Advantages of Calcium Hexaluminate in a Corrosive Environment

M. Schnabel, A. Buhr, G. Büchel, R. Kockegey-Lorenz, J. Dutton

A growing world population is demanding that all industries be-come smarter about how they use and reuse raw materials and en-ergy. It also demands that they recycle their waste material to helppreserve the environment. New technologies to increase the per-formance of processes or alternative designs are being pushed for-ward vigorously. Projects to reduce energy losses and therefore CO2-emissions by better thermal insulation have high priority throughoutall industries. In refractory applications, longer life of units will reducematerial consumption in the linings. This in turn will save raw mate-rials, some of which already face shortages around the world. Moreinnovative new concepts and product solutions will be required toachieve these set targets.

hexaluminate when compared to other min-erals of similar densities. The dense Bonite is produced by the sameprocess used to produce tabular alumina.Because of the controlled sintering processand the subsequent processing steps it ispossible to produce a homogeneous, non-hydraulic CA6 aggregate with only traces ofother impurities such as metallic iron(< 200 ppm). SLA 92 is a super lightweight raw materialfor insulation applications. It results from a multi-step process with final calcina-tion temperatures > 1550 °C. Bonite andSLA 92 were discussed in depth in previouspapers. [1–6]Because of their identical chemical and min-eralogical composition, both calcium hexalu-minate aggregates can be used singularly orin combination. This allows for the formula-tion of “tailor made“ solutions taking intoaccount density, strength and thermal insu-lation.

2.1 Reaction with other refractory raw materials

Calcium hexaluminate is the most alumina-rich intermediate compound of the CaO-

Al2O3 system. It is therefore thermochemi-cally stable with corundum. The thermal ex-pansion coefficient of CA6 (8,0 · 10-6 K-1

from 20 – 1000 °C) [7] is similar to that ofAl2O3 indicating a low thermal mismatch be-tween both materials. This allows mixing ofboth materials in any ratio technically re-quired. For calcium hexaluminate based mix-es with micro silica addition in the matrix,the stable phase composition is mullite-anorthite-cristobalite with first melt forma-tion at 1345 °C [6]. This has to be taken intoconsideration when designing recipes forapplications where a high thermal stability

1 Introduction

About a decade ago new synthetic raw ma-terials, based on calcium hexaluminate(CA6), a highly refractory mineral, werelaunched by Almatis. These have proved ad-vantageous in various applications such assteel (in reheating furnaces and burner cov-er linings), in the petrochemical and chemi-cal industry, in ceramic kilns (lining and kilncars), and in the glass industry. This paperpresents a brief overview of the special char-acteristics of calcium hexaluminate in vari-ous corrosive environments and should helpdrive the developer to new innovative re-fractory solutions.

2 Properties of CA6 aggregates

Almatis offers two aggregates, SLA 92 andBonite, both available in a range of sizes. Themajor difference between the two productsis their density and therefore their insulatingproperties (Tab. 1).They are composed of about 90 % CA6 witha small corundum content and traces of cal-cium dialuminate (CA2). Both aggregatesshow the typical hexagonal plate-like crystalstructure, which is assumed to be the reasonfor the low thermal conductivity of calcium

refractories WORLDFORUM 3 (2011) [4] 87

Marion Schnabel, Andreas Buhr,

Gunter Büchel, Rainer Kockegey-Lorenz,

Jerry Dutton

Almatis GmbH

60 528 Frankfurt

Germany

Corresponding author: M. Schnabel

E-mail: Marion.Schnabel@almatis.com

Keywords: calcium hexaluminate, corrosion,

insulation

SLA 92 Bonite

Mineralogical composition

Main phase: CA6 (~ 90 %) CA6 (~ 90 %)

Minor phase: Corundum Corundum

Chemical analysis [mass-%]

