Calcination is a crucial process in limestone production, and its quality greatly influences the quality of subsequent products. The lime kiln is used for calcination of limestone , and inevitably, some problems occur during its operation, with the most common being the formation of the lime kiln ring. So, how does the kiln ring problem arise in the calcination of limestone?
Factors of Lime Kiln Ring formation
Generally speaking, there are some mainly factors which affect lime kiln ring formation, like influence of limestone raw material composition, particle size, powder content. Influence of calcination temperature, calcination zone length, coke size and so on.
Influence of limestone raw material composition
In addition to the main component calcium carbonate (CaCO3), limestone also contains some impurities such as magnesium carbonate (MgCO3), oxides of iron (Fe), aluminum (Al), and silicon (Si). The MgCO3 and oxides of iron and aluminum contained in limestone are strong fluxes. Their presence makes it easy for them to react with CaO during the calcination of limestone process to form fusible compounds such as XCaO·SiO2, XCaO·Fe2O3, XCaO·Al2O3, and CaSO4. These compounds form a liquid melt at high temperatures in the calcination zone of lime kiln, causing the limestone to agglomerate into large chunks and sinter into nodules. Some will stick to the kiln wall, leading to kiln ring. Therefore, based on the mechanism of lime kiln ring formation, low-impurity limestone should be selected as the raw material before calcination.
Influence of limestone raw material particle size
The particle size of limestone has a significant impact on the calcination rate of lime. Oversized particles will prolong the material residence time, affecting the kiln’s capacity. Conversely, smaller limestone particles have a shorter residence time in the kiln. However, reducing limestone particle size also has its limits. If the particle size is too small, it will increase the kiln’s internal resistance, disrupt gas distribution, damage the lime kiln calcination conditions, and lead to uneven burning, localized high temperatures, or the formation of “nodules,” increasing fuel consumption and reducing the lime kiln’s production capacity and CO2 concentration in the kiln gas. Therefore, limestone particle size should be uniform, and the kiln area should be stable, which is conducive to smooth operation and increased decomposition rate of the lime kiln.
Influence of limestone raw material powder content
The powder content of limestone raw materials will also cause kiln rings. The higher the powder content of limestone raw materials entering the kiln, the easier it is for the materials to react, melt, or bond to the kiln wall, leading to kiln ring formation. Generally, if there is more powder in the limestone raw material, it indicates that there are more impurities. As mentioned above, these impurities are more likely to react with CaO, forming various low-melting-point compounds that bind the limestone together, thus forming lime kiln rings.
Influence of calcination temperature
Poorly controlled calcination temperature can lead to kiln rings. The higher the calcination temperature, the easier it is for low-melting-point substances in the materials to melt into a liquid phase, causing the limestone to bond together or adhere to the kiln wall, gradually forming hanging wall kiln rings. Moreover, at higher temperatures, various components in the materials are more likely to react or undergo chemical reactions with the kiln lining, generating more low-melting-point substances, thereby promoting the formation of kiln rings. Therefore, the calcination temperature should be reasonably controlled according to the impurity content, generally around 1300°C for limestone with high CaCO3 content, 1050-1150°C for limestone with low CaCO3 content, and below 1050°C for limestone with high MgCO3 content. Of course, such temperature control is not fixed and should be adjusted according to the quality of the finished lime.
Influence of calcination zone length
The actual length of the calcination zone in the kiln also affects the temperature in the kiln. When the calcination zone moves down, it causes low top temperature, high ash temperature, ash discharge with fire, low CO2 concentration, and high lime burning rate. Conversely, when the calcination zone moves up, it causes high top temperature, high ash temperature, high CO2 concentration, and high lime burning rate. If the calcination zone is prolonged, it causes high ash and top temperatures, scattered temperature, and lime burning. If the calcination zone is shortened, it causes low top and ash temperatures, low CO2 concentration, and lime burning. If the above situations occur, blindly raising the kiln temperature will lead to lime overburning, eventually forming kiln rings.
Influence of coke size
The amount of fuel is crucial in the decomposition of limestone in the kiln. If the ratio is low, the temperature will not reach the required level, the calcination will be incomplete, and lime burning will occur. Conversely, a high coke ratio increases fuel consumption, increases the amount of air introduced for combustion, and brings in more nitrogen, leading to high ash and top temperatures, lime burning, and, in severe cases, ash discharge, kiln ring formation, and excessive air volume.
In addition, factors such as air supply and production operations can also cause kiln rings. During production operations, operators should undergo professional training to ensure their technical level. They should strictly control various technical conditions such as top temperature and ash temperature during operation. The lime layer should not be too high or too low. Timely feeding and ash removal should be ensured to maintain the height of the material layer and the position and temperature of the calcination zone, thus avoiding the occurrence of kiln ring problems.
