A double chamber reverberatory furnace for melting aluminum

1.1 The Role of Recycling Aluminum Smelting Equipment

Recycling aluminum smelting equipment includes the melting furnace, holding furnace, blower, combustion system, and more. Among these, the melting furnace and holding furnace (small enterprises may not have a holding furnace) are considered as a whole. There are various types of melting furnaces, but their roles are the same: first, to melt raw materials and additives, which include scrap aluminum, master alloys, industrial silicon, and pure aluminum ingots; second, to allow a series of chemical and physical reactions to occur between the molten materials at a certain temperature inside the furnace, causing impurities to form into dross or gases that can be removed; third, to adjust the composition so that the content of various elements in the alloy meets relevant standard requirements; fourth, to perform modifications on the molten alloy, refine and fine-grain it, so that the alloy meets the related physical properties. Therefore, the form 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 Smelting Equipment

The melting points of commonly used aluminum alloys are not high, and the smelting furnaces generally come in two types: crucible type and bath type. 1.2.1 Crucible Furnace

Crucible furnaces are common equipment for melting recycled aluminum alloys. Their advantages include low investment, convenient operation, and high metal recovery rate. However, their disadvantages are small production capacity, short lifespan, and unstable composition, making them difficult to compare with large-scale reflection furnaces. There are various forms of crucible furnaces, with cast iron and graphite crucibles being commonly used. When using a crucible furnace, the furnace body is fixed on a stove platform built with refractory materials, with the lower part and surrounding areas of the crucible furnace forming the combustion chamber. For larger crucible furnaces, considering the self-weight issue, the bottom of the furnace body should not be elevated but should rest on stable refractory materials, especially for large cast iron crucible furnaces, which can deform under high temperatures affecting their lifespan. Crucible furnaces have strong fuel adaptability, capable of using coal, coke, gas, etc., offering a wide range of fuel options. When using fuel oil or gas as fuel, there are nozzles beneath the crucible that spray fuel and air for combustion heating, which constitutes an oil or gas crucible furnace. When using electric heating, resistive heating elements (resistive wires or silicon carbide rods) are arranged around the crucible, forming a resistive crucible furnace. Fuel-operated crucible furnaces generally heat up quickly, but their temperature control is not very strict. Resistive crucible furnaces have a slower heating rate; electric wires can reach up to 900°C, while silicon carbide rods can reach up to 1200°C, which is slightly lower than fuel furnaces. Additionally, they are more expensive to set up, consume more electricity, and have higher smelting costs. However, they offer better production environments and working conditions, and the melting temperature can be precisely controlled, making them suitable for melting aluminum and magnesium alloys. The external heat source first heats the crucible, which then transfers heat to the metal charge or melt inside the crucible. According to this heat transfer characteristic, crucible furnaces are externally heated melting furnaces designed to improve thermal efficiency. Crucibles are made in dimensions with a smaller diameter relative to height to increase the contact area between the metal and the crucible walls. This reduces the contact area of the melted liquid metal with the external atmosphere, minimizing metal oxidation and gas absorption, which is beneficial for the metal. In the smelting of aluminum alloys, two types of crucibles are commonly used: graphite crucibles with high strength and refractoriness, and cast iron crucibles.

1.Graphite Crucible:

Graphite crucibles are produced and supplied by specialized refractory material manufacturers. There are many specifications for the size and capacity of the crucibles, with the crucible number indicating the kilograms of copper alloy it can melt. For example, a No. 50 crucible can melt 50 kilograms of copper. When melting aluminum, its capacity should be divided by a coefficient of 0.4. Graphite crucibles can be used multiple times but generally have a short lifespan. Additionally, as their usage time increases, the thermal conductivity of the crucibles decreases, affecting the thermal efficiency and production efficiency.

2. Cast Iron Crucible Furnace:

Due to the low melting temperature of aluminum alloys, typically between 700-800°C, metal crucibles are widely used, with cast iron crucibles being the most common. Ordinary cast iron crucibles are favored for their low cost, high strength, and good thermal conductivity, making them widely adopted in production. However, they have a short lifespan and require frequent replacement during production. To extend the lifespan of cast iron crucibles, heat-resistant cast iron or steel containing nickel, chromium, or aluminum can be used. The capacity of cast iron crucibles used for melting aluminum alloys is generally between 30-250 kilograms, usually not exceeding 300 kilograms, though large capacities can reach over 500 kilograms. To prevent iron from the crucible contaminating the molten aluminum and to protect the crucible itself, a protective coating must be sprayed on the inner walls of the crucible before use. Information about suitable coatings can be found in relevant materials. Large crucible furnaces are mostly fixed; after the melting process is complete, the molten metal can be poured out using a ladle or, for large castings, the entire crucible can be lifted out for pouring. Many medium and small-sized electric resistance crucible furnaces come with tilting mechanisms to pour out the solution.

Currently, crucible furnaces are evolving towards larger sizes and more mechanized control systems. Tiltable large crucible furnaces can now pour out solutions.

1.2.2 Reverberatory Furnace:

Melting equipment with a bath-type furnace chamber is known as a reverberatory furnace. The original reverberatory furnaces were coal-fired and featured a combustion chamber, with flames being reflected into the melting chamber through an arched furnace roof. As secondary aluminum technology has developed, many modern reverberatory furnaces no longer use coal as fuel but instead employ oil or gas, thus the concept of the traditional reverberatory furnace has been diluted, and they are generally referred to as flame-type melting furnaces today. Fuel-heated reverberatory furnaces mainly consist of a furnace bottom, walls, and roof forming the melting chamber. This creates a shallow but wide molten pool to hold metallic charge materials and melted metal. The front of the furnace wall has doors for adding materials and for operations. A standard melting furnace is equipped with a flue window to effectively improve the operating environment, save energy, and facilitate smoke pollution control. However, in practice, many industrial furnaces lack chimneys; some are open, while others have smoke collection hoods at the furnace door. In coal-fired furnaces, high-temperature furnace gas from the combustion chamber rushes into the melting chamber through side windows, whereas in oil or gas-fired furnaces, the flame is directly injected into the furnace, heating the roof and walls while also heating the charge material. Metal charge materials are heated and melted by the radiation from the high-temperature furnace gas and the heated furnace roof and walls. Due to different fuel usage, reverberatory furnaces have significant structural differences. Because of their large furnace capacity, which can reach several dozen tons, current furnaces for melting aluminum alloys can be over 50 tons. Thus, they are suitable for melting various types of charge materials and are very suitable for secondary aluminum enterprises with large production volumes. Currently, reverberatory furnaces are the main equipment for melting aluminum alloys. Reverberatory furnaces come in rectangular and circular shapes, but most adopt a rectangular design because this type of furnace is easier and cheaper to construct. Circular reverberatory furnaces are more expensive and less convenient to maintain, but they have higher thermal efficiency because the surface area of a circle is greater for the same circumference, hence providing a larger heat absorption area and higher thermal efficiency for furnaces with the same circumference. In production, reverberatory furnaces have high thermal efficiency due to direct metal heating, shallow charge materials, and molten liquid, resulting in fast temperature rise and high productivity. Additionally, it is relatively easy to remove impurities from within the furnace. However, since metal comes into direct contact with combustion gases, there is significant oxidation and gas absorption, leading to more impurities and affecting the quality of the melt. Furthermore, because the flame comes into direct contact with the charge material, there is greater aluminum burnout compared to crucible furnaces, resulting in a lower recovery rate. Reverberatory furnaces can also use electric resistance heating, known as resistance reverberatory furnaces, where resistance wires (strips) or silicon carbide rods are suspended from the furnace roof, relying on high-temperature electric heating elements and roof radiation to transfer heat to the metal on the furnace floor. They are suitable for melting low-melting-point aluminum alloys. Resistance reverberatory furnaces offer better working conditions and produce higher quality aluminum alloy melts, but their significant drawback is high electricity consumption. Reverberatory furnaces are widely used in the secondary aluminum industry and have given rise to many furnace types.                                                                 

