1. Determination of crucible mold

The crucible of the medium frequency furnace is an important part of the medium frequency furnace. It must have the functions of high temperature resistance, insulation, heat insulation and energy conduction. It must have the ability to resist washing and compression. In addition, it must also have electrical characteristics and mixing ability that does not affect the magnetic field. The size and depth of the mold determine the thickness of the furnace wall and bottom. If the mold is too large, the furnace wall will become thinner, and its static pressure will increase with the capacity of the crucible. The increase of the scouring force borne by the crucible wall will affect the furnace life, but it can improve the power factor. The mold is too small, the furnace wall becomes thicker, the static pressure decreases with the crucible capacity, and the scouring force borne by the crucible wall decreases, which is conducive to extending the furnace life, but the resistance also increases, reducing the power factor and increasing the production cost. Therefore, it is very important to choose a suitable mold.

2. Selection of crucible charge

In addition to the required physical and chemical indicators, the smaller the expansion coefficient of the furnace charge, the better, so as to avoid expansion cracks caused by repeated temperature difference effect in use and affect the service life of the crucible. It is determined according to the model of equipment, the pH of smelting materials and the temperature control during smelting. At present, quartz sand and high alumina are mostly used as masonry materials for medium frequency furnace crucibles in China. High alumina clay is more suitable for medium frequency furnaces below 500kg. Dry quartz sand is used for crucibles of medium frequency furnaces above 500kg, and quartz sand is the most suitable for 2T smelting furnaces. Quartz sand material is made of natural ore, and its quality mainly depends on the content of SiO2 and its particle size formula. Changing the manufacture of crucible can improve the furnace life. Acid crucible is a crucible made of acid oxide. The oxide used to make acid crucible is mainly SiO2. Almost all acid crucibles are made of SiO2 based refractories, especially suitable for large induction furnaces with a capacity of 5~30t. Through the test, in Table 1, the expansion coefficient of the first kind of physical and chemical indexes of quartz sand is 7% ～ 9%, and that of the second kind is 0.5% ～ 2%. The second kind is suitable for particle size.

3. Tamping process

The tamping process mainly depends on the uniformity and compactness of tamping. ① The tamping site must be clean and free of sundries. ② Before tamping, the tools used shall be checked, and there shall be no rust and slag falling. ③ Check the fire-resistant protective layer of wire turns. If there is any defect, it should be repaired with refractory materials to protect the wire turns and prevent the molten liquid from contacting the wire turns after intrusion. ④ Install the boiler leakage alarm system to ensure the normal operation of the test system. ⑤ Lay the stove. The furnace wall is paved with two layers of asbestos cloth longer than 300mm deep, the interfaces are staggered, and the expansion ring is used to support it tightly. ⑥ The furnace charge shall be carefully selected before use. When in use, the furnace charge cannot be unpacked and directly added into the furnace. It is best to prepare a container (with a capacity of about 15 bags of furnace charge), pour multiple bags of furnace charge together, mix them thoroughly, pick out the paper pieces, thread ends and other sundries that may exist in the furnace charge, and use a high magnet to process the furnace charge. ⑦ Build the bottom of the furnace. The temperature borne by the furnace bottom is high, and the furnace bottom will form a groove after a certain period of use, and the upper part of the groove will become the furnace wall. If the tamping is not dense, it is easy to cause furnace leakage. ⑧ When using the iron crucible mold, the rust must be removed first, and its air hole is less than 4mm, which must be adhered with transparent adhesive tape to prevent quartz sand leakage and unconsolidated tamping. When the furnace bottom reaches the required height, scrape the bottom flat, and then place the crucible mold. It should be ensured that the crucible mold and the induction ring are concentric, vertical from top to bottom, and the shape should be closely combined with the furnace bottom. After the crucible is adjusted to be concentric, fix the crucible to avoid the displacement of quartz sand when the furnace wall is knotted. The service life of the furnace lining is also affected by the non concentric placement of the crucible (the center deviation is not greater than 5mm). ⑨ For furnace wall tamping, the contact surface shall be scraped before each feeding, and the feeding depth of the bottom layer shall not be greater than 150mm, so that it can be better integrated with the bottom. Add 100 ~ 130mm of materials above the bottom layer, vibrate for about 30min, and vibrate successively until the completion of furnace building. First use a fork and then a shovel. The fork should be vertically forked down and pulled up without shaking. Each circle is divided into three forks, one by the crucible, one by the clay, and one in the middle. The tamping personnel shall work in a cross way with even force.

