Shaft Seals Reduce Carbon Dust Emissions

Shaft Seals Reduce Carbon Dust Emissions Several producers of carbon anodes for use in the aluminium smelting process have begun using MECO shaft seals on their mixers to reduce carbon dust escape. The induction furnaces in aluminium smelters consume large numbers of carbon electrical anodes. It is common for the smelter to have an anode manufacturing facility on site to meet this need. These 'green mills' produce a finely-ground carbon/pitch mixture which is formed into dense blocks and fitted with electrodes to become the finished anode for use in powering the furnaces. In the process, three grades of carbon powder are mixed with molten pitch at high temperature to form a sticky paste, which is formed into solid blocks in a vibratory compactor and then baked. The correct ratio of carbon powder to pitch is essential to the success of the aluminium smelting process, so no dry powder can be allowed to escape.Dry material for the anodes is prepared in a sigma arm or paddle mixer, and these can produce tremendous powder leakage. When plants used braided packing in stuffing boxes, shaft run-out would cause packing to become compressed on two sides, creating a leakage pathway. The leaked carbon dust posed a serious inhalation hazard to mill personnel, in addition to being a damaging abrasive which caused severe bearing wear.Housekeeping and packing gland maintenance were demanding many hours per week.The environment was conducive to dust explosion. In several plants where maintenance crews used to take wheelbarrow loads of the hazardous carbon powder away daily, leakage has been eliminated by replacing packed glands with MECO Custom Shaft Seals. MECO seals have been operating very successfully since 1999 in one Australian smelter and since 2001 in another.The MECO EAS model seal was installed first on one mixer and then all eight in the first plant. A nitrogen purge was specified in this application to minimise the risk of dust explosion.Due to the seals' high shaft run-out tolerance, the coke dust was contained successfully, solving the problem.In both mills, all the carbon mixers have since been fitted with MECO seals.Evaluation seals operated with great success at a smelter facility in New Zealand for over a year.The facility is now in the process of retrofitting all of their mixers ; 22 seals have been purchased to date.The MECO EAS seals work in a similar way to a thrust bearing, rotating in a plane perpendicular to the shaft. A driving elastomer rotates with the shaft, turning a pair of rotors inside the seal housings.No motion takes place between the seal and shaft, so friction is minimised and the shaft is protected from abrasion.Purge gas consumption is very low.Seals are rebuilt when sacrificial wear parts (rotors and elastomers) wear beyond a certain point, indicated by monitoring a purge pressure gauge.These seals have run in most cases for six to twelve months between rebuilds, eliminating frequent packing maintenance.Seals are designed fully-split, so retrofit installations and seal rebuilds are accomplished quickly without removal of bearings or drives.