Aluminum smelters use very high voltage direct current in the smelting of aluminum. The levels are such that it is advised that metallic objects are not bought into the general area due to magnetic affects. People with medical implants are also banned.
So the two main issues with the control system was to ensure that emergency procedures are carried out if a pot is in danger of cooling down for whatever reason. While the second was to control of the actual process of smelting the aluminium including the raising and lowering of the electrodes.
So while the bulk of the equipment was COTS, the device drivers for the pots were hand crafted in order to reduce latency.
One issue that arose that did prove problematical was related to the high DC Voltages used. So even though the SCR's were rated at 1000 Volts DC and performed quite well, the layout of the silicon internally was such that the magnetic field distorted the internal fields to such as an extant that the life of the SCR's was greatly reduced.