Protection of the firing zone in a rotary kiln with refractory bricks

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Protection of the firing zone in a rotary kiln using refractory bricks. Keywords: cement rotary kiln; refractory bricks; firing zone; protection. The main function of refractory bricks in a rotary kiln is to protect the kiln shell from damage caused by high-temperature gases and materials, thereby ensuring the smooth progress of production. In industrial production, the service life of refractory bricks in the firing zone is very short, often leading to unplanned shutdowns for maintenance; this is a key factor affecting the high quality, high productivity, low energy consumption, and annual operational rate of cement kilns. I. Erosion mechanism of refractory bricks: In both wet-process kilns and modern dry-process rotary kilns, during the clinker calcination process, since the temperature of the gases inside the kiln is much higher than that of the material, the kiln lining surface is subjected to periodic thermal shocks with temperature variations ranging from 150 to 250°C. This results in thermal stresses within the 10–20 mm outer layer of the kiln lining. The kiln lining also withstands the radial and axial mechanical stresses resulting from the rotation of the kiln, which cause alternating stresses on the brick structure, as well as the abrasive wear caused by the materials being calcined. Due to the simultaneous formation of silicate melts, these melts can easily interact with the surface of the refractory bricks lining the furnace at high temperatures, forming an initial layer. They also penetrate into the interior of the refractory bricks through their pores, bonding with them and causing changes in the chemical composition and phase structure within a range of 10–20 mm from the surface of the bricks, thereby reducing the technical performance of those bricks. When the sintering range of the material is narrow or rapid burning with a short flame results in localized high temperatures, the lowest temperature on the surface of the kiln lining exceeds the liquidus temperature of the material. As a result, the surface layer of the kiln lining changes from solid to liquid and falls off, and a new initial layer of the kiln lining forms as it develops from the surface inward. When this situation occurs repeatedly, the lining of the firing zone gradually becomes thinner, or even completely falls off, resulting in the local exposure of the furnace cylinder and causing the furnace to turn red. In fact, this is exactly the case for the damage to the kiln lining during firing: in the high-temperature areas, the thickness of the remaining bricks follows an arc with a relatively large radius of curvature, and sometimes the bottom of this arc lies on the inner surface of the kiln body. II. Protection of refractory bricks 1. Impact on the physical properties of refractory bricks. Slag resistance refers to the ability of refractory materials to resist chemical erosion; it is particularly important when forming the initial layer of the kiln lining, as well as in situations where the material is sticky or where local high temperatures cause the kiln lining to peel off. Porosity and thermal conductivity play an important role in the formation of the initial layer of the kiln lining; moreover, when parts of the kiln lining come off, refractory materials with higher porosity and thermal conductivity help to restore the lining promptly. But at the same time, it can also have a highly destructive effect, causing thin layers of refractory bricks to peel off. During the production of refractory bricks, their physicochemical changes generally do not reach an equilibrium state at the firing temperature. There are also refractory bricks that have not been fully fired; as a result, when exposed to high temperatures again in the rotary kiln, most of these bricks undergo irreversible remelting shrinkage due to the formation of a liquid phase within them and the filling of their pores. Therefore, high-temperature volume stability must be taken into account when selecting refractory bricks for the firing zone. Thermal surface delamination is the main form of damage to the kiln lining in the firing zone of rotary kilns due to thermal shock ； If local sloughing of the kiln lining occurs simultaneously, it will significantly shorten the service life of the refractory bricks. 　　 2. The effect of combustion and fuel nozzles on refractory bricks: When coal is used as fuel, its volatiles and ash play a decisive role, directly affecting the shape of the flame. Coal powder with a high volatile content and low ash content can shorten the black flame tip, resulting in low-temperature, long-flame calcination. It is generally beneficial for protecting the kiln lining, but excessive volatiles lead to rapid ignition, causing the temperature of the clinker exiting the kiln to exceed 260°C and the temperature of the secondary air to surpass 900°C. This can easily damage the nozzles, causing them to deform or crack, resulting in an irregular flame pattern that damages the kiln lining before those nozzles can be replaced. The volatiles content in coal is too low (less than 0%), and the ash content is too high (greater than 28%). The incomplete combustion of a large amount of coal powder leads to combustion within the material, releasing large amounts of heat that can also damage the kiln lining. The structure of fuel nozzles often does not receive sufficient attention during production. The shape of the nozzle and its outlet dimensions mainly affect the degree of mixing between coal powder and primary air, as well as the spraying speed. Sometimes, to improve the mixing of air and coal, air fins can be installed inside the nozzle; however, care must be taken to avoid excessive rotation of the swirl air, which could damage the furnace lining. 3. Impact of fluctuations in raw material composition on refractory bricks: When the aluminum content is too high and the viscosity of the liquid phase is high, large amounts of slag fall off from the kiln lining, making it difficult to control the operation process; this is detrimental to the protection of the kiln lining. In practical production, the aluminum content is generally kept between 1.3 and 1.6 ； When a high saturation ratio, high silicon content, and low liquid phase ratio are used in the formulation, it is easy for sticky materials to cause erosion of the kiln lining; this can lead to severe thinning of the kiln lining and damage to it. In practical production, when the silicon content is 2.5, the saturation ratio should not exceed 0.92, while when the silicon content is 2.8, the saturation ratio should not exceed 0.90. Fluctuations in the feed amount of raw material cause significant damage to the kiln lining. When too much material enters the kiln, it becomes necessary to reduce the exhaust volume at the kiln’s rear end and increase the amount of coal powder used in order to force a stronger burning process; this leads to a rapid increase in the thermal load on the firing zone, causing severe damage to the kiln lining. When too little material enters the kiln, the coal powder flame tilts significantly downward; as a result, the kiln lining in that area loses its thickness due to the high temperatures and spreads over the thinner layer of material. If the air volume and coal consumption are not adjusted promptly, it is very easy to damage the kiln lining and the refractory bricks. Furthermore, fluctuations in the feed rate of raw materials can lead to instability in the thermal conditions inside the kiln; excessive temperatures may cause the kiln lining to peel off or get damaged. Based on the above analysis, when the temperature of the clinker exiting the kiln exceeds 1260°C and the temperature of the secondary air is above 900°C, the nozzles are highly prone to damage; they may become deformed or cracked, resulting in an irregular flame pattern that further accelerates the wear and damage of the kiln lining. The three ratios of clinker are generally controlled at KH 0.91±0.01, silicon ratio at 2.6±0.1, and aluminum ratio between 1.3 and 1.6; this is highly beneficial for extending the service life of refractory bricks and improving the strength of clinker.