Chapter 9: Kiln Protective Shield

Refractory is a ceramic material used in lining furnaces, kilns, fireboxes, and fireplaces. Due to the high temperature process the rotary cement kilns have a metal shell internally lined with refractory materials.

Talking about refractories is a long topic. In this article we will not discuss all the details about refractory materials.

What is a refractory lining used?

Refractories fulfill two basics functions:

1) Refractoriness – Protect kiln shell (ferrule) of thermal and chemical attacks that are dangerous to maintain metallic resistance.

2) Conductivity – Reduce heat insulation or thermal energy of the kiln metal plate, preheaters, ducts, and coolers.

Specifically, inside the kiln, refractory linings protect the rotary kiln shell from:

1) The flame (format, temperature, ashes, atmosphere, fuel),

2) The fired material (solid phase, liquid phase, coating, abrasive grains, volatile elements),

3) Deformations (temporary or permanent deformations of the ferrule due to the rotation of the kiln).

So, we can summarize that the refractory lining protects the kiln shell from thermal, mechanical, and chemical stresses.

What are the characteristics of an efficient refractory lining?

We mention the most relevant:

1) Refractoriness (insulation and thermal protection)

2) Chemical resistance (abrasion, reactions, and combustion)

3) Cold mechanical resistance (handling and transport)

4) Hot mechanical resistance (abrasion and wear resistance)

5) Thermal stability (temperature variation, contraction, and expansion)

6) Thermal conductivity (reduce thermal energy insulation, facilitate coating)

7) Excellent return on investment (complete its life cycle)

What types of refractories exist?

Modeled refractories: bricks and blocks with specific shapes and measurements.

Moldable refractories: mortars, concrete, shotcrete mixes, etc.

How is the most suitable type of refractory lining determined?

It is common to choose the type of refractory material to be used based on the process characteristics, lining features, and the geometry of the metallic component.

Refractory Lining Zones

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About Refractory Bricks

Whether patterned or castable, refractory linings are of varied chemical compositions. We will mention the most common refractory bricks used to line and protect the rotary kiln shell.

Silica Bricks Features

• They contain 60%-65% silica (SiO2)
• High porosity, high insulation
• High thermal conductivity
• Good thermal shock resistance
• Low dilation
• Sensitive to alkali attacks
• Volatile elements penetrate into the pores, causing alkali dilation and brick fragmentation

Installation Zone

Installed only in the calcination zone (kiln entrance). If there are alkalis in raw meal, it is recommended to install aluminous bricks in this zone.

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Alumina Bricks Features

• They contain 35%-85% alumina (Al2O3), depending on the composition.
• More alumina content, more refractoriness
• Medium porosity, low thermal conductivity, low insulation
• Low dilation
• High mechanical resistance
• Sensitive to K2O

Installation Zone

They are installed in the calcination zone, safety zone, and upper transition zone. It reacts with raw food forming liquid C4AF and C3A. For that reason, this brick should not be installed in the sintering zone.

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Magnesite Bricks Features

They contain:

1. Magnesian chrome 60%-85% MgO + 4%-15% Cr2O3
2. Magnesium spinel approx. 88% MgO + 12% Al2O3

• High resistance to thermal shock and abrasion
• Low porosity, high thermal conductivity, low insulation (does not favor coating)
• High expansion (need expansion joints)
• Magnesian chrome bricks sensitive to chemical attacks and atmospheres

Additionally, we will comment that there are three classes of spinel magnesium bricks:

1st Class: Contains 7% Al2O3

2nd Class: Contains 10% alumina

3rd Class: High porosity favors the formation of coating

Some magnesium spinel bricks are enriched with zirconium to improve thermal shock resistance. Operating cost criteria determine the choice of this type of brick.

Installation Zone

Magnesian chrome bricks are installed in the upper transition zone, sintering zone, and lower transition zone. Magnesium spinel bricks are installed in the upper and lower transition zones. Due to their high thermal conductivity, these bricks should not be installed between riding rings (tires).

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Dolomitic Bricks Features

• They contain 95%-98% lime conjoint with magnesium
• High mechanical resistance (enriched with zirconium) and thermal shock
• High porosity, favors coating formation by reacting with raw food (forming C3S from C2S)
• They are sensitive to the moisture contained in the magnesium. It can swell and burst the brick.

Installation Zone

Dolomitic bricks are installed in the sintering zone.

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Summary

Obviously, for the choice of the most suitable refractory brick for each area, the physical and chemical characteristics of the raw meal and fuel, the operating costs and the recommendations of the refractory lining supplier are considered.

I wait for you in the next article The Cement Newsletter...!!

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