Abstract
The raw gas composition from primary aluminium production depends mostly on the process technology applied and the composition of the raw materials. At steady state, a stationary condition is established among the material sources, gas production and sinks: the gas treatment centre and escaping gases. Only a few papers discuss the off-gas composition itself; most papers deal with the gas composition from laboratory-scale experiments performed under inert conditions. In this article, an overview of the literature describing gas production from aluminium electrolysis is given. Effects of temperature and chemical equilibrium on the stationary condition are also discussed. The typical chemical composition of the raw materials is presented to evaluate their input into the gas composition, especially with respect to their impurity levels.
Highlights
The Smelting ProcessAluminium (Al) is produced by electrochemical reduction of alumina (Al2O3) dissolved at about 950°C in a molten salt based on cryolite (Na3AlF6) with aluminium fluoride (AlF3) and calcium fluoride (CaF2) to reach an NaF (sodium fluoride) to AlF3 molar ratio of about 2.2
The gases originating from the reduction process are evacuated from the reduction cells by suction and diluted by draught
Since the power used by the fans to manage good off-gas suction, and the energy input to the anode production is not reported in the kWh/kg aluminium, the total energy consumption considering the whole plant operation is higher
Summary
Aluminium (Al) is produced by electrochemical reduction of alumina (Al2O3) dissolved at about 950°C in a molten salt based on cryolite (Na3AlF6) with aluminium fluoride (AlF3) and calcium fluoride (CaF2) to reach an NaF (sodium fluoride) to AlF3 molar ratio of about 2.2. Direct current (DC) is provided as the energy required by the reduction reaction as well as maintaining the cryolite bath temperature through resistive heating and compensate for heat losses. Carbon (C) anodes are consumed by the anode reaction as a reducing agent, forming mainly carbon dioxide (CO2) besides some carbon monoxide (CO). Graphite is most commonly used as current conductor to the cathode in modern electrolysis cells (the graphite is commonly denoted as the cathode in principle, the aluminium pad on top of the graphite is the cathode). The produced metal has a higher density than the electrolyte and is regularly tapped (siphoned) from the aluminium pad over the bottom of the cell. Whereas open electrolysis cells were historically used, modern technology makes use of a superstructure to contain the gases and dust produced during smelting. Large fans are used to extract these gases along with air from the draught, the draught serving to cool the off-gas as well as the superstructure
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