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The methods for melting cast iron are classified according to the furnace equipment used, including cupola melting, induction furnace melting, arc furnace melting, reverberatory furnace melting, as well as combinations of certain methods such as cupola-arc furnace or cupola-induction furnace systems. 1. Cupola melting method (1) Structure of the cupola The basic structure of the cupola is shown in Figure 1. The furnace body, bellows, flue, etc. are welded together from steel plates. The interior of the furnace is usually lined with layers of refractory bricks to withstand the high temperatures generated by the combustion of coke. To store molten iron, most cupolas are equipped with a forehearth. (2) Principle of cupola melting: During the melting process, the lower part of the furnace is filled with coke, which is known as bottom coke. Above the bottom focal point, layers of iron materials (pig iron, scrap steel, recycled metal, ferroalloys, etc.), coke, and fluxes (limestone, fluorite, etc.) are arranged alternately. By blowing air, the bottom charge burns intensely, and the resulting hot furnace gases rise along the height of the furnace, melting the layer of iron material above it. (3) Advantages, disadvantages, and applications of cupola melting. The cupola is the most widely used equipment for melting cast iron. It uses coke as fuel; the heat generated by the combustion of coke is used directly to melt the charge and raise the temperature of the molten iron, making it more energy-efficient than electric arc furnaces and other types of furnaces. Moreover, the equipment is relatively simple and can be used in both small and large factories. However, the cupola furnace also has certain disadvantages; mainly because the molten iron comes into direct contact with coke, carbon and sulfur content in the molten iron increase during the melting process. A combined melting method using a cupola and an electric arc furnace, or a cupola and an induction furnace, is adopted to make full use of the advantages of the cupola’s high melting efficiency, as well as the strong ability of electric arc furnaces and induction furnaces to overheat the molten iron and the ease of controlling its chemical composition. Figure 1: Schematic diagram of the cupola furnace. 2. Melting in an induction furnace (1) Structure and working principle of the induction furnace: An induction furnace is a type of furnace that uses electrical current induction to generate heat, thereby heating and melting iron materials. The structure of furnaces is divided into two types: core-type (Figure 2) and coreless-type. In a coreless induction furnace, the iron material in the crucible generates an induced current under the action of an alternating magnetic field, which in turn produces heat that melts the iron material and heats the molten iron. In a core-type induction furnace, molten iron melted in other furnaces (such as cupolas) must be added. The alternating magnetic field generated within the ring-shaped core causes the molten iron in the grooves to move, and the circulation between the molten iron in these grooves and that in the pool above them is used to heat the entire amount of molten iron. Coreless induction furnaces have the ability to melt solid feed materials, whereas cored induction furnaces can only heat up molten iron; however, in terms of energy consumption for heating molten iron, cored induction furnaces are more efficient. Figure 2 shows a schematic diagram of the structure of the core-type induction furnace body (capacity: 20 t). 1 – Induction coil; 2 – Yoke; 3 – Refractory material; 4 – Molten iron; 5 – Slag. (2) Advantages, disadvantages, and applications of induction furnace melting. Compared with cupola melting, the advantages of induction furnace melting are that there is no carbon or sulfur addition during the melting process, and slag can be formed to cover the molten iron, which helps to prevent the oxidation of silicon, manganese, and other alloying elements in the molten iron. It also reduces the absorption of gases by the molten iron from the furnace atmosphere, thereby resulting in a purer molten iron. The disadvantage of this melting method is high electricity consumption. Induction furnaces are suitable for melting high-quality gray cast iron, alloy cast iron, ductile iron, and nodular iron. Coreless induction furnaces can directly melt solid feedstock, and they are easy to start up and shut down, making them suitable for intermittent production conditions. Core-type induction furnaces are inconvenient to start and stop, making them suitable for continuous production. This type of furnace has low thermal efficiency in melting solid feedstock, but high efficiency in heating superheated molten iron; therefore, it is suitable for use in conjunction with a cupola furnace. Currently, both types of induction furnaces are used in cast iron production. 3. Arc furnace melting (1) Structure and working principle of arc furnaces. Arc furnace melting utilizes the heat generated by the arc formed between graphite electrodes and iron material (molten iron) to melt the iron material and superheat the molten iron. Three-phase arc furnaces are commonly used in production, and the structure of their furnace body is shown in Figure 3. During the arc furnace melting process, once the iron material has been melted, the operations to further increase the temperature and adjust the chemical composition are carried out with the slag covering the molten iron. Arc furnaces are divided into acidic and basic types depending on the properties of the slag and the refractory materials in the furnace lining. Basic arc furnaces have the capability to remove sulfur and phosphorus. (2) Advantages, disadvantages, and applications of arc furnace melting. The advantage of arc furnace melting is its strong ability to melt solid feedstock; moreover, the molten iron is overheated and its chemical composition is adjusted under the cover of slag, which helps to prevent gas absorption by the molten iron as well as oxidation of elements to a certain extent. This creates favorable conditions for melting low-carbon cast iron and alloy cast iron. The disadvantage of electric arc furnaces is their high power consumption; from the perspective of melting, they are less economical than cupola furnaces. Therefore, in cast iron production, a combined cupola-electric arc furnace method is often used for melting. Due to the poor resistance of basic arc furnace linings to sudden temperature changes, their lifespan is short under intermittent melting conditions, which leads to high melting costs; therefore, acid arc furnaces are often used in combination with cupolas. Figure 3: Schematic cross-section of the three-phase arc furnace body