glass industry process

1

GLASS INDUSTRY

by Douglas W. Freitag

Glass is an important component of the U.S.economy, employing more than 150,000 people inskilled jobs and generating more than 21 millionmetric tons of consumer products each year withan estimated value of $22 billion. Glass productionis energy intensive and accounts for 12% of thetotal cost of sales. In theory, about 2.2 million Btuof energy are required to melt a ton of glass. Inreality, it takes twice as much energy because ofinefficiencies and loses. The glass industry consistsof four major segments: container glass (bottles,jars, etc.); flat glass (windows, windshields,mirrors, etc.); fiberglass (building insulation andtextile fibers); and specialty glass (cookware, flatpanel displays, light bulbs, fiber optics, medicalequipment, etc.). Flat glass accounts for 17% of U.S.production by weight, container glass 60%,fiberglass 9% and specialty glass 4%. While thecontainer, fiber, and flat glass industries focus onproducing soda-lime glass for large, broad-basedmarkets, the specialty glass industry focuses onhigher temperature glasses and produces over60,000 products. Examples of productsmanufactured by the glass industry are shown inFig. 6.1. Key drivers for investment in the glassindustry include cost, quality, and increasedproductivity.

The desired state of the glass industry in twentyyears has been formulated in a vision documentprepared cooperatively with the U.S. Departmentof Energy (DOE). Future technology challengesand research opportunities were identified bycomparing the vision of the future with the cur-rent state of the glass industry. Technology chal-lenges are broadly categorized into four areas:• advances in melting and refining processes and

in fabricating/forming;• technology development of new glass-making

techniques, processing controls, and computersimulations to model new processes;

• systems improvements in emission controls,recycling methods, and solid waste manage-ment; and

• product development of innovative new usesfor glass.The following sections describe current glass

manufacturing processes and how ceramic-basedmaterials might contribute to achieving the glassindustry vision. Materials commonly used inglass-processing equipment include fused silica,graphite, precious metals, and water-cooled fer-rous alloys. Ceramics are primarily used as refrac-tories, with ceramic coatings increasingly beingused for wear resistance. Advanced ceramics arerarely used because of their high cost. Moreover,owing to the lack of suitable higher-temperature-capable materials, fluxes are commonly added toreduce glass-processing temperatures and permitthe use of conventional materials.

Fig. 6.1. Examples of glass industry products.Source: Marketing brochure from Praxair, Inc.,Danbury, Conn.

CHAPTER SIX

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Chapter 6

The discussion is organized according to thefour primary operations for glass manufacturing:batching, melting, refining, and forming. Batching,melting, and refining operations are common toall glass manufacturing processes with some varia-tion in furnace type. Various postprocesses areused, depending on the end product.

6.1 BATCHING

Raw material selection is based on chemistry,purity, uniformity, and particle size. Additionalorganic, metallic, and ceramic contaminants canbe introduced throughout the delivery, storage,mixing, and sizing processes, as well as throughthe use of cullet (recycled glass). Because of theeffects of contamination, the use of cullet varieswithin the industry according to the desired endproduct. Flat glass manufacturers recycle up to30% of their own cullet. Ceramic contaminantsundergo little reaction with the glass melt and re-side as inclusions (stones) in the finished product.Metal and organic contaminants create instabili-ties during glass processing (through reduction/oxidation reactions) and degrade the quality of theglass. Organics present in the batch are a sourceof increased emissions and added cost for flue gascleanup. Delivery, mixing, and sizing processes arehighly abrasive, and equipment used commonlycontains ceramic-coated wear surfaces or ceramicliners manufactured from alumina, silicon carbide,or tungsten carbide. While generally cost-effectiveand with adequate performance, lower cost solu-tions offering lower risk of contamination continueto be sought.

6.2 MELTING

There are about 600 glass melt furnaces inNorth America. The distribution of these furnacesis as follows: container, 210; fiberglass, 110; flatglass, 45; and specialty, 235; The life of a glass-melting furnace between rebuilds varies, but 7–8years is not uncommon. The cost of rebuilding afurnace can easily exceed $1 million.

Glass furnaces can be divided into those heatedelectrically and those heated primarily bycombustion. Often electrical heating of the melt isused in combination with fuel firing (electric boost)to improve heating uniformity, provideintermittent increases in melt capacity at minimumcost, increase melt efficiency, reduce gas

consumption, and lower temperatures above themelt for reduced emissions.

Combustion-Heated Furnaces

Combustion-heated furnaces can be furtherdivided according to the method used to recoverexhaust waste heat and the way the fuel is burned(with air or pure oxygen). Heat recovery is impor-tant because only 10% of the heat generated is ac-tually used to melt the glass, whereas 67% is lostthrough the exhaust. Recovery of exhaust wasteheat is performed with a regenerator orrecuperator. Regenerator furnace designs repre-sent one of the oldest and most universal devicesfor saving energy in the glass industry.

An example of a regenerative furnace is shownin Fig. 6.2. The batch introduced at one endimmediately begins to melt. As glass is extractedfrom the opposite end, the newly introduced glassflows along the melt zone where it becomes com-positionally homogeneous and bubble-free. Afterleaving the melt zone, the molten glass enters therefining zone where it becomes thermally homo-geneous. After leaving the refining zone, the glassenters the forehearth, where it can be distributedto one or more forming machines.

Refractories commonly used in regenerativeglass melt furnaces are shown in Fig. 6.3. Alumina-zirconia-silica (AZS) is the material of choice forthe sidewall, with the zirconia content rangingfrom 33 to 41%. Various chrome-containing refrac-tories are commonly used in the fiberglass indus-try because of their superior erosion resistance butare gradually being replaced because of environ-mental concerns about chrome. Even though su-perior in performance, chrome-containing refrac-tories are not widely used in the flat, container,and specialty glass industries because chrome im-purities result in glass discoloration. At the frontof the furnace where the charge is introduced, ero-sion and thermally induced damage are the mostsevere and limit furnace life. Materials that aremore corrosion resistant are sought for thesidewalls. Cladding refractories with platinum ormolybdenum is considered too expensive. Notshown are the AZS electrode and burner blockslocated on the sidewall below the melt line. Theseblocks support the electric boost heat electrodesand burner nozzles, both of which experience highlocal temperatures and thermal stresses that re-sult in reduced life.

