This article describes the damage mechanism of furnace lining, the selection of furnace lining refractory materials and the selection of furnace lining structure.
Keywords: furnace lining; refractory materials; damage mechanism
Submerged arc furnace is an important equipment for the production of various ferroalloys and calcium carbide. According to a research report on the development of China’s ferroalloy industry, my country has the ferroalloy production system with the largest output and the most complete varieties in the world. In recent years, the overall ferroalloy production has remained above 30 million tons. In recent years, “carbon peaking” and “carbon neutrality” have become the goals for the transformation and development of traditional industries in various countries around the world. The current ferroalloy production is also a high-energy-consuming and high-emission industry. Under the current “dual carbon” background, energy conservation, emission reduction, and high-quality development will be the only way out for the ferroalloy industry. In order to achieve this goal, governments at all levels have formulated a number of policies to actively promote the development of submersible furnaces in the direction of large-scale development. Large-scale submersible arc furnaces have the advantages of higher thermal efficiency, higher output, lower unit power consumption, and more stable operation. They are also conducive to the purification and recovery of smoke and dust and the utilization of waste heat.
The submerged arc furnace is a complex metallurgical equipment system, and its service time mainly depends on the life of the furnace lining. Before the submerged arc furnace was enlarged, due to the small amount of furnace lining refractory materials and the low cost of building the furnace, the ferroalloy industry did not pay enough attention to the furnace lining life. As submerged arc furnaces develop towards larger sizes, the cost of furnace lining shutdown and repair or re-laying has increased significantly. Therefore, how to obtain a long-life submerged arc furnace lining has become particularly important.
According to actual production experience, it is found that the main reasons for submerged arc furnace lining damage include: improper selection of refractory materials, unreasonable furnace lining structural design, quality problems in furnace construction, and irregular production operations. Among them, quality problems in furnace construction and irregular production operations can be avoided through institutionalization and standardization. Therefore, the correct selection of refractory materials and reasonable design of the furnace lining structure are important guarantees for the long-life operation of the submerged arc furnace lining. Based on the research results and industrial use of submerged arc furnace linings at home and abroad in recent years, the author introduces the damage mechanism of the furnace lining, analyzes the basis for selecting refractory materials for the furnace lining, and focuses on the research progress of the furnace lining structure, including the structural types of commonly used furnace linings, Corresponding working principles and usage conditions, and finally put forward rational suggestions on the selection of furnace lining structure based on the actual industrial production.
Damage mechanism of furnace lining
The submerged arc furnace uses three self-baking electrodes to heat the charge, reduce the metal elements from the oxide ore and smelt them. The temperature in the furnace can reach a maximum of 2000~3000°C. Submerged arc furnace lining generally consists of three parts: outer layer, middle layer and inner layer. The outer layer functions as thermal insulation and is rarely damaged, and is called the permanent layer; the middle layer plays the role of preventing furnace leakage and is relatively safe, also called the safety layer; the inner layer is also called the working layer, which is the working condition of the refractory material in this layer. It is the worst part and is the most fragile part of the furnace lining. Therefore, the damage to the furnace lining is mainly the damage to the refractory material of the working layer. The reasons include physical, chemical, mechanical and other factors, as follows:
(1) Corrosion: When the working temperature of the refractory material in the working layer exceeds its refractoriness, corrosion will occur. The load softening temperature of the magnesia furnace lining is about 1550°C. Corrosion is most likely to occur in higher temperature locations such as the furnace wall near the arc zone and the furnace bottom below the self-baking electrode.
(2) Chemical erosion: The refractory material of the working layer may react chemically with slag, molten metal, reaction atmosphere, etc. 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 carbonaceous furnace lining for submersible arc furnaces will undergo oxidation reactions at temperatures above 400°C, and may even cause “reverse oxidation”, causing internal cavitation and material damage. Carbon-deficient operations in the smelting process also consume carbonaceous materials as reducing agents. The service temperature of magnesia furnace lining is often higher than its load softening temperature. When it is in the reflow state, it will accelerate the chemical erosion of slag and molten alloy.
(3) Mechanical erosion: The refractory material of the working layer will be subject to mechanical effects such as the impact of pouring metallurgical raw materials, erosion of metal melt and slag. The strength of refractory materials in areas with higher operating temperatures will be reduced or even softened to varying degrees, making them more susceptible to damage when subjected to mechanical action.
