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What are the technical indicators for measuring the performance of refractory materials?

Dec 06, 2023

Refractory materials should be able to be used for a long time without damage under certain high temperature operating conditions. Generally, its performance is measured by the following technical indicators.

 

 

01

Refractory

When the refractory material is exposed to sufficiently high temperatures, it will gradually soften and melt into a viscous liquid. Therefore, refractory refers to the temperature at which a refractory material can withstand high temperatures without melting. It is one of the main aspects of performance of refractory materials.

 

Refractoriness is measured by using a sample of specified size and shape at a certain heating rate. This standard specimen is a triangular pyramid with a height of 30 mm, a lower base of 8 mm, and an upper edge of 2 mm. Under the influence of high temperature, the sample gradually softens and tilts toward the bottom under the action of its own weight according to the reduction in viscosity after the solution is formed. The temperature at which the top of the specimen is lowered to its bottom plane is used as the assumed "melting" temperature of the refractory material. The heating rate of the specimen can influence this "melting" temperature, so a certain heating rate must be observed.

 

The measurement of the sample temperature is compared with a standard temperature measuring cone under the same heating conditions. This "temperature measuring cone" is made of a mixed material of kaolin, alumina and quartz, and fusible substances are also added to the low temperature cone. In this way, temperature measuring cones with different bending temperatures are formed.

 

At present, it is generally recognized that the refractory bricks have a refractory degree above 1580°C.

 

 

02

Structural strength at high temperatures

The high-temperature structural strength of refractory products is generally expressed by the temperature of a certain amount of deformation caused by the refractory material under a static load of 2 kilograms per square centimeter. According to the amount of deformation, it is divided into starting deformation temperature deformation of 4% to 10%, and final deformation temperature deformation of 20% to 40%.

 

The load softening temperature of refractory materials mainly depends on the chemical-mineral properties of natural refractories (that is, the existence of certain crystalline phases), the characteristics of the crystalline structure of finished bricks, the ratio between crystals and glass phases (amorphous), and the glass The viscosity of a phase at a certain temperature. The generally visible particle structure of the product also has a certain impact on it. Dense and solid products have a higher starting softening point. In addition, increasing the amount of fusible materials will also reduce the deformation temperature of the refractory material. The amount of reduction in deformation temperature mainly depends on the chemical composition of the fusible material and its ratio. The most influential thing 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 used as mineralizers that can increase and improve the crystallization of bricks can promote an increase in softening temperature.

 

The actual load on the vertical wall of an industrial kiln is far lower than the load of 2 kilograms per square centimeter adopted during inspection. Only under special circumstances, it can reach 0.5~1.0 kilograms per square centimeter. And when the side of the brick lining is heated, the weight of the load is borne by the cooler side of the masonry. The pillars and vaults have a greater impact. In most cases, slag, fuel ash, mineral powder, gas, etc. are the main factors that damage refractory bricks. Due to the effects of these things, the chemical-mineral composition of refractory bricks can be changed, significantly reducing its structural strength.

 

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 and has a very heavy weight. Masonry is heated on both sides and is often subject to mechanical friction. Therefore, the operating temperature needs to be kept low and the heating regime must be strictly followed to maintain the life of the furnace bricks.

 

 

03

Volume fixity at high temperatures

When refractory bricks remain at high temperatures for a long time, some remaining phase components and structures will continue to change, resulting in recrystallization and sintering. The occurrence of these chemical changes causes changes in the volume of refractory products. These irreversible dimensional changes are called residual expansion or contraction of the refractory material.

 

These residual expansions or contractions are caused by insufficient firing of the refractory product. Therefore, under sufficient firing temperature and firing time, the refractory material can achieve the highest volume fixation. However, when bricks are fired at too high a temperature, it may cause deformation of the bricks and vitrification of the structure. A large amount of waste is caused by the deformation of the bricks, and the vitrification of the tissue will reduce its resistance to sudden temperature changes.

 

When the remaining shrinkage is too large, it will cause cracks in the joints of the masonry bricks, destroying the tightness of the masonry, resulting in loosening and destruction of the masonry structure.

 

Residual expansion is less harmful. However, if it is too large, it will also cause swelling of the masonry, destroying its geometry 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 based on the requirements and usage conditions of the product. For clay and semi-siliceous refractory materials in various states, it is 1250~1450℃, and for silica bricks it is 1250~1450℃. 1450℃. The volume change before and after calcination is measured, converted into linear shrinkage, and expressed as a percentage. The allowable number of residual shrinkage or expansion of various refractory materials depends on the nature of their use, and generally should not exceed 0.5~1.0.

 

 

04

Resistance to sudden temperature changes

In industrial kilns that operate intermittently, due to changes in heating temperature, or in equipment that operates continuously, due to temperature fluctuations, cracks will occur in refractory products and even bricks may peel off parallel to the heating surface. The ability of refractory products to resist repeated temperature fluctuations without damage is called resistance to sudden temperature changes.

 

The reason for this kind of 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.

 

The use of computational methods to determine resistance to sudden temperature changes is complex and incomplete. Therefore, the direct measurement method is generally used. This method is to place one end of a standard-sized 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 temperature change resistance of the refractory material is indicated by the number of rapid cooling and heating times that the peeled part can withstand before the peeled part reaches 20% of the initial total weight.

 

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 for inspection.

