01. Damage mechanism of lining
The lining of a submerged arc furnace generally consists of three parts: the outer layer, the middle layer and the inner layer. The outer layer is used for heat preservation and insulation, and is rarely damaged, so it is called the permanent layer; the middle layer is used to prevent leakage, and is relatively safe, so it is also called the safety layer; the inner layer is also called the working layer, and the refractory materials in this layer have the worst working conditions and are the most vulnerable part of the lining. Therefore, the damage to the lining is mainly caused by the damage to the refractory materials in the working layer. The reasons include physical, chemical, mechanical and other factors, as follows:
(1) Melting effect: When the working temperature of the refractory materials in the working layer exceeds its refractoriness, melting will occur. The load softening temperature of the magnesium lining is about 1550℃. Melting is most likely to occur in high temperature areas such as the furnace wall near the arc zone and the furnace bottom under the self-baking electrode.
(2) Chemical erosion: The refractory materials of the working layer may react chemically with slag, molten metal and reaction atmosphere. The furnace environment is different during the smelting process of different ferroalloys, and the chemical reactions that occur are also different. The most commonly used carbon lining of the submerged arc furnace will undergo oxidation reaction above 400℃, and even "reverse oxidation phenomenon" will occur, causing cavitation inside and material damage. Carbon deficiency operation during the smelting process will also cause the carbon material to be consumed as a reducing agent. The operating temperature of the magnesium lining is often higher than its load softening temperature. When it is in a soft melting state, it will accelerate the chemical erosion of slag and molten alloy on it.
(3) Mechanical erosion: The refractory materials of the working layer will be subjected to mechanical effects such as impact of metallurgical raw materials poured in, erosion of metal melt and slag. The strength of the refractory materials in the area with higher working temperature will be reduced to varying degrees or even softened, and it is more susceptible to damage when subjected to mechanical action.
02. Forms of damage to the refractory lining
The most critical part of the lining is the working layer, and material selection is the most important. Regarding the selection of refractory materials for furnace linings for the production of different ferroalloys, usually, the smelting temperature of metallic silicon, ferrosilicon, calcium carbide and silicon-chromium alloy is relatively high, and the working layer needs to use carbon lining; the slag generated by high-carbon ferromanganese and manganese-silicon alloy is acidic, and carbon lining is also mostly used; while the slag of low-carbon ferromanganese and medium-low carbon ferrochrome is alkaline, so magnesia lining is generally used.
In the same submerged arc furnace, the working environment of furnace walls and furnace bottoms of different heights is quite different, and their damage mechanisms and damage speeds are different, so refractory materials with different physical and chemical properties need to be selected. Usually, the furnace wall near the slag line and below the furnace bottom electrode are damaged faster, especially the tap hole is the most vulnerable part of the furnace lining structure, and the selection of materials at these locations is the most critical.
The normal service life of the submerged arc furnace lining can reach 10 years or even longer. However, the tap hole furnace eye bricks often fail prematurely in less than a year. Current research shows that the short-term damage of submerged arc furnaces is mainly caused by local damage to the tap hole. When the ore-fired furnace discharges slag and molten metal, it will cause mechanical scouring, chemical erosion and repeated thermal shock to the refractory material at the tap hole, making it the most vulnerable part of the furnace lining. The tap holes of traditional ferrosilicon, industrial silicon and ferromanganese silicon are mostly made of carbon eye bricks. Recent studies have shown that silicon carbide and silicon carbide combined with silicon nitride have better high-temperature mechanical properties, thermal physical properties and chemical stability. The use of such ceramics to replace carbon bricks for the tap hole has shown excellent use effects.
03. Lining structure type
1. Insulation lining
The traditional lining structure used in my country's ore-fired furnace is an insulation lining, as shown in Figure 1. This lining structure has achieved good results in the long-term smelting process of various ferroalloys. Its principle is to achieve long-term thermal insulation and heat preservation by improving the quality of refractory materials and increasing the thickness of refractory materials. The working layer of the insulation lining is mostly built with carbon bricks or magnesia bricks. In the smelting process of ferroalloys, furnace bottom burnout is the most common production accident. Through the dissection of the submerged arc furnace, it was found that the furnace bottom was burned through because the slag, molten metal and reaction gas penetrated into the joints between carbon bricks or magnesia bricks, thereby increasing the damage rate of the furnace lining.
2. Condensation type furnace lining
As the size of the submerged arc furnace becomes larger and the smelting temperature becomes higher, the working conditions of the refractory materials of the furnace lining become more and more harsh, even reaching the physical and chemical performance limit of the refractory materials. Simply increasing the thickness of the furnace lining and improving the quality of the refractory materials can no longer meet the needs of modern smelting. This design allows the slag to form a layer of solidified shell on the inner wall of the furnace lining, so it has the function of "self-protection". Compared with the insulation type lining, the structural characteristics of the condensing lining give it the following advantages: ① The requirements for the performance of refractory materials are reduced, and the life of the lining is greatly extended, thereby reducing the economic losses caused by furnace construction and shutdown and maintenance, and significantly reducing the maintenance of the taphole; ② The thermal conductivity of the solidified shell formed by the slag is small, which can effectively reduce the energy loss caused by heat conduction and reduce the unit power consumption; ③ The thin lining of the condensing lining increases the volume of the working area of the ore-fired furnace, which can increase the smelting output; ④ The condensing lining is usually equipped with a thermocouple when the furnace is built, which can monitor the candle erosion of the lining in real time to ensure production safety.
3. Temperature-controlled lining
In view of the current situation where the overall damage of the lining is mostly caused by the erosion of the furnace bottom by high-temperature molten iron, Lushan Tiannuo Furnace Lining Materials Co., Ltd. has designed a temperature-controlled lining. The main innovation of the temperature-controlled lining is the furnace bottom air cooling system, which is conducive to the rapid extraction of heat from the furnace bottom lining and forms an "iron retention layer" at the furnace bottom to protect the furnace bottom.
04. Conclusion
The key to the success of the condensing type lining structure is to form a low thermal conductivity slag self-protection layer, so it is not suitable for ferroalloy production by slag-free smelting. The two structures of insulation lining and condensing lining basically meet the smelting needs of all ferroalloy products. According to the actual production situation, a more reasonable lining structure is designed, and combined with new high-quality refractory materials, a more reasonable lining structure is provided for future submerged arc furnaces.
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