Modeling Cathode Cooling after Power Shutdown
M. Dupuis, Jonquière; A. Tabereaux, Muscle Shoals
Introduction
Experience has shown that shutting down and restarting aluminum electrolysis cells due to extended
electrical power interruptions at aluminum smelters causes irreversible and non-repairable damage
to the cathodes, and consequently shortens pot-life. The average loss in pot life due to shutdown and
restart of individual potlines is estimated to be about 200 days, but varies from 100 to 400 days at
different aluminum smelters. Cooling cells to ambient temperature causes the formation of
numerous, and often deep cooling cracks on the top surface of the carbon cathode lining, (in the
cathode blocks as well as the seams between blocks). The mechanism for the formation the cooling
cracks has not previously been determined.
Although there have been numerous publications regarding the preheating of cathode lining of
aluminum electrolysis cells, this work represents the first to effectively model the cooling of cathode
linings due to a power outage, and report the extent and consequence of thermal gradients formed
in the cathode lining during cooling causing crack formation in the cathode lining in aluminum
cells
Power Interruptions at Aluminum Smelters:
During the past ten years there has been an increase
in the shutdown and restart of aluminum potlines due to long duration power interruptions, (> 3
hours) at aluminum smelters. [Reference 1] Aluminum companies have been very successful in
using amperage creep to increase productivity and profitability at most existing aluminum smelters,
but it has come at a price, as it tends to shorten the lifetime of transformer/rectifiers systems. For
instance, the majority of smelters that experienced shutdown of potlines due to long power
interruptions were due to failure of the transformer/rectifier systems, especially at older aluminum
smelters built 20 to 40 years ago. Harsh weather conditions such as ice storms, snow and high wind
velocity are also major factors in causing shutdown due to long duration power interruptions, and
are frequent in China during winter. A somewhat surprising development is that several new
modern high-amperage smelters, (e.g., Fjardaal, Qatar and Dubal) have experienced recent
shutdown of potlines due to the temporary loss of power at their power generation stations and/or
national grid system.
Cooling the electrolyte below 850 °C results in the solidification of bath and eventually the
shutdown of the operating cells in the potlines. It requires a great deal of effort, pre-planning and
experience to survive power interruptions that last longer than 3 hours. However it is astonishing
that are a few instances in which potlines have been reported to survive power interruptions up to 8
hours.
Cathode Cooling Cracks:
The rapidly cooling of aluminum cells from an operating electrolyte
temperature 960 °C to ambient 25°C due to potline shutdown results in the generation of cooling
cracks on the cathode surface; this phenomena is observed in most all cells in which the metal pads
are removed and the surface cleaned for inspection. The cracks are formed in the cathode block
during cooling and not during cell operation as indicated by the absence of bath or a yellow film of