(1) Dual-chamber Reverberatory Furnace

Dual-chamber reverberatory furnace: The dual-chamber reverberatory furnace is a specialized piece of equipment for melting recycled aluminum alloys. It is widely adopted by some recycled aluminum enterprises in Europe and America due to its low energy consumption, low burnout rate, and high metal recovery rate. However, due to technical barriers between countries, the dual-chamber reverberatory furnace is rarely used in China. As the name implies, the dual-chamber reverberatory furnace consists of two melting chambers, with various forms available, but generally comprising an inner and an outer chamber. There is a specially designed passage between the two chambers for the circulation of aluminum liquid. The outer chamber of the dual-chamber reverberatory furnace primarily serves the purpose of melting scrap aluminum, while the inner chamber is used for smelting. In practice, scrap aluminum is directly added to the molten aluminum in the outer chamber and quickly submerged by the overheated molten aluminum. Since the scrap avoids direct contact with the flame, the burnout of scrap aluminum is very low, significantly improving the recovery rate of aluminum. The volume of the inner chamber is larger than that of the outer chamber; its main function is to heat the molten aluminum while smelting aluminum alloy. It is evident that the dual-chamber reverberatory furnace combines the advantages of crucible melting furnaces (where scrap does not come into contact with flames, reducing burnout) and reverberatory furnaces (which have a larger volume and higher thermal efficiency). Common dual-chamber reverberatory furnaces are equipped with a combustion system in the inner chamber, while the outer chamber lacks such a system. Scrap aluminum is added to the outer chamber, directly immersed in the overheated aluminum liquid, and melted. The temperature of the aluminum solution drops as it is melted, then enters the inner chamber through a circulation pump. The molten aluminum is heated in the inner chamber, then returned to the outer chamber under the action of the circulation pump to continue melting scrap aluminum, thus creating a continuous cycle.

During the melting process, a significant amount of dross is generated in the outer chamber. Due to the small volume and surface area of the outer chamber, compared to other melting furnaces, there is a noticeable reduction in the amount of additives (mainly flux) required, and it also facilitates the removal of dross, reducing labor intensity.

Ceramic or graphite circulation pumps are generally used. According to information provided, the consumption of additives in dual-chamber reverberatory furnaces is only half to one-third of that of other reverberatory furnaces, and the recovery rate can be improved by 2 to 5 percentage points. Energy consumption can also be reduced by 20-30%.

The advantages of the dual-chamber reverberatory furnace are more pronounced when dealing with scattered scrap aluminum and aluminum chips. However, the disadvantages are also apparent. When a certain amount of scrap aluminum is melted, reaching the design capacity of the furnace, it is necessary to stop adding materials to adjust the composition and refine, degas, etc. After standing still, casting is performed. If all the molten aluminum in the furnace is cast, then during the next melting cycle, a portion of the initially added scrap aluminum will still come into contact with the flame, leading to burnout issues. To avoid this problem, some companies reserve a portion of molten aluminum in the furnace during the final stage of casting, so that the next melting cycle can proceed without burnout. However, this reserved molten aluminum has already been refined, making it necessary to refine it again when mixed with new scrap aluminum. This not only wastes labor time but also increases energy consumption and the amount of additives required, reducing production efficiency and being economically unfavorable.

To address these issues, some companies build a separate holding furnace, where the dual-chamber reverberatory furnace only serves for melting and composition adjustment, while most refining processes are carried out in the holding furnace.

(2) Feed Well Type Aluminum Smelting Furnace:

This type of smelting furnace is also a dual-chamber reverberatory furnace, consisting of a feed well smelting furnace and a magnetic pump, forming a circulatory system as shown in the diagram. During production, aluminum scraps are continuously added to the feed well, melted by the overheated molten aluminum, and then enter the reverberatory furnace under the action of the magnetic pump. This process repeats to achieve the smelting objective. The advantages include low burnout, high metal recovery rate, suitability for handling fragmented scrap aluminum, and greater adaptability for processing aluminum chips. The form of the smelting furnace can be square.

(3) Reverberatory Furnace with Electromagnetic Stirring System:

In the process of melting recycled aluminum alloy in a reverberatory furnace, stirring is necessary to promote heat exchange, accelerate the melting speed of aluminum, increase reaction rates, and ensure the uniformity of the aluminum solution composition. Each stirring action disrupts the protective layer of aluminum oxide on the surface of the liquid, increasing aluminum burnout. For this reason, many entities are researching stirring technologies. Despite the emergence of mechanical rakes and other methods, none have proven entirely satisfactory. The electromagnetic stirring system is a technology developed by British companies and is suitable for various types of reverberatory and holding furnaces. The principle behind electromagnetic stirring involves installing induction coils beneath or at the side of the furnace, which generate a traveling wave magnetic field when electric current is applied. The stirring (movement) of the aluminum alloy solution within the melt pool relies on the interaction between the electromagnetic field and the conductive metal solution. This mechanism is similar to that of an electric motor, where the stator acts as the stirrer and the rotor corresponds to the melt pool. Electromagnetic stirring can significantly reduce burnout, lessen operational intensity, purify the environment, decrease the amount of slag produced, and result in a uniformly composed aluminum alloy solution. The cost of an electromagnetic stirring system is high, requiring substantial investment capacity from enterprises to implement.

