Introduction

There are several parameters that may determine thetype of raw material to use in a refractory product.Figure 1 lists a few of the possible servicerequirements that a designer may need to consider.No single refractory material has ideal properties e.g.Sintered alumina may have excellent corrosionresistance but poor thermal shock properties. Thismeans a wide range of alumina raw materials need tobe made available to refractory organisations.

In general alumina based materials are defined bytheir Al2O3 content, other elements such as Fe2O3,

SiO2, CaO, K2O and Na2O are important factorsduring the selection process. Also, bulk density,porosity and high temperature volume stability of araw material have a significant effect on theproperties of the final refractory product. Except for

light insulating aggregates, which are not discussedin this paper, alumina refractory materials can beclassed under three headings; refractory clays,bauxite and sintered/fused alumina. For eachclassification the chemical & physical properties willbe discussed and examples of typical applicationshighlighted. In addition, a brief note on how thenew REACh regulations affect alumina materials willbe made at the end of the article.

Chamotte (Fireclay)/Mullite

Refractory Clays and Chamotte's with varyingalumina contents are produced by blending alumina

rich raw mineral clays (commonly Kaolinitematerials) and calcining these at 1500- 1600°C. Atthis temperature the clays react together to producethe refractory material which is typically a mixture ofmullite, quartz, cristobalite and a glassy phase

which contains the impurities from the rawmaterials. By controlling the amount of each mineralclay in the original blend a refractory product withthe desired alumina content can be produced.However for the higher alumina refractory clays,Bauxitic minerals can be blended with the Kaolintematerials or a blend of Bauxitic mineral and silicasand can be used to produce a refractory with analumina content of 60 -70%. A general description ofthese refractory Clays and Chamottes is shown infigure 2.

The main sources of high quality refractory clays arethe USA, China, France and South Africa. Typicalproperties and applications are shown in Figure 3. Acommon feature of all sources of refractory clays islow thermal expansion, and good thermal shockresistance. Choosing the correct grade of clay isdependent on the application, for instance theMULCOA branded products are favoured for COresistant applications.

Within the 40-70% alumina range there is alsoAndalusite, sourced in South Africa & France.Andalusite is unique in that it does not require anycalcination, mining & beneficiation produces amaterial with exceptional thermal shock properties.

Refractory Grade Bauxite

The main use of the bauxite mined globally is in theproduction of aluminium metals, however, the ore

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An Overview ofRefractory RawMaterials – Part 1Alumina

Melvyn Bradley, Technical Manager, Minelco Ltd

Dr Tony Hutton, Assistant Technical Manager, Minelco Ltd

itself is classified by the end use and the materialrequirements that his implies, Figure 4. Bauxite is analuminium hydroxide material and the term bauxitecovers a range of raw mineral forms, Gibbsite thetrihydrate form, AlO(OH)3, and diasopore andbeohmite both monohydrate forms, AlO(OH).

Refractory grade raw material bauxite must containhigh Alumina and very low alkali content, Figure 5.The levels of Iron Oxide and Titania in the ore arealso very important as these impact on the hightemperature strength properties of the refractory. To

produce the bauxite material used in refractories theores are calcined at 1600 – 1650°C to produce denseand more volume stable aggregate.

There are very limited sources of refractory bauxitethe predominant two being China and Guyana.Although chemically similar the physical propertiesof gibbsitic & diasporic bauxites are quite different.Gibbsitic bauxite tends to expand slightly above1600°C whereas diasporic bauxite will shrink at thistemperature. The Guyana branded bauxite RASC hasalways been the favoured bauxite amongst refractory

Abstract

As a leading mineral processing organisation Minelco supply a variety ofraw materials to the refractory industry. The purpose of this two-parttechnical paper is to review the sourcing, properties and applications ofthe major refractory raw materials supplied by Minelco. In Part 1 the focuswill be on alumina based products and Part 2 will concentrate on basic(magnesia) raw materials.

• Maximum Service Temperature • Hot Load Strength

• Chemical Attack Resistance • Thermal Conductivity

• Thermal Shock Resistance • CO Resistance

• Slag Resistance • Volume Stability

FIGURE 1

FIGURE 2

Clays and Chamottes

Manufacturing Process Blended clays calcined at 1500 – 1600°C

Alumina Content 40% – 70%

Other Chemistry Low Iron (Fe ≈1%)

Service Limit Up to ≈ 1600°C

Thermal Behaviour Typically expand on heating

Properties:Low thermal expansion coefficient, lowthermal conductivity, low specific gravity,CO resistantTypical Applications:Ladle safety lining, tundish safety lining,coke oven, annealing furnace, reheatingfurnace, blast furnace, hot stove, soakingpit, cement kilns, incinerators

FIGURE 4

Metallurgical Refractory* Abrasive* Cement Chemical

Al2O3 50 - 55% >85* 80 -88% 45 - 55% 55% (Min)

SiO2 0-15% 7.5% (Max) 4 - 8% 6% (Max) 5 - 18%

TiO2 0 - 6% 4% (Max) 2 - 5% 3% (Max) 0 - 6%

Fe2O3 5 - 30% 2.5% (Max) 2 - 5% 20 -30% 2% (Max)

Other Chemical High alumina, lowiron and silica andvery low alkali con-

tent required

* These figures are for calcined material

Iron added to con-trol Fe2O3:SiO2 ratioin ferrosilicon slag

Gibbsitic form ofbauxite with verylow iron required

Diaspore (preferred)

FIGURE 3

product is a higher purity than BFA with an Al2O3content of ~ 99.5%. WFA is also a denser than BFAand with its large crystal size means that it hasexcellent corrosion resistance to molten metal slag.Like WFA, Sintered Alumina is also produced fromcalcined alumina, however, in this case the materialis pelletized and then passed slowly through avertical rotary furnace, discharging a dense highpurity aggregate.

