AP technology and the aluminum smelting process
How aluminium is made
Aluminium is produced by the electrolytic reduction of alumina through a process known as the Hall-Heroult process. In this process, an electric current breaks down the alumina, causing the aluminium and oxygen molecules to separate. This reaction takes place in large cells or ‘pots’, through which an electrical current is passed. The bottom and sides of each pot act as the cathode or negative electrode and contain the molten bath, composed mostly of cryolite and aluminium fluoride, in which the alumina is dissolved. Carbon blocks suspended just above the cathode, serve as the anode or positive electrode. When the electrical current passes through the mixture, flowing from the anode to the cathode, the molten aluminium molecules settle to the bottom of the pot while the oxygen combines with the carbon of the anode. The carbon anode is continuously depleted by the reaction and must be replaced. The molten aluminium deposited at the bottom of the pot is removed regularly using a vacuum siphon. It is then transferred to a holding furnace where it is alloyed, fluxed and degassed to remove trace impurities. From the holding furnace, the aluminium is cast in different shapes depending on end-use and customer requirements.
There are two different technologies of the Hall-Heroult process - Söderberg technology (originally used in Kitimat) that ‘creates and bakes’ the anodes in place in each pot. In this process, the tops of the pots are uncovered and the sides, although normally closed, must be opened regularly for a variety of process operations. The second, known as Aluminium Pechiney (AP) or pre-bake technology creates and bakes the anodes in separate facilities. The pots are totally enclosed and most process operations occur without opening the pot enclosure.
AP Technology was originally developed by Aluminium Pechiney (AP) and acquired by Alcan Inc. in 2004, and more recently by Rio Tinto Plc. for the modernization of Kitimat Operations.
Key benefits of AP Technology are:
- extremely high energy efficiency;
- leading-edge digital monitors and computer control systems;
- the lowest greenhouse gas emissions in aluminium production in the world (based on International Aluminium Industry 2003 Survey results);
- revolutionary efficiency and performance in its dry scrubbing (cleaning) systems; and
- high levels of material recovery to maximize raw material yield and recycling opportunities.
Rio Tinto Alcan’s AP Technology leadership surpasses all records for reliability, efficiency and productivity. Offering world-class environmental, health and safety improvements, AP Technology is the aluminium industry's cleanest technology with the lowest cost. Worldwide, Rio Tinto Alcan delivers the industry's most comprehensive and sustainable smelting technology and engineering with a century of aluminium production experience.
Smelter modernization technology
The Kitimat Modernization Project team will use the newest, most modern AP pre-bake technology. The modernization will consist of up to 384 pots housed in four buildings, increasing Kitimat’s current production capacity to 420,000 tonnes per year. The pots would operate at optimum amperage of 405 kA and would be serviced by overhead cranes; increasing overall operating efficiency considerably. By way of comparison, the existing Kitimat smelter has a total of 900 smaller pots in 15 buildings with a total rated capacity of about 280,000 tonnes per year.
Raw materials that would be used in the process include: alumina from Australia; liquid pitch from Korea; and calcined coke.
Alumina is stored in silos until it is required. Before being used in the process, the alumina is fed through devices known as dry scrubbers where it functions as a scrubbing medium to clean emissions.
The petroleum coke (carbon) and liquid pitch will be mixed together to form green anodes. Emissions from this process will be collected and dry scrubbed (cleaned) using coke as a scrubbing medium. The coke would then be reused in the process.
The green anodes will be baked using natural gas fires. All emissions will be collected, and any tars present, incinerated in the natural gas fires, then dry scrubbed using alumina. The alumina will then be reused in the process.
The baked anodes will be used to replace those that have been consumed in the pot. Partly consumed anodes (called butts) would be recycled back into the process as carbon.
In a pre-bake pot, alumina additions and anode effect treatments will take place within the totally enclosed pot. Removable doors will provide access for anode replacement. Emissions generated by the process will be contained within the enclosed pot and drawn off by an emission collection system. Treatment will be carried out in dry scrubbers. Emissions captured by the scrubbing process will be returned to the production process for re-use.
Aluminium smelting is a dry process with water being used only for cooling purposes. Sanitary sewage discharge will be pumped into the District of Kitimat sewage treatment facility while the surface runoff will be collected in retention ponds and then released.