Technical Processing: From Electrolytic Aluminum to Aluminum Strip Production
Aluminum is a key metal in modern industry, and its production involves four main stages: mining of bauxite, production of alumina, electrolytic smelting of aluminum, and final processing into aluminum products. The process is highly efficient, with approximately two tons of bauxite yielding one ton of alumina, and two tons of alumina producing one ton of primary aluminum.
There are several methods for extracting alumina from bauxite or other aluminum-containing materials. However, due to technical and economic constraints, only a few have been widely adopted. These include the alkali method, acid method, acid-base combined method, and thermal method. Among these, the alkali method is currently the most commonly used for large-scale industrial production.
Bauxite is the primary source of aluminum globally, followed by other materials like alunite and nepheline. In most parts of the world, especially in China, bauxite is the main raw material for alumina production. Bauxite consists mainly of minerals like gibbsite, boehmite, and diaspore, and its composition varies significantly. The ratio of aluminum oxide (Al₂O₃) to silica (SiO₂), known as the aluminum-silicon ratio, is a key indicator of bauxite quality.
In the alkaline process, such as the Bayer process, bauxite is treated with sodium hydroxide (NaOH) to dissolve the alumina, while impurities like iron and silica remain as a residue called red mud. This red mud is later processed or disposed of, while the purified sodium aluminate solution is used to produce aluminum hydroxide, which is then calcined to form alumina.
The Bayer process, developed by Carl Josef Bayer in the late 19th century, remains the most widely used method for alumina production. It involves leaching, precipitation, and recycling of the mother liquor. The process is particularly effective for high-grade bauxite with a good aluminum-silicon ratio. However, for lower-quality bauxite, a combination of the Bayer and sintering processes may be used to optimize efficiency and cost.
Globally, annual alumina production exceeds 55 million tons, with China being a major producer. In China, about 6.8 million tons of alumina are produced each year, with Henan Province playing a significant role.
For primary aluminum production, the Hall-Héroult process is the standard method. Invented in 1886 by Charles Hall and Paul Héroult, this electrolytic process uses alumina dissolved in molten cryolite. At around 950–970°C, the alumina is decomposed into aluminum metal at the cathode and oxygen gas at the anode. Each ton of aluminum produced generates about 1.5 tons of CO₂, and the energy consumption is roughly 15,000 kWh per ton.
Industrial electrolytic cells are categorized into self-baking, pre-baked, and side-plug types. With growing environmental concerns, self-baking cells are gradually being replaced by more energy-efficient pre-baked cells. Globally, primary aluminum output is around 28 million tons annually, with China producing approximately 7 million tons.
Once produced, primary aluminum can be refined for higher purity or alloyed to create various aluminum products. Aluminum alloys are commonly manufactured through melting, casting, rolling, and forging techniques. These materials are used in a wide range of applications, from construction and transportation to electronics and packaging.
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