The refractory though unseen from outside, plays a crucial role during plant operation, since the materials that are used to line different vessels from inside are generally not visible. There are two different areas of application: one used during moving the equipment, and the other used in static equipment. Rotary kiln and coolers are a part of moving/rotating equipment, while preheaters and precalciner are static hardware. However lining of a kiln still remains a challenging job - both with respect to materials and application.
The availability of a rotary kiln is highly dependent on the state of the refractory lining. Depletion of the refractory lining can lead to significant failures with fallout of bricks or castable that may require shutdown of the production. Unplanned shutdowns can cause very high production losses. Refractory products are often used in harsh service environment and therefore are prone to degradation. Tolerance to temperature, mechanical loads, thermal cycling, wear and chemical resistance are some of the common causes of degradation. Generally, the lining of kiln lasts for a period of minimum 10-11 months but there can be a few exceptions, subject to the operating conditions and other effects.
It has been experienced that however good the material may be but if not applied correctly will not give satisfactory results. This is more so in case of castable and other type of monoliths. The application of refractory is more of a civil engineering job, and therefore many plants employ civil engineers as refractory engineers. Over a period of time, many changes have happened in the materials of linings. Multi-location large plants have standardised the shapes so as to have minimum inventory. Earlier the use of alumina bricks was common because of easy availability. Later it was changed to mag-chrome bricks, which were found environmentally hazardous. Therefore slowly the mag-chrome bricks have been phased out and have been replaced with magnesite bricks. Though in Europe and the US these have been banned long ago.
Today in the industry, the trend is to source magnesite bricks from 3-4 worldwide suppliers. These bricks mainly go in the burning zone of a kiln. Alumina bricks still find their use in other part of the kiln and also in the coolers and preheaters and precalciner zone. Talking about the lining job itself, as stated in the article of Bricking Solutions, there have been a lot of improvement in the technique including that of the safety measures. It is very much appreciable that a number of accidents during lining jobs have gone down considerably because of availability of suitable devices and safety gadgets.
Apart from bricks, monolithic refractories like castable and plastic materials are also used at different locations. Generally, plastic refractory materials are used in repairing jobs, where without stopping the equipment, a job can be carried out, whereas in case of castable there has been a wide range of materials that available depending on the location of application. The important aspect of castable is the application technique, which is very important to follow. Here the role of the application engineer plays a significant role for the success of material. In case of low cement or cement-less castable, the water addition beyond a limit can create havoc and will never give the desired results. It is better to hire an expert agency to carryout the job.
In case of lining job also, there are mechanised ways of doing the job through brick laying machines. These machines are also used for removal of old lining from the kiln, which saves a considerable time. There are safety cages and movable platforms for use which makes the job a lot easier.
Actually the performance of lining depends on several factors. Assuming that the correct materials are used and right procedure is still followed, the performance can be way below the expectations.
A large number of kiln now use alternative fuels. Many of them substitute these alternative fuels through the main kiln burner as well as in the preheater/precalciner. The chemical reactions caused by fluctuations in the fuel(s) mix has a major influence on the coating and atmospheric conditions existing through the rotary kiln into the precalciner. The other factor that needs attention is the chemistry of raw meal and that of coal as fuel. Certain ingredients present in coal or in raw meal can be deleterious to the refractory lining.
The kiln lining over a period of time developes a coating on the brick lining and reduces its damage. When the lining is new, efforts are taken by plant personnel to form good uniform coating on the new lining so as to enhance its life. Premature failure of refractory lining has always been a subject of research. In situ testing and some laboratory tests can be the useful to find the root cause. The durability of the rotary kiln lining generally determines the time length of kiln operation. Whilst great efforts are made to ensure appropriate refractory material selection, installation, deployment (zoning) and the optimisation of the kiln light-up, etc., it is primarily the nature and stability of the subsequent kiln operation that influences rotary kiln lining life. Today the consumption of refractory materials is around 300-400 gm per tonne of cement produced, which is quite comparable to the world´s best figures. In deed the performance of men and materials is appreciable.