"If you don't prebake, the bottom gets gloopy" — that dreaded g-word again.
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In keeping with more of Baldwin's guidance, though, prebake the bottom before filling the pie.
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For fillings that take less than 30 minutes to cook, I prebake the base to biscuity completion.
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In 1977 approval was given to the Smelter to expand production to a second potline, expanding smelting capacity to 90,000 t/y of aluminium and adding 120 centre-worked prebake cells with supporting facilities. Supporting facilities included casting and anode manufacturing facilities and storage facilities for raw materials. This Line II expansion also involved the fitting of dry scrubbers to remove fluoride from waste gases. In 1981 a third potline was approved, increasing production capacity to 135,000 t/y of aluminium and adding another 120 centre-worked prebake cells with anode manufacturing capabilities, pollution controls and support facilities. This was the final stage of the Smelter’s expansion, generating its greatest capacity and efficiency. This third potline reached full production capacity in 1986, increasing the Smelter’s annual production capacity to 150,000 tonnes of aluminium.
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After exposure the SU-8 needs to be baked again to complete the polymerization. This baking step is not as critical as the prebake but the rising of the temperature (again to 95 °C) needs to be slow and controlled. At this point the resist is ready to be developed. The main developer for SU-8 is 1-methoxy-2-propanol acetate.
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Alumina is added to the electrolyte from between the anodes in prebake cells. Prebaked anodes contain a smaller percentage of pitch, as they need to be more solid than Söderberg anodes. The remains of prebaked anodes are used to make more new prebaked anodes. Prebaked anodes are either made in the same factory where electrolysis happens, or are brought there from elsewhere.
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During early production, the Smelter had a production capacity of 30,000 (t/y) of aluminium metal. The first potline was progressively expanded to a capacity of 45,000 t/y and consisted of 120 side worked prebake cells. A potline is a series of pots or electrolytic cells where alumina is melted with molten cryolite. These pots are situated in a long building or spread across several buildings.
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The former, also called Söderberg anodes, are less power-efficient and fumes released during baking are costly to collect, which is why they are being replaced by prebaked anodes even though they save the power, energy, and labor to prebake the cathodes. Carbon for anodes should be preferably pure so that neither aluminium nor the electrolyte is contaminated with ash. Despite carbon's resistivity against corrosion, it is still consumed at a rate of 0.4–0.5 kg per each kilogram of produced aluminium. Cathodes are made of anthracite; high purity for them is not required because impurities leach only very slowly.
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Each anode is lowered vertically and individually by a computer, as the bottom surfaces of the anodes are eaten away during the electrolysis. Compared to Söderberg anodes, computer-controlled prebaked anodes can be brought closer to the molten aluminium layer at the bottom of the cell without any of them touching the layer and interfering with the electrolysis. This smaller distance decreases the resistance caused by the electrolyte mixture and increases the efficiency of prebaked anodes over Söderberg anodes. Prebake technology also has much lower risk of the anode effect (see below), but cells using it are more expensive to build and labor-intensive to use, as each prebaked anode in a cell needs to be removed and replaced once it has been used.
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