It all starts at the Alba Petroleum Coke Calciner plant. Our strategy of a self reliant supply system has led to the setting up of the most modern calciner in the world. This has eliminated the need to import calcined coke, the major raw material for the production of carbon anodes used in the aluminium smelting process. The aim of this strategy is to control the quality of the basic green calcined petroleum coke by producing it in-house and, thus control the quality of the aluminium we produce.
The aluminium metal is made of alumina which is composed from aluminium and oxygen. During the smelting process, aluminium and oxygen are separated to produce aluminium metal. To produce one tonne of aluminium, about two tonnes of alumina is required.
Electrical energy is used in the smelting process to separate the aluminium metal from the oxygen. This is done in large steel, carbon-lined furnaces called reduction cells. When alumina is fed into these cells, it gets dissolved in molten cryolite, a liquid which can dissolve alumina and conduct electricity at 970°C. Electricity is introduced into each cell via carbon blocks called anodes that we manufacture in-house. All reduction cells in the smelter are connected in a series by an aluminium busbar, which carries electrical current to the cells.
The process is continuous with an electrical current of 100,000 to 320,000 amp flowing from the anode through the alumina and cryolite mixture, to the carbon cell lining, to the anode of the next cell, and so on. The electrical current causes the alumina in this mixture to react with the carbon anode, forming aluminium and carbon dioxide. In its molten form, the aluminium sinks to the bottom of the reduction cell, while the carbon dioxide and other gaseous by products form at the top of the cell.
In our efforts to save natural gas, necessary for generating electricity, we use the waste heat generated in the smelting process to produce steam for producing additional electricity in what is known as the combined cycle effect. The generated heat from this cycle is delivered to the pots. This process saves vast amounts of precious natural gas while keeping production at optimum levels of efficiency. Molten aluminium is siphoned from the bottom of the cell by a process called tapping. From the reduction lines, liquid metal is transported to the casthouse where it is poured into mixing furnaces where elements such as silicon, magnesium, copper, iron, titanium or boron are added to meet the customer required alloy specifications.
The prepared aluminium is then cast either in solid mould ingots or direct chill (DC) machines. The metal received from the reduction rooms is then cast into four different products. These are Standard or T-ingots used for remelting by other industries, Extrusion billet for the manufacture of extruded products such as window frames and structural supports, Rolling slabs that are used in the production of aluminium sheets and foil and Propertzi ingots that are used as wheel alloys. Finally, the remainder is supplied in molten form to local industries located in the immediate vicinity of the Alba plant.