Induction furnace equipment structure
Induction furnace is the use of electromagnetic field induction principle of electrical energy into thermal energy, so that the metal charge heat melting high temperature melting equipment. According to the structure, there are no core induction furnace (crucible type) and core induction furnace (trench type) two categories.
Induction furnaces have the advantages of high efficiency, energy saving, low pollution, easy adjustment of composition (coreless furnaces), easy control of atmosphere, etc. Therefore, they have been widely used in foundry, steelmaking, non-ferrous melting, precision alloy production, etc. However, the development of induction furnace is limited to some extent by high power supply and long-life refractory materials. With the development of high-power thyristor frequency conversion power supply and the improvement of reliability, the medium frequency induction furnace gradually replace the industrial frequency induction furnace and induction furnace, to the direction of large capacity, high power development, the application fields are constantly expanding, the working conditions are increasingly harsh, so that the refractory material puts forward higher requirements.
Requirements for refractory materials for induction furnaces
Refractories for induction furnaces are used in relatively thin sections and in the highly aggressive environments of many melting operations, and are often the cause of furnace shutdowns. This indicates that induction furnaces have become one of the most challenging of all refractory applications.
Induction furnace steelmaking method is divided into acid smelting method and alkaline smelting method according to the nature of refractory materials. Acid smelting method smelting acid slag, so the refractory material is mainly quartz sand and zircon, its knotting, baking, sintering process and smelting cast iron is similar. Quartz sand or zircon furnace lining is generally used for low carbon steel or low alloy steel smelting. Alkaline smelting method makes alkaline slag when smelting, so the refractories are neutral, alkaline refractories or graphite crucibles, etc., which are suitable for smelting of high alloy steels, special steels, precision alloy steels and other steel grades.
Historically, several series of refractories have been used for induction furnace linings, of which SiO2 and zircon have been occasionally used due to their acidity and continue to be used to this day, but the more popular refractories for steelmaking in induction furnaces are mainly high alumina, MgO-CaO, and refractories consisting of Spinel.
Induction furnaces require a variety of high-performance refractory materials due to the diversification of their functions. That is to say, refractories in the induction furnace application, not only need to consider the furnace type, furnace structure and melting process, but also to consider the refractory material and its mineral phase from room temperature to the working temperature between the mineral phase, the physical properties of the change process and change mechanism; at the same time, but also according to the requirements of the application, especially the application of the innovative equipment for the selection of refractory materials and the improvement of material properties.
In today’s steelmaking applications, where furnaces are becoming larger and more powerful, the focus is on using A12O3-based refractory materials and maximizing their performance. Due to the aggressive environment and severe thermal shock conditions of the “batch melting” (using directly reduced iron pellets as additives, etc.) coreless induction furnaces, it is necessary to optimize the properties of the A12O3-based refractories by incorporating MgO, chromite (including Cr2O3 powder), and pre-synthesized Spinel into the refractories.
Refractories for Furnace
In the core induction furnace smelting gray iron or ductile iron, different from the coreless induction furnace, the furnace life depends mainly on the refractory materials used and the reaction between the aggressive components in the slag, while the physical properties of the refractory materials themselves on the life of the furnace life of the influence of the secondary position. For gray iron holding furnace, melting furnace, the slag is acidic; for ductile iron holding furnace, melting furnace, the slag of the acidity and alkalinity of the smelting process with the change of the alternating changes, but the prevailing situation, the slag of its spheroidization process for the acidic, which suggests that the furnace lining should be mainly selected neutral refractory materials.
Smelting gray iron and ductile iron core induction furnace operating temperature of 1450 ~ 1550 ℃, furnace lining damage is mainly due to slag line parts of the slag infiltration and erosion led to the deterioration of the lining, resulting in a metamorphic layer, its density, strength and Young’s modulus and the original layer are different, resulting in cracks and structural flaking or the formation of a low-melting phase that leads to the melting of refractory materials lining, loss. For example: iron inlet and outlet parts of the liquid iron due to scouring, slag erosion, especially inlet parts of the damage is serious, often leading to the collapse of the partition wall or baking furnace sintering during the serious cracks or spalling; halfway to replace the induction ring, pouring furnace empty iron, temperature drop (700 ~ 1200 ℃), but also prone to thermal shock damage.
All of the above shows that the lining of the core induction furnace should be made of refractory materials with high resistance to corrosion, scouring, penetration and chemical stability. Usually, electrically fused corundum is used as the aggregate, with an appropriate amount of sintered alumina fines, and low and ultra-low cements with high-alumina cement as the binding agent are used as the refractory castables. In order to improve the thermal shock resistance of the lining and considering the situation that the furnace is mostly reducing atmosphere, a certain amount of SiC with low iron content can be added to the castables to obtain high-tech AI2O3-SiC refractory castables with SiC. This kind of high-tech refractory castables as core induction furnace chamber lining refractory material can obtain high service life, but he depends on the life of the induction ring and the number of times of replacement, the general induction ring two years to replace a time.
Refractories for inductors
Refractories for inductors include refractories for the melt channel, sealing materials and isolation materials. The thickness of the thinnest place between the molten channel and the water jacket is about 70mm, and the temperature there is the highest, up to 1600~1700JD, the inductor shell and the coil are forced water-cooled, thus the temperature gradient inside the furnace lining is very large. In addition, the groove material is also subject to high-speed liquid iron scouring, strong electromagnetic stirring and liquid iron static load, therefore, this part of the refractory material is in the harsh operating conditions, so the requirements of the lining refractory material has a high temperature strength, high chemical stability, volumetric stability and good and dense and so on. Considering the complex shape of the melt channel and the working environment of the refractory material, the refractory castables or knotting materials (mainly dry knotting materials) with electrofused magnesium sand or Al2O3-MgO sand as raw material are generally used.
The design of the sealing material is the same as that of the lining material of the melting trench. Sealing material is mainly to make the docking surface of the inductor at room temperature with a certain degree of strength, docking so that the induction furnace and furnace body tight seamless. When A12O3 sealing material is selected, high alumina cement or H3PO4 can be used as the binding agent, while H3PO4 is used as the binding agent when the inductor is changed quickly, because of its thermosetting property, it can effectively prevent the most likely to occur on-site bursting of the bonding material, and improve the docking speed and efficiency.
The use of Cr2O3 powder for isolation materials is due to its high sintering temperature with the furnace lining material (Al2O3-based) or the inductor material (Al2O3-MgO-based), which essentially does not produce a ceramic sintered layer at operating temperatures, thus facilitating the separation of the refractory materials in the throat of the inductor and the furnace.
Refractories for Furnace Covers
Furnace cover is generally composed of heat preservation layer and hot surface layer, the former is built with high alumina lightweight brick masonry or Al2O3 hollow ball particles as the main raw material, and is constructed with high alumina cement as the bonding agent of refractory castables; while the latter is built with AS-85/SO clinker as the aggregates and fines, and with appropriate amount of magnesium sand fines, and is constructed with high alumina cement as the bonding agent of refractory castables. The actual lining is supplemented with anchoring bricks or heat-resistant anchors to improve the ability of the inner cover lining to resist the erosion of high-temperature airflow, improve thermal shock resistance and durability, and prolong the service life.
Refractories for smelting non-ferrous metals
In non-ferrous metal processing plants, core induction furnaces can be used for their melting and processing. When the core induction furnace refining copper, its operating temperature between 1200 ~ 1350 ℃), the inductor melting groove part of the temperature up to 1350 ~ 1450 ℃. Due to the low melting point of copper, copper liquid viscosity is small (close to the viscosity of water), the density is large, has a strong permeability. Moreover, the penetration of copper is easy to oxidize, accompanied by an increase in volume, when adding the right amount of steel fibers to this material can improve its anti-spalling properties.
Melting Cu core induction furnace inductor melting groove parts of the refractory material is selected dense bauxite clinker as the main raw material, taking into account the strong permeability of Cu, to the ingredients to add fine-grained SiC, in order to improve the material’s resistance to wettability. The lining masonry methods are knotted (with H3PO4 as the binding agent) or dry vibration molding (with sintering agent).
The solubility of A1 and Zn is low, the viscosity of the solute is small, and the penetration ability is very strong. Aluminum’s chemical activity is high, and it is a strong reducing agent at high temperature, which is easy to have chemical reaction with refractory components. Especially in the refractory material an aluminum liquid a gas phase three-phase junction, Al and its alloying elements, such as Mg, Si, Mn, etc. are easy to penetrate into the refractory substrate, redox reaction, resulting in refractory material deterioration, structural laxity and damage, therefore, the general selection of bauxite (AS-85/88) as the raw material for the manufacture of refractory building lining. Usually casting molding is used to obtain a dense overall furnace lining. The lining of channel-type induction furnace for aluminum refining can also be built with bauxite clinker-MgO (2%) bricks with A12O3 content of 60%.
Induction furnaces are one of the most challenging of all refractory applications. Successful assembly of high-volume induction steelmaking plants has led to an urgent need to improve the performance of refractories, and as a result, has contributed to the continuous improvement of the performance of refractories for induction steelmaking.
Refractories for induction furnaces need to be selected according to the furnace type, furnace structure, the type of steel to be melted, the melting process and operating conditions, etc. The process and mechanism of change of the physical phase and physical properties of the refractory material from room temperature to working temperature, as well as the conditions of application of the refractory material, etc., should also be considered.
