Refractory materials should be able to be used for a long time without damage under certain high temperature operating conditions. The performance is generally measured by the following technical indicators.
(1) Refractoriness
When the
refractory material is under sufficient high temperature, it will gradually soften and melt into a certain viscous liquid. Therefore, refractoriness refers to the temperature at which a refractory material resists high temperatures without melting. It is one of the main aspects of performance of refractory materials.
The refractoriness is measured with a sample of specified size and shape at a certain heating rate. This standard sample is a triangular pyramid with a height of 30 mm, a bottom of 8 mm and an upper side of 2 mm. Under the influence of high temperature, the sample gradually softens, and under the action of its own weight, it tilts toward the bottom according to the decrease in viscosity after the formation of the solution. The temperature at which the apex of the sample is lowered to its bottom plane is used as the assumed "melting" temperature of the refractory material. The heating rate of the sample can affect this "melting" temperature, so a certain heating rate must be observed.
The measurement of the sample temperature is compared with the standard temperature measuring cone under the same heating condition. This "temperature measuring cone" is made of a mixture of kaolin, alumina and quartz, and fusible substances are added to the low temperature cone. In this way, temperature measuring cones with different bending temperatures are formed.
At present, the generally recognized refractory bricks should have a refractoriness above 1580℃.
(2) Structural strength at high temperature
The high-temperature structural strength of refractory products is generally expressed by the temperature of a certain amount of deformation caused by refractory materials under a static load of 2 kg per square centimeter. According to the amount of deformation, it is divided into the initial deformation temperature deformation of 4%~10%, and the final deformation temperature deformation of 20%~40%.
The load softening temperature of refractories mainly depends on the chemical-mineral properties of natural refractories (that is, the presence of certain crystalline phases), the characteristics of the crystalline structure of the product bricks, the ratio between the crystal and the glass phase (amorphous), and the glass The viscosity of the phase at a certain temperature. The general visible particle structure of the product also has a certain influence on it. The denser and firmer products have a higher initial softening point. In addition, increasing the number of fusible materials will also reduce the deformation temperature of refractory materials. The amount of reduction in the deformation temperature mainly depends on the chemical composition and the ratio of the fusible material. The most influential is the oxide that can increase the amount of liquid phase and reduce its viscosity. This oxide is Na2O for clay bricks and Al2O3 for silica bricks. However, oxides that are used as mineralizers and can increase and improve the crystallization of bricks can promote the increase of softening temperature.
The actual load on the vertical wall of the industrial kiln is much lower than the load of 2 kg per square centimeter adopted during the inspection. Only under special circumstances can it reach 0.5 to 1.0 kg per square centimeter. And when one side of the lining brick is heated, the weight of the load is borne by the colder side of the brickwork. The pillars and the top of the arch have a greater impact. In most cases, molten slag, fuel ash, mineral powder, gas, etc. are the main factors that damage refractory bricks. Due to the effect of these things, the chemical-mineral composition of the refractory brick can be changed, and its structural strength can be significantly reduced.
The heating conditions of the masonry on the coke oven are different from those of other industrial furnaces. The entire furnace body is composed of nearly 10 meters high refractory masonry, which is very heavy. The masonry is heated on both sides and is often subjected to mechanical friction. Therefore, it is necessary to maintain a low operating temperature, and the heating system must be strictly followed to maintain the life of the furnace bricks.
(3) Volume fixity at high temperature
When refractory bricks are kept at high temperature for a long time, it will cause some remaining phase components and structures to continue to change, resulting in recrystallization and sintering. The appearance of these chemical changes causes the volume of refractory products to change. These non-reversible dimensional changes are called residual expansion or contraction of refractory materials.
These residual expansions or shrinkages are caused by insufficient firing of refractory products. Therefore, under sufficient firing temperature and firing time, the refractory can achieve the highest volume fixation. However, when bricks are fired at an excessively high temperature, the deformation of the bricks and the vitrification of the structure may be caused. Due to the deformation of the bricks, a large amount of waste products are caused, and the vitrification of the structure will reduce its resistance to sudden changes in temperature.