The main source of a problem is above the glassmelt where temperatures are the hottest and

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Glass Industry

products of combustion reside. Superstructurerefractories can experience temperaturesapproaching 1600°C when air-fuel fired, and muchhotter when oxy-fuel fired. Silica is preferred forthe crown because of its low cost ($646m 2) and itsability to be dissolved into the melt if dislodged.However, silica does not perform well under theincreasingly harsh environment of oxy-fuel firing.Mullite has been used for the crown in processinghigher temperature glasses but can createinclusions in the glass. AZS, which has beenconsidered for the crown, costs much more

($10,764/m2) and is much heavier. Because of achemical incompatibility between the silica crownand the AZS sidewall, a buffer layer of zircon(ZrSiO4) is used.

Water-cooled, stainless steel burners are locatedon the sidewall of the furnace above the melt. In aregenerative furnace, firing alternates from sideto side, and intermittent firing is used to heat theregenerator brickwork, which then preheats thecold combustion air when firing is reversed. Thestainless steel burner orifice can be either water-or air-cooled and frequently overheats. When

Fig. 6.2. Typical side-fired flat glass melt furnace. Source: “Development of Improved Refractories forGlass Industries,” U.S. Department of Energy fact sheet.

Fig. 6.3. Refractories commonly used in a soda-lime glass melt furnace.

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Glass Industry

A cost of $400 for a 6.4-cm-diam, 15.2-cm-longtube with a 1.3-cm-thick wall is desired.

In addition to those for furnace temperature,sensors are being developed to measure refractorythickness during furnace operation. These sensorsare based on acoustic, eddy current, or electricalcapacitance measurements that change over time.Sensors imbedded directly into the refractory arealso being considered. Ceramic shields are desiredto protect the sensors from the harsh environment.

Particulate emissions contained in the flue gasare removed by either cloth filters or an electro-static precipitator. A conventionally fired regen-erative furnace can generate particulate emissionsapproaching 9kg/h at the particle separator . Fansare commonly used to maintain the desired levelof gas velocity and thus ensure the particles re-main in the gas stream and avoid fallout. Whilethis system is adequate for current furnace designsand regulatory guidelines, future furnace designsand increasingly stringent guidelines provide anopportunity for advanced ceramics with thehigher temperature capability and improved cor-rosion resistance sought for the filter media, fans,and ducts leading to the particle separator.

The maximum production output of a glassfurnace is limited by the amount of energy that canbe supplied inside the furnace to the melt. The limitis reached when the burners are at maximum heatoutput and electrical boost is at maximum power.Further increases in heat input at the melt furnaceare limited by the temperature capability of thematerials used to construct the furnace and theincreased emissions (NOx) that would be generated.

One approach to overcoming these limitations is topreheat the batch/cullet mixture. As shown inFig.6.5, the hot flue gas passes on one side of theheat exchanger while the batch powders fall counterto the flue gas stream. Preheating can be providedby using a separate burner or using the heatavailable in the exhaust stream of the furnace. Inboth cases, a fan is used to pull the hot gases throughthe heat exchanger. Operating conditions of thepreheater include inlet gas temperatures of up to982°C and exposure to the by-products ofcombustion, which with the flue gas can containhigh levels of moisture and corrosive vapors.Metallic designs currently used require frequentmaintenance and are temperature limited. By usinga ceramic heat exchanger, life could be extended,weight reduced, and temperature capabilityincreased. Similar benefits exist with a ceramic fan.One benefit of this approach is the ease of retrofittingexisting furnaces.

A second approach to increasing furnace effi-ciency is to introduce oxygen for firing. Theimprovement in furnace efficiency is a result ofthe higher flame temperature and diminished ex-haust losses caused by the reduction of nitrogenflow through the combustion system. The oxygenfor firing can be introduced locally through lances(tubes) immersed in the melt or by substitutingair at the burners for oxygen. Lances are con-structed from water-cooled metals, precious met-als, or, more recently, molybdenum disilicide.While the least expensive and simplest to setup,oxygen injection can result in local hot spots anddamage to the furnace walls, burners, etc.

Fig. 6.5. Batch and cullet preheatingsystem. Source: “Integrated Batch and CulletPreheater System,” U.S.Department ofEnergy fact sheet.

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Chapter 6

Pure oxygen–based combustion introduced atthe burners is gaining growing acceptance by theglass industry, with 87 furnaces, mostly in thespecialty glass and fiberglass industry, currentlyin operation. Benefits include greater flamestability, greater glass throughput, reduced fuelconsumption, reduced furnace cost, reduced NOxand particulate emissions, and high efficiencieswithout the use of costly regenerators,recuperators, or electric boosts. An importantdisadvantage is the change in characteristics of theexhaust gases. A typical regenerative furnace hasflue gas temperatures of 315–482°C at the exhausttakeoff, with moisture content of 8–10%.Recuperated furnaces are slightly higher in bothexhaust temperature and moisture content. Oxy-fuel-fired furnaces exhaust gas at 1204–1426°C andhave a moisture content of 55–67%. Because of thehigh flame temperatures, corrosive volatiles (e.g.,boron) are present in the exhaust gases. The highfiring temperatures and harsh exhaust gasespresent with oxy-fuel furnaces provide a numberof opportunities for ceramic materials.

A number of alternate combustion cycles arebeing developed in unison with oxy-fuel firing.The goal is to arrive at alternatives that are lessdifficult and costly to integrate into existing fur-naces while still achieving reduced emissions andincreased efficiency. Many of these concepts re-quire advanced ceramics for burners capable ofhigher temperatures, heat exchangers for preheat-ing combustion gases, particle separators for emis-sion control, and fans to move flue gases.

