Aspects of Elastification Reactions in Basic Cement Kiln Bricks

refractories WORLDFORUM 5 (2013) [4] 53

1 Introductionand historical background

The production of cement in highly efficientrotary kilns would not be possible todaywithout the development of sophisticatedelastified basic bricks. Only their installationoffers a superior economic and ecologicalkiln lining. To further increase the productiv-ity of cement production from the refrac-tories side, refractory producers have putand continue to put efforts into the develop-ment of even more advantageous products.

hibited bad performance due to their poorstructural flexibility [1]. The peak level ofmagnesia chromite bricks usage in the hotsections of the kiln was roughly between1940 and 1990.At the same time, dolomite bricks were in-stalled in the main burning zone with similarpositive results as magnesia chromite bricksin case of moderate climatic environmentalconditions. Dolomite bricks are sensitive toCO2, sulphur and humidity, which unfortu-nately affects their performance in hotclimatic zones or when secondary fuels areused [2].In the mid-seventies, trials with chromite-free magnesia spinel brick grades werecarried out in Japanese cement rotary kilns,primarily in the coating-free transition zones[3]. Use of magnesia spinel bricks in Ger-many started in the early 1980s [4]. In thiscontext, ALMAG® 85 became the mostpopular and successful magnesia spinelbrick for cement rotary kilns. Although coat-ing-friendly magnesia chromite bricks anddolomite bricks are still used in the burningzone, the environmental impact caused byrefractory products containing chromite(hexavalent alkali chromate formation) re-sulted in an increasing demand for chromite-free basic bricks in the cement industry.

Regarding the brickwork installation overthe recent 100 year history of cement rotarykilns, fireclay bricks and high alumina brickswere primarily used in the hottest areas ofthe kiln until the 1940s.Basic bricks were firstly installed approxi-mately 75 years ago. First trials were carriedout with magnesia and magnesia chromitebricks in the burning zone during the 1930sand 1940s. Magnesia chromite bricks be-came the standard basic brick in the hotsection of the kiln, while magnesia bricks ex-

Aspects of Elastification Reactions inBasic Cement Kiln Bricks

J. Södje, S. Uhlendorf, H.-J. Klischat

Johannes Södje, Stefan Uhlendorf,

Hans-Jürgen Klischat

Refratechnik Cement GmbH

37079 Göttingen

Germany

Corresponding author: Hans-Jürgen Klischat

E-mail: [email protected]

Keywords: chromite-free brick, magnesia,

spinel mineral, cement rotary kiln

To increase the flexibility and elasticity of a basic refractory brick sys-tem, beneath the resistor MgO a second component is necessary, theso-called elastifier. Spinel minerals have proven to be an economi-cally and technically suitable elastifier material. The behaviour ofspinel minerals in the magnesia system are highlighted and focussedin this paper, as other (like zirconia) are of less importance nowadays.The behaviour of the crucial spinel types can be subdivided into threegroups regarding their reaction potential with the basic brick mater-ial, the resistor MgO. The selection of the appropriate elastifier fromthe spinel group, its content and specific performance influences theproperties and behaviour as well as the resistivity of the basic bricksystems.

FFiigg.. 11 Use of different brick grades in the main burning zone (BZ) of rotary cement kilnssince 1900 (schematic diagram) (copyright: Refratechnik Cement GmbH)

54 refractories WORLDFORUM 5 (2013) [4]

Consequently, the refractory industry intro-duced new chromite-free bricks with modi-fied spinel types, structural properties andenhanced physical characteristics [5–12].Some of these brick grades were optimizedfor specific kiln sections, like tire area, se-verely thermally loaded zones and the burn-ing zone. Besides magnesia spinel brickswith varying spinel content and spinel types(sintered spinel, in situ spinel, fused spinel),magnesia hercynite bricks and magnesiapleonaste bricks have been developed forrotary kilns [8, 12, 13].

position. Two types of occupation of the octahedral and tetrahedral gaps can be dis-tinguished:

Normal spinel (A2+B23+O4)

From the 24 cations per unit cell 8 A2+ ionsare in the tetrahedral, and 16 B3+ ions are inthe octahedral coordination.

