Heating equipment required for scrap aluminum smelting

In addition to saving a large amount of resource consumption, scrap aluminum smelting can also reduce the social accumulation of recyclable non-ferrous metals by re-melting and processing waste aluminum products. Developing recycled aluminum resources and implementing a circular economy for waste aluminum not only brings economic benefits such as saving funds for the country and enterprises, improving resource utilization, conserving energy, and protecting the ecological environment, but also helps reduce many social problems. The recycling of aluminum metal can accelerate the comprehensive, coordinated, and sustainable development of the aluminum industry.

Scrap aluminum smelting equipment includes smelting furnaces, holding furnaces, fans, and combustion systems. Among them, for recycled aluminum smelting and holding furnaces, the smelting furnace and holding furnace can thus be considered as one unit.

There are many types of melting furnaces, but their functions are essentially the same. First, they melt the charge and additives, with the charge including scrap aluminum, intermediate alloys, industrial silicon, and pure aluminum ingots. Second, they allow a series of chemical and physical reactions to occur between the molten materials at a certain temperature in the furnace, removing impurities as slag or gas. Third, they adjust the composition to ensure that the content of various elements in the alloy meets relevant standards. Fourth, they perform treatments such as modification on the molten alloy to refine the grains, enabling the alloy to meet the required physical properties. Therefore, the form and structure of an aluminum melting furnace play a crucial role in the production capacity, cost, product quality, and environmental protection of recycled aluminum alloys.

1. Main Melting Equipment

The melting points of commonly used aluminum alloys are not very high, and there are basically two types of melting furnaces: crucible type and pot type.

2. Crucible Furnace

A crucible furnace is commonly used for melting recycled aluminum alloys. Its advantages are low investment, easy operation, and high metal recovery rate. However, its disadvantages include low production capacity, short lifespan, and unstable composition, making it difficult to compare with large reverberatory furnaces. Crucible furnaces come in various forms, with cast iron crucibles and graphite crucibles being commonly used.

When using a crucible furnace, the furnace body is fixed on a hearth built with refractory materials, and the lower part and the surrounding area of the crucible furnace serve as the combustion chamber. When using a larger crucible furnace, due to considerations of the furnace's own weight, the bottom of the furnace should not be suspended and should rest on stable refractory materials. This is especially true for large cast iron crucible furnaces, where high temperatures can deform the furnace body and affect its lifespan.

Crucible furnaces have strong fuel adaptability and can use coal, coke, gas, etc., providing a wide choice of fuels. When using oil or gas as fuel, there is a nozzle below the crucible to inject fuel and air for combustion heating; this is called an oil-fired or gas crucible furnace. When using electric heating, the resistance heating elements (resistance wire or silicon carbide rods) are arranged around the crucible, forming a resistance crucible furnace. Crucible furnaces using fuel generally heat and reach temperature quickly, but their temperature control is not very precise. Resistance crucible furnaces heat up more slowly; when using resistance wire, they can reach 900°C, and with silicon carbide rods, up to 1200°C, which is slightly lower than fuel-fired furnaces. At the same time, their equipment cost is higher, electricity consumption is greater, and smelting costs are higher. However, they provide a better working environment and labor conditions, and the melting temperature can be precisely controlled, making them suitable for melting aluminum and magnesium alloys.

An external heat source first heats the crucible, and once the crucible is heated, it transfers heat to the metal charge or molten metal inside. Based on this heat transfer characteristic, the crucible furnace is an externally heated melting furnace. To improve thermal efficiency, crucibles are made with a relatively small diameter-to-height ratio to increase the contact area between the metal and the crucible walls. 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 refractory properties, 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 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 overall their lifespan is relatively short. Moreover, as usage time increases, the crucible's thermal conductivity decreases, affecting thermal efficiency and production efficiency.

(2) Cast Iron Crucible Furnace: Because aluminum alloys melt at a relatively low temperature, usually between 700–800°C, metal crucibles are widely used, with cast iron crucible furnaces being common. Ordinary cast iron crucibles are low in cost, have high strength, and good thermal conductivity, so they are widely used in production, but they have a short lifespan and need to be frequently replaced. 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 kilograms, usually not exceeding 300 kilograms, with larger capacities reaching over 500 kilograms.

(3) To prevent iron from 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 to the inner wall of the crucible before use. The coating details for crucible furnaces can be found in the relevant materials. Large crucible furnaces are mostly stationary. After the melting process is completed, the molten metal can be scooped from the crucible with a ladle for pouring, or for large castings, the crucible can be hoisted out for pouring. Many medium and small-sized electric resistance crucible furnaces are equipped with tilting mechanisms, allowing the molten solution to be poured out by tilting the crucible. Currently, crucible furnaces are developing towards larger sizes and mechanical control systems. For large tiltable crucible furnaces, the molten metal can be poured simply by tilting the furnace body.