Operation of quartz sand lining in induction smelting furnace

Determination of 1 crucible mould

Crucible is an important part of induction furnace. It should have the functions of high temperature resistance, insulation, heat insulation, energy conduction, anti flushing and anti compression. In addition, it must also have electrical characteristics and stirring ability that does not affect the magnetic field. The size and depth of the mould determine the thickness of the furnace wall and bottom. If the mold is too large, the furnace wall will become thinner, the static pressure will increase with the capacity of the crucible, and the erosion force on the crucible wall will increase, which will affect the furnace life, but can improve the power factor. If the mold is too small, the furnace wall becomes thicker, the static pressure strength decreases with the capacity of the crucible, and the erosion force on the crucible wall decreases, which is conducive to prolonging the furnace life, but the resistance also increases, which reduces the power factor and increases the production cost. Therefore, it is very important to select the appropriate mold.

2. Selection of crucible burden

In addition to the required physical and chemical indexes, the smaller the charge expansion coefficient is, the better, so as to avoid the expansion crack caused by repeated temperature difference effect in use and affect the crucible life. According to the model of the equipment, the pH of the melting material and the temperature control during melting. At present, quartz sand and high bauxite are mostly used as masonry materials for the crucible of SCR medium frequency melting furnace in China. The high alumina material is suitable for the SCR medium frequency electric furnace below 500kg. The dry quartz sand is suitable for the crucible of medium frequency electric furnace above 500kg, and the quartz sand for 2T smelting furnace is the most suitable. The quality of quartz sand material is mainly determined by the content of SiO2 and its particle size formula. The furnace life can be improved by changing the making of crucible. The acid crucible is made of acid oxide. The main oxide used for making acid crucible is SiO2. Almost all the acid crucibles are made of SiO2 based refractories, especially suitable for large induction furnaces with 5 ~ 30t capacity. According to the test, in Table 1, the expansion coefficient of the first kind of physical and chemical index of quartz sand is 7% ～ 9%, and that of the second kind is 0.5% ～ 2%.

3. Tamping technology

Tamping technology mainly depends on the evenness and compactness of tamping. ① The tamping site must be clean and free of debris. ② Before tamping, the tools used should be checked, and there should be no rust and slag falling. ③ Check the fire-resistant protective layer of wire turns. If there is any defect, the refractory material shall be used to repair and protect the wire turn to prevent the molten liquid from invading and contacting the wire turn. ④ Install the leakage alarm system to ensure the normal operation of the test system. ⑤ Lay the stove. Lay two layers of furnace wall with asbestos cloth 300 mm longer than furnace depth, stagger the joints, and support with expansion ring. ⑥ The burden should be carefully selected before use. When in use, the furnace charge cannot be directly added into the furnace after being unpacked. It is better to prepare a container (with a capacity of about 15 bags of charge), pour multiple bags of charge together, mix and thoroughly stir, pick out the paper pieces, thread ends and other sundries that may exist in the charge, and treat the charge with a high magnet. ⑦ Build the bottom of the furnace. The furnace bottom bears the highest temperature, and it will form a groove after a certain period of use, and the upper part of the groove becomes the furnace wall. If the ramming is not dense, it is easy to cause furnace leakage. ⑧ When using the iron crucible mold, the rust should be removed first, and the air hole should be less than 4mm, which must be adhered with transparent adhesive tape to prevent the leakage of quartz sand and make the tamping not dense. When the furnace bottom reaches the required height, scrape the bottom flat to place the crucible mould. It should be ensured that the crucible mold and the induction ring are concentric, vertical up and down, and the pattern should be closely combined with the furnace bottom. After the crucible is adjusted to be concentric, the crucible is fixed 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 different placement of the crucible (the center deviation is not more than 5mm). ⑨ When ramming the furnace wall, scrape the contact surface before each feeding, and the feeding depth of the bottom layer shall not be greater than 150 mm, so as to better integrate with the bottom. The upper part of the bottom layer is filled with 100-130mm materials, tamped for about 30min, and vibrated in turn until the end of furnace construction. Use fork first and then shovel. The fork should be pulled up vertically without shaking. Each circle is divided into three layers, namely, one layer by crucible, one layer by mortar and one layer in the middle. The tamping personnel shall cross work and apply force evenly.

