minor glass making ingredients. A dolomite having an average (statistical mean) particle size less than 16 microns, preferably less than 7 microns, and a limestone having an average (statistical mean) particle size less than 250 microns is also obtained. All of the dry materials are mixed in proper proportions so that a desired final glass composition may be obtained when these materials are treated with a caustic soda solution. The dry mixture is con-tinuously fed to a pelletizing device at which a pre-determined amount of caustic soda is continuously added thereto. The dry mixture is pelletized by wetting it with the caustic soda on the pelletizing device. A pelletized product is continuously removed from the pelletizing device and this product is dried to produce glass batch materials in which the reaction of the caustic soda with both the dolomite and the limestone has given the product sufficient strength to withstand crushing and abrading forces normally encountered in handling operations.

Description

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:.f This invention is directed to a process for pro-ducing pre-treated glass batch materials and, more particular-ly, to a process for producing pre-treated glass batch -materials in a pelletized form having sufficient strength to withstand both the crushing and the abrading forces encountered in handling operations required between pellet manufacture and pellet use.
~ nvestigations have shown that the maximum amount of glass which may be drawn from a glass producing furnace can be increased if the batch materials melted to form the : glass (including caustic soda, soda ash, sand, limestone, dolomite, salt cake and minor ingredients such as carbon ; and rouge) are added to the furnace in a granulated or "
pelletized form instead of the conventional loose batch -form. Basically, pelletizing is a process in which finely divided materials are placed on a pelletizing device such 1~ as a rotatingj inclined pan, drum or cone and sprayed with a liquid for the purpose of forming pellets. The raw ~i;::
~r,- materials to be pelletized are fed into the pelletizing '~f'.; 20 device and carried under a liquid spray. Seed pellets are formed when droplets of the sprayed liquid draw a few raw `!if~ material particles together. In some cases this process is ~ aided by a compacting action of scrapers acting against the -~ sides and the bottom of the pelletizing device. Seed '.?. pellets are carried under the raw material feed and are i:~ coated with the new feed materials. The alternate wetting '~r and coating of the individual pellets p]us the rolling and ~i~ tum~ling action imparted to the pellets by the pelletizing device causes the pellets to grow in size. The size to -which the pellets grow is controlled by controlling the varia~les of the pelletizing process.
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~054164 Tl1e ability to form pellets of proper size with an adequate green strength (st:ength when pellets are wet) does not necessaril~ mean that when the pellets are dry they will have adequate crush strength to res~st both the crushing and the abrading forces encountered during the handling associated with glass manufacturing operations.
It is necessary to thoroughly dry the glass batch pellets ., .
- prior to placing them in a melting furnace as even a little residual moisture can cause the pellets to explode in the furnace. The dry pellets must be strong enough to with- ~-: .:
stand the drops, tumbling and storage conditions encountered in a pellet handling and storage system in which the pellets are moved from a pellet dryer to storage and then to a t melting furnace.

The prior art teaches that soda ash in the glass batch may be replaced, in whole or in part, by caustic soda.
The caustic soda provides the sodium oxide for the final glass which previously had been provided for by the soda ash. This prior art is best illustrated in U.S. Patents :
3,542,534 and 3,726,697 and Canadian Patent 923,309. These patents shown the mixing of glass batch materials and the use of caustic soda as a replacement for soda ash. In this prior art, the glass batch materials and the caustic soda ; are mixed together prior to introduction of the mixture to , . .
a pelletizing device. The patents also indicate ~hat it is necessary to reduce the particle size of all the materials involved in the process in order to carry it out.
~`; In accordance with the present 1nvention, a r~,` process for producing pre-treated glass batch materials in pelletized form has the following steps. Commercially available sand, sodium carbonate (soda ash), sodium sulfate ~, . . -. - ' ~:

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(salt cake), and other minor glass making ingredients are obtained. A dolomite having an average (statistical mean) ; particle size less than 16 microns, preferably less than -~ 7 microns, and a limestone having an average ~statistical ;
`, mean) particle size less than 250 microns is also obtained.
- All of the dry materials are mixed in proper proportions so that a desired final glass composition is obtained when these materials are treated with a caustic soda solution ~
and the dry materials have a dolomite content of at least `
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j 10 10~ by weight. The premixed materials are fed continuously .... .
`; to a pelletizing disc at which a predetermined amount of caustic soda is continuously added. The dry mixture is pelletized ln the pelletizing device by spraying the caustic soda solution on the dry mixture as it moves over the 5~ surface of the pelletizing disc. A pelletized product is continuously removed from the pelletizing device and dried to produce glass batch materials in which the reaction of the caustic soda with both the dolomite and the limestone giues the product .j7,, ' "

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~; sufficient strength to withstand the pressures and the forces encountered in subsequent han~ling operations. Because of ` the careful control of the particle size of the dolomite and ; limestone, all of the other materials employed in the process are the commercially available materials now used in glass "' ' ~ making process. Thus, the cost of making pellets is not :. .
- increased by requiring that all the materials of the glass .. .
~ batch material be ground to a finely divided state. By j,: .
. using the size range of both dolomite and limestone specified, i 10 a prolonged drying time for drying the green pellets is not required to develop high strength pellets. The stength of the pellets can be developed in a conventional conveyor dryer in less than one hour drying time, which strength normally . .
~; exceeds a resistance to a crushing force in excess of 400 ,.
~ lbs. per square inch.
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` We have discovered that by using the above des-cribed process, one hour drying time in a conventional con-~: veyor dryer for the pellets is more than adequate to produce `
~ strong, dry pellets. The use of the finely divided dolomite ~~;' 20 is an essential element in permitting the rapid development , of strength in the pellets. The larger dolomites commonly used by glass makers are of no value in our process. While not used in conventional glass batches, dolomite of the .:, .
required average particle size for the process disclosed here-in is available at the present time in commercial quantities at reasonable prices. While a reaction also takes place between the caustic soda and the limestone, this reaction is secondary and is of less importance for pellet strength.

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, ~` 1054164 Therefore, a somewhat larger average particle size is .~. .
tolerable for the limestone without substantially affecting `.~ pellet strength. Alsc, while the limestone used for the disclosed process is not the limestone used i~ conventional ` glass batches, limestone of the required average size is also available in large quantities at reasonable prices.
The invention defined herein is unique because it ~ teaches the lmportance of controlling the sub-sieve average !:i (statistical mean) particle size of the reacting dolomite 0 ' and, of less importance, of controlling the average (statistical mean) particle size of the limestone. Processes ~`~ which were developed and disclosed in the prior art con-.;, ~: centrated on increasing caustic percentages and adjusting -., .
reaction temperatures to achieve desired reaction rates and pellet abrasion resistance. In the process taught in U.S. Patent 3,726,697, all materials used are minus 100 mesh.
This process would, therefore, require the grinding of all materials employed in the process. In the process of U.S.
Patent 3,542,534 the dolomite and the limestone used in-cluded material that was 22~ greater than 100 mesh. If such materials were used in the process disclosed herein, ~ ;
a short drying period of less than one hour in a conventional conveyor dryer would be insufficient to develop the pellet strength required for handling operations.
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`-- 1054164 :~, The method of this invention will be described in more detail in conjunction with a glass which is melted ; for use in supplying molten glass to a float glass chamber.
The invention is, however, applicable to the manufacture of pre-treated glass batch materials in pelletized form for use in the manufacture of any glass composition desired so long as the glass composition is one which contains percen-tages of alkali oxide, calcium oxide, and magnesium oxide sufficient to allow adequate quantities of caustic soda, limestone and dolomite to achieve the development of reguired bond strength in pelletiæing and drying.
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; The amount of caustic soda solution whlch may be used in the process disclosed herein is a variable. The !;
amount of caustic soda used is best linked to the total weight of dry material in the feed. The minimum amount of caustic soda which will pelletize glass batch materials is ,-;
about 1 pound of caustic solution per 24 pounds of dry ~; ingredients. If a 50~ caustic soda solution is used, thelower limit of caustic soda becomes about 1 pound of caustic solution to 16 lbs. of dry ingredients. The upper limits for each of the two different solutions is about 1 pound of caustic solution to 4 pounds of dry ingredients. Beyond this upper limit, too much liquid is generally supplied to ; a pelletizing device and the size of the pelletized product is not controllable.
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The main component essential to the production of ; strong pellets is the finely divided dolomite. The dolomite should be present in the glass making materials in a minimum amount. In order for proper bond formation to occur to produce : '~
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~ pellets having adequate strength and abrasion characteristics, .
the dolomite preferably should make up at least about 10~ by `~ weight of the total dry feed. The upper limit on the dolomite, as well as the upper and lower limits on the remaining raw materials of the batch are not explicitly limited by pellet-` izing. The limits of these materials are set by the desired ;~' final glass composition. The glass composition can range through the compositions acceptable for manufacturing both ;
~ flat glass and container glass provided the above minimum . ~, .
dolomite restriction is generally adhered to.

