​Characteristics of Ferrosilicon Smelting

Characteristics of Ferrosilicon Smelting

I. Overview

With my country's energy shortages and increasing environmental pressure, ferrosilicon smelters, as energy-intensive industries, must shift from their existing extensive, highly polluting practices. Currently, ferrosilicon smelting capacity in my country is primarily below 36,000 kVA, resulting in high energy consumption and a lack of energy recovery, generating significant pollution. To address this, major ferrosilicon producers are gradually implementing large-scale furnaces, waste heat recovery, and new environmental protection technologies to reduce energy consumption and achieve the goal of "lucid mountains and clear waters are invaluable assets."

II. Key Trends

1. Larger Furnace Sizes

When ferrosilicon is smelted in submerged arc furnaces of varying capacities, the electrical-heat conversion patterns within the furnaces vary. The smelting process operating procedures vary between furnace types, and raw material conditions vary slightly from region to region. These factors can significantly vary the melting characteristics of the furnaces, making certain smelting parameters difficult to accurately measure. To further understand the changing patterns in the smelting characteristics of submerged arc furnaces, it is necessary to combine actual factory production with statistically analyzed data to summarize the internal patterns and approximate numerical values.

A company in Inner Mongolia has already installed 12,500 kVA, 25,000 kVA, 36,000 kVA, and 81,000 kVA ferrosilicon submerged arc furnaces for production. Analysis of nearly two decades of long-term data shows that thermal efficiency increases to varying degrees with increasing furnace size. The maximum increase in thermal efficiency is 10% when selecting an 81,000 kVA submerged arc furnace.

In addition, the 81,000 kVA submerged arc furnace provides stable furnace temperature and operation, consistent product quality, and reduced smelting power consumption. Furthermore, the increased furnace capacity facilitates mechanization and automation of raw material transportation and operation, improving labor productivity and reducing operating costs.

2. Application of Refining Technology

Ferosilicon smelting utilizes a high-quality composite carbonaceous reducing agent as raw material. Using metallurgical coke as the primary fuel, combined with gas coke, semi-coke, or bituminous coal as a reducing agent, can save 200-300 kWh/t of ferrosilicon production. Calculated at 0.4 yuan per kWh, the annual electricity cost savings are: 158,004 x 200 x 0.4 = 12.64 million yuan.

Improve ore preparation technology for incoming ore. Ensure the charge has the appropriate size to ensure good air permeability and achieve optimal smelting performance.

3. Adopt a Fully Automatic Charge Distribution System

Traditional charge distribution methods rely on manual labor, which is labor-intensive and creates a poor working environment. To address this, some companies have adopted fully automatic charge distribution, eliminating manual labor and improving the workshop environment. These fully automatic charge distribution devices, arranged at 120° on the roof of the submerged arc furnace, can provide circular, sector-shaped, and fixed-point charge distribution within the 120° sector. The chute's B angle (the circumferential angle of the distribution chute) is rotated by a motor, driving the pinion and large ring gear. The motor rotates forward and reverse, achieving a 120-degree rotation of the chute. Within the distribution range, the chute's α angle (the inclination of the distribution chute) is tilted by a motor-driven screw lift, which in turn drives a connecting rod. This changes the chute's angle, distributing the material onto different annular surfaces for uniform distribution. The flow rate of material at different distribution locations is controlled by a throttle valve below the hopper.

This device enables automatic charging at any point within the submerged arc furnace, ensuring more uniform material distribution, facilitating larger submerged arc furnaces, and reducing power consumption.

4. Application of On-site Low-Voltage Compensation Technology

Currently, the natural power factor of submerged arc furnace equipment is generally low. For most submerged arc furnace smelting processes, the natural power factor is between 0.5 and 0.7. To improve the natural power factor of the submerged arc furnace, a reactive power compensation device must be installed. Currently, a more mature compensation method involves connecting a large-capacity, ultra-low voltage, high-current power capacitor bank to the secondary side of the submerged arc furnace transformer using appropriate automatic control and a specific short-circuit connection method for reactive power compensation. This compensation reduces reactive current consumption in the short-circuit, submerged arc furnace transformer, and power supply network.

5. Cascaded Utilization of Waste Heat from Flue Gas

During production in a semi-enclosed submerged arc furnace, coal gas combusts with air to form flue gas. This flue gas volume is 10-15 times that of a closed system, resulting in high volume and high dust content. This makes purification difficult, hinders energy recovery, and results in long-term environmental pollution and energy loss. Therefore, a targeted design utilizes the high-temperature submerged arc furnace flue gas for waste heat recovery, followed by dry bag dust removal.

The use of semi-enclosed waste heat boilers and power generation has become the primary method for utilizing waste heat from this type of furnace. [The heat from the high-temperature flue gas from the submerged arc furnace is recovered and used for power generation, achieving energy conservation, emission reduction, comprehensive utilization, and environmental protection.] The process flow is as follows: High-temperature smelting furnace flue gas (around 650°C) is fed through two or three smelting furnace flues into a waste heat boiler (horizontal or vertical). After heat exchange in the waste heat boiler, the generated high-temperature, high-pressure steam is transported via a steam pipeline (limited to less than 1 km in length) to a steam turbine generator set, which then generates electricity.

After heat exchange in the waste heat boiler, the high-temperature smelting furnace flue gas is cooled to around 250°C, allowing its energy to be utilized. Currently, the main practical utilization method is to generate feed steam for the waste heat boiler.

In addition, the outlet flue gas temperature of the waste heat boiler already meets the requirements of the bag filter, eliminating the need for additional cooling equipment (such as an air cooler), thus reducing project investment.

III. Comparison of Key Economic and Technical Indicators

The following table compares indicators with domestic peers in the same industry. Analysis of process equipment advancement, resource and energy consumption, and other indicators, as well as comparisons with domestic and international peers, indicates that the production level of ferrosilicon smelted using large furnaces is at the domestic advanced level.

IV. Conclusion

Through the above analysis, the use of large-scale submerged arc furnace smelting, mainly using metallurgical coke, combined with gas coal coke, semi-coke or bituminous coal as reducing agent, and the use of fully automatic distribution technology, the high-temperature flue gas of the submerged arc furnace is cooled by the waste heat boiler, and the flue gas waste heat is recycled in a step-by-step manner, which will become a new feature of the ferrosilicon smelting industry in the future.

References:

[1] Guo Xin. Smoke dust characteristics and waste heat recovery of ferrosilicon smelting furnace [J]. Applied Energy Technology, 2014(5).

[2] Zhu He. Low-voltage reactive compensation of submerged arc furnace [J]. Industrial Heating, 2008(3).

[3] Wu Rongyang, Wu Guohua. Comprehensive utilization of submerged arc furnace flue gas resources [J]. Comprehensive Utilization, 2011(8).