Furnace lining refractory material selection
There are many types of ferroalloys. Different ferroalloys have different working temperatures during smelting, and the properties of the metal melt and slag in the furnace are different. Therefore, the selection of furnace lining refractory materials is also different. The most critical thing in furnace lining is the working layer, and material selection is the most important. Regarding the selection of furnace lining refractory materials for the production of different ferroalloys, usually, the smelting temperatures of metallic silicon, ferrosilicon, calcium carbide and silicon-chromium alloys are relatively high, and the working layer needs to use carbonaceous furnace linings; high-carbon ferromanganese and manganese-silicon alloys are produced The slag is acidic, and carbonaceous furnace linings are often used; while the slags of low-carbon ferromanganese and medium-low carbon ferrochromium are alkaline, so magnesia furnace linings are generally used.
In the same submerged arc furnace, the working environments of furnace walls and furnace bottoms of different heights are quite different, and their damage mechanisms and damage speeds are different. Therefore, refractory materials with different physical and chemical properties need to be selected. Usually, the furnace wall near the slag line and under the furnace bottom electrode are damaged quickly. In particular, the taphole is the most vulnerable part of the furnace lining structure, and the material selection at these locations is the most critical. Taking ferrochromium alloy as an example, the furnace wall uses carbonaceous and alumina materials, the furnace bottom uses carbonaceous and magnesia materials, and the tap hole uses carbonaceous and silicon carbide materials.
The normal service life of the submerged arc furnace lining can reach 10 years or even longer. However, taphole eye bricks often fail prematurely in less than a year. Current research shows that short-term damage to submerged arc furnaces is mainly caused by local damage to the taphole. When the submerged arc furnace discharges slag and metal melt outwards, it will cause mechanical erosion, chemical erosion and repeated thermal shock to the refractory material at the taphole, making it the most vulnerable place to damage in the furnace lining. Carbon furnace eye bricks are mostly used in tapholes of traditional ferro-silicon, industrial silicon and manganese-silicon iron alloys. Recent studies have shown that silicon carbide and silicon carbide combined with silicon nitride materials have better high-temperature mechanical properties and thermophysical properties. and chemical stability, using such ceramics
Replacing carbon bricks in tapholes has shown excellent results. However, silicon carbide furnace eye bricks are not suitable for use in all ferroalloy tapholes. A.V.L. Narasimham pointed out based on his more than 30 years of production experience that damage to chromite ore furnaces also occurs at the taphole, but silicon carbide furnaces Eyebricks erode quickly in ferrochrome furnaces and offer no advantage.
Furnace lining structure types
Insulation type furnace lining
The traditional furnace lining structure used in submerged arc furnaces in my country is an insulation lining, as shown in Figure 1. This furnace lining structure has achieved good results in the long-term smelting process of various ferroalloys. Its principle is to achieve long-term heat insulation effect by improving the quality and thickness of refractory materials. The working layer of the thermal insulation furnace lining is mostly made of carbon bricks or magnesia bricks. During the ferroalloy smelting process, furnace bottom burnout is the most common production accident. Through the dissection of the submerged arc furnace, it was found that the furnace bottom burned through because the slag, metal melt and reaction gas penetrated into the joints between the carbon bricks or magnesia bricks, thereby aggravating the damage rate of the furnace lining.
Figure 1 Thermal insulation furnace lining structure
In order to solve the problem of joints between refractory bricks, Elkem Company proposed an integrally lined furnace lining structure, as shown in Figure 2. Its working layer is made of amorphous refractory materials (cold rammed paste) and is integrally pounded into shape, and then passed through the oven. Sinter it into a whole. In the 1990s, Elkem developed the world’s earliest cold ramming paste product, which showed excellent performance in Elkem submersible furnace tests and was adopted by many ferroalloy customers. At present, the overall lining mostly uses carbonaceous cold-rammed paste, which is made of anthracite, graphite, etc. as aggregates, asphalt as binder, coal tar, resin, etc. as additives, and is a mixture of various materials. The physical and chemical properties of the integral rammed lining are the same as those of carbon bricks, but this integral structure can minimize the gaps between the carbon bricks, reduce the risk of lining leakage and furnace bottom perforation, thereby effectively extending the life of the furnace lining. In order to solve the problem of poor oxidation resistance of carbonaceous materials, silicon carbide ramming materials have been gradually developed, and the overall lining made of silicon carbide ramming materials has greatly improved its antioxidant performance. At the same time, silicon carbide ramming materials use silica sol as a binding agent, which can form a large number of SiC whiskers in situ during high-temperature use, and the overall strength is significantly improved.
