Refractory for kilns

Refractory raw material forging kiln

In order to ensure the stability and high strength of raw materials, many natural and synthetic raw materials need to be calcined at high temperature to make clinkers. There are mainly two kinds of equipment for calcining refractory raw materials: shaft kiln and rotary kiln.

(1) Shaft kiln

Shaft kiln is one of the raw material calcination equipment, commonly used to calcine dolomite, magnesite, clay, high bauxite and other raw materials.

Shaft kiln is a cylindrical kiln body, material from the kiln top added, and then the bottom discharge. The air needed for fuel combustion is sent in from the bottom of the kiln, and the combustion products are discharged from the top of the kiln. Therefore, vertical kiln belongs to counterflow thermal equipment.

Materials in the vertical kiln need to go through three belts, namely, pre-tropical belt, calcining belt and cooling belt. The lining of calcined belt is easy to be damaged, which is the key of lining selection in shaft kiln. In the same shaft kiln because each section of the working layer under the temperature, chemical erosion, mechanical wear, temperature change and material impact and so far, so, according to the specific circumstances to choose the lining material.

In the pre-tropical material with the help of the heat of flue gas preheating, and in the high temperature of the raw material decomposition and other chemical reaction, in this area furnace lining is mainly affected by the material block impact, friction; The scour and thermal shock when the furnace dust rises; Or it can be damaged by the chemical attack of the gas and the deposition of carbon.

In the calcination zone, the material is calcined with the help of the heat released by the combustion of fuel. In this area, the lining is mainly subjected to high temperature action, and the furnace dust is subjected to strong chemical erosion and thermal impact under high temperature. The lining material of this part is usually selected according to the condition of calcining raw material.

The calcined materials in the cooling belt are heat-exchanged with the cold air pumped in, and the materials are cooled, while the air is heated and enters the calcined belt as combustion air. The working environment is similar to the pre-tropical zone.

In general clay vertical kilns, the calcining temperature is relatively low, generally 1400℃, so the lining can be made of clay bricks. High alumina brick should be used for calcining belt and cooling belt in vertical kiln of high alumina clay, and clay brick can be used for other parts. For vertical kilns calcined with alkaline refractory raw materials, the calcination belt is generally built with high-quality magnesia bricks and magnesia aluminum bricks due to the high temperature of calcination, and can also be built with vaporized cooling walls. Other parts can be built with high strength clay bricks and high alumina bricks.

(2) Rotary kiln

Rotary kiln is a kind of advanced equipment with large production capacity and high mechanization degree. Because of its strong adaptability to raw materials, and the convenience of calcined synthetic products and products with special use requirements, it is increasingly used. Compared with shaft kilns, it has the following characteristics: it can calcine crushed materials and make full use of mine resources. The lumpiness of the ore calcined in shaft kilns is at least 25~30mm above, while the lumpiness of rotary kilns can be reduced to 5mm. Rotary kiln can calcine difficult to sinter and easy to lump raw materials, such as high alumina content of super grade and high grade bauxite, when calcined in shaft kiln, high temperature is easy to stick kiln lump, too low temperature is easy to burn, and with rotary kiln has no such disadvantage. But the rotary kiln capital construction investment is large, the equipment weight is large, the exhaust gas dust content is large, the product raw material and the combustion unit consumption is large.

The lining of clay and high aluminum rotary kilns is generally made of clay bricks and high aluminum bricks. The lining of magnesia and dolomite rotary kiln is made of magnesia or magnesia chrome brick in the firing belt, and other parts are made of clay brick or clay pouring material.

 

Refractory materials are fired in kilns

Refractory kiln can be divided into two types: continuous kiln (tunnel kiln) and intermittent kiln. The intermittent kiln includes square inverted kiln, round inverted kiln, shuttle kiln, bell jar high-temperature kiln and other intermittent kiln, its kiln lining with the same refractory material as tunnel kiln.

(1) Tunnel kiln

Tunnel kiln is a kind of can continuous production of high degree of automation, environmental protection, good, modern advanced kiln, tunnel kiln (continuous kiln) because of its flue gas discharge temperature generally is not high and 250 ℃, compared with the intermittent kiln, it is a kind of energy-saving kiln, is the most economical and most efficient kiln, is an important thermal technology equipment in refractory industry. Its firing temperature is 1200~1900℃, some even higher.

The tunnel kiln is composed of pre-tropical zone, firing zone and cooling zone. After the brick is installed outside the kiln, the kiln car burns through each zone in turn.

In general, hot gases introduced from the firing zone in the opposite direction from the kiln car advance are used to gradually heat the brick in the pre-tropical zone, during which the refractory products remove the adsorbed water and crystallized water, and the organic substances are decomposed.

Direct combustion of air and fuel heats the refractories to the specified temperature in the firing band and holds the heat, then enters the cooling band.

In the cooling zone, the cold air blown in from the outlet of the kiln is exchanged for heat with the burnt product to achieve the purpose of cooling. When the kiln car arrives at the outlet, the firing process of the refractory is completed.

At present, most of the tunnel kiln pre-tropical and cooling belt service life is very long. However, the high temperature of the sintering zone, coupled with the impact of flame, the erosion of volatile gas, the high-temperature creep of refractories, and the thermal shock spalling caused by temperature fluctuation make the service life of the sintering zone shorter, so it is necessary to make a correct and reasonable choice of refractory materials.

During the operation of the tunnel kiln, the working lining material of the firing belt should bear the effect of high temperature for a long time (1200~1900℃); Due to the diversity of refractory products, the operation system of tunnel kiln often changes, and the temperature frequently fluctuates, so that the lining of kiln is damaged by spalling caused by thermal shock. At the same time, due to the uneven distribution of temperature in the kiln, the linear expansion coefficient of each part in the kiln is different, which leads to greater thermal stress. Due to the failure of operation such as loading, the kiln liner is often physically damaged, and the kiln is forced to stop for repair. Kiln stoppage -- repair -- kiln opening also causes damage to the kiln body, reducing the service life of the kiln body; The corrosion of the lining material by the vaporization and combustion gas produced by the volatile components such as alkali metal and binder in the fired products; Under the action of high temperature and atmosphere, the mg-Cr refractories used in the lining of ultra-high temperature tunnel kiln evaporate and undergo chemical variation. Causing structural changes and layered cracks; Occurrence caving, drop block, decrease in strength, etc.

Therefore, the working conditions for the lining of tunnel kiln are continuous operation for a long time. In order to ensure the normal temperature and pressure system of the kiln, the requirements for the tightness of masonry for the working lining materials of the firing belt of tunnel kiln are as follows:

(1) Good resistance to gas erosion.

(2) It has high high temperature strength and good thermal shock resistance.

(3) Good volume stability at high temperature, the kiln lining refractory should be fired at no lower than the working temperature of the kiln, the refractory brick should not shrink or show a small amount of expansion after heating.

Refractory materials used in the high temperature zone of tunnel kiln shall be consistent with the characteristics of kiln temperature operation and with the products fired. Refractory clay bricks are used at temperatures below 1300℃ and high alumina bricks are used at 1300~1400℃. Silicon brick is used at 1400~1500℃. Magnesium aluminum brick is used at 1500~1600℃. Corundum bricks are used below 1800℃.

Tunnel kiln of silicon brick and clay brick, the firing temperature is around 1400℃. The lining of silicon brick with high temperature volume and high temperature strength shall be built, and other parts shall be built with clay brick. High aluminum brick tunnel kiln, firing temperature around 1550℃, with high aluminum brick masonry; For the high temperature tunnel kiln of firing alkaline refractory, the working lining of firing belt should choose high quality alkaline refractory.

(2) Inverted flame kiln

Inverted flame kiln is the flame in the kiln from the top to the bottom, so it is called inverted flame kiln. Inverted flame kiln is intermittent operation, which is divided into two kinds of round kiln and square kiln, the round kiln is used more because of the uniform temperature distribution. The masonry of inverted flame kiln is subjected to the scouring action of high temperature and combustion airflow, as well as the influence of temperature change, so the lining material of kiln should have high mechanical properties of high temperature and good thermal shock resistance.

(3) Selection of refractory materials for shuttle kiln (intermittent kiln)

Shuttle kiln is a kind of rotary inverted kiln, its structure is similar to the traditional rectangular inverted kiln.

The general principle of material selection for shuttle kiln is to use full heat preservation type refractory, that is, the kiln body, kiln car, kiln lower flue all use light refractory. Because shuttle kiln for intermittent operation, the requirement of rapid heating, rapid firing, good temperature uniformity. In order to achieve rapid firing and reduce energy consumption, it is necessary to reduce the heat storage of kiln body and kiln car. Kiln car has less heat storage, and the temperature uniformity of upper and lower in the kiln is good. In order to reduce the volume of concrete in the foundation part of the kiln, light refractory materials are also used in the flue part abroad, because the masonry with heavy refractory materials must be heavier, the size of the foundation part is also increased. In addition, the use of light refractory materials, in the furnace at the low temperature heating stage can also ensure that the temperature of flue gas to reduce less, achieve the effect of reducing energy consumption. However, from the current situation, the quality of light refractory for shuttle kiln, especially for shuttle kiln at high temperature (1500~1800℃), is not ideal, especially the poor thermal shock resistance, which limits the development of shuttle kiln at high temperature.