Melting equipment for recycled aluminum
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It is the same as primary aluminum. However, due to the varying shapes and sizes of scrap aluminum, large amounts are lost during the melting of small pieces; in fact, they oxidize completely even before they can be melted in the furnace. Therefore, equipment for melting scrap aluminum must take into account oxidation burnout and the various requirements imposed on such equipment as a result. Although aluminum has a relatively low melting point (660°C), it possesses a high latent heat of fusion and specific heat; as a result, the heat required to melt aluminum is even greater than that needed for copper (with a melting point of 963°C). This shows that energy savings in aluminum melting are very important. Aluminum melting furnaces include reverberatory furnaces, induction furnaces, resistance furnaces, etc. Refining furnaces are further divided into radiant and convective aluminum melting furnaces. The fuels used in reverberatory furnaces include natural gas, coal gas, heavy oil, etc. Classified by geometric shape, there are side-fed rectangular furnaces and top-fed circular furnaces, etc. Currently, apart from resistance furnaces, the use of reverberatory furnaces and induction furnaces is continuously expanding and being improved. In many rural areas of our country, individual enterprises still use traditional pit-type crucible furnaces that operate on coal as fuel; as a result, these furnaces lead to high energy consumption, severe wear and tear, low yield, high labor intensity, and low productivity – making them methods that should be phased out. Induction melting furnace for aluminum: There are two common types, coreless and cored. The advantages of this type of furnace are: ① Less gas absorption and less oxidation loss; generally, the loss of aluminum metal is only around 0.5%. When the raw materials are crushed, it’s not 1% either; for scrap aluminum, it’s at most 5%. ②During melting, strong magnetic field lines cause stirring, resulting in uniform composition and fast melting, which is highly beneficial for crushed materials. ③It has low power consumption per unit and high thermal efficiency. Under normal conditions, the power consumption is 380–450 kilowatt-hours per ton, with a thermal efficiency of up to 70%. The resistance furnace corresponds to 430–600 kilowatt-hours/ton and 50%. ④It requires less space and offers a good operating environment; the downside is that when changing the type of product in a trough-type induction furnace, it is necessary to remove the aluminum from within the trough, which poses difficulties. Secondly, the refractory materials in the molten pool are eroded and stripped by the molten aluminum, contaminating the melt. Refractory furnace: The refractory furnace is the most commonly used type of furnace at present. The radiant aluminum melting furnace relies on flames to heat the furnace walls, thereby enabling high-temperature radiant heat transfer that melts the material inside the furnace. This type of furnace uses radiant heat to melt aluminum; the amount of heat absorbed by the feed material is limited, resulting in a thermal efficiency of only 8%. Therefore, convective aluminum melting furnaces have seen increasing use in recent years. A convective aluminum melting furnace is a melting furnace that primarily relies on convective heat transfer. High-speed or ultra-high-speed nozzles are commonly used in furnaces such as rapid aluminum melting furnaces. The flame speed reaches 120–150 meters per second; the use of high-speed nozzles significantly improves heat transfer efficiency, which is more than 2/3 higher than that of radiant types. Secondly, the metal burnout is significantly reduced due to the lower furnace temperature and the strong convective flow of hot furnace gas. Therefore, it has been widely applied. Convection reverberatory furnaces have different structures: Such furnaces are characterized by the fact that they lack a furnace chamber; as aluminum melts, it flows out of the furnace along the inclined bottom and into the stilling furnace. During cooling start, aluminum alloy can be obtained 5–10 minutes after melting. After the furnace is shut down, the molten aluminum can be completely drained within 1–2 minutes. It is easy to operate and can continuously produce molten aluminum. The combustion speed of the burner is adjustable, and this type of furnace consumes less energy. This type of furnace can be used specifically for melting waste beverage cans, as well as melting waste from extruded profiles. Figure 4 shows that the flow in the rapid aluminum melting furnace consists of a tower-shaped preheating zone and a box-shaped heating zone. The feed material is fed into the tower furnace from the top of the tower using a bucket-type inclined bridge elevator; the waste gas comes into contact with the feed material, preheating it to temperatures of 300–400°C. High-speed nozzles are installed at the lower part of the tower furnace to melt the feed material. With an air flow velocity of 100–300 meters per second, the charge is subjected to impulsive heating, causing it to melt rapidly; the molten aluminum then flows along the inclined furnace bottom into the heating zone. A burner with low flame spread is installed at the top of the heating chamber to heat the molten aluminum to the casting temperature, after which it is allowed to stand still. With this furnace, the metal loss when processing 73% recycled material and 27% aluminum ingots is only 1.5%. Another advantage of this type of furnace is that as the furnace charge descends from the top of the furnace, it is continuously dried, preheated, softened, and sinks; volatile substances and water vapor are expelled along with the flue gas, and substances such as paint can also be burned and removed before melting. Therefore, the quality of the molten aluminum can also be improved. Hiroshi Oku of Japan proposed a structure for an open-pit reflective furnace for remelting aluminum scrap. This furnace consists of a pre-melting chamber, a melting chamber, a material pressing device, and an electromagnetic stirrer, among other components. This furnace is characterized by an open molten pool that facilitates the loading of waste materials, and allows the feed materials to come into direct contact with the melt in the pool for rapid melting; above the open pit, there is a pressing device used to force the waste materials into the melt. The open well is connected to the melting chamber, separated from it by a furnace door; the molten pools of the two are in communication below the furnace door. The melting chamber is enclosed by a furnace door and a furnace roof, with burners inside used to heat the molten aluminum. An electromagnetic stirrer for stirring the melt is installed outside the bottom of the furnace lining in the melting chamber. The position of the electromagnetic stirrer is adjustable. This type of furnace is most suitable for remelting metal aluminum waste, especially empty aluminum cans and aluminum shavings, without the need for traditional pressing into packs. After the aluminum waste is loaded into the open well, it is forced into the melt by a pressing device, which speeds up the melting process. This reduces the burning loss of aluminum materials such as aluminum foil, and the exhaust gases generated by the combustion of paint and coatings on the surface of the waste hardly enter the melting chamber; instead, they disperse above the open area or are released after being purified in a dust collector. Electromagnetic stirring increases the melting rate and homogenizes the composition, thereby improving product quality. Modern reverberatory furnaces mainly burn gas and oil, with the structure of the burner being the key factor. The domestic \"Yan’s rapid aluminum melting furnace\" and \"Yan’s burner\" have been widely adopted and achieved good results. The Shanghai Aluminum Wire Factory uses coal-fired vertical aluminum melting furnaces, which provides some reference value for central enterprises that do not have such facilities. At this point, coal is added to the combustion chamber for burning, with a blower used to assist the combustion process. The flames generated by this burning penetrate into the furnace through the flame deflector walls, thereby delivering heat to the furnace chamber to melt the aluminum material. Resistive furnaces: At present, resistive furnaces are only used in some small enterprises; due to their shortcomings such as low thermal efficiency and slow melting speed, large-scale furnaces have gradually been replaced by reverberatory furnaces and induction furnaces.