;~ Salt cake (sodium sulfate) is used primarily as an ~, aid for melting the glass batch. Sulfate usage in the past ~-~ has generally been in the range of from five to twenty percent by weight of the dry materials in ~lat glass manufacture.
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f,` The higher sulfate content is generally used for manufacturing tinted glass. Sodium sulfate may, however, be objectionable , because its use results in the emission of gaseous and solid sulfur compounds when the batch materials are melted. It ~:.
has been found that by using pellets as the batch material . ~
for a glass melting furnace, the sodium sulfate content of ;` the pellets may be reduced to 0.25 percent. These pellets still melt within a reasonable period of time to form homogeneous glass free of unmelted batch even though the ~; sulfate level has been reduced. We have also found that up - -to 5% of the soda of the final glass composition can be derived from sodium sulfate without detrimental effect on overall melting characteristics and emission levels.
Table 1 shows the compositios of clear and tinted ; glasses manufactured by the Glass Division of Ford Motor ; 30 Company.
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lOS41~4 The materials employed to make the above composi-tions are s~t forth in Table 2. Table 2 discloses the general size range of the various materials emplo~ed in the manufacture of such a glass composition.
Sand, soda ash and salt cake are all commercially available glass making materials. These materials are the standard materials generally available throughout the United States for manufacturing glass. The dolomite and limestone normally used in glass manufacture is generally larger in average particle size than the materials specified for use in the process described herein. The particular dolomite and limestone used in our process are available in commercial quantities at reasonable prices. The limestone and dolomite may be purchased from commercial raw material suppliers to the glass industry.
The caustic soda used in the method of this inven~
tion may be any concentration of a liquid caustic soda solution. However, the most readily available concentra-tions are a 50% caustic solution and a 73~ caustic solution.
As described above, either one of these two solutions may be used and may be used preferably within the limits described.
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`; The invention is described further, by way of - illustration, with reference to the accompanying Figure 1 which is a schematic diagram of an apparatus for carrying out the process of the invention.
; Reference is made to Figure 1 which shows a ~ schematic form of an apparatus which may be used in prac-. ., ticing the process of this invention. The apparatus employed in the described embodiment of the invention is generally identified by the numeral 10 in Figure 1. The raw materials making up :,' . , ~ ' ' , .
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`~- 10541~4 the glass batch composition as described in the previous section of this specification are stored in large storage bins (not shown). The raw materials are metered from their storage bins in a prescribed amount, for example, in amounts to make the final glass compositions described onto a feed-~- belt 11 which by the end of its length contains all of the materials in a side by side or overlaying manner. The feed-belt 11 delivers the raw materials to a material mixer 12.
The mixer 12 mixes the materials and then delivers them to a feedbelt 13. The belt 13 delivers the mixed materials to . ..................................................................... .
a hopper 14. A bucket conveyor 16 lifts the mixed raw ,:
~ materials for delivery through a discharge chute 18 to a -~
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raw material storage hopper 20.
The raw material storage hopper 20 includes a metering device 22 which delivers prescribed quantities of the mixed raw materials to a feedbelt 24. The feedbelt 24, :
in turn, delivers the mixed raw materials to a charging "~,,;, chute 26 which delivers the materials to a rotating ` pelletizing disc 28. At the pelletizing disc, a solution o~ caustic soda delivered to a spray nozzle 30 through a .,;, , ;~ suitable piping system (not shown) is sprayed on the raw .,." ~ , materials in order to cause them to pelletize. The caustic solution has a temperature generally in the range from 100F
to 180F. A full description of the manner in which the caustic soda and dry materials are pelletized on the ~` pelletizing disc is contained in U.S. Patent No. 3,969,100 assigned to Ford Motor Company. The continuous delivery of caustic soda to the disc along with the continuous delivery of dry ingredients produces a continuous stream of formed pellets of a prescribed size. The pellet size is controlled . .
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:-11 10541~4 11 . . ., ~' l 11 by controlling the variables of the pelletLzing disc as is

