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Comparison between cupola and induction melting furnace

Time:2021-02-16   Nums:3225

 1. Introduction:

The development of iron casting production has fully proved that the correct selection of smelting equipment is of great significance. Cupola as the primary equipment of cast iron smelting continues to today, still responsible for about 70% of the smelting task of iron castings. Cupola technology keeps up with the development of machinery manufacturing industry, and has made great progress with the goal of energy saving, high quality and efficiency, safety and environmental protection. In developed countries, a large capacity, long furnace life, external hot blast cupola as the main body of cast iron smelting system has been formed, but the cupola with small capacity, short furnace life and normal temperature air supply is still very common in China. In recent years, China's cupola capacity smelting has begun to embark on the development road of large capacity, long furnace life and external hot air. The long furnace life and external hot air cupola with independent intellectual property rights adapting to China's production conditions is meeting the needs of casting production development. The recommendation of the three ministries and commissions of the state on the hot blast cupola indicates the development direction of the cupola in China. China's cupola smelting is moving towards the goal of energy saving, high quality, high efficiency, safety and environmental protection.

Induction furnace has been used as smelting equipment in cast iron smelting and duplex smelting for decades. China has rich experience in manufacturing and applying power frequency furnace and induction furnace, and has made remarkable progress in power supply structure, frequency conversion technology, integrated control, circulating cooling technology, etc. More and more factories use induction furnace for melting and heat preservation.

The melting mechanism of cupola and induction furnace is different, and their properties are different, so their adaptability to production is also different. The type (grade, output, machining requirements, etc.) and production mode of iron castings, the type and state of furnace charge, the level of smelting operation and production management level will put forward different requirements for the performance of smelting equipment. Only when the performance characteristics of smelting equipment are consistent with the production conditions, can the production process with high quality and efficiency, energy saving and consumption reduction, safety and environmental protection be realized, and the high technical and economic indicators be achieved.

In the melting of cast iron, cupola or induction furnace should be used? How to choose cupola and induction furnace? Only after understanding the melting characteristics of cupola and induction furnace can we make a correct decision.

2. Melting characteristics

(1) Process performance:

1) The production capacity and adaptability of cupola and induction furnace are different. At present, the melting rate of cupola is from 1t / h to 100t / h, and it can be less than 1t / h and more than 100t / h if needed in production. The melting rate of any kind of cupola can be adjusted within the corresponding range without affecting its normal melting. The adjustment range can be increased or decreased by 10% ~ 20%. Therefore, it can meet the needs of any production scale in terms of output. Because the comprehensive benefit of cupola increases with the increase of melting rate, and the comprehensive benefit of large capacity, long furnace life and external hot air cupola is higher, it is more suitable to use Cupola for mass production. At present, the single capacity of induction furnace for smelting can reach nearly 100 tons, which can meet the needs of large-scale production. However, the expansion of power installed capacity is often an obstacle for many enterprises. Therefore, induction furnace is more suitable for small and medium-sized production.

2) The adaptability to the product is different. Cupola can melt cast iron, including gray cast iron, nodular cast iron, vermicular cast iron, white cast iron and some alloy cast iron and ferroalloy. It can also be used to melt some stone. The tapping temperature of cupola can be controlled at about 15500c, which can meet the casting quality from several grams to dozens of tons. Induction furnace can melt all kinds of steel and nonferrous metals. As far as the production of iron castings is concerned, the adaptability of induction furnace to products is better than that of cupola. Induction furnace can smelt many kinds of high alloy cast iron, such as high chromium cast iron, but cupola can't.

3) The adaptability to the production mode is different, cupola is continuous smelting, and the molten iron flows out continuously from the furnace with a certain flow rate, while induction furnace is intermittent smelting, which provides molten iron equal to the furnace capacity at intervals. It can be seen that cupola is more suitable for the production mode of parallel operation, and induction furnace is more suitable for the production mode of stage working system. The combination of multiple induction furnaces and the control of smelting and tapping sequence can also be used in parallel operation. However, the combination of multiple induction furnaces will increase the energy consumption, supporting facilities and floor area, and increase the burden of production management.

(2) Impact on the environment

1) Both cupola and induction furnace have dust and harmful gases. The difference is that the proportion of large particles in cupola dust is very large, mainly coke particles, The smoke and dust of induction furnace are oxide fine particles; due to the combustion of coke in the cupola, the emissions of SO2 and CO2 in the flue gas are more, and the amount of flue gas is also larger; the cupola has no electromagnetic pollution, but the induction furnace has strong electromagnetic pollution; the thermal radiation pollution of induction furnace is heavier than that of cupola; the pollutants of cupola are discharged outside the plant, and the pollution of induction furnace is all inside the plant. From the perspective of environmental protection, the dust removal device can completely solve the pollution problem of cupola. At present, the developing technology of dust back blowing and slag granulation has opened up a feasible way for pollution control of cupola, which is harmless and resource-based. At present, the pollution control of induction furnace does not have such advantages.