4 oven roasting

The process of lining baking and sintering is mainly the transformation process of quartz → quartz → phosphorous quartz → cristobalite. With the change of heating rate, when the temperature is below 500 ℃ in the first stage, it is mainly to remove water, including the release of crystal water from boric acid to boric anhydride. The quartz is in a loose state, and the water vapor is easy to penetrate, but the surrounding of the furnace lining prevents the escape of steam. Therefore, the early speed can be faster. Because about 400 ℃ is the heat preservation and exhaust stage, it should be maintained for 1H. When entering 500 ~ 650 ℃, boric acid begins to change, low-temperature quartz begins to change, and liquid phase appears around. In order to prevent the evaporation and transfer of boric acid, the heating rate should be accelerated. At 850 ~ 1250 ℃, quartz begins to transform into scaly quartz and enters preliminary sintering. The heating rate should be slowed down. Quartz at 1250 ℃ is violently transformed into scaly quartz, and gradually transformed into cristobalite at more than 1470 ℃, with a great tendency of expansion and cracking, so it should be slowly heated up. Heat preservation for 2 ~ 3H at 1500 ℃ ~1550 ℃. During baking, pay attention to the following matters. ① After the oven drying, the polycrystalline transformation of SiO2 is very slow. The molten iron is the sintering layer, the middle is the transition layer, and the near inductor is the loose layer. Even if the oven sintering is completed, the surface layer is very thin. Before use, the temperature change should be strictly controlled to prevent temperature fluctuations, cracks and affecting the service life of the crucible. This structure can prevent through burning cracking and maintain the integrity and reliability of the furnace lining. ② The furnace charge should be low-carbon and less rusty, with slow temperature rise at low temperature and full furnace sintering at high temperature. When pouring out a furnace of molten iron, first pour out 1/3 of the capacity and then refill, then pour out 1/2 of the capacity and refill, and pour it all for the third time. Seal the furnace mouth with asbestos cloth after shutdown to reduce the water supply, delay the cooling speed and avoid cracks. ③ The newly built furnace lining must be used continuously for more than 7-8 heats to facilitate the formation of sintering layer. At this stage, because there is still water vapor in the furnace lining, the sintering layer is not completely formed. In order to prevent the influence on the insulation of the induction coil, the strength of the furnace lining is poor, so the power transmission does not exceed 85% of the rated power, so as to reduce the scouring of the crucible by the molten metal during the magnetic field stirring.

5 post management

The following aspects should be done well for the newly built furnace lining. ① The molten material should be clean, dry and oil-free, so as to build the sintering layer of the furnace wall. ② The first few furnaces avoid high-power power transmission smelting, which will produce a great electromagnetic stirring force and wash away the sintering layer of the furnace lining that is not yet completely firm. ③ The iron should be added lightly, and the iron should be thrown evenly. Touching the furnace wall is easy to damage the very thin sintering layer, causing consumption and falling off, affecting the service life of the furnace lining. Evenly adding iron can balance the furnace temperature and prevent crusting. ④ The slag shall be fished frequently in use. The melting point of the slag is higher than that of the molten material. The slag crusts and the molten material cannot be in contact with the solution, which makes it difficult to melt. The bottom of the furnace lining crucible is eroded by high temperature. ⑤ Remember to avoid high temperature during smelting. The temperature is directly proportional to the consumption of blast furnace burden. The higher the temperature, the faster the consumption of blast furnace burden, and the higher the risk. The discharging temperature of molten iron should be controlled below 1450 ℃, the smelting temperature should be controlled around 1300 ℃, and the insulation temperature should be controlled at the liquid level without crusting. ⑥ Daily inspection of furnace lining, check the furnace wall for cracks and falling blocks when opening the furnace, and deal with them in time if any.

6 Summary

① For the service life of the lining of the medium furnace, the material selected for furnace building is very critical, and the proportion of physical and chemical indicators and particle size should be paid attention to.

② Strictly control the furnace building process. Although the process is not complex, it is essential.

③ Reasonably control the oven temperature to ensure that the objectives of each stage can be achieved.

④ Pay attention to the later management, strictly control the physicochemical indexes and reasonable proportion of smelting, carefully implement the operating procedures of smelting workers, carefully maintain and scientifically operate, which can prolong the service life of crucible and reduce the production cost.