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Effect of the change of control logic on the process efficiency

As a last test, the average energy efficiency and current efficiency of a 1 day run

was compared when only the type of feed control logic is changed. Figure 22 presents
the results obtained using a typical continuous tracking control logic. The average
dissolved alumina concentration obtained is 2.41 % with a standard deviation of 0.20 %,
the average power efficiency is 13.21 kWhr/kg with a standard deviation of 0.19
kWhr/kg and an average current efficiency obtained is 94.62 % with a standard deviation
of 0.20 %.

Figure 22: Process efficiency results during a typical continuous tracking logic

Figure 23 presents the results obtained using the new In Situ control logic. The

average dissolved alumina concentration obtained is 2.22 % with a standard deviation of
only 0.03 %, the average power efficiency is 13.16 kWhr/kg with a standard deviation of
0.12 kWhr/kg and an average current efficiency obtained is 94.77 % with a standard
deviation of 0.12 %. Further In Situ optimizing efforts are expected to produce a wider
divergence in production efficiencies.

Therefore the drastic reduction of the process variation of the concentration of the

dissolved alumina leads to the possibility of reducing the average value without
increasing the risk of anode effect. This result had a positive impact on both the current
efficiency and the power efficiency.