Inward bending of anode stubs, generally known as “toe-in“, is a well understood occurrence in all pre-bake aluminium smelters.
The difference in linear expansion of the carbon anode and the steel yoke or spider of the anode rod at high temperatures in the reduction cell means that the yoke expands more than the anode, bending the stubs which are anchored in the anode block.
Stubs become bent inwards just above the iron thimble, changing the geometry of the yoke, and stubs can no longer be correcly located in the anode holes during subsequent rodding. Ultimately the stubs will no longer fit into the holes.
The machine to be supplied to Kubal will be designed for the 2x2 150mm stub diameter techonology in use at that smelter and will be delivered in mid-2012.
VHE of Iceland has proven solutions suitable for different reduction technologies. For smaller diameter stubs, ambient temperature straightening is an economical approach. For larger stub diameters, hot straightening is often a better solution.
Stubs heated to 650°C need considerably less straightening force, and the cost of an induction pre-heating unit is to a large extent off-set by savings in the heavy duty steelwork and hydraulic systems which would otherwise be needed.