Some Important Figures
From the electrochemistry laws we know that, if I is the current (in kA) flowing into a cell, the production of aluminum in 24h is equal to:
Theoretical Amount [in kg] = 8,0533 * I
For example, for a pot with a 200 kA flowing current the daily production should be equal to:
Theoretical Amount = 8,0537 * 200 = 1610.74 kg
But the actual weight of the aluminum produced is always lower than the theoretical amount. This is because together with the alumina reduction reaction:
Is always present a parasite reaction, called “back reaction”:
which reoxidizes to the state of alumina part of the aluminum produced.
Is called current efficiency and is always less than 100%. The best in class aluminum smelters operate with current efficiencies of 95 – 96%, while normal figures for current efficiencies range between 90 ÷ 94%.
Specific Energy Consumption
For the current to flow we need to apply a voltage V to the pot. The specific energy consumption, expressed in kWh/kg Al produced, from the electrochemistry laws is equal to:
This equation tell us that to reduce the specific consumption we need either to reduce voltage and/or increase current efficiency. In the aluminum industry specific energy consumption ranges between 13 ÷ 15 kWh/kg Al.
The theoretical amount of energy needed to produce aluminum at 100% current efficiency is equal to 6.34 kWh/kg. The difference between this theoretical and the actual amount in the real world is due to the fact that:
- Current efficiency is never 100%
- More than 50% of the electrical power given is lost as heat escaping from the pot itself
Specific Carbon Consumption
As we have seen, in the alumina reduction it is involved also the carbon. Theoretically, at 100% current efficiency, to produce 1 kg of aluminum 0,333 kg of carbon are needed.
The real consumption is higher than the theoretical because current efficiency is always less than 100% and also because of the oxidation of the anodes to the air.
Practically, the specific carbon consumption is equal to: