An overview of extracting aluminum direct from the ore
Aluminum is a precious metal that came to the limelight in the year 1825 thanks to Hans Christian Oersted, a Danish chemist, whose concerted efforts in an attempt to extract it from its mineral ore (alumina) marked a major milestone in its production. Nonetheless, its extraction process, in the better part of the nineteenth century, was extremely difficult rendering it a rare metal, and hence expensive. For its value, it was, in most cases, owned by the rich, and in particular, the kings. It was meant for making ornaments and utensils in palatial homes. Of note, aluminum does not occur naturally as a pure metal, it occurs inform of compounds including oxides (bauxite) and chlorides. As such, its extraction involves a complex task of separating it from these compounds.
Oersted’s works attracted interests from scientists including Friedrich Wohler, a German, who managed to extract the metal in a powdery form in the year 1827. Moreover, he further proceeded to establish its density, a core characteristic vital in its application in aerodynamics. In the year 1954, a French scientist, Henri Sainte-Claire Deville, commercialized aluminum production after improving Wohler’s methodology. In the meantime, because of its unavailability, aluminum had been ranked among the most precious metals found on the planet, relegating both gold and platinum in value. At one point, “in the year 1855, it was exhibited in a Paris Exhibition” (Harris, McLachlan & Clark 1998). However, a decade on following its commercial production, its value depreciated by more than 90%. Its value would further depreciate but gradually with advances in technology, and by the year 1985, worldwide production was achieving an output of 15 tonnes per annum.
Nonetheless, the actual revolution in extraction was witnessed a year later when two production processes were invented, simultaneously, in America and Europe. Paul Louis Toussaint Heroult, a French, and Charles Martin Hall, an American, isolated a pure aluminum metal from its molten state courtesy of electrolysis method. Consequently, this process, which is applicable hitherto, would later be christened after their surnames- Hall-Heroult Method.
Advances in technology would not cease yet. Four years later, “Karl Josef Bayer developed a novel and efficient method (the Bayer process) of production of aluminum from bauxite” (Harris, McLachlan & Clark 1998). This functioned to further reduce the price of aluminum significantly. As a result, commercial production increased and in the year 1900 “8 thousand tonnes were produced, in 1946 the output was 681 thousand tonnes, and in 1999 24 million tonnes of aluminum was made” (Harris, McLachlan & Clark 1998).
Aluminum is an excellent metal owing to its unique properties that include among others non-corrosive, and it is light. As a result, it has found application in a diversity of field including automotive, aerodynamics and artworks among others. Nevertheless, it has drawbacks derived from its health risk factors, for instance, it is alleged to be associated with Alzheimer’s disease. However, this remains to be an allegation that is yet to be proved. As such, the limitations of aluminum are rather insignificant hence; they can’t affect its preference. Vitally, its strength can be enhanced to any degree when mixed with other metals to form alloys. Noteworthy, aluminum is the most abundant metal compound present in the Earth’s crust, accounting for approximately 8% of the crust.
An overview of extracting aluminum directly from ore
Aluminum can be extracted directly from its ores that include among others feldspar, bauxites, clays, slates, shale, by-products derived from the Bayer process, and slimes derived from phosphate rocks. Bayer process is the initial process vital in extracting the alumina that is fed onto the Hall-Heroult process to obtain a pure metal of aluminum. Bayer process is a purification process that is aimed at separating the alumina from the ore thanks to caustic soda that, actively, attacks the complex compound. Nonetheless, the efficiency of the Bayer process is largely influenced by the nature of the ore. Because of the nature of the oxides formed among a diversity of elements and aluminum in the ore, the efficacy of the attack is compromised.
To this end, halogens that include chlorine and bromine serve as substitutes to caustic soda. When halogens are applied, halides of aluminum and other elements are formed and separated in a fashion similar to fractional distillation. Consequently, pure metal of aluminum is extracted from its halides via a thermal operation (disproportionation). As such, Hall-Heroult process is rendered immaterial.
How the ore is converted to the metal
The processes that result in the production of aluminum metal from its ore are rather complex than what is described in the preamble above. First, in order to determine the process to be executed it is necessary to know the nature of the raw material at hand. Bayer process can be used as a preliminary, purification process for Hall-Heroult process, or it can solely be used to extract the metal from the ore. The literature that follows will evaluate the two processes involved in the extraction of aluminum from bauxite (Othmer 1972).
