The primary function of refractory bricks in a rotary kiln is to protect the kiln shell from damage caused by high-temperature gases and materials, ensuring normal production. In industrial production, the service life of refractory firebricks in the firing zone is very short, often leading to unplanned kiln downtime and maintenance. This is a key factor affecting the high quality, high production, low energy consumption, and annual operating rate of cement kilns.
Slag resistance refers to the ability of refractory materials to resist chemical attack. It is extremely important during the formation of the initial kiln lining layer and when high viscosity or localized high temperatures cause the kiln lining to fall off.
Porosity and thermal conductivity play a crucial role in forming the initial kiln lining layer. In the event of localized kiln lining loss, refractory materials with higher porosity and thermal conductivity can facilitate timely repair of the kiln lining. However, they can also be extremely destructive, causing the thin layer of refractory bricks to fall off.
During the production process, the physical and chemical changes of refractories bricks generally do not reach equilibrium at the firing temperature. Some refractory fire bricks are not fully fired. Consequently, when subjected to high temperatures in a rotary kiln, most refractory firebricks undergo irreversible reburning shrinkage due to the generation of a liquid phase and the filling of pores. Therefore, high-temperature volume stability must be considered when selecting refractories bricks for the firing zone.
Hot surface delamination is the primary form of damage to the lining of a rotary kiln's firing zone after thermal shock. If this occurs simultaneously with localized lining shedding, the service life of the refractory bricks will be significantly shortened.
When coal is used as a fuel, its volatile matter and ash content play a decisive role, directly influencing the flame shape. Pulverized coal with a high volatile matter content and a low ash content can shorten the black flame head, resulting in a low-temperature, long-flame calcination. This generally protects the kiln lining. However, excessively high volatile matter content can lead to rapid ignition, resulting in clinker temperatures exceeding 260°C and secondary air temperatures exceeding 900°C. This can easily damage the nozzles, causing them to deform or break, resulting in gaps and a distorted flame shape, damaging the kiln lining before it can be replaced.
If the volatile matter of coal is too low and the ash content is too high (greater than 28%), a large amount of incomplete combustion of pulverized coal will settle within the material, burning and releasing a large amount of heat, which can also damage the kiln lining. Fuel nozzle structure is often not given sufficient attention in production. Nozzle shape and outlet size primarily affect the degree of mixing of pulverized coal with the primary air and the discharge velocity. Sometimes, fins are added to the nozzle to enhance air-coal mixing, but be careful not to over-rotate the swirl air, which can damage the kiln lining.
When the aluminum content is too high and the liquid viscosity is high, the kiln lining can collapse significantly, making it difficult to control and detrimental to the protection of the kiln lining. In production practice, the aluminum content is generally controlled between 1.3 and 1.6. When a high saturation ratio, high silicon content, and low liquid content are used, sticky bulk material can easily scour the kiln lining, abrading the kiln lining and causing severe damage. In production practice, when the silicon content is 2.5, the saturation ratio should not exceed 0.92, and when the silicon content is 2.8, the saturation ratio should not exceed 0.90. Fluctuations in raw meal feed rate can significantly damage the kiln lining. When the kiln is overloaded with raw material, the exhaust air volume at the kiln tail must be reduced, and the amount of pulverized coal used must be increased to force the fire. This rapidly increases the heat load in the firing zone and severely damages the kiln lining.
When the kiln is underloaded with raw material, the pulverized coal flame significantly tilts downward, causing the kiln lining in this area to peel off and thin out under the high temperature, striking the thinner layer of raw material. If the air volume and coal usage are not adjusted promptly, the kiln lining and refractory bricks can be easily damaged.
In addition, fluctuations in the raw material feed rate can lead to unstable thermal systems and excessive temperatures in the kiln, causing the kiln lining to peel or become damaged. Therefore, when the temperature of the discharged clinker reaches over 1260°C and the secondary air temperature exceeds 900°C, the nozzles are easily burned, deforming or cracking, resulting in a chaotic flame pattern and extremely damaging the kiln lining. The three rates of clinker are generally controlled at KH 0.91±0.01, silicon rate 2.6±0.1, and aluminum rate between 1.3~1.6, which is extremely beneficial to protecting the service life of refractory bricks and improving the strength of clinker.