Figure 2 Thermal insulation furnace lining structure with integral lining
Condensing furnace lining
As submerged arc furnaces become larger and larger, smelting temperatures get higher and higher, and the working conditions of the furnace lining refractory materials become more and more demanding, even reaching the limits of the physical and chemical properties of the refractory materials. Simply increasing the thickness of the furnace lining and improving the quality of the refractory materials is no longer enough. Adapt to the needs of modern smelting. In response to this problem, in 1995, SAMANCOR Company began research on the condensing furnace lining structure using a water cooling system and a high thermal conductivity carbon composite lining. In 1996, this condensing furnace lining structure was applied to ferromanganese production and achieved success. The concept of condensing furnace lining is completely different from that of traditional thermal insulation furnace lining. It uses high thermal conductivity refractory materials with thin masonry thickness. During use, it is combined with forced convection cooling of the furnace wall or furnace bottom. This design allows the slag to form a solidified shell on the inner wall of the furnace lining, so it has the function of “self-protection”, as shown in Figure 3. Compared with the insulating furnace lining, the structural characteristics of the condensing furnace lining give it the following advantages: ① The requirements for the performance of the refractory material are reduced, and the service life of the furnace lining is greatly extended, thereby reducing the economic losses caused by furnace construction and shutdown maintenance, and significantly reducing Maintenance of the taphole; ② The thermal conductivity of the solidified shell formed by slag is small, which can effectively reduce energy loss caused by heat conduction and reduce unit power consumption; ③ The thin lining of the condensing furnace lining increases the volume of the submerged arc furnace working area, which can improve smelting Output; ④ When building a condensing furnace lining, it is usually equipped with a thermocouple, which can monitor the furnace lining intrusion in real time to ensure production safety.
Figure 3 Schematic diagram of condensing furnace lining structure
The cooling methods of condensing furnace lining mainly include copper cooling stave, furnace wall water cooling and forced air cooling. Copper cooling staves have the best cooling effect, but due to the cost of furnace construction, copper cooling staves have not been promoted on a large scale. The cooling effect of forced air cooling is not good, which will affect the formation of self-protecting furnace shell. At present, the condensing furnace lining mainly uses spray water for cooling. As a traditional standard cooling medium, water will cause the risk of explosion once leakage occurs during use, causing serious damage to the submerged arc furnace. Therefore, for safety reasons, the area cooled by spray water is limited to the outer area close to the steel shell and not close to the hot surface of the refractory material. The Austrian company Mettop GmbH has developed a new ionic liquid cooling technology. This ionic liquid is non-flammable, non-corrosive and non-toxic, and can minimize the risk of explosion due to its low vapor pressure. It has gradually replaced Cooling water trends and prospects. With the in-depth understanding of condensing furnace linings, Elkem developed an improved condensing furnace lining. Its structure is shown in Figure 4. The characteristics of this structure are: first, forced air cooling is installed on the outermost layer of the furnace wall and furnace bottom; secondly, at the highest temperature, forced air cooling is installed. The lower part of the furnace lining and the bottom of the furnace are pounded with thicker carbonaceous cold ramming paste to form an integral lining; in addition, a layer of silicon carbide ramming layer is laid under the carbonaceous cold ramming paste, which can not only prevent corrosive The gas and metal melt pass downward, and because of its higher thermal conductivity, it can also increase the heat transfer outward and improve the condensation effect.
Figure 4 The condensing furnace lining structure with integral lining developed by Elkem
At present, condensing furnace linings are mainly used in submerged furnaces for alloy smelting such as manganese silicon, ferrochrome, ferronickel, ferromanganese, etc. Regarding the cost of using condensing type furnace linings and thermal insulation type furnace linings, Yang Lizhong conducted calculations on a high-carbon ferromanganese ore furnace and found that after the lining of the submersible furnace was transformed from the thermal insulation type to the condensing type, the annual cost of the submersible furnace lining increased during the three years of operation. The economic benefits are increased by 14.83 million yuan, including the benefits from reduced masonry costs, increased output, reduced power consumption and reduced coke consumption, and the annual income will further increase with the extension of the furnace lining life. Kang Guozhu compared imported and domestic refractory materials and pointed out that imported refractory materials have high thermal conductivity and are easy to form thicker solidified shells, which provide better protection. However, the quality of domestic refractory materials is poor, the solidified shell formed is thin, the furnace lining is easily eroded, and the advantages of the condensing furnace lining are not fully utilized.
