One. Definition of Refractory

Refractory material refers to the refractoriness of not less than 1580 ℃, has good resistance to thermal shock and chemical erosion ability, low thermal conductivity and low expansion coefficient, inorganic non-metallic materials. It includes natural ores and various products made by certain processes according to certain purposes. At present, refractory materials generally refer to inorganic non-metallic materials used in metallurgy, petrochemical, cement, ceramics and other production equipment lining.

II. Classification of refractory materials

There are many kinds of refractory materials, which are usually divided into acid refractory materials, neutral refractory materials and alkaline refractory materials according to chemical characteristics. In addition, there are refractory materials for special occasions.

Acid refractories are mainly composed of silicon oxide, and silica bricks and clay bricks are commonly used. Silica brick is a silicon product containing more than 94% silicon oxide. The raw materials used are silica, waste silica brick, etc. It has strong resistance to acid slag erosion, high load softening temperature, no shrinkage or even slight expansion after repeated calcination. However, it is vulnerable to the erosion of alkaline slag and has poor thermal shock resistance. Silicon bricks are mainly used in coke ovens, glass furnaces, acid steelmaking furnaces and other thermal equipment. Clay brick with refractory clay as the main raw material, containing 30% ~ 46% of alumina, is a weak acid refractory material, good thermal vibration resistance, corrosion resistance to acid slag, widely used.

Neutral refractories are based on alumina, chromium oxide or carbon. Containing more than 95% alumina corundum products is a wide range of high-quality refractory materials. The chromium brick with chromium oxide as the main component has good corrosion resistance to steel slag, but poor thermal shock resistance and low deformation temperature under high temperature load. Carbonaceous refractory materials are carbon bricks, graphite products and silicon carbide products, its thermal expansion coefficient is very low, high thermal conductivity, good thermal shock resistance, high temperature strength, resistance to acid and salt erosion, especially weak acid and alkali has good resistance, not by metal and slag wetting, light weight. Widely used as high temperature furnace lining material, also used as petroleum, chemical autoclave lining.

Alkaline refractory materials with magnesium oxide, calcium oxide as the main component, commonly used is magnesium brick. Magnesia bricks containing 80% to 85% magnesia have good resistance to alkaline slag and iron slag, and their fire resistance is higher than clay bricks and silica bricks. Mainly used in hearth furnace, blowing oxygen converter, electric furnace, non-ferrous metal smelting equipment and some high temperature equipment.

Three. Properties of refractory materials

1. Thermal properties and electrical conductivity of refractories

1.1 thermal expansion

The GB/T7320 standard has two definitions: linear expansion rate (the relative rate of change of the length of the sample from room temperature to the test temperature, expressed in%), and average linear expansion rate (the relative rate of change of the length of the sample for every 1 ℃ increase in the temperature from room temperature to the test temperature, in units of 10-6/℃).

The determination principle of GB/T7320 standard: the sample is heated to the specified test temperature at the specified heating rate, the change value of the sample length with the increase of temperature is measured, the linear expansion rate of the sample with the increase of temperature and the average linear expansion coefficient of the specified temperature range are calculated, and the expansion curve is drawn.

1.2 thermal conductivity

Thermal conductivity is defined as the amount of heat transferred through a unit area of a material in the direction of heat flow at a unit temperature gradient per unit time.

The principle of determining thermal conductivity is as follows: according to the basic principle of stable thermal conduction process of Fourier one-dimensional plate, the heat absorbed by the water flow flowing through the central calorimeter after the heat flow in the one-dimensional temperature field per unit time in steady state flows longitudinally through the hot surface of the specimen to the cold surface. The heat is proportional to the thermal conductivity of the sample, the temperature difference between the cold and hot surfaces, and the area of the heat absorption surface of the central calorimeter, and inversely proportional to the thickness of the sample.

The physical meaning of thermal conductivity refers to the heat passing through a unit vertical area in a unit time under a unit temperature gradient. Thermal conductivity is a physical indicator of the thermal conductivity of refractory materials, and its value is equal to the heat flux divided by the negative temperature gradient.

1.3 heat capacity

Any substance heats up when heated, but different substances of the same quality need different heat to heat up 1 ℃. It is usually expressed as the heat (KJ) required to heat 1kg of material at normal pressure to raise the temperature by 1 ℃, which is called heat capacity (also known as specific heat capacity).

1.4 conductivity

Refractory materials (except carbon and graphite products) are poor conductors of electricity at room temperature. As the temperature increases, the resistance decreases and the conductivity increases. Above 1000 ° C., the increase is particularly pronounced, and when heated to a molten state, a large electrical conductivity is exhibited.

2. Mechanical properties of refractory materials

The mechanical properties of refractory materials refer to the strength, elasticity and plasticity of materials at different temperatures. Usually by testing pressure, bending, wear resistance and high temperature load soft creep and other indicators to judge the mechanical properties of refractory materials.

Mechanical properties of 2.1 at room temperature

2.1.1 Normal temperature compressive strength

It refers to the maximum pressure on the unit area of the refractory material at room temperature, if it exceeds this value, the material is destroyed. If A is used to represent the total area of the sample under pressure and P is used to represent the ultimate pressure required to crush the sample, then there is: normal temperature compressive strength = P/A (Pa)

In general, the refractory material is rarely damaged by the static load at normal temperature during use. However, the room temperature compressive strength mainly indicates the sintering of the product, as well as the properties related to its organizational structure, the determination method is simple, so it is a common test item to judge the quality of the product.

2.1.2 Tensile, flexural and torsional strength

Refractory materials in use, in addition to compressive stress, but also by tensile stress, bending stress and shear stress, the main factor affecting the tensile and flexural strength of refractory products is its organizational structure, fine particle structure is conducive to the improvement of these indicators.

2.1.3 Wear resistance

The wear resistance of refractory materials depends not only on the density and strength of the product, but also on the mineral composition of the product, the organization and the firmness of the material particles. Room temperature high compressive strength, low porosity, compact and uniform structure, good sintering products always have good wear resistance.

2.2 high temperature mechanical properties

2.2.1 High temperature compressive strength

High temperature compressive strength is the ultimate pressure that a material can withstand per unit section at high temperatures. As the temperature increases, the strength of most refractory products increases, with clay products and high aluminum products being particularly pronounced, reaching a maximum at 1000-1200°C. This is due to the higher viscosity of the melt produced at high temperatures than the brittle glass phase at low temperatures. But the bond between the particles is stronger. As the temperature continues to rise, the strength drops sharply. The high temperature compressive strength index of refractory material can reflect the change of the bonding state of the product at high temperature.

2.2.2 High temperature flexural strength

High temperature flexural strength refers to the ultimate bending stress that a material can withstand per unit section at high temperatures. It characterizes the ability of a material to resist bending moments at high temperatures.

High temperature flexural strength is also called high temperature bending strength or high temperature modulus of rupture. Determine the maximum load that a cuboid specimen of a certain size can bear when bending on a three-point bending device at high temperature.

The high temperature strength of refractory is closely related to its practical use. In particular, high-temperature flexural strength is an important property for evaluating the quality of an alkaline direct bond brick. Such as alkaline direct bonding brick high temperature flexural strength, the resistance due to the temperature gradient of the shear stress is strong, so the product in use is not easy to produce peeling phenomenon. Products with high temperature flexural strength will also improve the impact and abrasion of their materials and enhance slag resistance. Therefore, high temperature flexural strength is used as an indicator of the strength of the product.

The high temperature flexural strength index of refractory materials mainly depends on the chemical mineral composition, organizational structure and production process of the products.

2.2.3 High temperature creep

When the material is subjected to a constant load less than its limit at high temperature, plastic deformation occurs, and the deformation will gradually increase with time, and even damage the material. This phenomenon is called creep. Therefore, for materials at high temperatures, the strength cannot be considered in isolation, but the factors of temperature and time should be considered at the same time as the strength. For example, the damage of the lattice brick of the hot blast stove working at high temperature for a long time is due to the plastic deformation caused by the gradual softening of the brick body, and the strength is significantly reduced or even destroyed. This creep phenomenon of the lattice brick becomes the main reason for the damage of the furnace.

It is generally believed that the factors affecting high temperature creep are: 1) use conditions, such as temperature and load, time, atmosphere properties, etc.; 2) materials, such as chemical composition and minerals; 3) microstructure. The high temperature creep curve of the material is divided into three stages. The first stage is slow creep (short time), the second stage is uniform creep (minimum creep rate), and the third stage is accelerated creep (rapid increase of creep rate).

3. High temperature use properties of refractory materials

3.1 refractoriness

The nature of refractoriness that resists high temperature without melting under load is called refractoriness. For refractory materials, the meaning of refractoriness is different from the melting point. The melting point is the temperature at which the crystalline phase of a pure substance is in equilibrium with its liquid phase. But the general refractory material is composed of a variety of minerals multiphase solid mixture, not a single-phase pure substance, so there is no certain melting point, the melting is carried out in a certain temperature range, that is, only a fixed start melting temperature and a fixed melting end temperature. In this temperature range both a liquid phase and a solid phase are present.

