Thread Content
The melting equipment for recycled aluminum is the same as that for 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 and burnout, as well as 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 extremely 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; therefore, they represent a technology 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 furnace types 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, with this efficiency being more than 2/3 higher than that of radiant systems. Secondly, the metal burnout is significantly reduced due to the lower furnace temperature and the strong convective flow of hot furnace gases. 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 a static 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 burner’s combustion speed 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 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 loaded 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 furnace charge is subjected to impulsive heating, causing it to melt rapidly; the molten aluminum then flows along the inclined bottom of the furnace toward 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 settle. 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 gases, and substances such as paint can also be burned and removed before melting. Therefore, the quality of molten aluminum can also be improved. Japanese engineer Oku Kunio proposed a structure for an open-pit reverberatory 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 heating 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 accelerates 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 holds certain reference value for central enterprises that lack 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 wall, 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.