Roles Of Containment - Monofrax LLC

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bulletinAMERICAN CERAMIC SOCIETYemerging ceramics & glass technologyMARCH 2018Fusion cast refractories:Roles of containmentMarch 2018 ACerS Bulletin reprint

bulletinFusion castrefractories:Roles ofcontainmentcover storyThe role of glass in modern society is evidenteverywhere—from windows and winebottles to car windshields and durabledevice touchscreens.Industrial glass manufacturers require highly engineered high-temperature furnaces to contain glass in itsmolten state (3,000 F–3,200 F, or 1,600 C–1,800 C)so that convections in the melter allow proper mixingas well as melting of incoming raw batch materials.After the refining process, in which dissolved gases areallowed to escape, the glass has a chemical homogeneity ready for formation of the final article. This moltenglass must be contained by a refractory lining in thefurnace to allow safe operation over an extended timeperiod to economically and efficiently manufacturehigh-quality glass products.Credit: MonofraxEvolution of glass furnace refractory liningsBy Kevin SelkreggRefractory linings in glass furnaces are a critical component of glass-basedapplications, including encapsulation of nuclear waste through vitrification.Careful design of these lining materials can ensure safe and long-lastingmethods of nuclear waste storage.Reprinted with ACerS permissionToward the end of the 19th century, fireclay, a bonded alumina refractory, was the glass furnace refractorylining of choice. This progressed to a better quality offireclay, and later, the refractory lining package includedbonded silica brick, which easily dissolved but did notaffect glass quality. However, the furnace life of glasscontact silica refractory was only 8–12 months.In the early 20th century, sillimanite (Al2O3 SiO2)and then mullite (3Al2O3 2SiO2) found their way intouse as bonded refractory materials that performed betterthan fireclay and silica bricks.1,2 Typically, these refractorymaterials are pressed with binders to maintain geometryand fired at high temperatures to create a bonding phasefor strength. These refractory bonded shapes typically havea porosity ( 10–15%) that will severely reduce corrosionresistance in contact with a slag or glass at high temperatures, not to mention high solubility of their components.Enter the advent of a refractory manufactured byfusing molten oxide powders at high temperatures( 3,800 –4,000 F). The process of fusion casting bypasses conventional bonding of refractory bodies mentionedearlier by developing crystalline intergrowths capableof exceptional corrosion resistance due to high densityof the body. The batch, after dry blending, is fed to anelectric arc furnace for fusion by energy released in arcresistance paths. The furnace melting the material tiltsto pour this liquid into molds designed for final applications.3 Monofrax LLC pioneered this technology in thelate 1930s with high-alumina fused cast refractory materials and, later, many compositional evolutions.

Fusion cast refractories: Roles of containment(B)Credit: Monofrax(A)(C)Figure 1. (A) New AZS furnace lining during inspection before delivery and installation. (B) CorrodedAZS furnace lining after 6.5 years of service in a soda-lime glass furnace. (C) Close-up of a corroded AZSfurnace lining panel after 6.5 years of service in a soda-lime glass furnace.Glass quality in soda-lime, borosilicate,and high alumina–silica glass compositions is critical to achieve clarity andstrength because, without these properties, the items of interest will fail in theirdesigned applications. This places highdemand for quality refractories in contactwith glass to not alter critical propertiesof the glass by refractory defects and dissolved refractory components. The imagesin Figure 1 of corroded AZS fused castrefractory lining are a revealing testamentto the erosion of refractory linings duringa glass furnace campaign.The final glass article, be it a bottle orwindow, will actually have some trace ofthe refractory components (e.g., 0.07%ZrO2) dissolved in its structure, althoughat a level that does not affect requiredglass clarity and strength. Refractory lining in a typical glass furnace is designedto account for the types of corrosionencountered at molten glass contactor by corrosive vapor species in nonglass contact regions at temperatures of 2,700 F (1,500 C).The philosophy in glass furnacerefractory design is to ensure corrosion equivalency of differing refractorymaterials inthe wholefurnace, sothe term ofthe campaignis not prematurely interrupted dueto a singlerefractoryregion failure.There aremany compositionalvarieties offusion-castrefractories available beyond AZS,such as high zirconia, high alumina,magnesium spinel, and chrome–magnesium–aluminate castings. MonofraxLLC supplies several compositionalgroups ( 12 currently) to diverse glassmanufacturing industries, including flatglass, containers, fiberglass, and, morerecently, tough, thin glass touchscreensurfaces for electronic devices.AZS fused cast materials such asMonofrax CS-3 and CS-5 are typicalglass contact and non-glass contact mate-CONTAINMENTDESIGNLONGEVITYDisposal of nuclear waste is a complexAs with industrial glass furnaces, refractory designsNuclear applications have already generatedproblem—one solution is vitrification, in whichfor nuclear waste vitrification melters call for athousands of tons of nuclear waste, and thatglass is used as a containment medium tovariety of refractories that corrode equivalently.amount will continue to increase. Althoughstabilize radioactive waste.Refractory linings in the glass furnace are aoperational challenges still exist in melters,critical component of molten glass containmentvitrification provides a proven method offor glass articles.nuclear waste storage.After the Second World War, therefractory of choice for lining glass furnaces soon became a material called AZS, anacronym for a composition consisting ofalumina, zirconia, and silica. Manufactureof fusion-cast AZS resulted in a refractorymaterial with low porosity ( 1%), highdensity, and good corrosion resistance—critical factors to extend life of the glassfurnace. The material increased furnacelife from 18 months to 3–5 years, allowing furnaces to operate at higher temperatures and at greater throughput.1,2Corrosion resistance of AZS resultsfrom its low porosity and high density, aswell as the presence of zirconia, a highlyinsoluble phase. Even though the AZSrefractory lining in contact with glassextends high-temperature glass furnace lifedramatically over bonded refractory materials, furnace life cannot continue indefinitely. Corrosion and erosion of the liningwill occur, eventually curtailing furnaceoperation until the lining is repaired orreplaced. Figure 1A shows a new AZS furnace lining before delivery and installation,contrasted with a corroded AZS furnacelining after 6.5 years of service in a sodalime glass furnace in Figures 1B and 1C.Capsule summaryReprinted with ACerS permission

