The aluminum liquid sucked out of the electrolytic cell contains various impurities, so it needs to be purified before casting. In industry, clarification, flux, gas and other purification methods are mainly used, and some trial directional solidification and filtration methods for purification.

1. Flux purification flux purification is to use the flux added to the aluminum liquid to form a large number of fine droplets, so that the oxide in the aluminum liquid is wet adsorption and dissolution of these droplets, forming a new droplet rise to the surface, the scum formed after cooling is removed. The purifying flux is composed of salt with low melting point, low density, low surface tension, high activity and strong adsorption ability to oxide slag. When in use, the small flux is first put into the cage, then inserted into the bottom of the mixing furnace and stirred back and forth until the flux is melted and the cage is taken out, and the cage stands still for 5 ~ 10 minutes. The surface scum can be cast by removing it. The flux can also be removed to cover the surface as needed.

2. Gas purification gas purification is a main method of primary aluminum purification, the gas used is chlorine, nitrogen or chlorine nitrogen mixture gas.

(1) chlorine gas purification. Previously used the active gas chlorine gas as a purifying agent (chlorination method) . In chlorination, when chlorine gas is passed into the liquid aluminum, many unusually fine ALCL3 bubbles are formed, which are fully mixed in the liquid aluminum. The dissolved hydrogen and some mechanical inclusions are adsorbed on the ALCL3 bubbles, and are discharged as the ALCL3 bubbles rise to the surface of the liquid aluminum. Chlorine can also make some more negative than aluminum element chlorination, such as calcium, sodium, magnesium and so on because of the chlorine into the corresponding chloride, can be separated out. Therefore, chlorination process is a very effective method of primary aluminum purification. Chlorine is used at a rate of 500-700 g per ton of aluminium. In order to avoid air pollution and reduce the production cost of aluminum ingot, chlorination process has been replaced by nitrogen purification process in modern aluminum industry.

(2) nitrogen purification method. Also known as smokeless continuous purification method, with alumina ball (418mm) as the filter media. N 2 goes directly into the molten aluminum. The aluminum liquid is continuously fed into the purification furnace, through the alumina ball filter layer, and is washed by nitrogen gas, so that the non-metallic inclusions and dissolved hydrogen in the aluminum liquid can be removed, and then continuously discharged, therefore, the fine nitrogen bubbles are uniformly distributed in the treated aluminum solution to play a purification role. Nitrogen has no pollution to the atmosphere, and the purification capacity is large, every minute can treat 200 ~ 600 kg aluminum liquid, the aluminum loss caused by the purification process is relatively reduced, so now widely used. But it does not remove calcium, sodium and magnesium from the aluminum solution as chlorine does.

(3) mixed gas purification method. The mixture of chlorine gas and nitrogen gas is used to purify aluminum liquid, which can remove hydrogen gas and separate oxides on the one hand, and remove some metal impurities (such as magnesium) in aluminum on the other hand. There are also 10% chlorine + 10% carbon dioxide + 80% nitrogen. This effect is better, carbon dioxide can make chlorine and nitrogen good diffusion, can shorten the operation time.