Author: Lü Kedong 1.1 The role of recycled aluminum melting equipment Recycled aluminum melting equipment includes melting furnaces, holding furnaces, fans, combustion systems, etc. Among them, there is reprocessed aluminum melting and a holding furnace (small enterprises do not have holding furnaces); therefore, the melting furnace and the holding furnace can be considered as one unit. There are many types of melting furnaces, all of which serve the same purpose: to melt the feed materials and additives. These feed materials include scrap aluminum, master alloys, industrial silicon, and pure aluminum ingots ； Secondly, at a certain temperature within the furnace, a series of chemical and physical reactions occur between the molten materials, causing the impurities to form slag or gas and thus be removed ； Third, adjust the composition so that the contents of various elements in the alloy meet the relevant standard requirements ； Fourth, the alloy melt is subjected to treatments such as modification in order to refine the grain structure, enabling the alloy to meet the required physical properties. Therefore, the design and structure of the melting furnace play an important role in the production capacity, cost, product quality, and environmental protection of recycled aluminum alloys. 1.2 Main Melting Equipment The melting points of commonly used aluminum alloys are not high, and there are basically two types of melting furnaces: crucible type and bath type. 1.2.1 Crucible Furnace The furnace crucible is a commonly used device for melting recycled aluminum alloys. Its advantages include low investment, easy operation, and a high metal recovery rate; however, its disadvantages are low production capacity, short lifespan, and unstable composition, making it difficult to compete with large-scale reverberatory furnaces. There are various types of crucible furnaces, with cast iron crucibles and graphite crucibles being the most commonly used. When in use, the crucible furnace has its furnace body fixed on a hearth built with refractory materials; the lower part and surrounding areas of the crucible furnace constitute the combustion chamber. When using larger crucible furnaces, due to the weight of the furnace itself, its bottom cannot be left suspended in the air; it must rest on stable refractory materials. This is especially true for large cast-iron crucible furnaces, as high temperatures can cause the furnace body to deform, thereby affecting its lifespan. The crucible furnace has strong fuel adaptability, capable of using coal, coke, gas, etc., offering a wide range of fuel options. When fueled by fuel oil or gas, there is a nozzle beneath the crucible that injects the fuel and air for combustion and heating; this is what is known as a fuel-oil crucible furnace or gas furnace. When using electric heating, a resistive heating element (resistive wire or silicon carbide rod) is arranged around the crucible, which is known as a resistive crucible furnace. Crucible furnaces that use fuel generally heat up rapidly, but their temperature control is not very precise. The heating rate of resistance crucible furnaces is relatively slow; it can reach 900 degrees for electric heating wires and 1200 degrees for silicon carbide rods. The temperature obtained is lower than that of fuel furnaces. Additionally, such furnaces are expensive to operate, consume a lot of electricity, and result in high melting costs. However, it features a favorable production environment and working conditions, as well as precise control over the melting temperature, making it suitable for melting aluminum and magnesium alloys. An external heat source first heats the crucible; once the crucible is heated, the heat is then transferred to the metal charge or melt inside it. Based on these heat transfer characteristics, the crucible furnace is an externally heated melting furnace, in order to improve thermal efficiency. The crucibles are designed with a diameter that is smaller than their height, in order to increase the contact area between the metal and the crucible walls. In this way, the contact area between the molten liquid metal and the external atmosphere is relatively small, which reduces oxidation and gas absorption of the metal, benefiting it. When melting aluminum alloys, two types of crucibles are commonly used: one is the graphite crucible, which has high strength and heat resistance, and the other is the cast iron crucible. (1) Graphite crucibles: Graphite crucibles are produced and supplied by specialized refractory material manufacturers. There are many different sizes and capacities available for these crucibles; the number assigned to a crucible indicates the weight in kilograms of copper alloy that can be melted in it. For example, a crucible labeled 50 can melt 50 kilograms of copper. When melting aluminum, the capacity needs to be divided by a factor of 0.4. Graphite crucibles can be reused multiple times, but overall they have a short lifespan; as they are used for longer periods, their thermal conductivity decreases, which affects thermal efficiency and production efficiency. (2) Cast-iron crucible furnace: Since the melting temperature of aluminum alloys is relatively low, typically between 700–800°C, metal crucibles are widely used, with cast-iron crucible furnaces being the most common type. Ordinary cast iron crucibles are inexpensive, have high strength and good thermal conductivity, which makes them widely used in production; however, they have a short lifespan, requiring frequent replacement during manufacturing. To increase the lifespan of cast iron crucibles, heat-resistant cast irons or heat-resistant steels containing nickel, chromium, or aluminum can also be used to extend their service life. The capacity of cast iron crucibles used for melting aluminum alloys is usually between 30 and 250 kilograms, generally not exceeding 300 kilograms; those with larger capacity can reach over 500 kilograms. To prevent iron in the crucible from seeping into the aluminum melt during the melting process, and also to protect the crucible, it is necessary to apply a protective coating on its inner wall before use. Information regarding the coating used for crucible furnaces can be found in relevant materials. Large crucible furnaces are usually fixed; once the melting process is complete, the molten material can be poured out of the crucible using a pouring spoon. For large castings, the crucible can also be lifted out for pouring. Many medium and small resistance crucible furnaces are equipped with a tilting mechanism to pour out the solution in the crucible. Currently, crucible furnaces are evolving toward larger sizes and more mechanized control systems; large tilting crucible furnaces allow molten material to be poured by tilting the furnace body. 1.2.2 Reflecting Furnace The melting equipment with a pool-type furnace chamber is called a reflecting furnace. The original reverberatory furnace used coal as fuel; it had a combustion chamber, and the flames were reflected into the melting chamber through an arched roof. With the development of recycled aluminum technology, many modern reverberatory furnaces no longer use coal as fuel; instead, they rely more on oil and gas. As a result, the concept of reverberatory furnace has faded, and they are now generally referred to as flame melting furnaces. A fuel-heated reverberatory furnace is mainly composed of a furnace bottom, furnace walls, and a furnace top that form the melting chamber. A shallow but wide molten pool is formed to hold the metal charge and the melted liquid metal. There are furnace doors on the front of the furnace wall for feeding and operation. Standard smelting furnaces are equipped with chimneys, which help to improve the operating environment, save energy, and facilitate the control of exhaust gas pollution. However, in practice, many enterprises’ furnaces lack chimneys; some are open, while others have smoke collectors installed on their