In the Hall-Héroult process for primary production of aluminium, a considerable amount of anode carbon is lost through unwanted gasification in air and CO2. The carbon gasification reactions are catalyzed by a number of inorganic impurities normally present in the anodes. Some of these impurities follow the anode raw materials while others are introduced during the anode manufacturing process. The aim of this work is to obtain a fundamental knowledge of how the bath compounds: AlF3, Al2O3, NaF, Na3AlF6 and CaF2, which may be introduced in various amounts to prebaked anodes through the addition of recycled anode butts, influence the air and CO2 reactivity of anode carbon. In order to avoid the disturbing and possibly masking effect of other impurities normally present in industrial anode materials, this work uses cokes made by carbonization of high purity carbon precursors (tar oil/petroleum pitch) in a laboratory scale coke reactor. Known amounts of fluoride salts or the corresponding metal acetylacetonates are added to the liquid precursor prior to carbonization. “High”-sulfur cokes are prepared by also adding 4.5 weight percent dibenzothiophene to the precursor (corresponds to an addition of 1 wt% elemental sulfur). The calcined coke samples are characterized in terms of reactivity towards air and CO2 gasification, size and shape of optical texture units and degree of turbostratic order. Scanning electron microscopy and surface area measurements are used to study the surface textural changes resulting from the catalyzed gasification. The extent of gasification inside industrial prebaked and Søderberg anodes is investigated by characterizing anode core samples in terms of air permeabilities, contamination profiles and reactivities towards air and CO2 gasification. The characteristics are related to findings from electron microscopy examinations. COKES DOPED WITH SODIUM ACETYLACETONATE, SODIUM FLUORIDE AND CRYOLITE Sodium acetylacetonate decomposes completely to sodium carbonate during carbonization and calcination of the coke samples. At the initial stages of gasification, the sodium carbonate particles decompose to a sodium oxide phase, which catalyzes the air and CO2 gasification reactions strongly. Sodium fluoride and cryolite also act as strong gasification catalysts. Due to formation of higher amounts of inhibiting fluorine gases (COF2, AlOF2), the cryolite doped cokes are less reactive than the corresponding sodium fluoride cokes. The difference between the two coke series is especially pronounced during air gasification. The air reactivity of the sodium-doped cokes is markedly reduced when 4.5 weight percent dibenzothiophene is added to the coke precursors prior to carbonization. During carbonization, the sulfur is stabilized in large aromatic molecules and it is only liberated when the carbon matrix is gasified. The free sulfur adsorbs on the active sites of the sodium particles and lowers their catalytic activity. The Na-S adsorption complexes are thermally unstable at the CO2 gasification temperature (960 °C) and the CO2 gasification rates are therefore not affected by the dibenzothiophene additions. The additions of various amounts of sodium acetylacetonate, sodium fluoride or cryolite (> NaF > Na3AlF6 > Na2O >> Al2O3 100 100 26 21 15 3 Air gasification: NaF = Na2O >> Na3AlF6 >> CaF2 > Al2O3 > Na2O 100 100 57 29 24 19 “High”-sulfur cokes: CO2 gasification: CaF2 >> CaO >> NaF > Na3AlF6 > Na2O >> Al2O3 100 50 25 21 15 3 Air gasification: Na2O >> CaF2 = Al2O3 > NaF > Na3AlF6 = CaO 100 19 19 16 13 13 Aluminium fluoride inhibits the gasification reactions. There are no cocatalytic effects between calcium and sodium i.e. the catalytic activity of calcium is not affected by the concurrent presence of sodium and vice versa. GAS REACTIVITY OF INDUSTRIAL ANODES The carbon dioxide produced at the anode working surface, percolates through the open porosity of the anodes and reacts with accessible carbon in the lower parts. Airburn is mainly of concern at the exposed parts of prebaked anodes (anode tops and the sides near the tapping positions) and under the gas skirts of the Søderberg anodes. From air permeability measurements and electron microscopy examinations, internal CO2 gasification is found to occur in the lower 3 – 5 cm of prebaked anodes. Depending on the anode top surface temperature, air gasification may occur as deep as 4 cm below the anode top surface. In Søderberg anodes the extent of internal CO2 gasification strongly depends on the gas permeability of the anode. In high-permeability Søderberg anodes, CO2 gasification may occur as far as 60 cm above the working surface. Higher up, the temperature is normally too low for gasification. In some particular low-permeability Søderberg anodes (permeability resembling “bad” prebaked anodes), internal CO2 gasification is limited to the lower 15 – 20 cm of the anodes. In both prebaked and Søderberg anodes, the more reactive binder coke is selectively gasified. Since much of the binder phase in the lower parts of the high-permeability Søderberg anodes is consumed, substantial dusting is expected from the working surface of these anodes. In the prebaked anodes, the binder phase is mostly structurally intact. Catalytically active sodium and calcium impurities are mainly introduced to prebaked anodes via the addition of butts. Pot-room dust is an important contamination source in Søderberg anodes. Additionally, the lower parts of especially the high-permeability Søderberg anodes are contaminated by gaseous sodium and aluminium bath species that penetrates into the anode open porosity and condense within the anodes.