In the production practice of sintered bricks, the drying link is one of the key links to achieve enterprise benefits. Without the yield and quality of dry green bodies, there is no business benefit. The preparation and molding of the mixture at the front end of the drying process and the firing process at the back end of the drying process have a great impact on improving the efficiency of the enterprise, but the drying process has become the most important process at present.
Whether it is a small enterprise fired in a natural drying hoffman kiln, a tunnel kiln production line using secondary firing or a modern large-scale primary firing tunnel kiln production line, the drying process of wet blanks is still the key to restricting the output and quality of sintered bricks Factors play a decisive role in the level of enterprise efficiency. Therefore, the key link of increasing the output of sintered brick enterprises should be implemented in the drying link, not the roasting link.
(1) Source and form of moisture
The process of removing moisture from the green body is called drying.
The water in the green body is mainly composed of three parts, free water, adsorbed water and chemically bound water.
Free water is also called mechanical water or free water: it is distributed between particles and in capillaries, and the binding is relaxed and it is easier to remove. Free water is drained during the drying stage, and the volume shrinks.
Adsorbed water: attached to the surface of the particles, the quantity is related to the ambient temperature and humidity, and has a certain equilibrium relationship, that is, it changes reversibly with the surrounding medium conditions, and the volume remains unchanged.
Chemically bound water: It is contained in the molecular structure of minerals, and the binding is firm, and it requires a large amount of energy to be removed. The chemically bound water content is less, and the discharge temperature in the preheating section of the roasting kiln is generally 430℃~750℃.
Under the premise of fixed drying conditions, the moisture in the green body can be divided into free moisture and equilibrium moisture, and the free moisture must be removed to the maximum extent in the drying stage.
From a technological point of view, the moisture in the brick comes from the raw material itself containing water, the first stirring, the second stirring and the water addition of the extruder. The moisture content of bricks produced by different molding processes is quite different. In practice, people tend to pursue the output excessively, which reduces the extrusion pressure of the adobe, resulting in a decrease in strength and an increase in the moisture content of the adobe.
Some blind codes are high, which seriously exceeds the bearing capacity of the bottom bricks, and the yield of the fired products is very low. Under the premise of allowing, a molding process with low moisture content should be sought, which can ensure that the bricks are not deformed or cracked after they are placed on the kiln car, and the bottom bricks can withstand the pressure of the upper body.
(2) Green body drying mechanism
In the drying chamber, the drying medium (waste heat and hot flue gas) transfers heat to the surface of the green body through heat exchange. After the surface of the green body is heated, the heat is transferred to the interior of the green body by conduction, which is a heat transfer process. After the surface of the green body is heated, the surface water vaporizes and evaporates, and the water inside the green body moves to the surface due to the difference in material moisture, and then evaporates from the surface until the green body is dried, which is a mass transfer process.
It can be seen that the entire drying process is both a heat transfer process and a mass transfer process. The mass transfer process can be further divided into two processes, the process of surface water vaporization and evaporation to the air is called external diffusion, and the process of internal water moving to the surface is called internal diffusion. In the whole drying process, the heat transfer process, the external diffusion process and the internal diffusion process are interconnected and carried out simultaneously.
When the drying medium is stable and the green body does not undergo chemical changes, the green body drying can be divided into four stages:
New cars are constantly entering the drying chamber. At this stage, the surface of the green body is heated to a temperature equal to the wet bulb temperature of the drying medium, and the evaporation rate of water increases rapidly. This stage is short, and the amount of water removed is not large. The drying medium has the lowest temperature and the highest humidity.
②Isokinetic drying stage
The water evaporated from the surface of the green body is continuously replenished from the inside to the surface of the green body, and the surface of the green body is always kept moist. At this stage, the drying speed remains unchanged, the surface temperature of the green body remains unchanged, and the water evaporates freely. The humidity of the drying medium gradually increases.
③ Slow down drying stage
The surface stops shrinking, and the continued drying only increases the pores inside the green body. The drying speed decreases, the heat energy consumption decreases, and the surface temperature of the green body increases.
When the surface moisture of the green body reaches the equilibrium moisture, the drying speed is 0. Dry final moisture depends on the temperature and humidity of the drying medium. The drying stages are not fixed, and will be adjusted with the temperature and humidity of the drying medium. In practice, it is necessary to find out the drying curve suitable for the characteristics of the raw materials according to the specific situation.
By studying the drying mechanism and drying rate, it can be seen that the factors affecting the drying rate are essentially the factors affecting the heat transfer rate and diffusion rate. The factors affecting the heat transfer rate are the temperature, humidity and flow rate of the drying medium. Generally speaking, the unit How much heat is sent into the drying chamber over time.
In practice, this phenomenon often occurs. Some production lines send hot air into the drying room with a high temperature, but the drying speed is very slow and the drying effect is poor. This is because the air volume drawn by the fan is too small, and there is no air in the drying room. Get enough calories. In some production lines, the air supply temperature is lower, but the drying effect is very good. This is because the air volume is large and the drying room has obtained enough heat.