Aluminum Production: from bauxite mining to chemical processing

I have always been interested in metalworking and crafting, and one of the more prominent experiences I have had with this line of work was when I created an aluminum ingot in high school. The process in which this ingot was made involved the use of a homemade, charcoal-powered furnace. The process of constructing the furnace consisted of filling a bucket with a mixture of plaster, sand, and silica powder; these are all highly heat resistant materials and can withstand high temperatures. An indentation was made with a smaller vessel and left until the filling hardened somewhat. Afterward, a hole was drilled in the side, and once it was fully hardened, a metal tube with an air outtake was placed through the hole. Our crucible was essentially just a fire extinguisher cut in half; just a sturdy steel cup to hold the metal. We put charcoal at the bottom and used the air outtake to heat up the hot coals and bring the crucible to temperature and we put cans in until they melted. Once the metal was all liquid, we poured it into an ingot mold to cool. This was a simple process that some Highschool kids were able to complete, and It made me think of the larger process that goes into creating aluminum ingots on an industrial scale, and all of the uses it has.

My ingot was made of aluminum beer and soda cans that I found in the forest behind my home. When people think of aluminum, this is probably one of the first things that come to mind, and they are probably seen as nothing more than a commonplace object. Soda cans, “tin” foil, kitchen utensils, smartphone bodies, laptop frames, are all common uses for aluminum. 

The next question we can ask is, why aluminum? It really comes down to a matter of both chemistry and economics. Aluminum is famous for being extraordinarily lightweight, while also possessing a high tensile strength; “Aluminum has a tensile strength of 276 MPa and a density of 2.81g cm3.” while “Stainless steel has a tensile strength of 505 MPa and a density of 8 g cm-3” (Aluminum vs Stainless Steel). Going back to the example of a simple soda can, we can see that the structural ingenuity of these cans is quite a marvel. A room temperature soda can typically hold a pressure of around 50-60 psi and is rated for pressures of up to 90 psi, a pressure that is almost 6 times that of standard conditions. This is all possible despite the fact that an aluminum can is usually only a millimeter in thickness. A scuba tank shows us a similar example, these tanks are usually only around 15 millimeters in thickness but hold almost 3,000 psi of pressure. 

Aluminum’s lightweight nature has led it to be the favored material in airplanes, trains, and even space crafts for many decades. 

“Aluminum is also known as the ‘winged metal’ because it is ideal for aircraft; again, due to being light, strong and flexible. In fact, aluminum was used in the frames of Zeppelin airships before airplanes had even been invented. Today, modern aircraft use aluminum alloys throughout, from the fuselage to the cockpit instruments. Even spacecraft, such as space shuttles, contain 50% to 90% of aluminum alloys in their parts.” (Metal supermarkets)

This being said, there is one large exception to this trend, automobiles. Automobiles have traditionally used steel in their construction, and still do to this day. However, “experts predict that the average aluminum content in a car will increase by 60% by 2025” (Metal supermarkets), due to its cheapness and structural benefits. 

All of the previously mentioned examples show the physical and tensile properties of aluminum on a large scale. However, if we look at some of aluminum’s lesser-known uses, we can see that the chemical properties of this metal play a big role in its widespread use. Many metals are known to be “ductile”, that is to say, that they have the ability to be molded and shaped without being subject to brittle deformation. This is typically important when concerning the production of cables and wires. Most metals are highly conductive, and aluminum is no exception. While the ideal conductors used in electronics typically tend to be copper, silver, and gold, aluminum holds its own with its specific niche, cheapness. At the time of writing, the market price of gold is 1,791 dollars per ounce, silver is 23.27 dollars per ounce, copper is 27 cents per ounce, while aluminum is only 8 cents per ounce (Daily Metal Spot Prices). Silver is technically the best conductor of electricity, however, it is extremely prone to oxidation and tarnishes, making it unsuitable for typical electronics. Gold is highly resilient but is very expensive to use in large quantities. Copper is the prime choice for most electronics, but it too is susceptible to oxidation. Aluminum, while a worse conductor than copper, outshines it in the context of the conductivity to weight ratio. It is also very resistant to typical means of corrosion, this is due to the fact that aluminum forms a thin layer of oxide on its surface that keeps the corrosion from penetrating any deeper than the skin. Most electrical transmission cables are aluminum, as they are typically situated outside, and need to be able to deal with changing weather conditions (Why Are Power Lines so Dangerous?). Given all of this, it is easy to see why aluminum has become the dominant choice of material for many purposes, both industrial, and commonplace. 

Mining and Manufacture

The presence of aluminum is something that most people likely take for granted, after all, it is a clean metal that poses no health issues like lead or zinc. Most people likely have some idea as to the production process of aluminum, let alone any metal. A simplified explanation for the processing of most metals involves a few steps: location sampling, raw ore mining, and finally purification and smelting. However, like most things, the process is a little more complicated than it first seems.

