Use spray construction method to repair the unshaped refractory used in the lining of thermal equipment. It is composed of refractory aggregates, binders and admixtures (including coagulants, plasticizers, sintering aids and mineralizers, etc.) with a certain particle gradation. They are generally thermally hardened or air-hardened materials. Gunning repair is usually a repair method used when the lining is partially damaged prematurely and most of it is still in good condition. It can make the furnace lining reach equilibrium or close to equilibrium damage. It is characterized by simple construction, short construction period, no formwork and support for construction, and can effectively extend the life of the furnace lining and reduce the consumption of refractory materials. Generally, gunning materials should have the following basic properties:
①The adhesion rate is greater than 80%.
②The gunning surface does not burst.
High-quality gunning materials must meet the following requirements:
① When passing through the gunning equipment, the material has uniform flow properties;
②Good adhesion, curing and hardening;
③The amount of rebound is small;
④The gunning layer has good physical properties (large volume density, low porosity, high strength, etc.);
⑤Good sintering ability;
⑥ Good fire resistance;
⑦Good corrosion resistance.
It is not difficult to see that the gunning performance (adhesion, sintering, etc.) and corrosion resistance of the gunning material depend on the composition of the raw material, the particle size distribution, and the performance of the binder. The rebound rate directly affects economic indicators.
(1) Classification
According to the main material of the gunning material, it can be divided into magnesia, magnesia calcium, magnesia chromium, magnesium carbon, aluminum silicate and aluminum chloride-silicon carbide-carbon gunning materials, etc., according to the construction method can be divided into Wet method (mud method) gunning material, semi-dry method (nozzle mixing type) gunning material and flame method (melting method) gunning material, etc.
(2) Material selection
The key to the preparation of refractory gunning materials is to select the main material reasonably, select the appropriate binder, additives, etc., and determine the optimal particle size composition. The selection of the main material is based on the highest temperature possible in the kiln, the temperature fluctuation range and the material of the lining itself. The commonly used raw materials are magnesia, dolomite, magnesia chromium, aluminum silicate and zircon. The binders used are silicate, phosphate (including polyphosphate and phosphoric acid), chromate, sulfate, chloride, asphalt, resin, and cement. In order to make the refractory gunning material have good construction performance (such as conveying, wetting, plasticizing, adhesion and sintering, etc.). A variety of additives must also be added, such as sizing agents, coagulants, plasticizers, sintering aids and so on. Commonly used plasticizers are clay and carboxymethyl cellulose. Sintering aids include iron oxide, serpentine, olivine, borax, etc. The particle size composition and limit particle size of the gunning material should be determined according to the structure of the gunning machine, the thickness of the gunning layer and the gunning method. In general, too coarse particles or too many coarse particles will cause the adhesion rate to decrease. Semi-dry gunning material has more coarse particles than wet gunning material, and the critical particle size is also larger, sometimes up to 3~7mm or even larger.
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