2) The cupola uses coke as fuel, the coke consumption per ton of molten iron (85% of fixed carbon mass fraction) is about 100kg, and the heat of complete combustion is 2896mj; the induction furnace uses electricity as energy, and the power consumption per ton of molten iron is about 2160mj (600kwh). If 1kg coke needs 1.25Kg coal, it will consume 125kg coal per ton of molten iron; if 1 kwh power needs 0.36kg coal, it will consume 216kg coal per ton of molten iron. At present, the power structure of our country is mainly thermal power, and the consumption of non renewable resources by induction furnace is more than 70% of that of cupola. The resulting emissions of pollutants also exceed the cupola. Moreover, the volatiles of coking coal become chemical raw materials, while the volatiles of power coal which can be used as chemical raw materials are burned. With the change of power structure, the current situation will be changed, and the energy structure of cupola will also be changed. For example, the consumption of coke can be reduced by more than 60% in the cupola with plasma heating and air supply, and the corresponding pollutant emissions will be greatly reduced. The cupola which uses coke or carbon bed to achieve metallurgical purpose and uses non coke fuels such as natural gas, gas, electricity and fuel oil to provide heat has a successful precedent, and the research on improving performance and practical application feasibility is also in progress. On the premise of maintaining the melting performance, improving the energy structure of cupola will make the superiority of cupola more obvious.

(3) Influence of equipment structure and matching on smelting

The performance of cupola with different structure and matching is obviously different. During the production, the suitable cupola structure and supporting equipment can be selected according to the requirements of casting grade, charge composition, output and production mode. Cupola has a strong structure and matching adaptability. There are a variety of cupola systems to choose from, from the cupola with normal temperature air supply and constant lining, single shift manual operation to the long-life automatic cupola without lining. Induction furnace belongs to crucible furnace. Its basic function is melting and overheating. The change of structure has little effect on its basic function. Therefore, the production conditions can only be selected according to the basic function of induction furnace, but not according to the production conditions.

(4) Thermal efficiency and waste heat recovery:

From the perspective of production energy utilization, the thermal efficiency of cupola is about 40% - 60%, and that of induction furnace is about 60%, which is lower than that of induction furnace. However, from the perspective of resource energy consumption per ton of molten iron, cupola is much lower than that of induction furnace. The heat loss of cupola includes the physical heat taken away by flue gas and the latent chemical heat of CO, the heat loss of furnace body, the heat of slag and slag, etc.; the heat loss of induction furnace includes the heat loss of furnace body and furnace mouth, the resistance heat of induction coil taken away by cooling water, etc. For the recovery of these lost heat, cupola has more full feasibility and greater benefits than induction furnace. For example, the technology of hot air, long furnace life and flue gas waste heat boiler has been successfully applied in China, With the increase of the number of large capacity cupola, the energy consumption per ton of molten iron for cast iron smelting in China will decrease significantly, and the goal of catching up with and surpassing the world advanced level can be achieved.

(5) Metallurgical properties and quality of molten iron

It is generally acknowledged that the metallurgical performance of cupola is much better than that of induction furnace. The metallurgical environment composed of high temperature above 17000C and coke, slag and furnace gas in cupola plays an active role in the composition change of iron material and molten iron. Through process control, it can increase carbon and silicon, greatly reduce the burning loss of alloy elements and reduce the sulfur increasing rate. In particular, the cupola with large capacity and long life of external hot air has stronger ability to stabilize and improve the quality of molten iron. In the cupola, although the superheating time of molten iron is only a few minutes, the molten iron has sufficient surface area to contact with coke, slag and furnace gas due to its droplets and small streams, so as to achieve sufficient endothermic heating and material transfer. The results show that the superheating temperature of molten iron can reach above 16000c; the sulfur content of molten iron can be controlled below 0.06%; the fluidity of molten iron and slag is good, which is conducive to the transfer of metal oxides from molten iron to slag; the superheating time of molten iron in the furnace is short, there are about 0.005% oxygen and 0.06% sulfur in molten iron, and their high melting point compounds can be used as the nucleation substrate. After being discharged from the furnace, the self nucleation of molten iron is relatively low The effect of grain refinement by inoculation treatment is more significant for high grade gray cast iron.