Due to the high purity of sintered/fused alumina therefractory properties are superior to most other rawmaterials and as such they are used in highly criticalapplications, Figure 8. Generally BFA is used inapplications that require a superior performance tobauxite but offers a cost advantage againstWFA/sintered alumina. Since they are bothmanufactured from the same feedstock i.e. calcinedalumina, there are a number of similarities betweenWFA and sintered alumina. However, the type ofproduction process used gives raise to differentproperties which ultimately determines the mostsuitable material for a variety of applications.

Another class of materials within this group areAlumina-rich spinels. These normally contain 10-22%MgO which is added to the alumina during thesintering/fusion process. Alumina-rich spinels areoften used in high alumina based refractory productsto improve hot strength, slag resistance and thermalshock resistance.

REACh

Initially when the Registration, Evaluation,Authorisation and Restriction of Chemicalslegislation (REACh) was being discussed one area ofconcern to the refractory industry was the status ofthe calcination process. Under the originalinterpretation of the REACh legislation it appearedthat calcination could be considered a chemicalmodification and as such most, if not all, aluminabased refractories would need to be registered atpotentially very high costs. Thankfully because of thedescription of “not chemically modified” as definedin Article 3 (40) of Regulation (EC) No 1907/2006(REACh), Figure 9, it can be shown that calcinedkaolins and bauxites are exempted from REAChbecause to produce these refractory materials onlynaturally occurring kaolinitic materials or onlynatural bauxitic materials are heat treated andalthough some impurities are removed in the processthe alumina material itself is not modified. Theredoes, however, remain some discussion anddisagreement on the REACh status of the higheralumina refractory clays due to the way they can bemanufactured using a combination of kaolinic andbauxitic materials or bauxitic materials and silicasands and as such could be classified as syntheticmaterials by REACh. Therefore some of these productswould require registration before they can bereleased on to the market in Europe. Sintered andFused Alumina, as synthetic manufactured materials,

do fall under the scope of REACh and therefore canonly be sold in Europe if they have been registeredby a manufacturer or importer.

Conclusion

To meet the ever increasing demands on refractoriesit is vital that a wide variety of raw materials aremade available to refractory companies. Even withineach classification described in this article there canbe significant differences in the properties ofmaterials that on first glance appear very similar. Asa major supplier of refractory minerals it is importantthat Minelco supply materials that meet all thetechnical and legal requirements.

Acknowledgements:

Refractories Handbook, The Technical Association ofRefractories, Japan, June 1998

Practical Refractories, Dr J D Hancock, 1988

Industrial Minerals A Global Geology, P.W. Harben &M. Kužvart, 1996

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FIGURE 5

Bauxites

Manufacturing Process Mined ores calcined at 1600 - 1650°C

Alumina Content 82% – 92%

Other Chemistry Two forms: Gibbsitic (S. America) + Diasporic (China)

Service Limit Up to ≈ 1750°C

Thermal Behaviour Typically shrink on heating

technologists and in same applications gives superiorperformance to Chinese bauxite. However, in manyapplications Chinese and Guyana bauxite can be usedinterchangeably without any significant effect onperformance, Figure 6.

Sintered/Fused Alumina

The highest alumina content Refractories aresynthetic materials formed at very hightemperatures. There are three general categories forthese Sintered Alumina, Brown Fused Alumina (BFA)and White Fused Alumina (WFA), Figure 7. BFA with

the lower alumina content of ~95% Al2O3 is formedby fusing fired bauxite in an electric arc furnace. Thisallows the removal of impurities as a ferroalloy. Thefused alumina is then allowed to cool and solidifybefore processing into the desired size fractions. WFAalumina is formed in the same way as the BFA,however, the alumina source in this case is calcinedalumina from the Bayer process. In the Bayer processBauxite is treated with caustic soda to precipitateout alumina hydroxide which is then heated at 1200-1300°C to produce the calcined alumina product. Dueto higher purity of the starting material the WFA

FIGURE 7

Sintered + Fused Alumina

Manufacturing Process Sintered: Aluminium Oxide is sintered at up to ~2000°C

Brown Fused: Bauxite is fused/melted

White Fused: Calcined Alumina is fused/melted

Alumina Content 95% – >99%

Other Chemistry Low levels of impurities

Service Limit Up to ≈ 1850°C

Thermal Behaviour Volume stable on heating

Properties:High refractoriness, good mechanicalstrength, moderate slag resistanceTypical Applications:Ladle working lining, tundish furniture, Deltasections, general purpose repair, aluminiummelting furnace, mineral processing

FIGURE 6

FIGURE 6

Properties:High refractoriness, good slag resistance,high specific gravity, high thermal conduc-tivity, creep resistance, abrasion resistanceTypical Applications:Slide gate, flow control, ladle impact pads,launders, glass tanks, high temperaturekilns

FIGURE 8

“Not chemicallymodified substance:means asubstance whose chemical structureremains unchanged, even if it hasundergone a chemical process or treatment,or a physical mineralogical transformation,for instance to remove impurities;”