Electrically Heated Furnaces

Electrical heating is used exclusively in manysmaller specialty and fiberglass melting furnacesbecause of its lower initial cost and low emissions,even though energy costs remain high. Keydisadvantages of all electric furnaces are the highenergy cost and short furnace life. Electric furnacesare rebuilt as often as every six months, butbecause of their small size, down time is limitedto only about two days. An example of a bottom-entry electrode furnace with a refractory-lined,water-cooled wall is shown in Fig.6.6. T op-entryelectrodes are commonly used to simplifymaintenance because of the short life of materialscurrently used. Where bottom or sidewallpenetration is not desired, boost elements aresimply placed over the side of the tank, whenaccessible. In the furnace example shown inFig.6.6, the melt exits thr ough a submerged throatinto a separate temperature-conditioning chamber.Bubblers and stirrers are frequently used toimprove homogeneity of the melt and eliminatebubbles. Materials used in the construction ofsmall specialty glass and fiberglass furnacesinclude precious metals (for bubblers, stirrers,plungers) and molybdenum (for electrodes andflow-control hardware). Replacement materials aresought that are lower in cost and provide longerlife. Replacement materials are also sought forelectrode holders that do not require water cooling.Natural-gas-fired immersion heaters, which could

Fig. 6.6. Electric melt furnace for producing chopped fiber. Source:“Advanced High-Temperature Material for Glass Applications,” U.S.Department of Energy fact sheet.

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Glass Industry

ultimately replace electrical immersion heaters,will require advanced ceramic outer liners.

6.3 REFINING

The glass-conditioning operation (refining)occurs in the forehearth and provides molten glassthat is uniform in temperature. The forehearth isgenerally natural gas fired. Electric boosting canbe used to increase efficiency and improvetemperature uniformity. Water-cooled metal heatexchangers are used to help regulate temperatureand would benefit from advanced ceramics.Temperature sensing is critical in the forehearthand shares problems similar to those encountered

in the melt section. Plungers and nozzles used todistribute the molten glass are refractory ceramicsor molybdenum, but they experience high wearand erosion resulting from glass flow andcorrosion. For lower temperature glasses, Inconel600 is commonly used, with similar problemsreported. At higher temperatures, water coolingcan also be considered.

A number of opportunities for advanced ce-ramics were identified within the melting and re-fining operations of glass manufacturing and aresummarized in Table 6.1. Many of these opportu-nities directly relate to oxy-fuel-fired furnaces. Al-ternative combustion cycles are also being devel-oped in which advanced ceramics are consideredenabling.

Table 6.1. Opportunities for advanced ceramics in glass melt furnaces

Application Industry need Opportunities for ceramics

Refractories More corrosion resistance materials Incremental improvementspotentially through gradeddesigns

Flue gas ducts High-temperature-capable,corrosion-resistant materials

Fiber-reinforced ceramic compositeto accommodate thin wall designs

Particle separators High-temperature-capable bag filters

Greater corrosion resistance

Ceramic hot-gas filter

Burners Higher-temperature-capable burnernozzle with longer life

SiC or MoSi2 ceramic matrixcomposite, thermal barriercoatings

Sensor shields Uncooled thermocouple and IRcamera shields with longer life

Engineered material toaccommodate uniquerequirements; SiC (above themelt) or MoSi2 (below the melt)ceramic matrix compositepotential candidates

Immersion heaters Change from electric boost tonatural gas fired

Engineered material with a Mo outerliner below the melt

Electrode supports Longer life blocks, uncooled holders Incremental improvementspotentially through gradeddesigns

Stirrers Lower cost, uncooled materials MoSi2, ceramic matrix composite orcoating

Plungers/melt flow control Lower cost, longer life materials Engineering ceramic matrixcomposite. MoSi2 and mullitepotential candidates

Bubblers/lances Lower cost, uncooled materials Engineering ceramic matrixcomposite; MoSi2 and mullitepotential candidates

Glass stop insulation Longer life Incremental improvementspotentially through gradeddesigns

Heat exchangers Higher temperature capable, longerlife materials, uncooled materials

SiC or SiC ceramic matrixcomposite tubular structures

Fans for air flow control High-temperature-capable fanmaterials

SiC ceramic matrix composite withengineered attachment

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Chapter 6

6.4 FLAT GLASS

The U.S. flat glass industry comprises six ma-jor flat glass producers operating 28 furnaces in16states. These plants, which employ about 12,000skilled workers, produce 3.9 million tons of glassper year and achieve sales of $2.1 billion. Plantsare mainly located near sources of low-cost energy.In 1991 the industry used 55.2 trillion Btu of en-ergy derived primarily from natural gas and elec-tricity. Partly because of competitive pressuresfrom developing countries, increases in produc-tion efficiency and energy efficiency are continu-ally being sought. Over the past 25 years, energyefficiency has improved by over 50%, with muchof the energy savings resulting from the use of im-proved refractories. The typical flat glass plantcosts $100 million to build and has a campaignlife of 12 years.

The typical flat glass manufacturing plantconsists of upstream operations for batching,melting, refining, forming, and annealing.Downstream operations include reheating,reforming, coating, tempering, and laminating.Downstream operations can be performed on-siteor off-site. Upstream operations are virtuallyidentical for all manufacturing plants because all

use the float process to form molten soda-lime-silicaglass into a flat sheet. No applications for advancedceramics were identified within the flat glass coatingoperation, and that operation will not be discussedfurther. Unlike the melting furnaces used in otherglass manufacturing processes, those used forproducing flat glass are generally very large. Therefiner is equally large because of the need toeliminate seeds resulting from gas bubbles and otherdefects that degrade optical clarity.

