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Lining technology of induction furnace

Time:2021-02-10   Nums:3146

      

The medium frequency melting furnace has the advantages of simple operation and maintenance, accurate control and adjustment of liquid metal temperature and composition, less element burning loss, less noise and pollution, so it is more widely used than cupola and electric arc furnace. This paper discusses how to improve the lining life of medium frequency furnace.

1、 Preface

The high temperature performance of medium frequency furnace lining mainly depends on the physical and chemical properties and mineral composition of refractories. On the premise of selecting raw and auxiliary materials, sintering process is the key process to obtain good microstructure and give full play to its high temperature resistance. The densification degree of furnace lining sintering is related to the chemical composition, particle size ratio, sintering process and sintering temperature of refractory. Through more than 20 years of continuous exploration and production test, our factory has summed up the reasonable furnace building and knotting process and baking and sintering process, which has greatly increased the furnace life. The furnace life of 2T medium frequency furnace dry knotting lining (after one intermediate repair) is as high as 1157 heats, and has achieved remarkable economic benefits.

2、 Furnace building process:

2.1 remove the mica paper when building the furnace.

2.2 the crystal quartz sand for furnace construction is treated as follows:

2.2.1 manual selection: mainly remove lumps and other impurities;

2.2.2 magnetic separation: magnetic impurities must be completely removed;

2.2.3 dry ramming material: it must be dried slowly at 200 ℃ - 300 ℃ for more than 4 hours.

2.3 selection of binder: boric anhydride (B2O3) instead of boric acid (H3BO3) is used as binder, and the addition amount is 1.1% - 1.5%.

2.4 selection and proportion of furnace building materials:

2.4.1 selection of furnace building materials: it should be noted that not all SiO2 & Ge; 99% quartz sand can be used as furnace lining materials of induction furnace, but the important thing is the size of quartz grains. The larger the grains are, the less the lattice defects are, the better. (for example, the purity of SiO2 of crystal quartz sand is high, and the appearance is white and transparent.) The larger the furnace capacity is, the higher the requirement of grain size is.

2.4.2 ratio: quartz sand for furnace lining: 6-8 mesh 10% - 15%, 10-20 mesh 25% - 30%, 20-40 mesh 25% - 30%, 270 mesh 25% - 30%.

2.5 knotting of furnace lining: the knotting quality of furnace lining is directly related to the sintering quality. The results show that the distribution of sand particle size is uniform without segregation, the density of sand layer after knotting is high, and the probability of cracks after sintering is reduced, which is conducive to improving the service life of induction furnace lining.

2.5.1 dry knotted furnace lining (take 2T medium frequency coreless induction furnace as an example): application of coil insulating mortar: the induction coil of 2T medium frequency electric furnace is coated with insulating mortar layer. Compared with the common insulation materials such as mica and glass fiber cloth used in induction circuit, the coil insulation mortar has the following advantages: first, after drying, the coil insulation mortar layer with a thickness of 8-15mm has good insulation performance, which can completely replace mica and glass fiber cloth as the insulation protection layer between coil and furnace lining; the thermal conductivity of the mortar material is high, so it is unnecessary to worry about the relatively low thermal conductivity Thick cement layer will affect the three-layer structure of hot surface lining. Secondly, the mortar layer is located between the coil and the insulation layer. Under normal circumstances, the ambient temperature is very low (< 300 ℃), and occasionally when the liquid metal approaches its surface, the mortar layer will release a small amount of residual water, which will reduce the insulation resistance, and the system will provide early warning. Thirdly, the fire resistance of the mortar itself is higher than 1800 ℃. When the metal liquid occasionally seeps to its surface, the mortar can provide a protective barrier for the coil. When an alarm occurs, the mortar layer can provide a certain accident handling time. Fourthly, for the furnace with bottom ejection type, the cement is made into a shape with taper to avoid the friction between the lining and the coil. At the same time, the strength is used to fix the coil to avoid the deformation of the coil in the process of using, building and dismantling the furnace, and to extend the service life of the coil. Fifthly, the coil and cement layer as the permanent lining of the furnace, although the one-time cost is high and the construction period is long, but its service life can be the same as the coil, and it can also be repaired locally, so as to reduce the furnace construction cost on the whole.

Before the dry knotting furnace lining, first lay a layer of asbestos board and a layer of glass cloth in the insulation layer of the furnace coil. In addition to manually leveling and compacting each layer of materials, the spring coil should also be used to tighten up and down. When tamping quartz sand, move the spring coil from top to bottom one by one until the furnace lining is finished.

2.5.2 knotting furnace bottom: the furnace bottom is about 280mm thick, and sand is filled in four times to prevent uneven density when knotting manually, and the furnace lining after baking and sintering is not dense. Therefore, it is necessary to strictly control the feeding thickness. Generally, the sand filling thickness is not more than 100 mm per time, the furnace wall is controlled within 60 mm, many people operate in different shifts, 4-6 people in each shift, knot for 30 minutes each time, and rotate slowly around the furnace, so as to avoid uneven density.

When the knotting of furnace bottom reaches the required height, the crucible mould can be placed by scraping. In this regard, attention should be paid to ensure that the crucible mold and the induction ring are concentric, and the vertical adjustment should be made up and down. The pattern should be closely combined with the furnace bottom as far as possible. After adjusting the peripheral gap, three wooden wedges should be used to clamp tightly, and a heavy object should be lifted in the middle to avoid the displacement of quartz sand when the furnace wall is knotted.

2.5.3 knotting furnace wall: the lining thickness is 110-120mm, dry knotting material is added in batches, the distribution is uniform, the filler thickness is not more than 60mm, knotting for 15 minutes (manual knotting), until it is flush with the upper edge of the induction coil. The crucible mould will not be taken out after knotting, and it will play an induction heating role in drying and sintering.

2.6 baking and sintering specification: in order to obtain the three-layer structure of furnace lining, baking and sintering process can be roughly divided into three stages

2.6.1 baking stage: the crucible mould is heated to 600 ℃ at the speed of 25 ℃ / h and 50 ℃ / h respectively and kept for 4h, so as to completely remove the moisture in the furnace lining.

2.6.2 semi sintering stage: temperature rise from 50 ℃ / h to 900 ℃ for 3 h, temperature rise from 100 ℃ / h to 1200 ℃ for 3 h. the heating rate must be controlled to prevent cracks.

2.6.3 complete sintering stage: during high temperature sintering, the sintering structure of crucible is the basis of improving its service life. The thickness of sintering layer is insufficient and the service life is obviously reduced with different sintering temperature.

During the baking process of 2T medium frequency coreless induction furnace, about 950 kg iron was added to enhance the heating effect of induction ring. With the continuous baking and sintering, relatively stable electromagnetic force was generated by low-power power power transmission to stir hot metal, so that the upper and lower lining was heated evenly. Strictly control the temperature of three phase transformation zones of quartz sand, promote the phase transformation of quartz sand fully, and improve the first sintering strength of furnace lining.

3、 Summary

For the service life of medium and power frequency furnace lining, in addition to ensuring a complete and reasonable three-layer furnace lining, we should also pay attention to the usual operation. Building furnace is to select high-quality and clean building materials, reasonable proportion, careful knotting, scientific baking and sintering specification, strict operation process, which can extend the service life of furnace.


Yiphee Electric Furnace Co.,Ltd specializes in producing induction smelting furnace.Tel(WhatsApp):+ 86-13450756789, yiphee@yiphee.cn

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