doors. In coal-fired furnaces, during the melting process, the high-temperature furnace gases coming from the combustion chamber flow into the melting chamber through side openings; whereas in oil or gas-fired furnaces, the flame is injected directly into the furnace, heating the furnace roof and walls as well as the material being melted. Metal charges are heated and melted by high-temperature furnace gases as well as the radiation from the roof and walls of the furnace that have been heated to high temperatures. The structure of reverberatory furnaces varies considerably depending on the type of fuel used. Due to the large volume of the furnace chamber in reverberatory furnaces, their capacity can reach several dozen tons; currently, furnaces used for melting aluminum alloys can have a capacity of over 50 tons. Therefore, it can melt various types of feedstock, making it very suitable for recycling aluminum manufacturers with large production volumes. At present, the reverberatory furnace is the main equipment for melting aluminum alloys. Reverberatory furnaces come in rectangular and circular shapes, with the rectangular type being the most common; it is easier to construct and costs less. Circular reverberatory furnaces are costly and difficult to maintain, but they have a high heat efficiency. Since a circle has the largest surface area for a given perimeter, a furnace with a circular shape has the greatest surface area when the perimeter remains constant; this results in a larger area that is exposed to heat, thereby enhancing thermal efficiency. In production, the reverberatory furnace has high thermal efficiency due to the direct heating of the metal; moreover, with the charge and melt at a shallow level, it achieves rapid temperature rise and high productivity. At the same time, it is also easier for reverberatory furnaces to remove impurities from inside the furnace. However, due to the contact between the metal and the combustion gases, oxidation of the metal and gas absorption occur severely, resulting in many impurities that affect the quality of the melt. Furthermore, due to the direct contact between the flame and the charge, significant aluminum loss occurs, resulting in a lower recovery rate compared to crucible furnaces. Reverberatory furnaces can also use resistance heating, that is, resistance reverberatory furnaces. Resistance wires or strips, or silicon carbide rods are suspended from the furnace roof; heat is transferred to the metal at the bottom of the furnace through high-temperature electric heating elements and radiation from the furnace roof. It is suitable for melting aluminum alloys with low melting points; the working conditions in resistance arc furnaces are good, and the quality of the melted aluminum alloys is high. However, its major drawback is high power consumption. Reverberatory furnaces are widely used in the recycled aluminum industry, giving rise to many different types of furnaces. (1) Two-chamber reverberatory furnace: A two-chamber reverberatory furnace is a specialized device for melting recycled aluminum alloys. Thanks to its advantages of low energy consumption, low burn-off rate, and high metal recovery rate, it is widely used by various recycled aluminum manufacturers in Europe and the United States. However, due to technical barriers between countries, two-chamber reverberatory furnaces are rarely used in China. A two-chamber reverberatory furnace, as the name implies, is a melting furnace composed of two melting chambers. There are various types of such furnaces, but they generally consist of two chambers – an inner chamber and an outer chamber – with specially designed channels between them for the circulation of aluminum melt. The outer melting chamber of a two-chamber reverberatory furnace is primarily used for melting waste aluminum, while the inner melting chamber carries out the actual smelting process. In practice, scrap aluminum is directly added to the molten aluminum in the outer melting chamber, where it is quickly submerged by the superheated molten aluminum. Since the scrap aluminum does not come into direct contact with the flame, its burn loss is very low, which allows for a significant increase in the aluminum recovery rate. The volume of the inner melting chamber is larger than that of the outer melting chamber; its main function is to heat the aluminum melt while also melting aluminum alloys. It can be seen that the two-chamber reverberatory furnace combines the advantages of the crucible melting furnace and the reverberatory furnace (in the former, waste aluminum does not come into contact with the flame, resulting in low burnout) ； The latter has a larger volume and higher thermal efficiency). The melting chamber of a common two-chamber reverberatory furnace is equipped with a combustion system, while the outer melting chamber does not have one. The scrap aluminum is added from the outer melting chamber, where it is directly immersed in the superheated aluminum melt and thus melted. The temperature of the aluminum melt then drops, and it moves into the inner melting chamber via a circulation pump. There, the molten aluminum is heated again, after which it returns to the outer melting chamber under the action of the circulation pump to continue melting the scrap aluminum, in this repeated cycle. During the melting process, a large amount of aluminum dross is generated in the outer melting chamber. Due to the small volume and surface area of this chamber, it is possible to significantly reduce the amount of additives (mainly fluxes) that need to be used, compared to other melting furnaces. It also facilitates the removal of the aluminum dross, thereby reducing the workload for the workers. Circulation pumps generally use ceramic circulation pumps or graphite circulation pumps. According to the information provided, the consumption of additives in two-chamber reverberatory furnaces is only one-half to one-third that of other reverberatory furnaces; the recovery rate can be increased by 2 to 5 percentage points, while energy consumption can be reduced by 20–30%. The dual-chamber reverberatory furnace exhibits these advantages even more when dealing with fragmented waste aluminum and aluminum scrap. The disadvantages of the two-chamber reverberatory furnace are also evident: when a certain amount of scrap aluminum has been melted and the furnace reaches its designed capacity, feeding must be stopped to allow for composition adjustment, refining, degassing, etc., followed by casting after a period of settling. If all the aluminum melt in the furnace is cast into ingots, then during the next melting process, the initial portion of scrap aluminum added will still need to be in contact with the flame, and the problem of burnout still exists. To avoid this problem, some companies reserve a portion of the molten aluminum in the furnace at the end of the casting process, so that it can be used for the next melting cycle. However, this reserved amount of molten aluminum has already been refined; therefore, it needs to be mixed again with scrap aluminum before being refined once more. This approach not only wastes time but also increases energy consumption and the use of additives, while reducing production efficiency – making it economically unviable. To address the above issues, some companies build a separate holding furnace; the two-chamber reverberatory furnace serves only to melt the material and adjust its composition, while most of the refining processes take place in the holding furnace. (2) Aluminum melting furnace with a charging well: This type of melting furnace is also a double-chamber reflector furnace, consisting of a furnace with a charging well and a magnetic pump; the three together form a circulation system, as shown in the figure. During production, aluminum scrap is continuously added to the feeding well, where it is melted by the superheated aluminum melt; thereafter, it is pumped into the reverberatory furnace by a magnetic pump. This process repeats itself to achieve melting. The advantages are low burn loss, high metal recovery rate, suitability for processing crushed waste aluminum, and even greater suitability for processing aluminum shavings. The shape of the melting furnace can be square. (3) Refractory furnace with an electromagnetic stirring system: During the melting of recycled aluminum alloys in a refractory furnace, stirring is necessary to promote heat exchange, accelerate the melting rate of aluminum, increase the reaction rate, and ensure uniform composition of the aluminum solution. Each stirring damages the alumina protective layer on the liquid surface, increasing the loss of aluminum due to burning. To this end, many organizations are researching mixing techniques; although mechanical rakes and similar devices have been developed, they are not very satisfactory. The electromagnetic stirring system is a technology developed by British companies, and it is suitable for various reverberatory furnaces and static furnaces. The principle of electromagnetic stirring is to install induction coils at the bottom or sides of the furnace; when electricity is applied, a traveling magnetic field is generated. The stirring (flow) of the aluminum alloy solution in the molten pool is achieved through the interaction between the electromagnetic field and the conductive molten metal. This is similar to the principle of a motor, where the stator of the motor acts as a stirrer, and the rotor acts as the molten pool. Electromagnetic stirring can significantly reduce burn loss, lessen the operational workload, purify the environment, decrease slag formation, and yield an aluminum alloy solution with uniform composition. Electromagnetic stirring systems are very expensive, and only enterprises with strong financial capabilities can afford to install them. (4) Drop-type reverberatory furnace: The drop-type reverberatory furnace, also known as a mother-and-child furnace, is a type of reverberatory furnace setup that is quite suitable, especially for processing waste aluminum materials with a high iron content. The mother-son furnace consists of a melting furnace and a refining furnace, which are connected to each other with a certain height difference. The melting furnace serves only to melt the material; once the feedstock enters it, it melts rapidly. The molten aluminum then flows into the refining furnace, while impurities such as iron remain in the furnace and are removed manually. This reduces the time that iron is in contact with the molten aluminum solution, thereby minimizing the amount of iron that mixes into the aluminum alloy. The aluminum melt that enters the melting furnace is further melted. Since there are no impurities such as iron in the melting furnace, contamination of the aluminum melt by iron is avoided throughout the melting process, ensuring the quality of the aluminum alloy. The mother-and-child furnace is a type of furnace that is highly worth promoting; it comes in various sizes, requires low investment, and has broad applicability. Currently, many enterprises in the northern regions use this type of furnace. When using a mother-son furnace, once the charge has melted, the solution should be removed as quickly as possible to reduce the time it stays in the furnace, thereby minimizing the absorption of iron and other impurities into the aluminum melt. (5) Rotary Refractory Furnace There are various types of rotary furnaces; their characteristic is that the furnace body can rotate 360 degrees during the production process. This improves thermal efficiency, accelerates heat transfer, and virtually eliminates the need for stirring. Since the refractory material makes uniform contact with the molten aluminum, the corrosion of the furnace walls is even (generally, the most severely corroded area in a furnace is at the liquid level), which also results in a longer service life for the furnace. Rotary aluminum melting furnaces must use liquid or gas fuel. Koala 1.2.3 Induction furnace: This is a type of melting furnace that uses electromagnetic induction to heat metal. Induction furnaces can be classified into power-frequency furnaces (50–60 Hz), medium-frequency furnaces (1–10 kHz), and high-frequency furnaces [200–300 kHz], depending on the frequency of the alternating current applied to them. Induction furnaces can be classified into two types based on their structure: core-type induction furnaces and coreless induction furnaces. An industrial-frequency core induction furnace functions like a transformer, feeding alternating current into the primary winding outside the core to melt the material. In an induction furnace, alternating current with a frequency different from the supply frequency generates a large induced current in the metal within the molten pool, which is in contact with the secondary windings; this current is what heats the metal. An iron-coreless induction furnace is a crucible-type melting furnace in which a primary winding, namely the inductor, is placed outside the crucible; it is made of a hollow copper tube through which water flows for cooling. Once the sensor is powered, an induced current is generated in the metal inside the crucible, resulting in heat production. A choke is also arranged outside the crucible of the power-frequency induction furnace to improve the electromagnetic efficiency of the furnace. Power-frequency induction furnaces are very inefficient at melting small pieces of metal; in fact, it is difficult to melt them, and they are only suitable for melting large pieces of metal. Therefore, the capacity of power-frequency induction furnaces is relatively large, reaching several tons or even more. Due to the electromagnetic effect, which enables the liquid in the furnace to stir on its own, resulting in uniform composition and temperature, power frequency furnaces can be used to melt copper alloys, aluminum alloys, and other alloys with lower melting points. When starting a power-frequency core induction furnace, the melt pool should be filled with metal to form a closed circuit ； After each melting cycle, there should be a certain amount of metal remaining in order to ensure that the melt pool is filled and the furnace can continue to operate. In the lower melting groove, it may sometimes get blocked by slag and debris, affecting normal melting processes; therefore, a plug hole is provided on the lower side of the furnace to allow for timely cleaning of the melting groove. When melting aluminum alloys, the melt pool is prone to being clogged by aluminum oxides. Currently, core-type induction furnaces are mostly used to melt copper alloys. A power-frequency coreless induction furnace has no melting groove, involves fewer problems, and has a simple furnace structure, making it superior to cored furnaces. However, after each pouring, a remaining amount should also be left in the crucible to facilitate smooth continuation of work. Some coreless induction furnaces are equipped with cast iron crucibles inside, which can improve electromagnetic efficiency and are particularly suitable for melting aluminum alloys. When high-frequency alternating current passes through a metal, the \"skin effect\" occurs; that is, the current induced in the metal is not evenly distributed. The current density is highest at the surface of the metal, and it decreases as one moves inward. At a certain depth, there is almost no current left. Generally, the depth at which the current is concentrated is referred to as the \"penetration depth\". It can be calculated using the following formula: δ = ρ / μf. Where: 6 – current penetration depth, in centimeters ； p – the electrical resistivity of the metal, in ohms per centimeter;
u – the magnetic permeability of the metal;