Two float glass forming processes are in use, onedeveloped by Pilkington Brothers (PB) and onedeveloped by PPG Industries, Inc. Major differencesbetween the two processes involve how the glass isintroduced into the furnace. Essential features of thePPG process are shown in Fig.6.7. A typical floatzone furnace is 49 m long and 9 m wide and canhold 909 metric tons of glass. In the PPG process,the refined glass flows in a continuous, full-widthribbon onto a molten bath of tin contained in an air-cooled, refractory-lined steel shell. In the PB process,the molten glass enters a much narrower region ontoa molten bath of tin and takes a very complicatedflow path before it reaches its full width. In bothprocesses, glass enters at a temperature of 1040°Cand exits at a temperature of 600°C. The tin bathis maintained at a temperature of 815°C, whereas

Fig. 6.7. PPG float glass process. Source: Adapted from F. V. Tooley, Handbook of Glass Manufacture, 3rd

ed., Vol.2, Books of the Glass Industry Div ., Ashlee Publishing Co., New York, 1984, pp. 714–15.

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Glass Industry

the steel shell air is cooled to 100°C. An atmosphereof N2plus 5–8% H 2 is used to prevent tin oxidation.Alternate materials considered for replacement ofthe refractory-lined steel/tin bath shell (e.g.,tungsten and graphite) are either costly, difficultto fabricate into the desired shape, or lackoxidation resistance. Few material candidates existbecause of the highly corrosive nature of tin.

The flow of glass into the float zone furnacefrom the refiner is regulated by a fused-silica gatecalled a tweel. The tweel meters the glass flowinginto the float zone furnace and thereby helps con-trol the finished product thickness. The finishedproduct thickness is also varied by the glass vis-cosity, surface tension, and, most important, thetraction forces applied to the edge of the glass rib-bon by water-cooled stainless steel gears (stretch-ing drives). Thicker glass can be formed by flow-ing the glass against nonwetting, water-cooledgraphite barriers at the edges.

Because the life of the tweel is only about tenmonths, alternate materials are being sought toincrease life. In additional to molten glass com-patibility, requirements for the tweel include high-thermal-shock resistance and load-carrying capa-bility at molten glass temperatures. Replacementsfor water-cooled materials are also sought becauseof the high level of maintenance and added costfrom corrosion. More corrosion-resistant metal al-loys (e.g., stainless steel) have been used but areundesirable because nickel gets into the recycledglass and forms nickel sulfide during annealing.Conditioned water is commonly used throughouta glass manufacturing plant to minimize corrosion.

The glass temperature and tin melt are main-tained by several thousand silicon carbide globarslocated along the length of the furnace. As the glassmoves down the tin bath, it cools and increases inviscosity, thus allowing lifting from the tin bathby transport rolls. The transport rolls that carrythe glass to the annealing furnace are engineeredto avoid deforming the glass under its own weight.The rolls, about 30cm in diameter and 4.3 m long,are constructed of asbestos or water-cooled metal.

Instrumentation used to monitor the processincludes optical pyrometers for measuring surfacetemperature of the glass and tin bath and shieldedthermocouples for measuring the subsurface tem-perature of the tin bath. Measurements of local ve-locity in the tin and glass and of glass thickness,as well as visualization of surface defects andstresses, are desirable but have met with limitedsuccess. Cooled instrumentation used in the roofis commonly a source of glass defects because con-

densates of tin sulfide vapor can dislodge and fallon the glass surface. Current procedures call forintermittent cleaning of the furnace roof and dis-posal of the contaminated glass. Globars create asimilar problem, but a potential solution is to re-place them with radiant burners.

From the float zone furnace, the glass sheetenters the annealing furnace, more commonly re-ferred to as the lehr, to remove internal stressesresulting from thermal gradients generated dur-ing the plate-forming process. The lehr operatesat temperatures of 200°C, with heating providedby gas combustion or electricity. In an electricallyheated lehr, the atmosphere is air; in a gas-firedlehr, the atmosphere contains by-products of com-bustion. The glass is transported through the lehron water-cooled stainless steel, fused-silica, or as-bestos-containing rollers with dimensions of upto 5 cm in diameter by 2.5 m long (as illustrated inFig. 6.8). Problems associated with supporting theglass sheet in the lehr include roller surface dam-age, roller marking, sag between rollers, rollerwave distortion, and roller contact heat transfer.Uniform size and speed of the rollers is critical toavoid scratching the glass surface. Radiant burn-ers are used in many gas-fired lehrs and can be asource of high maintenance. Upon cooling to roomtemperature, the glass is cut and packaged fordownstream processing.

Downstream operations can include a numberof reheating, reforming, tempering, and coatingsteps. Reheating and tempering occur in furnacessimilar to the lehr and only vary in temperature.Large volumes of heated air are frequently circu-lated in the lehr to help reduce thermal gradients.The fans are produced from high-temperature steelwith water-cooled drives. Fan distortion reduceslife at the higher operating temperatures. After theglass has been reheated it can be reformed bybending on cast fused-silica molds with integralheating elements. Because of their poor structuralproperties, molds are easily damaged. While theycan be repaired, repairs are time consuming andcan be costly, both in down time and the actualmaintenance. Release coatings or barriers of ce-ramic or metal films are used to inhibit sticking ofthe glass to the mold. Because the glass relaxesafter molding, die design is an iterative processthat can occur over an extended period and becostly.

A summary of opportunities for advanced ce-ramics in flat glass manufacturing is shown inTable 6.2. Opportunities for the batch, melt, andrefinement operations were previously described.

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Chapter 6

Table 6.2. Opportunities for advanced ceramics in flat glass manufacturing

Operation Industry need Opportunities for ceramics

Float zone furnace 1. Longer life tweel

2. Replacement of asbestos transportrolls

3. Improved materials for stretchingdrives

4. Replacement of refractory lined tinbath

5. Uncooled sensors

6. More efficient gas-fired radiantburners

1. Incremental improvements

2. Ceramic matrix composite or ametal/ceramic hybrid

3. SiC or Si3N4 monolithic ceramic

4. Engineered material withanticipated bath redesign

5. Ceramic matrix composite or Si3N4

6. SiC or MoSi2 ceramic matrixcomposite. Thermal barriercoatings.

Annealing, reheating,tempering

1. Longer life air circulation fans

2. Improved hot handling materials

3. High-temperature-capablenoncontact sensors

4. Low-maintenance radiant burners

1. SiC ceramic matrix composite withengineered attachment

2. Ceramic matrix composite orcermet

3. Si3N4 or ceramic matrix composite

4. SiC or SiC ceramic matrixcomposite

Reforming 1. Improved mold materials andmanufacturing methods

2. Improved mold release coatings orfilm barriers

3. High-temperature-capablenoncontact sensors

1. Engineered materials manufacturedusing 3-D rapid prototypingtechnology

2. Ceramic-coated metal or ceramiccloth

3. Si3N4 or ceramic matrix composite

Fig. 6.8. Fused silica rollers used in a lehr. Source: Marketing brochure from AirLiquide America Corp., Walnut Creek, Calif.

