What process is used to achieve 99.9% pure aluminium?

The method used to achieve 99.9% pure aluminium is zone refining. This process is based on the principle of fractional crystallization. In zone refining, a molten zone is moved along a solid metal bar (in this case, aluminium), which gradually melts and then solidifies as it passes through the material.

Impurities are often more soluble in the liquid phase than in the solid phase. As the molten zone travels and solidifies, it leaves impurities behind in the liquid that remain in front of the advancing solidification front, effectively concentrating the purifying effect as the process continues. This allows for the separation of pure aluminium from its alloy and residual impurities, leading to a high purity level, which can reach up to 99.9% under optimal conditions.

The other methods listed, such as electrolysis, are used for the initial extraction of aluminium from bauxite ore and are effective but do not achieve the same level of purity as zone refining. Similarly, reduction with carbon is also involved in the reduction process of aluminium oxides but again is not specifically aimed at reaching the high purity levels that zone refining can achieve.