Sintering technology of ceramic kiln

1, the concept of sintering atmosphere The sintering atmosphere of ceramic products refers to the percentage of free oxygen and reducing components contained in the combustion products in the kiln during the sintering process. Generally, the firing atmosphere is divided into two types: oxidizing atmosphere and reducing atmosphere. The atmosphere with free oxygen content above 8% is called strong oxidation atmosphere, the atmosphere with free oxygen content between 4% and 5% is called ordinary oxidation atmosphere, and the atmosphere with free oxygen content between 1%- 1.5% is called neutral atmosphere. When the free oxygen content is less than 1% and the co content is less than 3%, it is called weak reducing atmosphere, and when the co content is higher than 5%, it is called strong reducing atmosphere. In actual production, what kind of atmosphere system is used to fire ceramic products depends on the composition of raw materials in the product formula and the physical and chemical reflection of each stage in the firing process. When the raw material contains less organic matter and carbon, low viscosity, weak adsorption and high iron content, it is suitable for sintering in reducing atmosphere, but suitable for sintering in oxidizing atmosphere. 2. Influence of sintering atmosphere on product performance As we all know, the atmosphere will affect the physical and chemical reaction speed, volume change, grain size and pore size of ceramic body at high temperature, especially the color, transmittance and glaze quality of ceramic body. ① Influence on valence states of iron and titanium In actual production, when sintering in oxidizing atmosphere, the melting degree of fe2o3 in the green body is very low in the glass phase with low alkali content, and colloidal fe2o3 can be precipitated to make the green body yellow. When sintered in reducing atmosphere, feo formed melts into light blue in the glass phase. In addition, when the iron oxide content in the green body is constant, if the green body is fired in an oxidizing atmosphere, a part of fe2o3 surrounded by the glaze layer will react with sio2 to generate fayalite and release oxygen, and the reaction is as follows: (2fe2o3+2SiO2 → 2 (2feo. Oxygen produced by the reaction of SiO _ 2)+O2 ↑) will cause bubbles and holes in the glaze, while the remaining Fe2O3 will make the green body yellow. For the blank with high titanium content, sintering in reducing atmosphere should be avoided, otherwise part of tio2 _ 2 will turn into blue to purple ti2o3, and black 2feo ti2o3 spinel and a series of iron-titanium mixed crystals may be formed, thus deepening the color. (2) Reducing sio _ 2 and decomposing CO. At a certain temperature, reducing atmosphere can reduce sio _ 2 to gaseous siO, and at a lower temperature, it will decompose according to 2SIO → SiO _ 2+Si, thus forming black spots of Si on the surface of products. At a certain temperature, co in the reducing atmosphere will decompose in the order of 2co → co2+C. At 400℃, co2 is stable, while at 1000℃, there is only 0.7% CO2 by volume. Below 800℃, the decomposition rate of co is faster, and above 800℃, a certain catalyst is needed. Although carbon also has catalytic effect, it needs a certain surface area, and the catalytic effect of free iron oxide has nothing to do with the surface area, so it is likely that carbon decomposed by carbon monoxide is deposited on the glaze in reducing atmosphere to form black spots. If the sintering temperature continues to rise, the carbon seal will be oxidized into co2 in the green body, which will form bubbles, especially for the green body with strong adsorption performance. 3. Influence of sintering atmosphere on product defects A series of physical and chemical reactions will occur in ceramic products during sintering, such as evaporation of water, decomposition of salts, oxidation of organic matter, carbon and sulfide, transformation of crystal form and formation of crystal phase. The speed of these physical and chemical reactions is not only affected by temperature, but also by the atmosphere. If it is not properly controlled, it will cause various defects of ceramic products. The following are the most common defects. (1) The black core of black-core ceramic products refers to the phenomenon of organic matter, sulfide and carbide. During the sintering process of the green body, carbon particles and iron reducing agent are not fully generated due to oxidation, resulting in black, gray and yellow in the middle of the green body. The existence of black core defects will affect the strength, water absorption, color and other performance indexes of ceramic products. The key to the black core defect of ceramic products is insufficient oxidation of organic matter, carbide and sulfide. In the low temperature stage of the firing process, ceramic products undergo the decomposition of organic matter and the following oxidation reactions: (FeS2+O2 → FeS+SO2 = (350 ~ 450℃); (4 FeS+7 O2→2 fe2o 3+4so 2 ↑( 500 ~ 800℃)； (C+O2→CO2 ↑( 600)。 In actual production, in order to eliminate the "black center" of products, it is necessary to start the combustion of organic matter at 600~650℃, and fully oxidize organic matter, iron compounds and carbon at 300~850℃, that is to say, to ensure a strong enough oxidation atmosphere in the preheating zone. In addition, in the low temperature stage of sintering, co in flue gas will decompose, and the reaction formula is as follows: (2co→ 2cleft+O2 ↑) This decomposition will be obvious above 800℃, and it will also be obvious when there is a catalyst below 800℃ (free feo is a good catalyst). If the oxidizing atmosphere in the kiln is insufficient at the low temperature stage and there is a reducing atmosphere, co will decompose violently and C will precipitate due to feo in the reducing atmosphere. At the low temperature stage, due to the high porosity of the green body, the precipitated C is easily adsorbed on the surface of the green body pores, forming black spot defects. ② In the low temperature stage of the firing process, bubble and pinhole ceramic products are accompanied by carbonate decomposition besides the above oxidation reaction: (MgCO3 → MgO+CO2 = (500 ~ 750℃)), (CaCO3 → Cao+CO2 = (550 ~1000℃) The speed and completeness of these reactions are all affected by the atmosphere. When the sintering process enters the high temperature stage, liquid phase appears in the green body, and the gas produced by the reaction cannot be