Al2O3 91 91

CaO 8,5 7,6

Femag 0,01

Impurities 0,5 1,3

Physical properties

Bulk density [g/cm3] 3,0

Lose bulk density [kg/l] 0,5 – 0,6

Apparent porosity [vol.-%] 70 – 75 9,8

Tab. 1 Properties of SLA 92 and Bonite

88 refractories WORLDFORUM 3 (2011) [4]

The catalytic reaction follows a vapour-liq-uid-solid mechanism where the carbon isabsorbed in a liquid iron droplet until satu-ration. Thereafter the carbon is segregatedon the surface of the droplet in at least twodifferent types of structures: So called“bucky onions” decreasing the total grow-ing rate and carbon nanotubes, which en-courages further carbon deposition [13]. Thedestructive effect of these carbon structuresdepends on the predominant carbon modifi-cation with nanotubes being the much more critical form. Although the detailedwear mechanism of carbon disintegration is still under discussion a selection of appro-priate refractory materials, especially withregard to low iron content will result in ahigh resistance in CO-bearing atmosphere.Because of the requirement for low silicaand low iron content, only high purity re-fractory raw materials such as tabular alumi-na or premium grade white fused aluminaare suitable aggregates for the formulationof dense castables or bricks. Insulating ma-terials contain either bubble alumina or cal-cined alumina in the case of high purity fire-bricks.Calcium hexaluminate is an interesting alter-native due to its purity combined with itslow thermal conductivity when comparedwith typically used raw materials. In the frame of a German public sponsoreddevelopment project [14], extensive calcula-tions of the thermochemical stability of cal-cium hexaluminate were performed consid-ering an atmosphere typical for petrochemi-cal applications (Tab. 2).It was concluded, that under a reducing at-mosphere, calcium aluminate phases arestable over the entire temperature range. Inthe presence of water vapour and with in-creasing pressure the stability ranges of theCA phases do not change above 700 °C. Be-low 700 °C, only slight phase transforma-tions result in equilibrium conditions [15]. Inanother project, the CO-resistance acc.ASTM C288-87 of a medium weightcastable, (32 vol.-% porosity) based ondense calcium hexaluminate was tested andrated class A (highest resistance class) afterpre-firing at 540 °C [3]. SLA 92 as an aggregate for insulating mate-rials is of special interest because of its verylow thermal conductivity in these mixeswhen compared to bubble alumina contain-ing castables (Fig.1).

> 1350 °C is required. However, the factthat CA6 creates liquid phases at lower tem-peratures can also be used to formulate re-fractory products with a self-coating effectthat reduces further penetration of corrosivemedia or to increase the elasto-plastic be-haviour of a high alumina formulation.

3 Specific advantages of CA6 inselected application areas

3.1 Stability in H2 / CO atmosphere

Various industrial processes operate underatmospheres that require special attentionwhen selecting the right refractory liningmaterial for a particular plant. In parts of thechemical and hydrocarbon processing indus-try, for example, steam reformers which areused to produce the required process gasesfrom natural gas, the refractory material is indirect contact with the synthesis gases athigh temperatures up to 1400 °C under ex-treme hydrogen and carbon monoxide con-ditions [8].Also industrial furnaces, used for the treat-ment of metals or the sintering of metalsand ceramics, are increasingly operating un-der inert gas atmospheres (H2/N2) demand-ing optimised refractory linings [9]. Reducingatmospheres can also be found in combus-

tion processes for heat generation e.g. boil-ers with circulating fluidised beds (CFBs)[10].

3.1.1 Hydrogen attack

At temperatures > 1200 °C oxides with low-er stability such as SiO2, either as tridymite,cristobalite or in silicate form, react with hy-drogen according to the following reaction:

SiO2 (s) + H2 (g)←→ SiO(g) + H2O(g) [11]

The extraction of silica weakens the refracto-ry materials by reducing their strength andmay cause premature lining failures. Further-more the gaseous SiO can be carried down-stream in the process where it will condensein areas of lower temperature leading to po-tential fouling in heat exchangers or con-tamination of the product. [12]

3.1.2 CO-resistance

Recent studies carried out at the ResearchAssociation for Refractories (FGF) inBonn/DE describe the catalytic carbon depo-sition in refractory materials under a CO atmosphere. Metallic iron, hematite or magnetite act as catalytic reactive particlesleading to carbon deposits in the lining following the Boudouard balance reaction 2 CO←→ CO2 + C.