2 1¦ well known in the pelletizln~ art~ The coltlnuou~ly developed

3 ¦ pellets are discharged frorn the pelletizir,s disc 28 through ¦ a discharge chute 32. The discharge chut~, ln turn, dellvers the green pel]ets to a feedbelt 34 which d~liver~ the pellets ; 6 through a spreader devlce (not 3hown) to the fir~t of a pa~r ;~ 7 of dryer belts 36 and 38 contained ln a ml~lti-stage drying furnace 40. When a 16.5 foot diameter pelletizing dlsc is ~ ¦
9 used, typically 2.45 tons/hr of caustic solution may be `` lO sprayed on l6.35 tons/hr. of raw material to produce 20 tons/hr. ;
ll of green pellets. ` jir 12 In order to make the process a commercially feasible i l l3 one, the time Or drylng and amount of ene1gy employed in ~ ~ `~-; drying the pellets has to be reduced to a~l absolute mlnimuni, ~ 15 ~ We have found that by using dolomite and ~lmestone of the ~
.-~ l6 specified average particle slze, the drying of the pellets; ~ ,r ;~ 17 can be accomplished on a conveyor type dr~er in less than one~ ~ ,~;
18 hour to produce pellets having the streng1h required for ~
-, subsequent handling operations. While the drying of pellets ;' !~ ~ may take more or less time in other types of dryer~, the 21 drylng tlme in all cases will be commercially reasonab1e iif ; ~i 22 the method of this invention is followed. The pellet~ conveyed i~, -23 by the feedbelt 34 to the first dryer bell; 36 and subsequently :j24 to the second dryer belt 38 are dried on ~he dryer belts as ~;
they move through the multi-~tage drying furnace 40. On the ; i .,., . , , j ,.,~.,.
26 dry~r belt 36, typically a thrce inch la~r o~ pellet~ 18 ;
27 sub~ected to a 250F updraft alr rlow at a rate of 250 cu. ~t,/mi 1 ~i 28 for 5 m utc~ ~d a 275F downdra~t air ~10w Or abou~ 250 u. ,~

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` ~ 1054164 ft./min. for 6 minutes. The semi-dry pellets are then dis-tributed onto the dryer belt 38 to form a nine inch thick layer. On this second belt the pellets are subject to a -350F updraft of about 250 cu. ft./min. for 12 minutes and a 370F downdraft of about 250 cu. ft./min. for a period of 18 minutes to dry the pellets. Because of the particular sizes selected for the dolomite and the limestone, as pre-viously stated, the drying action can take place in a relatively short period of time thereby requiring a reason-ably sized dryer facility. The product also has a strength sufficient for subsequent handling. The reasons why a certain level of strength is required, and how that strength is measured will be described below.
After the pellets are dried in the multi-stage furnace 40, they are discharged from the end of the dryer belt 38 onto a slide 42 which delivers them to a bucket conveyor 44. The bucket conveyor 44 lifts the pellets to its top for placement in one of two areas. For a first placement, the pellets can be delivered to a charging con-.;
veyor belt 46 which leads to the glass melting tankassociated with either a float glass furnace or any other glass using the structure. In this case the handling of the pellets is direct from the slide 42 to an end use in a melt-ing furnace. As an alternate, the bucket conveyor 44 can deliver the pellets to a discharge chute 48 which, in turn, delivers the pellets to a storage bin 50 for temporary storage.
When pellets are demanded at a rate greater than the pellet production rate from the multi-stage drying furnace 40, the storage bin 50 deliver pellets through a metering device 52 .

