Effect of the modification of a high-QI pitch with an additive on anode quality

Abstract

Prebaked carbon anodes are manufactured from a dry aggregate (calcined petroleum coke, recycled butts, and rejected green and baked anodes) and a pitch as the binder. The process involves the mixing of the dry aggregate and the pitch according to an appropriate recipe followed by the compaction of the resulting paste and its baking to produce baked anodes for use in electrolysis cells for aluminum production. A good bonding between pitch and dry aggregate particles improves the anode quality which leads to reduction in process cost, energy and carbon consumptions, and emission of greenhouse gases (GHG). One of the potential avenues to improve such bonding is the modification of pitch by an additive. It enriches the surface functional groups of pitch and makes it more compatible with coke. The objective of the current study is to investigate the effect of the modification of a pitch containing a high amount of primary quinoline insolubles (HQI pitch) on the anode quality.The wettability of coke by modified and non-modified pitches was measured using the sessile drop method. Fourier Transform Infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses of these pitches were carried out to determine the change in their surface chemistry upon modification. Laboratory anodes were produced using the non-modified and modified HQI pitches at different pitch levels and additive percentages. The anodes were characterized by measuring their density, electrical resistivity, air and CO2 reactivities, and permeability. Then, these properties were compared for anodes containing non-modified (standard) and modified pitches. In general, the results showed that the modification improved the anode properties compared to those of the standard anodes. The most suitable additive percentage was 2 % and the best pitch percentage was 16 % to obtain good quality anodes. The wettability tests showed that the HQI pitch modified with 2 % additive wetted the coke best. XPS and FTIR tests showed the chemical changes occurring on the pitch surface after the modification with the additive. The results show the potential for the improvement of anode quality based on the pitch modification which renders pitch more compatible with coke.