Fig. 1 Thermal conductivity of high temperature insulating materials

Tab. 2 Typical synthesis gas composition

Pressure Temperature Gas composition [vol.-%]

[hPa] [°C] H2 CH4 CO CO2 H2Osteam

30 500 700 – 800 36,52 6,24 4,86 6,18 46,10

refractories WORLDFORUM 3 (2011) [4] 89

The improved insulation behaviour is pro-nounced at elevated temperatures wherethe thermal conductivity is dominated by ra-diation. Here, the micro porosity of SLA 92shows a clear benefit compared to the bigpores in bubble alumina grains. Furthermorethe SLA 92 based castables are easier tohandle and gunning mixes show significant-ly lower rebound due to the better embed-ding of the SLA 92 grains in the matrix.

3. 2 Resistance against alkali attack

Many high temperature applications such ascement kilns, incinerators, blast furnaces,gasifiers and glass furnaces face corrosionby alkalis. These can be in either vapour formor the corrosion can occur by direct contactwith alkali rich melts or slags.Although in literature it is often referred toas alkali corrosion in general, in reality thesituation is more complex, since the corro-sive compounds and the process tempera-tures vary depending on the application. Al-kali sulphates are the typical salts found incement plants, whereas carbonates are moreprominent in CFBs and gasifiers. The burningof municipal waste, either in incinerators orits use as a secondary fuel in cement kilnsleads to increased emission of chlorides andheavy metals such as zinc or lead. This fur-ther increases the corrosive potential due toformation of eutectic compounds with verylow melting points (Tab. 3).Dependent upon the alkali compounds andtemperature in a given application, the de-structive corrosion progresses with varyingspeeds but follows a similar mechanism.At a temperature near the boiling point ofthe salt mixtures, the alkalis will be vapours.

Initially the gaseous compounds diffuse intothe refractory lining at a rate determined bythe porosity and permeability of the materi-al. In lining areas with lower temperaturesthe salts condense and recrystallize, densify-ing the porous structure of the refractory.Differences in the E-modulus of the originaland densified material will lead to structuralspalling when the temperature changes(Fig. 2).The often quoted mechanism of “alkalibursting” is related to the reaction of the alkali compounds with the refractorymaterial. New mineral phases within theNa2O/K2O –Al2O3–SiO2 system are formed.The density of the reaction products is lower than the original phases in the refrac-tories. This leads to considerable volume

expansion. Corundum reacts with alkalis to form “ß-alumina” (K2O·11 Al2O3 orNa2O·11 Al2O3) giving expansions of 26,4 %for KA11 and 29,6 % for NA11. Andalusiteand mullite form low density alkali sili-cate phases such as kalsilite or nepheline(NaAlSiO4). The stresses induced by the ex-pansion of the refractory material ultimatelylead to cracking and spalling of the lining.The mechanism of alkali bursting is dis-cussed more in detail by [17] and [3]. In ar-eas where the refractory lining material is indirect contact with an alkali rich melt thesolubility of the refractory compound in themelt is the most important criteria. In prac-tice all corrosion mechanisms are often pres-ent in different zones of a unit or throughoutthe thickness of the refractory lining. The

Melting point (MP) [°C] Boiling point [°C]

NaCl 801 1461

KCl 772 1500

CaCl2 (H2O free) 782 >1600

Na2SO4 888 Decomposition at MP

K2SO4 1069 1689

CaSO4 700 Decomposition at MP

K2CO3 891 Decomposition at MP

Na2CO3 851 Decomposition at MP

ZnSO4 >680 Decomposition at MP

FeCl2 (H2O free) 674 1026

Melting point of typical salt mixtures [°C]