freely discharged from the green body, so defects such as pinholes and bubbles appear. It is impossible to oxidize and decompose all the gas components in the green body at the low temperature stage, because carbonate and fe2o3 will be decomposed at a temperature higher than 1300℃ in the oxidizing atmosphere. However, in such a high temperature area, the liquid phase already exists in the green body, and the viscosity decreases, and the decomposed bubbles will break through the liquid phase and escape, resulting in uneven glaze surface or remaining in the glaze layer, forming bubble defects. In order to solve this problem, before the high temperature (about 1000℃), the sintering atmosphere should be controlled as reducing atmosphere, and fe2o3 and sulfate should be reduced and decomposed into: (Fe2O3+Co→ 2feo+CO2 =), (CaSO4+Co→ CaSO4+CO2 =), (CaSO4 → Cao+SO2 =). In the raw materials of ceramic bodies and glazes, some iron and titanium compounds are always introduced more or less. In the sintering process, different sintering atmospheres will affect the valence states of iron and titanium, and different valence states of iron and titanium will have different colors. When the sintering atmosphere is unstable, the color of the green body will change accordingly, thus forming the color difference of the product. At present, the vanadium-titanium metal brick popular in the market has a high titanium content in the blank. For example, in reducing atmosphere, part of tio2 _ 2 will be transformed into blue to purple ti2o3, resulting in color difference, and black feo ti2o3 spinel and iron-titanium mixed crystals may also be formed, thus deepening the color of iron and forming brick surfaces with different shades. The reaction formula is as follows: (tio2+co→ti2o3+co2↑), (FeO+2TiO 2+Co→ FeO Ti2O3+CO2 ↑) 4. The control of firing atmosphere is limited by the kiln structure and equipment configuration, such as the size of fan air volume, the size of air duct diameter, the setting of smoke exhaust port, heat exhaust port and moisture exhaust port, etc. But the most important thing is to stabilize the pressure system and operate the burner reasonably. ① The pressure change of the pressure stabilizing system will affect the gas flow state, so the fluctuation of the pressure system in the kiln will cause the fluctuation of the atmosphere. Controlling the atmosphere requires a stable pressure system, and the key of the stable pressure system is to control the zero pressure surface. In the preheating zone of the kiln, the pressure is relatively lower than the pressure outside the kiln. In contrast, the air pressure in the kiln is in a negative pressure state, and cold air is blown into the cooling zone to cool the products, which is relatively higher than the environment outside the kiln. In contrast, the air pressure in the kiln is in a positive pressure state, and there is a zero pressure surface between the positive pressure and the negative pressure, and the sintering zone is between the preheating zone and the cooling zone, so the movement of the zero pressure surface will cause the change of the atmosphere of the sintering zone. When the zero pressure surface is located in the front of the firing zone and between the firing zone and the preheating zone, the air pressure in the firing zone is in a slightly positive pressure state, and the atmosphere is a reducing atmosphere. When the zero-pressure surface is located at the back of the firing zone, the firing zone is in a slightly negative pressure state, and the atmosphere is an oxidizing atmosphere. (2) Whether the fuel burned by the reasonable operation burner is completely burned will affect the atmosphere in the kiln, especially the atmosphere in the firing zone. Therefore, it is an important means to control the atmosphere in the kiln to operate the burner reasonably and control the combustion degree of fuel. When the fuel is completely burned, all combustible components in the fuel can be completely oxidized under the condition of sufficient air, and there are no free combustible components such as C, co, h2 and ch4 in the combustion products, thus ensuring the realization of the oxidation atmosphere. When the fuel is not completely burned, there are some free C, co, h2 and ch4 in the combustion products, which makes the atmosphere in the kiln reductive. To make the fuel burn completely, we must pay attention to the following three points: ① ensure that the fuel and air are fully and evenly mixed; (2) to ensure adequate air supply and maintain a certain amount of excess air; ③ Ensure that the combustion process is carried out at a higher temperature.

5. Adjustment of sintering atmosphere in actual production Many people are well aware of the above theoretical points of stabilizing atmosphere, but in actual operation, the atmosphere in the kiln will change unconsciously because of solving some sintering problems, which is often overlooked. The following are frequently asked questions. ① changing the excess air coefficient to increase the sintering temperature; In order to maximize the output of a single kiln, some enterprises have continuously accelerated the sintering speed and shortened the sintering cycle. The most common means for operators is to increase the fuel supply, but after the fuel supply increases, the supply of combustion air and the adjustment of the main brake of combustion fan are often not adjusted in time, resulting in the combustion atmosphere changing from oxidizing atmosphere to reducing atmosphere. ② Changing the atmosphere to solve the defects in the preheating zone; Some operators in order to reduce the temperature in the back section of the preheating zone, reduce the opening of the smoke exhaust door, affect the pressure balance and gas flow in the kiln, and weaken the oxidizing atmosphere in the preheating zone. If the control is not good, it will easily lead to the bad combustion state of the forehearth and make the atmosphere fluctuate. (3) In order to solve the defects of the cooling zone, changing the cold air volume not only affects the change of the whole kiln pressure system, but also changes the atmosphere. For example, if cold air is added, the zero pressure surface will easily move to the preheating zone, otherwise, the atmosphere will be changed. In order to stabilize the pressure, the opening of the extraction gate must be adjusted accordingly to balance the air inlet and outlet of the whole kiln and stabilize the zero pressure surface.