The furnace building and sintering process of induction smelting furnace are the key processes to make the furnace lining resistant to high temperature, corrosion and washing. The compactness of lining sintering is related to the chemical composition, particle size ratio, sintering process and temperature rise curve of refractory materials.

1、 Determination of crucible mould

Crucible shall be resistant to high temperature, insulated and insulated The energy conduction function must have the ability to resist washing and compression. In addition, it must have electrical characteristics and the ability to stir without affecting 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 crucible capacity. The increase of the scouring force on the crucible wall will affect the furnace life, but it can improve the power factor. If the mold is too small, the furnace wall becomes thicker. The static pressure strength decreases with the crucible capacity, and the scouring force on the crucible wall decreases, which is conducive to extending the furnace life. However, the resistance also increases, reducing the power factor and increasing the production cost. Therefore, it is very important to select the appropriate 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 due to repeated temperature difference effects during use, which will affect the life of the crucible. It shall be 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 clay are mostly used as the masonry materials for induction furnace crucibles in China. Quartz sand material is carefully processed from natural ores, and its quality mainly depends on the content of SiO2 and its particle size formula.

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 debris.

② Before tamping, all tools used shall be checked once, and there shall be no rust and slag falling.

③ Check the fire-resistant protective layer of wire turns. If there is any defect, refractory materials shall be used to repair, and the wire turns shall be protected to prevent the molten liquid from invading the wire turns.

④ Large furnaces (generally more than 3 tons) shall be installed with furnace leakage alarm system to ensure normal operation of the test system.

⑤ Lay the furnace. Lay the furnace wall in two layers with asbestos cloth 300mm longer than the furnace depth, stagger the interfaces, and use expansion rings to support

⑥ The furnace charge shall be carefully selected before use. During use, the furnace materials cannot be directly added into the furnace after being unpacked. Pour the multi bag materials together, mix them thoroughly, pick out the paper scraps, thread ends and other sundries that may exist in the furnace materials, and treat the furnace materials with high magnets.

⑦ Build the furnace bottom. The furnace bottom bears the highest temperature, 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 ramming is not dense, it is easy to cause furnace leakage.

⑧ When using the iron crucible mold, the rust shall be removed first, and its air hole shall be less than 4mm, which must be stuck with transparent adhesive tape to prevent quartz sand leakage from making the tamping not dense. When the furnace bottom reaches the required height, scrape the bottom and place the crucible mold. It shall be ensured that the crucible mold and the induction ring are concentric and vertical from top to bottom. The pattern shall be closely combined with the furnace bottom as far as possible. After the crucible is concentric, fix the crucible to avoid displacement of quartz sand when the furnace wall is knotted. The service life of the furnace lining will also be affected if the crucible is not placed concentrically (the center deviation is not greater than 5mm).

For furnace wall ramming, the contact surface shall be roughened before each feeding, and the feeding depth of the bottom layer shall not be greater than 150mm, so as to better integrate with the bottom. Add 100~130mm materials above the bottom layer, tamp for about 30min, and vibrate in turn until the furnace building is completed. The fork shall be used first and then the shovel. The fork shall be vertically pulled down and shall not be shaken. Each circle shall be divided into three layers of forks, namely, one layer by the crucible, one layer by the clay and one layer in the middle. The tamping personnel shall cross work with even force.

4、 Oven roasting

The process of lining baking and sintering roasting varies with the heating rate. In the first stage, when the temperature is below 500 ℃, the main purpose is to remove water, including the release of crystal water from boric acid to boric anhydride. The quartz is loose and the water vapor is easy to penetrate, but the escape of steam around the lining is hindered. Therefore, the speed can be faster in the early stage. Because about 400 ℃ is the stage of heat preservation and exhaust, it should be kept for 1h. At 500~650 ℃, boric acid began to change, low-temperature quartz began to transform, and liquid phase appeared around 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 tridymite and enters into preliminary sintering. The heating rate should be slowed down. At 1250 ℃, quartz transforms into tridymite violently, and transforms into cristobalite gradually when the temperature exceeds 1470 ℃. There is a great tendency of expansion and cracking, and the temperature should be raised slowly. Keep the temperature at 1500 ℃~1550 ℃ for 2~3h. During oven drying, pay attention to the following matters:

① After the oven drying, the transformation of SiO2 polycrystals 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 shall be strictly controlled to prevent the temperature from rising and falling, causing cracks and affecting the life of the crucible. This structure can prevent cracking during penetrating combustion and maintain the integrity and reliability of furnace lining.

② The furnace charge shall be low carbon and less rust, and the temperature shall be slowly raised at low temperature, and full furnace sintering shall be conducted at high temperature. When discharging molten iron from a furnace, first pour out 1/3 of the capacity and then refill it, and then pour out 1/2 of the capacity and refill it again. The third time all the molten iron is poured out. After shutdown, seal the furnace mouth with asbestos cloth to reduce water supply and slow down the cooling rate to avoid cracks.

③ The newly built furnace lining must be continuously used 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 induction coil insulation, the furnace lining strength is poor, so the power transmission shall not exceed 85% of the rated power, so as to reduce the erosion of the crucible when the metal liquid is stirred in the magnetic field.

5、 Post management

The following work shall be done well for the newly built furnace lining:

① The molten materials shall 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 shall be avoided, which will generate great electromagnetic stirring force and wash away the sintering layer of the furnace lining that is not completely firm.

③ Iron shall be added lightly and thrown evenly. It is easy to damage the very thin sintering layer when touching the furnace wall, causing consumption and falling off and affecting the service life of the furnace lining. Uniform iron addition can balance furnace temperature and prevent crusting.

④ Slag shall be salvaged frequently during use. The melting point of slag is higher than that of molten materials. Slag crusts and molten materials cannot contact the solution. It is difficult to melt. The bottom of furnace lining crucible is eroded by high temperature.

⑤ Avoid high temperature during smelting. The temperature is in direct proportion to the consumption of blast furnace burden. The higher the temperature is, the faster the blast furnace burden is consumed, and the higher the risk is. The pouring temperature of molten iron shall be controlled below 1450 ℃, the smelting temperature shall be controlled around 1300 ℃, and the insulation temperature shall be controlled at the liquid level without crusting.

⑥ Daily inspection of furnace lining, check whether there are cracks and falling blocks on furnace wall when opening the furnace, and handle them in time if any.

6、 Summary

① For the life of furnace lining, the materials selected for furnace building are very critical, and the proportion of physical and chemical indexes and particle size should be paid attention to.

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

③ The oven temperature shall be controlled reasonably to ensure that the objectives of each stage can be achieved.

④ The service life of the crucible can be extended and the production cost can be reduced by paying attention to the later management, strictly controlling the physical and chemical indexes of smelting and reasonable proportioning, conscientiously implementing the operating procedures of smelting workers, careful maintenance and scientific operation.

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