MASTER'S THESISHigh-temperature Compression Strengthof High-alumina Refractory Bricks Used inRotary Kilns of LKABDmitrij RamanenkaMaster of Science in Engineering TechnologyMaterials Technology (EEIGM)Lule University of TechnologyDepartment of Engineering Sciences and Mathematics High-temperature Compression Strength of High-alumina Refractory Bricks Used in Rotary Kilns of LKAB Masters Thesis in Materials Science and Engineering Dmitrij Ramanenka September 2011 Supervised by: Marta-Lena Antti, Division of Material Science Lule University of Technology Department of Engineering Science and Mathematics Division of Material Science 1 Abstract Degradationofmaterialsisoneofthewell-knownchallengesthataremetinmanyindustrial situations.LKABasoneoftheworldsleadingproducersoftheupgradediron-oreproducts experiencesvarieddegradationconditionsduringtheprocessingoftheirproducts.Oneoftheir central processes is theheattreatment of the products. Sintering, the final heat treatment step for theproducts,isdoneinsocalledrotary-kilns.Theinteriorofarotary-kilnisisolatedbyaliningof refractorybricks.Thebricksinsidethekilnareconstantlyaffectedbymanydifferentdegrading processes,wherechemicalattacktogetherwiththermalshockareconsideredtobethemost importantreasonstoseveredegradation.Thisworkisfocusedonthemechanicalbehaviorofthe bricks and specifically their compression strength at room and service temperatures.Threealuminosilicate-basedbricks,belongingtothehigh-aluminacategoryweretested.Thebricks areproducedbyHgansBjufABandaremarketedunderthenames:VictorHWM,Silox60and Alex.VictorHWMcontainsapproximately79wt%alumina(Al2O3),whileSilox60andAlexcontain approximately59and54wt%aluminarespectively.TheprimarymineralphaseofVictorHWMis corundum(-Al2O3),Silox60isdominatedbymullite(3Al2O32SiO2),whileAlexisablendof corundum and mullite. The guideline value of room temperature compression strength is 80 MPa for Victor HWM, 70 MPa for Silox 60 and 50 MPa for Alex. Compressionstrengthtestswereperformedatroomtemperature(RT)andin700-1300Cinterval withavariationof100Cbetweenthetests.Itwasfoundthatthestrengthincreaseswith temperature up to 900-1000 C, where the peak of strength is noted for all the three materials. The maximum strength increase, relative to the RT compression strength, of Victor HWM was found to be approximately 65 %, for Silox 60 around 115 % and for Alex by more than 165 %.After passing 1000 C,thestrengthdecreaseisrelativelyrapidforallthethreematerials,butisfastestincaseofAlex andisslowestin caseofVictorHWM.At1300Cthecompressionstrengthdecreaserelativeto RT compressionstrengthisapproximately63%and60%forVictorHWMandSilox60respectively, whilethedecreaseisalmost86%forAlex.Relativelytothebrickshighestnotedcompression strength, the strength decrease at 1300 C is approximately 78 % and 81 % for Victor HWM and Silox 60 respectively, and around 95 % in case of Alex. 2 Acknowledgement IwouldliketoexpressmygratitudetomysupervisorMarta-LenaAnttiforthesupportduringthis MastersThesis.LargeappreciationtoStefanAdolfssonandLKABforgivingmetheopportunityto dealwiththisproject.Furthermore,thankstoEricaGranbergandHgansBjufABfortechnical informationandsupplyofthematerial.ThankstoJesperStjernbergatMaterialSciencedivisionat LTUforsharinghisexperiencewithme.MoreoverIamgratefultoJohnnyGrahnandLarsFriskat MaterialSciencedivisionatLTUfortheirhelpwith theequipment. ThankstoJanGranstrmatthe divisionofMechanicsofSolidMaterialsatLTU.AtlastbutnotleastIamthankfultoalltheother MastersThesesstudentsattheMaterialSciencedivisionforcreatingapleasantworking atmosphere. Dmitrij Ramanenka 3 Contents Abstract ................................................................................................................................................... 1 Acknowledgement ................................................................................................................................... 2 1Introduction ..................................................................................................................................... 5 1.1Background .............................................................................................................................. 5 1.1.1Description of the problem ............................................................................................. 6 2Objective.......................................................................................................................................... 6 3Theoretical background ................................................................................................................... 7 3.1Ceramics .................................................................................................................................. 7 3.2Refractory materials ................................................................................................................ 7 3.2.1History ............................................................................................................................. 8 3.2.2Manufacture .................................................................................................................... 9 3.2.3Classification and description of refractory materials .................................................. 11 3.2.4Mullite ........................................................................................................................... 14 3.3Mechanical testing ................................................................................................................ 16 3.3.1Cold crushing strength ................................................................................................... 16 3.3.2Compression strength at elevated temperatures ......................................................... 17 3.3.3Brittle fracture in compression...................................................................................... 17 4Scope of work ................................................................................................................................ 18 5Materials ........................................................................................................................................ 19 5.1General .................................................................................................................................. 19 5.2Notation and properties of the received materials ............................................................... 21 6Method and Experimental procedure ........................................................................................... 22 6.1Method for determination of hot compression strength ..................................................... 22 6.1.1Test specimens .............................................................................................................. 22 6.1.2Procedure of the hot compression tests ....................................................................... 23 6.2Preparation for the hot compression strength tests ............................................................. 24 6.2.1Test specimen preparation ............................................................................................ 24 6.2.2The equipment .............................................................................................................. 25 6.2.3Manufacture of the ceramic support ............................................................................ 27 6.2.4Planing of the ceramic rod ............................................................................................ 29 6.3Analysis of contaminated Victor HWM ................................................................................. 29 7Results and discussion ................................................................................................................... 31 7.1Characterization of the materials .......................................................................................... 