No matter how many syllables you use to express, aluminum is one of the most useful industrial metals we have. Aluminum is light in weight, high in strength, easy to alloy, high in conductivity and easy to process, cast and extrude. It can be used in almost all industrial processes and imaginable commercial products.
Without aluminum, modern life is impossible, but silver metal has only been widely used in the last 100 years. Not long ago, aluminum tableware was still a status symbol, and its value once exceeded the weight of gold. The reason it was once rare was the effort required to extract the rich elements from the rock that carried it, and the energy to do so. The forces that have locked aluminum out of human use until recently have been overcome, and it is worth studying the chemistry and engineering required to achieve this goal in our next part of "Mining and Refining".
Aluminum is the metal element with the highest content in the earth's crust. But for something that occupies an average of 8% of the ground under your feet, it is very difficult to obtain it in its elemental form. There are no outcrops or veins of metallic aluminum for mining; aluminum almost always exists in the form of various oxides and requires chemical release before it can be used as an industrial metal.
Although aluminum-bearing rocks are widely distributed, there are only a few aluminum ore deposits that have important economic significance: bauxite. The exact content of bauxite varies, but it is usually composed of alumina minerals and aluminum hydroxide, clay, quartz, and iron-containing minerals. Some of the largest and most abundant bauxite deposits are located in tropical regions, where high temperatures and abundant rainfall alternately occur, followed by prolonged droughts.
These conditions are conducive to chemical weathering and are actually the first step in aluminum processing-it breaks bauxite (already a very soft rock) into bite-sized pieces, which can be easily scooped up. Most bauxite mines are mined using open-pit mining technology. The current world leader in bauxite production is Australia, whose output accounts for about a quarter of the world's output. China ranks second, and West African country Guinea ranks third. There are also large amounts of bauxite in Brazil and the Caribbean, mainly in Jamaica.
Because bauxite is mined in only a few places in the world, the ore is often transported long distances for further processing. When ore is transported across the ocean due to liquefaction and dynamic separation, this may ultimately be a dangerous proposition. Bauxite usually contains a lot of clay, and when exposed to rain, it forms a quicksand-like suspension that resembles a liquid. When loaded into the cargo hold of a bulk carrier, bauxite that is too humid will slosh around, coupled with the upward migration of water in the mud, which will change the ship’s center of gravity, leading to catastrophic consequences.
The raw bauxite ore must be chemically treated to remove impurities and prepare for the aluminum contained in the smelting. The Bayer process is almost always used to achieve this, including cooking large quantities of crushed bauxite in a pressure vessel with caustic soda or sodium hydroxide solution. At 150°C to 200°C, aluminum oxide and hydroxide, which are usually insoluble in water, react with sodium in sodium hydroxide to form sodium aluminate:
This will dissolve the aluminum in the bauxite, but will not dissolve impurities that are mainly iron oxide. The insoluble matter is filtered together with excess sodium hydroxide into a waste called "red mud". Bauxite processing plants produce large amounts of red mud and store it in a lagoon, usually formed by flooding abandoned bauxite pits when the ore is processed near the mining site. The oxides in the red mud have economic value and can be recycled for industrial processes, including the recovery of trace rare earth elements that may be present in tailings. If not handled properly, red mud can also lead to disasters.
The final step in bauxite processing is to precipitate and purify the aluminum in the filtrate. This is achieved by adding highly purified aluminum hydroxide crystals to a solution containing sodium aluminate. This will cause aluminum hydroxide crystals to form and fall out of the supersaturated solution:
The aluminum hydroxide crystals are collected and processed in a high-temperature rotary kiln. In a process called calcination, aluminum hydroxide is thermally decomposed into pure white alumina crystals:
The next step is to actually smelt elemental aluminum from alumina. The process used to achieve this goal is the Hall-Héroult process, named after American chemist Charles Martin Hall and French scientist and inventor Paul Héroult, who invented the process independently and almost simultaneously in 1886. The process basically aims to eliminate the natural oxidation process that initially locks elemental aluminum in its oxide to form bauxite. It is carried out by electrolysis and therefore requires a lot of cheap electricity to be economically viable; this is why aluminum smelters are usually located near hydroelectric dams.
In order to electrolyze alumina powder, it must first be liquefied. It is not feasible to simply melt it because of its ridiculously high melting point (2,072° C). The key to the Hall-Héroult process was the discovery of cryolite, a salt of sodium, aluminum and fluorine. Cryolite lowers the melting point of alumina to about 900°C, making electrolysis possible. Cryolite exists naturally, but it is very rare, and it can be found in only a few places on the earth. Almost all cryolite used in aluminum smelting is produced synthetically.
On an industrial scale, the Hall-Héroult process is carried out on an almost ridiculous level, and the smelter is so large that it can be seen from space. Each steel reaction cell, called a pot, is lined with ceramics and has a graphite cathode at the bottom. The pot is filled with alumina powder and cryolite, and then the block composite anode is put into the mixture. The anode is mainly made of molten coke with a copper or steel frame to conduct the required current-hundreds of thousands of amperes-to electrolyze the solution.
The electrolysis reaction results in the formation of metallic aluminum at the anode of each cell. Molten metal is denser than electrolyte, so droplets will sink to the bottom of the pot and accumulate on the cathode there. The tank runs continuously, and it takes one to three days to accumulate enough molten aluminum. The liquid metal is discharged through the siphon, the expendable anode is replaced as needed, and another charge is added to the pot.
The aluminum coming out of the pot is about 99% pure aluminum and is usually cast into ingots or rods for further processing. Aluminum of this purity level is mainly used for food containers or as electrical conductors, such as overhead power lines. If higher purity metals are required, another electrolysis process called the Hoopes process can increase the purity to the "four nines" level (99.99%). All metals with a purity of 99% and above are called "1000 series" aluminum.
However, pure aluminum is generally not that useful in industry, so most aluminum alloys with other metals to obtain other properties. For example, 2000 series aluminum is mainly alloyed with copper to improve strength and toughness, and has found a way in aircraft manufacturing. 3000 series metals, such as 3003 alloy in piping systems and cookware, are alloyed with manganese to improve workability. Silicon and aluminum form an alloy to form 4000 series metals; the addition of magnesium will produce 6000 series metals, such as the popular 6061 and 6063, which appear in all products from aluminum extrusions to engine blocks.
I like articles like this. Why add bauxite? Especially considering that aluminum accounts for 8% of the earth's crust? The problem is that most of these 8% are locked in silicate minerals (such as feldspar in granite, etc.). Only where rocks like this have been weathered and the residue has accumulated as bauxite can we have economic deposits based on current mining technology.
It is also worth pointing out that aluminum is one of the most ideal recycled materials.
As an avid follower with obvious difficulties, I am happy to see their content enter one of my favorite sites!
Please be kind and respectful to help make the comment section great. (Comment Policy)
This website uses Akismet to reduce spam. Learn how to handle your comment data.
By using our website and services, you explicitly agree to the placement of our performance, functionality and advertising cookies. Learn more