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Refractories

Definition & Classification

Tina Stendal Svendsen

Product Manager

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Definition of Refractory

“non-metallic ceramic material or product whose chemical and physical properties allow it to be

used in a high temperature environment”

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Application of Refractory

Lining of plants for thermal processing (melting, firing and heat-treatment furnaces) e.g.:

• Iron and steel• Cement and lime• Glass and ceramics• Chemical and petrochemical• Heat- and power (waste and biomass)

Heat-insulation

Functional products: shaped products with an additional function during service

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Classification due to product type

Refractory

Monolithics

(unshaped)

Castables

Dense Insulating

Mortars

Heat-insulating

Bricks

(shaped)Functional products

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Classification due to bond type

and cement content Dense

castable

Hydraulic

bond

Conventional

Traditional castables Gunning

‘high technology’

Low cement

1.0-2,5%

CaO

Ultra low cement

< 1.0% CaO

No cement

< 0.2% CaO

Chemical

bond

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Classification according to type of chemical composition

Focus on Alumina-Silicate refractory products based on:• Al2O3 (alumina)

• SiO2 (sillica)

The variants of alumina-silica castables are formulated by:

• Different amounts of Al2O3 and SiO2 in raw materials

• Different crystal structures of raw materials

• Different properties of raw materials

• Grain size distribution and matrix products

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Composition of dense refractory castables

Aggregates (> 45 μm) 65-75%• Croase; build the foundation of the castable • Fine; fill the voids between the croase aggregates

Matrix products (< 45 μm) 25-35%• Fill the micron size voids, affects workability and strength• Binders; hold castable together until thermal sintering occurs• Additives; reduces water or control setting time

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Raw materials characterization

• Chamotte (calcined fireclay): 30-45% Al2O3 Alkali resistance

• Andalusite: 50-60% Al2O3 Shock resistance

• Bauxite: 80-90% Al2O3 Abrasion resistance

• Corrundum (alumina): 99% Al2O3 High refractoriness

• Syntetic mullite (calcined bauxitic clay): 60-80% Al2O3 Strength

• Silicon carbide: SiC High thermal conductivity

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High quality refractory raw materials:

Are subjected to Calcination (minerals fired at 1400-2000°C)• Reduces the amount of impurities• Low porosity• High density• Chemically stable• Increased refractoriness

Low iron (Fe2O3) content (minimize the risk of CO-attack)

Low alkali content (minimize the formation of low-melting-temp. glasses)

Be aware of recycled materials!

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Summary• Refractory materials can be classified according to e.g. type, cement

content, application/installation technique and the main raw material• We will focus on Alumina-Silicate castables• Dense castables contains approx. 70% aggregates and 30% matrix

products• The variants of alumina-silicate castables are achieved by a diversity

of different raw materials with different properties• Both natural minerals and calcined minerals are used• Fractions (and optimal particle distribution) are obtained by crushing,

sieving and grinding/milling • High quality raw materials performs better!