31 4 7.1.1X-ray diffractometry ...................................................................................................... 31 7.1.2Microscopy .................................................................................................................... 32 7.2High-temperature crushing tests .......................................................................................... 34 7.2.1Compression strength of the bricks .............................................................................. 34 7.2.2Fracture of bricks ........................................................................................................... 40 7.3Evaluation of test method and test data ............................................................................... 43 7.3.1Effect of the powder ...................................................................................................... 43 7.3.2Double peaks ................................................................................................................. 44 7.4Analysis of contaminated Victor HWM ................................................................................. 46 7.4.1Analysis of raw materials ............................................................................................... 46 7.4.2Analysis of sintered materials ....................................................................................... 49 8Summary and conclusions ............................................................................................................. 53 References ............................................................................................................................................. 54 Appendix A ............................................................................................................................................... i Appendix B ...............................................................................................................................................ii Appendix C............................................................................................................................................... iii Appendix D .............................................................................................................................................. iv Appendix E ................................................................................................................................................ v Appendix F ............................................................................................................................................... vi Proportion calculation ......................................................................................................................... vi Appendix G ............................................................................................................................................. vii 5 1Introduction Degradationofmaterialsinindustrialapplicationsisawell-knownphenomenon.Itsinfluencecan appear in different ways. It can be a question about lowered production rates, quality of the product, or safety, but it always diminishes the potential of an industrial process. 1.1Background LKAB,Luossavaara-KiirunavaaraAB,isoneoftheworldsleadingproducersofupgradedironore productsforthesteelindustrybutalsoasupplierofmineralproductstootherindustrialsectors. Theirmainmineralresourceismagnetite(Fe3O4)thatisfurtherprocessedintodifferentsuitable products for sale on the market. Ironorepelletsandsinterfinesarethe mainproductsofLKAB.LKABproduces twotypesofpellets; pelletsfortheblast-furnaces,whichisalsothemostcommonreductionmethodofironore,and pelletsforthedirectreduction.Pelletsarecircular,centimeter-largeiron-reachsolidpiecesmainly containing hematite (-Fe2O3) that are produced after oxidation and sintering of magnetitetogether withabinderandvariousadditives(e.g.olivine,dolomite,limestone).Theadditivesplayan important role in the behavior of pellets during reduction tosmelted iron and thatcan for example influence the stability of the reduction process and the quality of the end-product. Sinterfinesareiron-reachfinesofeithermagnetite orhematitethataremeanttobeusedinblast-furnaces.Therearealsoproductswhicharemixturesofbothpelletsandsinterfines,allthis depending on the further iron ore reduction process used by the customer. Inordertooxidizeandsintertheirproducts,LKABusesgrate-kilnsystemattheirproductionsites. This process consists mainly of three parts; a grate band, a kiln and a cooling part. The kilns are rotary kilns and the size varies between approximately 30-40 m in length and 5-7 m in diameter. These are heated by a flame directed from one end of the kiln that is fueled by coal or oil. The band of grates is a transport band for the raw material to the kiln. Furthermore, the system is made in such a way that large part of the heat from the flame is not only warming the kiln, but is also used in order to oxidize therawmaterialonthegrates.Thiscreatesatemperaturegradientalongthewholesystem.The end-part of the kiln, which is also the final step in the heat treatment, is the hottest spot in the chain, whilethebeginningofthegratebandisthecoolest.Theoxidationprocessstartsalreadyonthe grate-band, while sintering is finished in the rotary kiln. See Figure 1 for the illustration of the chain process. Figure 1. Illustration of the process in the grate-kiln system at LKAB. 1250 C40 C1120 C1000 C400 C Cooling Oxidation and sintering Grates Rotary kiln 6 1.1.1Description of the problem The interior of the rotary kiln is insulated by a lining of refractory bricks. Since the temperature varies along the kiln different qualities of bricks are used in different temperature zones. In general, bricks with higher alumina content are used in the warmest zones. The bricks inside the kiln are constantly affectedbydifferentmeans,suchas:hightemperatures,harshchemicalenvironment,wearand abrasionandveryoftenallthatsimultaneously.Howeveritisconsideredthatchemicalattack together with thermal shockis the primary sourceto degradation of the bricks. All these conditions make it unavoidable for the bricks to wear out. The best case scenario is when the bricks are wearing outinasteadyway.Inaverageacertainliningofthebrickslastsapproximatelyfiveyearsinthe rotary kilns of LKAB before it has to be replaced. Regular, planned service stops are made every 12-15 month in order to repair the lining. However, unplanned emergency stops due to the fall-outs of the refractory insulation are common and causes varied troubles to the company.Degradation of the lining is influenced by a large number of factors: thermal, chemical, physical and mechanical. Therefore there are many reasons to degradation, but also many ways to characterize a refractory brick in order to classify the properties of the bricks. One of the most important and used mechanicalcharacterizationmethodsistheCCS-test,coldcrushingstrengthtest,auniaxial compression strength test at room temperature (RT). This test is usually used as guidance to a bricks mechanical strength. Naturally, a brick with higher compression strength would mean a material that is moreresistantto mechanical stress. However, experience of LKAB has showed that cold crushing testsdonotalwaysreflecttheperformanceof thebricks.Nevertheless,theservicetemperature of thebricksisfaraboveroomtemperature.Forexample,recentemergencystopshavesuddenly occurredatLKABonlyfewweeksaftertheinstallationofstrongerbricksinakiln.Becauseofthe shorttimethecausesaremostlyofmechanicalcharacterandnotchemical.ExperiencefromLKAB has given rise to a couple of questions regarding the strength of the bricks, which will be also the aim of this project. 2Objective Themainaimofthisstudyistocharacterizecompressionstrengthatelevatedtemperaturesof refractories used at LKAB. An important aspect to investigate is how relevant it is to use cold crushing strength measurements as guidance to the mechanical strength of a brick at its service temperatures. Differentquestionswillbeconsidered.Howdoesthestrengthvaryoverthetemperature?Does rankingofthestrengthatRTremainatelevatedtemperaturesordothedifferencesdisappear? Simplymeaningdoesthestrongestbrickatroomtemperaturestillremainsthestrongestbrickat elevated temperatures? 7 3Theoretical background 3.1Ceramics Refractorybricksbelongtothematerialcategoryofceramics.Ceramicsareinorganicandnon-metallicmaterials.Theyareusuallycomposedofmetallicandnon-metallicelements.Theatomic bonding within these materials ranges from purely ionic to totally covalent and can be a combination of the two. [1] Generallythesecompoundscanbedividedinto threegroups:traditionalceramics,refractoriesand castables,andadvancedceramics.Stiffnessandstrengthsofceramicsareinmanycasesanalogous withthoseofthemetals.Butincontrasttometalsceramicslackductility,whichmakesthemvery brittle and defect sensitive. [1] 3.2Refractory materials Refractoriesarematerialswhichareusuallyusedwherethetoleranceofhightemperaturesis required. Most of the times they are used as a barrier between hot zones and relatively cold zones, in order to protectthe rest of a construction. Therefore, normally they function as thermal insulators, but can also be manufactured and used for heat transport. The temperature tolerance is seldom the solerequirementwhichhastobemetinindustry.Tolerancetothermalcyclingandwearcanbe someoftheotherrequests.Inothercasespropertiessuchaselectricalresistanceorelectrical conductionareneeded.Butprobablythegreatestchallengetorefractoriesisthetoleranceto corrosivemedia.Especiallyconsideringthatseverityofcorrosionattackisgenerallyincreasingwith elevatedtemperaturesandinpresenceofabrasive/erosiveconditions. [2]Refractoriesaretypically usedasliningsinfurnacesfortherefiningofmetals,butalsoinmanufactureofglassand metallurgical heat treatments. Refractoriesareusuallymadeofacomplexcombinationofhighmeltingcrystallineoxides.Their strengthincompressionismuchhigherthanintensionandtheyhaverelativelyhighmodulusof elasticity,butasallceramicstheylackductility.Refractoriesexhibitplasticdeformationathigh temperatures, but mechanisms of it, as well as mechanisms of heat and electricity conduction are in mostcasesnotgovernedbythesameprinciplesasinmetals.Duetothehighvariationoftheir compositiontheycanbedesignedinordertotolerateawidevariationofserviceconditions (basic/acidic,salts/metals,liquids/gases).However,refractoriesareseldomtotallyimmunetoa certain condition.Refractoriesarerarelypore-free.Togetherwithphasecompositionporosityisaveryimportant factortoconsiderduringthemanufactureofrefractoryproducts.Reductioninporosityincreases strength, load-bearing capacity and corrosion resistance. On the other hand typical characteristics of refractories,suchasthermalshockresistanceandthermalinsulationdiminishwithporosity reduction. By balancing between porosity and phase compositions a good combination of properties can be achieved for a specific service condition. [2] 8 3.2.1History Theuseofrefractoriescanbetracedtothousandsofyearsago,startingwiththediscoveryoffire. Unsurprisinglythefirstrefractorymaterialsweretakendirectlyfromnature.Easilyaccessible materialssuchasstoneandclayservedasprimaryrefractoriesforseveralthousandsofyears.Accordingly, development of refractories over the years has been rather slow. On the other hand as longastheprimarysourceoftheheatproductionwaswood,thenaturallyfoundrefractories functionedrelativelywellsincewood-firedovensbarelyreachedtemperaturesover800C.About 4000B.C.anewtypeoffurnaces,convection-draftandforced-draftfurnaces,madeitpossibleto reachtemperaturesaround1100C.Thiswasalsoastartingpointoftheeraofmetals,but neverthelessstone,clayandfiredclaycouldstillserveasadequaterefractories.Notevenwhen porcelain was made in China by the year 600 A.D. and required temperatures up to 1350 C, the need of other refractories raised. Nor when the first types of blast furnaces in Europe by about 1400 A.D. reached temperatures up to 1500 C it was necessary- stone and fireclay were still acceptable, even thoughthequalitywasrelativelybad.Oneofthebiggestreasonstothisacceptancewasthatthe productionrateswerestillrelativelylow.Probablythefirsttriggerforthedevelopmentof refractories was the invention of Bessemer Converter in 1856 for the production of steel, a new type offurnace.Thefurnaceoperatedattemperaturesover1600Candunderacidiccorrosive conditions. Butthe necessaryrefractories to withstand acidic attackwere not yet developed. Justa yearafter,anothertypeofsteelmakingfurnacewasinvented,Siemensopenhearthfurnace.This time reaching temperatures even higher than Bessemer Converter and operating with basic corrosive slagging. Also the production capacity of open hearth furnaceoutstripped the capacity of Bessemer Converter; thereforeanother requirement on the refractoriesbecame very important,namely their durability in mass production industry. Compared to before there was now a need of a radical change -consequently new types of refractories started to develop.Silica(SiO2)brickwasoneofthenewtypesofrefractoriesthatcouldwithstandacidicslags,high temperaturesandwearbetterthantraditionalfirebricks.Inthebeginningalotoffocuswas concentratedonthepurityofone-elementrefractories.Silicabrickwasneverthelessnotgood enoughinallconditions;asoneweaknessitcouldnotresistbasicslag whichcouldbefoundin the openhearthfurnaces.Thisopeneddoorforthedevelopmentofbasicrefractories.Magnesite (MgCO3)wassoonintroducedintotheliningsofopenhearth.LaterduringWorldWarIdolomite (MgO-CaO)bricksweredevelopedandintroduced,replacingmagnesitebrick.Howeverasthe developmentprogressedbasicrefractoriessuchasmagnesiteandmagnesite-chromebecamethe refractory of choice in the open hearth furnaces. In the meantime, while radical changes happened in thesteelmakingindustry,alsootherindustrieschanged.Copperandaluminumsmelting,glassand manyotherindustrieswereinneedofnewrefractories,sothevariationsofmagnesitebrickwere developed. Later during World War II, super duty silica brick replaced the traditional silica brick, now withhigherrefractorinessandlongerlifetime.Howeverafterawhile,basicandhigh-alumina refractories became the preferred alternatives. Thedevelopmentofthevariedrefractoriesduringthefirsthalfof20thcenturywassignificant,but the quality was relatively low, partly due to the low demands during the two World Wars. After the wartime a large demand raised on high-quality refractories which were specifically made for certain serviceconditions. Especially when the three-phasearc furnaceand the basic oxygen furnace(BOF) insteelmakingwereintroduced,before50sandintheendof50srespectively,anewstepinthe development of high-quality refractories was taken. [2], [3] 9 3.2.2Manufacture3.2.2.1Raw materials Thechoiceofrawmaterialforrefractoriesisdependentontheapplicationofthefinalproduct. Basicallytwosourcesofrawmaterialscanbedistinguished:non-claymineralsandclayminerals. Non-clays which are suitable for refractories include minerals such as: calcite (CaCO3) and aragonite (CaCO3)whichcanbefoundinlimestone;chromite(FeCr2O4),dolomite(CaMg(CO3)2),andalusite (Al2SiO5),magnesite(MgCO3)anddifferentfeldspars(KAlSi3O8-NaAlSi3O8-CaAl2Si2O8).Alsoboehmite (-AlO(OH)), diaspora (- AlO(OH)) and gibbsite (Al(OH)3) which can be found in bauxite ore are some otherwell-knownnon-claymineralsthatareusedintheproductionofrefractories.Claysare materials which have during the years been affected by different means, such as: water, temperature cycles, abrasion etcetera. The grain size of these materials is generally smaller than that in the non-clay group. Over the years and under various conditions the composition of these materials change; forexamplebychemicaldecomposition,lossofwaterand/oracid-solublecompounds.Therefore thereisawidevariationofclayminerals,butallofthemcontainsomechemicallybondedwater. Kaolinite(Al2Si2O5(OH)4)isthemostusedclaysintheproductionofrefractorieswithratioof 1:2of alumina to silica (Al2O3 : SiO2) content. [4] Refractories are seldom one-component products, it is usually a mixture of compounds where one or two components are dominating. Aluminosilicates are probably the most occurred compounds found inrefractoryproducts,alsosmallamountsofhematite(Fe2O3),rutile(TiO2),burntlime(CaO)and alkalisaresomeofthecommonconstituentsinrefractorieswhichnaturallyexistintheraw materials.Additionallyotheradditivescanbeusedinthefinalmaterialmixtureinordertogain specific properties. 