Prebake and Soderberg
The alumina reduction cells, and in a broader sense, the aluminum smelters can be divided into two big categories depending on how is arranged their anodic system.
In the alumina reduction cells the anode is a block of carbon made of petroleum coke and pitch. What differentiate the two technologies is the way this carbon block is produced.
In a so-called pre-bake cell, the petroleum coke is mixed with pitch, which acts as a binder. Then, at this mixture, usually called green paste, it is given a parallelpiped shape with either a press or a vibrocompactor. The formed carbon block is then baked into furnaces in order to be transformed into a solid carbon block. The electric current arrives to the carbon block through a rod linked to it through nippels. A pre-bake pot contains several single anodes (usually 14 ÷ 40, mainly depending on the line current), which stay on the pots for a fixed amount of days (generally 26 to 30 days). Then, before being completely consumed, they are removed together with the rod, and the remaining carbon reused to produce new anodes.
In a Soderberg smelter the basic idea is to eliminate the sub-plants which form, bake and join the carbon block with the rod. A Soderberg cell has only one big anode, housed in a steel container, which gives to the anode its shape. From the upper part of this container it is introduced the green paste. During its movement from the top to the bottom of the container the green paste is baked. Unfortunately, the quality of the baked Soderberg anode is lower than the quality of the prebaked one, hence the Soderberg cells are always characterized by a lower current efficiency and a higher pot voltage, needed also to produce the extra heat necessary for the anode baking.
Presently all the new built smelters adopt the pre-bake technology, because of the higher current efficiency, lower specific energy consumption and lower emission (especially PAH). However, a good number of Soderberg plants are still in operations, sometimes retrofitted with additional technology aimed at increasing current efficiency and reduce emissions.
Throughout the rest of this website we will refer only to the pre-bake technology, even though most of the topics apply both at pre-bake and Soderberg technology.