Properties of Refractory Materials
Important characteristics of refractory materials include chemical composition, bulk density, apparent porosity, apparent specific gravity, and resistance to atmospheric temperature. These characteristics are often used as “checkpoints” during manufacturing and quality control. The chemical composition is the basis for the classification of refractory materials, and the density, porosity, and strength are affected by many other factors. These include the type and quality of raw materials, the size and blending degree of particles, the water content during pressing, the pressure in the mold, the firing temperature, the duration of firing, and the cooling rate. The properties of refractory materials can be divided into four types of service stress, namely (i) chemical stress, (ii) mechanical stress, (iii) thermal stress, and (iv) thermo-technical stress. Only by accurately understanding the characteristics of refractory materials and the stress of refractory materials during use, can we correctly choose refractory materials for furnace lining.
Chemical composition:The chemical composition of refractory materials is very important for the erosion of liquid slag, combustion dust, molten glass, and steam. According to the behavior of contact reaction, refractory materials can be divided into
- Acidic refractory materials (silicon dioxide, silicon carbide, cristobalite zircon, and zirconium silicate, etc.),
- Alkaline refractory materials (dolomite, magnesite, etc.) Chrome magnesite, chrome magnesite, forsterite, etc.)
- Neutral refractory materials (alumina, carbon, refractory clay, chromite, etc.).
Gas permeability: Gas permeability describes the properties of a solid porous refractory material that allows gas to flow in one direction under the influence of a pressure gradient. The airflow can be carried out by a pressure pump or a suction pump. Methods for determining gas permeability are described in various standards. The air permeability size is cm2 or Perm. Since this unit is very large, we usually use these units divided by 10 to the 9th power of dimensional terms.
Refractories determine the furnace’s ability to withstand the difficulties of transportation and handling before the refractories are incorporated. Based on other properties such as bulk density and porosity, it can be considered as a useful indicator of the effectiveness of fire and abrasion resistance. Frost resistance is determined according to methods described in various standards. The resistance of the hot layer can be determined by adding it to the resistance of the cold layer to evaluate its behavior at operating temperature.
Wear resistance: The mechanical stress of refractories is not only caused by pressure, but also by the abrasion and erosion of solid fillers when they slowly pass through the masonry in the furnace. Mechanical stress may also be due to the effect of fast-moving gas filled with fine solid dust particles. The Bohme grinder simulates the abrasive stress well, but the results usually cannot be applied to the conditions that exist in high-temperature furnaces, especially when the resistance of refractory bricks changes due to chemical influences. There is no approved measurement method to measure the wear resistance, and the Baume wear coefficient is still used as the reference value.
Porosity and density: The low porosity of refractory bricks is desirable because it improves the mechanical resistance and other properties of refractory materials. The real porosity of refractory bricks is the ratio of the total pore space of the entity (ie, open and closed pores) to its volume, expressed as a percentage by volume. The formula for true porosity is as follows: true porosity = (S-R) / S X 100% volume, where S is density and R is apparent density.
Thermal stress Properties
High-temperature cone equivalent: Refractory materials gradually melt within a temperature range due to their chemical complexity. The refractoriness of refractory materials is one of the most important properties of refractory materials. Since refractories are rarely composed of a single compound, the reference is not to a specific melting point, but a softening area.
Thermal rupture modulus: The thermal rupture modulus is the ability to resist high temperature bending stress. The flexural strength of refractory products provides information about their deformation behavior at high temperatures. The test sample is a bar heated in an electric box furnace. To perform the test, the bar is placed on the supporting edge of the furnace, and pressure is applied until a fracture occurs by applying an increased load to the center of the bar at the test temperature.
Thermal expansion: All materials undergo volume changes under the influence of temperature. Refractory materials may shrink or expand during use. This permanent change in size may be due to (i) a change in the form of an allotrope that causes a change in specific gravity, (ii) a chemical reaction that produces a new material with a change in specific gravity, (iii) the formation of a liquid phase, and (iv) sintering The reaction, and (v) can occur due to the effect of flux or alkali with dust and slag on the refractory clay refractory, forming alkali aluminosilicate, causing expansion and cracking.
Thermal shock resistance: Thermal shock resistance is one of the most important performance properties. It characterizes the behavior of refractory materials on the sudden temperature shock that often occurs during the operation of the furnace. Temperature fluctuations will greatly reduce the strength of the brick structure and may cause the layer to collapse or peel off. There are two standard methods for testing thermal shock resistance. They are (i) water cooling and (ii) air cooling. In the water-cooling method, the test piece is a standard cylinder, heated to 950 degrees Celsius, and then cooled in flowing cold water.
Thermal stress characteristics Thermal conductivity: Thermal conductivity is defined as the amount of heat flowing normally to the surface per unit area at a given time using a known steady-state temperature gradient. It has the general properties of the heat flux of refractory materials and depends on the chemical and mineralogical composition and temperature of the coating. The unit of measure for the thermal conductivity of a refractory is W / K * m, and the thermal conductivity is determined using the hot plate, sphere, hollow cylinder, or wire method.
Specific heat: Specific heat is the energy component associated with temperature and material and is determined colorimetrically. This factor represents the amount of energy (in joules) required to raise the temperature of 1 gram of material by 1 degree Kelvin. Compared to water, refractory materials have a very low heat capacity.
Apparent Density: To determine heat build-up, you need to know the apparent density of the refractory material. The term bulk density refers to the degree of mass and volume, including pores. Bulk density is generally considered to be high porosity. It is a measurement of the weight of a specific refractory material. For many refractory materials, high density is a common indicator of product quality.
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