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