The very nature of Portland Cement is basic and hence the choice of kiln refractory for Rotary Cement Kiln is confined to mostly aluminous fireclay bricks or to basic refractory bricks.
Cement manufacturing process in a rotary kiln is such that according to temperature gradient in the kiln and the condition of raw materials inside, the kiln may be divided broadly into three zones. Properties and type of refractories for rotary cement kiln will depend on the conditions they will be exposed to in a particular zone of the kiln where it is to be used.
Resistance to High Temperature
The refractory has to withstand the temperatures that can prevail under adverse conditions as well as under normal conditions in the zone where it is being used. Not only does it require the ability to withstand high temperatures without melting (high PCE or refractoriness), it must also retain its shape & structure, maintain its structural strength below its melting point (HMOR - hot modulus of rupture), maintain a constant volume when exposed for prolonged times too high temperatures.
Spalling Resistance
Any shutdown, start-up, severe operating upset of rotary kiln is usually accompanied by large temperature changes in the kiln, and the refractory must possess the necessary thermal shock resistance to withstand such temperature variations. Failure to possess this property may cause the development of cracks, referred to as Spalling, horizontally along the refractory bricks. Spalling is a result of thermal shock and so, when a cold rotary kiln or any kiln is fired, the temperature must be raised slowly maintaining a time-temperature graph to avoid spalling.
Resistance to Chemical Attack
During the process of linearizing, ash, slag, and vapors formed during the combustion process can attack the refractory, reacting chemically with brick, depending on the type of fuel used. Furthermore, dust and alkalies entrained in the kiln gases can adhere to the refractory lining in the burning zone and react with the refractory. Refractories that do not have resistance to these chemical attacks will result in premature refractory failure.
Abrasion Resistance
Conditions encountered in a rotary cement kiln make it necessary that the refractory withstand the abrasive action resulting from the sliding kiln feed bed and also by dust entrained in the moving gas stream. This abrasion resistance is a prerequisite for all refractory bricks installed in front of and behind the burning zone where coating is not usually formed.
Coatability
One of the most important properties of refractories for the burning zone of rotary cement kiln where the highest temperatures exist is its ability to take on a good coating and to hold this coating for a prolonged length of time. Just as the refractory acts as a protection for the kiln-shell, so the coating, in turn, acts as a protection for the refractory, thus serving to prolong the campaign-life of the refractory lining in the burning zone of the cement kiln.
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