In furnace engineering, refractory castable materials are usually provided in the form of bulk mixtures and lined on site, mainly used for the renewal of kiln linings and the repair of old parts. The required materials must also be diversified to adapt to various complex environments.
1. Fine control of water consumption
In the construction process of castables, the role of water is crucial. By carefully adding the right amount of water, we can convert dry powder into a refractories castable slurry with good fluidity, and then smoothly carry out construction. Water not only serves as a medium for refractories castables, but also plays a bridging role in its hardening process, ensuring that the slurry has ideal thixotropy and fluidity, thereby forming a cast component or lining with a dense structure, tight bonding and high mechanical strength.
Although too much water seems to improve fluidity, it actually causes the separation and deposition of aggregates and powders, resulting in excessive pores when moisture escapes during the drying process, thereby affecting the structural tightness and performance of the cast component or lining. It is particularly noteworthy that excessive water consumption may also lead to serious problems such as reduced strength and reduced erosion resistance.
In order to ensure construction performance while limiting water consumption, we usually adopt professional solutions. This includes the use of surfactants, namely water reducers (plasticizers) or dispersants. These additives can effectively improve the thixotropy and fluidity of the refractory castable slurry, so that while reducing water consumption, it can still maintain good construction performance. In order to achieve the best water reduction effect, we often choose composite surfactants to achieve a comprehensive improvement in slurry performance.
2. Mixing of castables
During the construction process of refractory material castables, it is accurately mixed with water to form a uniform and smooth slurry. This process is carried out at a constant speed in professional mixing equipment to ensure that each mixing can achieve the best effect. As the mixing deepens, the mixing force gradually increases, and the time required for uniform mixing is correspondingly reduced. At the same time, the temperature of the castable slurry will gradually increase, laying a solid foundation for subsequent construction.
When water is initially added, the particles seem to be gently covered by a layer of liquid film, forming a subtle "liquid bridge" connection between each other. As the adsorption layers overlap, the adsorption force between particles gradually increases, and the strength of the agglomerates also increases. At this point, we need to cleverly adjust the amount of water added to ensure that the agglomeration between particles is optimal.
Further increasing the amount of water or improving the water distribution in the particles can make more particles wrapped by water, while increasing the torque and improving the mixing efficiency. However, when the water content reaches a certain critical level, a large number of "liquid bridges" between particles will be formed, and the shear resistance of the system will increase sharply. This shear resistance increases with the increase of particle surface area, which brings greater challenges to mixing.
But please note that when the water just fills the gaps between particles and covers the surface of the particles to a critical value, the capillary force reaches its strongest. At this point, if further water is added, the number of "liquid bridges" will decrease sharply, and the shear resistance of the mud will also decrease. Therefore, we need to accurately control the amount of water added to ensure that the rheological properties of the castable slurry are optimal.
3. Drying of refractory castables
In order to ensure that the castable can show the best performance, the selection of drying and heating conditions is crucial. Especially for high-aluminum castables, since they contain a large amount of free water and hydration water, these water will gradually release between 100~550℃. Therefore, we need to carefully design the drying and heating process to ensure that these water can be effectively removed while avoiding adverse effects on the performance of the castable.
The specific heating rate control recommendations are as follows: First, increase the temperature from ambient temperature to 110℃ at a rate of 15℃ per hour, and keep it warm for 1 hour for every 25 mm thickness at this temperature; then, continue to increase the temperature from 110℃ to 300℃ at a rate of 15℃ per hour, and keep it warm for 1 hour for every 25 mm thickness at 300℃; then, increase the heating rate to 25℃ per hour, so that the temperature rises from 300℃ to 550℃, and keep it warm for 1 hour for every 15 mm thickness at 550℃; finally, increase the temperature from 550℃ to the final use temperature at a rate of 75℃ per hour. Such fine heating rate control will ensure that the refractory castable is evenly heated and stably changed during the heating process, and ultimately presents excellent performance.