Inverse spinel (B3+A2+B3+O4)

In this case, 8 B3+ ions are in the tetrahedralas well as 8 A2+ ions, and 8 B3+ ions are inoctahedral coordination.The “A” position is usually filled by bivalentcations (Mg2+, Fe2+, Mn2+, Zn2+, Co2+, Ni2+,Cu2+, etc.), and position “B” by trivalentcations (Al3+, Cr3+, Fe3+, V3+, etc.). Titan (Ti4+)can be also embedded in the crystal struc-ture, e.g. the spinel type ulvite (Fe2TiO4).Fig. 2 shows the general spinel structure.All spinels form mixed spinel minerals, espe-cially at high temperatures. Tab. 1 shows anoverview of the most important minerals ofthe spinel group [14].The variety of end members indicates thatthe spinel structure can incorporate cationswith a wide spectrum of radii and valences.Additionally, a certain ratio of the octa hedraland tetrahedral gaps can be unoccupied.This is named defect spinel. Defect spinelsare formed at high temperatures and lowpressures.Regarding Tab. 1, some spinel grades fromthe spinel-gahnite series and the magnesio -chromite-zincochromite series are utilised inthe refractory industry producing elastifiedbasic magnesia bricks, whereas the spinelgrade chromite/magnesiochromite is theonly one which occurs naturally and is in-dustrially mined. The other utilised spinelgrades, like MA-spinel and hercynite, have tobe synthetically produced. The typical typesof spinel which are used in basic brickgrades are pre-synthesized sintered or fusedspinel grades. The so-called in situ spinel ormatrix spinel are formed in the texture of basic bricks during the manufacturingprocess (Chapter 3).Currently, magnesiochromite (MCr), spinel(MA), hercynite (FA) and the recently devel-oped and produced pleonastic spinel type([FexMg1–x]Al2O4) are the principal spinelgrades to elastify basic bricks. Basic brickselastified with the spinel grades gahnite and

Name Formula Type of Occupation a [Å]

Spinel-Gahnite Series

SpinelHercyniteGalaxiteGahnite

MgAl2O4FeAl2O4MnAl2O4ZnAl2O4

normalnormalnormalnormal

8,098,138,298,09

Magnesioferrite-Franklinite Series

MagnesioferriteMagnetiteJakobsiteTrevoriteCuprospinelFranklinite

MgFe23+O4Fe2+Fe23+O4Mn2+Fe23+O4Ni2+Fe23+O4Cu2+Fe23+O4ZnFe23+O4

inverseinversenormalinversenormalnormal

8,388,408,478,348,378,47

Magnesiochromite-Zincochromite Series

MagnesiochromiteChromiteManganchromiteCochromiteNichromiteZincochromite

MgCr2O4Fe2+Cr2O4Mn2+Cr2O4Co2+Cr2O4Ni2+Cr2O4Zn2+Cr2O4

normalnormalnormalnormalnormalnormal

8,368,368,478,298,328,35

Vuorelainenite-Brunogeierite Series

MagnesiocoulsoniteVuorelaineniteCoulsoniteQuandiliteUlvöspinel (Ulvite)Brunogeierite

MgV23+O4Mn2+V23+O4Fe2+V23+O4TiMg2O4TiFe22+O4Fe22+Ge4+O4

normalnormalnormalinversenormalnormal

8,388,488,308,408,518,41

FFiigg.. 22 General spinel structure (source: ruby.chemie.uni-freiburg.de)

TTaabb.. 11 Overview of the most important spinel group (endmember) [14]

Fig. 1 schematically illustrates the use ofbrick grades in the main burning zone of ro-tary kilns since 1900.

2 Classification of spinel minerals

Spinel minerals have proven to be most suit-able materials to act as elastifier in basicbricks. Various types of spinel are currentlythe most established elastifying componentfor basic brick grades in cement rotary kilns.Industrially important minerals of the spinelgroup are lithogenous and form deposits,like huge chromium ore deposits in SouthAfrica, the Philippines and Finland. Spinelminerals possess a highly low-energy struc-ture (cubic closed packed), which compre-hends characteristic high hardness, highdens ity, high melting temperature and highchemical resistance. The spinel structure is based on a highest cubic sphere packing of O2– ions. The cationsin the spinel structure are encased in the octahedral and tetrahedral gaps. The gener-al formula is AB2O4, and every elementarycell contains eight units of formula and ex-hibits different edge lengths between 8,09and 8,64 Å depending on the chemical com-


galaxite were installed in the cement indus-try to a lesser extent.

