Refractory in Cement Kiln -II

Question: What is the Functions of Refractory Lining?

·       Protection of the kiln shell and tires from overheating and subsequent deformation

·       Protection of the kiln shell from chemical attack

·       Protection of the kiln shell from mechanical abrasion

·       Thermal insulation

·       Increase of heat exchange

·       Control of material flow

·       Control of coating behavior

Question: What is the Refractory associated costs in Cement Plant?

2 to 4% of cement production costs.

Question: What Factors influencing lining durability?

·       Process type

·       Fluctuations in operating conditions

·       Selection and quality of refractory materials. 

·       Quality of installation, mechanical condition of kiln

·       Heating-up and cooling down procedures

·       Raw materials and burnability

Question: How to select calcination zone refractory?

The length of the calcining zone in long kilns is 6 to 9 kiln diameters.

Bricks in this area are mainly subjected to:

Thermal Shock

Abrasion

Alkali Attack

Fireclay bricks (for normal conditions)

Possess high thermal shock and abrasion resistance

Contain 30 to 45% Al2O3

Maximum operating temperature of 1200oC

Acid fireclay / light weight bricks (for heavy alkali attack)

Have an Al2O3 content under 30 %

Provide better insulation (light weight bricks)

React with alkalis to form a protective vitreous surface layer of 2 to 3 mm thickness

Question: How to select transition zone refractory?

The transition zone is located on both sides of the sintering zone, since the length of the sintering zone varies with process fluctuations, the transition zone is characterized by unstable coating formation. Usually, the upper transition zone is further subdivided into:

         Cold section - called the safety zone with a length of approx. 2 kiln diameters

         Hot section - called the proper transition zone with a length of approx. 2 to 4 kiln diameters.

·    Transition zone refractories are usually exposed to higher thermal loads than the sintering zone 

·    No or only unstable coating is formed in this zone.

·    Bricks are exposed directly to the flame radiation and to temperature variations

Usually lined with:

Magnesia-spinel bricks, usually based on synthetic magnesia sinter

Magnesia-chrome bricks are not recommended due to their inferior thermal and environmental properties.

Question: How to select Sintering / Burning zone refractory?

Maximum material temperature in the sintering zone is in the order of 1400 to 1500°C, located at the beginning of the cooling zone.

The length of the sintering zone is generally between 3 to 5 kiln diameters, depending largely on the shape of the flame and the type of fuel.

Coal flames generally give short, oil flames medium and gas flames long sintering zones.

The ‘sintering’ zone is covered by a stable coating made up of clinker with solidified liquid phase

The ‘sintering zone’ is the zone in which the main formation of C3S occurs.  

The bricks in the sintering zone are exposed to

Chemical attack by the clinker liquid phase

Infiltration by alkali sulphates and chlorides

High temperatures 

High thermal shock (loss of coating)

Basic brick types are used due to their high resistance against chemical attack

Magnesia-spinel bricks

Dolomite bricks

When selecting magnesia-spinel bricks, qualities particularly developed to improve coating adhesion should be chosen (based on natural magnesia sinter)

Dolomite bricks have the best coating adhesion but must be restricted to zones with permanent coating formation

The price for dolomite bricks is approx. 60% of that of magnesia based bricks.

Dolomite bricks are very sensitivity to moisture; magnesia-spinel types are also sensitive but to a lesser extent. For longer storage times, these brick types must be stored air-tight (Aluvac).  Dolomite bricks may be wax impregnated

During long kiln stops, the dolomite lining should be protected against moisture by isolating the section with plastic barriers and by putting a hygroscopic agent to absorb atmospheric water (e.g. burned lime).

Question: How to select Cooling zone refractory?

In this zone the clinker is cooled down from its maximum burning temperature of 1400 to 1500°C to approx. 1350oC in kilns equipped with a grate & CB cooler

The cooling zone length in grate cooler kilns is 0.5 to 1.5 kiln diameters

Dense high alumina bricks containing 80% Al2O3 are preferred.

Basic bricks are used only when severe chemical attack is encountered 

As the outlet zone is not coated, the kiln shell temperature is increased in this area, especially when basic bricks (with their higher thermal conductivity) are installed

Question: Kiln Nose Rings Refractory Selection?

The end of the cooling zone, the nose ring, is one of the most critical points of cement kiln linings. Its lifetime is nose ring is often only 6-9 months: lower than in the sintering zone

High alumina bricks (>90%) are normally placed within the nose ring section

Magnesia-spinel bricks are preferred when severe thermal and chemical attack is encountered

The casting segments should be air cooled. Requirement is about 1200 m3/h,m at 20 mbar

At secondary air temperatures of more than 1050 °C castable nose ring designs are recommended

Question: What is Requirements for High Quality Refractory Lining?

Complete removal of worn lining sections

Replacement of uneven shell sections

Tight installation of bricks (brick joints)

Good dimensional and geometrical accuracy of bricks

 Alignment with mortar; not steel shims

Correct compensation for thermal expansion

Correct closing of rings (use of correctly spaced key bricks)

Selection of suitable mortar for mortar joints.

Selection of the optimum lining method, so that kiln rotation in the cold state is minimized.

Question: How Lining of Static Sections carried out?

To achieve good insulating properties two or three layer linings are normally installed

Three layer insulating lining consists of:

• A backing layer of ceramic fibre board,
• An intermediate layer of insulting firebricks
• A dense working lining

To achieve gas tight linings, the working lining is laid with a chemically setting mortar

The working lining consists of abrasion resistant fireclay bricks with low porosity

Acidic firebricks with an Al2O3 content below 30% are installed when alkali attack is present

Preheater Cyclones

Are lined with bricks and an insulating back-lining

Complicated shaped parts such as the cyclone inlet and control openings are lined with unshaped materials (Castable)

Meal Chutes

Meal chutes should be lined with highly abrasion resistant materials.

Usually dense castables with metallic anchors or prefabricated materials are installed

Question: How to Heating-Up of Rotary Kiln Systems?

The kiln heating-up schedule determines the kiln temperature increase per unit of time and the timing of raw material feed. This schedule must consider the following main factors:

• Refractory type and age
• Kiln system design                                                                      
• Mechanical systems

The optimum heating schedule constitutes a compromise which satisfies the various requirements

The minimum heat-up period is indicated by the material which requires the longest heating period.

Heating specifications from the suppliers must be respected

The recommended heating schedule for the start-up of a new kiln or after a major overhaul is:

Maximum heat-up rate of 25°C/hr in the sintering zone up to a sintering zone temperature of 900°C

After reaching 900°C heating can continue at a rate of 50°C/hr up to the working temperature

The recommended heating schedule after minor repairs and not caused by refractory failure is:

50°C/hr up to the working temperature