3.2.2.2Processing After the raw materials have been extracted they need to be processed. This is usually firstly done by crushingandgrindingthematerialintosmallerparticlesandafterthatfollowedbycalcination. Calcinationisaheattreatmentwhichisusuallydonewellbelowthemeltingpointsofthe constituentsinthematerials.Thepurposeofthisprocessistobringthermaldecompositionofthe material, sometimes also recrystallization and crystal growth. Some common examples of calcination are calcination of hydroxides and carbonates, see (1) and (2) respectively for the reaction examples. 2Al(OH)3 Al2O3 + 3H2O (g)(1) Where (1) is dehydration of gibbsite and (2) is decarburization of calcite. Otherstepsintherawmaterialprocessingcanbesizeclassification,analysisofthematerial composition and drying.Followed after the raw material processing is batching; this is made in order to correlate the defined (by size and composition) raw materials to the desired final product. Simply meaning to find out the needed mixture of materials for the final product. Mixing of the materials itself is an important step for the quality of the final product; this can be done by dry or wet mixing. Processes following after themixingsteparedependentonthetypeofproducts.Someoftheproductsareunformed;then CaCO3 CaO + CO2 (g)(2) 10 theymaybereadytobepackageddirectlyaftermixing,whileothersarefurtherprocessedfor agglomeration or forming. However, most of the products are pre-shaped products, as in the case of bricks. After mixing they are formed to a desired shape,this can bedonein different wayssuch as: uniaxialdrypressing,isostaticpressing,andextrusionorevenbyhand.Theformedproduct,often calledgreenform,isthendriedorcured.Theseproducts(greenforms)caneitherbefurtherheat treatedbyfiringthem(burnedproducts)orbeleftuntreated(unburnedproducts),followedby eventualshapingandpackaging.Thefiringistheprocesswheretherefractoriesobtaintheir refractorypropertiesandinvolvesformationofceramicbonds.However,atsomestageeventhe unburned refractories will create ceramic bond, usually after being installed. [2],[4] In Figure 2 can be found a schematic of the manufacture process of refractories. Figure 2. Schematic over the manufacture process of refractory products. Raw material processing Crushing/grinding Drying Calcining Size classification Composition analysis Batching and mixing Unformed refractory-end product. Forming Green form Drying/curing Unburned formed refractory-end product. Shaping Burned formed refractory-end product. Shaping Fused-cast refractory- end product.Fused grains-further used. Sintered grains-further used. Firing Agglomerating Drying Firing

Drying Arc melting 11 3.2.3Classification and description of refractory materialsThemodernindustrytodayinvolvesawidevariationofserviceconditionsandmanyofthese industrieshaveprocessesthatoperateatelevatedtemperatures.Asknown,refractoriesareoften used in such conditions. Consequently there is a wide range of refractories available on the market. The classification of the refractory materials is usually based on theirraw material constituents, but alsoonthemethodsofmanufacture.Mostoftherefractoriesfallintothefollowingmaingroups: basic,high-alumina,silica,fireclayandinsulating.Therearealsoclassesofspecialrefractories; siliconcarbide,graphite,zircon,zirconiaandotherscanbefoundhere.Asmentionedpreviously mostoftherefractoriesareavailableinpre-formedshapes,suchasbricks.Buttheycanalsobe suppliedasclays,bondingmortarsorrammingmixes,andothers. [3]Duetothewidevariationof products, the content in this section will mostly refer to the refractories in form of bricks, since they arethemostcommonrefractoryproducts.InTable1anoverviewofageneralclassificationof refractory materials can be seen. Table 1. General classification of ceramics and refractory materials. 3.2.3.1Basic refractories Thenameisderivedfromthefactthattheserefractoriesexhibitresistancetocorrosionofbasic character at elevated temperatures. However, today this is not the whole truth, because some of the basic refractories are capable of withstanding rather acidic conditions also. One such type is a direct-bondedchrome-magnesitebrick.Magnesia(MgO)isalwayspresentinthisgroupofrefractories, therefore all the refractories rich in magnesia belongs to this group. But generally five categories can be distinguished, products based on [3]: deadburned magnesite1 or magnesia deadburned magnesite or magnesia + chrome containing materials deadburned magnesite or magnesia + spinel deadburned magnesite or magnesia + carbon Dolomite (CaMg(CO3)2)

1 Deadburned magnesite (inert) is produced by sintering raw magnesite at a controlled temperature of 1750 C. Ceramics Traditional ceramics Refractories and castables Basic Based on: - deadburned MgCO3 or MgO - deadburned MgCO3 or MgO + chrome - deadburned MgCO3 or MgO + spinel - deadburned MgCO3 or MgO + carbon - Dolomite ... High-alumina % alumina: - 50- 85 - 60- 90 - 70- 99 - 80 - Mullite brick - Chemically-bonded brick - Alumina-chrome brick - Alumina-carbon brick ... Silica Flux factor - Type A - Type B Fireclay - Superduty - High-duty - Regular - Spall-resistant - Slag-resistant - Medium-duty - Low-duty - Semi-silica Insulating Gr. No. _ Max bulk density (g/cm3) From fireclay base: -16_ 0.54 - 20 _ 0.64 - 23_ 0.77 - 26_0.86 From high-alumina base: - 28_0.96 - 30 _ 1.09 - 32 _ 1.52 - 33 _ 1.52 Advanced ceramics 12 Deadburned magnesite bricks have good resistance to basic slag and also to iron oxide and alkalis. By combining deadburned magnesite with spinel a better resistance to spalling and lower coefficient of thermalexpansioncanbeachieved.Intheearly60sitwassucceededtomakedirectbonded magnesite-chromebrick,comparedtoearliersilicate-bondedbrick,meaningthatathinfilmof silicate minerals surrounded and bonded magnesite and chrome. With a direct-bond, a ceramic bond betweenchromeandmagnesitewasachieved,resultinginproductsofhighstrengthatelevated temperatures and also exhibiting excellent slag and spalling resistance. Nevertheless chrome tends to be replaced due to the environmental concerns. Another way of improvement is a magnesite-carbon combination, resulting in increased corrosion resistance. This brick can sometimes replace the use of magnesite-chromebrick.Another,widelyusedbrickisadolomiticbrickandisagoodlow-cost choice. [3]

3.2.3.2Silica refractories Silica refractories, also sometimescalled acid refractories, are classified according to the amountof impuritiesinthematerialbyusingsocalledfluxfactor,whichperdefinitionisequaltothe percentageofaluminaplustwicethepercentageoftotalalkalis.Additionally,someinitialcontent conditionshavetobemetbyaproductinordertobeclassifiedassilicabrick.Theseare:Al2O3 contentlessthan1.50wt%,TiO2contentlessthan0.20wt%,FeO3 contentlessthan2.50wt%and CaO content less than 4.00 wt%. Also, average modulus of rupture should not be less than 3.45 MPa. There are two types of silica brick [5]: Type A. Flux factor 0.50 Type B. Flux Factor 0.50 Silicarefractorieshaverelativelyhighmeltingtemperatures(1600-1725C) andtheycanwithstand relativelyhighpressuresclosetotheirmeltingpointswithoutfailing.Accordinglytheirvolume constancyatelevatedtemperaturesisalsogood.Theyhaveverygoodresistancetoacidslagsbut insufficientresistancetobasicslagsandironoxides.Resistancetothermalspallingabove650Cis verygood,whilebelow650Cthethermalshockresistanceisdiminishing.Furthermore,silica refractories are sensitive to impurities. [3] 3.2.3.3Fireclay refractories Thistypeofrefractoriesconsistsmainlyofhydratedaluminosilicates,butsmallamountsofother mineralsarealsopresent.Oneofthemostcommonmineralsusedintheproductionofthese materials is kaolinite (2Al2O34SiO24H2O). [6] Fireclay bricks are in contrast to the above mentioned groups of refractories not classified according totheirchemicalcontent(exceptoneclass),butbytheirphysicalproperties-maximumservice temperatureandPCE2(PyrometricConeEquivalence).ThehigherPCEthehigherresistanceofa material to plastic deformation induced by temperature. They are divided into the following classes- according to ASTM standard [7]:Low-duty fireclay (max. 870 C; PCE 18-28) Medium duty fireclay (max. 1315 C; PCE 29)

