Drying of bricks

Oct 24, 2023

The purpose of drying the bricks is to increase the strength of the semi-finished products so that they can be safely transported, stacked and loaded into the kiln. After drying, the wet billet can also ensure rapid temperature rise in the early stages of firing. Especially for bricks with high moisture content, if they are not dried well, they will cause severe cracking and deformation during firing.

1. Dry working process of bricks

The drying process of bricks is essentially that the preheated hot air (or hot flue gas) transfers heat to the brick body. The brick body absorbs heat and increases the temperature, so that the water evaporates and escapes from the brick body, and is discharged with the hot gas for drying. device. The process of removing moisture from the green body is generally carried out in two stages, namely the constant speed drying stage and the decelerated drying stage.

In the constant-speed drying stage, the physical water on the surface of the brick is mainly eliminated, and the water evaporates on the surface of the brick. As the moisture is removed, the green body shrinks accordingly. Therefore, the drying speed at this stage should be slower to avoid rapid shrinkage of the green body and cracking. This is especially important for large or special products with high moisture content. When the constant-speed drying stage basically ends and enters the decelerated drying stage, the evaporation of water will gradually move from the surface of the green body to the inside of the green body. At this time, the drying speed of the green body is affected by factors such as temperature, the number and size of pores, and the bricks are loaded from the load. The heat absorbed in the hot body not only increases the temperature of the body, but also supplies water for evaporation. The drying speed of the bricks during this stage is related to the temperature of the bricks and the speed at which water vapor transfers outward from the pores. After the water evaporates in the pores, it moves from the inside of the body to the surface, and then diffuses into the heat transfer medium. When the temperature of the green body increases and the amount of water evaporation is exactly the maximum amount of water vapor transferred outward from the pores, the safe drying speed of the green body is the greatest at this time. Therefore, during this stage, if the temperature is too low, the evaporation of water will be reduced and the drying process will be delayed; if the temperature is too high, a large amount of water that has evaporated inside the brick will not be discharged in time, causing hairline cracks in the brick and even cracking. Therefore, when drying bricks, we must first select and control the appropriate temperature and humidity of the heat carrier to ensure the maximum safe drying speed of the bricks.

When bricks are dried, some physical-chemical changes often occur along with the evaporation process of water. For example, in the mud of aluminum silicate products, a small amount of sulfurous acid pulp waste liquid is usually added, which is concentrated during drying to cement the particles in the body to increase the strength of the body; silica bricks are made of colloidal Ca The transformation of (OH)2 into crystal hydrate Ca(OH)2H2O and the hydrous silicate (CaOSiO2nH2O) generated by its interaction with active SiO2, etc., all increase the strength of the silica brick body; unfired products combined with water glass, when dry The water glass undergoes condensation polymerization, which significantly improves the strength of the green body and so on.

2. Drying methods and drying equipment

At present, the drying methods used by refractory materials factories in my country mainly include the following.

1. Tunnel dryer drying

Tunnel dryer drying is the most commonly used drying method in large and medium-sized refractory plants. It is a drying equipment for continuous production. The heat source (heat carrier) is usually preheated with hot flue gas or a preheater, and is then sent into the dryer with a blower from one end (outlet end) and discharged from the other end (inlet end) using an exhaust fan. The temperature requirements of the heat carrier vary depending on factors such as product type, shape, size, and moisture content in the body. Generally, the supply air temperature is 120~200℃ and the exhaust temperature is 60~90℃.

The commonly used specifications of tunnel dryers are: 1.65m high and 0.95m wide. The length fluctuates greatly, depending on the specific conditions of each factory building, and is generally around 25-30m.

The drying time of bricks in the tunnel dryer is generally expressed in terms of cart time, and its fluctuation range is large. Usually the interval between carts is about 15 to 45 minutes. For large or special-type products, before entering the tunnel dryer, they should be naturally dried at room temperature for 24 to 48 hours, and then enter the tunnel dryer for drying to prevent cracking due to excessive shrinkage during drying.

The general requirements for residual moisture of bricks after drying are: 2 to 1.0% for clay products; less than 1 to 0.5% for siliceous products; and less than 1.0% for magnesia products.

2. Chamber dryer drying

The chamber dryer (or grid dryer) is an intermittent drying equipment. After the drying trolley is pushed in, the heat carrier is sent in for drying. Since the temperature and humidity of the heat carrier in this dryer are easy to adjust, it is suitable for drying large and special products. The heat source of this type of dryer generally uses air preheated by a preheater, and heating pipes are also installed on the inner wall of the dryer to directly heat the air in the dryer for drying.

Most refractory products are formed by machine pressing. After forming, the bricks have a low moisture content and do not shrink much during the drying process, so drying is relatively easy. Some factories even stack bricks such as clay bricks and high alumina bricks directly on the The firing is carried out on the tunnel kiln car in the drying section. In this way, the production cycle can be shortened and transportation waste can be reduced.



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