Refractory castables are generally used in the area from the kiln tail smoke chamber to the preheater cyclone. After the co-treatment, the increase of harmful components such as alkali, chlorine, and sulfur in the system leads to severe crusting. Generally, the kiln line will increase the crusting cleaning system. In addition to the erosion of harmful components such as sulfur and alkali, mechanical vibration (such as air cannon) also increases the load of the castable, and also increases the risk of separation between the insulation layer of calcium silicate board and the working layer of the castable.
The damage of castables is greatly affected by the construction quality. Common damage comes from problems in quality control such as anchor installation, moisture (mixing agent) of castable construction, mixing time, expansion joint reservation, and vibration uniformity. Here we mainly discuss the situation where the damage of metal anchors closely related to co-treatment leads to the failure of refractory castables. There are two common situations: one is that the metal anchor in the castable is completely burned; the other is that the metal anchor breaks in the castable. Under working conditions, anchors must not only bear the mechanical load generated by the weight of the castable and the insulation layer, but also withstand the thermochemical erosion of harmful salts such as alkali, chlorine, and sulfur. Since the porosity of the castable is greater than that of the refractory brick, the erosion rate of the castable by harmful components is greater than that of the refractory brick, resulting in the peeling of the working layer. Although damping components (SiC, SiO2, etc.) are added to the refractory castables, the effect of use is still unsatisfactory.
The erosion of metal anchors is more complicated, generally high-temperature corrosion under working conditions and low-temperature corrosion during kiln shutdown, among which high-temperature corrosion is the main damage factor. High-temperature corrosion mainly occurs in the form of metal oxidation under working conditions and harmful components destroying the oxide protective layer on the metal surface and then corroding the metal matrix, which is manifested in the peeling of the oxide skin. The speed of forming the oxide skin accelerates with the increase of the use temperature and the concentration of harmful components, and finally manifests as complete burning as shown in Figure 1. Low-temperature corrosion mainly occurs during the kiln shutdown period. Harmful components adhere to the materials and lining in the form of alkali, chlorine, and sulfur compounds, absorb moisture in the air, form an acid film to corrode the metal parts of the anchor and the shell, and generate rust. This phenomenon is more common in places with high air humidity. If hot and cold corrosion occurs repeatedly during the operation of the rotary kiln, the corrosion rate will be greatly accelerated.
If the insulation layer shrinks and separates from the working layer during operation, this gap will form a chimney effect, leading to a vicious cycle of gas inside the gap. With the enrichment of harmful components, the metal anchor will be directly corroded by the gas (the higher the temperature, the more obvious), and the inherent σ phase embrittlement of the metal anchor (occurring at 750~900 degree ) is superimposed, which is very easy to disconnect from the maximum stress point, and the anchor is disconnected from the middle. This corrosion is very likely to occur if the insulation layer uses thicker or multi-layer calcium silicate boards during construction, the expansion joints are not properly set, and flammable materials such as wooden boards are used as expansion joints.
