Calcination is a common process in the production of high alumina refractory brick products. During the calcination process, a series of physical and chemical changes such as decomposition, solid-phase reaction, and sintering occur inside the raw materials or the green body to obtain a relatively stable composition, structure, and sufficient strength. The physical and chemical changes that occur in different types of refractory firebrick raw materials and products during the calcination process are different. In order to ensure the smooth completion of these physical and chemical changes, a certain calcination system and corresponding thermal equipment are required.
The calcination system usually includes temperature system, atmosphere and pressure system. In the actual production process, the temperature, pressure and atmosphere system are interrelated. There are many types of fire brick calcining equipment, collectively referred to as thermal kilns. It can usually be divided into two categories: ① Raw material calcination kilns, such as downdraft kilns, vertical kilns, rotary kilns, boiling furnaces, suspended light-burning furnaces, etc.; ② Product firing kilns, such as downdraft kilns, tunnel kilns, and shuttle kilns. According to the operation mode, it can be divided into continuous production kilns and intermittent production kilns. The former include tunnel kilns, rotary kilns, etc., and the latter include shuttle kilns, downdraft kilns, etc. Due to energy consumption, environmental protection and other reasons, downdraft kilns, ring kilns and square kilns that were commonly used in the past are now used less and less.
Tunnel kilns are common calcining equipment that can work continuously in high alumina refractory brick production. They can be used to calcine raw materials and burn products. The kiln is divided into several zones such as preheating, burning, and cooling. The kiln car loaded with the blank enters the entrance of the kiln. Driven by the cart device, it goes through the preheating, burning, and cooling zones to complete the entire burning process, and then is pushed out from the exit of the kiln. The length of the tunnel kiln mainly depends on the burning system and output of the product. The burning system mainly depends on the physical and chemical changes of the product during the burning process. For example, silica bricks have more complex phase changes during heating and cooling, so the requirements for the burning system are strict, and the corresponding kiln is also longer. In addition, factors such as fuel and operation should also be considered. The height of the tunnel kiln mainly depends on the characteristics of the blank during the firing process and the allowable temperature difference between the upper and lower parts. As the kiln height increases, the temperature difference between the upper and lower parts increases, which can easily cause uneven quality of the fired products. The width of the kiln is related to the output of the kiln and the allowable temperature difference. The walls and roof of the tunnel kiln are built with refractory materials and thermal insulation materials.
The shuttle kiln is a commonly used intermittent kiln. Its working process is to load the blank onto the kiln car, push it into the kiln, and heat it according to a certain heating system, so that the blank goes through the stages of preheating, insulation and cooling, and the product is taken out after cooling to a certain temperature. Its characteristics are that the production is carried out in batches, the thermal system is easy to adjust, and the flexibility is large. And its volume can be determined according to the product output, firing system, and the characteristics of the combustion device. The vertical kiln is a cylindrical kiln body, and the cylinder body mainly has a variety of shapes such as straight cylinder, trumpet shape, dumbbell shape and rectangular cross-section shape. It is mainly used to calcine bauxite raw materials for producing high alumina refractory bricks. The material is added from the top by the distribution equipment and discharged from the bottom by the unloading equipment. The air required for fuel combustion enters from the bottom. When using liquid or gas fuel, primary air can be supplied by the burner, and the hot air brought by cooling is secondary air.
The vertical kiln is roughly divided into three zones from top to bottom: preheating, calcining, and cooling. The material is preheated in the preheating zone with the help of the heat of the flue gas; calcined in the calcining zone; cooled by the cold air blown into the bottom of the kiln in the cooling zone, and the heated air enters the calcining zone for combustion. To ensure good calcination of the material, the above three zones should be kept at a certain height and strive for stability. The vertical kiln has high thermal efficiency and energy saving, but its disadvantage is that it is easy to form uneven airflow distribution, local high temperature causes the material to agglomerate in the kiln, and the production process is difficult to control.