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.~,` l 'l ¦and a chute 54 to the bucket conveyor 44 to lift the previou~ly ~ 2 stored pellets to a loca~ion where they are di~chargable onto : ' 3 the charging conveyor belt 46. In general, the method of thls , ~nvention has been illus~rated in a schematic rorm. It is obvious from this descrlption that the pellets are handled -'~` 6 in a rough manner and that ~uch handling can cause problems :
7 if the pellets do not have an adequate ~trength to resist ,, 8 abradlng and crushing. ~hls facet of the subJect matter w1ll ' ,~
9 be discu~sed below. , , .,, . . , ~ ; ~
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ll , The reason that pellets must have adequate dry - "12 strength is that if material ls abraded from the surface of , ,~
j, , l3 the pellets ln conveying and/or storage, such abraded material , , ;'~' ,,, , 14 generally differs in chemical composition from the average, '~ ;- ' ~,i,~, , l5 theoretical pellet compositlon. When thls abraded materlal, , 16' enters the furnace, as for example when it collects ln the ;,, ;`;
,,,,,~,17 ' bottom of the storage bin and ls discharged therefrom ln a ' ;;~,~
, 18 , segregated condltlon, lt enters the glass meltlng furnace in , ' ,'~- ,:
,'-", l9 a non-unlform amount and has the potential to cause many ,,,,~', ,~
;; 20 defects in the flnlshed glass~ One such defect ls ream whlch ~ ,,; !l!~
, 21 18 a locallzed area of g:lass having a different chemlcal '~' ,;~
'"' 22 compo~ltion than the,bulk glas~ which can lower the glass , ';,i;
" 23 quallty to an unacceptable level. We have ~ound from our ' , ;,", 24 studies that' the abraded material, los~eA '~hould be 1cept below ; ,~
'7' 25 a level of about 0.75~ of' t,he total pellet welght so that ' ;' 26 the possibili~y o~ concentrations of abraded material enterlng ' , ,j,i:
" a productlon glass meltlng tank is substantlal1y reduced. ~ /, ,:-', I . -' . .'.,, ': ! . , . ' .

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~. i 10541~4 Since there is no known test for measuring abrasion of glas pellets, we devised our own test procedure. In our test, 20 lbs. of dried pellets (prescreened to remove all materials less than 1/4 inch in diameter) are dropped three consecutive times through a 15 ft. section of a four inch diameter plastic pipe. The first 5 ft. of the plastic pipe , .
lS vertlcal, the second 5 ft. is inclined from the first section of pipe at a 45 degree angle from the vertical and the third 5 ft. of the pipe is once again vertical. Before . ,.
a fourth drop of the previously dropped pellets through the pipe, pieces of glass cullet are hand mixed with the pellets.
~he amount of cullet added represents 30 weight percent of the total batch and cullet mi~ture, that is 8 lbs. 9 ounces of cullet to the 20 lbs. of dry pellets. The mixture is dropped through the plastic pipe system for the fourth time.

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; The pellet-cullet mixture from the last drop is screened to separate out all materials less than 16 mesh. The weight of the minus 16 mesh material is recorded and then expressed as a weight percentage of the original 20 lbs. of pellet passing through the plpe. By such a test we are able to determine .
the percent of abrasion of the pellets. As long as the pellets were well formed with relatively smooth surfaces, the amount of material abraded from the pellets correlated very well with pellet crush strength (i.e., stronger pellets had less material abraded). In general, our findings were that pellets had to have a crush strength of better than about 400 psi in :
order to achieve an abrasion loss less than 0.75%. For example, i if the pellets had a crush strength of 200 psi, the pellets generally had an abraslon loss in our test procedure of ~: 30 , .:
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greater than 1.7%. If the pellet strength was increased to ~`- 300 psi under a crushing force, ~he abrasion loss was still 1.2~ which is undesirable.
We have thus found that if one employs the normally ~-~; commercially available sand, salt cake, soda ash and other minor ingredients in forming pellets, and if one further ~- employs with these commercially available and generally used materials a dolomite having an average (statistical mean) particle size less than 16 microns, preferably less than 7 microns, and a limestone having an average (statistical mean) particle size less than 250 microns, and if one applies -; caustic soda at a pelletizing device to these dry materials in order to pelletize them one will produce a pellet which can be dried in less than one hour in a conveyor dryer to a crush strength better than 400 psi. Such a pellet can with-stand the forces and the pressures encountered in pellet handling without losing as abraded materials any more than 0.75% of its total weight. The introduction of concentrations of abraded material into the glass melting tank is eliminated ,,. ~.
; 20 and, therefore, the glass defects produced from the intro-. :.
: duction of such concentrations of material is also eliminated.