Na2SO4 + Zn2SO4 (eutectic) 472

NaCl + CaCl2 (49 % : 51 %) 500

NaCl + FeCl2 (25 %: 75 %) 156

Tab. 3 Melting and boiling points of typical corrosive compounds [16, 30]

Fig. 2 Spalling of incinerator lining due to alkali corrosion [16]

90 refractories WORLDFORUM 3 (2011) [4]

knowledge of the prevailing corrosion mech-anism in a given application will define theconcept used by the refractory materials de-veloper to improve the alkali resistance.For dense high alumina refractories the alka-li resistance can be improved by addition ofsilicon carbide to the refractory formulation.The protective mechanism is described byPoirier et al. as a protective silica layerformed by the oxidation of SiC limiting thediffusion of gaseous compounds [18]. How-ever, in practice, controlling the SiC oxidationto the right level is difficult. Unfavourableconditions such as increased temperatures,thermal cycling, water vapour or inconsisten-cy of the corrosive compositions could accel-erate the SiC oxidation leading to too muchmelt formation, resulting in damage to thelining. It could also lead to increased stickingof fly ash or clinker to the lining. The highthermal conductivity of SiC mixes is anotherpotential drawback. This is an advantage inboilers or incinerators with power genera-tion. However, in other applications, e.g. ce-ment plants, the higher conductivity leads toincreased energy loss when compared topure alumina linings if no counteraction istaken. Bonite and SLA 92 are available aspotential alternatives with high alkali resist-ance together with low thermal conductivity.The resistance of calcium hexaluminate toalkalis originates from the mineralogicalstructure of CA6. Calcium hexaluminate hasa crystal structure similar to “ß-alumina”(KA11 or NA11). Large cations Ca2+ are in-corporated between the planar alumina lay-ers (spinel structure type with vacant posi-tions). In these layers alkalis (Na+, K+) can beincorporated without significant change ofvolume. Therefore, calcium hexaluminate

based refractories show much higher volumestability when under alkali attack comparedto other high alumina refractories (Fig. 3).Thermochemical calculations have shownthat in high alkali conditions and with in-creasing temperature, hibonite may form ß-alumina. This was shown by a corrosion testof microporous CA6 (SLA 92) conducted at1250 °C with K2CO3. It was confirmed thatsmall amounts of hibonite decomposed andß-alumina and CA2 were formed, but withlittle crack formation in the cups. [3]. Thiswas explained by the fact that in calciumhexaluminate refractories, the formation ofKA11 and NA11 does not lead to a newcrystal structure, as CA6 belongs to the samefamily of crystal structures. [4].At a project undertaken at the Oak RidgeNational Laboratory, corrosion studies ofvarious refractories were carried out to se-lect the best suited material for back liningsin black liquor gasifiers. The corrosive mediacannot be fully blocked by the wear liningrefractories. Therefore an alkali resistantback lining material is required to guaranteestability of the whole lining. The test materi-als were suspended in a molten mixture ofsodium sulphide, sodium sulphate, and sodi-um carbonate at 1000 °C for 50 or 100 h. Itwas concluded that an alternative refractorymaterial based on calcium hexaluminate(CA6) was found to be highly resistant topenetration by the molten mixture andshowed minimal expansion [19]. A similarsituation can be found in the crown of glassfurnaces. Insulating mixes based on alkalitolerant calcium hexaluminate were pro-posed by Windle et al. as the insulation lay-er, especially under oxy-fuel conditions wheninterfacial temperatures are increasing [20].

