Refractory materials are inorganic non-metallic materials that can meet the use under high temperature conditions, such as lining structural materials in thermal equipment, lining materials for high-temperature containers, and components and parts materials in high-temperature devices.
Metallurgy has very strict performance requirements for refractory materials, that is, high refractoriness, good high-temperature strength, and the ability to withstand the erosion of slag and molten metal and rapid temperature changes. Some also require that they do not volatilize or decompose under high vacuum and high temperature. It can be said that the development of the metallurgical industry is inseparable from the progress of the refractory industry.
Types of refractory materials commonly used in nickel-iron smelting
According to the chemical properties of their raw materials, refractory materials can be divided into three categories: alkaline, acidic and neutral. The main components of alkaline refractory materials are MgO and CaO; the main components of acidic refractory materials are SiO2; neutral refractory materials are divided into Al2O3, C and SiC, etc.
According to the use temperature, it can be divided into ordinary refractory products (refractory 1580-1770℃), high-grade refractory products (refractory 1770-2000℃), and special-grade refractory products (refractory greater than 2000℃).
Refractory materials have the following important properties
01 Refractoriness and load softening point
Refractory is the performance of the material to resist melting at high temperature. Refractoriness is an important indicator to measure the quality of aluminum silicate refractory materials. The melting point is the solid-liquid phase equilibrium temperature of a pure substance, and the refractoriness is lower than the melting point of a pure substance. Refractory materials bear a certain load when in use. If the refractory material is deformed after being heated, the furnace body will inevitably be damaged. Therefore, the load softening point is another important quality indicator of refractory materials.
02 High-temperature flexural strength
Bricks with high high-temperature strength must have good slag resistance and strong resistance to rapid cooling and heating changes. Therefore, high-temperature flexural strength has become a key indicator for many refractory materials, especially alkaline refractory materials.
03 Thermal shock stability
The rapid temperature change to which refractory materials are subjected is called thermal shock (or thermal shock). Since refractory products are brittle materials with large expansion rate and low elasticity, internal stress is caused when subjected to thermal shock, causing the material to break or peel off.
04 Porosity and density
The pores of refractory materials are divided into two types: open pores that communicate with the outside world and closed pores that do not communicate with the outside world. The volume of closed pores is difficult to measure. Therefore, the porosity of the product is expressed as a percentage of the volume of open pores, which is called apparent porosity. The density of refractory products is related to the pores. The density that does not include any pore volume is called true density; the density that includes all pore volumes is called volume density; the density that only includes the closed pore volume but not the open pore volume is called false density. Porosity and density are indicators of the density of refractory products. High-density products have good mechanical properties and are also conducive to resistance to slag erosion and thermal shock. The key to obtaining high-density products is high-pressure molding equipment.
05 Slag resistance
For metallurgical refractory materials, slag erosion resistance is a very important property, which is related to the properties of slag and the types of refractory materials.
Aluminosilicate refractory materials are cheap, widely used and have many varieties. Their basic chemical composition is SiO2 and Al2O3. Clay bricks, high-alumina bricks and silica bricks are typical aluminum silicate refractory materials.
The main components of basic refractory materials are CaO-MgO eutectic composition, which is 67%CaO, 33%MgO, and the eutectic temperature is 2300℃.
Damage and life of refractory materials
There are many reasons for the damage of refractory materials, including problems with refractory material performance, quality and use conditions, as well as problems with furnace design, masonry quality and smelting operations. The damage of refractory materials is often the result of a combination of many factors. Generally speaking, the lining of basic materials reacts with slag during the smelting process, which will change the material composition and structure and cause damage to the refractory materials. Factors such as drastic fluctuations in operating temperature and the impact of molten metal, slag and airflow accelerate the damage of refractory materials.
Refractory spalling: Due to the capillary action of the open pores, slag can enter the interior of the brick and form a new phase dominated by silicates. At high temperatures, the expansion coefficients of the original brick and the newly formed metamorphic layer are different, which often produces thermal stress and cracks. The crack expansion causes the metamorphic layer to spall and damage. The large porosity, large pore diameter, and good wetting between the slag and the refractory material all exacerbate this damage.
In short, to improve the life of the smelting furnace lining, from the perspective of refractory performance, the "three high technologies" of high-purity raw materials, high-pressure molding, and high-temperature firing are required to obtain high-purity, high-density, and high-strength refractory materials. From the perspective of using refractory materials, it is necessary to correctly select the refractory masonry method and improve the smelting process to ensure balanced erosion of the furnace lining. It can be seen that the smelting process also has a great impact on the life of the furnace lining.
The amount of pyrometallurgical refractory materials is large, and their storage is also very important. They should be stored in a special warehouse in a classified manner to prevent them from being damp and deteriorating. Amorphous materials should be prevented from mixing to avoid reducing the quality or even deteriorating the materials.
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