The aluminum liquid sucked from the electrolytic cell contains various impurities, so purification is required before casting.   In industry, clarification, flux, gas and other purification methods are mainly used, and some are tried out by directional solidification and filtration methods. I also see some small methods on the international casting industry website.

1. Flux Purification Flux Purification is to form a large number of fine droplets by using flux added to aluminum liquid, so that oxides in aluminum liquid are wetted, adsorbed and dissolved by these droplets to form new droplets to rise to the surface, and scum is formed after cooling to remove.   The flux used for purification is composed of salt with low melting point, low density, low surface tension, high activity and strong adsorption capacity on oxidized slag.   When in use, a small piece of flux is firstly put into an iron cage, then inserted into the bottom of the mixing furnace to stir back and forth, the iron cage is taken out after the flux is melted, and the iron cage is still for 5 to 5~10min. The surface scum can be taken out and cast.   The flux can also be removed from the surface to cover the surface as required.

2. Gas purification is a major primary aluminum purification method. The gas used is chlorine, nitrogen or a mixture of chlorine and nitrogen.

(1) chlorine purification.   Previously, the active gas chlorine was used as a purifying agent (chlorination method).   In the chlorination process, when chlorine gas is introduced into the aluminum liquid, a lot of abnormally fine AlCl3 bubbles are generated and fully mixed in the aluminum liquid.   The hydrogen dissolved in the aluminum liquid and some mechanical inclusions are adsorbed on the AlCl3 bubbles and discharged as the AlCl3 bubbles rise to the surface of the aluminum liquid.   When chlorine gas is introduced, it can also chlorinate some elements with more negative electricity than aluminum, such as calcium, sodium, magnesium, etc. due to the introduction of chlorine gas, corresponding chlorides are generated and separated.   Therefore, chlorination is a very effective method for purifying primary aluminum.   Chlorine is used in an amount of 500-700g per ton of aluminum. However, because chlorine is toxic and relatively expensive, in order to avoid air pollution and reduce the production cost of aluminum ingots, the chlorination method has gradually been replaced by an inert gas-nitrogen purification method in the modern aluminum industry.

(2) Nitrogen purification method.   Also known as smokeless continuous purification, alumina balls (418mm) are used as filter media.   N2 is directly introduced into the aluminum liquid.   The aluminum liquid is continuously fed into the purification furnace, passes through the alumina ball filter layer, and is flushed by nitrogen, so that nonmetallic inclusions and dissolved hydrogen in the aluminum liquid are removed, and then are continuously discharged, so that fine nitrogen bubbles are uniformly distributed in the treated aluminum liquid to play a purification role.   Nitrogen has no pollution to the atmosphere and has a large purification capacity. It can treat 200~600kg of aluminum liquid per minute. The amount of aluminum loss caused in the purification process is relatively reduced, so it is now widely used.   However, it does not remove calcium, sodium and magnesium from aluminum liquid like chlorine.

(3) mixed gas purification method.   The mixture of chlorine and nitrogen is used to purify aluminum liquid. Its function is to remove hydrogen and oxide on the one hand, and remove some metal impurities (such as magnesium) in aluminum on the other hand. The common composition is 90% nitrogen +10% chlorine.   There are also 10% chlorine +10% carbon dioxide +80% nitrogen.   In this way, the effect is better, carbon dioxide can make chlorine and nitrogen diffuse well, and the operation time can be shortened.