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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 is one of the most commonly used techniques. Once the alloy is melted, it is slowly stirred using a bell jar that contains either hexachloroethane with a concentration of 0.1% to 0.2%, or a solution with 3% to 4% chlorine. This process helps remove impurities effectively.
In addition, fine ceramic filters can also be employed to purify metal solutions. Using a filter with an average particle size ranging from 2 to 35 micrometers and a layer thickness of about 105 micrometers, it is possible to eliminate up to 90% of oxides and 85% of metal compounds in ZA4-1 alloy. The filter is typically placed inside a bell jar, heated to 500°C, and then transferred into a holding furnace or ladle. Rare earth elements are known for their active chemical properties, which allow them to form low-density, high-melting-point compounds with impurities like oxygen, nitrogen, and hydrogen. These compounds are easily separated from the molten metal along with the slag, improving the purity of the final product.
Refining agents play a crucial role in casting furnaces by refining, protecting, and removing impurities from molten metals, leading to higher-quality 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 favored for its moisture resistance, ease of storage, and low corrosiveness. It can be used as an alternative to chlorine-based refining agents across various casting processes. Zinc oxide flux, on the other hand, is primarily used for deslagging in zinc alloys. Before use, it needs to be remelted and stored in a drying oven at temperatures between 100°C and 130°C. However, this method produces a significant amount of harmful gases, leading to poor environmental conditions. Its effectiveness in removing slag is limited, often resulting in residual impurities and bubbles in the alloy, increasing waste during die casting.
Rare earth elements are highly reactive and can form stable compounds with impurities such as oxygen, nitrogen, and hydrogen. These compounds are typically removed along with the slag, contributing to better purification. Inert gases like nitrogen are also used as refining agents due to their low cost and non-polluting nature, making them suitable for a wide range of alloys.
The use of degassing and slag removal agents has proven effective in improving the quality of alloy melts. According to some reports, applying a chlorinated bell jar at temperatures between 450°C and 470°C can remove approximately 80% of oxides and 70% of intermetallic compounds from the zinc alloy. However, experimental results have shown that adding 0.12% of chlorine to the refined material can increase melting losses and produce excessive smoke and ammonia, degrading the working environment. In many cases, achieving a dense, clean alloy structure without slag or inclusions can be done through simple settling and fining. Therefore, the decision to perform slag removal operations and the amount of chlorine required should be based on the quantity of recycled materials used in production.