High Purity Aluminum (HPA) is used for many applications due to the purity and conductivity of the various grades of aluminum. 4N (99.99%- 1199 alloy) is the highest purity that can be produced in standard primary aluminum electrolytic production in one of the two main processes:
(1) Fractional Crystallization method works by taking advantage of differences in solubility. This is more expensive process which produces pure metal with particularly low Copper (Cu) content (1 to 3 ppm), while most other elements are reduced in 4N production.
(2) Three-Layer method is the lower cost process where the higher purity aluminum segregates from the lower purity aluminum with the addition of trace amounts of Cu, but which results in slightly higher Cu content (20 to 40 ppm). The 3 layers are (a) Anode - bottom layer - which is a mixture of molten aluminum alloy with copper, (b) - Electrolyte - middle layer - molten mixture of aluminum with elements in defined proportions, and (c) Cathode - top layer - molten high purity aluminum.
Each process has an advantage depending on the final production chemistry desired.
Both 4N processes are produced in commercial quantities with typical production in metric ton (MT) quantities.
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