f – the frequency of the current, in hertz. As can be seen from the above formula, the penetration depth is proportional to the square root of the resistivity of the metal, and inversely proportional to the permeability coefficient and the square root of the current frequency. In other words, for a given metal, the higher the current frequency, the smaller the penetration depth. When a large current is passed through a very thin layer of metal, the heat generated becomes concentrated, which facilitates the heating and melting of the metal charge. Therefore, medium-frequency induction furnaces have much higher electrical efficiency than power-frequency furnaces, and they allow the use of relatively small pieces of metal. Medium frequency is suitable for coreless induction furnaces, as there is no need to install the choke required for low frequency; moreover, after each melting process, all of the molten metal can be poured out without having to retain any residue. It can be used to melt steel and aluminum, with high efficiency and quality, as well as favorable working conditions. However, medium-frequency induction furnaces require specialized frequency conversion equipment for power supply, which increases the cost of melting products. High-frequency furnaces are generally not used to melt steel and aluminum alloys. This type of furnace, which is heated by induced currents within the metal material, is an internal-heating melting furnace; its thermal efficiency is much higher than that of the crucible furnaces and reflector furnaces mentioned earlier. The lining of an induction furnace is generally made by compacting refractory materials, or pre-made crucibles can also be used. The melting process of aluminum alloys consists of two stages: melting the metal material and treating the molten metal. The melting stage is energy-intensive and time-consuming; measures should be taken to melt the material as quickly as possible in order to reduce metal loss. During the liquid treatment phase, steps such as melting and impurity removal, alloying, refining and degassing, and modification are generally carried out, depending on the characteristics of each molten alloy as well as the composition and quality of the feed materials. At large production scales, reverberatory furnaces are mostly used in a tandem process: first, the metal is rapidly melted in a large-capacity and high-efficiency reverberatory furnace, after which the molten metal is transferred to a resistance furnace or reverberatory furnace with strict temperature control for further processing and heat retention, before being cast. By using these two furnaces in combination, each leveraging its strengths, better economic and technical performance can be achieved. The specific type of furnace to be used and the melting process to be employed should be determined based on the requirements regarding the quality and output of the alloy to be melted. 1.3 Development of melting furnaces The advancement of melting furnace technology has progressed alongside the development of other industrial technologies, particularly electronics and new material technologies. In terms of furnace types, the double-chamber reflector furnace, the melting furnace with a charging shaft, the electromagnetic stirring reflector furnace, the rotary reflector furnace, and the tilting heat-resistant crucible furnace mentioned above are all directions for development. In terms of heating methods, high-energy beam heating sources such as lasers, electron beams, and ion beams are used. Melt protection is achieved using sealed containers with special structures to provide vacuum or gas shielding, thereby preventing the melt from being contaminated by the ambient atmosphere. From the perspective of energy conservation, new types of insulation materials are used in the insulation design of the furnace to fully improve energy efficiency. From an environmental protection perspective, a purification system for furnace gas and slag has been added. The crucible material is evolving from graphite to high-temperature-resistant alloy crucibles in order to increase the service life of the crucible furnace. In terms of stirring methods, both mechanical stirring and electromagnetic stirring are developing rapidly, with electromagnetic stirring being adopted quite soon. 1.4 Introduction to the Construction of Refractory Furnaces A refractory furnace, as the name implies, heats the charge using reflected heat, thereby melting and refining it. Traditional reverberatory furnaces use coal as fuel; a combustion chamber is built at one end of the furnace, and as the flames rise they encounter the arched ceiling, which reflects them into the melting chamber, thereby achieving the purpose of melting. Reverberatory furnaces are not only used in aluminum recycling enterprises but are also widely applied in the non-ferrous metal industry, such as copper metallurgy and lead metallurgy. With the advancement of metallurgical technology, reverberatory furnaces have developed rapidly, especially due to improvements in fuels – the widespread use of oil, gas, and other fuels has led to significant enhancements in these furnaces. The role of reflection in their operation is diminishing; nowadays, the design of such furnaces takes into account not only the reflection of flames but also the improvement of heat efficiency and the reduction of losses during heating. 1.5 Thermal Mechanics of Refractory Furnaces 1.5.1 Heat Transfer in Refractory Furnaces Heat transfer is a complex physical phenomenon, which is generally divided into three modes: conduction, convection, and radiation. The furnace body of a reverberatory furnace is mainly composed of the furnace roof, furnace walls, and furnace bottom, all of which play an important role in heat transfer within the furnace. The three modes of heat transfer coexist in a reverberatory furnace, which is generally referred to as combined heat transfer. In actual production, the flame radiates heat toward the scrap aluminum and the four walls of the furnace; these walls then conduct (or radiate, reflect) the heat to the aluminum. At the same time, some of the heat is lost from the system through the walls, which is the main cause of heat loss. Therefore, it is important to pay attention to insulating the furnace walls when constructing the furnace. Heat convection can occur only when there is a temperature difference; if the temperature is uniform throughout the furnace, then no convection takes place. In reality, however, there is a significant temperature difference within the furnace. Before the material inside the furnace melts, the air surrounding it is much cooler than the temperature of the flame, which leads to convective heat transfer between the flame and the surrounding air. After the material melts, there are large temperature differences across different parts of the molten liquid, and therefore convective heat transfer plays a key role in the heating of the molten aluminum. In a reverberatory furnace, the furnace walls play an important role in heat transfer; their functions include absorbing part of the heat and simultaneously radiating part of it to the furnace charge ； Directly reflect the heat of the flame back onto the charge ； Transfer heat to the charge through the wall. Therefore, during the design and construction of a reverberatory furnace, it is necessary to take into account the three types of heat transfer as much as possible, giving full consideration to the structure of the furnace walls and the materials used. 1.5.2 Fuel and Consumption of Refining Furnaces The main fuels for refining furnaces include coal, gas, diesel, heavy oil, and natural gas. Regardless of the fuel used, the form of heat transfer remains essentially the same; the differences lie in the thermal efficiency, the melting rate, and the fuel costs, which can vary significantly. The fuel consumption per ton of aluminum in conventional reverberatory furnaces is as follows: coal-fired reverberatory furnaces consume around 200-300 kilograms of standard coal ； The fuel consumption of a heavy oil-fired reverberatory furnace is approximately 60-80 kilograms ； Diesel-fired reverberatory furnaces consume approximately 50 kilograms of fuel, while the most advanced models have reduced this amount to 30 kilograms ； The coal consumption of semi-gas reflective furnaces is approximately 260–300 kilograms. Fuel consumption is related to the production capacity of the reverberatory furnace; generally, the larger the furnace, the lower the fuel consumption per unit. Below are some examples of fuel consumption for commonly used reverberatory furnaces. (1) Technical parameters of a 15-ton heavy fuel oil furnace in a certain enterprise (2) Coal consumption in semi-gas reflective furnaces 1.5.3 Thermal efficiency of reflective furnaces The thermal efficiency of reflective furnaces is generally not very high, especially those of the flame-type. Under normal circumstances, the thermal efficiency of different types of such furnaces is as follows: The low thermal efficiency of reflective furnaces means that a large amount of heat is wasted. Given the characteristics of melting processes in these furnaces, the distribution of heat in them is as follows: (1) Approximately 25-30% of the heat is used directly for melting; this heat is mainly utilized for melting aluminum ； (2) Heat lost through the outer surface of the furnace wall and the furnace door accounts for generally 15-25%, and sometimes can exceed 30% ； (3) Approximately 40-50% of the heat is carried away by the flue gas and slag. During the melting of aluminum alloys, a small amount of slag is produced, and it is not in a molten state; therefore, the amount of heat carried away by the slag is very small, with the majority of the heat being carried away by the flue gas ； To improve the thermal efficiency of the reverberatory furnace, heat loss must be reduced through various methods, such as thickening the furnace walls and roof and adding insulation layers ； Minimize the number of times the furnace door is opened ； When designing a furnace, it is necessary to minimize its surface area and increase its volume as much as possible. 