How the ore is converted to the metal through Bayer process
Bayer process can solely be used to extract aluminum from bauxite. Contemporary processes require that the extraction happens at temperatures ranging between 1100 and18000 C, preferably greater than 15000C. The rational in this is that the temperature regime results in the creation of chlorinating agents vital in the reduction-chlorination reaction that follows, where Al2O3 is converted AlCl. Ideally, the reactions that happen in the chamber are shown by chemical equations below:
Cl2 + Al2O3 + 3C 2AlCl + 3CO
SiCl4 + 2Al2O3 + 4C 4AlCl + 4CO + SiO2
AlCl3 + Al2O3 + 3C 3AlCl + 3CO (Othmer 1972)
The reactions happen in the presence of carbon that also supplements on the heat requirements (when it reacts with O2) of the chamber vital in sustaining the reaction. Vitally, in the course of the reaction, care should be taken when adding chlorine and oxygen to the ore to maintain stoichiometric balance lest the process backfires (the resulting aluminum might be oxidized or chlorinated). To obtain the aluminum, the temperature of the chamber is decreased to below 10000 C where mono-valence halide of aluminum is converted to tri-valence plus aluminum metal. Notably, the reaction is exothermic as heat is liberated and might cause a temperature increase (above 10000C) in the chamber thereby reversing the reaction. The equation below is a manifestation of the reaction that happens in the chamber:
3AlCl (g) AlCl3 (g) + 2Al(s) + Heat (Othmer 1972)
The aluminum obtained flows into molds at temperatures above 6600C (melting point), cooled to form bars or sheets. The resulting gaseous halide (AlCl3) is condensed and then recycled back to the system to form more mono-valence halides. Nonetheless, the non-condensed halide is fed into the ore to accomplish an oxidation-chlorination reaction where iron is eliminated from the ore (see the equation below).
2AlCl3 + Fe2O3 2FeCl3 + Al2O3 + Heat
Nevertheless, the above reaction does not achieve 100% elimination of iron from the ore hence; it is necessary that additional chlorine be added to the reaction to achieve absolute elimination. The resulting compound of Al2O3 is fed back into reduction-chlorination reaction where the process recurs once more. Noteworthy, condensation of AlCl3 should be instantaneous lest undesirable reactions happen. To this end, equipment designed to achieve flash condensation are designed to avert such consequences.
How the ore is converted to the metal through Hall-Heroult process
Hall-Heroult process comes into effect when the raw material in question is alumina. This process is achieved in a reduction plant where a mixture of alumina and cryolite (sodium-aluminum fluoride), a molten concoction at a temperature of about 9800 C, serves as an electrolyte. Electrolysis happens in a series of electrolytic pots also referred to as a potline where the metal is extracted. The features of the pot include a carbon cell that acts as an anode and the pot lining that acts as a cathode. When in operation, a high amperage current flows between the two terminals (anode and cathode), consequently achieving its function of separating the metal from the compound. Ideally, at the anode, oxygen is liberated, reacts with carbon to form carbon dioxide whereas aluminum is formed at the bottom of the cell (cathode). Since aluminum is denser than the electrolyte, it forms at the bottom where it is siphoned away (see below) ().
Figure 1: Hall-Heroult process.
The overall reaction that takes place in the cell is shown by the equation below:
2Al2O3 (l) + C (s) 4Al (s) + 3CO2 (g)
An overview of the manufacturing process.
The Bayer process is a core industrial method for purifying bauxite to obtain alumina (Al2O3), a raw material for Hall-Heroult process necessary for extracting aluminum metal. Ideally, bauxite contains alumina in the range of between 30 to 54%. The rest are extraneous compounds of silica, iron and titanium. The initial step involves mixing of previously milled bauxite with hot, caustic soda, at temperatures of 1750 C (see in the flow diagram below).
This is achieved in a mixing tank that is supplied with steam at high pressure. As such, the bauxite will gradually dissolve forming a liquid compound, sodium aluminate (2NaAl (OH) 4), while the rest remain as residues (red mud). Remember, this is when halogens are not used. The corresponding chemical equation of whatever happens in the tank is shown below:
Al2O3 + 2NaOH + 3H2O 2NaAl (OH)4
Clarification by filtration is the next step where the red mud is eliminated. Sodium aluminate is then cooled, and as such, aluminum hydroxide precipitates leaving caustic soda as an aqueous solution. See in the equation below:
NaAl (OH)4 (aq) Al(OH)3 (s) + NaOH(aq)
To obtain alumina, the precipitate is heated to a temperature of 9800 C where all the water evaporates (see the equation below). The alumina then proceeds to the Hall-Heroult process to obtain aluminum.