Smelting High Silicon Silicon-manganese Process in Large Enclosed Submerged Arc Furnace (2)

Smelting High Silicon Silicon-manganese Process in Large Enclosed Submerged Arc Furnace (2)

3. Conditions for producing high-silicon silicon-manganese in large enclosed electric furnaces

3.1 Composition and grade of high silicon silicon-manganese alloy

3.2 Reasonable combination of raw materials

The raw materials required for high-silicon silicon-manganese smelting are the same as those for ordinary silicon-manganese smelting, namely manganese ore, coke, and silica.  However, smelting high-silicon silicon-manganese requires more careful ore blending than smelting ordinary silicon-manganese. The grade, impurity content, melting point, hardness, mineral structure, and mineral composition of manganese ore all have an impact on the smelting process.  While fully considering the metallurgical properties of the ore, it is also necessary to consider the blending of lump ore, fine ore, clinker, and raw materials to ensure the permeability and reducibility of the furnace charge. When selecting high-grade ore for charging, it is also important to consider that the ore properties of the raw materials should be good, and the reduction of manganese ore is a gradual process from high-valent manganese to low-valent manganese.

       527℃      900℃     1172℃

    MnO2→Mn2O3→Mn3O4→MnO

From MnO2 to Mn2O3, to Mn3O4, and then to MnO, each step requires the consumption of coke. Therefore, manganese ore with high oxygen content requires more coke consumption. For example, using brown manganese ore with low oxygen content yields better smelting results than using pyrolusite and psilomelane;  sulfur is a surface active substance in slag melt, which catalyzes the smelting reduction of MnO; MnCO3 can be directly decomposed into MnO and CO2, thereby reducing the amount of coke used in the reduction process. Therefore, manganese carbonate and high-sulfur ore significantly improve the indicators. The grade of high-silicon silicon-manganese ore entering the furnace must be guaranteed to be above 36%.

3.3 Selection of reducing agent (coke)

The coke required for high silicon manganese production should possess both good chemical and physical properties.  Specifically, it should exhibit excellent reactivity, high specific resistance, high fixed carbon content, high strength, appropriate lumpiness, good permeability and thermal stability, and be cost-effective.

The reducing agent (coke) used in smelting high-silicon silicon-manganese at a certain ferroalloy plant is primarily metallurgical coke.  It can also be combined with silica coke and gas coal coke.  The use of these coke types has shown good results in several large electric furnaces over the past few years.  With the coke type being fixed, various electric furnaces have continuously explored and experimented with different particle sizes.  We believe that for electric furnaces with a capacity of 12500-16500KVA, the coke particle size should be between 18-25mm, while for electric furnaces with a capacity of 25000KVA, the particle size should be between 22-35mm or 25-40mm.

3.4 Selection of slag type

There are three types of slag for high silicon silicon-manganese and ordinary silicon-manganese production: magnesium slag, calcium slag, and aluminum slag.  Theoretically, using magnesium slag and aluminum slag is beneficial for silicon reduction, which is advantageous for producing high silicon content.  However, the ore used by a certain ferroalloy plant has low magnesium oxide and aluminum oxide content, so it is necessary to add additional materials with higher magnesium oxide and aluminum oxide content to increase the magnesium oxide and aluminum oxide content in the slag.  Moreover, the plant produces a large amount of medium manganese slag, so considering comprehensive utilization and cost reduction, calcium slag is chosen.  Although SiO2 and CaO tend to form low-melting silicates that inhibit silicon reduction, which is not conducive to the production of high silicon silicon-manganese, practical production has shown that using calcium slag can still meet the production requirements for high silicon silicon-manganese.  The silicon content in the alloy can reach up to 36%.

The slag compositions, advantages, and disadvantages of calcium slag, magnesium slag, and aluminum slag are shown in the table below:

3.5 Control of slag alkalinity

The quality of high-silicon silicon-manganese slag type needs to be controlled by the slag's alkalinity, and both high and low alkalinity have their advantages and disadvantages.  Although operating with high alkalinity results in good slag fluidity and low manganese loss in the slag, it also increases slag conductivity, making it difficult for electrodes to be inserted, leading to high furnace mouth temperature, severe manganese volatilization, large heat loss, and significant equipment damage.  High alkalinity also severely erodes the furnace wall, especially the taphole, which can easily cause burnthrough of the taphole.  Additionally, the utilization rate of high-silicon silicon-manganese slag decreases with high alkalinity.