4 oven baking

The process of lining baking and sintering roasting is mainly the transformation of quartz rarr; quartz rarr; phosphorite quartz rarr; cristobalite. With the change of heating rate, when the temperature is below 500 ℃ in the first stage, the main reason is to remove water, including boric acid changing into boric anhydride releasing crystal water, quartz in loose state, water vapor is easy to penetrate, but the surrounding of furnace lining hinders the escape of steam. Therefore, the early speed can be faster. Because about 400 ℃ is the stage of heat preservation and exhaust, it should be kept for 1 hour. During the temperature range from 500 ℃ to 650 ℃, boric acid begins to change, low temperature quartz begins to change, and liquid phase appears at the boundary. 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 lepidolite and enters into primary sintering, so the heating rate should be slowed down. Quartz is transformed into squamoskite at 1250 ℃ and squamoskite at 1470 ℃. The temperature should be raised slowly. The heat preservation time was 2-3 hours at 1500 ℃ ~ 1550 ℃. During oven drying, the following items should be noted. ① After drying out, the polycrystalline transformation of SiO2 is very slow, with hot metal as sintering layer, transition layer in the middle and loose layer near the inductor. Even if the oven sintering is completed, the surface layer is very thin. Before use, the temperature change should be strictly controlled to prevent the temperature from rising and falling, causing cracks and affecting the life of the crucible. The structure can prevent through burning cracking and keep the integrity and reliability of the lining. ② The burden should be low carbon and less rust, low temperature should be raised slowly, high temperature should be full furnace sintering. When the hot metal is discharged from a furnace, first pour out 1 / 3 of the capacity and then refill it, then pour out 1 / 2 of the capacity and refill it again. The third time is all finished. After shutdown, the furnace mouth is sealed with asbestos cloth to reduce water supply, delay 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 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 erosion of the crucible when the molten metal is stirred in the magnetic field.

5 post management

The following work should be done well for the newly built furnace lining. ① The melting material should be clean, dry and oil-free, so as to build the sintering layer of furnace wall. ② In the first few heats, high-power power transmission smelting is avoided, and high-power electromagnetic stirring force will be generated to wash away the sintering layer of furnace lining which is not completely firm. ③ Adding iron should be light and evenly, and the thin sintering layer is easy to be damaged when touching the furnace wall, resulting in consumption falling off and affecting the service life of furnace lining. Adding iron evenly can balance furnace temperature and prevent crust. ④ The melting point of the slag is higher than that of the melting material. The melting material of the slag crust can not contact with the solution. It is difficult to melt. The bottom of the lining crucible is eroded by high temperature. ⑤ Avoid high temperature during smelting. The temperature is directly proportional to the burden consumption. The higher the temperature, the faster the burden consumption and the higher the risk. The pouring temperature of molten iron should be controlled below 1450 ℃, the melting temperature should be controlled around 1300 ℃, and the heat preservation temperature should be controlled at the liquid level without crusting. ⑥ Daily inspection of furnace lining, check whether there are cracks and blocks on the furnace wall when the furnace is opened, and deal with them in time if any.

6 Summary

① For the service life of medium furnace lining, the selection of materials for furnace construction is very important, and the proportion of physical and chemical indexes and particle size should be paid attention to. ② Although the process is not complicated, it is very important to strictly control the furnace building process. ③ Reasonable control of oven drying temperature, to ensure that the goal of each stage can be achieved. ④ Paying attention to the later stage management, strictly controlling the physical and chemical indexes and reasonable proportion of smelting, conscientiously implementing the operation regulations of smelting workers, careful maintenance and scientific operation can prolong the service life of crucible and reduce the production cost.