(4) Drop-type Reverberatory Furnace: The drop-type reverberatory furnace, also known as the parent-child furnace, is a relatively suitable type of reverberatory furnace group, particularly effective for processing scrap aluminum with high iron content. The parent-child furnace consists of a melting furnace and a refining furnace, which are interconnected and have a certain height difference. The melting furnace serves only to melt the material; once the charge enters the melting furnace, it melts quickly, and then the molten aluminum flows into the refining furnace, leaving impurities like iron in the furnace to be manually removed. This reduces the contact time between iron and the molten aluminum solution, minimizing the incorporation of iron into the aluminum liquid. The molten aluminum entering the refining furnace undergoes further refining. Since there are no impurities such as iron present in the refining furnace, contamination of the molten aluminum by iron is avoided throughout the refining process, ensuring the quality of the aluminum alloy. The parent-child furnace is a highly recommendable furnace type, available in various sizes and at low investment costs, with strong applicability. Many enterprises in northern regions currently adopt this furnace type. When using the parent-child furnace, it is important to quickly drain the solution after the charge has melted to reduce the residence time of the aluminum solution in the furnace, thereby minimizing the dissolution of iron and other impurities into the molten aluminum.

(5)Rotary Reverberatory Furnace: The rotary reverberatory furnace comes in various forms, characterized by the ability to rotate 360 degrees during the production process. This feature enhances thermal efficiency and speeds up heat transfer, essentially eliminating the need for stirring operations. As the refractory material comes into uniform contact with the molten aluminum, the corrosion of the furnace walls is even (the most severe corrosion in conventional furnaces typically occurs above the liquid line), resulting in a longer furnace lifespan. Rotary aluminum melting furnaces must use either liquid or gaseous fuels.

1.2.3 Induction Furnace: This is a type of melting furnace that uses electromagnetic induction to heat metals. Induction furnaces can be categorized into industrial frequency furnaces (50—60 Hz), medium frequency furnaces (1—10 kHz), and high-frequency furnaces (200—300 kHz) based on the power frequency used. There are two types of induction furnaces in terms of construction: coreless induction furnaces and those with an iron core. The industrial frequency cored induction melting furnace acts like a transformer, delivering alternating current at industrial frequencies through the primary winding outside the iron core to the melting furnace. In the secondary winding connected to the molten pool, a large induced current is generated within the molten metal, thus heating it. Coreless induction melting furnaces are crucible-type melting furnaces where the primary winding, known as the inductor, is placed outside the crucible. The inductor is made of hollow copper tubes with water cooling inside. When the inductor is powered, the metal inside the crucible generates an induced current, producing heat.

