Recommendations on the analysis of spalling of refractory castable linings in cement rotary kilns

Recommendations on the analysis of spalling of refractory castable linings in cement rotary kilns

Cement rotary kilns belong to the category of building materials equipment and are a type of lime kiln. They are key equipment in both dry and wet process cement clinker production lines. Many companies, in their efforts to improve the operating rate and production efficiency of rotary kilns, and even reduce production costs, have led to numerous problems in the production process for most cement manufacturers. Some problems can be resolved through repairs, but major issues necessitate kiln shutdowns for maintenance.

Analysis of the Bursting Mechanism

Currently, refractory castings in the kiln head hood, grate cooler hot section, and kiln inlet of Chinese cement production lines are prone to bursting. The main reason for this is that during the kiln drying stage, the rapid temperature rise causes a rapid increase in the kiln temperature. Free water within the refractory material changes from liquid to gas in a short time. As the lining temperature rises, a certain vapor pressure is formed. When the water vapor inside the lining cannot diffuse in time, the pressure increases. When the water vapor pressure inside the lining becomes unbalanced, the refractory material peels off, flakes off, or even bursts.

To address the bursting problem of refractory castings, refractory manufacturers in the cement industry commonly use the addition of 0.1% to 0.3% organic fiber to prevent bursting. Organic fibers, when melted or burned at approximately 160°C, form interconnected and evenly distributed micro-sized, narrow pores. This formation increases the permeability of the lining material, accelerating drainage and buffering water vapor pressure, thus preventing lining cracking caused by rapid temperature changes during baking and kiln loading. However, the use of organic fibers also presents challenges such as difficulty in dispersing the castable, the need for large addition amounts, and reduced strength.

Five Suggestions for Solving the Cracking Problem

It should be pointed out that as long as cement manufacturers reasonably arrange the construction time of various parts of the cement kiln, strictly control the construction quality of refractory castables, and carry out kiln drying and feeding according to the temperature rise curve, the spalling, peeling, and cracking of refractory castables can be completely avoided. Based on experience in guiding the construction of refractory castables in cement kilns, the following five suggestions are made:

① Since there are overlapping operations of equipment maintenance and castable construction during the maintenance period of various cement production enterprises, all departments must coordinate well and make good arrangements for the construction schedule of castables in various parts of the cement kiln. For castables in parts that are difficult to bake (grate cooler, kiln head hood, and front kiln inlet), priority should be given to casting construction, and sufficient curing time should be reserved; at the same time, supervision of the construction unit should be strengthened, and all aspects of castable mixing, water addition, formwork, and curing should be strictly carried out in accordance with the construction instructions. ② When constructing the kiln head hood, it is best to leave vent holes. Specific drilling requirements: spacing 300mm x 300mm, with Ф5mm wooden strips embedded to a depth half the height of the castable refractory.

③ After sealing the casting holes in the kiln head hood, retain 8 vent holes and ensure waterproofing. Simultaneously, to ensure the refractory castable at the kiln inlet does not crack during baking, it is best to also leave vent holes using Ф3mm bamboo sticks, with 100 vent holes per square meter.

④ For intermediate and major repairs of cement kilns, before using burner oil spraying for kiln baking, some special parts (kiln head hood, grate cooler, kiln inlet) should preferably be baked with firewood for at least 12 hours. During the baking process, the temperature rise rate should not be too fast. For low-cement series refractory castables with an inner lining thickness of 200mm–400mm, an optimal temperature rise rate of no more than 15℃ per hour is sufficient to meet the lining’s heating needs.

⑤ When there is no time for baking during emergency repairs or kiln shutdowns due to accidents, phosphate-bonded refractory castables, such as PA-852 phosphate castable, can be used. These products have the advantage of not requiring baking, and their strength increases continuously with rising temperature. However, it should be noted that these products also have their own weaknesses, such as poor wear resistance and short service life. The problem of castable bursting is complex and closely related to many factors, especially during winter construction.