Starting with the process of mining, we need to look at the locations from which aluminum ore is primarily sourced as well as the nature of the ore itself. The primary ore of aluminum is a rock called Bauxite.

Bauxite is a sedimentary rock, rather unremarkable in appearance, with a rusty red to brown color; this red hue comes from iron ores mixed in with the aluminum. The most prominent feature of it is the “nodules” of aluminum minerals found inside. Bauxite as a whole is composed of many minerals, the main constituents are Iron and Aluminum Oxides, but there are a few notable trace elements like lead, titanium, mercury, etc.

 The 5 largest producers of Bauxite in the world are, Australia, China, Guinea, Brazil, and India; of these countries, Guinea has the largest reserves at almost seven million, thousand tonnes (Bauxite and Alumina). That being said, the prime minister of Vietnam claims that they have nearly 11,000 megatons of bauxite ore, making it, possibly, the largest untapped reserve of bauxite in the world.

Fig 1b. Bauxite ore and Bauxite compositions (Review of Bauxite Residue)

Bauxite is mined through fairly conventional means, primarily strip mining. Strip mining is a blanket term for any mining that involves a “full” excavation of the soil and whatever lies underneath. Most ore is found near the surface, so it is simply removed from the ground through diggers and trucked off to a processing facility.

Fig. 2: Bauxite mine in Guinea (Alufer Mining, Bel Air mine)

It is at this stage where we start to see the beginnings of controversy. Bauxite mining is done primarily in underdeveloped countries, and as such are highly subject to corporate exploitation and exploitation, and endangerment of the community as a whole. 

We can take a look at one particular instance of mining to get a good picture of some of the negative effects that come from bauxite mining. A study done in 2016 by doctor Noor Hisham Abdullah looks at the “Potential Health Impacts of Bauxite Mining in Kuantan”. This study was conducted in Malaysia, a country not typically known for high bauxite production, but still a third-world country in many regards. Bauxite mining, like many other forms of resource acquisition, is a highly profitable business, so it is easy to see why companies would flock to pursue such an endeavor. 

“Bauxite mining in Kuantan offers some exciting economic opportunities for various parties including individual landowners. Nevertheless, the “bauxite boom”; the extensive and uncontrolled mining activities have great potential to cause adverse impacts on the environment, health and quality of life of the people living in the affected areas.” (Abdullah)

This study observed a few key points, but the ones of interest are its impact as a pollutant, and subsequently its impact on health.

Pollution is something that must always be considered when undertaking any sort of mining project. In the United States, most mines are far from the public eye, and they would face harsh legal and social repercussions if they were not. However, this is not the case in these third-world countries.

Fig. 3 “Mining activities occurring close to school area” (Abdullah)

Fig. 4 “Dust deposited on floor of the school” (Abdullah)

In these two photos, the direct impact of Bauxite mining pollution is undeniably clear. The local school is right next to some of the mining operations, and it is clear that large amounts of dust, and silt are accumulating within the building itself. This brings up the question of toxicity and potential harm. Abdullah states:

“The processes of excavating, removal of topsoil and vegetation, transportation of bauxite and unwanted elements and stockpiling of bauxite cause degradation of air quality mainly related to dust pollution. Dust is a solid particulate matter, in the size range of 1 to 75 microns in diameter. Dust smaller than 10 micrometers in diameter, known as particulate matter PM10 and PM2.5 are of great health concern because it can be inhaled deep into the respiratory system” (Abdullah)

These small particles are often unseen by the human eye; the visible particles are seen mostly as a nuisance as they coat nearly everything in an orange-brown skin. This is far more prominent when looking at the water supply:

Fig. 5: “Bauxite washing pond showing red water” (Abdullah)

The health impact of heavy metals in water is extremely well documented and this case is no exception, 

“Because of its composition, aluminum and iron are the main contaminants that pollute the water resources but depending on the geological characteristics of the land and surrounding land use activities, other toxic metals such as arsenic, mercury, cadmium, lead, nickel, and manganese may also contaminate drinking water resources when the natural ecosystem is aggressively removed and excavated. Chronic exposure to toxic metals may cause multiple organ toxicity and increase cancer risk. Whereas, high-level exposure to aluminum in the stomach prevents the absorption of phosphate, a chemical compound required for healthy bones and may cause bone diseases in children.” (Abdullah) 

This is but one documented case of Bauxite mining and its potentially life-threatening consequences. The study looked mainly at society as a whole, but it is clear that individuals directly involved in these processes, such as miners and workers, suffer far more from these negative effects. It is true that many of these workers make their living from their work in the mines, but the potential health risks may quickly overturn any “profit” there is to be made. Governments in these countries are often more concerned with their own revenue and production than the safety of their workers and people. In the country of Guinea, it is clear that “the government’s focus on growing the bauxite sector has at times appeared to take priority over social and environmental protections” (What Do We Get out of It). 

