Gas Anodes for Electrowinning of Aluminium

Abstract

Aluminum is today commercial produced by the Hall-Haroult process using consumable carbon anodes. Consumable anodes have several drawbacks such as CO2 emission, continuous anode-cathode distance adjustments and replacements of anodes. Replacing the consumable anodes with inert anodes has been a topic for many decades without commercial success. Using porous inert gas anodes where natural gas or hydrogen take place in the anode reaction has been shown in laboratory tests to reduce the anode potential and reduce the CO2emission. However, formation of water results in evolution hydrogen fluorides which must be solved. Different type of anodes have been prepared and evaluated as possible gas anodes for this concept. Three different graphite materials with different pore sizes and porosity, commercially available, have been used to fabricate gas anodes. Porous SnO2 based materials have been prepared using either foam preforms or coarse SnO2 based particles sintered together with fine SnO2 based powders. These fabrication routes results in significant different materials. The materials are evaluated as possible porous anodes for this concept. Laboratory experiments using porous depolarized SnO2-based with CH4 and H2-gases have been performed with off-gas analysis and with special attention to hydrogen fluoride evolution. An theoretical mass balance with focus on HF emission has been performed and two different scenarios are presented 1) gas anode process and 2) conventional Hall Heroult cell. In addition we present simulation of the gas transport of a possible industrial gas anode. The challenging side effects of HF emission might be utilized in the process and is discussed.