rials, while high-alumina materials such as Monofrax M andH are used in lower temperature glass contact refiner and distributor regions. Chrome/alumina/magnesia-bearing materials,such as Monofrax K-3 and E, are often used in weir walls andthroat cover blocks, which require the highest level of corrosion resistance and can tolerate potential chrome coloration.Clarity to containmentOn December 2, 1942, a team of 49 scientists, led by EnricoFermi, proved that a self-sustaining nuclear chain reaction couldbe initiated. Conducted under Stagg Field of the Universityof Chicago, this experiment, called the Chicago Pile-1 reactor,became the integral first step of the Manhattan Project to develop the atomic bomb.4In the midst of this dash to successfully create a controllednuclear chain reaction, there is no known reference that any ofthese scientists foresaw the immensity of the amount of nuclearwaste that harnessing such energy in weaponry and power generation would create.However, as of January 2009, the amount of spent nuclearfuel from the 104 nuclear reactors operating within the UnitedStates alone reached 64,000 metric tons.5 In the U.S., defenseand weapon-related activities are another source of waste,with the largest quantities created in the early days of nuclearweapon development and testing.The U.S. Department of Energy officially discontinuedreprocessing spent nuclear fuel in 1992, although the U.S.has generated 347,300 m3 of waste incidental to reprocessing.6 Most of this liquid high-level waste (HLW) is stored inunderground tanks at the Hanford site in Richland, Wash.,and the Savannah River site in Aiken, S.C. Another portion ofHLW was calcined to a dry powder and is stored at the IdahoNational Laboratory in Idaho Falls, Idaho.Weapons-grade plutonium production stopped in the1980s. However, the consequence of this material lingers onin the form of waste. The current emphasis of nuclear fissionis electricity generation in the U.S., but not to the extent of itsrole in other countries, such as Canada and China.In the public sector, developing uranium fuel to producepower from nuclear plants generates different forms of waste(e.g., mine mill tailings, conversion, enrichment)—which allwill need disposal. Rod Ewing7 states that “ the complexity ofthe nuclear waste disposal problem has delayed final choices ofwaste disposal sites in most countries that have nuclear wasteinventories. So much so that, there are, at present, no operating [geologic] nuclear waste repositories for spent nuclear fuelfrom commercial nuclear power plants or for HLW from thereprocessing of spent fuel.”Complexity in the disposal of nuclear waste is partially due tothe variety of waste compositions that ultimately drive the needfor different glass containment formulations. Waste containment plant designs are dictated by radioactive loads, which mayrequire fully remote designs or permit a hands-on approach. Atthe Hanford and Savannah River sites, HLW is further separated into a smaller volume containing most of the radioactivityand a larger volume of contaminated liquid with much lowerReprinted with ACerS permissionradioactivity (low activity waste, or LAW), which has a different disposal strategy. However, each facility treats LAW differently—Savannah River grouts LAW, while Hanford vitrifies it.Regardless of the means, nuclear waste must be reduced to asolid form before disposal and must resist leaching.VitrificationThe term vitrification connotes involvement of glass, whichserves as a host medium to stabilize radioactive waste. Durabilityis the top priority for containing radioactive waste for thousandsof years. This contrasts with other applications, such as commercial glasses designed for optical clarity.Table 1. Soda-lime glass composition typical for flat glass andexamples of vitrification melter glass chemistriesWt%SiO2Soda-limea71.7HLWbWDVREF6c 1.6Na2O14.111.6K O5 1.5Fe2O30.13.412.210.817.5CeO2 1.90.19Cr2O30.30.30.16SO30.7 0.50.08NiO0.41.5Sb2O50.50.40.10.30.30.80.1P2O5 1.3 1.70.31ZrO2 1.40.63.97TiO2 O20.1CuO0.04Other99.8599.599.999.4Flat glass industrial furnace, Glaverbel S.A. Belgium.bChina simulated HLW waste, Karlsruhe Nuclear Research Center, Germany.18cWest Valley, NY.19dSavannah River, EA Glass.19eHanford Low Activity Waste Vitrification Project, 2002.19fPNNL, glass for research scale melter test.a6.02.399.6100.0