2 PCE-test is made by heating cone-shaped samples of the test material together with different standard cones that are already defined by PCE-number. When the tip of the test cone reaches its base simultaneously as a standard cone then the PCE-number is found. [3]

13 High-duty fireclay (max. 1480 C; PCE 31) Super-duty fireclay (max. 1480-1619 C, PCE 33) Semi-silica fireclay (SiO2 content min. 72 %) Amongtheseclasseshigh-dutyandsuper-dutyfireclaysaresubdividedintoregular,spall-resistant and slag-resistant fireclays. Ingeneral,themeltingtemperatureandthereforeservicetemperatureisincreasingwithhigher aluminacontent.Consequentlythehighestaluminacontentisinsuper-dutyfireclays,with40-44 wt% of alumina. Low-duty firebrick is often used for backing up of other bricks of higher refractoriness while medium-dutybrickcanoftenresistabrasionbetterthanhigh-dutybrick. [3]High-dutybrickshavegood thermal-shockresistanceandarewidelyused.Super-dutyfireclaybrickshaveevenbetterthermal shock resistance and therefore also resistance to spalling. Semi-silicabrickshavesomecharacteristicsofsilicarefractories,suchasexcellentload-bearing strength and volume stability at relatively high temperatures. [3] 3.2.3.4Insulating brick As the name reveals this products are used for insulation. Insulating brick can be made from different typesofoxides,andfireclayisoneofthemostwidelyused.Thetypicalcharacteristicsofthese products are light weight and low thermal conductivity thanks to the high porosity of these materials. Thereforeevenclassificationisdoneaccordingtotheirbulkdensities,whilethegroupnumbers multiplied by hundred reveals their nominal service temperature in degrees of Fahrenheit.According to ASTM standard the groups are [8]: Based on fireclay, group No: 16 (845 C); 20 (1065 C); 23 (1230 C); 26 (1400 C) Based on high-alumina, group No: 28 (1510 C); 30 (1620 C); 32 (1730 C); 33 (1790 C) Insulating bricks are mainly used as back-up materials sincethey do nottolerateabrasion and wear very well. The advantages of theseproducts can befaster heating of furnaces,lighter constructions and lower heating costs. 3.2.3.5High-alumina refractories Refractories with alumina (Al2O3) content above 47.5 wt% are called high-alumina refractories; this is accordingtoASTMstandardandcanthereforebesomewhatdifferentinothersources.The classification within this group is primarily based on the alumina content in the material, resulting in seven classes, as following [3], [7]: -50, -60, -70, -80 ( 2.5 % Al2O3) -85, -90 ( 2 % Al2O3) -99 (minimum 97 % of Al2O3) Someotherspecialclassesofhigh-aluminarefractoriesaremullite(e.g.3Al2O3-2SiO2),chemically-bonded, alumina-chrome and alumina-carbon products.High-aluminabricksaresomeofthemostusedrefractoryproducts.Bauxiteoreandandalusiteare the most common raw materials when producing high-alumina refractories. The dominating mineral 14 phasesareusuallyeithercorundumormullite.Therefractorinessofthesematerialsisingenerala functionofaluminacontent,increasingwithhigheraluminacontent.Silicaistheothermain componentinthistypeofmaterials;thereforetheslagresistancewillbepartlydependentonthe amountofit.High-aluminaproductswhicharehighinsilicawillresistacidslagsbetterthanbasic slags.However,slagresistanceisgenerallyaffectedpositivelybyincreasingaluminacontent.Also creepresistanceisrelatedtothealuminacontent.Sincealuminahasthehighestmeltingpoint amongthemainconstituentscreepresistancewillbeaffectedpositivelybyincreasedalumina content. High-aluminaproductswithaluminacontentbetween70wt%to78wt%wherethedominating mineralphaseismullitebelongtothecategoryofhigh-aluminamulliterefractories.Thisisthe highestpossibleamountofaluminainpuremulliteproducts.SeeFigure3forthebinaryphase diagramofalumina-silicasystem.Theseproductsareoftenusedwherehighpurityandcreep resistance is required.Whenhighpurityalumina andchromium oxideare firedtogetherasolidsolutionbonddevelopsin thematerial.Alumina-chromebrickexhibitexceptionalstrengthatawiderangeoftemperatures. Additionally, the corrosion resistance is in many cases very good. High-aluminachemically-bondedbrickisusuallymadefrombauxiteinadditionwithsoluble phosphates, creating an aluminum-phosphate bond at 320-540 C. But also creation of ceramic bond ispossibleathighertemperatures.Phosphatebondedhigh-aluminabricksareoftenusedin aluminum industry due to their resistance to wetting and penetration by aluminum and its alloys.Alumina-carbonbricksarebondedbycarbonaceous bondcreatedduringheat treatmentbyspecial thermosettingresinswheregraphiteisusuallythebondingagent.Naturally,theseproductsare often used in reducing environments. [3]

3.2.4Mullite Mullite is a solid solution phase of alumina and silica, regard Figure 3. It is a very important material andisamajor mineralphasein manyceramicproductsandisalwayspresentinclayproducts.The generalcompositionofmulliteisAl4+2xSi2-2xO10-xwherexdenotesthenumberofmissingoxygen atoms per structural unit. Stable mullite exists in the range of 0.17 x 0.59 and up to a temperature around1850C.Therearetwomullitecompositionswhichareknowntobestableundernormal conditions.Thefirstoneissintered-mullite(x=0.25),3Al2O32SiO2,alsocalled3/2-mullite,with aluminacontentmaximum72wt%.Asthenamedenotestheproductionofmulliteisdoneby sintering-processes.Thealuminacontentinthistypeofmullitewillbegovernedbyvariedfactors suchassinteringtemperature,thedurationofheattreatment,theinitialcompositionofstarting material, particle sizeand others. The second type of stable mulliteis fused-mullite(x=0.40) or 2/1-mullite(2Al2O3SiO2),aluminacontentmaximum78wt%.Thistypeofmulliteisproducedeitherby melting (> 2000 C) a raw material or by crystal growth techniques (Czochralski techniques). [10] 15 Figure 3. Alumina-silica binary phase diagram. [11] Mulliteexhibitshighstrengthathightemperatures,lowthermalexpansionandthereforegood thermal shockresistance,and good chemical stability. SeeTable2 wheresomeof the properties of mullite are compared to the properties of corundum. Mullite (3Al2O32SiO2)Corundum (-Al2O3) Tm (C)18302044 (g/cm3)3.153.95-4.10 Linear thermal expansion (10-6/C) 20-1400 C 3.5-5 5-10 Thermal conductivity (kcal/m/h/C) at RT at 1400 C 6 3 26 4 Compression strength (MPa) at RT at 1400 C [9] 1300 800 2600-3000 250 Fracture toughness K1c, at RT (MPam0.5) 2.5 4.5 Table 2. Properties of mullite and corundum. [12], [10] Mullite is very rare in the earth crust; hence the formation of it in industry always involves processing of other raw materials. Some of the most used starting materials for mullite formation in refractory productsare:bauxiteorestogetherwithsilica;andalusite,kyaniteandsillimanite;kaoliniteand pyrophilite. Kaolinite and pyrophilite are used in fireclay refractory products, among which kaolinite isthemostused.Whiletheotherabovementionedmineralsareusedintheproductionofhigh-alumina refractories. [6],[10] When kaolinite is fired itloses firstitsabsorbed water at 100-200 C and after thatit dehydratesat about 500-600 C. Depending on the firing temperature different chemical changes happen. At 500-600 C, after the dehydration, metakaolin (2Al2O32SiO2) is formed. At 800 C formation of one of the followingmineralstakesplace,allwiththechemicalformulaAl2SiO5;andalusite,kyaniteor sillimanite.Iffiringiscontinuedandreachestemperaturesabove1000C,theformationof3/2-16 mullite occurs. The microstructure of fireclay products consists mainly of 3/2-mullite, cristobalite and amorphous silica. [6] Andalusite together with kyanite and sillimanite are the three polymorphs of Al2SiO5. The difference lies in their crystal structures and that is dependent on temperature and pressure conditions. Among these three forms andalusite is the less dense form, while kyanite is the densest. [13]The beginning of transformationofkyanite,andalusiteandsillimaniteto3/2-mulliteoccursat1150C,1250Cand 1300Crespectively(thisdataisspecificallyforpowdersofgrainsize Al6Si2O13 + SiO2.(3) Someofthecharacteristicsofmullitethatisformedfromandalusiteisitsrelativelylowflux (impurities) content and low porosity. Reason to low porosity is partly due to the fact that large part oftheamorphoussilicathatisformedduringtransformationistrappedinthecapillariesofmullite grains. Mullite formation from andalusite is stable and has low volume expansion, which is favorable during manufacture of refractory products.[15]

