In addition to saving a large amount of resource consumption, the re-melting and processing of废铝 (waste aluminum) can also reduce the social accumulation of recyclable non-ferrous metals. Developing recycled aluminum resources and implementing a circular economy for废旧铝 (waste aluminum) not only provides economic benefits such as saving money for the country and enterprises, improving resource utilization, saving energy, and protecting the ecological environment, but also helps reduce many social issues. The recycling of aluminum metal can accelerate the comprehensive, coordinated, and sustainable development of China's aluminum industry.
The waste aluminum smelting equipment includes smelting furnaces, settling furnaces, fans, combustion systems, etc. Among them, for recycled aluminum smelting and settling furnaces (small enterprises do not have settling furnaces), smelting furnaces and settling furnaces can therefore be regarded as one.
There are many types of melting furnaces, and their functions are the same. First, they melt the charge and additives, which include scrap aluminum, intermediate alloys, industrial silicon, and pure aluminum ingots; second, they enable a series of chemical and physical reactions between the molten materials at a certain temperature to remove impurities as slag or gas; third, they adjust the composition to ensure that the various elemental contents in the alloys meet relevant standard requirements; fourth, they process the molten alloy for modifications, refining the fine particles to ensure the alloy meets relevant physical performance standards. Therefore, the form and structure of the aluminum melting furnace play an important role in the production capacity, cost, product quality, and environmental protection of recycled aluminum alloys.
1. Main smelting equipment
The melting points of commonly used aluminum alloys are not high, and the forms of melting furnaces are basically two types: crucible type and melting bath type.
2. Crucible Furnace
The crucible furnace is a commonly used equipment for melting and recycling aluminum alloys. Its advantages include low investment, convenience of operation, and high metal recovery rate, but its disadvantages are small production capacity, short lifespan, and unstable composition, making it difficult to compare with large reflector furnaces. There are various forms of crucible furnaces, with cast iron crucibles and graphite crucibles being the most commonly used.
When using a crucible furnace, the furnace body is fixed on a potting table made of refractory materials, with the lower part and the sides of the crucible furnace being the combustion chamber. When using larger crucible furnaces, considering the self-weight of the crucible furnace, the bottom of the furnace body should not be suspended and should rest on stable refractory materials. This is especially important for large cast iron crucible furnaces, as high temperatures can cause deformation of the furnace body, affecting its lifespan.
The crucible furnace has strong fuel adaptability, capable of using coal, coke, gas, etc., providing a wide selection of fuel. When using fuel oil or gas, there are nozzles under the crucible that inject fuel and air for combustion heating, hence it is called a fuel oil crucible furnace or gas furnace. When using electric heating, the resistance heating elements (resistance wire or silicon carbide rods) are arranged around the crucible, thus it is termed a resistance crucible furnace. Crucible furnaces that use fuel generally heat up quickly, but their temperature control is not very strict. Resistance crucible furnaces heat up more slowly; the electric heating wire can reach 900 degrees, and the silicon carbide rod can reach 1200 degrees, which is lower than the temperatures of fuel furnaces. However, their equipment costs are high, they consume a lot of electricity, and the melting cost is also high. But they offer better production environments and working conditions, and the melting temperature can be accurately controlled, making them suitable for melting aluminum and magnesium alloys.
The external heat source first heats the crucible. Once the crucible is heated, it then transfers heat to the metal charge or molten liquid inside the crucible. Based on this heat transfer characteristic, the crucible furnace is an externally heated melting furnace designed to improve thermal efficiency. The crucibles are made in a small dimension with a greater height-to-diameter ratio to increase the contact area between the metal and the walls of the crucible. This way, the contact area between the molten metal and the external atmosphere is relatively small, which can reduce the oxidation and gas absorption of the metal, benefiting the metal.
There are two commonly used types of crucibles for melting aluminum alloys: one is a graphite crucible with high strength and refractoriness, and the other is a cast iron crucible.
(1) Graphite Crucibles: Graphite crucibles are produced and supplied by specialized refractory material manufacturers. There are many specifications for the size and capacity of the crucibles. The number of the crucible indicates the kilograms of melted copper alloy, for example, a No. 50 crucible can melt 50 kilograms of copper. When melting aluminum, the capacity should be divided by a coefficient of 0.4. Graphite crucibles can be reused multiple times, but generally have a shorter lifespan, and as the usage time increases, the thermal conductivity of the crucible decreases, which affects thermal efficiency and production efficiency.
(2) Cast Iron Crucible Furnace: Due to the melting temperature of aluminum alloy being relatively low, typically between 700-800℃, metal crucibles are widely used, with cast iron crucible furnaces being common. Ordinary cast iron crucibles are low in price, high in strength, and have good thermal conductivity, making them widely adopted in production; however, they have a short lifespan, leading to frequent replacement during production. To extend the lifespan of cast iron crucibles, crucibles made of heat-resistant cast iron or heat-resistant steel containing nickel, chromium, or aluminum can also be used. The capacity of cast iron crucibles for melting aluminum alloys generally ranges from 30 to 250 kilograms, typically not exceeding 300 kilograms, with larger capacities reaching over 500 kilograms.
(3) To prevent the iron from infiltrating the aluminum melt during the melting process and to protect the crucible, a protective coating must be sprayed on the inner wall of the crucible before use. The coating conditions of the crucible furnace can be found in relevant materials. Large crucible furnaces are often fixed; after the melting process is completed, the molten liquid can be taken out with a ladle for pouring, and for large castings, the crucible can also be lifted out for pouring. Many medium and small resistance crucible furnaces are equipped with tilting mechanisms that can tilt the solution out of the crucible. Currently, crucible furnaces are developing toward larger sizes and mechanization of control systems; large tilting crucible furnaces can pour out the molten liquid by tilting the furnace body.
