Electrodeposition of Aluminium and Co-Deposition of Alloying Elements Titanium, Manganese and Silicon in Molten Fluoride Electrolyte

Primary aluminium is produced by the Hall-Heroult process which is based on electrolysis in molten fluoride electrolyte, Na3AlF6-AlF3, at ~960 oC in which the raw material alumina is dissolved and decomposed into pure aluminium and CO2 gas due to the use of carbon anodes. Alloys are made by adding pure metals prior to the casting process. An alternative approach to produce alloys by co-deposition of alloying elements during electrolysis, where metal oxides of alloying elements are added to the electrolyte, is under study. Laboratory experiments were carried out to quantify the impact of co-deposition of alloying elements of titanium, manganese and silicon. The experimental set-up was dedicated to determine the current efficiency of aluminium and aluminium based alloys during galvanostatic electrolysis. Current efficiencies above 90 % were obtained for electrodeposition of aluminium alloys with titanium, manganese and silicon. It was found that the current efficency decreasde a few percentage points compared to the current efficiency for deposition of pure aluminium. The content of deposited allloying element was determined by ICP analysis of deposits. By adding from 1 - 3 wt% of metal oxide to the electrolyte the content of alloying element was found to be typically ~ 5 - 10 wt% after electrolysis. Experiments were carried out by changing temperature and amount of added metal oxide. The proposed method for producing aluminium based alloys directly in the Hall-Heroult process may give substantial benefits in terms of cost and efficiency. Challenges are related to metal oxide solubilities and control of the alloy composition.