1.5.4 Utilization of waste heat in reverberatory furnaces To improve the efficiency of heat utilization, it is necessary to consider the use of waste heat, especially that carried away by flue gases. The main current method for utilizing the waste heat from flue gas is to use it to preheat the fuel (gas) and air, achieving good results. Building a waste heat boiler is also a good option. 1.6 Environmental and geological conditions for furnace construction 1.6.1 Understanding of the site conditions To ensure the longevity and safety of the furnace, it is essential to conduct a thorough investigation of the environment surrounding the site, including the site conditions and wind patterns, before construction. This includes checking whether there are any rivers or lakes in the vicinity, as well as any areas prone to subsidence. At the same time, it is necessary to have an understanding of the geological conditions at the construction site; if needed, refer to the data provided by geological authorities to gain a detailed understanding of those conditions. It is also necessary to understand the wind direction during the monsoon season. 1.6.2 Foundation To ensure the safety, reliability, and long-term use of the furnace, it is necessary for the melting furnace to have a stable foundation; therefore, the construction of the foundation is very important. The foundation must be constructed on solid soil layers, and the following principles should generally be taken into account: (1) The foundation must not be built on loose soil or sand layers; if this is unavoidable, measures must be taken ； (2) When constructing the foundation, special attention must be paid to ensuring that it is built below the disturbed layer, on the original soil ； (3) In the north, the foundation must be laid below the frost layer. 1.6.3 Layout of the melting workshop and furnace body The location of the furnace needs to take into account the following factors: 1. Surrounding space and operating area: it should be convenient for operation, with enough space for feeding materials and carrying out maintenance; transportation should be easy. If the conditions are not available yet, space should be reserved for the ingot casting machine, with long-term considerations in mind to ensure a rational layout. The smelting workshop should be built downwind (companies should be located downwind of a certain area or below residential zones to avoid unnecessary problems). 1.7 Form and Structure of the Furnace Body 1.7.1 Shape of the Furnace Body The main types currently in use are circular and rectangular reflector furnaces. Circular furnaces, such as reverberatory furnaces, are expensive and difficult to maintain, but they have a high heat efficiency. Since a circle has the largest surface area for a given perimeter, circular furnaces have a larger surface area compared to rectangular furnaces of the same perimeter; this results in a greater area that can be heated, higher efficiency, and less heat loss from the furnace’s surface. Rectangular oil reflection furnaces have low costs and are easy to maintain; however, their thermal efficiency is slightly lower than that of circular furnaces. Its furnace doors can be either two doors on each side or two doors on one side, and the oil nozzles consist of 2 to 3 nozzles on one side or two nozzles at a diagonal position. Other large domestic companies in the recycled aluminum industry include Shunsi Oil Reflected Blast Furnaces. 1.7.2 Selection of furnace type: The design of the furnace is determined based on the manufacturer’s requirements, as well as factors such as the site conditions, thermal energy sources, and process parameters. There are a wide variety of furnaces used for melting aluminum alloys, and there are many types of reverberatory furnaces. Commonly used pool-type reverberatory furnaces include heavy oil reverberatory furnaces, diesel reverberatory furnaces, resistance furnace reverberatory furnaces, gas reverberatory furnaces, semi-gas reverberatory furnaces, and coal-fired reverberatory furnaces. Based on the company’s own conditions and local advantages, such as fuel resources and transportation, it decides on its own type of furnace. It should be noted, however, that reverberatory furnaces fueled by heavy oil are declining in use, as heavy oil has high viscosity and a low freezing point (30), making it difficult to transport. It also needs to be preheated before combustion in order to improve its fluidity and atomization; further preheating is required before it enters the nozzle (110-120), which increases the cost of the equipment. When heavy oil is burned without proper atomization, it produces large amounts of black smoke, polluting the environment, and at the same time, the efficiency of heat utilization is low. At present, large enterprises recommend building reverberatory furnaces equipped with gas generators; the initial investment is higher, but the long-term benefits are significant, as these furnaces are pollution-free, have high heat utilization efficiency, and are easy to operate. Small enterprises are advised to build semi-gas reverberatory furnaces. 1.7.3 Surface area and depth of the molten pool The surface area and depth of the molten pool are important parameters for reverberatory furnaces, as the heat in the furnace is primarily conducted through the liquid surface and the walls. Theoretically, a larger surface area and wall area result in higher heat conduction efficiency. However, the greater the surface area, the more severe the surface oxidation becomes, and it also leads to an uncontrolled expansion of the molten pool, increasing energy consumption and investment costs. On the other hand, if the molten pool is too deep, it affects heat conduction. Therefore, it is necessary to take various factors into consideration—such as energy savings, reduced investment costs, and ease of maintenance—to determine the length, width, and height of the molten pool. Generally, the depth of the melt pool in furnaces of 15 tons or less is around 500 millimeters. 1.7.4 Structure of the furnace The dimensions of the structure of a reverberatory furnace are closely related to the scale of production of the furnace; therefore, it is difficult to establish a universal formula. Here, a 10-ton reverberatory furnace is used as an example. The calculation of the combustion space is based on the heat required to melt aluminum and the volume of gas that burns per unit of time; in order to ensure complete combustion of the fuel, an air excess factor is generally taken into account during such calculations. If the space is too large, it affects heat generation and reduces thermal efficiency; if it is too small, it causes flames to be ejected outward, and the smoke stays in the furnace for a short time, resulting in energy waste. Therefore, in addition to theoretical calculations, practical experimentation is also necessary.
u – the magnetic permeability of the metal;
