Electrochemical Co-Deposition of Alloying Elements during Aluminium Electrolysis in a Laboratory Cell

Abstract

Aluminium alloys are mainly produced by the so-called melt mixing at which an alloying element is added to the molten aluminium before casting. This approach suffers from several drawbacks such as the high requirements of purity for the alloying element, which means high energy consumption and high price, holding times for dissolution and mixing of master alloys, and the possibility of segregation. It would be of great interest if aluminium alloys can be efficiently produced directly in the Hall–Heroult process with acceptable current efficiencies for alloy deposition.

This thesis reports the direct production of aluminium alloys during the electrodeposition of aluminium in a standardized laboratory cell designed for aluminium current efficiency measurements. Parameters such as temperature, bath chemistry, and cathodic current density were consistent with current industrial standards. Investigated alloying elements were titanium, manganese, and silicon.

The electrochemical production of aluminium-titanium alloys was experimented. It was possible to electrodeposit aluminium-titanium alloys with various contents of titanium in the alloy keeping quite good current efficiencies both for aluminium and for the produced alloys. The content of titanium in the produced alloys was in the range of 1–7 wt. %. The current efficiency for the produced alloys was as high as 96 % and that for aluminium at a minimum value of 89 %.

The production of aluminium-silicon alloys was possible but rather inefficient to produce in conditions described especially when the content of the alloying element was high. Analysis of some samples proved the formation of the alloy but with a significantly low current efficiency. The presence of silicon in the melt decreased the apparent current efficiency by 9 % for every 1 wt. % silicon added.

Aluminium-manganese alloys were successfully produced. Manganese could form alloys in the range from 8‒21 wt. % with aluminium. Current efficiencies for aluminium and aluminium-manganese alloys were generally quite good compared to the values of current efficiencies conducted for tests without the presence of manganese.

Achieving even current distributions, and thus reliable current efficiencies was a challenge. This was attributed to the quality of the synthetic cryolite used to conduct the work. The challenge was overcome by using cryolite based on the pure constituents NaF and AlF3.