Short notes on Refractory

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General:-

The brickwork in burning zones of rotary kilns normally consists of
magnesite. .ie., basic bricks. In order to meet the increased demands on
the refractory lining, test methods were developed from the samples
received from the kilns to find out the materials that cause the wear. The
wear to the magnesite bricks can be attributed to continuous and
discontinuous causes.

Continuous wear:

The continuous wear component covers attrition by the material being
burnt, initially in the form of limestone and after calcinations as quick lime.
Because of the long service lives of the refractory linings abrasion is now
one of the most important wear factors in continuously operated kilns. As
limestone has considerably more abrasive action than quicklime the
abrasion occurs to a greater extent in the pre-heating zone and at the start
of the burning zone.

Dis-Continuous wear:

Discontinuous wear in the form of spalling is observed mainly in kilns that
are operated in campaigns. When there is NO coating over the refractory,
these bricks will be exposed to a very high temperature of the order of
1200 to 1400 * C for long periods. This thermally overstresses the brick’s
microstructure which then loses its flexibility, i.e., recrystalization with
grain growth causes the fine magnesia particles contained in the brick to
coalesce into coarser particles.

Chemical Effects:

The changes in the brick’s microstructure caused by thermal effects are
often accompanied by migration of material which sometimes causes
fundamental changes in the brick chemistry. Al2O3, SiO2, and Fe2O3 prove
to be mobile oxides which are carried away from the parts of the brick
near the hot face. The thermal and chemical changes produce
embitterment in the brick’s microstructure so that the hot face of the brick
tends to spall when the operation is interrupted.

Because of their comparatively high levels of Al2O3 , spinel bricks exhibit
intensified contact reactions with CaO at elevated temperature. These
reactions either remove some of the bricks spinel content with the kiln
feed or infiltrate into the microstructure of the brick as low melting calcium
aluminates. Both the cases have a significantly detrimental effect on the
stress-relieving ability of the brick’s microstructure. For this reason the
spinel bricks now normally used in the cement industry with their high
levels of Al2O3 are of only limited suitability for use.

Basic Requirements of bricks for use in cement:

The following are the basic requirements of chrome-free bricks for use in
cement kilns:

1 Abrasion resistance
2 Hot strength
3 Thermal shock resistance
4 Smoothest possible brick surface

Abrasion Resistance:

In principle, a high abrasion resistance can be achieved in two ways
– by a high firing temperature with heavy sintering of the brick
Components associated with high strength.
– by producing a brick matrix which has a high fracture toughness
and at the same time is elastic.
For use in the burning zone , preference is given to the second type of
brick mentioned .This matrix is made of the high melting silicate phase
forsterite ( Mg2SiO4) and spinel ( MgAl2O4). Its toughness prevents the
coarse grains from being broken out by the passing kiln feed and it is not
itself abraded because of the effect of the spinel.

Hot Strength :

At the operating temperatures of 1200 to 1400 *C normally found in a kiln
the association of the periclase , forsterite and spinel phases also exhibits
a significantly superior hot strength to other conventional types.
The recent developments move towards the low-iron materials based on
natural sintered magnesia in order to increase the hot strength still further
and ensure higher operational reliability.

Thermal Shock Resistance :

The thermal shock resistance in chrome -free bricks is achieved primarily
by the spinel components. Bricks for the cement industry often contain
pre-prepared spinel for this purpose. In normal coke fired kilns movements
of the burning zone can also occur which are difficult to control and are
accompanied by fluctuations in temperature.

Surface Smoothness :

Spalling of the refractory lining leads to the formation of “pockets” in the
brickwork which prevent the column of lime from sinking uniformly and , in
the worst case, can even lead to bridging. The forsterite-spinel matrix in
the above mentioned magnesite bricks particularly seals the brick surface
during operation and prevents any deep penetration of foreign phases and
brick densification which leads to spalling. The surface of this brick
therefore remains comparatively smooth during its entire service life and
allows the kiln to operate uniformly.

Conclusion :

The forsterite-bonded , chrome –free, magnesite brick with low Al2O3
content has been found successful in cement industries but has limitations
in usage inspite of higher hot strength. This was eliminated in the case of
chrome-free, high grade spinel bricks which have been adapted very
advantageously to the specific operating conditions.
Out of the properties such as Thermal shock , Abrasion , Overheating ,
Alkali attack , Sulphate attack and lime infiltration , the magnesia –
chromite brick is by far superior in overheating only. In addition to these
advantages the used bricks removed from the kiln can be disposed off
without problems in the case of chrome-free , forsterite & spinel bonded
brick.

Acidic and Basic bricks – Damage mechanisms;

Chemical Damage:

Liquid Infiltration (Clinker / Coal ash)
 Lower porosity due to penetration
 Change of colour
 Change of mineralogy
 Increase in CaO , SiO2 , Al2O3 , Fe2O3

Alkali Condensation

 Increase in Na2O , K2O , Cl , SO3 levels
 Porosity increase
 Presence of light colored layers
 Alkali bursting ( Al2O3 Refractories )
 Change of mineralogy
CO2 / SO3 Attack ( On CaO containing compounds )
 Decrease of porosity
 Presence of white colored layers
 Increase in CO2 , SO3
 Change of mineralogy

Thermal Damage:

Over Heating
 Melting of hot face
 Change in type of porosity ( Increase / Decrease )
 Elongation of periclase crystals
 Reduction of Chrome content
 Diffusion of low melting silicates , Aluminates
 Change of color
Shock
 Spalling of hot face

Mechanical Damage:

Kiln Deformation / Thermal expansion
 Porosity unchanged
 Chemistry / mineralogy unchanged
 Spalling over wide areas
 Crack formation on micro to micro scale