When the residual shrinkage is too large, it will cause the cracks of the masonry brick joints, destroy the tightness of the masonry, and result in the loosening and destruction of the masonry structure.
The remaining swelling is less harmful. However, when it is too large, it will also cause swelling of the masonry, destroying its geometric shape and evenly distributed stress.
The residual shrinkage and expansion of refractory materials are measured by repeatedly calcining them at a certain temperature. The verification temperature of each group and each product is determined according to the requirements and use conditions of this product. For clay and semi-silica refractory materials in various states, it is 1250~1450℃, and for silica bricks it is 1450°C. The volume change before and after calcination is measured, converted into linear shrinkage, and expressed as a percentage. The allowable figures for residual shrinkage or expansion of various refractory materials depend on the nature of their use and generally should not exceed 0.5 to 1.0.
(4) Resistance to sudden temperature changes
In intermittently operated industrial kilns, due to changes in the heating temperature, or temperature fluctuations in continuously operating equipment, refractory products will crack and even spall parallel to the heating surface. The resistance of refractory products to repeated temperature fluctuations without damage is called the resistance to sudden changes in temperature.
The cause of this cracking is due to the stress generated by the temperature difference inside the product after the heating temperature changes. It is directly related to the thermal expansion performance and thermal conductivity of refractory products.
It is very complicated and not easy to use calculation method to determine the temperature sudden change resistance. Therefore, the direct measurement method is generally used. This method is to place one end of a standard-size refractory product in an electric furnace and rapidly heat it to 850$, and then cool it in flowing water. According to the standard method of OCT-3267, the rapid cooling and rapid heating times that can be withstood before the peeled part reaches 20% of the initial total weight are used to indicate the temperature sudden change resistance of the refractory.
Such a measurement method is obviously inconsistent with the operating conditions of industrial kilns. However, due to the limitation of test time, this method is generally used to test.
In addition, there are some other test methods. From the results of these tests, the performance of a certain refractory material can be provided. When selecting the refractory material used in various parts of the coke oven, it should be considered as one of the important conditions. .
(5) Some physical properties The main physical properties are as follows:
1. Thermal expansion:
Refractory products are the same as all physical objects-they expand when heated, and shrink when cooled. This kind of expansion is a reversible physical change, which is different from the aforementioned "survival expansion". The expansion of the former is an irreversible change caused by changes in phase composition and structure, while thermal expansion depends on the chemical-mineral composition of the material, and the structure, density and strength of the brick have no effect.
To evaluate the properties of refractory materials, not only the expansion coefficient must be considered, but also the balance in the entire expansion process. Especially in coke ovens that require compact masonry structure, long life, and use silica bricks, thermal expansion becomes even more important.
2. Thermal conductivity:
The thermal conductivity coefficient of refractory products is expressed in terms of thermal conductivity ". Its unit can be calculated using the technical unit-kilocalories/meter•hour•degree, or the physical unit-millicacal/cm•second•degree.
The thermal conductivity λ value increases with the increase of heating temperature. For example, the lambda value of silica brick at room temperature is about 1 kcal/m•hour•degree, and it will increase to 1.5 kcal/m•hour•degree at 1000~1200℃. The value of clay bricks also changes similarly. But for some refractory products with crystalline structure, when the temperature increases, the lambda value decreases instead. For example, the lambda value of magnesia brick is 4~5 kcal/m•hour•degree at room temperature, and it drops to 2~3kcal/m•hour•degree at 1000℃. Silicon carbide bricks are particularly noticeable.
Thermal conductivity decreases as the porosity of refractory products increases. For example, a compact clay brick with a volume specific gravity of 1.95 has a lambda value of 0.9 kcal/m•hour•degree, but when the volume specific gravity increases to 2.2, the lambda value increases to 1.10 kcal/m•hour•degree.
Thermal conductivity is a very important technical index for the heating wall of the coke oven carbonization chamber.
3. Heat capacity:
The heat capacity is expressed in kilocalories/kg, degrees. It is useful in calculating the heat content of the checker bricks and furnace wall bricks of the coke oven regenerator. It can indicate the ability of the masonry to absorb heat from the exhaust gas.
