Suggestions for Solving the Spalling Problem of Refractory Castables Used in Cement Kilns
Spalling of Castables
To improve the operational efficiency and production rate of rotary kilns while reducing production costs, many cement manufacturing enterprises face the challenge of heavy production loads and short kiln shutdown periods for maintenance. For these enterprises, the duration of intermediate and major overhauls is generally controlled within 8 to 15 days. However, most cement kilns in China use low-cement series refractory castables
that are primarily bonded by hydraulic binders. The construction, curing, and baking processes of these castables must follow specific time cycles.
As a result, under tight construction schedules and rapid kiln baking conditions, spalling of castables is likely to occur. In particular, castables in certain critical areas may spall during the baking and feeding processes of the production line. This forces enterprises to shut down the kiln for re-casting, leading to unnecessary losses.
01. Causes of Spalling
①Heavy production loads: All cement production enterprises face heavy production demands. For a production line with a daily output of 5,000 tons of cement, the daily production value is approximately 1 million yuan. As a result, most cement plants find themselves racing against time during maintenance periods. In order to resume kiln operation and production as quickly as possible, they often have no choice but to shorten the curing and baking times of the castables.
②Material and binder issues: Currently, the refractory castable materials used in cement kilns in China are mainly based on the SiO₂-Al₂O₃ system, with binders that are rapid-hardening and early-strength cements, primarily consisting of aluminate cement. This requires the castables to undergo sufficient curing time and a reasonable baking process after construction, which typically takes a relatively long time.
③Impact of maintenance work and workforce quality: Maintenance work in China's cement industry is generally entrusted to furnace construction companies. The professional skills of the construction team and whether the construction process is standardized significantly impact the quality of the castable installation and its long-term performance.
④Difficult baking in special kiln areas: Some special parts of the cement kiln, such as the front kiln inlet, kiln head hood, and the top and side walls of the grate cooler, present significant challenges when it comes to the baking process.
For some of these parts, certain cement enterprises do not use firewood at all for baking, while others only perform short-term baking. During the kiln baking stage using burners, hot air flows propagate and radiate toward the kiln tail system at a certain speed due to the control of cement kiln process parameters. This leaves only partial radiant heat energy for the kiln head hood and the grate cooler. The amount of heat is insufficient to fully expel the free water in the castables.
When production resumes with material feeding, the hot air in the system flows back to the kiln head in a short period of time. At the same time, high-temperature clinker (above 1300°C) begins to accumulate at the bottom of the kiln head hood, causing a sharp rise in the temperature of the kiln head hood and grate cooler. This creates the conditions for spalling and the detachment of castables.
These multiple factors make the spalling problem of castables during maintenance particularly prominent in various cement plants. Every year in China, some cement plants experience spalling and collapse of castables in the kiln head hood and grate cooler due to inadequate anti-spalling measures and unreasonable baking systems.
02. Mechanism Analysis of Spalling
Currently, spalling of castables is prone to occur in the kiln head hood, the hot section of the grate cooler, and the front kiln inlet of cement production lines in China. The main cause of spalling can be traced to the rapid temperature rise during the kiln baking stage, which leads to a sharp increase in the temperature inside the kiln. The free water in the refractory material changes from liquid to gas in a short period of time. As the temperature of the lining rises, a certain vapor pressure is generated. If the water vapor inside the lining cannot diffuse in a timely manner, the pressure will build up. When the vapor pressure inside the lining reaches an imbalance, spalling, peeling, or even bursting of the castables may occur.
To address the spalling problem, refractory material manufacturers in the cement industry generally add 0.1% to 0.3% organic fibers to the castables. When these organic fibers are heated to approximately 160°C, they melt or burn, creating fine, elongated pores that are evenly distributed throughout the material. These pores improve the air permeability of the lining material, which helps accelerate water drainage and buffer the water vapor pressure. As a result, this reduces the risk of lining spalling caused by sharp temperature changes during the kiln baking and kiln startup-feeding stages. However, the use of organic fibers presents challenges such as difficulty in dispersing the fibers evenly within the castables, the need for relatively large quantities of fiber, and potential negative impacts on the strength of the castables.
03. Five Suggestions for Solving the Spalling Problem
It should be noted that if cement plants reasonably arrange the construction time for each part of the cement kiln, strictly control the construction quality of refractory castables, and follow the temperature rise curve during kiln baking and material feeding, issues such as spalling, peeling, and bursting of refractory castables can be completely avoided. Based on the operational guidelines for refractory castable construction in cement kilns and the author's experience, the following five suggestions are offered:
①Coordinating the Construction Schedule: Since cement production enterprises often face overlapping tasks during maintenance periods, including equipment maintenance, castable construction, and other operations, it is essential for all departments to coordinate and properly schedule the construction of castables in each part of the cement kiln. For areas that are difficult to bake (such as the grate cooler, kiln head hood, and front kiln inlet), priority should be given to casting construction, ensuring sufficient curing time. At the same time, strict supervision of the construction unit is necessary, ensuring that all aspects of the castable construction process-such as mixing, water addition, formwork erection, and curing-are carried out according to the construction instructions.
②Vent Hole Reservation During Kiln Head Hood Construction: When constructing the kiln head hood, it is advisable to reserve vent holes. The specific requirements for drilling are as follows: the spacing between holes should be 300mm × 300mm, and wooden strips with a diameter of 5mm (φ5mm) should be embedded, with a depth of half the thickness of the castable.
③Vent Hole Reservation for Kiln Head Hood and Front Kiln Inlet: After sealing the casting holes of the kiln head hood, 8 ventilation holes should be reserved, and waterproof measures should be properly implemented. Additionally, to prevent the refractory castables at the front kiln inlet from bursting during baking, it is recommended to reserve vent holes using 3mm bamboo sticks (φ3mm) during construction. The density of the vent hole arrangement should be 100 holes per square meter.
④Baking Method for Special Parts: For intermediate and hot maintenance of the cement kiln, before using the burner to inject oil for kiln baking, it is preferable to use firewood to bake the special parts (kiln head hood, grate cooler, and front kiln inlet) for no less than 12 hours. During the kiln baking process, the temperature rise rate should not be too fast. For low-cement series refractory castables with a lining thickness of 200mm to 400mm, an optimal temperature rise rate of no more than 15°C per hour will fully meet the temperature rise requirements for the inner lining.
⑤Use of Non-Baking Refractory Castables for Emergency Repairs: In the event of an emergency repair or kiln shutdown for maintenance due to an accident, where there is no time for baking, phosphate-bonded refractory castables (such as PA-852 phosphate castable) can be used. This type of product has the advantage of being non-baking, and its strength increases with temperature. However, it should be noted that such products also have some drawbacks, such as poor wear resistance and a shorter service life.
The spalling problem of castables is complex and closely related to many factors. Special attention is required, especially during winter construction.
In conclusion, adding 0.1% to 0.3% anti-spalling fibers to the castables can improve their anti-spalling performance. For areas prone to castable spalling, such as the front kiln inlet, grate cooler, and kiln head hood, as long as reasonable construction arrangements are made, quality control during the construction process is properly implemented, and all aspects such as reserving vent holes and baking are handled appropriately, spalling of the refractory linings can be completely avoided.