1. Thermal expansion
Refractory products are like all physical objects – they expand when heated and contract when cooled. This kind of expansion is a reversible physical change, which is different from the aforementioned “residual 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, while the structural properties, density and strength of the bricks have no effect.
To evaluate the properties of refractory materials, not only the size of its expansion coefficient, but also the balance in the whole expansion process should be considered. Especially in coke ovens that require dense masonry structure, long service life, and are made of silica bricks, thermal expansion is even more important.
The thermal conductivity coefficient of refractory products is expressed by “thermal conductivity”. Its unit can be calculated by technical unit – kcal/m•hour•degree, or physical unit – mcal/cm•second•degree.
The value of thermal conductivity λ increases with the increase of heating temperature. For example, the λ value of silica bricks at room temperature is about 1 kcal/m·hour·degree, and it increases to 1.5 kcal/m·hour·degree at 1000~1200°C. The values of clay bricks also change similarly. But for some refractory products with crystalline structure, when the temperature increases, the λ value shrinks instead. For example, the λ value of magnesia bricks at room temperature is 4~5, kcal/m•hour•degrees, and it is reduced to 2~3 kcal/m•hour•degrees at 1000°C. Silicon carbide bricks are particularly notable.
Thermal conductivity decreases with increasing porosity of refractory product. For example, the dense 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 indicator for the heating wall of the coke oven coking chamber.
Heat capacity is expressed in kcal/kg, degree. It is useful in calculating the heat content of coke oven regenerator checker bricks and furnace wall bricks, which can indicate the ability of masonry to absorb heat from exhaust gas.
4.Density index of brick tissue
The granular structure and mechanical strength of refractory products are another important aspect that indicates the performance of refractory materials. The increase in tissue density and strength means that this refractory product can withstand relatively harsh production and operating conditions without damage.
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 material according to the prescribed procedures, and the fluctuation of the product volume density is not large, so the firing of refractory products, the quality of raw materials or the quality of raw materials can often be judged according to the volume weight value Whether other production processes are good.
The volumetric weight is the unit volume weight of the material, including voids, expressed in g/cm3, and is measured by the static water weighing method after the small bricks are unloaded from the bricks and saturated in water.
The amount of water absorbed by the bricks after boiling is called water absorption and is expressed as a percentage of the dry weight of the bricks.
The ratio of the volume occupied by the boiling water to the entire volume of the brick is called the apparent porosity. If the specific gravity of water is 1.0, the apparent porosity is the result obtained by dividing the weight of absorbed water by the volume of the brick (expressed as a percentage).
The true porosity is the sum of all the voids—including the pores that can be penetrated by boiling water and the closed pores, the ratio of its specific gravity, expressed as a percentage, and its calculation method is as follows:
The weight per unit volume (excluding voids) of a material is called the true specific gravity.
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 pores in the refractory product. This performance of refractory products is called air permeability, which decreases with the decrease of porosity of refractory fire bricks. In addition, air permeability also depends on the size of the pores and their interconnection. 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 regenerator wall and the carbonization chamber wall. In order to ensure the tightness of these masonry during production, it should be required that the refractory bricks have a minimum air permeability.
In most industrial kilns and coke ovens, the load on refractory products is not large, generally no more than 1~2 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 mainly the main index indicating the processing quality of the molding material, the uniformity of the brick structure and the good degree of firing. Some products with higher strength often must have a high firing temperature in order to complete recrystallization, brick sintering, reduce residual shrinkage process, etc. In order to resist friction, impact and other mechanical actions, high compressive strength is also required.
The compressive strength complements 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 the special test, it is determined that the strength of most refractory products increases with the increase of temperature, and reaches the highest strength at 1000 ° C ~ 1100 ° C. This highest value may be 200~300% of the value obtained at normal temperature. But it decreased significantly with the increase of temperature.