3.3 Resistance against metal andmetal slag

The containment, handling, and processingof liquid metal are key parts of several met-allurgical processes. These include the melt-ing and casting of aluminium alloys. Although the process temperatures in thealuminium industry are low when comparedto iron or steel the requirements of refracto-ry materials for direct contact with alumini-um and aluminium alloys are many and var-ious.The refractory materials in transfer ladlesshould preserve energy during transport,protect the underlying insulation materialsfrom infiltration by molten metal and protectthe steel casing. The chosen lining concepthas to guarantee the operating safety, espe-cially for transportation on public roads. Inmelting furnaces, used to re-melt aluminiumalloys or scrap, the refractory materials in theupper structure have to withstand tempera-tures that can easily exceed 1100 °C. Withthe use of oxy-fuel burners to increase themelting efficiency the temperatures can evenbe higher. Alkali salts, such as chlorides andfluorides of sodium and potassium may alsobe added to reduce oxidation of aluminiumby the atmosphere. The refractory liningshould resist increased chemical attack bythese components. One of the most criticalareas in aluminium furnaces is the belly-band area. Refractory materials installed inthis zone are exposed to the high tempera-tures in the upper furnace, having contact toliquid aluminium and salts. Mechanical re-sistance is also required to withstand charg-ing and cleaning procedures. Because of thelow viscosity of aluminum alloys, furnacebuilders and refractory producers pay special

Fig. 3 Test cups after corrosion test with K2CO3 (left: Bonite, right: andalusite)

refractories WORLDFORUM 3 (2011) [4] 91

attention to the design of the furnace lin-ings. By the selection of a multi-layer designwith dense, medium-dense and insulatingmaterials the solidification point of the alloywill occur in the wear or safety layer to avoidmetal break through.Refractory linings in contact with aluminiumgenerally require high chemical purity rawmaterials. Because of its high oxidation po-tential, molten aluminium will reduce impu-rities like SiO2, Fe2O3 and TiO2 to their metal-lic state. The most common of such reactionsis the reduction of silica by aluminium with avery high negative free energy change(Fig. 4). Alumino-silicates such as kyaniteand mullite also lead to the formation ofcorundum, which is the major contributor torefractory damage.Corundum formation is a major problem inaluminium furnaces and occurs by two com-monly accepted processes.External corundum growth occurs at thetriple point junction of the aluminium, re-fractories and atmosphere. The outwardcorundum growth is explained by Allaire

atmosphere, where it oxidizes to form morecorundum. If magnesium is present, the“mushroom” growth is accelerated. Pres-ence of fluxing agents and salts also en-

[21] as direct metal oxidation. Some alu-minium penetrates the refractory and movesup into the refractory by capillary action. Thepenetrated metal is then exposed to an open

Fig. 4 Free energy of typical minerals used in refractory lining materials for aluminiumand aluminium alloys [25]

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hances corundum growth. This corundumbuild-up reduces the furnace capacity caus-ing frequent downtime of production lines,and severely reduces energy efficiency.The internal corundum growth process is ex-plained as an oxido-reduction mechanism.The corrosion of the refractory first leads tothe formation of an Al2O3/Al composite. Theformation of this composite is favoured athigher temperatures and lower oxygen par-tial pressure. At low temperatures, silica re-duction is considered to be the rate-control-ling process and is more favoured in thepresence of magnesium. At high tempera-tures metal penetration is considered to bethe rate-controlling process [21].Internal corundum growth causes extremedamage to the refractory wall over time. Thisis due to stresses created by the aluminasurface concretion and creation of porosityinside the castable structure. There is also acorresponding difference in thermal expan-sion behaviour, which leads to mechanicaldamage during thermal cycling. Further-more, the thermal efficiency of the lining isreduced leading to greater heat losses andless heat retained in the process.Classical refractories used so far in the alu-minium industry are high alumina materialsbased on alumino-silicate or bauxite refrac-tory aggregates. Often, additives like BaSO4,CaF2, SiC or phosphate are added to im-prove the resistance against molten metal orslag.

