Einfluss der Brennstoffwahl auf die Schmelzequalität und Oxidation beim Aluminiumrecycling

The combustion of hydrogen as an energy carrier is considered one of the most promising solutions for reducing fuel-related greenhouse gas emissions in high-temperature processes. In aluminium recycling, natural gas is currently predominantly used to heat energy-intensive melting, holding, and casting furnaces. A potential substitution with hydrogen presents a reasonable alternative from both a process and plant engineering perspective on the path to climate neutrality. However, the metallurgical effects of a changed fuel selection on the liquid metal are still largely unknown. Since combustion of hydrogen, compared to natural gas, not only eliminates carbon dioxide as desired but also increases the water vapour concentration in the furnaces, potential interactions between H₂O and aluminium melts need to be evaluated.
This study provides an overview of the impact of water vapour and complex furnace atmospheres on the melt quality and oxidation behaviour of aluminium melts. Experiments ranging from 0.5 g to 60 t were conducted to explore both the fundamental mechanisms and assess the actual effects on industrial processes. The research covers two main aspects, the processes in conventional hearth furnaces for processing pure scrap into wrought alloys and the role of hydrogen in melting organic-contaminated or oxide-containing materials in tilting rotary drum furnaces using melting salts.
The results show that retrofitting furnaces in the secondary aluminium production with hydrogen burners is an equivalent alternative to conventional natural gas heating, both in terms of process technology and achievable product quality. Increased hydrogen contents in the liquid metal, resulting from hydrogen combustion, can still be reduced to the usual level through proper sized degassing systems. The evaluation of alternative furnace atmospheres on oxidation behaviour is considerably more complex. However, the experimental results show that increased H₂O levels and the removal of CO₂ do not lead to increased metal loss. If a salt slag is present, it remains an effective protection for the liquid metal even under the altered conditions of hydrogen heating, largely preventing reactions with water vapour.