There has been disclosed herein a method of making pellets of pre-treated glass batch materials. The method ; taught is one which produces strong and abrasion resistant ~, . .
pellets. These pellets are produced within a reasonable drying time. The method of the invention also permits the replacement of part or all of the soda ash of the batch materials by caustic soda and also permits the reduction in 5~ the amounts of salt cake (sodium sulfate) used in the glass ~ 30 ,, ~ .

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materials as a melting aid because the pellets themselves - melt in a more rapid and homogeneous manner than the pre-;; viously used loose glass batch materials. The reduction in the salt cake also reduced the amount of undesirable '; emissions which can be generated when this material is present. We have also found that since one may melt pellets in a melting furnace in a more rapid manner than the pre-~ viously employed loose batch materials, one may obtain a ; greater tonnage of glass from the same melting facility in the same period of time thereby reducing the cost per ton for melting, which reduction in cost of melting helps to offset the cost of making the pellets. One may also melt . . .
the same tonnage of pellets using less fuel than required to melt the same amount of loose batch materials.
When one skilled in the art views the method of this invention, many modifications thereof will be apparent ,; to him. It is desired that all such modifications which fall within the true spirit and scope of this invention be included within the scope of the appended claims.

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Claims (4)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for producing pre-treated glass batch materials in pelletized form which comprises:
obtaining commercially available sand, sodium carbonate (soda ash), sodium sulfate (salt cake) and other minor glass making ingredients;
obtaining a dolomite having an average (statistical mean) particle size less than 16 microns;
obtaining a limestone having an average (statistical mean) particle size less than 250 microns;
mixing all of the obtained, dry materials in proper proportion to produce a desired final glass composition when the dry materials are mixed with a caustic soda solution and the dry materials have a dolomite content of at least 10% by weight;
feeding continuously a predetermined amount of the dry mixture to a pelletizing disc;
feeding continuously a predetermined amount of a caustic soda solution to the pelletizing disc;
pelletizing the dry mixture by spraying caustic soda solution on the dry mixture as it moves over the surface of the pelletizing disc;
removing continuously a pelletized product from the pelletizing disc; and drying the pelletized product to produce glass batch materials in which the reaction of the caustic soda with both the dolomite and the limestone has given the pelletized product sufficient strength to withstand crush-ing and abrading forces encountered in normal handling operations.

2. The method of Claim 1 wherein: the dolomite obtained has an average (statistical mean) particle size less than about 7 microns.

3. In a process for producing pre-treated glass batch materials in which the principal dry glass forming materials include sand, sodium carbonate (soda ash), sodium sulfate (salt cake), dolomite and limestone and other minor glass forming materials, and in which a caustic soda solution is used as a replacement for at least a portion of the soda ash; wherein the dry glass forming materials are mixed together; wherein the caustic soda solution is added to the dry glass forming materials; wherein the glass batch materials produced from the caustic soda and the dry glass forming materials are dried before storage, the improvement comprising:
selecting for use as the dolomite of the glass forming materials only dolomite having an average (statistical mean) particle size less than 16 microns;
selecting for use as the limestone of the glass forming materials only limestone having an average (statistical mean) particle size less than 250 microns;
selecting for use as the other dry glass forming materials, materials normally available for glass making processes on the commercial market;
utilizing sufficient dolomite to provide at least 10% by weight of the mixed dry glass forming materials;
spray applying the caustic soda solution to the mixed dry glass forming materials as the dry materials are being moved about the surface of a pelletizing disc;
removing pellets of the glass batch materials from the pelletizing disc; and drying the glass batch pellets for a period of no greater than one hour before storage.

4. The method of Claim 3 wherein: the dolomite selected for use as the dolomite of the glass forming materials has an average (statistical mean) particle size less than 7 microns.