Kiln construction method
If all refractory bricks are used for construction, dry or wet construction is generally used. The brick lining and the kiln barrel must be reliably concentric, and the brick lining should be built as tight as possible so that the top of the brick is closely attached to the barrel and the wedge-shaped surfaces of adjacent bricks in the lining are in complete contact.
If dry-built, the number of steel plates driven between bricks should be minimized; if wet-built, the joint between bricks should be 1-1.5 mm. The shape and specifications of the bricks are determined according to the diameter of each zone in the kiln, which is generally fan-shaped. The bottom arc of the brick is the same as the arc of the kiln body, and the thickness is appropriate, neither too thick nor too thin. For dry masonry, if the quality of refractory materials selected is low, and there are mistakes in masonry, brick falling and collapse accidents will often occur.
For wet masonry, the choice of fireclay or mud is critical. If the bonding is good, the lining effect will be better. Therefore, some units have changed from dry masonry to wet masonry. Nowadays, energy saving and environmental protection are advocated.
The rotary kiln is usually built with lightweight heat-insulating products and ceramic fibre modules close to the shell, and then heavyweight refractory products are built—heavy-duty one-piece composite brick. So far, cement rotary kilns use brick linings and composite bricks commonly used in pre-tropical and firing zones.
Some cement kilns, petroleum coke kilns, and nickel-iron kilns use spray paint or castables to form integral linings. To improve the service life of the rotary kiln lining, in addition to improving the masonry method or using a fully poured lining construction, a new kiln construction method is adopted in the oxidation pellet kiln, lime dolomite kiln, petroleum coke kiln, etc., that is, vertical masonry Castable prefabricated blocks with anchor nails, the masonry method of pouring castables in alternate rows, is explicitly to use refractory mud to vertically build prefabricated blocks with anchor nails along the direction of the centre line of the rotary kiln, and then weld the anchor nails on the blocks to the shell finally, refractory castables are poured in the gap between the two rows of prefabricated blocks.
This method does not need moulds; masonry, anchoring and pouring can be carried out simultaneously. It is a composite construction method with the characteristics of a simple construction method, time-saving and labour-saving, good integrity of kiln lining, and masonry is not easy to loosen, crack, deform and fall off. At the same time, the prefabricated block is a composite of lightweight and heavyweight, so the thermal insulation effect in the kiln is also good.
This technology has been widely used and achieved good results. In addition, the combination of anchor welding masonry and castables requires less water, which overcomes the problems of easy falling off of single brickwork and significant shrinkage of single castables. Lining maintenance and repair Because of the different types of calcined materials in the rotary kiln, the operating conditions of the lining are also different. The firing belt must be hung for kilns such as cement, dolomite, active lime, alumina, chemicals, waste, etc…
Stable kiln skin protects the lining bricks from drawing lots and wear resistance, while kilns burning metallized pellets, oxidized pellets, etc., do not hang the skin. In addition to choosing wear-resistant refractory materials for the lining, Consider preventing loops, patching, etc., to protect the lining.
01 Hanging kiln skin
The kiln skin refers to the protective layer that the material adheres to the surface of the kiln lining brick. It can prevent chemical erosion and mechanical wear of high-temperature materials and play a role in heat insulation. It is an essential factor to stabilize the thermal system and increase the service life of the lining. Hanging the kiln skin is a complex physical and chemical process.
When the material is close to the burning zone, the flame will gradually shorten, and the exhaust and combustion volumes will be increased appropriately to control the high-temperature flame. The chemical components in the clinker and the chemical components in the clinker react and bond. The reacted products and bricks agglomerate when condensing to form a kiln skin.
It is required that the kiln skin has a small and uniform particle size, good integrity, and is firm and does not fall off. If it cannot be hung, the semi-molten material will seriously erode the inner lining; if it is not hung firmly, it will fall off frequently, and the inner lining will be subjected to rapid cooling and heat, seriously damaging the inner lining.
Therefore, the problem of hanging kiln skin in the firing zone of the lining of the rotary kiln is generally paid attention to. For example, the firing zone of the cement kiln has been using magnesia-chrome bricks for many years because it will cause hexavalent chromium poisoning in the human body. Hanging the kiln skin well like magnesia-chrome bricks has become the primary condition.
After many experiments and studies, it has been determined that magnesia-iron-aluminium spinel bricks are more suitable. Of course, from the perspective of hanging the kiln skin, the materials for firing are different, and the lining bricks are also different. For example, andalusite bricks and mullite bricks are better for calcined dolomite kilns, and alumina kilns are used for secondary bauxite clinker.
As an aggregate of bauxite, clay and other composite powder ingredients, machine press moulding anti-stripping high-alumina bricks fired at high temperatures is better.
02 Repair
As mentioned earlier, the pellet kiln cannot hang the kiln skin. In the 600,000-ton pellet rotary kiln, Laiwu Steel casts 5 round casting belts from the kiln head to the kiln tail, one at each end and 3 in the middle; 6 casting belts in the north-south direction, one for every six bricks; the rest are made of refractory bricks.
The castable will fall off after eight months of use, and it will be repaired by spraying, and the life of the kiln lining will be more than three years. The first step of the repair is to clean up where the refractory material falls off and the surrounding refractory brick kiln skin. At the place where the refractory material falls off, two holes with a depth of 30-50mm are drilled according to the size of the anchor nails to fix the anchor nails and fix them on the refractory bricks.
On the brick. The second step is to pour the corundum castable on the damaged part and cover the nails, the height of which is 15-20mm from the normal refractory brick, and vibrate to compact it. The third step is to spray a layer of 15-20mm high-alumina spray coating on the top of the castable and intact refractory bricks. The properties of the spray coating are: w(Al2O3+SiC)>70%, bulk density ≥2.4g/cm3, flexural strength ≥5MPa, compressive strength ≥40MPa. Spraying three times a year greatly improves the service life of the inner lining, and the ringing phenomenon is also effectively curbed to ensure the normal production of pellets.
Masteel’s lime kiln is built with clay bricks, and the kiln shell is hot due to high temperature. The thinnest residual brick is less than 50mm, and it has been used for four years continuously by spraying. First, blow off the floating ash of the inner lining with high-pressure air, remove the severely damaged bricks, and drive anchor nails into the joints of the bricks.
Test spraying with a gunning machine, slowly adjust the water volume and wind pressure; when the material is slurry and the adhesion rate is high, lock the water volume and wind pressure. Use multi-layer gunning, first spray the dents and bricks, and then spray the whole after filling up, each time the thickness does not exceed 50mm until it meets the requirements; after spraying and curing for 48 hours, ignite the kiln, after the kiln, it can be quickly Feeding to regular operation.