The unfortunate reality is that these mining operations are likely to continue into the far future. Aluminum is too precious of a commodity for it to ever go out of demand, and even if we are able to stop, or regulate mining in countries like Australia, countries like Guinea and China will have no qualms about continuing their mining spree. In a way, our demand for almost any inorganic product fuels this process: cars, phones, planes, trains, computers, electrical wires, construction materials, these are all arguably essential in our modern lives, and there will never cease to be a demand for these products.

One thing to consider with the production of aluminum is the economic viability of these mining operations. Bauxite ore falls in the price range of about 50$ per metric ton, and Guinnea exported around 88 million tons of ore last year bringing the total price to more than four billion dollars (Bauxite prices). When looking at the processing end of things, most figures show that it takes “approximately 4 to 5 tonnes of bauxite ore to produce 2 tonnes of alumina. In turn, it takes approximately 2 tonnes of alumina to produce 1 tonne of aluminum” (Aluminum facts). The highest demand for aluminum actually comes from Asia, mainly China. China alone makes up more than 56% of the world’s aluminum demand. This is reasonable given the large amount of industrial activity that China promotes. We have seen the exploitative measures that are taken in the mining industry, and it is clear that a similar process is occurring in the manufacturing side of things as well.

Even if we can reduce our consumption of these goods, the very structure of our society depends on aluminum and bauxite resources. That being said, there it is important to recognize all aspects of production and consumption, mining is simply one step of the process. Processing and manufacturing is an entirely different sequence that must be considered, and perhaps through this, we can evaluate the process as a whole.

Processing and Production:

We have seen the process of mining up to this point, but that is but one part of the sequence. Next, we can look at the actual process of the production of aluminum. At this point, we have a large amount of raw Bauxite, and there are a few steps that must be taken to reach the purified metal. First, the bauxite must be crushed and washed, then any excess silica is removed. At this point, the ore is mixed with a soda solution and heated in a pressure tank. The following processes are quite complex and an explanation of the entire chemical process is unnecessary in this context. A short description of the chemical processes involved is shown here:

Al2O3 + 2 NaOH > 2 NaAlO2 + H2O (This is the process of dissolution with the soda)

Once this reaction has occurred, bauxite residue can be separated from the solution through a sedimentation process.

The alumina can then be crystallized from the solution via a precipitation process which carries out the following reaction:

Al(OH)4– + Na+ → Al(OH)3 + Na+ + OH–

Coarse crystals are then removed through classification, and processed in a calciner or rotary kiln to remove bound moisture and yield alumina in the following reaction:

2Al(OH)3 → Al2O3 + 3H2O (Feeco)

The resulting aluminum is pure enough to be melted and cast into whatever shape is required. This process is the standard procedure for aluminum production and it is known as the Bayer Process, but it too has its downsides, “For every ton of metallic aluminum produced, around two tons of red mud are also produced, with annual production at around 30 million tons per year” (Feeco). This so-called “red mud” is a highly alkaline slurry of various oxides and is quite toxic to most organic life. This red mud can be dealt with in a couple different ways but the most traditional method is simply to keep it in a large vat or holding area. These areas were often the remnants of other mines, or ponds and lakes. Before 2016, large quantities of red mud were simply discharged into estuaries or directly, but this was put to a stop due to the environmental repercussions. 

Fig 6. Red Mud Pit in Germany 

At the current moment, there is not much use for this red mud, and it has become a major environmental problem. There is a great deal of ongoing research to try to find a use for red mud, such as element recovery, or use in cement, and ceramics, but for the most part, red mud remains a massive environmental concern. 

Production and Manufacturing

Aluminum manufacturing is fairly simple, it is a metal with a low melting point and can be molded and cast into whatever shape is necessary. That being said, many aluminum products are actually made of alloys, mixtures of metals. Various alloys are used for different needs; alloys are classified with a 4 digit number, the first digit indicates their general use cases. For example alloys with the numbering of 2, 3, 4, or 7 are suitable for general purpose castings: “Aluminium ingots are produced in various shapes and sizes depending on their end-use. They may be rolled into plates, sheets, foil, bars, or rods. They may be drawn into the wire which is stranded into the cable for electrical transmission lines. Presses extrude the ingots into hundreds of different useful and decorative forms or fabricating plants may convert them into large structural shapes.” (Aluminum Association). These ingots are shipped to factories around the world and molded into the many products that are sold to both industry and consumers. 

Overall, the importance of aluminum as a resource is something that may be apparent at first glance. There are many debates in our current day over switching to “green” alternatives when considering things like energy. In the case of clean energy, one can argue that there are acceptable, theoretically practical, alternatives. Solar and wind power may not be the best choice at the current moment, that being said, further development of these technologies may produce a viable alternative. But with the case of mining products, such as aluminum, there is no alternative. These resources will always be in demand, and while we may find ways to make the process more “eco-friendly”, it is unlikely to counteract the exponential increase in demand for these resources. Aluminum is one of the backbones of our society, and it likely isn’t going anywhere in the foreseeable future. 

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