In addition to the crucible, an industrial frequency induction furnace is also equipped with a magnetic yoke to enhance the electromagnetic efficiency of the furnace. Industrial frequency induction furnaces are inefficient when melting small pieces of metal and may even struggle to melt them, making them more suitable for melting large blocks of metal. Therefore, industrial frequency cored induction melting furnaces generally have larger capacities, reaching several tons or more. Due to electromagnetic effects, the liquid metal in the furnace can self-agitate, ensuring uniform composition and temperature distribution. These furnaces are used for melting copper alloys, aluminum alloys, and other low-melting-point alloys. When starting an industrial frequency cored induction furnace, the melting channel should be filled with metal to form a closed circuit; after each melting cycle, a certain amount of metal should remain to ensure the melting channel is filled, allowing the furnace to continue working. In the lower melting channel, it can sometimes become blocked by slag and impurities, affecting normal melting operations. Hence, there are plug holes on the side of the furnace to facilitate timely cleaning of the melting channel. When melting aluminum alloys, the melting channel can easily become blocked by aluminum oxide. Currently, cored induction furnaces are mostly used for melting copper alloys. Coreless industrial frequency induction furnaces do not have a "melting channel," reducing complications and simplifying the furnace structure compared to cored furnaces. However, after each pouring cycle, a residual amount of metal should also be left in the crucible to facilitate smooth continuation of work. Some coreless induction furnaces use cast iron crucibles in the middle, which improves electromagnetic efficiency and is particularly suitable for melting aluminum alloys. When a high-frequency alternating current passes through metal, it causes the "skin effect," where the induced current in the metal charge is not uniformly distributed but concentrated on the surface, with the current density decreasing towards the interior. The depth at which the current is concentrated is known as the "penetration depth." It can be calculated using the formula: δ=pμf, where: δ—current penetration depth in centimeters; p—the resistivity of the metal in ohm·cm; μ—the permeability of the metal; f—the current frequency in Hertz. From this formula, it is evident that the penetration depth is directly proportional to the square root of the resistivity of the metal and inversely proportional to the square root of the permeability and the frequency of the current. That is, for a given metal, a higher current frequency results in a smaller penetration depth. Consequently, a large current passing through a very thin layer of metal generates concentrated heat, which is beneficial for heating and melting the metal charge. Therefore, medium frequency induction furnaces have much higher electrical efficiency than industrial frequency furnaces and allow the use of smaller metal charges. Medium frequency is suitable for coreless induction furnaces, eliminating the need for magnetic yokes required by industrial frequency furnaces, and allows all molten metal to be poured out after each melting cycle without retaining any residue. They can be used for melting steel and aluminum with high efficiency and quality and better working conditions. However, medium frequency induction furnaces require specialized frequency conversion equipment for power supply, increasing the cost of their smelting products. High-frequency furnaces are generally not used for melting steel and aluminum alloys. This type of furnace, heated by internal induced currents in the metal charge, is an internally heated melting furnace and has much higher thermal efficiency than the previously mentioned crucible and reverberatory furnaces. The lining of an induction furnace is generally made of refractory materials or can use prefabricated crucibles. The melting process of aluminum alloys includes two stages: metal charge melting and liquid metal treatment. The melting stage is energy-intensive and time-consuming, so measures should be taken to expedite melting and reduce metal loss. The liquid metal treatment stage varies according to the characteristics of different melting alloys and the composition and quality of the charge, generally including steps such as refining, purification, alloy adjustment, degassing, and modification. In large-scale production, reverberatory furnaces are often used in a dual-furnace method, where metal is quickly melted in a high-capacity, efficient reverberatory furnace and then transferred to an electric resistance furnace or reverberatory furnace with strict temperature control for liquid metal treatment and holding before casting. This combined use of two furnaces leverages their respective strengths, achieving better economic and technical indicators. The specific furnace type and melting process method to be adopted should be determined based on the quality and output requirements of the alloy being melted.

1.3 Development of Smelting Furnaces

The development of smelting furnace technology has evolved alongside other industrial technologies, particularly electronics and new materials. In terms of furnace types, the previously mentioned dual-chamber reverberatory furnaces, charging well smelting furnaces, electromagnetic stirring reverberatory furnaces, rotary reverberatory furnaces, and tiltable heat-resistant crucible furnaces are all directions for development. Regarding heating methods, they primarily utilize high-energy beam sources such as lasers, electron beams, and ion beams. For melt protection, special structures of sealed containers are used to achieve vacuum or gas protection, preventing the melt from being contaminated by the environmental atmosphere. From the perspective of energy conservation, new insulating materials are used in the thermal design of the furnace body to fully improve energy utilization efficiency. Environmentally, purification treatment systems for furnace gases and slag have been added. Crucible materials are evolving from graphite to high-temperature-resistant alloy crucibles to extend the service life of crucible furnaces. In terms of stirring methods, both mechanical and electromagnetic stirring are rapidly developing, with electromagnetic stirring expected to be adopted soon.

1.4 Introduction to the Construction of Reverberatory Furnace Bodies

A reverberatory furnace, as the name suggests, heats the charge through reflected heat to melt and smelt the material. Traditional reverberatory furnaces are coal-fired and have a combustion chamber at one end. The flame rises and meets the arched furnace roof, which reflects it into the smelting chamber, achieving the purpose of smelting. Reverberatory furnaces are not only used in recycled aluminum enterprises but are also widely applied in the non-ferrous metal industry, such as copper metallurgy and lead metallurgy. With the development of metallurgical technology, reverberatory furnaces have evolved rapidly, especially with improvements in fuel, extensively adopting oil and gas. This has significantly improved reverberatory furnaces, reducing the reliance on reflection. Currently, furnace design focuses not merely on reflecting flames but emphasizes improving thermal efficiency and minimizing burnout.

