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The Czech GlassSociety’s 13thconference on electricand other highly

efficient ways of glass meltingwas held at Plzen, in the CzechRepublic, in September. Sosuccessful was the event that theorganisers had to find a largervenue at very short notice toaccommodate the 140 peoplewho came to find out moreabout the latest technologies.

Electric melting uses energythat is often more expensive thangas, but uses less energy. It waspatented as a technology in 1902but the technique is thought todate back to 1899.

Josef Smrcek, managingdirector of Electroheat, sayselectric boosting increases theefficiency of gas furnaces but toachieve an all-electric furnaceconsumption of 0.7kWh/kg it isnecessary to slow the flow of theglass melt. Slow flow currentsallow lower meltingtemperatures, increase pull, andallow the use of little or norefining agents. They also allowshallower melting tanks.

“Furnaces with slow currentsmake it possible to decrease thetemperature to 1400°C, needing

important to useall electricfurnaces for themelting of crystalglass. “The qualityof produced glassmelt is one of themost importantreasons,” he says.

Professor Kasapresentedexamples showingthe importance ofmodellingmethods for theinfluence of thepositioning,connecting andthe shape of Moelectrodes on thedistribution ofcurrent density ontheir surfaces.“This distributionrepresents one of the mostimportant data in assessing theoptimum frequency of feedingcurrent if the method of lowfrequency current melting is usedto protect Mo electrodes againstcorrosion,” he says.

Professor Kasa compared theperformance of furnaces withhorizontal electrodes, verticaltop-electrodes and oblique top-

electrodes. Hefound it was notpossible for rodelectrodes to reachuniform loadalong their entirelengths, that thetip of the electrodealways bears thegreatest currentload, that themagnitude ofelectrode loaddepends onwhether themeasured place ofthe electrode is

a passage time of two hours,”Mr Smrcek says. “It allowspulling twice as today’s passagetime is sufficient, it allows ashallower basin and it meansyou can use less or even norefining agents and use onlyStokes’ law on ascendingbubbles.”

The multiple pull rate up to5t/m2 per day decreases heatlosses to 20% and the meltingheat by 0.03 kWh/kg with a cutof 0.7 kWh/kg possible, helpingincrease furnace life by cuttingcorrosion. Such a furnace wouldbe ecologically friendly becauseit would use less or no refiningagents. The furnace would alsobe compact with no largeauxiliary equipment, withnatural heat recuperationthrough the batch layer. Itwould also be possible to lowerthe temperature of the emergingglass by cooling the walls belowthe electrodes, and the bottom,with air and by creatinginsulated channels in the wallsto prevent radiation. This wouldgive hotter air that could beused in preheating.

Professor Stanislav Kasa, fromthe Institute of ChemicalTechnology in Prague, says it is

Conference examines highly efficientmelting methodsGlassmakers from 17 different countries attended the13th Czech conference on glass melting.

turned away from otherelectrodes and turned towardsthe electrode with the samephase and that the uniformdistribution of electrode load canbe reached only by changing theelectrode’s shape.

“Very close correspondencebetween the technological datafrom the model and thatobtained from a furnace, shows itis possible to use modellingmethods as significant sources ofdata for designers of glassmelting furnaces,” ProfessorKasa says. When comparingelectric melting with other formsof melting using fossil fuels,Professor Kasa says it shows anumber of advantages.

The environment in andaround glassworks is improvedby the removal of combustionproducts from a furnace and byreducing the amount of dust.Volatile components in the batchare reduced by up to 40% forfluorides and 10-20% for leadoxide.

Glass produced is morehomogenous because allelectrically heated glass meltshave the same thermic course,which leads to the reduction of

MELTING TECHNOLOGY

� The magnificentcathedral in thecity of Plzen.

� An attentiveaudience listens toone of thepresentations at theconference.

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poor quality products by morethan 20%. The proportion ofproducts rejected because ofstones is reduced using electricmelting compared with othertypes of melting.

Electric melting gives moretolerance for mistakes madeduring the preparation of thebatch. Such errors areinadmissible when using gas-firedfurnaces because they increasecords in the glass.

Melting with all-electricfurnaces also creates fewerdifficulties when heating up thefurnace to full productivity anduseable production after aninterruption for holidays, repairsand so on. Such heating up andpreparation usually takes betweenhalf a day and a full day for a fossilfuel furnace but electric furnacescan often start productionimmediately with no delay.

Professor Kasa also claims thatelectric furnaces can be rebuiltmuch quicker than can othertypes of furnace, because of theirsimplicity. “The shut-down of anelectric furnace does not usuallylast longer than 10-20 days,” hesays.

Because an electric furnacecan usually react faster and easierto outside changes than canother furnaces, it is easier to keepits pull constant. And ProfessorKasa says electric furnacesdemand less space in the meltinghall. “The all-electric furnace hasa simpler design as it consists ofa melting tank, a throat, riser,and a forehearth only,” ProfessorKasa says.