The highest temperature of induction furnace is about 16000c, which is lower than that of cupola. When using carburizing agent, it is difficult to highly dissolve and disperse, and it is easy to form flake graphite during crystallization. The superheating time of molten iron is as long as one hour, and there is electromagnetic stirring, which can greatly reduce the number of foreign nuclei as eutectic crystallization due to dissolution and reaction. The sulfur content is about 0.03%, and the oxygen content is about 0.002%. Because the nucleation base is too little, the supercooling degree of molten iron is increased. Under the same carbon equivalent, it is 400-500c higher than that of cupola. The thin and thin parts of the casting are easy to appear white spots, and the cutting performance is poor. The shrinkage tendency of the casting is large, and the thick part is easy to produce shrinkage porosity and shrinkage cavity. The effect of the usual inoculation method is not obvious, so it is necessary to adopt large dose and strong inoculation And inoculated for many times.

Due to the poor metallurgical capacity, the adaptability of EAF to burden is far less than that of cupola. Clean and rust free burden must be used, and the composition of the burden should meet the requirements of the composition of the molten iron, and it is not expected to achieve the composition of the molten iron through smelting. When melting high carbon cast iron with low carbon charge, the properties and amount of carburizing agent are strictly limited, so the proportion of scrap in the charge is also limited, but the cupola can melt high carbon cast iron with scrap as charge; in the burden of induction furnace, the proportion of original pig iron cannot be high, and the coarse graphite flakes in the original iron are difficult to completely dissolve in the molten iron at the temperature of induction furnace, When the molten iron crystallizes, the primary graphite is formed as the crystal core. These graphite are flakes, which have a bad effect on the mechanical properties of cast iron. Usually, when the amount of pig iron used in induction furnace is about 10%, and more than 20%, the mechanical properties of cast iron will be difficult to guarantee.

In order to avoid heredity in the production of ductile iron and high grade gray cast iron, the induction furnace is more sensitive to the proportion of return material than the cupola.

     
       

Melting method Cupola meltingInduction furnace meltingCupola induction furnace duplex
Type of cast ironGray cast iron; nodular cast iron; malleable cast iron; some medium and low alloy cast ironAlloy cast iron; gray cast iron; nodular cast iron; malleable cast iron Alloy cast iron; gray cast iron; nodular cast iron; malleable cast iron
Requirements for chargeThe results show that it is feasible to use different proportions of scrap, chips, raw pig iron and recycled materials, the content of C and Si in the proportioning can be quite different from that of molten iron, slag iron and some pellets can be used, and purification treatment is not needed It is not suitable to use a large amount of primary iron and recycled charge, the ingredient of burden should be consistent with that of molten iron, purification treatment should be carried out, and low-quality charge should not be used Cupola melting
Product batch Mass production Medium and small batch Various batches
Production scalelarge-scale Medium and small scalelarge-scale
Mode of productionParallel working system, continuous working system and stage working system Stage work system Parallel working system, continuous working system and stage working system
 Degree of modeling mechanization high low high
Electricity supply less large secondary
Technical requirements for operation higher Not high higher
Equipment capacitylargeno large large
Quality of molten ironThe quality and stability of molten iron vary greatly with different types of cupola The quality of molten iron is not high, but its stability is easy to controlThe quality of molten iron is high and the stability is easy to control

 


    The development of cast iron smelting equipment and technology at home and abroad and a large number of production practice provide us with experience that can be used for reference. The status of cupola as the most important basic equipment for cast iron smelting will not change in the next few decades at least. With the development of casting industry, the number of cupola with large capacity, long furnace life, external hot air and intelligent operation will continue to increase, with its low energy consumption and low material consumption The excellent properties of low pollution, high quality and high efficiency play a key role in changing the backward face of China's iron casting production. In mass production, cupola induction furnace duplex melting should be popularized as the first choice. On the premise that the production conditions such as electric power and furnace charge can meet the requirements, it is also a feasible choice to use induction furnace to melt small and medium-sized iron castings with high added value.


    

2. Selection of smelting equipment

The correct choice of smelting equipment has a great influence on the comprehensive benefit of production. China is a big country of casting production, but the cast iron smelting equipment in China is very backward, and the quantity of small capacity cupola is too much, resulting in the situation of high energy consumption, high material consumption, high pollution, low quality and low benefit. Especially in recent years, the price of coke and burden has risen inappropriately, and the environmental protection requirements have become more and more strict, which has caused considerable difficulties to the survival of small and medium-sized enterprises with low efficiency. It is undoubtedly right to take the road of reform, including changing the melting method of cast iron, to improve efficiency and to seek survival and development. The problem lies in how to choose the melting method of cast iron. Table 1 mainly compares the characteristics of cupola and induction furnace from the aspect of production technology for reference. Production technology is closely related to comprehensive benefits, and the fundamental basis for selecting equipment is the comprehensive benefits of equipment used in production. In order to get the best plan, we should analyze and calculate the comprehensive benefits from all aspects according to the actual situation and expected development of production.



 

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


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