 

There are also some other test methods. The results of these tests can provide the performance of a certain refractory material in this regard. This should be considered as one of the important conditions when selecting refractory materials used in various parts of the coke oven. .

 

 

05

Main physical properties

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 "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, and the structural properties, density and strength of the bricks have no effect.

 

When evaluating the properties of refractory materials, we must not only consider the size of its expansion coefficient, but also consider the balance during the entire expansion process. Especially in coke ovens that require dense masonry structures, long service life, and are made of silica bricks, thermal expansion becomes even more important.

 

2. Thermal conductivity:

 

The thermal conductivity coefficient of refractory products is expressed as "thermal conductivity coefficient". Its unit can be calculated in technical unit - kcal/m·hour·degree, or in physical unit - millicalorie/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 increases to 1.5 kcal/m·hour·degree at 1000~1200°C. A similar change occurs in the values for clay bricks. However, 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·degree, but it drops to 2~3 kcal/m·hour·degree at 1000°C. Silicon carbide bricks are particularly noticeable.

 

Thermal conductivity decreases as the porosity of refractory products increases. For example, the λ value of a dense clay brick with a volume specific gravity of 1.95 is 0.9 kcal/m·hour·degree, but when the volume specific gravity increases to 2.2, the λ value increases to 1.10 kcal/m·hour·degree.

 

Thermal conductivity is a very important technical indicator for building the heating wall of the coke oven carbonization chamber.

 

3. Heat capacity:

 

Heat capacity is expressed in kcal/kg, degrees. It is useful when calculating the heat content of coke oven regenerator checker bricks and furnace wall bricks. It represents the ability of the masonry to absorb heat from the exhaust gas.

 

 

06

Density index of brick tissue

The particle structure density 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 harsh production and operating conditions without being damaged.

 

1. Density:

 

The density of refractory products is expressed by the following values: water absorption, bulk density, apparent porosity and true porosity. Volume density and apparent porosity are important indicators for evaluating various refractory materials.

 

The same type of bricks, especially those made in the same factory with the same raw materials according to prescribed procedures, have little fluctuation in the volume density of the products. Therefore, the firing of refractory products, the quality of the raw materials, or the quality of the raw materials can often be judged based on the volume weight value. Are other production processes in good condition?

 

The volumetric weight is the unit volume weight of the material, including voids, expressed in grams/cm3, and is measured using the hydrostatic weighing method after saturating the bricks with small bricks taken from the bricks.

 

The amount of water absorbed by the brick after boiling is called water absorption and is expressed as a percentage of the dry weight of the brick.

 

The ratio of the volume of a brick occupied by boiling water to the entire volume of the brick is called apparent porosity. If the specific gravity of water is 1.0, the apparent porosity is the weight of absorbed water divided by the volume of the brick (expressed as a percentage).

 

True porosity is the sum of all pores - including pores that boiling water can penetrate and those that are closed, and its ratio to its specific gravity, expressed as a percentage. The calculation method is as follows:

 

The weight per unit volume of a material (excluding voids) is called the true specific gravity.

 

2. Breathability:

 

When both sides of a 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 property of refractory products is called air permeability, which decreases as the porosity of refractory bricks decreases. In addition to this, breathability 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.

 

Most of the coke oven masonry is under airflows of different pressures, such as regenerator walls, carbonization chamber walls, etc. In order to ensure the tightness of these masonry products during production, the refractory bricks should be required to have a minimum air permeability.

 

3. Compressive strength:

 

In most industrial kilns and coke ovens, the load endured by 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 designed to resist the static load generated on the furnace wall. High compressive strength is mainly an indicator of the processing quality of the molding material, the uniformity of the brick structure and the degree of firing. Certain products with higher strength often must have high firing temperatures in order to complete recrystallization, brick sintering, reduce residual shrinkage processes, etc. High compressive strength is also required to resist friction, impact and other mechanical effects.

 

The compressive strength supplements the porosity and becomes a reliable indicator for checking the uniformity of the product structure and the correctness of the operating process. Therefore, every refractory product must be tested for compressive strength.

 

According to special tests, it is determined that the strength of most refractory products increases as the temperature increases, reaching the highest strength at 1000°C ~ 1100°C. This maximum value may be 200~300% of the value obtained at normal temperature. But it decreases significantly with the increase of temperature.

 

 

07

Chemistry - Mineral Properties

The chemical and phase composition of bricks and the structural characteristics of crystallization determine the various properties of refractory products. The physical properties of refractory products are also limited by chemical-mineral properties to a certain extent. The structural strength of refractory products at high temperatures, volume fixation during firing and slag resistance are also affected to a large extent by chemical-mineral properties.

 

08

Correctness of overall dimensions

According to some damaged industrial kilns, the brick joints of refractory masonry are usually the starting point of damage. Therefore, good quality masonry should ensure the width and uniformity of brick joints, which is especially important for coke oven refractory masonry. Maintaining the correctness of brick joints depends on the accuracy of the dimensions of the refractory bricks.

 

It is very difficult to maintain the correct shape and size of products, especially special-shaped products with large unit weights, because the entire production process is accompanied by shrinkage and expansion from the time the refractory products are formed to the final firing. . There are many factors that affect the shrinkage and expansion values. For example: raw material composition, particle gradation, degree of wetting, degree of uniform distribution, pressing pressure and firing temperature, etc., all of which will affect the accuracy of the product's appearance and dimensions.

 

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