What are the reasons for the excessively high underburning rate in lime kilns
Lime kilns, as the key equipment for the thermal decomposition of calcium carbonate into calcium oxide, often encounter the problem of excessive underburning during the production process. Underburning refers to the incomplete decomposition of limestone, resulting in unreacted calcium carbonate mixed in the produced quicklime. This not only reduces the quality of the lime but may also have adverse effects on subsequent production processes and product applications. The following is a detailed analysis of the reasons for the excessively high underburning rate in lime kilns.
I. Raw Material Factors
The particle size of limestone has a direct impact on its calcination speed. If the particle size is too large, heat cannot penetrate the interior of the stone easily, resulting in a slower calcination speed and an increased likelihood of underburned core formation. For ordinary shaft kilns, the particle size of limestone should be controlled within the range of 40-80mm. If the kiln capacity is larger, it can be appropriately relaxed to 50-150mm. Additionally, uneven quality of limestone is also one of the reasons for underburning. If limestone contains a relatively high amount of impurities such as silicon, aluminum, and iron, these impurities can form low-melting-point compounds at high temperatures, leading to lumps that impede the smooth descent of materials and ventilation within the kiln, thus preventing some limestone from being fully calcined.
II. Fuel and Air Supply Issues
Insufficient fuel supply or a fuel calorific value lower than the standard will fail to provide adequate heat for the complete decomposition of limestone. For instance, a mixed-fuel kiln typically requires a coal calorific value of at least 5,500 kcal. If the fuel calorific value does not meet the standard or the fuel ratio is improper, it will result in a combustion temperature that is insufficient to complete the calcination of limestone. Additionally, insufficient air supply or uneven air distribution will prevent the fuel from burning fully, leading to uneven heat transfer within the kiln and causing local underburning.
III. Operational and Management Factors
The calcination temperature is a key factor determining the degree of limestone decomposition. Excessively high temperatures can lead to over-decomposition of limestone, resulting in overburned lime; while temperatures that are too low can cause incomplete decomposition of limestone, leading to underburned lime. If the temperature monitoring equipment malfunctions or the operators fail to control the temperature properly, underburning problems may occur. In addition, issues such as uneven distribution of materials, untimely removal of ash, tilting or deformation of the kiln body, and failure of the sealing device can all affect the quality of lime calcination, causing an increase in the underburning rate.
The reasons for the excessively high underburning rate in lime kilns involve multiple aspects such as raw materials, fuel and air supply, operation and management. To reduce the underburning rate and enhance the quality and output of lime, targeted improvement measures should be taken from these aspects.