The function of the non-wetting additives isstill not fully understood. Aguilar-Santillan[22] concluded that the non-wetting theoryfor the improvement of aluminium refracto-ries by BaSO4 additions does not appear tobe the correct explanation. Afshar et al. pos-tulated that the efficiency of non-wettingadditives is in their potential role in convert-ing silica to some alumino-silicate basedcrystalline phases that are more resistant toaluminium attack [23].Whatever the mechanism, the practical ex-perience has proven the function of theseadditives for temperature ranges below1000 °C. At higher temperatures their effi-ciency is reduced and corundum formationwill appear. But sometimes even at low tem-peratures corundum formation can be ob-served. A possible explanation is given byRichter et al. [24]. According to their teststhe coarse refractory aggregates do not ben-efit from non-wetting additives and somecorrode in contact with aluminium, even en-hancing the corrosion of the matrix. As aconsequence the development of an opti-mum non-wetting matrix is only part of thesolution to improve the corrosion resistanceof refractory materials to molten aluminium. With aggregates based on calcium hexalu-minate it is possible to use aluminium resist-ant fractions throughout the whole product.The high resistance of calcium aluminatesagainst metal and metal slags was alreadydescribed in the literature [7]. In contact

with aluminium, calcium hexaluminate ex-hibits a high stability with a significantlylower Gibbs energy for the reaction com-pared to silica and mullite (Fig. 4).The advantage of dense calcium hexalumi-nate based refractories over conventionalhigh alumina refractories containing antiwetting agents such as BaSO4, has alreadybeen described in detail in by Buhr et. al. Inan enhanced aluminium resistance test atthe Corus Research Centre in Ijmuiden/NL itwas demonstrated that the superior resist-ance of calcium hexaluminate remains at el-evated temperatures up to 1400 °C, whileconventional bauxite/BaSO4 refractories losetheir protective properties [1, 2].In the frame of a project supported by theU.S. Department of Energy, Office of EnergyEfficiency and Renewable Energy [26], calci-um hexaluminate was tested together withother raw materials such as mullite and SiCas new potential lining material for alumini-um furnaces. It was concluded that high-alu-mina based materials, in particular calciumhexaluminate (Bonite), are superior to tradi-tional bauxite type materials. No metal pen-etration was observed (Fig. 5). Another advantage of calcium hexaluminatethat was emphasised in this work was thefact that calcium hexaluminate (Bonite), dueto its inherent low thermal conductivity, in-creases the thermal efficiency of a furnacewith the same lining thickness when com-pared to traditional refractory materials.

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This effect could be further optimized by theuse of the super lightweight aggregateSLA 92 or combinations of Bonite withSLA 92. The advantage of the low thermal conduc-tivity of calcium hexaluminate based aggre-gates in combination with their high resist-ance against CA-rich steel ladle slags waspresented in detail in a previous paper [6].

4 Summary and outlook

Synthetic, calcium hexaluminate based rawmaterials exhibit various advantages whencompared to traditionally used refractory ag-gregates in a given application: • high refractoriness • low thermal conductivity • stability in H2/CO atmosphere• resistance against alkali attack • resistance against metal and metal slag

attack • fibre free, therefore environmentally

friendly.But, it is the combination of the presentedproperties that makes calcium hexaluminateaggregates unique as refractory raw materi-als. The availability of two commerciallyavailable products, Bonite as a dense aggre-gate and SLA 92 as a super lightweight ma-terial, allows the developer to create “tailor-made” solutions taking into account density,strength and thermal insulation. Successful industrial scale applications in thealuminium, glass and cement industry havealready shown the potential of dense, calci-um hexaluminate based Bonite in new inno-vative refractories. The light-weight aggre-

gate SLA 92 has been successfully used insteel reheating furnaces significantly improv-ing the energy performance of the units. Ithas also been used in petrochemical appli-cations. Ceramic kiln linings have also beeninstalled with this insulating material[27–29].Both aggregates, SLA 92 and Bonite are pro-duced by Almatis in Ludwigshafen/DE, whichguarantees state-of-the art productionprocesses, high quality and secure supply.

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