Whenbauxiteoreisusedforformationofmulliteithasfirsttobemixedwithsilicaorsilicarich minerals.Then,whensintered,theformationof 3/2-mullitestartsatabout1100C.Thisgivesalso high-aluminarefractories.Thefinalmicrostructureoffiredhigh-aluminabricksoftenconsistof mullite and/or -Al2O3 (corundum) grains interlocked by the amorphous silica phase. [6] 3.3Mechanical testing 3.3.1Cold crushing strength Cold crushing strength test or CCS-test is a uniaxial compression strength test (Figure 4) performed at room temperature that is often used in order to characterize strength of refractory materials. Figure 4. Illustration of a uniaxial compression strength test. The test is performed until failure, meaning collapsing of the specimen or reduction of the specimen heightto90%ofitsoriginalvalue.Highestappliedloadisnotedandthecompressionstrengthis calculated by: F Sample 17

(4) Wherec(MPa)iscompressionstrengthofthematerial,F(N)isthemaximumnotedappliedload during the test; A (mm2) is average of the areas of the top and bottom of the specimen perpendicular to the applied load direction. [16] 3.3.2Compression strength at elevated temperatures Compressionstrengthtestsathigh-temperaturesarerare;noactualstandards existforthis kindof test. It requires non-ordinary equipment and are very time consuming. Normally if high-temperature strengthisofinterestthentheclosestalternativetocompressiontestsaresocalledhotMOR (modulusofrupture)tests(bendingtest).Thisrequiresalsospecialequipmentandistime consuming, but the maximum applied loads are much lower compared to compression tests.Thegeneralbehaviorofthematerialsatelevatedtemperatures,asknown,isthatthestrength diminishes. However, there are cases when this is not true; the strength can not only remain but also augment with temperature. See Figure 5 where hot MOR of two refractory materials is presented. Figure 5. Dependence of bending strength on temperature for two refractory materials. [17] 3.3.3Brittle fracture in compression Ithasbeenshownthatmaterialssuchasbricks,concrete,rocksandevenglassbehaveverysimilar when regarding their fracture modes. There are many evidence confirming that the dominating crack grows in these materials is of mode I type (cleavage mode). This is the same mode that occurs in pure tension.Therefore,propagationofcleavagecracksisthemostnoticedeventduringacompression test.However,manyauthorsreferalsotoanoverallmodeoccurringincompression,socalled splitting-modethatcannotbeclearlyreferredtotheclassicalmodeI,IIorIII. [18]Inanycase, separation of the material and formation of opened visible cracks is a fact. Not surprisingly different kindofflows(cracks,pores,voids)willfunctionasstressraisersinthematerialsandthecrack growth will originate from them. [19] It has been found that in a material such as granite most of the cracksareinitiatingatthe grainboundaries.Morespecifically thosegrainboundariesthatseparate quartz grains are extra prone to crack initiation. Also, in general, the grain boundaries that run 20-40 from the applied load direction were found to be most prompt to start new cracks. [20] 18 4Scope of work In this work room temperature and high temperature compression(uniaxial) strength tests of three differentmaterialswereperformed.Scanningelectronmicroscope(SEM),electrondispersive spectroscopy(EDS)andx-raydiffractometry(XRD)wereusedascharacterizationmethods.Mineral phasesoftheas-receivedmaterialswereanalyzed.Morphologyofthebrickswasstudied.Phase transformationandstrengthvariationafteralongsoakingatelevatedtemperaturewasdetected. Deformationmodesatdifferenttemperatureswereinvestigated.Tworawmaterialsforoneofthe materials were analyzed and a contaminated brick-sample made from one of the raw-materials was studied.

19 a)c) b)1mm1mm 1mmd) 5Materials 5.1General HgansBjufABisthesupplieroftherefractorybrickstoLKABthatareusedintheirrotarykilns. Three qualities (products) were considered in this study. Their trade names are: Victor HWM Silox 60 Alex MostofthefocuswasaimedtoVictorHWMandSilox60,whileAlexwastestedforthesakeof comparison. Below can be found light microscope images of the three brick types. Figure 6. Light microscope images (x20) of Victor HWM (a), Silox 60 (b) and Alex (c). d) Represents Viktor HWM (x15) from a different batch compared to a). These are all aluminosilicate-based bricks, belonging to the category of high-alumina bricks, but that havebeenmanufacturedfromdifferentrawmaterials.VictorHWMisbasedonbauxite,Silox60is basedonandalusiteandAlexisbasedonchamottereinforcedbybauxite.Theyallhavethesame main chemical constituents (preferred metal oxides) but in different proportions (note these are not their crystalline phase composition). See Chart 1 and Table 3 for the chemical analysis and guideline values of the properties for these three bricks.1mm20 Chart 1. Chemical analysis (wt %) of Victor HWM, Silox 60 and Alex representing the abundance of the most preferred oxides. Victor HWMSilox 60Alex Bulk density (BD) (g/cm3)2.70 (2.65-2.75)2.45 (2.40-2.50)2.32 (2.25-2.40) Apparent porosity (AP) (%)18 (17-21)17 (15-19)19 (17-21) Cold crushing strength (CCS)(MPa)80 (60-100)70 (50-90)50 (30-70) Refractoriness under load -T0.5 -T5 1400 1590 1500 1650 1380 1470 Thermal shock resistance>30 cycles>30 cycles>30 cycles Linear thermal expansion at 1000 C (%) 0.6 0.6 0.6 Thermal conductivity (W/(mK)) Measured at: -500 C -750 C -1000 C -1250 C 1.8 1.9 2.1 2.0 1.2 1.4 1.9 2.0 1.8 1.6 1.5 1.6 Table 3. Guideline values of the properties of Victor HWM, Silox 60 and Alex. Tests according to DIN-standards. InFigure7canbefoundQemScanimagesofthepolishedsurfacesofthethreeasfabricated materials representing distribution of the mineral phases. Figure 7a-b. QemScan images of Victor HWM (a), Silox 60 (b) and Alex (c). [21] 54% 40% 2.1% 1.4% 0.3% 1.3% Alex 79% 17% 2.2% 1% 0.2% 0.5% Victor HWM 59% 37% 1.5% 0.9% 0.1% 0.5% Silox 60 Al2O3 SiO2 TiO2 Fe2O3 CaO Alkaliesa)b) c)2 mm2 mm2 mm CorundumMulliteMullite (low Al)Andalusite 21 All the three types of bricks are dry-pressed at approximately 60-100 MPa while the water content is held at 2-3 wt % during the compression state. The burning of the bricks takes place in a tunnel kiln at 1350-1360 C. These materials arehighly heterogeneous and as can beseen in thetableabove a property such as cold crushing strength varies significantly within the same product type. One of the main reasons to thevariationinpropertiesliesinthemanufactureofthebricksandmorepreciselyduringtheir sinteringprocess.Whenthegreenbodiesarefiredinthetunnelkilntheyarestackedoneon another, as can be seen in Figure 7. The geometry of the kiln and the location of the burners in the kiln create an uneven heating of the heat chamber. Due to this there is a temperature difference of approximately100C,atthemost,betweenthebottomandtheupperpartofthekiln-hencea variation in compression strength appears due to these dissimilar burning conditions. Figure 8. Picture of stacked refractory bricks when sintered in a tunnel kiln. 5.2Notation and properties of the received materials In Table 4 indication of the properties on the received samples from Hgans Bjuf AB can be found. Every sample notation represents samples from the same batch.QualityCCS (Mpa)BD (g/cm3)AP (%)Sample notation Viktor HWM73,32,7119,41-5v Victor HWM66,32,7320,49-16v Victor HWM97,92,8617,217-24v Victor HWM61,62,7219,635-40v Silox 6066,92,4617,91-5s' Silox 6056,62,4119,11-8s'' Silox 6050,72,4419,225-29s Silox 6066,22,4618,530-34s Silox 6052,92,518,847-54s Alex45,12,3517,141-46a Table 4. Specifications and notation of the received samples. (Measured by Hgans Bjuf AB) 22 6Method and Experimental procedure 6.1Method for determination of hot compression strength Nostandardsforhotcrushingtestsofrefractoryproductswerefoundtoexist.Inordertofinda proper methodology- standards for cold crushing tests,hotMORtests and recommendations from other works were considered. 6.1.1Test specimens AccordingtoASTMC133,standardtestmethodforcoldcrushingstrengthofrefractories,the specimensfrombricksofdensityabove1.60g/cm3arerecommendedtobeinshapeof51mm- cubes or cylinders of 5151 mm dimension. The height should be parallel to the original direction of pressing of the brick. EuropeanstandardEN993-5,methodforthedeterminationofcoldcrushingstrengthofdense shapedrefractories,recommendstestpiecesofeithercylinders(500.5500.5mm)orcubes (500.5mm).Itisalsosaidthatifitisnotpossibletoobtainthissizefromthetestitem,then cylinders360.3mmindiameterand360.3mminheightcanbeused.Furthermore,thetopand the bottom ends ofthetestpiecesshould beflat and also parallel, with height difference not more than 0.2 mm between the load-bearing faces. In this study, the tests were limited by the existing equipment; the available frame was limited to 100 kNloads.Thereforetherecommendedlargespecimensizesdescribedabovecouldnotbeused, which would haverequired crushing loads by estimate up to 200 kN. Even though it wassuspected thatthecompressionstrengthofthetestedrefractorieswouldconsiderablydecreaseatelevated temperatures-itwasconsiderednecessarytodiminishtherecommendedspecimensize.Itwas decided to take the maximum room temperature compression strength of Victor HWM (100 MPa) as a required upper limit. By calculating (Areamax=Forcemax/Strengthmax) it was found that the area of the samplescouldbemaximum1000mm2.Thatisacylindricalsampleofapproximately35.7mmin diameter or a cube with a side length of approximately 31.6 mm. The size of the cylinder would be in the vicinities of the acceptable sizeby EN 993-5 standard, described above. Fortunately,it was also found out that Hgans Bjuf AB had possibilities of drilling cylindrical samples of 35 mm in diameter. Therefore,forthesakeofsimplicityitwaschosentopreparesamplesofcylindricalshapewith diameter of 35 mm and height of 35 mm, as a slight modification to the European standard EN 993-5, see Figure 9. Figure 9. Illustration of specimens dimensions. 35 mm 35 mm// 0.2 mm 23 6.1.2Procedure of the hot compression tests ASTMC583standard,testmethodformodulusofruptureofrefractorymaterialsatelevated temperatures, was considered in order to find appropriate heating procedure for the test specimens. The standard quotes that the rate of heating of burned refractory products shall not exceed 330 C/h fromroomtemperatureupto980C,andshallnotexceed110C/hfrom980Cuptothetest temperature. The test temperature should be maintained for a minimum of 3 h. In case of this studythe heating rate was set to 300 C/h up to 1000 C and from 1000 C to the test temperature the heating rate was set to 150 C/h. This is slightly above the recommended heat rate, but it was believed that this would not have any significant influence in this case since the materials andequipmentcantoleratethat.Thetesttemperaturewasmaintainedfor2h,withrespecttoa three times smaller volume than for the specimens in the standard described above. The cooling rate was set to 150 C/h. Rotary kilns of LKAB can experiencelocal temperaturesup to 1350C, but vary along the kiln. Also, naturally the temperature of the bricks varies along the thickness too. In order to find relevant test-temperatures tests were firstly performed at 1200 C, followed by increasing or decreasing the next test-temperatureby100C.Stepbystepafollowingtest-temperaturerangeandsequencewas found to be reasonable and of interest:700-900-1000-1100-1200-1300 C. ASTMC133 standard,standardtestmethodfor cold crushingstrengthofrefractories,recommends loadingratesofeither12MPa/min,31.2kN/minor1.3mm/minforbrickswithdensityover1.60 g/cm3. Loading at a constant stress rate is however preferable if possible. Thecompressionrateforthetestswaschosentobe1mm/minforthewholerangeoftest temperatures. Thereis a certain possibility that the materials exhibit somevisco-plastic behavior at temperaturesover1000C,meaningthatthestrengthdeterminationisalsoload-rateaffected, however the chosen compression rate is probably relevant even in that case. ASTMC133standardalsohighlyrecommendsusageofasphericalloadbearingthatcandistribute loadevenlyifthesamplesarenotperfectlyparallel.Inpracticesuchbearingatplannedtest temperatures wasfoundtobedifficultandexpensivetoimplement.Ithasto beofaheattolerant material and has to be specially manufactured, therefore expensive. It has to be relatively large (large curvature radius) in order to function well and it has to be quite massive, since the loads on it would bepartlyofthebendingcharacter.Attemptstomanufacturesuchbearinginthefacilitiesofthe university were done but all unsuccessfully. However, it was important to find a replacement for the bearing in order to avoid possible bending stresses on the sensitive equipment (pushing bars) during tests. Aluminum oxide powderwas found to be a relevantalternative that could beused under the samplesinordertoadjustthedifferencesinparallelism.Thepowderwasevenlyplacedunderthe sampleswithinitialthicknessofapproximately5mm,aftertheappliedcompressionthethickness was not more than 1 mm.A pre-load during the heating up of the samples was necessary. This is required in order to compress the alumina powder below the sample but also simply in order to keep the contact with the sample and keep the sample steady during the heating. There exist no recommendations about the applied pre-load. ASTM C133 states that for high density refractory samples it is possible to increase load as fastasitiswishedupto50%oftheexpected maximumstrengthandthenusetherecommended 24 compression rate. This implies that high-density refractories are relatively insensitive to loads. It was chosentosetthepre-loadoftheteststo1kNduringtheheating,whichis1-2%ofthemaximum roomtemperaturestrengthofthetest-bricks.Thesamepre-loadwasappliedinthewhole temperature interval. In reality the pre-load was somewhat lower during the heating, since the load cell was affected by the heat and measured approximately 300 N above the real values.6.1.2.1Summary-test procedure Test temperatures: RT, 700-900-1000-1100-1200-1300 C. Pre-load: 1 kN. Heating rate: 300 h/C up to 1000 C. 150 h/C after 1000 C. Compression rate: 1mm/min. Cooling rate: 150 h/C. 6.2Preparation for the hot compression strength tests 6.2.1Test specimen preparation Samples were received from Hgans Bjuf AB in form of cylindrical rods, with diameter of 35 mm and length of approximately 60 mm. The samples were drilled out from the refractory bricks that already passed the sintering process. In the best cases maximum eight samples could be drilled out from one brick.Only thesamplesfromthe samebrickwereconsideredasthesamplesfromthesamebatch. Thedrilled,longsideofthecylinderswassmoothandnopreparationofthatwasconsidered tobe necessary.Acoupleofmethodsweretriedoutfortheheight,planingandparallelismpreparing.It was found that simple cutting by the facilities of university was not reliable as a method, due to bad parallelism.Therefore,alsogrindingofthesampleswasmade.Thecuttingofthesampleswas performed in Discotom 2 (Struers) cutting machine using a diamond cutting wheel and water cooling. The force during the cutting was applied manually and one cut engaged less than a minute. Polishing was done in Phoenix 4000 (Buehler) semi-automatic polishing machine. The samples were fixed in a holder at the long sides, while the sides to be polished were on the same level relative to each other. Threesamples,120betweeneachother,weregrindedatatime.Acoarsediamondgrindingdisc was used together with water cooling, grinding direction was clockwise (150 rpm/min) and only very little pressure was applied. As a matter of fact, the pressure was so low that the machine registered zero applied pressure; this is of course not true in the reality. Due to the samples nature, the grains in the material can easily detach if too much pressure during coarse grinding is applied. The grinding rate was approximately 1 mm in 20 min by the applied pressure.Cutting was firstly done on one side and then polished until the perpendicularity wassatisfied. That waschecked by abevel protractor, this can bedone without taking samples outfrom the holder. If necessary the samples haveto bere-fixed in the holder. It required some degree of practicebefore gettingthedrilledsidesofthesamplesparalleltothewallsoftheholder-whichisofcoarse necessaryinordertogetthegrindedsidesperpendiculartothedrilledsides.Whenthe perpendicularitywassatisfiedthesecondcutwasmadeapproximately36-37mmfromtheother end.Afterapproximately10minutesofgrindingtheparallelismofthesampleswascheckedby caliper.Ifsatisfyingthenthesamplesweregrinded untilapproximately35 mm.Ifparallelismisnot satisfyingthesampleshavetobere-fixedintheholder.Dummieswereusedinordertoreplace samples that were finished before others.25 Allthesamplespreparedinthisstudyhadload-bearingsidesparalleltoeachotherwithheight difference of not more than 0.2 mm. Some of the samples were not successfully prepared and were thereforenotaccepted.Mostofthetimethiswasduetoheavydetachingofthematerialduring polishing. Victor HWM (9-16v) in the middle-strength category was specifically prone to that; half of the samples in this batch were excluded. Figure 9 shows a final prepared sample. Figure 9. Picture of a prepared sample. 6.2.2The equipment Fortheperformanceofthewarmcompressiontestsafurnace(SSL)andacompressiontesting machine were available. The furnace was equipped with water cooling system and can according to themanufacturer beused up to 1600C. Thecompression testing machine was fitted with 100 kN-loadcellandtheframewaslimitedtoloadsofmaximum100kN.Theheat-tolerantcompression equipmentwasmadeofaluminumoxidethatwasfixedintowatercooledsteelsupports.Detailed description of the equipment and the preparation of it can be found further down. 6.2.2.1The furnace Thefurnaceismadeoftwosemi-circularpartsthatarefixedontheframeofthecompression machine.Whenthefurnaceisinusethetwopartsareclosedtogethercreatingacylinder-like heatingchamber(useablespace:D=120,h=260)thatiswarmedupbysixMoSi2 heatingelements. Twooftheheatingelementshadtobereplacedpriorthetests.Twoentrancesoppositetoeach otherarerunningparalleltothefurnaceslongsideandarecenteredovertheheatingchamber. These entrances are used for the fitting of the desired equipment. They were broadened to 48 mm in diameterinordertofittheusedequipment,whichisalsothemaximumpossiblediameteronthe upper part of the furnace. Parts of the insulation in the furnace had to be repaired; this was done by a high-temperature spackling paste Kleber and a hardener.The two parts of the furnace have to be well centered to the centrum of the machine and also fit well together.Hightemperaturecottonwasusedtosealtheexistingopenings.Ifthetwohalvesofthe furnacearenotfittedwelltogethertheheatflowfromthefurnacecanbesignificant,especially abovethefurnacewheretheloadcellissituated.Thiscreatesariskofdamagingtheloadcellbut also the cell starts to measure incorrectly due to its own abnormal thermal expansion. A steel sheet with an on-glued insulating layer (see Appendix A) was placed on the top of the furnace to block the heat flow additionally. Furthermore, a ventilation output was placed close to the top of the furnace. Power supply cabling for the heating elements is situated on top of the furnace, when the steel sheet isplacedontopofthefurnacethecablingissubjectedtohighertemperaturesthanusually. However,itshowedthattheblockagesheetcouldinsulatefromtheheatflowwithoutcausingany troubles with the cablings below it. 26 The power control box of the furnace was not connected to the furnace and that had to be installed. The maximum output power supply can be regulated by a switch inside the power control box. This shouldbecheckedoutinordertobesurethattheoutputpowerisnotlimitingthepossibilityof reachingthedesiredtemperature.Theoutputpowerswitchcanbeusedwhennoautomatic temperature control system exists, which was not in this this case. The temperature control system (Eurotherm) of the furnace was equipped with a limited number of controlfunctions.Theramp-functionwasavailable,butnodwellorstep-functionsoftwarewas installed. Temperature measurement was done by two thermocouples placed close to the sample.Prior to the warm tests the water cooling was tested during a long run (>12h) of the furnace at 1200 Canditwasconfirmedtofunctionwellbyregularlycheckingtheoutputwatertemperature.Itis recommendedtodoregularcontrolsofoutputwatertemperaturetobesurethattheflowis sufficient and free. 6.2.2.2The compression equipment The design of the heat-tolerant equipment was planned to be in form of pushing bars that could be fitted to the furnace.Duetothehightesttemperaturesaceramicmaterialwaschosenasheat-tolerantequipment. Alumina as one of the cheapest and well-performing options was the candidate in this case. At room temperature high purity alumina (> 96 %) exhibits compression strength of around 2000-2700 MPa. [12]Thisissignificantlyabovetherequiredaround100MPaattheroomtemperaturetestsofthe refractorysamples.Maximumservicetemperatureofhigh-purityalumina(>96%)liesgenerally around 1550-1700C withoutapplied load. [12] According to[9] the compression strength of 99.8 % aluminaat1400Cisapproximately250MPa.Attheplannedtestconditions(max1300C)the compression strength of alumina should of course be higher, but it is difficult to say to what degree. Inanycase,sincetheframeislimitedto100kNloads,themaximumpossiblepressureonthe equipment is around 100 MPa independent on the test-temperature. Thebottompartofthehigh-temperatureequipment(aluminatube+steelsupport)wasavailable. Thedimensionsofitwereacceptable,theequipmentwasfirstlyfoundtobesufficientlystrongfor the planned tests. However, after approximately ten tests the steel support for the bottom alumina tubestartedtobuckle.Thiswassolvedbyinstallinganextrasupportunderthealuminatubethat could unburden the primarysteelsupport. Also the whole bottom part had to bere-aligned, in this case simply by placing steel sheets between the steel support and the rig. Fortheupperpartofcompressionequipmentsolidaluminabar(FrialitF99.7)fixedintoawater cooled steel support was designed and used.With total length of approximately 70 cm the pushing equipment could be easily affected by bending stresses if not properly aligned, which was also one of the challenges. Thesupplyofwatertothefurnaceandcompressionequipmentwasnotpreinstalled;thereforea newmainwaterconnectionhadtobesetup.Thiswasdonebysplittinganalreadyexistingwater junction.Alsosomeoftheoldexistinghosesandhoselinkshadtoberepairedduetoleakage.For theillustration of theequipmentset-upseeFigure10.Additionalpicturesoftheequipment canbe found in Appendix A. 27 Figure 10. Illustration of the equipment set-up. Notations: 1: Al2O3-pushing bars; 2: sample; 3: water-cooled steel supports; 4: load cell; 5: thermocouples; 6: heating elements. 6.2.3Manufacture of the ceramic support Ceramicsupportisapartmadeofaluminathatwasplacedbetweenthesampleandthelower pushingtube,seeAppendixA,Figure1.Duetoanaccidenttheexistedceramicsupporthadtobe replaced. In order to save time it was chosen to manufacture such ceramic support in the facilities of the university. The material of choice was alumina (Al2O3); the specifications of the powder material can be seen in Table 5. MaterialProducerProduct nameParticle size (m) Al2O3Baikowski (France)CR10