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[Technology] Classification and selection of furnaces for smelting non-ferrous alloys (1)
Metal smelting is a crucial step in the casting process, as it directly determines the quality of the casting material. This process is also closely tied to various factors such as the quality, production cost, output, energy consumption, and environmental impact of the final castings. Smelting furnaces are essential equipment in foundries, and selecting the right type can significantly improve metallurgical quality, reduce energy usage, lower costs, and increase productivity.
First, let's look at the classification of melting furnaces. Non-ferrous alloy melting furnaces can be broadly divided into two categories: fuel-fired furnaces and electric furnaces. Fuel-fired furnaces use coal, coke, natural gas, fuel oil, or other fuels. They include types like cupola furnaces and reverberatory furnaces. Electric furnaces, on the other hand, convert electrical energy into heat and are categorized based on their heating method. These include resistance furnaces, induction furnaces, and arc furnaces. Resistance furnaces can further be divided into muffle-type, reflector-type, and box-type furnaces. Induction furnaces can be classified by frequency—industrial, intermediate, or high-frequency—and whether they operate with intentional or unintentional currents. Arc furnaces are typically divided into non-consumable and consumable types.
Each type of furnace is suitable for different alloys. For example, resistance furnaces are often used for small-scale non-ferrous metal melting. The increased eddy current resistance furnace, also known as a muffle furnace, uses electric heating elements made of materials like iron-chromium alloys or silicon carbide. These furnaces generally have a capacity ranging from 30 to 400 kg. The design includes a fixed or rotating structure, with fixed types being more common due to the risk of damage from movement. Small and medium-sized castings can be poured directly from the crucible using a ladle, while larger ones may require lifting the furnace for pouring.
This type of furnace is compact and cost-effective, making it ideal for small-scale operations. However, it has a major drawback: long melting times. For instance, melting 150–200 kg of aluminum might take 5–5.5 hours, leading to high power consumption and low efficiency. As technology advances, larger models of these furnaces are gradually being replaced by industrial frequency induction furnaces.
Another common type is the reflector resistance furnace, which uses heating elements located at the top of the furnace. The molten metal is heated from above through radiation. To protect the heating elements from splashing and erosion, they are usually placed in grooves on the roof. The furnace chamber consists of a bath and one or two sloped front chambers where raw materials are placed. As the melting progresses, the melt flows into the bath, while oxides remain in the front chamber. These furnaces come in fixed or tilting designs. Fixed models pour metal through an overflow port or siphon device, while tilting models allow for easier pouring.
Reflector resistance furnaces offer advantages such as stable furnace gases, reduced oxidation, clean melts, and large capacities, making them suitable for mass production. However, they cannot use fluxes or perform refining inside the furnace. One of their main disadvantages is long melting times and high energy consumption. For example, melting aluminum alloys can require over 600 kWh per batch, and the heating elements have a short lifespan. To extend their life, silicon-platinum rods can be used instead of resistance wires. These rods are installed in holes on the side walls, and their performance degrades over time, requiring adjustments to the voltage or wiring configuration (e.g., from star to delta). Silicon-platinum rods last longer and are easier to replace, reducing maintenance and improving efficiency.
In summary, while traditional resistance and reverberatory furnaces still have their place, modern trends favor more efficient and versatile technologies like induction and arc furnaces. Choosing the right furnace depends on the specific needs of the foundry, including the type of alloy, production scale, and efficiency requirements.
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