The aluminum liquid sucked from the electrolytic cell contains various impurities, so it needs to be purified before casting. In the industry, aluminum liquid purification methods such as clarification, flux, and gas are mainly used, and some try directional solidification and filtration methods for purification.
Aluminum Liquid Purification Methods
Flux purification uses the fluxes added to the molten aluminum to form a large number of fine droplets, so that the oxides in the molten aluminum are wetted, absorbed, and dissolved by these droplets, forming new droplets that rise to the surface, and form scum to be removed after cooling.
The flux used for purification is composed of salt with a low melting point, low density, low surface tension, high activity, and strong adsorption capacity for oxidized slag. When using, first put a small piece of flux into the iron cage, then insert it into the bottom of the mixing furnace and stir it back and forth. After the flux is finished, take out the iron cage and let it stand for 5-10 minutes. Remove the scum on the surface to be cast. If necessary, the flux can also be removed on the surface to cover it.
Gas purification is the main primary aluminum purification method. The gas used is chlorine, nitrogen, or a mixture of chlorine and nitrogen.
(1) Purification of chlorine gas. In the past, the active gas chlorine was used as the purifying agent (chlorination method). In the chlorination method, when chlorine gas is passed into the molten aluminum, a lot of abnormally fine AlCl3 and bubbles are generated, which are fully mixed in the molten aluminum. The hydrogen dissolved in the molten aluminum and some mechanical inclusions are adsorbed on the AlCl3 bubbles and are discharged as the AlCl3 bubbles rise to the surface of the molten aluminum. When chlorine gas is introduced, some elements that are more negative than aluminum can be chlorinated, such as calcium, sodium, magnesium, etc., which are separated into corresponding chlorides due to the introduction of chlorine gas. Therefore, the chlorination method is a very effective primary aluminum purification method. The amount of chlorine gas is 500-700g per ton of aluminum. However, because oxygen is poisonous and expensive, in order to avoid air pollution and reduce the cost of aluminum ingot production, the modern aluminum industry has gradually abolished the chlorination method and replaced it with an inert gas-nitrogen purification method.
(2) Nitrogen purification method. Also known as the smokeless continuous purification method, alumina balls (418mm) are used as the filter medium. N2 is directly passed into the molten aluminum. The molten aluminum is continuously fed into the purification furnace, passes through the alumina ball filter layer, and is flushed by nitrogen, so the non-metallic inclusions and dissolved hydrogen in the molten aluminum are removed, and then continuously discharged, so that the fine nitrogen bubbles are evenly distributed Play a purifying effect in the treated aluminum liquid. Nitrogen has no pollution to the atmosphere, and the purification capacity is large. It can process 200~600kg aluminum liquid per minute, and the aluminum loss caused by the purification process is relatively reduced, so it is now widely used. But it is not as capable of removing calcium, sodium, and magnesium in molten aluminum as chlorine gas.
(3) Mixed gas purification method. A mixture of chlorine and nitrogen is used to purify molten aluminum. Its role is to remove hydrogen and separate oxides on the one hand, and to remove some metal impurities (such as magnesium) in aluminum on the other. The commonly used composition is 90% nitrogen + 10% chlorine. . There are also 10% chlorine + 10% carbon dioxide + 80% nitrogen. This effect is better, carbon dioxide can diffuse chlorine and nitrogen well, which can shorten the operation time.