Introduction

About 2M tons of aluminium is currently generated by the plants in Siberia. Unfortunately only a minor part is generated using the currently environmentally-appropriate "prebaked" technology, which ensures low specific yields of tar that contain carcinogenic polycyclic aromatic hydrocarbons (PAHs) (0.06 kg/t Al based on data from the Pechiney Company). Instead the majority of aluminium is generated in Soderberg cells with self-baking anodes, which generates the highest carcinogenic risk

in aluminium production. The specific tar yields are considerably higher for aluminium productions that use Soderberg technology, producing 0.21 kg/t Al with a 'dry' gas cleaning system (Reynolds Company) and up to 0.65 kg/t Al with a 'wet' gas cleaning system at the Krasnoyarsk aluminium plant (Kaiser Aluminium & Chemical Corporation) (Johnson and Lobachev, 1998). The reduction of carcinogenic PAH emissions is ecologically very significant for large centres of aluminium production using Soderberg technology (Krasnoyarsk, Bratsk, Irkutsk, Novokuznetsk).

When determining specific yields of tar and carcinogenic PAHs using Soderberg technology, emissions from the gas cleaning system and gas escapes from under the gas skirt are traditionally considered while the process stage of current-carrying stud repositioning is not. It is important to evaluate the contribution of two major sources of atmospheric PAH emissions: 1) gases formed during slow (0.01-0.1°/min) carbonisation of 'dry' anode paste and anode body formation; and 2) gases released from the open hole during stud repositioning, when rapid heating (70-100°/min) pyrolysis of the studhole anode paste takes place, and 'secondary' anode formation.

The use of high-softening-point pitches (HP) instead of medium-softening-point pitches (MP) to prepare the anode paste is considered one of the ways to reduce the environmental danger. Such a replacement is associated with economic and technological advantages due to a decrease in anode paste consumption and a decrease in specific tar yields, which should in turn result in a decrease of carcinogenic PAH yields. However, it has not been proven that a switch from MP to HP will decrease the carcinogenic danger. As such it is important to compare the carcinogenic PAH yields of carbonisation products of MP, HP and anode pastes based on MP (APMP) and HP (APHP).

The objectives of this work are to evaluate the contributions of the main sources of carcinogenic PAH emissions and to compare their carcinogenic danger during aluminium production using Soderberg cells.

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