Causes Of Peeling, Cracking And Spalling Of Castable Lining For Cement Kiln Burner

The burner is a key piece of process equipment in the clinker burning system of a cement kiln, significantly affecting clinker output and quality, the service life of the refractory materials inside the kiln, clinker coal consumption, and environmental emissions. The primary factor influencing the service life of the burner is the damage to the castable lining. Therefore, it is crucial to explore ways to extend the service life of the castable lining for the burner.

A domestic cement manufacturer adopted Al₂O₃-SiC series low-cement refractory castables as the protective lining for its four-channel pulverized coal burner, with a lining thickness of 100 mm. Severe damage occurred on the burner's head and bottom after only two months of operation, primarily characterized by peeling, cracking, and spalling. This paper investigates and analyzes the causes of the damage to the burner's castable lining.

01 Test Content and Characterization

A piece of the lining castable, sampled from the burner in a full-thickness section from the outer surface to the inner core, was taken as the test specimen for overall morphological observation.The residual lining specimen was divided into four layers, from outer to inner, based on the color differences.

An X-ray diffractometer (XRD) was used to analyze the mineral phase composition of each layer, and an X-ray fluorescence spectrometer (XRF) was applied to determine the chemical composition of each layer, respectively.

02 Results and Analysis

2.1 Appearance Analysis of Castables

Observation of the specimen's cross-section revealed the following characteristics for each layer:

Layer 1: Pale yellow with obvious corrosion, indicating the formation of a liquid phase on the castable surface.

Layer 2: White and with the loosest structure; the castable had completely deteriorated and was primarily composed of accumulated alkali salt crystals.

Layer 3: Dark gray and dense in structure, with visible aggregate particles from the castable components.

Layer 4: Gray and dense, with no crystalline substances detected; both the matrix and aggregate particles of the castable were clearly observable.

2.2 Phase Composition Analysis of Castables

Figure 1 presents the XRD patterns of the lining castable specimens. As shown in the figure, the mineral composition of Layer 1 is primarily composed of anorthite (CaAl₂Si₂O₈), wollastonite (CaSiO₃), and potassium feldspar (KAlSi₃O₈) phases.

The analysis indicates that the wollastonite (CaSiO₃) present in this layer results from the adhesion of cement clinker to the surface of the burner lining castable. The formation of anorthite (CaAl₂Si₂O₈) may be attributed to the infiltration of calcium-containing substances from the cement clinker into the lining castable, where they then react with the matrix components of the castable. The generation of potassium feldspar (KAlSi₃O₈) is likely as follows: alkaline substances in the raw fuels volatilize to form potassium-containing gaseous species during high-temperature calcination. These gaseous species condense and deposit on the surface of the burner castable upon contact, and subsequently react with the castable matrix at high temperatures to form potassium feldspar.

Both minerals are low-melting-point substances. They melt to form a liquid phase under high-temperature conditions, which causes corrosion on the lining surface, reduces the surface strength of the castable, and accelerates its wear.

No silicon carbide (SiC) phase was detected in the mineral composition of Layer 1, indicating that the SiC in this layer underwent oxidation under high-temperature conditions. The mineral composition of Layer 2 was dominated by corundum, silicon carbide, leucite (KAlSi₄O₁₀, with a volume expansion of 29%), and potassium corundum (KAlO₂, with a volume expansion of approximately 27.8%). Analysis suggests that potassium (K) and sodium (Na) from the raw fuel materials reacted with α-Al₂O₃ and silica fume in the castable at high temperatures according to the following reaction:

3Al₂O₃+3K₂O+8SiO₂→3(K₂O•Al₂O₃•4SiO₂),This reaction results in a 29% volume expansion. Potassium corundum (KAlO₂) forms through the reaction between Al₂O₃ in the castable matrix and K₂O that penetrates into the castable, theoretically causing a 27.8% volume expansion. At high temperatures, the reaction between Al₂O₃ and K₂O may also generate KAl₉O₁₄, which transforms into KAl₁₂O₁₉ during the cooling process, inducing a certain degree of volume change. The combined effects of these two volume-expanding reactions lead to structural damage and subsequent deterioration of the lining castable.

The presence of the SiC phase in Layer 2 can be attributed to the formation of an SiO₂ film on the lining surface following the oxidation of SiC in Layer 1, which inhibits the oxidation of the internal SiC phase. Theoretically, the SiO₂ film formed by the oxidation of SiC in Layer 1 should also prevent alkali vapor from penetrating into the castable interior; however, alkali metals were also detected in Layer 2. This phenomenon is likely due to the burner's location within the kiln hood space: the lining castable is continuously scoured by high-temperature secondary air carrying fly ash particles over long periods of operation, which damages the SiO₂ film on the lining surface and allows alkali vapor to infiltrate the castable interior, resulting in continuous erosion.

The main mineral phases in Layers 3 and 4 are corundum (Al₂O₃) and silicon carbide (SiC), which are consistent with the primary mineral composition of Al₂O₃-SiC series castables. This indicates that Layers 3 and 4 were not subjected to alkali salt erosion.

2.3 Chemical Composition Analysis of Castables

Table 1 shows the results of the X-ray fluorescence spectrometry (XRF) analysis of the lining castable specimens. As shown in the data from Table 1, Layers 1 and 2 contain substantial amounts of potassium (K) and sodium (Na) salts, while the original aluminum (Al) and silicon (Si) contents in the castable are significantly lower than the standard values, indicating that these layers have suffered severe erosion by alkaline substances.

The CaO content in Layer 1 is notably higher than the standard value. Analysis suggests that the calcium oxide originates from the cement clinker. The possible mechanism is as follows: under the influence of substances such as K and Na, the surface layer of the burner lining castable forms a liquid phase at high temperatures, leading to the adhesion of cement clinker fly ash onto the castable surface.

From Layer 1 to Layer 3, the Na content decreases significantly, but a considerable amount of K remains in Layer 3. This phenomenon implies that K has stronger permeability than Na. In Layer 2, the K content is far higher than that of Na, suggesting that potassium salts are the primary substances responsible for chemically eroding and damaging the internal matrix of the castable.

The destructive effect of potassium salts stems from two aspects.

On the one hand, potassium salts penetrating into the castable matrix react with the matrix to form new compounds such as KAlSi₂O₆ (leucite) and KAlSi₄O₁₀ (kalsilite), which induce volume expansion, alter the composition and structure of the matrix, and thus lead to castable damage.

On the other hand, after potassium salts permeate into the castable matrix, they gradually cool and deposit during the temperature drop. Due to their low bulk density, volume expansion occurs after deposition, causing damage to the castable matrix. In addition, since their mechanical properties are completely different from those of the original matrix, they may crack under thermal stress during temperature fluctuations or cause damage to the castable matrix due to volume changes.

As for other oxides such as Fe₂O₃, Na₂O, and MgO listed in Table 1, no relevant mineral phases were detected in the XRD mineral composition analysis, indicating that they exist in the castable in the form of a glass phase.

03 Conclusions

(1) Low-melting-point anorthite and potassium feldspar formed on the outer surface of the burner lining castable. These minerals melted into a liquid phase under high-temperature conditions, reducing the surface strength of the castable and causing corrosion damage to the material.

(2) The SiO₂ film, formed by the oxidation of SiC on the castable surface, was damaged by the scouring action of high-temperature secondary air. This allowed alkali salts to penetrate into the interior of the castable, resulting in continuous erosion.

(3) The chemical erosion and damage to the burner were primarily caused by potassium salts, stemming from two aspects. First, potassium salts that permeated into the castable matrix reacted with the matrix to form new compounds, disrupting the original composition of the matrix. Second, due to the difference in bulk density of potassium salts, volume expansion occurred after the salts cooled and deposited inside the castable matrix. The combined effects of these two factors led to the damage of the castable lining.