f – the frequency of the current, in hertz. As can be seen from the above formula, the penetration depth is proportional to the square root of the resistivity of the metal, and inversely proportional to the permeability coefficient and the square root of the current frequency. In other words, for a given metal, the higher the current frequency, the smaller the penetration depth. When a large current is passed through a very thin layer of metal, the heat generated becomes concentrated, which facilitates the heating and melting of the metal charge. Therefore, medium-frequency induction furnaces have much higher electrical efficiency than power-frequency furnaces, and they allow the use of relatively small pieces of metal. Medium frequency is suitable for coreless induction furnaces, as there is no need to install the choke required for low frequency; moreover, after each melting process, all of the molten metal can be poured out without having to retain any residue. It can be used to melt steel and aluminum, with high efficiency and quality, as well as favorable working conditions. However, medium-frequency induction furnaces require specialized frequency conversion equipment for power supply, which increases the cost of melting products. High-frequency furnaces are generally not used to melt steel and aluminum alloys. This type of furnace, which is heated by induced currents within the metal material, is an internal-heating melting furnace; its thermal efficiency is much higher than that of the crucible furnaces and reflector furnaces mentioned earlier. The lining of an induction furnace is generally made by compacting refractory materials, or pre-made crucibles can also be used. The melting process of aluminum alloys consists of two stages: melting the metal material and treating the molten metal. The melting stage is energy-intensive and time-consuming; measures should be taken to melt the material as quickly as possible in order to reduce metal loss. During the liquid treatment phase, steps such as melting and impurity removal, alloying, refining and degassing, and modification are generally carried out, depending on the characteristics of each molten alloy as well as the composition and quality of the feed materials. At large production scales, reverberatory furnaces are mostly used in a tandem process: first, the metal is rapidly melted in a large-capacity and high-efficiency reverberatory furnace, after which the molten metal is transferred to a resistance furnace or reverberatory furnace with strict temperature control for further processing and heat retention, before being cast. By using these two furnaces in combination, each leveraging its strengths, better economic and technical performance can be achieved. The specific type of furnace to be used and the melting process to be employed should be determined based on the requirements regarding the quality and output of the alloy to be melted. 1.3 Development of melting furnaces The advancement of melting furnace technology has progressed alongside the development of other industrial technologies, particularly electronics and new material technologies. In terms of furnace types, the double-chamber reflector furnace, the melting furnace with a charging shaft, the electromagnetic stirring reflector furnace, the rotary reflector furnace, and the tilting heat-resistant crucible furnace mentioned above are all directions for development. In terms of heating methods, high-energy beam heating sources such as lasers, electron beams, and ion beams are used. Melt protection is achieved using sealed containers with special structures to provide vacuum or gas shielding, thereby preventing the melt from being contaminated by the ambient atmosphere. From the perspective of energy conservation, new types of insulation materials are used in the insulation design of the furnace to fully improve energy efficiency. From an environmental protection perspective, a purification system for furnace gas and slag has been added. The crucible material is evolving from graphite to high-temperature-resistant alloy crucibles in order to increase the service life of the crucible furnace. In terms of stirring methods, both mechanical stirring and electromagnetic stirring are developing rapidly, with electromagnetic stirring being adopted quite soon. 1.4 Introduction to the Construction of Refractory Furnaces A refractory furnace, as the name implies, heats the charge using reflected heat, thereby melting and refining it. Traditional reverberatory furnaces use coal as fuel; a combustion chamber is built at one end of the furnace, and as the flames rise they encounter the arched ceiling, which reflects them into the melting chamber, thereby achieving the purpose of melting. Reverberatory furnaces are not only used in aluminum recycling enterprises but are also widely applied in the non-ferrous metal industry, such as copper metallurgy and lead metallurgy. With the advancement of metallurgical technology, reverberatory furnaces have developed rapidly, especially due to improvements in fuels – the widespread use of oil, gas, and other fuels has led to significant enhancements in these furnaces. The role of reflection in their operation is diminishing; nowadays, the design of such furnaces takes into account not only the reflection of flames but also the improvement of heat efficiency and the reduction of losses during heating. 1.5 Thermal Mechanics of Refractory Furnaces 1.5.1 Heat Transfer in Refractory Furnaces Heat transfer is a complex physical phenomenon, which is generally divided into three modes: conduction, convection, and radiation. The furnace body of a reverberatory furnace is mainly composed of the furnace roof, furnace walls, and furnace bottom, all of which play an important role in heat transfer within the furnace. The three modes of heat transfer coexist in a reverberatory furnace, which is generally referred to as combined heat transfer. In actual production, the flame radiates heat toward the scrap aluminum and the four walls of the furnace; these walls then conduct (or radiate, reflect) the heat to the aluminum. At the same time, some of the heat is lost from the system through the walls, which is the main cause of heat loss. Therefore, it is important to pay attention to insulating the furnace walls when constructing the furnace. Heat convection can occur only when there is a temperature difference; if the temperature is uniform throughout the furnace, then no convection takes place. In reality, however, there is a significant temperature difference within the furnace. Before the material inside the furnace melts, the air surrounding it is much cooler than the temperature of the flame, which leads to convective heat transfer between the flame and the surrounding air. After the material melts, there are large temperature differences across different parts of the molten liquid, and therefore convective heat transfer plays a key role in the heating of the molten aluminum. In a reverberatory furnace, the furnace walls play an important role in heat transfer; their functions include absorbing part of the heat and simultaneously radiating part of it to the furnace charge ； Directly reflect the heat of the flame back onto the charge ； Transfer heat to the charge through the wall. Therefore, during the design and construction of a reverberatory furnace, it is necessary to take into account the three types of heat transfer as much as possible, giving full consideration to the structure of the furnace walls and the materials used. 1.5.2 Fuel and Consumption of Refining Furnaces The main fuels for refining furnaces include coal, gas, diesel, heavy oil, and natural gas. Regardless of the fuel used, the form of heat transfer remains essentially the same; the differences lie in the thermal efficiency, the melting rate, and the fuel costs, which can vary significantly. The fuel consumption per ton of aluminum in conventional reverberatory furnaces is as follows: coal-fired reverberatory furnaces consume around 200-300 kilograms of standard coal ； The fuel consumption of a heavy oil-fired reverberatory furnace is approximately 60-80 kilograms ； Diesel-fired reverberatory furnaces consume approximately 50 kilograms of fuel, while the most advanced models have reduced this amount to 30 kilograms ； The coal consumption of semi-gas reflective furnaces is approximately 260–300 kilograms. Fuel consumption is related to the production capacity of the reverberatory furnace; generally, the larger the furnace, the lower the fuel consumption per unit. Below are some examples of fuel consumption for commonly used reverberatory furnaces. (1) Technical parameters of a 15-ton heavy fuel oil furnace in a certain enterprise (2) Coal consumption in semi-gas