Fusion cast refractories: Roles of containment(A)(B)Credit: Monofrax(A)Figure 2. (A) A melter box consisting of Monofrax K-3 during inspection before shipment to the Hanford site. (B) Low activity melterunit containing K-3 melter box being readied for use at Hanford.At Savannah River and Hanford sites,radioactive waste is transitioned into amolten borosilicate glass through a variety of steps involving a liquid slurry withdry additives that form a blanket on theglass called a cold cap. The bottom portion of this cold cap melts into a foamyglass and ultimately melts into the pool,which is poured into a robust stainlesssteel canister ( 1–3 m high) and allowedto cool, forming a solid matrix.Containers are welded shut, ready forstorage and final disposal. This encapsulation in molten glass and solidification infinal storage containers is called vitrification and is a suitable and adequate process for management of ILW and HLW.Figure 2A shows a K-3 melter box inthe setup area at Monofrax with a similarlayout to soda-lime industrial glass tanks.The melter box contains glass slurry asnuclear waste is encapsulated within theglass. This box is a portion of the largerunit at the Hanford site (Figure 2B).Refractory design experience, philosophy, and technology for melting ofindustrial commercial glasses (e.g., sodalime, borosilicate, and high-aluminacover glasses) has been transferred ina similar fashion when designing thenuclear waste vitrification melter. Inthis case, the design uses another property of glass.Unlike the clarity and strength necessary in soda-lime and borosilicate glass,the chemistry of encapsulating glass innuclear waste treatments is unique inits ability to immobilize radionuclides.Specific oxides determine various properties in soda-lime glass, such as meltingpoint, mechanical properties, or color.For example, iron is incorporated at lowlevels (0.1–2.0% iron oxide) in soda-limeglass to reduce the effect of harmful UVrays for construction glass.8Design of the glass composition necessary for nuclear waste encapsulationinvolves a complicated selection processwith non-radioactive glass-forming additives. These chemistries are tailored tocreate a favorable viscosity–temperaturerelation, meaning radionuclide volatilities are not in play.9 In this case, boronhas an important role in reducing glassviscosity at temperatures below radionuclide volatility temperatures of 1,200 C.Vitrification is a particularly attractiveimmobilization route because the glassyproduct has high chemical durability.10Borosilicate glass contains waste materialthrough direct chemical incorporationinto the glass structure (i.e., dissolution),although some studies also have evaluatedthe feasibility of physically encapsulatingsolid wastes. The durability of borosilicateglass allows storage for thousands of years,even under conditions of irradiation byincorporated radioactive materials, whichdo not crystallize the oxide glass.The temperatures encountered in vit-Types of nuclear wasteHLW High level waste—highly radioactive due to reprocessed nuclear fuelILW Intermediate level waste—requires shielding when handlingLLW Low level waste—contaminated by radioactive materials, but not inherentlyradioactiverification melters ( 1,050 C–1,200 C)are considerably lower than incommercial soda-lime glass tanks( 1,500 C–1,600 C). Table 1 lists sodalime glass compositions typical for flatglass, alongside some examples of vitrification melter glass chemistries.There are numerous critical components of the vitrification melter used toheat glass to 1,050 C–1,150 C, not theleast of which is the refractory lining.Monofrax has manufactured a chromebearing fused cast refractory designed forthis lining for over 30 years, since thebeginning of the process of encapsulating nuclear wastes. In one instance atSavannah River National Laboratory, thedesigned life of this lining was estimatedto be 2–6 years.11 However, in actualpractice at SRNL, the life of Melter #1was 8.5 years and Melter #2 was 14years, eventually shutting down due tomechanical failures that were not refractory related.12Corrosion in soda-lime glass tanksCorrosion kinetics and byproductsof fused cast refractories in contact withsoda-lime glasses of the container andflat glass industries are well known.AZS refractories have three microstructural components: zirconia dendrites, acoprecipitate component of zirconia andcorundum, and a high-alumina glass.When the AZS lining interacts withmolten glass, there is typically a corrosionreaction layer at glass contact that remainsattached to the lining. Continued corrosion takes place by erosion of this layerand, in some cases, may “peel” off, creating some glass quality problems.Reprinted with ACerS permission