3 Elastification in basic bricks

The main component of basic bricks is the re-sistor MgO, which is thermochemically resist-ant to attack by cement clinker or related ma-terials. This resistor can be either extractedfrom natural magnesia-rich rocks (coarselyand finely grained magnesite) or syntheticallyproduced from sea water and salt brines [15].Unfortunately, products made only from MgOare characterised by a poor thermal shock resistance (due to their rigidity and high modulus of elasticity), a high thermal conduc-tivity, and a high thermal expansion.For an appropriate performance of basicbricks, a second component has to be intro-duced into the structure, the so-called elasti-fier. This elastifier increases the structural flex-ibility of the lining, the ductility, and its frac-ture toughness by developing a micro cracknetwork in the microstructure. This results ina high resistance to mechanical stresses bylowering of the modulus of elasticity E and si-multaneously reducing the thermal expansionand the thermal conductivity. The formerlybrittle MgO bricks turn into elastic, some-times even thermoplastic high-tech products,which can manage the complex stresses pre-vailing in a cement rotary kiln. These aremainly mechanical stresses (by kiln shell oval-ity, thermal shocks, thermal expansion, con-densing infiltrates), thermal stresses (by over-heating), and chemical stresses (by cementclinker, redox conditions, etc.).Spinel minerals have proven to be the mostsuitable materials to act as elastifier in basicbricks. They are compatible with the sur-

Thus, the crucial spinels can be subdividedinto three groups regarding their reactionpotential with MgO, which also influencestheir content in the basic brick (Tab. 2).Group I with the classical mineral spinelMgAl2O4 is characterised by little or no inter -action between the elastifier and the MgOmatrix, Fig. 3. Spinel is located, almost unaf-fected, beneath MgO. The elastification re-sults from the thermal expansion mismatchbetween spinel (8,8 · 10–6 K–1) and magne-sia (13,85 · 10–6 K–1) creating a microcracksystem in the brick with a reduction of themodulus of elasticity (Young’s modulus) anda dramatic advance of the mechanical prop-erties, like improved thermal shock resist-ance, lower thermal expansion, and less inter nal stresses in the brick. Similarly, the newly developed pleonasticspinel is acting [12, 18]. As this pleonasticspinel contains a considerable amount ofMgO, and alumina and iron are tightlybound in the crystal structure, the diffusionpotential of ions is largely suppressed andthe pleonastic spinel also elastifies largely bya thermal mismatch pattern (Fig. 4).Because of absent reactions between resis-tor and elastifier, the elastifier needed

rounding magnesia matrix, they induce a microcrack system in the brick, and aremostly highly resistant to chemical attack bycement clinker or related materials at regu-lar service conditions and temperatures, al -though their resistance is naturally lowerthan that of MgO. Known from the past, MA spinel (MgAl2O4)can be introduced into the brick matrix intwo ways: as a presynthesized material (ei-ther sintered at temperatures of approx.1700–1800 °C or fused at temperatures ex-ceeding 2135 °C) or from an in situ reactionof alumina additives with MgO of the sur-rounding brick matrix during brick firing at1500–1800 °C [16, 17].The performance of the elastifier in the mag-nesia matrix is highlighted by consider -ations, which focus only on the spinel miner-als, as other elastifiers (like zirconia) are ofless importance nowadays. As target valuesreflecting the structural flexibility, for thecold crushing strength σ ~70–80 MPa and for the modulus of elasticity E ~20– 22 GPa are assumed, as bricks withthese values are known to deal well with themechanical requirements of typical rotarykilns.