The refractoriness is a technical index, and its measurement method is a truncated triangular cone made of test materials. The length of each side of the upper bottom is 2mm, the length of each side of the lower bottom is 8mm, and the height is 30mm. (The included angle between one side and the vertical direction is 80) The section forms an equilateral triangle. When heated at a certain heating rate, it gradually deforms and bends due to the influence of its own weight. When it bends until the vertex contacts the chassis, it is the refractoriness of the sample.

3.2 high temperature volume stability

When the refractory material is used for a long time at high temperature, its shape and volume remain stable without change (shrinkage or expansion) performance is called high temperature volume temperature. It is an important index to evaluate the quality of products.

During the firing process of refractory materials, the physical and chemical changes during the firing temperature generally do not reach the equilibrium state. When the product is used for a long time and is subjected to high temperature, some physical and chemical changes will continue. On the other hand, in the actual firing process, due to various reasons, there will be insufficient firing products. When such products are used in the kiln and then subjected to high temperature, due to some firing changes, the volume of the products will change-shrinkage or expansion. This irreversible volume change is called residual shrinkage or expansion, and is also weighed as burning shrinkage or expansion. The size of the re-burning volume change indicates the high temperature volume stability of the product.

3.3 thermal shock stability

The ability of refractory materials to resist sharp changes in temperature without damage is called thermal shock stability. As we all know, the material expands or contracts with the rise and fall of temperature. If the expansion or contraction is restricted and cannot develop freely, stress will occur inside the material. This internal stress caused by thermal expansion or contraction of the material is called thermal stress. Thermal stress is not only generated under the condition of mechanical constraints, but also the temperature gradient in homogeneous materials, the difference of thermal expansion coefficient between phases in heterogeneous solids, and even the anisotropy of thermal expansion coefficient in single-phase polycrystals are the root causes of thermal stress.

The thermal shock damage of refractory materials can be divided into two categories: one is instantaneous fracture, called thermal shock fracture; the other is under the action of thermal shock cycle, the first cracking, spalling, and then chipping and deterioration, and finally to the overall damage, called thermal shock damage.

Four. Prospects for the development of refractory materials

In the future, the main direction of the development of refractory technology should be to comprehensively improve the quality according to the characteristics of China's raw material resources, develop high-quality and efficient new products at the end, adopt advanced technology, technological innovation and update equipment, so as to improve the use effect, reduce the use consumption, and adapt to the harsh requirements of high temperature technology. Specific future development prospects are as follows:

This is the basis for the development of high-quality refractory materials. A lot of research should be done in mineral processing, purification, synthesis process, exercise technology and so on. The purity of raw materials is improved, the degree of sintering is improved, the composition structure is uniform, and the performance is stable, which provides the basis for the development of high-quality, high-purity and high-efficiency products. The proportion of natural refractory raw materials used in this case will gradually, and after beneficiation and synthesis of high purity, high density and uniform quality of refractory raw materials proportion continues to increase. In addition, in the development of new varieties of refractory materials, should also be based on China's resources and needs, research and development of high-quality, efficient, high aluminum and alkaline products.

2. The transformation to quality type is characterized by energy saving, labor saving, multi variety, high quality, low yield and high profit, which accelerates the transformation of refractory materials from quantity to quality, from labor-intensive to technology-intensive, and requires the transformation of quantity type to variety quality type.

3. Develop new varieties including: high temperature refractory products; low energy consumption refractory products; high-tech, high-performance refractory products.

4. The new process strictly requires fine material, fine matching, high pressure, high temperature, composite technology, and the use of ultra-fine powder. By adjusting and controlling the microstructure characteristics, the high temperature performance can be improved and optimized, especially the physical performance, thermal shock resistance and anti-invasion performance. In addition, in order to make the direct combination and re-combination of bricks to higher purity, higher density and better organizational structure development, the use of beneficiation purification, two-step calcination, high pressure into the ball, artificial synthesis and high pressure molding and high temperature firing and other new processes.

The comprehensive consumption of refractory materials is a comprehensive reflection of all the above-mentioned technological advances. The continuous reduction of the comprehensive consumption of refractory materials (the ratio of refractory production to steel production) is an important symbol of the development of refractory materials.