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Spesial refractory coatings

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These coatings extend service life of the refractory materials, extend service life of the thermal generating units even upon application of the low-cost refractory materials instead of expensive ones, improve strength properties of the lining and prevent penetration of moisture.

Coatings are applied by means of pressure pulverizers, brushes or rollers to a base surface (standard refractory bricks lining) in several steps until a desired layer thickness of 1-1.2 mm is achieved. The coating applied to the refractory bricks should be left to dry under atmospheric conditions until it hardens. As a result, the slip layer becomes strong enough due to a proper adhesion between the coating and the base. The material of a lining or masonry joint shall demonstrate its final performance characteristics upon the drying in the course of heating-up of the thermal generating unit under process conditions. Protective and strengthening features of the coating applied to the surface of the refractory material or product (brick, slab, bar, etc.) is in many ways due to properties of the base, and, primarily, its density and porosity. Dried products with the hardened coating are exposed to burning in special furnaces with controllable heating rate until the process of heterogeneous combustion starts in the coating layer. The moment when the SHS process starts can be monitored visually (using inspection window of a furnace), by means of video recording or by readings of thermocouple sensors which show temperature perturbation in the reaction zone of the combustion wave. The temperature point when the SHS process starts in the protective coating layer is based on composition of the blend, and highly depends on density of the base material as the rate of heat removal from the reaction zone to the base material may differ. If the density of base material increases, the temperature point of SHS process grows. Temperature for lightweight chamottes is 650-800оС, while the temperature for high-density chamottes increases up to 950оС.

Compounds being produced as a result of the synthesis in the combustion wave, may include (in addition to oxides of heat-proof metals) carbides, borides, nitrides, silicides of metals which are the high-heat-resistant compounds. These compounds degrade into constituents under the temperature over 2,000оС.

Exact composition of the compounds produced as a result of the synthesis is to be determined by means of X-ray-structural analysis of the known spectra of individual chemical compounds. Certainly, carbides, borides and nitrides may be produced only when the original coating blend contains carbon, boron, nitrogen or the compounds that include these elements, for example, borax, hydrazin, nitrates, etc.

When protective and strengthening coatings are being applied on lightweight, porous surfaces of refractory or thermal-protective materials, the surface shall be pretreated with a liquid slip and dried.

When the slip is applied on the lining of any thermal generating unit in operation, the protective and strengthening coating is usually applied by means of pressure pulverizers or sprayers. In such a case, appropriate consistency (viscosity) of the slip is provided by means of dilution of liquid glass (binding agent) with water. This protective coating applied on the thermal generating unit lining shall be left to dry under atmospheric conditions for several days until the full cure. Later on, the normal operation of the thermal generating unit (its heating-up) induces SHS process in the protective coating layer which triggers the production of mullite-type compounds.

No.

Name

Operating temperature [°С]

Applications

Consumption rate

Note

Binding agent

Material

1

Oxide ceramic SHS coating M-1

1,800°С

Coating of linings of melting furnaces crowns, ladles, trunks, heating, thermal furnaces, resistor furnaces, salt-bath furnaces, boilers, and so on.

Liquid glass

ρ=1.42-1.46 g/cm3

 

0.6 l/kg of blend

1.5 kg/m2

To be used for chamotte, mullite, mullite and corundum, corundum refractory materials protection

2

Oxide ceramic coating M-KhP-1

 

1,900°С

Coating of masonry of ladles, heating furnace hearths, arc furnace crowns, lime burning furnaces, vacuum vessels and so on.

Liquid glass

ρ=1.42-1.46 g/cm3

 

0.6 l/kg of blend

1.5 kg/m2

To be used for periclase, chrome-periclase, periclase-chrome refractory materials protection.

3

Oxide ceramic coating M-D-1

1,800°С

Coating of masonry of crowns, arc furnaces, coke-chemical furnaces

Liquid glass

ρ=1.42-1.46 g/cm3

 

0.6 l/kg of blend

1.5 kg/m2

To be used for silica, electrical silica refractory materials protection.

As stated above, after SHS process the coating binds with the base as a result of capillary penetration of the liquid phase of blend into the open pores and defects of the masonry lining. This contributes to the integrity of coating and prevents the peeling and spalling of the coating in the course of continuous heating and cooling of the thermal generating unit. Application of protective coatings to the hot surfaces of aluminium silicate masonries extends the service life of the thermal generating unit by several times.

Protective and strengthening coatings may be useful in modification of different refractory, heat-protective and heat-insulating materials of heating boilers of thermal power plants, metallurgical furnaces, molten baths and melting pots, reactors in chemical and petrochemical industries, different types of waste incinerators, furnaces for burning construction materials and in many other industries.

Video: Behaviour of innovative special coating under abrupt temperature differences serving as an indicator of material performance characteristics.

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