While aluminium is the Earth's most abundant metal, it doesn't exist naturally in its pure form. Instead, aluminium readily combines with other metals to create compounds. Unlike iron, which can be isolated through straightforward furnace melting of its compounds, aluminium's production process is considerably more intricate, demanding substantial electrical power. Consequently, aluminium smelting facilities are strategically located near clean energy sources, typically hydroelectric power plants, to minimise environmental impact. But let's begin at the outset.
The aluminium production process consists of three main stages. First, bauxites, which are rich in aluminium, are extracted from the Earth. Second, these bauxites are refined into alumina or aluminium oxide. In the third stage, pure aluminium is produced through electrolytic reduction, where aluminium oxide is broken down into its components using electrical current. Approximately 4-5 tonnes of bauxite are processed into 2 tonnes of alumina, from which about 1 tonne of aluminium can be derived.
While various minerals can be used to extract aluminium, bauxite is the most common and preferred source. Bauxite is primarily composed of aluminium oxide, often mixed with other minerals. Bauxite is considered high quality if it contains over 50% aluminium oxide. These bauxite deposits exhibit a wide range of characteristics; they can be solid, dense, or crumbly and their colours can vary from brick red, flaming red, or brown due to iron oxide to grey or white in cases of low iron content. Bauxites with hues like yellow, dark green and multi-coloured varieties with bluish, purple, red and black streaks can be found.
Around 90% of global bauxite resources are situated in tropical and subtropical regions, with the majority, 73%, concentrated in just five countries: Guinea, Brazil, Jamaica, Australia and India. Guinea boasts the largest bauxite supply, totalling 5.3 billion tonnes, accounting for 28.4% of the global supply. Guinean bauxites are renowned for their exceptional quality, characterised by minimal impurities and their proximity to the surface simplifies the mining process.
The subsequent production process involves converting bauxite into alumina, also known as aluminium oxide (Al2O3), a white powder. The predominant method for producing alumina from bauxite is the Bayer process, a century-old technique still widely employed today, with approximately 90% of global alumina refineries utilising this method. The Bayer process proves highly efficient but necessitates high-quality bauxite with relatively low impurities, mainly silicon.
The fundamental principle of the Bayer process is as follows: the crystallised aluminium hydrate present in bauxite readily dissolves in concentrated caustic soda (NaOH) at elevated temperatures. Aluminium hydrate crystallises upon cooling and subsequent solution concentration, while the other elements present in the bauxite (referred to as ballast) either remain undissolved or recrystallise and settle at the bottom long before aluminium hydrate crystallises. Consequently, after dissolving aluminium hydrate in caustic soda, the ballast can be effortlessly separated and removed, resulting in a byproduct known as red mud.
Following bauxite mining and alumina production, the final stage involves electrolytic reduction to create aluminium. The heart of an aluminium smelter, the reduction area, differs markedly from traditional steelworks. It consists of expansive rectangular buildings, some exceeding a kilometre in length, housing numerous reduction cells or pots connected to power sources via massive cables.
Operating at constant voltages between 4 and 6 volts, with amperages reaching 300 to 400 KA or more, electric current powers the highly automated production process, requiring only a minimal workforce. Within each reduction cell, aluminium is produced from alumina through an electrolytic reduction in a 950°C molten cryolite bath, with the cell's bottom acting as the cathode and large cryolite-carbon blocks serving as anodes.
An automated alumina feeding system introduces fresh alumina into the cell every thirty minutes. Electric current breaks down aluminium-oxygen bonds, accumulating aluminium at the cell's base, forming a 10-15 cm layer. At the same time, oxygen combines with carbon in the anode blocks, creating carbon dioxide. Aluminium is extracted from the cell using specialised vacuum buckets two to four times daily. A hole is punched in the surface cryolite crust, allowing a pipe to draw in liquid aluminium. On average, each reduction cell yields about 1 tonne of metal, while a vacuum bucket can hold up to 4 tonnes of molten aluminium before transport to the casthouse.
The aluminium production process emits 280,000 cubic metres of gases per tonne of aluminium produced, necessitating gas removal systems in every reduction cell. These systems direct emitted gases to a gas treatment plant, where modern dry gas treatment employs alumina to filter out toxic fluoride compounds, creating a closed-loop system. Due to the substantial electrical power required for aluminium reduction, using eco-friendly renewable sources is crucial.
Hydroelectric power plants are the primary choice, delivering ample power without environmental pollution. For example, in Russia, 95% of aluminium smelters rely on hydroelectric power. However, regions still dependent on coal-fired generation, such as China, see 93% of aluminium production powered by coal plants, resulting in significantly higher carbon dioxide emissions—21.6 tonnes per tonne of aluminium produced, compared to just 4 tonnes with hydroelectric power.
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