1.5 Thermal Process of Reverberatory Furnace

1.5.1 Heat Transfer in Reverberatory Furnaces

Heat transfer is a complex physical phenomenon, generally divided into three modes: conduction, convection, and radiation. The body of a reverberatory furnace mainly consists of the roof, walls, and bottom, all of which are significant for heat transfer in the furnace. These three modes of heat transfer coexist within the reverberatory furnace, typically referred to as combined heat transfer. In actual production, flames radiate heat to both the scrap aluminum material and the four walls of the furnace, which then conduct (or radiate or reflect) heat to the aluminum material. At the same time, some heat is lost through the walls to the outside environment, which is a major factor in heat loss. Therefore, during furnace construction, it is important to focus on the insulation of the furnace walls. Convection occurs only when there is a temperature difference; if the temperature inside the furnace is uniform, there would be no convection. However, in reality, significant temperature differences exist within the furnace. Before the furnace charge melts, the air surrounding the charge is significantly cooler than the flames, resulting in convective heat transfer between the flames and the surrounding air. After the furnace charge melts, different parts of the molten liquid have significant temperature differences, so heat transfer within the molten aluminum primarily relies on convection. In the reverberatory furnace, the walls play an essential role in heat transfer, serving to: absorb part of the heat while radiating part of it to the furnace charge; directly reflect the flame’s heat to the furnace charge; and conduct heat through the wall to the furnace charge. Therefore, in the design and construction of reverberatory furnaces, it is crucial to consider the relationship among these three modes of heat transfer and to carefully consider the structure of the furnace walls and the materials used.

1.5.2 Fuel and Consumption of Reverberatory Furnaces

The primary fuels used in reverberatory furnaces include coal, gas, diesel, heavy oil, and natural gas. Regardless of the type of fuel, the form of heat transfer remains essentially the same; what differs are the thermal efficiency, melting rate, and fuel costs. Generally, the fuel consumption per ton of aluminum in a reverberatory furnace is as follows: coal-fired furnaces consume about 200-300 kilograms of standard coal; heavy oil-fired furnaces consume approximately 60-80 kilograms; diesel-fired furnaces consume around 50 kilograms, with some advanced ones achieving 30 kilograms; semi-gas reverberatory furnaces consume about 260-300 kilograms of coal. The consumption of fuel is related to the production capacity of the reverberatory furnace. Generally, the larger the furnace, the lower the unit fuel consumption. Below are several common examples of fuel consumption in reverberatory furnaces: (1) Technical parameters of a 15-ton heavy oil furnace in a certain enterprise; (2) Coal consumption of a semi-gas reverberatory furnace.

1.5.3 Thermal Efficiency of Reverberatory Furnaces

The thermal efficiency of reverberatory furnaces is generally not very high, especially for flame-type reverberatory furnaces. Under normal circumstances, the thermal efficiencies of several types of furnaces are as follows: The low thermal efficiency of reverberatory furnaces means that a large amount of heat is wasted. According to the melting characteristics of reverberatory furnaces, the distribution of heat is mainly as follows:

(1) Heat directly used for melting accounts for about 25-30%, and this portion of heat is primarily consumed for melting and smelting aluminum;

(2) Heat lost through the exterior walls and doors of the reverberatory furnace generally accounts for 15-25%, and sometimes it can exceed 30%;

(3) Heat carried away by flue gases and slag accounts for about 40-50%. During the smelting process of aluminum alloys, the amount of slag produced is relatively small and it is not in a molten state, so the heat carried away by slag is minimal, with the majority being carried away by flue gases. To improve the thermal efficiency of reverberatory furnaces, various methods should be employed to reduce heat loss, such as thickening the furnace walls and roof, adding insulation layers; minimizing the frequency of opening the furnace door; and when designing the furnace, efforts should be made to minimize the surface area of the furnace and increase its volume.

1.5.4 Waste Heat Recovery from Reverberatory Furnaces

To improve the utilization rate of heat, it is essential to consider waste heat recovery, particularly the utilization of heat carried away by flue gases. The primary method currently used for recovering waste heat from flue gases is to use the flue gases to preheat fuel (gas) and air, achieving good results. Installing a waste heat boiler is also an effective solution.

1.6 Furnace Construction Environment and Geological Conditions

1.6.1 Understanding the Site Conditions

To ensure longevity and safety in furnace construction, it is crucial to thoroughly understand the surrounding environment, site conditions, wind directions, etc., before commencing construction. For example, whether there are nearby rivers or lakes, or if it is a subsidence area. Additionally, it is important to be aware of the geological conditions at the construction site. When necessary, consult with the geological department for detailed information on the geology. It is also essential to understand the seasonal wind directions.