Group Ino or Little Reactionwith MgO Matrix

Group IILimited Reactionwith MgO Matrix

Group IIIComplete Reactionwith MgO Matrix

spinelMgAl2O4pleonastic spinel (Mg,Fe)(Al,Fe)2O4

chrome spinel(Mg,Fe)(Cr,Al,Fe)2O4hercyniteFeAl2O4galaxiteMnAl2O4

aluminaAl2O3

TTaabb.. 22 Classification of spinel minerals regarding their reaction potential with MgO mat rix (chrome spinel is a synonym for chrome ore in basic bricks)

FFiigg.. 33 Microstructure of a magnesia spinel brick with fusedspinel grains (arrows) embedded in a magnesia matrix(copyright: Refratechnik Cement GmbH)

FFiigg.. 44 Microstructure of a magnesia spinel brick with fusedpleonastic spinel grains (arrows) embedded in a magnesia matrix(copyright: Refratechnik Cement GmbH)


FFiigg.. 55 Microstructure of a magnesia chromite brick with chromespinel grains (arrows) embedded in a magnesia matrix; notableare the gaps around the chrome spinel grains(copyright: Refratechnik Cement GmbH)

FFiigg.. 66 Microstructure of a magnesia hercynite brick with fusedhercynite grains (arrows) embedded in a magnesia matrix; notable are the holes in the hercynite grains due to diffusion of iron ions into the surrounding magnesia matrix(copyright: Refratechnik Cement GmbH)

FFiigg.. 77 Electron microprobe element maps for Fe, Cr and Mg of chrome spinel grains in an MgO matrix burned for 4 h at 1400 °C (left),1500 °C (centre) and 1600 °C (right) illustrating the diffusion processes between chrome spinel and matrix as well as pore space formation around the chrome spinel grains [27]


for elastification typically amounts to14–18 % and can even be as high as 28 %for special applications.Group II, which is mainly represented bychrome spinel and hercynite as importantspinels, is characterised by moderate reac-tions between the elastifier mineral and thematrix (Fig. 5, 6). These reactions are pro-nounced enough that a publication desig-nates these minerals as active spinels [19].When compared to the classical spinel(MgAl2O4) and pleonastic spinel((Mg,Fe)(Al,Fe)2O4) from Group I, chromespinel and hercynite contain significantlyless MgO. This leads to higher concentrationgradients between elastifier and MgO matrixand thereby favours diffusion processes athigh temperatures. Reaction tests withchrome spinel and magnesia exhibit diffu-sion of Fe(II)/Fe(III), Mn(II)/Mn(III), and/orCr(III) ions from the elastifier into the matrixand, vice versa, the diffusion of Mg(II) intothe elastifier [20–22]. These diffusionprocesses lead to further elastification in add ition to the thermal expansion misfit.In case of chrome spinel, a well-reviewedmaterial, the interaction with an MgO matrixis described by various authors [23–26]. Attypical burning temperatures (1400–1600 °C) significant amounts of Fe(II)/Fe(III)enrich in the contact area between chromespinel and magnesia and subsequently dif-fuse into the MgO matrix (Fig. 7). The lack ofFeO/Fe2O3 in the chrome spinel relic is par-tially substituted by MgO. But as mass trans-fer is mainly directed to the matrix, porespace is generated around the shrinking

Because of the thermochemical reactionsbetween resistor and modifier, the elastifierneeded for sufficient elastification typicallyamounts to 6–10 %. For special applica-tions, magnesia chromite bricks may evencontain 15–25 % chrome spinel (the use ofchrome spinel in cement kiln refractories isdecreasing due to environmental aspects).Although a reaction takes place, the originalelastifier is still present in the most part ofthe core area of the elastifier (topochemicalreaction).Group III is characterised by extensive reac-tions between the elastifier mineral and thematrix. Typically, no considerable amounts ofthe original elastifier substance remain, acomplete mineral conversion takes place(Fig. 9). Even if some original alumina is left,the thermal mismatch between alumina andmagnesia still provides a sufficient elasti -fication due to the similar thermal expansion coefficient of alumina and spinel. Up to now,of economic importance is only the additionof alumina (Al2O3) to an MgO-based brick,the reaction product is magnesium aluminaspinel. Due to the diffusion paths of Al2O3into the MgO matrix, even hollow grains canbe formed, with a pore on the site of the for-mer alumina grains, surrounded by a shell ofthe newly formed spinel. Sometimes the coreof the spinel shell is filled with low-meltingreaction products from secondary mineralsof the magnesia with alumina, in most casescalcium aluminates. The reaction formula is simple:

MgO + Al2O3 → MgAl2O4

chrome spinel grains (Fig. 7). This pore spaceis supposed to inhibit crack propa gation andis therefore advantageous for the mechani-cal flexibility of the brick (reduction ofYoung’s modulus). Cr(III) also significantlydiffuses into the matrix, especially at tem-peratures above 1500 °C at which it in-creasingly turns into gaseous state. Duringcooling, the solubility of the iron and chro -mium ions in the periclase crystals of themagnesia decreases and secondary spinels[Mg2+(Fe3+,Cr3+)O4] precipitate on grainboundaries and inside periclase crystals, alsocontributing to brick elastification (Fig. 8).In case of hercynite and galaxite, the diffu-sion of Fe(II)/Fe(III) or Mn(II)/Mn(III) into theMgO matrix can be observed as well andalso leads to the formation of secondaryspinel in the periclase crystals. The loss ofFeO in the hercynite or of MnO, respectively,in the galaxite is compensated by MgO fromthe magnesia matrix, which results in somekind of thermochemically more stablepleonastic spinel according to the reaction:

FeAl2O4 + MgO → (Fe,Mg)Al2O4

In case of galaxite in the brick, the followingreaction can take place:

MnAl2O4 + MgO →(Mn,Mg)Al2O4 + MgAl2O4 [28].

The mass transfer is lower compared tochrome spinel and usually does cause only alittle pore space formation around the her-cynite grains.

FFiigg.. 88 Backscattered electron image micrograph of the contactzone between chrome spinel (top, left) and MgO matrix (bottom,right) showing formation of secondary spinels in the periclasecrystals and on grain boundaries [27]. Burning conditions were4 h at 1600 °C

FFiigg.. 99 Microstructure of a magnesia spinel brick with an in situspinel grain (ellipse) embedded in a magnesia matrix; notable isthe vacant core hole within the spinel grain due to diffusion ofaluminium ions into the surrounding magnesia matrix with subsequent spinel reaction (copyright: Refratechnik Cement GmbH)


The use of other X2O3 sesquioxides likeFe2O3, Y2O3, or even Mn2O3 and Cr2O3 in-stead of Al2O3 did not serve to create brickswith properties suitable for rotary kiln instal-lations. Because of the intensive reaction betweenresistor and modifier, the elastifier neededfor elastification typically amounts to only3–5 %. With this technology, the MgO content of the brick can be increased up to 95 %, but also with the disadvantage of a higher thermal conductivity and a high-er thermal expansion. This has to be consid-ered when installing bricks or a thermal in-sulation or when providing space for expan-sion.By adding spinel minerals to a coarse- andfine-grained magnesia matrix, it is possibleto create bricks, which can withstand themechanical tensions in hot rotating units. Bymeans of the appropriate mineral from thespinel group, also the thermochemical resist -ivity of a brick can be adjusted.

4 Chemical stability of elastifiers in basic bricks against cement clinker

Known from numerous investigations onused basic bricks, in most cases the elastifier(=spinel mineral) is corroded whereas theresistor (=MgO) is rather unaffected. Especially when alternative fuels are fired,high thermal und thermochemical stresseswith increased temperature load to the lin-ing are caused by abnormal burning condi-tions [29]. Also the use of low-grade coalcan change the flame shape, which can

