Short Notes on Kiln Refractory

Short Notes on Cement Kiln Refractory
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  Short notes on Refractory: 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. Al 2 O 3 , SiO 2 , and Fe 2 O 3 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 Al 2 O 3 , 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 Al 2 O 3  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 f  orsterite  ( Mg 2 SiO 4 ) and spinel  ( MgAl 2 O 4 ). 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 Al 2 O 3  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 , SiO 2  , Al 2 O 3 , Fe 2 O 3   Alkali Condensation   Increase in Na 2 O , K 2 O , Cl , SO 3  levels   Porosity increase   Presence of light colored layers   Alkali bursting ( Al 2 O 3  Refractories )   Change of mineralogy CO 2  / SO 3  Attack ( On CaO containing compounds )   Decrease of porosity   Presence of white colored layers   Increase in CO 2  , SO 3     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
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