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Glass Industry

Fig. 6.9. Single gob feed cycle. Source: Reprinted from F. V. Tooley, Handbook of Glass Manufacture, 3rd

ed., Vol.2, Books of the Glass Industry Div ., Ashlee Publishing Co., New York, 1984, p. 601.

6.5 CONTAINER GLASS

The container glass industry, which is made upof 64 plants in 25 states, employs 30,000 skilledworkers, produces 12.3 million metric tons of glassper year, and achieves sales amounting to $5 bil-lion annually. Plants are located mainly near prod-uct markets. In 1992 the industry used 119 trillionBtu of energy, 79% of which was derived fromnatural gas. Competitive pressures occur frommarket preferences for lighter weight containermaterials (e.g., plastic and aluminum), the highcost of end product distribution resulting from theweight of glass containers, and the lower cost oflabor in developing countries. The market sharelost to alternate materials is being reduced by regu-latory concerns and the higher cost of “productstewardship.” The distribution cost of glass con-tainers is benefiting from the development ofhigher strength glass and improved manufactur-ing methods developed to produce thin-wall,light-weight containers. Increased production andenergy efficiencies are continually sought to effec-tively compete against developing countries. Thetypical container glass plant costs $70 million tobuild. The last container plant was constructed in1981. Because of their high level of maturity, manu-facturing processes are essentially the same in allplants.

The typical glass container manufacturingplant consists of upstream operations for batching,melting, conditioning, and forming and down-stream operations for coating and annealing.

Downstream operations can be performed on-siteor off-site. The batching, melting, conditioning,and annealing operations are similar to those dis-cussed previously. No applications for advancedceramics were identified within the coating opera-tion, and thus that operation will not be discussedfurther.

Forming

After conditioning to the desired temperatureand uniformity, the molten glass is transferred tothe forming operation through the gob feeder. Thegob feeder is an integral part of the forehearth andincludes a mixer, plunger, orifice, and shears, asshown in Fig 6.9. The gob feeder controls theweight, temperature, and shape of the gob—all ofwhich are critical to container quality. Gob-form-ing rates can easily approach 300 gobs per minute.The materials of construction for the gob feederinclude water-cooled metals (cut-off blades), pre-cious metals (stirrer), and refractories (plunger) toaccommodate glass gob temperatures approach-ing 1100°C.

The gob next enters the forming operationwhere the heat is extracted from the glass in a con-trolled manner during shaping. Several processesare available to form the gob; the process selectedis based on composition, container shape, size, andproduction rate. The press-and-blow process com-monly used to produce large-mouth containers isillustrated in Fig.6.10.

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Chapter 6

In the press-and-blow forming operation, thegob enters the mold and is first pressed into ablank. During the pressing operation, plungertemperature approaches 550°C. Because theplunger material is not as hard at thesetemperatures, the plunger surface is easily scoredby the chilled glass at the interface duringextraction. The blank is removed from the pressmold, reheated, and then placed into a secondmold for blowing into final shape. When removedfrom the blow mold, the glass cools to 780°C.Production rate is limited by the rate of heatextraction and can approach 100 bottles per minutewhen multiple molds are used with a single gobfeed system. The forming operation is the singlelargest contributor to both the cost and quality ofthe finished glass container, and molds representa large part of that contribution.

Requirements for mold materials include lowcost, capability for producing complex shapes, lowdistortion in use, ability to take and retain a finesurface finish, low thermal expansion, adequatehot structural properties, scale and oxidationresistance, high thermal diffusivity, and highresistance to thermal cycling. The primary moldmaterial is cast iron. Low-pressure air-cooling isused to accelerate heat extraction and improvetemperature control. However, because air cooling

generates a high level of noise, liquid cooling maybe employed as an alternative. A disadvantage ofliquid cooling is the increased maintenance neededfor leakage and corrosion. To increase cycle times,mold temperature can be further reduced byprecooling the air or liquid. Mold materials havinghigher thermal diffusivity have also been used (e.g,copper-based alloys in neck rings). To reducemold-to-glass adhesion, glass marking, andoxidation of the metal mold, the mold is swabbedwith a lubricant between cycles. Because moldtemperatures approach 500°C, the lubricants usedgenerate smoke and vapor and sometimes ignitespontaneously. Alternate approaches availableinclude nonmetallic solid film lubricants or theinjection of acetylene into the hot mold betweencycles, which cracks and forms a thin deposit ofcarbon throughout the mold interior.

Alternate materials of construction for moldscurrently being considered include nickel-basedsuperalloys, graphite, and ceramic coatings.Asbestos-containing materials were used for aperiod but have been replaced by graphite forreasons of regulatory compliance. Nickel-basedsuperalloys are being used in areas where higherhot hardness and strength is desired (e.g.,plungers, baffles, funnels, blow heads, bottomplates, and plugs). As with flat glass, nickel

Fig. 6.10. Container press-and-blow forming operation. Source: Reprinted from F. V. Tooley, Handbook ofGlass Manufacture, 3rd ed., Vol.2, Books of the Glass Industry Div ., Ashlee Publishing Co., New York, 1984,pp.616–17.

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contamination can be a problem with containerglass.

