Refractories Refractories for firing kilns

Refractory firing kiln can be roughly divided into continuous kiln (tunnel kiln) and intermittent kiln two categories. Intermittent kiln also includes square inverted flame kiln, circular inverted flame kiln, shuttle kiln, bell jar type high temperature firing kiln. Inverting flame kiln, shuttle kiln, bell jar type high temperature firing kiln and other intermittent kiln, the kiln lining with the same refractory materials as tunnel kiln.

(1) Tunnel kiln

Tunnel kiln is a kind of can continuous production of a high degree of automation, good for environmental protection, modern advanced kiln, tunnel kiln (continuous kiln) because of its flue gas discharge temperature generally is not higher than 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 in 1200~1900℃, some even higher.

The tunnel kiln is composed of pre-heating zone, firing zone and cooling zone. After the kiln car loads the bricks outside the kiln, the kiln is fired through the above zones in turn.

Usually, in the pre-tropical with kiln car forward in the opposite direction from the introduction of hot gas sintering zone to make the brick gradually heating and heating, refractory products during the removal of adsorption water or crystal water, and the decomposition of organic materials.

Refractories are heated to a specified temperature by direct combustion of air and fuel in the firing zone and then enter the cooling zone.

In the cooling zone, the cold air blown from the exit of the kiln is exchanged with the fired products to achieve the purpose of cooling. When the kiln car arrives at the exit, the burning process of refractory materials is completed.

At present, the service life of most tunnel kilns pre-heating and cooling zones is very long. The sintering zone temperature is high, coupled with the impact of flame, volatile gas erosion, high temperature creep of refractory materials, temperature fluctuations caused by thermal shock peeling, so that the sintering zone service life is short, which requires a correct and reasonable choice of refractory materials.

During the operation of tunnel kiln, the working lining material of firing zone should bear the effect of high temperature (1200~1900℃) for a long time. Due to the diversity of refractory products, the operation system of tunnel kiln often changes, and the frequent fluctuation of temperature makes the kiln lining peeling damage 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 induces the greater thermal stress. Due to the operation such as loading fault, often cause physical damage to the kiln lining, forced to stop the kiln maintenance, kiln stop - maintenance - open kiln also cause damage to the kiln body, reduce the service life of the kiln body; The evaporation and gasification of volatile components such as alkali metal, binder and combustion gas in the fired products corrode the lining material; Under the action of high temperature and atmosphere, Mg-Cr refractories used for ultra-high temperature tunnel kiln lining will evaporate and change chemically. Cause structural change, produce lamellar cracks; Caving, falling, strength reduction, etc.

Therefore, the working condition of tunnel kiln lining is continuous operation for a long time. In order to ensure the normal temperature system and pressure system of the kiln, the requirements of masonry tightness for working lining materials of tunnel kiln firing zone are :(1) good resistance to gas erosion. (2) It has high high temperature strength and good thermal shock resistance. (3) good high temperature volume stability, kiln lining refractory material should be fired at no lower than the working temperature of the kiln, refractory brick after heating should not shrink or show a small amount of expansion.

The use of refractory materials in the high temperature zone of the tunnel kiln should be consistent with the characteristics of the kiln's gentle operation and with the products fired. In the temperature below 1300℃ use refractory clay brick, 1300~1400℃ use high aluminum brick; The silicon brick is used at 1400~1500℃; Magnesia-alumina brick was used at 1500~1600℃; Corundum bricks are used below 1800 ° C.

Silica brick and clay brick tunnel kiln, the firing temperature is about 1400℃, the lining of the sintering zone is constructed with silicon brick with good volume and high temperature strength, and the other parts are constructed with clay brick; High alumina brick tunnel kiln, firing temperature at about 1550℃, with high alumina brick masonry; For the high temperature tunnel kiln firing alkaline refractories, the lining of firing belt should choose high-quality alkaline refractories.

(2) inverted flame kiln

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

(3) shuttle kiln

The shuttle kiln is a kind of kiln car type inverted flame kiln, and its structure is basically similar to the traditional rectangular inverted flame kiln. The general principle of the selection of shuttle kiln refractory material is to use the full insulation refractory material, that is, kiln body, kiln car, kiln flue are light refractory material. Because shuttle kiln is intermittent operation, it requires rapid heating, rapid firing and 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. The heat storage of kiln car is less, and the upper and lower temperature uniformity in the kiln is good. In order to reduce the concrete volume of the foundation part of the kiln body, foreign countries also use light refractory materials in the flue part, because the masonry with heavy refractory materials must be weighted, remember that part of the size is also increased. In addition, the use of light refractory materials, in the low temperature heating stage of the kiln can also ensure the temperature reduction of flue gas less, to achieve the effect of reducing energy consumption. However, from the current situation, the quality of lightweight refractories for shuttle kilns, especially those for high-temperature shuttle kilns (1500~1800℃), is not ideal, especially the poor thermal shock resistance, which limits the development of high-temperature shuttle kilns.