In the construction of submerged arc furnaces, the selection of furnace bottom materials is crucial, as it is directly related to the service life of the furnace and the ore refining efficiency. Self-baked carbon bricks and pre-baked carbon bricks are two common furnace bottom materials. Although they are both carbon materials, they have significant differences in production processes, performance characteristics and use effects.
Both self-baked carbon bricks and pre-baked carbon bricks are carbon materials widely used in submerged arc furnaces, but there are significant differences in their heat treatment processes.
The production of prebaked carbon bricks involves a delicate roasting process. In this process, green carbon bricks are placed in a roasting furnace, and specific fillers are used to isolate oxygen and prevent the carbon bricks from oxidizing at high temperatures. Subsequently, the roasting furnace heats the carbon bricks at a strictly controlled temperature rise rate. The temperature of this roasting process usually does not exceed 1200°C to ensure that the physical and chemical properties of the carbon bricks are optimized.
In contrast, the heating process of self-baking carbon bricks in submerged arc furnaces has its own uniqueness. It is not done in a special roasting furnace, but in the actual operating environment of the submerged furnace. As the internal temperature of the submerged arc furnace increases, the self-baking carbon bricks gradually undergo a self-baking process. It is worth noting that since the internal temperature of the submerged arc furnace is usually higher, the highest temperature experienced by self-baking carbon bricks often exceeds the baking temperature of pre-baked carbon bricks.
Heating methods are different
Prebaked carbon bricks enjoy a precise and uniform heating environment during the baking process. In the roasting furnace, the prebaked carbon bricks are closely surrounded by fillers. These fillers not only play the role of isolating oxygen, but more importantly, they serve as excellent heat conduction media and can absorb the heat generated by the multi-sided heating fire channels in the roaster. Heat is transferred to each side of the charcoal brick evenly and efficiently. This heating method ensures that the six sides of the prebaked carbon bricks can be evenly heated during the baking process, so that their physical and chemical properties can reach the best state.
In contrast, the carbonization process of self-baking carbon bricks in the submerged arc furnace relies on the temperature gradient of the submerged arc furnace itself. The heat source mainly comes from the high-temperature area above the furnace, and the heat is gradually transferred from top to bottom to the carbon bricks through thermal conduction. Therefore, the roasting process of self-baking carbon bricks is not carried out at the same time, but carbonization is carried out layer by layer from top to bottom as the heat is transferred. This unique carbonization method makes self-baked carbon bricks exhibit unique layering and gradient characteristics in performance.
Heating rates vary
Prebaked carbon bricks have significant advantages during the roasting process, mainly reflected in their controllable heating rate. Thanks to the support of external heat sources, we can precisely control the temperature changes during the roasting process. This flexibility allows us to develop corresponding heating curves according to different stages of carbon brick roasting to optimize the roasting effect. Specifically, before 300℃, we will adopt a faster heating rate to reduce the risk of deformation of carbon bricks; when the temperature rises to the range of 300~750℃, we will reduce the heating rate to ensure that the asphalt is fully coked and improve The quality of the carbon bricks; once the temperature exceeds 750°C, we will speed up the heating rate again to promote the smooth completion of the roasting process.
In contrast, the roasting process of self-baking carbon bricks is limited by the temperature conduction in the smelting bath. Due to the lack of external heat source, we cannot control the heating rate as precisely as prebaked carbon bricks. In addition, the thermal conductivity of self-baking carbon bricks is relatively low, only 3W/m·k, which is much lower than the 16W/m·k of pre-baked carbon bricks. This means that the heat conduction speed in the self-baked carbon bricks is slow. Although this ensures the coking value of asphalt to a certain extent, it also increases the risk of deformation of the carbon bricks. Therefore, when baking self-baking carbon bricks, we need to pay special attention to their deformation to ensure the baking quality.
The roasting of charcoal bricks and joint paste is different
When discussing self-baking carbon bricks and pre-baked carbon bricks, we have to pay attention to the joint processing between them. Self-baking carbon bricks use carbon mortar to build joints. This delicate processing method ensures that the width of the joints usually does not exceed 1mm, thus ensuring a tight connection between the carbon bricks.
Relatively speaking, the joint processing of prebaked carbon bricks is more extensive, usually using a wide joint construction method of 40 to 50mm. In these wide gaps, cold tamping paste or electrode paste needs to be tamped to fill the gap. However, with the advancement of technology, some large-scale submersible furnaces have begun to try to use carbon cement between the finished pre-baked carbon blocks to improve the sealing and strength of the joints.
During the heating process of the submerged arc furnace, the paste between the seams of the prebaked carbon bricks will be roasted and adsorbed on the carbon blocks. However, it is worth noting that due to the difference in shrinkage coefficients between the joint paste and the prebaked carbon bricks, the joint strength of the prebaked carbon bricks is relatively low. Therefore, these gaps often become the weak links in the furnace lining that are most susceptible to molten iron erosion.
Unlike pre-baked carbon bricks, self-baking carbon bricks and their seam paste will undergo the roasting process at the same time, and because they are made of the same material, they can be closely combined during the roasting process. When the self-baking carbon block heats up to 100°C, it begins to soften. At this time, its plastic deformation can effectively fill the excessive gaps that may occur during the furnace masonry process. This characteristic makes the sintering strength of the self-baking carbon brick seam paste reach 2 to 3 times the strength of the pre-baked carbon brick seam paste, thereby significantly improving the durability and stability of the furnace lining.
Self-baking carbon bricks are baked in a pressurized atmosphere
Coal pitch plays a vital role in the charcoal brick making process. However, under normal pressure roasting conditions, more than 30% of the carbon component in coal pitch will be lost due to the escape of volatile matter. This phenomenon not only leads to the formation of numerous pores inside the carbon bricks, but also directly affects a series of physical and chemical properties of the carbon bricks. In order to improve this situation, we can increase the coking amount of coal pitch by increasing the environmental pressure, thereby effectively increasing the volume density of the carbon bricks and optimizing their internal structure.
In submerged arc furnaces, the baking environment for self-baking carbon bricks is very special. Due to the surrounding refractory bricks and furnace shell, as well as the pressure of the upper charge, the baking process of self-baking carbon bricks presents the characteristics of pressure and single-sided heating. Under such conditions, the volatile matter in the carbon bricks and their joints is not easily discharged. Instead, a polymerization reaction occurs, producing coke with excellent performance. This coke formation can not only effectively reduce the porosity of carbon bricks, but also significantly improve the overall performance of self-baking carbon bricks.
In contrast, pre-baked carbon bricks do not have the same favorable conditions as self-baked carbon bricks during the roasting process. Although the atmosphere in which the seam paste is baked is similar to that of self-baking carbon bricks, due to the relatively small amount of seam paste, its performance after baking is often not as good as that of self-baking carbon bricks.
Self-baked carbon bricks and pre-baked carbon bricks each have their own advantages and applicable scenarios. When selecting furnace bottom materials, we need to fully understand their characteristics and performance differences, and make decisions based on the actual needs of the enterprise. Only in this way can we select the furnace bottom material that is most suitable for our company and provide a strong guarantee for the efficient and stable operation of the submerged arc furnace. I hope the introduction in this article can be helpful to everyone!
HOME