(6) Density index of brick structure
The compactness and mechanical strength of the granular structure of refractory products are another important aspect of the performance of refractory materials. The increase in tissue density and strength means that this refractory product can withstand harsher production and operating conditions without being damaged.
1. Density:
The density of refractory products is indicated by the following values: water absorption, bulk density, apparent porosity and true porosity. Bulk density and apparent porosity are important indicators for evaluating various refractory materials.
The same type of brick, especially in the same factory, is manufactured with the same raw materials according to the prescribed procedures, and the fluctuation of the product volume density is not large. Therefore, the firing of refractory products, the quality of raw materials or the quality of raw materials can often be judged by volume and weight values. Whether other production processes are good.
Volume weight is the unit volume weight of the material, including voids, expressed in grams/cm3. It is measured by the hydrostatic weighing method after the bricks are saturated with water.
The amount of water absorbed by the bricks after boiling is called water absorption, which is expressed as a percentage of the weight of the bricks when they are dry.
The ratio of the volume of the bricks occupied by boiling water to the entire volume of the bricks is called apparent porosity. If the specific gravity of water is 1.0, the apparent porosity is the result of dividing the weight of inhaled water by the volume of the brick (expressed as a percentage).
The true porosity is the sum of all voids-including the pores that can be penetrated by boiling water, and the ratio of their specific gravity, expressed as a percentage,
The weight per unit volume of the material (excluding voids) is called the true specific gravity.
2. Breathability:
When the two sides of the refractory product are in contact with gases of different pressures, the gas will flow from the side with higher pressure to the side with lower pressure through the air holes in the refractory product. This property of refractory products is called air permeability, and it shrinks as the porosity of refractory bricks decreases. In addition, air permeability also depends on the size of the pores and their relationship with each other. Therefore, in addition to indicating the amount of pores, air permeability can also indicate the nature of pores.
In the masonry of the coke oven, most of them are under the airflow of different pressures, such as the wall of the regenerator and the wall of the carbonization chamber. In order to ensure the tightness of these masonry during production, refractory bricks should be required to have minimum air permeability.
3. Compressive strength:
In most industrial kilns and coke ovens, the load on refractory products is not large, generally not exceeding 1 to 2 kg/cm². In fact, the compressive strength of most refractory products is between 250 and 350 kg/cm² or higher. Therefore, the compressive strength of the product is by no means to resist the static load generated on the furnace wall. High compressive strength is the main indicator of the processing quality of the molding material, the uniformity of the brick structure and the good degree of firing. Certain high-strength products must often have a high firing temperature in order to complete recrystallization, brick sintering, and reduce residual shrinkage processes. In order to resist friction, impact and other mechanical effects, high compressive strength is also required.
The compressive strength supplements the porosity and becomes a reliable indicator for checking the uniformity of the product structure and the correctness of the operation process. Therefore, each refractory product must be tested for compressive strength.
According to a special test, it is determined that the strength of most refractory products increases with the increase of temperature, reaching the highest strength at 1000℃~1100℃. The highest value may be 200~300% of the value obtained at room temperature. But as the temperature increases, it decreases significantly.
(7) Chemistry-mineral properties
The chemical and phase composition of bricks and the structural characteristics of crystals determine the various properties of refractory products. The physical properties of refractory products are also limited to a certain extent by chemical-mineral properties. The structural strength of refractory products at high temperatures, the volume fixation during firing, and the slag resistance fixation, etc., are also largely affected by the chemical-mineral properties.
(8) The correctness of external dimensions
According to some damaged industrial furnaces, the brick joints of refractory masonry are generally the starting point of damage. Therefore, good quality masonry should ensure the width and uniformity of the brick joints, especially in the coke oven refractory masonry, which is more important, and the correctness of the brick joints depends on the correctness of the external dimensions of the refractory bricks.
It is very difficult to maintain the correct shape and size of the product, especially the special-shaped product with a large unit weight, because the whole production process is accompanied by shrinkage and expansion from the beginning of the refractory product to the final burning. . There are many factors that affect the shrinkage and expansion values. For example: the composition of raw materials, particle gradation, degree of wetting, uniformity of distribution, pressing pressure and firing temperature, etc., all of which will affect the correctness of the product dimensions.