Graphite is a preferred material for hot han-dling because it does not wet, mark, or adhere toglass; provides inherent lubricity; does not scale;retains its strength at temperature; and is low cost,easily machined, and environmentally safe. Ap-plications include blow-head inserts, take-outtongue inserts, sweep-outs, lehr-loading bars, gobdeflectors, and conveyor guides. Grades of graph-ite used are selected based on their application.Porous graphite is desirable in the lehr to avoidthermal checking, which is caused by rapid heattransfer from glass containers. Higher densitygraphite is used in molds where increased strengthand hardness are desired. While graphite offersmany advantages, a key disadvantage is its lim-ited life due to poor oxidation resistance and lowstrength. A greater number of glass defects havebeen seen when ceramics have been used.

As containers become thinner, higher hardnessmaterials are desired for mold surfaces to reducecontamination and maintain surface finish andcritical dimensions. Higher hardness materials alsoprovide reduced die wear in machines where pro-duction rate is being increased in place of addedcapacity. One solution is the application of hardcoatings (e.g., tungsten carbide and titanium car-

bide) onto existing hot handling surfaces, espe-cially the plunger, by a variety of processes. Whilemore difficult and costly to machine, resistance tooxidation and scaling occurs when applied tographite and cast iron, respectively. Spallation ofthe coating can also occur, thus increasing the riskof glass contamination.

After molding, the containers are placed intothe lehr for removing stresses (see Fig. 6.11). Thelehr is similar to that previously discussed for flatglass; the primary difference is the use of a con-veyor belt to move batches of the container glassthrough the hot zone in a controlled manner. Theconveyor is fully contained within the furnace toreduce heat loss and increase energy efficiency.

With the exception of coatings, no evidence wasfound of advanced ceramic materials usage in theforming of glass containers. Opportunities exist,however, because of the increasing emphasis onlightweight glass containers, continuous produc-tion, and improved energy efficiency as competi-tion increases from abroad. Specific opportunitiesinclude replacement of graphite in areas of hothandling, with alternate materials providinghigher toughness, greater hardness, and improvedoxidative stability. Lower cost materials are soughtto replace precious metals used in the forehearthand gob feeder. Materials with higher thermal

Fig. 6.11. Gas-fired glass annealing furnace with an internal belt return. Source: Re-printed from F.V .T ooley, Handbook of Glass Manufacture, 3rd ed., Vol.2, Books of the GlassIndustry Div., Ashlee Publishing Co., New York, 1984, p. 682.

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diffusivity and hot hardness are sought for moldsto extend life and reduce cycle time without ex-ternal cooling. Higher-temperature-capable mate-rials are desired throughout the mold machine forreduced maintenance and improved quality. Newsensors and sensor shields are sought to measuregob temperature, temperature uniformity, andheat extraction during shaping. In all cases, ad-vanced ceramics are central to the solution. Be-cause they are compatible with molten glass, ad-vanced ceramics of choice are oxide-based or ce-ramic composites having predominate oxidephases. Leading candidates are alumina, mullite,and silicon carbide–alumina composites. Molyb-denum disilicide–based composites are also goodcandidates for many of these applications. Silicon-based ceramics are candidates for hot applicationsrequiring optimum structural properties that arenot in contact with molten glass.

A summary of opportunities for advanced ce-ramics in container glass manufacturing is shownin Table 6.3.

6.6 FIBER GLASS

The fiber glass industry is made up of twoprimary markets: wool insulation and textilefibers. Wool insulation is a short-fiber product usedby the construction industry. Textile fibers are acontinuous-fiber product principally used as apolymer matrix composite reinforcement.Together, the four major producers of wool andthe six major producers of textile fibers employabout 16,000 workers. A typical producer canoperate as many as 27 glass melt furnaces. In 1991,fiber glass made up 9% of all glass produced andaccounted for 21% of the total market value. In1981, 71% by weight of all glass fiber shipped was

wool. Total energy consumption for the fiber glassindustry in 1991 was 59.5 trillion Btu, a valueexceeded only by the container industry. Unlikecontainer or flat glass, a large amount of energyconsumed is used to fabricate fiber from moltenglass. Whereas both markets are capital intensive,the textile fibers industry is more so because of itsgreater dependence on precious metals. Glassquality requirements also vary, but textile fibersproduction requires much higher quality glass andgreater use of process control. Key drivers for theuse of new technology in the fiber glass industryinclude reduced cost, increased throughput,reduced emissions, and a competitive advantage.Competitive pressures primarily exist from organic-based products. Processes used to manufacture fiberglass can be divided into batching, melting, andfiberization. Each process and opportunities foradvanced ceramics is discussed in detail in thefollowing sections.

Batching

The batching operation is similar to that usedfor producing other glass products with theexception of different compositions and finer rawmaterial particle size. Because particle size is finer,emissions control and contamination by ceramics,metals, organics, and alloys are of greater concern.A key ingredient for manufacturing fiber glass isboron, which provides reduced melt viscosity andimproved glass durability but is volatile andcondenses at undesirable locations. The contentof boron in glass fiber ranges from 5 to 10 w %and represents 50–75% of the total cost. A highpercentage of recycled container and flat glass isused to produce wool fiber glass, for which qualityrequirements are less stringent. Recycled fiberglass is not currently used because of the boronpresent.

Table 6.3. Opportunities for advanced ceramics in container glass manufacturing

Application Industry need Opportunities for ceramics

Radiant burners Longer life burners SiC or SiC ceramic matrixcomposite

Hot handling Higher toughness materials withimproved oxidation resistance

Ceramic matrix composite, cermet,or carbide coatings

Gob feeder Lower cost stirrer materials, reducederosion plunger materials, anduncooled cutter materials

Engineered ceramic matrixcomposite plunger, cutter, andstirrer; mullite ceramic compositeplunger

