REFRACTORIES (PART 1) General Introduction

Through this article we're going to arrange a short introduction about refractories that can find application in different environments such as steel, cement, aluminum, glass productions and so on.

1. DEFINITION

In general, we can call "refractories" any INORGANIC NON METALLIC MATERIAL which can withstand high temperature (> 600 °C) without undergoing physic-chemical changes while remaining in contact with elements such as molten slag, metals and gases.

2. GENERAL REQUIREMENTS

Generally, refractories require the following abilities:

• Ability to WITHSTAND HIGH TEMPERATURE;
• Ability to WITHSTAND sudden CHANGES OF TEMPERATURE;
• Ability to WITHSTAND LOAD at service conditions;
• Ability to RESIST at the CONTAMINATION of the material with which it comes into contact;
• Ability to MAINTAIN SUFFICIENT DIMENSIONAL STABILITY at high temperature and after/during repeated thermal cycling;
• Ability to CONSERVE HEAT;

3. GENERAL CLASSIFICATION

Refractories can be classified in different ways as indicated here below:

3.1 CLASSIFICATION BY PHYSICAL FORM

• SHAPED REFRACTORIES, those which have fixed shaped (like bricks);
• UNSHAPED REFRACTORIES, they are without definitive form and are only obtaining shape upon application. They're also known as monolithics refractories.

3.2 CLASSIFICATION BY ALKALINITY

• ACIDIC REFRACTORIES, are those which are attacked by alkalis (like basic slag). They're normally used in areas where slag and atmosphere are acidic. They're normally composed by SiO2 and Al2O3 oxides. Examples are Alumina, Silica and Fireclay (mix of SiO2 and Al2O3 oxides);
• NEUTRAL REFRACTORIES, they are chemically stable to both acids and bases and are used in areas where slag and atmosphere are either acidic or basic. They're normally composed by Al2O3, ZrO2 and Cr2O3 oxides. Examples are Silicon Carbide, Zirconia, Chromite, Carbon / Graphite;
• BASIC REFRACTORIES, Are those which are attacked by acid slags but stable against alkaline slag. They're normally composed by CaO and MgO oxides. Examples are Dolomite and Magnesite.

3.3 CLASSIFICATION BY CHEMICAL COMPOSITION

• SINGLE OXIDE REFRACTORIES, Made up of single oxide (such as Alumina or Silica);
• MIXED OXIDE REFRACTORIES, Made up of more than one oxide (such as Fireclay or Dolomite);
• NON OXIDE REFRACTORIES; Made up of non oxides (such as Carbon / Graphite, Carbides SiC).

3.4 CLASSIFICATION BY FUSION TEMPERATURE

• EXTRA HIGH FUSION TEMPERATURE REFRACTORIES, When melting is starting at temperatures more than 2.000 °C;
• HIGH FUSION TEMPERATURE REFRACTORIES, When melting is starting at temperatures between 1.750 and 2.000 °C;
• NORMAL FUSION TEMPERATURE REFRACTORIES, When melting is starting at temperatures less than 1.750 °C.

4. OXIDES - MAIN PROPERTIES

As indicated before refractories are generally composed by different oxides. Here below we can see a table with the most common oxides and their properties:

5. TYPES OF REFRACTORIES

There're many types of refractories classified based on the chemical composition as indicated in the list here below:

• SILICA REFRACTORIES, content of SiO2 is normally at least 90%;
• SILICA - ALUMINA REFRACTORIES (Fireclay), content of Al2O3 is normally less than 40%;
• ALUMINA REFRACTORIES, in this case the content of Al2O3 is above 40%. We can have low, medium or high alumina refractories (until 80-90% Al2O3);
• MAGNESITE REFRACTORIES, Content of MgO oxide is generally more than 85%; Usually they can be burned or carbon bonded (MgO-C);
• DOLOMITE REFRACTORIES, they're a mix of CaO and MgO oxides. Usually they can be burned or carbon bonded (DOL-C);
• CHROMITE REFRACTORIES, they're a mix of Cr2O3 and MgO oxides. We can have a Chrome - Magnesite refractories (where Cr2O3 is 18 - 35% and MgO is 42 - 50%) or a Magnesite - Chrome refractories (where Cr2O3 is 8 - 18% and MgO is > 60%);
• CARBON REFRACTORIES where Carbon is the principal component;
• ZIRCONIA REFRACTORIES that contain ZrO2 oxide;

6. APPLICATIONS

Refractory products find many applications in various industries such as:

• IRON & STEEL INDUSTRIES;
• CEMENT & LIME INDUSTRIES;
• NON FERROUS METALS INDUSTRIES;
• ENVIRO - ENERGY - CHEMISTRY INDUSTRIES;
• GLASS INDUSTRIES;
• CERAMIC INDUSTRIES;
• OTHER INDUSTRIES;

Here below it shown a graph with the consumption of refractories by the end market:

In the next article we'll explain the main properties of refractories. Thank you for your attention!

Author: Eng. Matteo Sporchia 

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