“All-electric furnaces also reachhigher specific pull than gas-firedones and feeding sources areusually situated outside the hall.”

Electric melting wasintroduced to the CzechRepublic in the 1960s,concentrating on lead-freecrystal production. Initial resultswere not successful and sodevelopment work continued.“At the beginning of the 1970sthe essential change of thedesign of electric furnaces forcrystal glass was made byreplacing vertical Mo electrodeswith horizontal electrodes,”Professor Kasa says.

“A reliable all-electric meltingfurnace with very goodperformance was the result.”

The next step was thedevelopment of electric meltingof 24% lead crystal with Moelectrodes. Gradual

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modernisation and optimisationof furnace designs increased theirlives and decreased energyconsumption, and a series of all-electric furnaces pulling 5-25tonnes/day has been developed.

“The longevity of thesefurnaces reached more than fiveyears with specific consumptionof energy 0.9 kWh/kg,” ProfessorKasa says. “However, exceedingthe limits of the specific pull ofthis type of furnace can shortenthe longevity of its meltingparts.”

Now a new type of all-electricmelting furnace has beendeveloped for melting crystalglass, as part of a new researchproject. “The project has resultedin an all-electric furnace with topelectrodes passing into the meltperpendicularly through thebatch,” Professor Kasa says.

“The furnace longevity will belonger than five years with amaximum pull of 25 tonnes/day.Specific energy consumption willrange from 0.9 to 1 kWh/kg. Thedisadvantage is that the Moelectrodes cannot be moved, butthis disadvantage has beenremoved in later developments ofthe furnace using oblique orshaped top electrodes.”

The Novy Bor factory ofBohemia Crystalex Trading hasbeen using electricmelting for more than30 years, and lastmonth the companystarted up its thirdelectric melting tank.The company has beenusing top electrodesfor more than fiveyears but it is still tooearly to analyse thecorrosion of themelting basin and ofother parts of the tank.

“We think topelectrodes are a goodsolution for cold topmelting furnaces,”says the company’s

JaroslavSaroch. “Weonly use oneglasscompositionand our tanks

have very similar pulls, butSorg’s top electrode VSM melterswork with different glasscompositions and can pullbetween five and 180 tonnes ofglass per day.

“Have no fear of top electrodes.They are a flexible, user-friendlysystem in comparison tohorizontal electrodes for electricmelting tanks.” But Mr Sarochsays electrodes can be devastatedby alloys of molybdenum andantimony.

Czech glassmaker SklarnyKavalier has been using electricmelting since 1969. Ales Selzak,from the company, says controlof electric furnaces depends onthe collection of a very largeamount of data about thecondition of the furnace. “It ispossible to anticipate futurebehaviour of the melt from pasttrends,” he says. Informationthat must be collected includestotal pull rate, powerconsumption, thickness of thecold layer, arch temperature,glass melt temperature, and theelectrical resistance of themelting area. But people are stillvital. “In spite of considerableprogress, melting of glass in anall-electric furnace is notpossible without an operator,”Mr Slezak says.

Another leading Czechglassmaker, Preciosa, uses anelectric furnace with tin oxideelectrodes supplied by IAL of theUK. Ladislav Novak of Preciosasays the company tries to keep aconsistent pull to stabilise theperformance. Its nominal furnaceoutput is ten tonnes in 24 hours,melting 30% lead crystal for usein costume jewellery andtrimmings for chandeliers.

Antonin Lisy, from theInstitute of Chemical Technologyin Prague, says the distributionof temperature in the glass meltand the generated flow connectwith released electric energyaround the electrodes, but alsobetween them. “Temperaturedistribution and generated floware in principle influenced by thelocation of electrodes and theirconnection to the power supply,”he says.

“Measuring temperature fieldsby thermo vision camera is fastand accurate and this methodcan also be used for quantifyingpower relations in chosensections of the furnace.” Theymight also be used to evaluatethe character of the glass flow.

Dr Petr Schill, from GlassService in the Czech Republic,was presented with an award atthe start of the conference for hiscontribution to the mathematicalmodelling of glass melting. In hispaper at the conference, Dr Schillshowed a mathematicalsimulation for the evaluation ofphysical fields influenced by theelectrical conducting sludge layerthat accumulates in avitrification melter.

The model quantifies theincrease of maximum refractorytemperature and the decrease ofJoulean heat generated insidemolten glass, dependent on

sludge layerthickness and shape.The critical sludgelayer shape leadingto the side electrodeshas been calculated.

“It results in a veryhigh refractory andsludge temperature of1350°C which canresult in rapid meltercorrosion,” Dr Schillsays. “This criticalconfiguration couldhappen in a realmelter and indicatesweak points of melterdesign andoperation.”

� Dr Petr Schill, from Glass Service, received an awardfor his leadership in mathematical modelling of glassmelting.

� ProfessorStanislav Kasashowed theimportance ofmodellingmethods.