Molds Higher hardness materials, higherthermal diffusivity

Engineered material or coating

Sensors Longer life thermocouple shields Si3N4 or ceramic matrix composite

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Melting

In addition to batching, the other major processthat fiber glass shares in common with other glassindustries is the melting operation. The differencesare limited to processing challenges resulting fromglass compositions and an overall smaller furnacesize. The low alkali content of textile fibers requireshigher melting temperatures and greater fiberparticle size to ensure homogeneity without thebenefit of fluxing additives. Both textile and woolproducts include boron, which can foul furnacehardware and increase particulate emissions. Glassmelters include regenerative, direct-fired, andrecuperative furnaces. Large recuperative gas-firedfurnaces are still the primary source of melters. Thecampaign life of gas-fired furnaces is 7–10 years;rebuilds are costly and time consuming. Someconversion to electric and electric-boosted gas-firedfurnaces has occurred to reduce gas consumption,increase melt efficiency, and reduce emissions(SOx, NOx, particulate). Electric furnaces use top-or bottom-entry electrodes, the former being easierto maintain. The campaign life of a electric furnaceis six months; atypical r ebuild takes two days tocomplete. An example of a cooled metal wall,bottom-entry electrode melter is shown in Fig. 6.12.The melt exits through the submerged throat into a

Fig. 6.12. Example of a bottom entry electrode electric glass melter. Source: “AdvancedHigh-Temperature Material for Glass Applications,” U.S. Department of Energy fact sheet.

homogenization section followed by a hearth fordistribution to the different fiber-formingoperations. Conversion to oxy-fuel firing is alsooccurring as a result of demonstrated improvementsin energy efficiency and emissions.

A number of opportunities exist for advancedceramics within the melter. Higher temperaturesof the melt in combination with the low alkalicontent force the use of environmentally sensitivechrome-containing refractories. Chrome-containing refractories are also electricallyconducting, drawing current away from the glassand reducing refractory life. The by-products ofcombustion, and higher temperatures ofcombustion with oxy-fuel firing, also limit refractorylife. Improved glass stop refractories are sought forreduced maintenance. Precious metals (platinum/rhodium) are used throughout the melter (gasbubblers, plungers, thermocouple shields) with aper furnace cost approaching $10 million. Platinum/rhodium is used because of its high-temperaturecapability and stability when in contact with glassat temperatures exceeding 1371°C. Disadvantagesinclude high cost, low creep strength, and uncertainsupply. Alternatives to platinum/rhodiumevaluated and deemed unsuccessful includegermanium-doped molybdenum disilicide andInconel. Ceramic-based material options include

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Chapter 6

high alumina or zirconia composites with highpurity and density. Ternary phase oxides couldalso be considered, but phase instability is ofconcern. Another option is higher creep resistantnonoxide composites (i.e., silicon carbide with aplatinum/rhodium coating for molten glasscompatibility).

Molybdenum is also a primary material used infiber glass melters. Applications include electrodesand glass flow control devices. For lowertemperature applications (< 1315°C), Inconel 600has been used. In both cases, premature failureshave occurred due to excessive corrosion and wear.Advanced ceramics are sought that can operateuncooled, above or below the melt line, and exhibithigh-temperature creep resistance. Opportunitiesfor advanced ceramics also exist for oxy-fuel-firedburners and in shields for sensors being developedto detect refractory wear.

Fiberization

Molten glass leaves the hearth and enters thefiberization process directly or after anintermediate marblizing process. Marblizing iscommonly used for textile fiber production whenthe forming process is not located in closeproximity to the melter. Fiberization occurs byattenuating a melt steam with air, steam, orcombustion gas (for wool) or by drawing moltenglass through precious metal bushings (for24textiles). For the wool pr ocessing example

Fig. 6.13. Example of a wool forming process. Source: Reprinted from F. V. Tooley,Handbook of Glass Manufacture, 3rd ed., Vol.2, Books of the Glass Industry Div ., AshleePublishing Co., New York, 1984, p. 721.

shown in Fig. 6.13, a single stream of molten glassfalls into a rapidly rotating cylinder (centrifuge)made of a high-temperature base metal alloy. Thecylinder has a 12- to 36-in. diameter and contains10,000–30,000 laser-drilled holes having diametersof 0.1–0.015in. Operating conditions includesteady state temperatures of 2000°F while exposedto air. Typical lifetime is 40–100 h. The base metalalloys used are generally proprietary, investmentcast, and extensively recycled to reduce cost.Alternate materials are sought with increasedcreep strength, resistance to corrosion (sulfidation)and oxidation, and reduced cost. As the moltenglass is driven through the perforated cylinder,downwardly directed high-velocity gas jetsattenuate the flow and form fibers of varyinglengths. The fibers are sprayed with organicbinders and collected onto a moving belt wherethey are transported to a curing oven. Alternateapproaches to the rotary process for forming woolinclude attenuation of a molten stream passedthrough sieve-like platinum alloy bushings andremelting of drawn fiber by a high-velocity burner.

In the continuous textile forming processshown in Fig. 6.14, glass marbles are fed throughorifices in an electrically heated platinum alloybushing. A typical bushing has a cross-section of25–50 cm by 18–20cm with a depth of 5 cm. About100 dies are used with a single melter, and a typicalproduction site contains four melters. Eachproduction unit processes 400–3,000 filamentssimultaneously, with fiber diameters ranging from

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Glass Industry

Fig. 6.14. Example of a textile forming process.Source: Reprinted from F.V. Tooley, Handbook ofGlass Manufacture, 3rd ed., Vol.2, Books of the GlassIndustry Div., Ashlee Publishing Co., New York,1984, p.725.

Fig. 6.15. Ceramic textile fiber handlingguides. Source: Reproduced from Kyocera’s Web siteat http://www.kyocera.com.

3.5 to 20 µm. Bushing life is about six months, atwhich time the bushing is recycled because of thehigh cost of raw material. Life is limited by holewear and sag. Operating conditions include steadystate temperatures of 1200°C in air. Forced-convection air cooling is used prior to the sizingapplication to increase productivity. A windingdrum collects the filaments and applies a tractionforce to stretch the fibers as they are pulled througha mechanical device that applies an organic sizingand twists multiple filaments into a single strand.Depending on the application, additionalprocessing is performed on the multiple strands.Materials commonly used to handle and guide hotfilaments are carbon based. Alumina and siliconnitride guides are used for sizing application andtwisting where high wear can occur. Typical fiberhandling guides are shown in Fig. 6.15. Alternatematerials are sought for fiber handling that exhibitmany of the characteristics of monolithic ceramicsbut have improved toughness for increased

reliability. Lower cost solutions are also sought forthe platinum sieves and bushings. Oxidesdispersion strengthened with yttria have beenevaluated as a precious metal replacement withpromising results. Platinum-coated molybdenumwas also considered but failed during testing.