Using a low-alkalinity operating electrode and inserting it properly results in relatively less loss at the tap hole and a high utilization rate of silicon.  However, the slag has poor fluidity, making it difficult to discharge slag from the tap hole, and the manganese loss in the slag is high.  Therefore, a suitable slag alkalinity is necessary for high-silicon silicomanganese.  Through practice, it is believed that an alkalinity between 0.6 and 0.75 is appropriate.  

Slag type and alkalinity table:

3.6 Selection of secondary voltage

Under the premise of maintaining good furnace conditions, the secondary voltage should not be set too low, so as to maximize the overall efficiency of the furnace and achieve good technical and economic indicators.

When the coke in the furnace charge has different properties, if its resistance is low, the voltage used should be lower.  Conversely, if the resistance of the coke is high, the voltage used should be higher.

The secondary voltage chosen for furnaces with different capacities varies.  The larger the capacity, the higher the secondary voltage used; for furnaces with smaller capacities, the voltage should be lower.

3.7 Establish a reasonable power supply system to improve power factor

A reasonable power supply system can not only ensure the effective utilization of electric energy in electric furnaces but also guarantee good economic and technical indicators for the furnaces. Electrodes used in the production of high-silicon silicon-manganese are difficult to insert, and the power factor is low.  However, low-voltage reactive power compensation equipment can simultaneously address these two issues and create conditions for smelting high-silicon silicon-manganese in enclosed electric furnaces.

3.8 Selection of parameters

Reasonable electric furnace parameters are a prerequisite for smelting any variety. Since smelting high-silicon silicon-manganese requires a higher furnace temperature, a high furnace temperature is beneficial for the reduction of silicon dioxide and the improvement of alloy silicon.  Therefore, the appropriate diameter of the electrode circle for high-silicon silicon-manganese is smaller than that for ordinary silicon-manganese and larger than that for silicon-chromium.  Generally, an electrode circle power of 2100 kW/m² is sufficient to meet the requirements.  A larger electrode circle results in a lower furnace temperature, a lower silicon reduction rate, and difficulty in increasing silicon content in the alloy;  a smaller electrode circle results in excessive furnace power, difficulty in inserting electrodes, large heat loss, and increased power consumption.

3.9 Maintenance of taphole

The high-silicon silicon-manganese furnace operates at high temperatures, resulting in high slag and iron temperatures, which severely erode and scour the taphole.  Its utilization rate is only one-third of that of ordinary silicon-manganese.  To address these characteristics, the following effective measures have been taken:

1 Shorten the time of taking out from the furnace

2 Under normal circumstances, it is not allowed to open the eye with electricity, and it is required to use an eye opener to open the eye.

3 The hole should be plugged deeply, and the outer opening should also be sealed with mud balls to reduce oxidation.

4 Adjust the slag type, reasonably control the slag basicity, and properly manage the electrode working end and electrode position.

3.10 Overload operation of gas system

Due to the high temperature in high-silicon silicon-manganese smelting, manganese volatilization is significant, resulting in excessive dust and high pressure on the gas system and the ascending flue. The flue, especially the gas downpipe, often gets blocked, which is even more severe in winter.  Considering the cold climate and the high investment required for water treatment in wet gas recovery, it is recommended to use a dry method for manganese dust recycling and utilization.

4 Conclusion

1. The grade of the material entering the enclosed furnace for smelting high-silicon silicon-manganese must be guaranteed to be above 36%, and the burdening of the furnace must be reasonable, ensuring good comprehensive ore properties.

2. The coke particle size is appropriate, with a range of 22-40mm for a 25000KVA electric furnace.

3. The alkalinity should be controlled between 0.6 and 0.75.

4. The electrode's pole circle should not be too large, with a reasonable voltage and a reasonable power supply system.

5. Strengthen the maintenance of the tap hole, and use silicon carbide bricks to construct the furnace eye bricks to prevent oxidation.

6. Dry dust removal is more suitable for smelting high-silicon silicon-manganese alloy in enclosed electric furnaces.