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OCTOBER 2004 287 GLASS

Mathematical modellingshowed that the replacement ofbottom electrodes by refractorymight not bring any advantagebecause the operationalparameters would not change.The same goes for the materialfrom which bubbler tubes aremade, and the electricalconductivity of the tube materialwill not affect melter operation.

Frantisek Novotny, also from theInstitute of Chemical Technology,recommends regulation of Scott’stransformers and has developed amodel of regulation options for usein glass melting furnaces.“Providing both ends are perfectlybalanced, the transformers willwork optimally,” he says.

Another expert from theInstitute of Chemical Technology,Jiri Matej, explains that thecorrosion of molybdenumelectrodes is still not fullyunderstood despite years ofinvestigation. “In particular, themechanism of the action exertedby alternating current still eludesdescription,” he says.

furnaces are high temperaturesin melting and refining, coveredlevel in the melting tank, airatmosphere in the working endand vertical movement of theworking flow. “Hightemperatures support fastrefining, intensive foaming isdangerous in the batch layer,and higher concentrations of

Mr Matej surveyed recent dataon the effects of alternatingcurrent. “Alkalis play an importantrole in the mechanism ofmolybdenum corrosion in glassmelts,” he says. “The alkalicontent also exerts influence onthe frequency corresponding tothe maximum suppression ofcorrosion and it influencesprobably the onset of the anodicpassivation too.” Layers ofreaction products on the electrodesurface play a significant role inthe corrosion mechanism and inaffecting the corrosion byalternating current, Mr Matej says.

Milan Hajek, from the Institute ofChemical Process Fundamentals,examined the production of basaltfibres by microwave melting. “Themicrowave method of providinghigh homogenous melt for theproduction of basalt fibres isbelieved to open new industrial fibreproduction from a readily availablesource of raw material,” he says.

Lubomir Nemec, from theCzech Academy of Science, saysthe important features of electric

sulphates and chlorides plusvertical circulation might leadto agent precipitation,” he says.“The flexible character ofelectric heating predetermines itto new arrangements of meltingand refining.”

Wolfgang Simader, fromPlansee, discussed specialcoatings including his

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� Jaroslav Baranek (left), a float glass pioneer, and Vlastimil Dvorak(right), organising secretary of the Czech Glass Society, received lifemembership of the Society in a special ceremony.

company’s SIBOR coating. Healso looked at glass tankreinforcements and meltingelectrodes. Jan Valenta, from theVSS engineering company inPrague, looked at how thecooling and design of electrodeholders can influence theirlifetimes.

Bernhard Fleischmann, fromthe German glass technologyassociation HVG-DGG,introduced a device to measureemitted heat radiation in thecombustion chamber ofoperating glass furnaces.“Methods to increase the directheat transfer of the flame can betransferred from the laboratory toindustrial scale furnaces,” hesays. Methods have beenprovided to increase the flameemissivity locally due to localsoot formation, increasing directheat transfer without increasingthe crown temperature.

Davor Spoljaric, from the gasesgroup Messer, discussed thepossibilities for NOx reduction inoxy-fuel combustion. “Theoscillation of natural gas in

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foam and lowersoverall oxygencosts,” Mr Vidunasays. A detailedeconomic modelpredicts costsavings of €4-9.6per tonne compared toconventional air-fuel furnaces.

Miloslav Franek, also from AirProducts, examined oxygen forglass melting and itsenvironmental impact. “Burnertechnology is one of the mostimportant factors influencingemissions,” he says. “Inconjunction with an appropriateoxygen generator, emissions canbe eliminated.”

Vlastimil Nevyhosteny, fromPromat, looked at a newgeneration of microporous

different types of oxygen burnershas shown a 50% decrease ofNOx values,” he says. “This canbe used for burners like P-LONbut they have to be operated withat least two pieces to stoppulsation in the furnace. Thedilution burner obtained by farthe lowest NOx values, less than100ppm at a furnace temperatureof 1550°C.

Air Products, a sponsor of theconference, presented threepapers to delegates. Jan Vidunadiscussed oxy-fuel burners, inparticular his company’sCleanfire HR design. “Resultsshow the Cleanfire HR is suitablefor all types of furnaces and giveshigher radiation, better coverage,reduced flame momentum andlower NOx emissions,” he says.Mr Viduna also looked at thehybrid furnace design developedby Air Products, which uses oxy-fuel burners over the unmeltedbatch and air-fuel burnersdownstream. “Compared to oxy-fuel alone the hybrid furnacedelivers similar production levels,improves glass quality, reduces

materials for high temperatureinsulation. The company modifiedthe microporous matrix by usinga new inorganic structurethrough mineral synthesis toallow its products to cope withmuch higher temperatures.

“The representativeapplications seem to be meltingtanks, feeders and feeder bowlinsulation,” Mr Nevyhosteny says.“Much lower heat losses help tostabilise the glass temperature inthe feeder.”

� Jaroslav Saroch,from BohemiaCrystalex Trading,discussed thesuccessful use ofelectric melting athis company.

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