Heating equipment required for aluminum scrap smelting

There are many types of melting furnaces, but their functions are the same: first, to melt the charge and additives, where the charge includes scrap aluminum, intermediate alloys, industrial silicon, and pure aluminum ingots, etc.; second, to allow a series of chemical and physical reactions to occur between the molten materials at a certain temperature in the furnace, so that impurities can form slag or gas and be removed; third, to adjust the composition so that the content of various elements in the alloy meets the relevant standards; fourth, to treat the molten alloy to alter its properties and refine the grain, so that the alloy can meet the relevant physical performance requirements. Therefore, the form and structure of an aluminum melting furnace play an important role in the production capacity, cost, product quality, and environmental protection of recycled aluminum alloy.

1. 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 molten pool type.

2. Crucible Furnace

The crucible furnace is commonly used for melting recycled aluminum alloys. Its advantages are low investment, convenient 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 reverberatory furnaces. There are various types of crucible furnaces, with common ones being cast iron crucibles and graphite crucibles.

When using a crucible furnace, the furnace body is fixed on a hearth built with refractory materials, and the lower part and the sides of the crucible furnace form the combustion chamber. When using a larger crucible furnace, considering the weight of the crucible furnace, the bottom of the furnace should not be suspended but should rest on stable refractory materials. This is especially important for large cast iron crucible furnaces, as high temperatures can cause the furnace body to deform and affect its service life.

Crucible furnaces have strong fuel adaptability and can use coal, coke, gas, etc., providing a wide range of options for fuel selection. When using oil or gas as fuel, there are nozzles under the crucible that inject fuel and air to burn and heat, which is called an oil crucible furnace or a gas furnace. When using electric heating, resistance heating elements are arranged around the crucible, which is called a resistance crucible furnace. Crucible furnaces using fuel generally heat up quickly, but their temperature control is not very precise. Resistance crucible furnaces heat up more slowly; with electric wires, they can reach 900°C, and with silicon carbide rods, they can reach 1200°C, slightly lower than fuel furnaces. At the same time, their equipment cost is higher, electricity consumption is large, and smelting cost is high. However, they provide a better production environment and working conditions, and their melting temperature can be accurately controlled, making them suitable for the smelting of aluminum and magnesium alloys.

The external heat source first heats the crucible, and once the crucible is heated, it then transfers heat to the metal charge or molten metal inside the crucible. According to this heat transfer characteristic, a crucible furnace is an externally heated melting furnace. To improve thermal efficiency, crucibles are made in a form with a small diameter and relatively tall height to increase the contact area between the metal and the crucible wall. In this way, the contact area between the molten metal and the external atmosphere is relatively small, which can reduce metal oxidation and gas absorption, benefiting the metal.

When smelting aluminum alloys, there are two commonly used types of crucibles: one is a graphite crucible with high strength and refractoriness, and the other is a cast iron crucible.

(1) Graphite Crucible: Graphite crucibles are produced and supplied by specialized refractory material manufacturers. There are many specifications for crucible sizes and capacities. The number of the crucible corresponds to the weight of the 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 factor of 0.4. Graphite crucibles can be used multiple times, but generally speaking, their lifespan is relatively short. Additionally, as usage time increases, the crucible's thermal conductivity decreases, affecting heat efficiency and production efficiency.

(2) Cast Iron Crucible Furnace: Because the melting temperature of aluminum alloys is relatively low, mostly between 700–800℃, metal crucibles are widely used, the most common being cast iron crucible furnaces. Ordinary cast iron crucibles are inexpensive, have high strength, and good thermal conductivity, making them widely used in production, but they have a short lifespan and need to be frequently replaced during production. To extend the life 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 is generally 30–250 kg, usually not exceeding 300 kg, while large-capacity crucibles can reach over 500 kg. 

(3) To prevent iron from the crucible from penetrating into the molten aluminum during melting, and to protect the crucible, a protective coating must be applied to the inner wall of the crucible before use. The coating specifications for crucible furnaces can be found in related materials. Large crucible furnaces are mostly fixed types. After the melting process is completed, the molten metal can be poured using a ladle from the crucible; for large castings, the crucible can also be lifted out for pouring. Many medium- and small-sized resistance crucible furnaces are equipped with a tilting mechanism, allowing the molten solution to be poured out. Currently, crucible furnaces are developing toward larger sizes and mechanized control systems. Large tilting crucible furnaces can pour out the molten metal by simply tilting the furnace body.