1. The mechanism of damage of refractory bricks in rotary kiln1. Mechanical stress damage
1) Thermal expansion.
When the temperature of the refractory bricks rises to a certain extent in the kiln, the thermal expansion will produce pressure in the axial direction of the kiln, causing the adjacent refractory bricks to squeeze each other, and when the pressure is greater than the strength of the refractory bricks, it will lead to the peeling off of the refractory brick surface. In view of this situation, the following measures should be taken: dry laying refractory bricks, setting up reasonable side cardboard, wet laying refractory bricks to leave 2mm fire mud joints;Leave a suitable brick retaining ring.
2) Ovality stress.
The gap between the kiln wheel belt and the horn increases, resulting in a large ovality, resulting in the extrusion of refractory bricks. In view of this situation, the ovality of the cylinder should be checked regularly, and the backing plate should be replaced or the gap of the pad adjustment wheel should be increased.
3) The lining bricks are misplaced.
Due to the loose masonry, the kiln is frequently opened and stopped, and the kiln cylinder is deformed, which makes the relative movement of the kiln cylinder and the cold surface of the lining brick cause the lining brick to twist and dislocate and the brick surface to burst and fall off the corner. The following measures should be taken in response to this situation: When masonry, the large surface of the refractory bricks should be hammered with a wooden hammer, and the lock bricks should be locked, and the wedge iron should be added carefully for the second timeMaintain a stable thermal system;The deformed part of the kiln cylinder is leveled with high-temperature cement.
4) Lock iron stress extrusion.
When locking bricks, too much lock iron is too tight, which will lead to the formation of brick grooves at the lock mouth. The following measures should be taken in response to this situation: No more than 3 locking irons at the same locking opening;The spacing of the lock iron is spread as far as possible;When locking the bricks, the inner and outer mouths are tightened consistently;Keep the lock iron away from the thin lock bricks as much as possible.
5) Extrusion of brick retaining ring.
The retaining bricks (special-shaped bricks) at the retaining ring of refractory bricks are broken and cracked due to extrusion, and the single-channel brick-retaining ring should be changed to a double-track retaining brick ring in view of this situation, and the whole brick should be laid on the retaining ring to avoid processing special-shaped bricks.
2. Thermal damage
1) Overheating.
The local overheating of the temperature in the kiln leads to the melting of refractory bricks and the formation of pits. In order to avoid this, the burner should be adjusted correctly and a reasonable refractory material should be selected in different parts.
2) Thermal shock.
Due to the sudden change in temperature, the thermal stress causes the brick surface to peel and crack, which is mainly caused by the frequent opening and stopping of the kiln and the rapid cooling and heating. Production operations should be stabilized, and a reasonable system for heating up cold kilns should be formulated.
3. Chemical erosion damage
1) Alkali erosion.
Alkali erosion vapor phase alkali salt compounds infiltrate into the void of the brick body to condense and solidify, forming a horizontal permeability layer of alkali salt in the brick, and the alkali salt content of the kiln should be reduced in production.
2) Chromium erosion.
Chromium erosionUnder the erosion of excessive alkali salts, chromium ore and free (K, Na)2O form K2(Na2Cr6), and the hexavalent alkali chromate produced by chemical reaction not only causes damage to the bricks, but also pollutes the environment and seriously poisons health.
3) Redox bursting.
In this process, the magnesia-chrome brick gradually fades to dark brown, the brick structure bursts, and the coal dust is deposited in the cracks, resulting in the bursting of the brick structure.
4) Hydration phenomenon.
MGO reacts with water to form Mg(OH)2, which increases in volume and destroys the overall structure of refractory bricks. Because refractory bricks containing MGO and CAO will have hydration reactions, it should be ensured to avoid moisture, waterproof and rainproof during storage, transportation and masonry of refractory bricks.
2. Common cases of damage to refractory bricks in rotary kilns
1. Mechanical stress damage - thermal expansion
Phenomenon:The top of the refractory brick presents convex spalling on both sides longitudinally.
Symptoms:The axial pressure generated by the expansion exceeds the strength of the brick, causing the brick to peel off. Reason:The expansion gap is not enough. Cardboard is not set. The kiln is frequently stopped after the cardboard is burned out. Improvements:Set up the cardboard according to the drawing requirements. Use refractory bricks with cardboard.
2. Mechanical damage - misalignment of lining bricksPhenomenon:The relative movement of the kiln cylinder and the cold surface of the lining brick causes the friction damage of the lining brick, resulting in the spiral skew and misalignment of the brick. Symptoms:The relative movement between the kiln barrel and the lining causes the lining bricks to be misaligned.
Reason:The masonry is too loose. The ovality of the kiln cylinder increases. Frequent kiln shutdowns cause shrinkage and expansion of the lining bricks. Kiln skin changes. The kiln barrel is damaged. changeEnter:Keep the operation of the kiln stable. Flatten the protruding part of the kiln cylinder. Control the ovality of the kiln body. Appropriately reduce the gaps and adopt the method of staggered masonry.
3. Thermal damage - molten pitsPhenomenon:The pit is damaged and resembles a duck's nest (what appears to be a eutectic mixture of high-alumina bricks).
Symptoms:Overheating of the temperature damages the brick structure of the hot surface.
Reason:Refractory bricks with medium refractory resistance are not protected by a kiln skin during production operations, and the flame burns directly onto the lining bricks. Improvements:Regulate the burner. Use mafic spinel lining bricks that are easy to hang on the kiln skin. Spinel magnesia bricks are used for non-kiln skin parts.
4. Thermal damage - thermal shock
Phenomenon:Each layer is 2-3cm peeling and cracking. Symptoms:The sudden change in temperature produces thermal stress, which causes the brick surface to peel and crack horizontally. Reason:Heats up quickly. Sudden cooling. The kiln skin collapsed.
Improvements:30 h reasonable temperature increase. Cool down slowly. Stable kiln skin operation. Use a magnesia brick that is stable to 80 for thermal shock.
5. Chemical erosion damage - alkali salt penetration and metal cylinder corrosionPhenomenon:Cutting open the brick body can reveal the horizontal permeability layer of alkali salts and the kiln barrel corroded by alkali salts. Symptoms:The vapor phase alkali salt compound penetrates into the pores of the brick body, condenses and solidifies.
Reason:The lower transition zone is mainly K2SO4 permeable deposition, while the upper transition zone is KCl permetic deposition. Improvements:Reduce the alkali salt content into the kiln. The sulfur-base ratio is controlled at 08~1.2。Use bricks with good elastic properties. Use special coatings or materials to protect the kiln barrel. 6. Chemical damage - redox burstingPhenomenon:In this process, the magnesia-chrome brick gradually turns brown to dark brown, and the brick structure bursts. Pulverized coal is deposited in the cracks, causing the brick structure to burst.
diseaseShape:The repeated changes of the oxidation atmosphere and reducing atmosphere in the kiln are called gasification reduction, and the volume of ferric and ferric in the brick changes accordingly in this process. Reason:The short-lived reduction atmosphere in the kiln is mainly as follows: incomplete combustion. Excessive ash in the fuel. Coal and petroleum coke particles are too large. Use alternative fuels.
Improvements:O2 required to ensure combustion. Improve the fineness of pulverized coal. Use low-iron chrome-free bricks.