Table 2. Chemistry of glass in the AZS glass phase, soda-limetank glass, and passivation layer(A)Wt%GCredit: Monofrax(B)Figure 3. Electron micrographs of (A) virgin AZS and (B)corroded AZS.This thin reaction layer, often called the passivation layer("G" in Figures 3A and 3B) because it serves to “passivate” further corrosion, is a byproduct of incongruent dissolution intothe tank glass. Alumina in the coprecipitate alumina–zirconiacomponent of AZS goes into solution at the glass–refractoryinterface, creating a layer of highly aluminous glass with undissolved zirconia (Figure 3 and Table 2).Corrosion with soda-lime glass is not restricted to the immediate glass contact, however, as the glass phase component ofAZS provides a pathway for diffusing alkali and alkaline earthspecies, such as potassium, sodium, magnesium, and calcium.Alkalis are more rapid diffusers than alkaline earths, as observedby Kasselouri et al.13 as well as others—potassium and sodiumspecies migrate to deeper depths than magnesium and calcium.Consequently, alkalis such as potassium and sodium promotecorrosion of the corundum primary phase at depths into theAZS body beyond the immediate glass–refractory contact.Corrosion in vitrification meltersFacilities active in vitrification of nuclear waste cannotafford failure of the melter due to refractory lining failureeither by excessive corrosion or spalling. During the life ofthe melter, different glasses formulated due to differing wasteReprinted with ACerS permissionAZSglass 30.00.2MgO1.0ZrO21.81.9Total100.0100.0Sodalimetank glass99.5compositions can have a variable impact on the refractory corrosion rate. Care must be taken to not formulate glasses thatwill be highly aggressive to the refractory.Some of the most corrosion-resistant refractory materialsavailable contain chromium oxide as a major component (e.g.,Monofrax K-3 and E). Since the beginning of vitrification ofnuclear waste, Monofrax K-3 chrome refractory has been arefractory of choice for lining melters in the U.S. and, in lateryears, Japan.Chromium oxide is more insoluble than even zirconia inmost glasses, making it a desirable component of refractory lining. Potential coloration of the glass by chrome refractories isa concern in soda-lime container and flatglass industries, but isnot an issue for nuclear waste glass.What are the chemical and microstructural factors thatmake a chrome refractory, such as Monofrax K-3, perform sowell as the glass contact refractory liner in vitrification reactors?As aforementioned, fused cast materials such as K-3 havelow porosity and an interlocking, tight microstructure. Thetypical microstructure of Monofrax K-3 is a binary phaseassemblage primarily of an (Mg,Fe)O (AlCr)2O3 spinel, and anR2O3 (Cr2O3-Al2O3 solid solution) phase, with minor glassyphase and low level reduced iron as free metal at grain boundaries. Further, the R2O3 phase is present as chemically inhomogeneous cored grains, with relatively Cr2O3-rich centers andrelatively Cr2O3-poor rims (Figure 4).When in contact with melter glass, Monofrax fusedcast chrome materials (K-3 and E) react with glass in anincongruent fashion (as in AZS in contact with soda-limeglass), leaving a byproduct at the glass–refractory interface.Magnesium and aluminum are the most soluble componentsof K-3, generally leaving the most insoluble component, Cr2O3,behind at the corrosion interface.Monofrax K-3 in contact with waste glass simulant at theSRNL melter was characterized af

Mar 08, 2016 · Evolution of glass furnace refractory linings Toward the end of the 19th century, fireclay, a bond-ed alumina refractory, was the glass furnace refractory lining of choice. This progressed to a better quality of fireclay, and later, the refractory lining package included bonded silica brick,

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