Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world’s main source of aluminium. In 1821 the French geologist Pierre Berthier discovered bauxite near the village of Les Baux in Provence, southern France. Australia is the largest producer of bauxite, followed by China. Bauxite is usually strip mined because it is almost always found near the surface of the terrain, with little or no overburden. As of 2010, approximately 70% to 80% of the world’s dry bauxite production is processed first into alumina and then into aluminium by electrolysis (see below for Primary Aluminum Smelting). Bauxite rocks are typically classified according to their intended commercial application: metallurgical, abrasive, cement, chemical, and refractory. Usually, bauxite ore is heated in a pressure vessel along with a sodium hydroxide solution at a temperature of 150 to 200 °C (300 to 390 °F). At these temperatures, the aluminum is dissolved as sodium aluminate (the Bayer process). Eventually, the Bayer process will crate the aluminum oxide compound (Alumina) and is shipped to the primary smelting furnace for processing.
In 2017, China was also the top producer of aluminium with almost half of the world’s production, followed by Russia, Canada, and India. Although aluminium demand is rapidly increasing, known reserves of bauxite ore are sufficient to meet the worldwide demands for aluminium for many centuries. Increased aluminium recycling, which has the advantage of lowering the cost in electric power in producing aluminium, will considerably extend the world’s bauxite reserves.
Depending on the mining site location, the bauxite may be shipped after crushing or it may be processed as the crystallization seed or dried and shipped as Aluminum Oxide (Al2O3).
Primary Aluminum Smelting
Aluminum is a very common element that can be extracted from earth materials (bauxite) using the long standing “Hall-Héroult” industrial electrolysis process. In this system, a cryolite material is created and subjected to electricity passing from an anode to a cathode. In the Hall-Herloult process illustration above, the minerals are processed in a high temperature furnace so the pure aluminum can be electrolytically extracted from the Cryolite solution. Molten cryolite (Na3AlF6), the major constituent of the aluminum smelting cell electrolyte, has the unique ability to dissolve oxides including alumina. Other important physical properties of the electrolyte include high electrical conductivity for low power consumption, low liquidus temperature for low heat loss and improved current efficiency, and low density to prevent the aluminum product from floating.
The mineral cryolite is a double fluoride of sodium and aluminium and has a stoichiometry very near the formula Na3AlF6 and a melting point of about 1010 °C. It has been found in substantial quantities only in Greenland, and was mined extensively there in the early twentieth century, but the mine is now essentially exhausted. Synthetic cryolite can be produced by reacting hydrofluoric acid with an alkaline sodium aluminate solution:
Due to the rarity of known cryolite deposits, it is possibly the only mineral on Earth ever to be mined to commercial extinction. While not commercially mined, Cryolite has also been reported at Pikes Peak, Colorado; Mont Saint-Hilaire, Quebec; and at Miass, Russia. It is also known in small quantities in Brazil, the Czech Republic, Namibia, Norway, Ukraine, and several U.S. states.
6 HF + 2 NaOH + NaAlO2 = Na3AlF6 + 4 H02O
The actual aluminium is made by dissolving the oxide into the molten salts, at about 960°C, and applying a current to help break down the oxide. As previously described, this is the Hall-Heroult process, which dominates world aluminium production at the moment. The process typically loses around 50% of the incoming energy as low grade heat, which is due in part to the fact that the salts required to dissolve the oxide are so corrosive that there no practical way to keep the heat in. There are also great challenges in the anode and cathode technology in the Hall-Heroult process that result in large resistance losses. At the time of writing this article, the lowest energy consumption process on record is the HAL4e process piloted in 2017 by Hydro Aluminum company in their Norwegian Karmøy Technology Pilot plant. Boasting a 15% improvement and lower CO2 emissions, the Karmøy plant operates with an energy consumption of somewhat below 12 kWh/kg Al produced in the 75,000 Ton/yr plant, using 60 electrolytic cells.
Primary aluminum smelting plants are, by necessity, located near abundant sources of lower cost energy resources. In the NAFTA region, we see most primary smelters located where water power has been harnessed to produce electricity that can be readily applied to the electrodes in the smelting furnaces.
Continuously cast T-Bar Ingots
Primary aluminum smelting plants that produce aluminum for remelting at casting operations use a continuously cast process that generates a "T-Bar" shape. The continuous bar is cut across the section to a length where the weight is typically 750 Kg (1650 lb) per ingot. This shape allows for safe handling as the ingot is placed inside a reverberatory melting furnace, where the ingot is set onto a shelf and allowed to heat up before pushing it into the molten bath of aluminum.
Secondary Aluminum Smelting
Secondary smelting operations are considered much more energy efficient and operate at temperatures where resources such as gaseous fuels (natural and propane), electricity (via resistance heating elements) or even oil-based fuels. These plants are designed to recycle aluminum scrap and prepare an alloy composition consistent with customer needs. During my work in industry, I was able to witness the efficient recycling of consumer goods (beverage cans), automotive related castings gathered at junk yards (wheels, suspension, engines and transmissions), and machining chips. Well organized secondary smelting plants have the ability to segregate iron-base materials by using magnetic separation. Water and oils are removed by lower temperature drying ovens and non-metals are removed by hand or cautiously burned away. In the NAFTA region there are many, many scrap collection alternatives for individuals to sell their scrap metals. These collection sites separate materials (often based on spectrographic analysis results from direct measurements) and wait until a reasonable amount of material has been collected before selling it to a smelter or another broker. The impressive economic fact about scrap metal collection sites is they are working business entities and present no community burden, as a garbage collector would.
Beverage can recycling is frequently used as an example of how a disposable metallic packaging commodity can have very high recycling rate.
In a 2017 study commissioned by the Can Manufacturers Institute (CMI), Beverage Can Makers Europe (BCME) and Abralatas in Brazil, the global weighted average recycling rate for aluminum cans was 69%. By country, the study identified aluminum recycling rates at:
- Brazil: 98%
- Poland: 79%
- Japan: 77%
- Italy: 72%
- USA: 55%
Considering that an aluminum beverage can is considered disposable, these recycling rates are excellent. According to the Aluminum Association, “recycling aluminum saves more than 90 percent of the energy that would be needed to create a comparable amount of the metal from raw materials.” As a result of outstanding efforts in collecting aluminum scrap for recycling, nearly 75 percent of all aluminum produced is still in use today. While some of this success is attributable to government and community collection programs, its the high scrap value for aluminum (about 50% of buying new) that promotes the desire to recycle.
Because the value of aluminum wheel scrap is double the amount for sheet stock and old parts, the recovery rate is very high. Since all aluminum wheels are cast from A356 alloy, and it is very easy to remove iron contamination by magnetic separation, crushed wheels are commonly recycled in a reverberatory furnace with no requirement to modify the composition.