1.6.2 Foundation

To ensure the safety, reliability, and long-term use of the furnace body, a stable foundation is imperative; hence, constructing the foundation is very important. The foundation should be built on solid soil layers, considering the following principles: (1) Avoid building on loose soil or quicksand layers, and if unavoidable, take appropriate measures; (2) Pay special attention during foundation construction that it must be below the disturbed layer, built on undisturbed ground; (3) In the north, the foundation must be constructed below the frost line.

1.6.3 Arrangement of the Smelting Workshop and Furnace Body

The position of the furnace should consider the following issues: 1) Surrounding space and operational plane: Facilitate operation, leave space for adding materials and maintenance, ensure convenient transportation, and if not immediately possible, reserve space for the casting machine's location, allowing for long-term planning to achieve a reasonable layout. The smelting workshop should be built downwind (the enterprise should be located downwind of a certain region or residential area to minimize unnecessary trouble).

1.7 Form and Structure of the Furnace

1.7.1 Furnace Shape

Currently, the main types used are circular and rectangular reverberatory furnaces. Circular furnaces have higher costs and are inconvenient to maintain, but they offer higher thermal efficiency because the surface area of a circle is greater than that of a square with the same perimeter. Therefore, for furnaces with the same circumference, a circular furnace has a larger surface area, resulting in greater heat reception, higher thermal efficiency, and less heat dissipation from the furnace surface. Rectangular oil reverberatory furnaces are cost-effective and easier to maintain, though their thermal efficiency is slightly lower than that of circular furnaces. They may have doors on opposite sides or two doors on one side, with single-sided 2-3 nozzles or diagonally opposite 2 oil nozzles. Larger domestic recycled aluminum enterprises also use vertical oil reverberatory furnaces.

1.7.2 Choice of Furnace Type

Based on the manufacturer's needs, the choice of furnace shape should be made according to site environment, thermal materials, and process conditions. There are many types of furnaces for melting aluminum alloy, and various kinds of reverberatory furnaces are commonly used, including heavy oil reverberatory furnaces, diesel reverberatory furnaces, electric resistance reverberatory furnaces, gas reverberatory furnaces, semi-gas reverberatory furnaces, and coal reverberatory furnaces. Depending on the enterprise’s situation and local advantages such as fuel resources and transportation, the appropriate furnace type should be decided. However, it should be noted that heavy oil-fueled reverberatory furnaces are declining due to the high viscosity and low freezing point (30) of heavy oil, which makes transportation difficult. Heavy oil requires preheating before combustion to improve its fluidity and atomization, and further preheating (110-120) before entering the nozzle, increasing equipment investment. If the heavy oil is not well atomized during combustion, it produces a large amount of black smoke, polluting the environment and resulting in low thermal efficiency. Currently, it is recommended that large enterprises construct reverberatory furnaces with gas generators, which require a larger initial investment but offer long-term benefits such as no pollution, high thermal efficiency, and simple operation. Small enterprises are recommended to build semi-gas reverberatory furnaces.

1.7.3 Surface Area and Depth of the Melting Pool

The surface area and depth of the melting pool are important parameters of reverberatory furnaces because the heat in the furnace is mainly conducted through the liquid surface and four walls. Theoretically, the larger the surface area and wall area, the higher the thermal conduction efficiency. However, a larger surface area leads to more severe oxidation at the surface and can result in an unrestricted increase in the size of the melting pool, increasing energy consumption and investment. Conversely, if the melting pool is too deep, it affects thermal conduction. Therefore, it is necessary to consider various factors comprehensively, such as energy saving, investment saving, and maintenance convenience, to determine the length, width, and height of the melting pool. Generally, for furnaces under 15 tons, the depth of the melting pool is around 500 millimeters.

1.7.4 Structure of the Furnace Body

The dimensions of the reverberatory furnace structure are greatly related to the production scale of the furnace, making it difficult to obtain a uniform formula. Taking a 10-ton reverberatory furnace as an example: the calculation of the combustion space is based on the heat required to melt aluminum and the volume of gas combusted per unit time. To ensure complete combustion of the fuel, an excess air coefficient is generally considered in the calculations. If the space is too large, it affects the generation of thermal energy, reducing thermal efficiency; if too small, it can lead to fire spraying outward, shortening the residence time of flue gas in the furnace, causing energy waste. Therefore, in addition to theoretical calculations, practical exploration is also needed.