A summary of opportunities for advanced ce-ramics in fiber glass manufacturing is shown inTable6.4. Opportunities for the batch operation ar esimilar to those discussed for container and flatglass.

6.7 SPECIALTY GLASS

This market segment is a catchall for glassproducts not contained in the flat glass, containerglass, and fiber glass market segment. An impor-tant difference is that the specialty glass produc-ers, unlike the other glass producers, work withhigher temperature glasses, aluminosilicates, andborosilicates, as opposed to soda lime glass. Whilecompositions vary, processing methods are simi-lar to those previously discussed. The marketsserved are very broad, with as many as 60,000 dif-ferent products, but volumes are small. A largenumber of specialty glass producers are small,owner-managed, and labor intensive. Because ofthe nature of the business, the technology beingused is frequently developed in-house and re-mains highly proprietary.

Specialty glasses have very specific chemicalcompositions with clearly defined physical andchemical properties. Furnaces are generally specialdesigns and principally oxy-fuel fired. To furtherimprove energy efficiency, cullet preheaters arebeing evaluated. Melter capacities of 45 metric tonsare considered large. Because of the high melting

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Chapter 6

Table 6.4. Opportunities for advanced ceramics in fiber glass manufacturing

Operation Industry need Opportunities for ceramics

Melting 1. Improve refractories

2. Precious metal replacement

3. Longer life electrodes

4. Longer life flow control materials

5. Sensor shields

6. Oxy-fuel-fired burner nozzles

1. Incremental improvements

2. Mo based engineered material



5. Mullite or Mo based engineeredmaterial

6. SiC or MoSi2 ceramic matrixcomposite; thermal barrier coating

Wool forming 1. Low-cost centrifuge material

2. Precious metal replacement forsieves and bushings



Textile forming 1. Precious metal replacement forbushings

2. Improved fiber-handling materals


2. Ceramic matrix composite

temperatures, refractories and thermocouplesheaths are a problem. Infrared cameras arefrequently used in place of short-livedthermocouples for temperature control, and fiberoptic systems are being developed. Because oftheir small size, convection currents are used inplace of bubblers to homogenize the glass. A rangeof high-temperature base metal alloys andprecious metals are used throughout the melt andforming steps (e.g., platinum paddles used in themelter for homogenization). After the meltingprocess, the glass goes in several differentdirections for forming, depending on the final

product. During the forming operation, molds aregenerally superalloys. Ceramic molds wereconsidered but shown to increase the presence ofstones in the finished glass. Natural-gas-firedmetal radiant burners, which are commonly usedduring forming and annealing, continually requirea high degree of maintenance.

A number of niche opportunities exist withinthe specialty glass industry. Opportunitiescommon among glass market segments also exist,but operating conditions both above and belowthe melt are much harsher in specialty glass.

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6.8 REFERENCES

Publications

DOE/Conf-970292 December 16, 1997. Oxy-FuelIssues for Glassmaking in the 90s: WorkshopProceedings, documenting a workshopsponsored by the U.S. Department ofEnergy’s Office of Industrial Technologiesand cosponsored by 22 companies in theU.S. glass industry, held in Arlington, Va.,February 27, 1997.

Energetics, Incorporated September 8, 1994.“Materials Needs and Opportunities in theGlass Industry: Schuller International, Inc.”

Energetics, Incorporated October 27, 1994.“Materials Needs and Opportunities in theGlass Industry: Libby Owens Ford Com-pany.”

Ensor, T. F. June 1990. “Mould Materials,” GlassTechnol. 31(3), 85–88.

Ensor, T. F. October 1978. “Mould Materials,”Glass Technol. 19(5), 113–19.

Geiger, G. 1990. “Recent Advances in GlassManufacturing,” Ceramic Bull. 69(3), 341–44.

Geiger, G. 1993. “Recent Advances in GlassManufacturing,” Ceramic Bull. 72(2), 27–33.

Glass Technology Roadmap Workshop September1997. Proceedings of the Glass TechnologyRoadmap Workshop held in Alexandria, Va.,April 24–25, 1997, prepared by Energetics,Incorporated, Columbia, Md.

Gridley, M. May 1997. “Philosophy, Design andPerformance of Oxy-fuel Fired Furnaces,”Ceramic Bull. 76(5).

Pfendler, R. D. 1995. “Glassmakers Face NewEmissions Concerns,” Ceramic Bull. 74(4),95–99.

Tooley, F. V. 1984. The Handbook of Glass Manufac-ture, 3rd ed., Vols 1–2, Books for the GlassIndustry Div., Ashlee Publishing Co., NewYork.

U.S. Department of Energy, Office of IndustrialTechnologies January 1995. A TechnologyVision and Research Agenda for America’s GlassIndustry.

U.S. Department of Energy, Office of IndustrialTechnologies January 1996. Glass: A ClearVision for a Bright Future.

Information Supplied by Academia and theGlass Industry

Scott Anderson, Combustion-Tec, Inc.Bernard Conner, Accutrue International

CorporationPeter Gerhardinger, Libby Owens Ford

CompanyDr. Foster Harding, Schuller InternationalVince Henry, Ford Motor CompanyTheodore Johnson, U. S. Department of EnergyDr. Walter Johnson, Schuller InternationalDan Labelski, Libby Owens Ford CompanyPat Moore, Combustion-Tec, Inc.Dr. Robert E. Moore, University of Missouri-

RollaFrederic Quan, Corning, Inc.Phil Ross, ConsultantDr. Robert Speyer, Georgia Institute of

TechnologyGreg White, Praxair

glass industry process

Documents