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Zinc alloys can be purified through several methods, including settling clarification, chlorine salt treatment, inert gas blowing, and filtration. Among these, the chlorine salt treatment method is one of the most commonly used. Once the alloy is melted, it is gradually stirred using a bell jar that contains either hexachloroethane at a concentration of 0.1% to 0.2%, or a solution with a 3% to 4% concentration of chlorinated material. This process helps remove impurities effectively.
In addition to chemical treatments, fine ceramic filters can also be employed to purify metal solutions. Filters with an average particle size ranging from 2 to 35 micrometers and a layer thickness of 105 micrometers can efficiently eliminate oxides (up to 90% by mass) and metal compounds (up to 85% by mass) in ZA4-1 alloys. These filters are typically placed inside a bell jar, heated to around 500°C, and then introduced into a holding furnace or ladle for use. Rare earth elements are particularly useful in this context due to their high reactivity. They can form low-density, high-melting-point compounds with impurities like oxygen, nitrogen, and hydrogen, which are easily separated from the molten metal along with slag.
Refining agents play a crucial role in casting processes by refining, protecting, and removing slag from the molten metal, ultimately leading to high-purity products. Commonly used refining agents for zinc alloys include hexachloroethane, zinc oxide, rare earth elements, and noble gases. To prevent contamination, especially from iron, all tools must be coated before the refining process. The coating typically consists of zinc oxide, fossil powder, water glass, and water.
Hexachloroethane is a popular choice because it is moisture-resistant, does not require remelting, and is less corrosive, making it easy to store and suitable for various casting applications. On the other hand, zinc oxide flux is often used for deslagging in zinc alloys. It needs to be remelted and dried at temperatures between 100°C and 130°C before use. However, its application has some drawbacks, such as the release of harmful gases, poor slag removal efficiency, and the potential for introducing bubbles and impurities into the alloy.
Rare earth elements are known for their reactive nature, allowing them to form stable compounds with impurities like oxygen, nitrogen, and hydrogen. These compounds are easily removed during the slag separation process, enhancing the overall purity of the alloy. Inert gases, such as nitrogen, are also used as refining agents. They are cost-effective, non-polluting, and applicable across a wide range of alloys.
The effectiveness of degassing and slag removal agents is well-documented. For instance, using a chlorination bell jar at temperatures between 450°C and 470°C can remove up to 80% of oxides and 70% of intermetallic compounds in zinc alloys, significantly improving quality. However, experimental results show that adding 0.12% of chlorinated material can increase melting losses and produce excessive smoke and ammonia, negatively affecting the working environment. In many cases, a dense, clean alloy ingot without slag or inclusions can be achieved through simple settling and refining. Therefore, the decision to perform slag removal operations and the amount of chlorine needed depends on the quantity of recycled materials used.