cause local or general overheating. As a re-sult thermochemical reactions between thecement clinker and the basic refractory brick,and here especially the elastifier (spineltypes), are probable.Minerals from Group I with the spinelMgAl2O4 are characterised by the formationof low melting calcium aluminates, mostlymayenite (12CaO · Al2O3) or ye’elimite(4CaO · Al2O3 · SO3, in the presence of sul-phur oxide from the kiln gas atmosphere) oncorrosion by overheated liquid cement clink-er or unsuitable low-melting clinker compos -ition. Primary (from infiltration) and second-ary melts (formation of calcium aluminates)are absorbed by the surrounding brick struc-ture and solidify during migration to thebrick cold face or when the kiln is shutdown. Cavities or pores remain on the site ofthe corroded spinel particles. In the centre ofthese pores may remain small relics of theoriginal spinel grain. Due to the presence ofprimary and secondary melts, the brick struc-ture at the hot face is highly densified, whichresults in spalling of the infiltrated area. Theuse of fused spinel may be advantageousdue to less grain boundaries and higher pri-mary crystal size (Fig. 10), whereas a sin-tered material provides a more homoge-neous spinel phase, which may be better forelastification [30]. The selection of the spineltype also requires knowledge about the areaand conditions of brick installation.In case of pleonastic spinel, calcium alumin -ates may form as well, also the formation ofcalcium ferroaluminates CaO(Al2O3,Fe2O3) ispossible (Fig. 11).

As an important additional aspect, reactivityand surface area have to be considered: notonly the chemical composition determinesthe corrosion behaviour, but to a similar ex-tent also the structural properties do. Gener-ally, fused materials with high grain size anda low porosity are more resistant to chem icalattack than sintered ones with many grainboundaries (as first point of attack) andsmall crystal sizes.In case of spinels, fused ones are increasing-ly used in cement kiln refractories to preventa premature corrosion reaction, especially ifthe necessary amount for elastification israther high. In case of iron-containing fusedpleonastic spinel, the usually mobile ironions are tightly bound in the crystal struc-ture. When bricks containing minerals fromGroup II (hercynite and chrome spinel), char-acterised by a mandatory amount of6–10 % needed for elastification, are cor-roded by overheated cement clinker, the cor-rosion products are mainly dicalcium ferrite2CaO · Fe2O3 (melting temperature1435 °C) and/or 4CaO(Al2O3, Cr2O3)Fe2O3,the latter in the event of magnesia chromitebrick corrosion. This reduces the brick’s re-fractoriness, and as a consequence of a sub-sequent densification a spalling of the infil-trated brick horizon and therefore a pre ma -ture wear can take place (Fig. 12, 13).In case of magnesia hercynite bricks, dical -cium ferrite and calcium aluminates, espe-cially mayenite and ye’elimite (in the pres-ence of SOx), are the main corrosion prod-ucts as well. Generally, a susceptibility of

FFiigg.. 1100 Initial corrosion of a fused spinel grain leaving a porenear the corrosion area (~3–5 mm from the hot face side, arrows) (copyright: Refratechnik Cement GmbH)

FFiigg.. 1111 Initial corrosion of a pleonastic spinel grain leaving secondary melts in the surrounding magnesia matrix; only initialpore formation is observed (~3–5 mm from the hot face side, arrows) (copyright: Refratechnik Cement GmbH)


hercynite (and galaxite) to the presence ofSOx is observed [31]: in several cases, KFeS2as reaction product of hercynite degradationaccording to the reaction

4MgO + 2K2O + 4Fe(II)O · Al2O3 + 8SO2 →4KFeS2 + 4MgO · Al2O3 + 11O2

is found under reducing burning con di-tions. For magnesia galaxite bricks, a similar reac-tion may take place, with K2Mn3S4 as corro-sion product, although this mineral has notbeen explicitly found up to now. Althoughthe corrosion mechanisms for these brickgrades have not been investigated fully yet,it is possible that some low melting eutecticsin the quaternary system MnO-CaO-Al2O3-SiO2 (approx. 1150 °C) may occur resultingin a premature wear by loss of refractoriness. Although [28] mentions that a loweramount of the elastifier from Group II, com-pared to magnesium aluminium spinel fromGroup I, is needed to achieve a similar struc-tural flexibility (which should give the brickalso a higher resistance to chemical attack),it has to be considered that the elastificationis lost more easily when the already reducedamount of elastifier is even more reduced bykiln feed corrosion.In case of minerals from Group III (i.e. alu -mina, either sintered or fused), the lowestamount is needed for elastification, whichalso gives the lowest amount of elastifier(a.k.a. spinel) in the magnesia spinel brick.The reaction products are chemically the

