Though refractory brick lining is installed with a purpose to protect the metallic shell of the kiln but often it is practised to have a good layer of coating on the brick lining. A majority of experts attribute it to the skills of a kiln operator.
A good protective coating on the refractory lining in the burning zone is always preferred to prolong the life of a refractory. Replacement of refractory bricks costs a large amount of money, loss in production while the kiln is down for lining replacement. In short replacement of refractory is an undesirable condition in any kiln.
Although refractories in the burning zone have to be replaced from time to time, a kiln operator has the capacity to increase or decrease the life of the lining by his ability to control the coating in the burning zone.
Types of coatings
Coating is a mass of clinker or dust particles that sticks to the wall of the kiln, having changed from a liquid or semiliquid to a solidified state. The solidified particles stick to the surface of the coating (CS in Fig. 1), or the refractory surface (BS) when no coating exists, as long as the temperature of the surface of the coating is below the solidifying temperature of the particles. Coating continues to form until its surface reaches this solidifying temperature. When the kiln operates under such condition at equilibrium, the coating will maintain itself.
There is a temperature drop between the coating surface (CS) and the kiln shell (KS), the heat flowing in the direction indicated by the arrow (Fig. 1) (Heat always travels from a place or body of high temperature to a place of body of lower temperature). This heat transfer is governed to a great extent by the conductivity of the refractory and the coating. The better the conductivity of the refractory, the better the chance of coating formation, explained by the fact that the more heat that travels in the direction of the arrow. A kiln feed with a high liquid content at clinkering temperatures is more effective in coating formation than a feed low in liquid.
Kiln feeds with a high liquid phase (easy-burning mixes) have a high content of fluxes: the iron, alu¡mina, magnesia, and alkalis. On the other hand, hard-burning mixes (low in iron, alumina, magnesia, and alkalis, and high in silica and lime) do not have a favourable influence on coating formation. Alkalis entrained in the gas stream promote the formation of coating and rings also because of their high fluxing characteristics.
Because the surface temperature is probably the most important factor in the formation of a coating, it is obvious that the flame itself has a significant effect on coating formation because the shape of the flame directly governs the surface temperature at any given point in the burning zone. A flame that is too short, snappy, and wide can erode the coating because of the great heat released over a short area with this kind of flame. A long flame is more favourable to coating formation inòthe burning zone. It is observed that short flames are desirable for better control of the burning operation, but the flame should be shortened only to the extent that it will not harm the coating.
Once we ensure all favourable factors for good coating formation, it is then up to the kiln operator to control the coating during operation of the kiln. It is his responsibility to form and maintain a good solid coating in the burning zone.
Operating conditions are just as important for coating formation as all the other factors mentioned above. Assume that a kiln will be operated from one extreme of temperature to the other, that is, a cold, a normal, and a badly overheated kiln; that the same kiln-feed composition is burned in all three examples; that the solidifying temperature is around 1,3000 C; and that 24 per cent liquid is formed at the point of investigation, under ideal operating condition.
First, consider the cold kiln (Fig. 2). In this case almost no coating is formed. The coating surface temperature as well as the feed temperature is too low to produce the necessary amount of liquid matter that would promote coating formation. The condition in this example is commonly referred to by kiln operators as the kiln being in a 'hole.' This example also supports the widely known fact that no new coating can be formed while the kiln is cold.
In the normal kiln (Fig. 2), enough liquid (24 per cent) is present to form a coating. Temperature of the coating when it emerges from the feed bed, as well as when in contact with the feed, is below the solidifying temperature of the feed particles. The particles will adhere to the wall and solidify, and will continue to do so as long as the surface temperature of the coating remains below the solidifying temperature of 2,4000 F (1,3150 C). Whenever the wall reaches this temperature no new coating will form. The coating is in equilibrium.
In the hot kiln (Fig. 2), because of the extremely high temperatures of the feed and the coating, too much liquid is formed. As all temperatures are above the solidifying temperature, the coating transforms from a solid back to a liquid again. In such a condition, coating will come off, and the feed because of its high liquid content will 'ball up.' Needless to say, this condition is extremely harmful to the kiln and to the refractory.
Most basic refractories, are not able to withstand prolonged exposure to the high flame temperatures without this protective coating. As was mentioned in the previous chapter, the burning zone is divided into three subzones namely the upper-transition 1 the sintering, and the lower-transition zones. Because of the lower liquid content in the feed and because of the frequent temperature changes, the upper- and lower-transition zones are areas where formation and maintenance of coating is the most unstable.
Shifting burning zone locations produce a similar shift in the location where coating is formed; thus, unstable coating conditions are most frequently observed in the upper and lower end of the burning zone. This is clearly supported by the fact that most rotary kilns experience the most frequent refractory failures in these two critical areas. It should be noted that since the upper and lower burning zones are also within the vicinity of the first and second tires, brick failures are not only the result of variations in burning-zone conditions but, are also often the direct result of excessive tire clearance and shell ovality. Both the frequent falling out of coatings in these areas and the formation of too much coating can lead to troublesome ring formations.
Ring formations in the lower-transition zone (i.e., at the kiln discharge) are referred to as nose rings. Others refer to these as ash rings when the kiln is coal fired. Ring formations in the upper-transition zone are referred to as clinker rings. These ring formations can in many instances be so severe that they force operators to shut down the kiln and shoot these rings out with an industrial gun.
The possible causes are many and no one single factor has yet been found that would be the main cause for all the rings formed. What seems to be true for one particular kiln might be completely wrong for another kiln. On many coal-fired kilns, operators have found a relation¡ship between the fusion temperature of the coal ash and the frequency of ring formation. There appears to be more ring formation when the fusion temperature is low, i.e., when the ash contains larger amounts of fluxing iron and alumina and less silica. However, this could not be the only cause for such ring formations because natural gas- and oil-fired kilns, which have no ash deposits in the burning zone, can have just as many ring problems as the coal-fired kilns.
Hence, solutions for the elimination of rings in the burning zone are predominantly found by a process of elimination. First, all probable causes are listed and then each suspected cause is eliminated or changed until hopefully an answer is found.
It is of interest that half of the causes of ring formation can be somehow controlled by the kiln operator and action taken to stabilise the flame and the kiln operation that might be beneficial in lessening the frequency of ring formation.
Less frequent but nevertheless equally troublesome are the so-called feed rings that form in the calcining zone of the rotary kiln. It has been found that the majority of these rings and heavy coatings in kilns are associated with one of the following factors: