- Time:Oct 10, 2023
The refractory materials inside the cement kiln are subjected to thermal stress, mechanical stress, thermochemical stress, and material abrasion, among other interactions. With the continuous improvement of technologies for co-processing various types of waste in high-temperature cement kilns, the use of alternative fuels such as municipal solid waste and urban pollutants is increasing. Whether these materials are introduced into the kiln system through a decomposition furnace or burned separately in dedicated containers with exhaust gases entering the kiln system, the volatile harmful components will circulate and accumulate in the system, leading to an increasing load on the refractory materials.
I.The main phenomena and causes of damage to refractory materials during co-processing
The refractory material configuration in the cement rotary kiln system mainly includes high-alumina bricks, magnesia bricks, alkali-resistant bricks in the preheater, and refractory castables. Since the temperature in the alkali-resistant brick usage area is relatively low and its service life is long, its influence under co-processing conditions can be negligible.
1.Damage to high-alumina bricks
The cooling zone, safety zone, precalcining zone, and partially upper transition zone of the rotary kiln mainly use anti-spalling high-alumina bricks, GUIMO bricks, GUIMO red bricks, and composite structure GUIMO bricks. The damage modes mainly include high-temperature erosion and abrasion, which generally occur in the cooling zone (Figure 1), and interlayer fracture, which usually occurs in the safety zone and the upper end of the transition zone (Figure 2).
Fig.1 Refractory bricks subjected to high-temperature erosion and washout
Fig.2 Damaged high alumina bricks in safety zone
The damage to high-alumina bricks caused by high-temperature erosion and abrasion is due to significant temperature variations in the operating conditions, which prevent the refractory bricks from adapting to the high temperatures in their respective areas. The usage temperature is close to the load softening temperature of the refractory bricks, causing a rapid decrease in their strength while being subjected to the scouring of clinker particles, resulting in typical smooth pits resembling chicken nests.
This type of damage is often caused by factors such as the burner at the kiln head having an excessively coarse flame, using coal with a calorific value greater than 24.2 MJ/kg, setting the burner thrust too high, or undergoing process changes without adjusting the refractory brick configuration.
The damage to high-alumina bricks near the safety zone is mainly due to the accumulation of large amounts of harmful elements such as alkalis, chlorides, and sulfurs brought into the kiln during co-processing. These elements adhere to dust and materials in various states. There is a concentration gradient on the hot face (in contact with the kiln lining and materials) and the cold face (in contact with the kiln shell). Harmful components present in the gas phase migrate into the interior of the refractory bricks through the open pores and contact gaps between them. Therefore, co-processing imposes stricter requirements on equipment management.
2. Damage to magnesia bricks
Magnesia bricks are mainly used in the lower transition zone, burning zone, and upper transition zone. Common varieties include magnesia spinel bricks, magnesia-alumina spinel bricks, magnesia-iron-alumina spinel bricks, and less commonly used magnesia-manganese spinel bricks, magnesia-zirconia spinel bricks, as well as directly bonded magnesia-chrome bricks that have been phased out of the cement market due to environmental concerns.
The failure of magnesia bricks due to interaction with the operating medium is primarily manifested through lining brick damage:
(1) Erosion by harmful salt compositions
Similar to high-alumina bricks, the penetration and erosion of harmful elements such as alkalis, chlorides, and sulfurs on magnesia bricks occur. However, the strength of magnesia bricks is generally about 20 MPa lower than that of high-alumina bricks, resulting in a higher probability of brick fracture.
（Corrosion damage to magnesia-alumina bricks with a depth of breakage of up to 80-100 mm）
(2) High-temperature liquid-phase corrosion
In co-processing kilns, the introduction of harmful components reduces the burnability of raw materials to some extent and leads to early liquid-phase formation. The liquid-phase content fluctuates significantly with temperature changes, and the low viscosity of the liquid phase makes it difficult for clinker to agglomerate. Therefore, for kilns with unstable operation and fluctuations, drift easily occurs in the hot end of the upper transition zone, leading to the erosion and spalling of lining bricks due to high-temperature liquid-phase attack, while the risk is relatively smaller in the lower transition zone and burning zone.
The erosion of lining bricks by high-temperature liquid phase primarily occurs through the blocking of pores on the hot face of the lining brick, as well as the destruction of the crystal lattice of magnesia-iron or magnesia-alumina spinel, resulting in the densification of the microstructure of the lining brick, causing it to lose mechanical flexibility. As the system temperature changes, the differential expansion between the densified layer and the base layer generates internal stress. When the internal stress exceeds the bonding strength, the densified layer will detach. Lining bricks damaged by high-temperature liquid phase have a relatively small thickness, generally not exceeding 20mm, distinguishing them from the corrosion caused by harmful components.
（Refractory bricks damaged by high temperature liquid phase erosion）
II. Countermeasures
1. Optimization of kiln system operation:
(1) Strictly control the quality of various raw and fuel materials, ensuring that the volatile component content of co-processing materials meets the quality control requirements.
(2) Homogenize the co-processing materials to avoid significant fluctuations in harmful component concentrations during kiln feeding, which can exacerbate crust formation.
(3) Properly control the fineness of the raw feed, with a residue of 12%-14% on an 80μm sieve and not exceeding 1.5% on a 200μm sieve.
(4) Control the sulfur-to-alkali ratio within the range of 0.8-1.2.
(5) Stabilize the flow rate fluctuations of raw feed and fuels entering the kiln, with short-term standard deviation not exceeding 2%.
(6) Use high-thrust burners at the kiln head, optimize the internal and external air distribution to form a fine and short flame, and distribute the thermal zones inside the kiln appropriately.
2. Optimization of kiln mechanical management:
(1) Regular/online measurement of the slip amount of each wheel belt, with an ideal range of 12-15mm per revolution during normal operation. If it exceeds 25mm per revolution for an extended period, additional shims must be added.
(2) Regular monitoring of the deviation degree of the kiln's central axis. Generally, when cold, the lateral deviation should not exceed ±5mm, and the vertical deviation should not exceed ±3mm.
(3) Regular monitoring of the equipment shell thickness at critical locations.
(4) In areas prone to heavy/rainstorms, install rain shelters above the kiln shell to prevent thermal shock.
3. Selection and construction of refractory materials:
(1) Prioritize refractory materials that possess inherent properties to resist harmful gas penetration and choose products with a porosity rate of less than 15%.
(2) During refractory material construction, pay attention to filling all gaps with refractory mortar to prevent gas leakage. Properly design expansion joints and select suitable materials for filling them to prevent gas short-circuiting into the lining.
(3) Select insulation layer materials with low shrinkage rates. For insulation layers with high shrinkage rates, single-layer arrangement is not recommended. Install isolation layers every 2-3 square meters in the insulation layer to prevent the formation of continuous gaps due to insulation layer issues.