FFiigg.. 1122 Corrosion of a hercynite grain leaving secondary melts,and calciumferritic melt in the surrounding magnesia matrix; notably, a significant pore formation is observed (~3–5 mm fromthe hot face side, ellipse), which gives rise to further infiltrationand corrosion (copyright: Refratechnik Cement GmbH)

FFiigg.. 1133 Corrosion of chromite grains leaving chrome-containingsecondary calcium alumino-ferritic melts in the surroundingmagnesia matrix. Here, also a significant pore formation is observed (~3–5 mm from the hot face side, ellipses and arrows),which can promote further infiltration and corrosion(copyright: Refratechnik Cement GmbH)

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same ones as for presynthesized spinel fromGroup I. Due to the in situ reaction, the new-ly formed spinel is rather fine-grained (crys-tal size <10 µm) and thus less corrosion re-sistant than e.g. a fused spinel. This can bealso clearly seen by a post-mortem analysisof a used brick containing in situ formedminerals (Fig. 14): a complete corrosion ofthe in situ spinel took place and caused acomplete loss of structural flexibility andbrick spalling. The structure is infiltrated pri -marily by liquid cement clinker and second-arily by melts formed by spinel corrosion. Themagnesia grains still remain chemically un-affected by cement clinker, as predicted bytheory.So although the amount of spinel as a pri -mary point of attack is much lower than thatof minerals of Group I, the degradation takesplace much faster due to the smaller crystalsize. For performance of basic bricks, not only thechemical composition of the elastifier

“spinel mineral” has to be considered, butalso mineralogical, structural and steric rea-sons. For a high corrosion resistance, whichis needed in thermally loaded transitionzones of cement kilns, a dense materialshould be used to minimize a thermochem -ical attack by kiln feed. Usually, these arefused products essentially from Group I witha high crystal size and few grain boundaries.If only an elastification is needed withouthigh requirements on the chemical resist-ance (e. g. for installations in the main burn-ing zone with a stable protective coating),sintered/fused material from Group I or insitu material from Group II can be used. Ma-terials from Group III, essentially spinel froma reaction of alumina with the surroundingmagnesia matrix, nowadays are less com-mon, also as both the spinel with its finecrystalline structure and the unreacted alu-mina relics show a low corrosion resistanceto calcium-containing materials.

An example shows the different reactivity ofpresynthesized and in situ spinel: Fig. 14shows a brick which combined presynthe-sized and in situ elastification. Both elasti -fication types are located right beneath each

FFiigg.. 1144 Corrosion of in situ spinel grains leaving corrosion holes as big pores in the structure(~3–5 mm from the hot face side, ellipses). Simul -taneously present presynthesized sintered spinel inthe structure remains rather unaffected (arrows)(copyright: Refratechnik Cement GmbH)

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other. Due to corrosion by overheated ce-ment clinker, the in situ spinel completelydisappeared leaving only pores in the struc-ture, whereas the presynthesized spinel re-mains rather unaffected in the structure. For a proper selection of brick grades withthe spinel minerals from Group I, II or III, be-neath the selection of the resistor magnesia,the conditions in the kiln regarding the kilnitself, the raw materials and the fuel have tobe known and analysed to install a brick lin-ing with the highest efficiency and best per-formance.

5 Conclusion

In the past, the development of basic brickswas given a lot of attention by refractorymanufacturers. New brick grades with spe-cific properties for the various requirementsin cement kilns, kiln zones and miscel -laneous kiln conditions have been devel-oped. The concept is mostly based on thecombination of magnesia with at least onemineral from the spinel group. Spinel min -erals can be introduced into the brick aspresynthesized material, so that no inter -action with the magnesia brick matrix takesplace, as material with a limited interaction,and as material with a complete conversionreaction with the brick matrix. The amountneeded for elastification is highest for thefirst group and lowest for the latter one. Re-garding brick choice for cement kiln installa-tion, not only the elastifying effect, reductionof Young’s modulus, but also the thermo-chemical resistance to cement clinker attackhas to be considered.It goes without saying that research on basicbricks will not cease, but new products withnew properties of the elastifier (not limitedto minerals from the spinel group), de signedto improve the ambitious process of cementmanufacturing, for the benefit of the cementproducer are in progress.

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Aspects of Elastification Reactions in Basic Cement Kiln Bricks

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