Selection of furnace lining structures in different ferroalloy industries
There are many types of ferroalloys. How to choose the appropriate furnace lining structure for different ferroalloy production? Is the currently promoted condensing furnace lining suitable for the production of all ferroalloys? In response to this problem, Li Jianwei believes that not all ferroalloy production is suitable for condensing furnace linings, and the furnace lining structure The choice should depend on the density of the ferrous alloy. Ferroalloys with low density are preferred for insulating furnace linings, such as calcium carbide, ferrosilicon alloy and metallic silicon; while ferroalloys with high density are suitable for condensing furnace linings, such as high carbon ferromanganese, high carbon ferrochromium, manganese silicon alloy, ferrochrome, ferronickel , nickel-chromium, etc. Li Jianwei also pointed out that in addition to the density of ferroalloy, whether to choose a condensing furnace lining is also directly related to whether the smelting process is slag-containing smelting or slag-free smelting.
In response to this problem, the author conducted a survey. Currently, both ferrosilicon and calcium carbide production use an insulated furnace lining structure. Under the premise of correct operation, the average life of the furnace lining can reach more than 10 years. Take ferrosilicon smelting as an example. Ferrosilicon smelting is a slag-free smelting. Every time 1 ton of ferrosilicon is produced, only 20~60kg of slag/t is produced. The success of the condensing furnace lining depends on the formation of a slag insulation shell on the furnace wall. Therefore, it is difficult for ferrosilicon smelting to achieve the effects of other slag-containing smelting by using condensing furnace linings. In addition, the amount of melt in the furnace is small during ferrosilicon smelting. Taking a certain model of ferrosilicon ore furnace as an example, the furnace body diameter is 11m, and 11t of iron is discharged every two hours. Assuming that the ferrosilicon melt is laid flat on the bottom of the furnace, the output The deepest point of the melt in front of the iron is only 3.6cm. In other words, the height of the contact between the melt and the furnace lining is very limited. In the actual production of ferrosilicon, the melt is not spread flat on the furnace bottom, but is mainly concentrated under the three electrodes, so the ferrosilicon melt hardly contacts the inner wall of the furnace lining. It can be seen that ferrosilicon is not suitable for the use of condensing furnace linings, and traditional insulation lining structures should be selected. Through temperature measurement and heat dissipation calculation comparison, Kang Guozhu believes that the water-cooled condensing furnace lining has better thermal insulation performance, so he speculates that the use of condensing furnace linings for ferrosilicon is more suitable for ferrosilicon production. However, the author’s analysis shows that his speculation does not take into account actual production conditions.
Conclusion
(1) As submerged arc furnaces develop towards larger sizes, the operating temperature and usage environment in the furnace are becoming more and more demanding. The service life of the furnace lining directly determines the service life of the submerged arc furnace. Causes of damage to the furnace lining include softening and corrosion of high-temperature refractory materials, chemical erosion of the refractory lining by slag, molten metal and reaction atmosphere, and mechanical erosion of the charge and slag. The coupling effect of these three aspects will accelerate the damage rate of the furnace lining.
(2) There are many types of ferroalloys. The furnace conditions during smelting of different ferroalloys are different. There are also great differences in the working environment at different locations in the same submersible furnace. Therefore, refractory materials must be reasonably selected according to specific conditions. The working environment at the slag line at the lower part of the furnace wall and under the furnace bottom electrode is the worst. Carbonaceous materials and magnesia materials are mostly used. The taphole location, which is most vulnerable to damage, usually uses carbon or silicon carbide materials.
(3) Thermal insulation furnace lining is currently the most widely used furnace lining structure. In order to improve the service life of the furnace lining, the integral lining gradually replaces the refractory brick masonry. With the continuous improvement of ferroalloy smelting requirements, the concept of condensing furnace lining emerged. The condensation furnace lining uses high thermal conductivity refractory materials and thin furnace lining, and cooperates with the forced cooling of the furnace wall to make the slag form a self-protecting solidification shell on the inner wall of the furnace lining. It has been successfully used in a variety of ferroalloy submerged arc furnaces at home and abroad. This furnace lining structure can extend the service life of the furnace lining and reduce production costs. It is the main development direction of submerged arc furnace linings in the future.
The key to the success of the condensation-type furnace lining structure is the formation of a self-protective layer of slag with low thermal conductivity, so it is not suitable for the production of ferroalloys smelted by slag-free smelting. The two structures of insulation furnace lining and condensing furnace lining basically meet the smelting needs of all ferroalloy products. Based on actual production conditions, a more reasonable furnace lining structure is designed, combined with new high-quality refractory materials, to provide a more reasonable furnace lining structure for future submerged arc furnaces.