Formation of Carbon Build-Up on the Flue Wall of Anode Baking Furnace

Abstract

A hard carbon build-up layer often forms on the flue wall surface in anode baking furnaces. The layer accumulates over thermal circles and needs to be mechanically removed regularly to ensure sufficient space for the anodes between flue walls. The underlying mechanisms are still unknown and the extent of the carbon build-up varies from plant to plant. The build-up on the flue wall, taken from an autopsy of an open top furnace, has been examined. Microstructure and phase compositions of the carbon build-up, especially towards the refractory interface, were studied by optical microscopy, X-ray computed tomography (CT), SEM/EDS, and XRD. Pyrolytic carbon was found to be the main part of the carbon build-up layer in addition to packing coke particles. The transport of silicon from the refractory material, condensating on the flue wall surface, is found as nucleation sites for the formation of carbon build-up. Formation mechanisms of the carbon build-up are proposed with reaction schemes supported by thermodynamic calculations.

Autopsy of refractory lining in anode kilns with open and closed design

“…The preliminary study confirmed the presence of carbon in the outer layers facing the anode pit, with a significant reduction in carbon level farther into the lining . This corresponds to investigations on similar samples showing carbon build‐up on the flue wall . The overall stability of the flue wall during thermal cycling is maintained by the use of mortar‐filled gaps in the wall.…”

“…At higher flue wall temperatures, parts of the hydrocarbons are likely to crack at the refractory surface, especially in areas with low suction, leaving solid carbon at the surface and in the open porosity, and consequently increased density. The reduction in density after heat treatment in air confirmed oxidation of carbon, hence supporting the proposed carbon cracking reaction . The preliminary study confirmed the presence of carbon in the outer layers facing the anode pit, with a significant reduction in carbon level farther into the lining .…”

“…The thermodynamics show that a transport of silica from the bottom section to the top section in the open furnace is possible considering local equilibrium in the lower and upper part. Investigations of similar samples have shown that both silica depletion and deposition could occur in the same region, given that the local conditions favor both reactions . In the closed furnace, on the other hand, oxygen intrusion is reduced by the use of section covers, resulting in a significantly lower oxygen partial pressure in the flue gas below the section covers, compared to the surrounding air.…”

“…In addition, the gaseous environment surrounding the refractories at the anode side may also contribute. Green anodes are made of 20%‐25% recycled anode butts, which contains traces of the electrolyte and impurities like Na and F. The presence of cryolite will cause volatile species like NaF and NaAlF 4 to form during baking, which may react with the refractory materials resulting in degradation of the wall . Typically, CO, CH 4 , H 2 , as well as other hydrocarbons, are formed during baking, either from the coke itself or the coal tar pitch.…”

Synthesis of Magnetic Activated Carbon Treated with Sodium Dodecyl Sulphate

“…Next, there were some white spots scattered around the surface of each magnetized sample. The white spot was believed to act as the silica particles and MAC_B2III contained more white spots because of the addition of D350 chemical that were contained in silica particles (Wang, et al, 2017). X-Ray Diffraction (XRD).…”