reflective furnaces 1.5.3 Thermal efficiency of reflective furnaces The thermal efficiency of reflective furnaces is generally not very high, especially those of the flame-type. Under normal circumstances, the thermal efficiency of different types of such furnaces is as follows: The low thermal efficiency of reflective furnaces means that a large amount of heat is wasted. Given the characteristics of melting processes in these furnaces, the distribution of heat in them is as follows: (1) Approximately 25-30% of the heat is used directly for melting; this heat is mainly utilized for melting aluminum ； (2) Heat lost through the outer surface of the furnace wall and the furnace door accounts for generally 15-25%, and sometimes can exceed 30% ； (3) Approximately 40-50% of the heat is carried away by the flue gas and slag. During the melting of aluminum alloys, a small amount of slag is produced, and it is not in a molten state; therefore, the amount of heat carried away by the slag is very small, with the majority of the heat being carried away by the flue gas ； To improve the thermal efficiency of the reverberatory furnace, heat loss must be reduced through various methods, such as thickening the furnace walls and roof and adding insulation layers ； Minimize the number of times the furnace door is opened ； When designing a furnace, it is necessary to minimize its surface area and increase its volume as much as possible. 1.5.4 Utilization of waste heat in reverberatory furnaces To improve the efficiency of heat utilization, it is necessary to consider the use of waste heat, especially that carried away by flue gases. The main current method for utilizing the waste heat from flue gas is to use it to preheat the fuel (gas) and air, achieving good results. Building a waste heat boiler is also a good option. 1.6 Environmental and geological conditions for furnace construction 1.6.1 Understanding of the site conditions To ensure the longevity and safety of the furnace, it is essential to conduct a thorough investigation of the environment surrounding the site, including the site conditions and wind patterns, before construction. This includes checking whether there are any rivers or lakes in the vicinity, as well as any areas prone to subsidence. At the same time, it is necessary to have an understanding of the geological conditions at the construction site; if needed, refer to the data provided by geological authorities to gain a detailed understanding of those conditions. It is also necessary to understand the wind direction during the monsoon season. 1.6.2 Foundation To ensure the safety, reliability, and long-term use of the furnace, it is necessary for the melting furnace to have a stable foundation; therefore, the construction of the foundation is very important. The foundation must be constructed on solid soil layers, and the following principles should generally be taken into account: (1) The foundation must not be built on loose soil or sand layers; if this is unavoidable, measures must be taken ； (2) When constructing the foundation, special attention must be paid to ensuring that it is built below the disturbed layer, on the original soil ； (3) In the north, the foundation must be laid below the frost layer. 1.6.3 Layout of the melting workshop and furnace body The location of the furnace needs to take into account the following factors: 1. Surrounding space and operating area: it should be convenient for operation, with enough space for feeding materials and carrying out maintenance; transportation should be easy. If the conditions are not available yet, space should be reserved for the ingot casting machine, with long-term considerations in mind to ensure a rational layout. The smelting workshop should be built downwind (companies should be located downwind of a certain area or below residential zones to avoid unnecessary problems). 1.7 Form and Structure of the Furnace Body 1.7.1 Shape of the Furnace Body The main types currently in use are circular and rectangular reflector furnaces. Circular furnaces, such as reverberatory furnaces, are expensive and difficult to maintain, but they have a high heat efficiency. Since a circle has the largest surface area for a given perimeter, circular furnaces have a larger surface area compared to rectangular furnaces of the same perimeter; this results in a greater area that can be heated, higher efficiency, and less heat loss from the furnace’s surface. Rectangular oil reflection furnaces have low costs and are easy to maintain; however, their thermal efficiency is slightly lower than that of circular furnaces. Its furnace doors can be either two doors on each side or two doors on one side, and the oil nozzles consist of 2 to 3 nozzles on one side or two nozzles at a diagonal position. Other large domestic companies in the recycled aluminum industry include Shunsi Oil Reflected Blast Furnaces. 1.7.2 Selection of furnace type: The design of the furnace is determined based on the manufacturer’s requirements, as well as factors such as the site conditions, thermal energy sources, and process parameters. There are a wide variety of furnaces used for melting aluminum alloys, and there are many types of reverberatory furnaces. Commonly used pool-type reverberatory furnaces include heavy oil reverberatory furnaces, diesel reverberatory furnaces, resistance furnace reverberatory furnaces, gas reverberatory furnaces, semi-gas reverberatory furnaces, and coal-fired reverberatory furnaces. Based on the company’s own conditions and local advantages, such as fuel resources and transportation, it decides on its own type of furnace. It should be noted, however, that reverberatory furnaces fueled by heavy oil are declining in use, as heavy oil has high viscosity and a low freezing point (30), making it difficult to transport. It also needs to be preheated before combustion in order to improve its fluidity and atomization; further preheating is required before it enters the nozzle (110-120), which increases the cost of the equipment. When heavy oil is burned without proper atomization, it produces large amounts of black smoke, polluting the environment, and at the same time, the efficiency of heat utilization is low. At present, large enterprises recommend building reverberatory furnaces equipped with gas generators; the initial investment is higher, but the long-term benefits are significant, as these furnaces are pollution-free, have high heat utilization efficiency, and are easy to operate. Small enterprises are advised to build semi-gas reverberatory furnaces. 1.7.3 Surface area and depth of the molten pool The surface area and depth of the molten pool are important parameters for reverberatory furnaces, as the heat in the furnace is primarily conducted through the liquid surface and the walls. Theoretically, a larger surface area and wall area result in higher heat conduction efficiency. However, the greater the surface area, the more severe the surface oxidation becomes, and it also leads to an uncontrolled expansion of the molten pool, increasing energy consumption and investment costs. On the other hand, if the molten pool is too deep, it affects heat conduction. Therefore, it is necessary to take various factors into consideration—such as energy savings, reduced investment costs, and ease of maintenance—to determine the length, width, and height of the molten pool. Generally, the depth of the melt pool in furnaces of 15 tons or less is around 500 millimeters. 1.7.4 Structure of the furnace The dimensions of the structure of a reverberatory furnace are closely related to the scale of production of the furnace; therefore, it is difficult to establish a universal formula. Here, a 10-ton reverberatory furnace is used as an example. The calculation of the combustion space is based on the heat required to melt aluminum and the volume of gas that burns per unit of time; in order to ensure complete combustion of the fuel, an air excess factor is generally taken into account during such calculations. If the space is too large, it affects heat generation and reduces thermal efficiency; if it is too small, it causes flames to be ejected outward, and the smoke stays in the furnace for a short time, resulting in energy waste. Therefore, in addition to theoretical calculations, practical experimentation is also necessary.