Why is bauxite red

Elements, school book

124 5 LARGE-SCALE CHEMISTRY Aluminum Although aluminum is the most common metal in the solid earth's crust, its extraction from the ubiquitous aluminosilicates is not economical. Bauxite, an aluminum oxide or hydroxide with heavy impurities, mainly SiO 2, iron oxide and titanium oxide (Al 2 O 3 content approx. 60%), is used as ore for the extraction of aluminum. Since aluminum is one of the very base metals, it can only be obtained by electrolysis. To do this, the bauxite must first be converted into pure aluminum oxide, since otherwise the more noble impurities with aluminum would be electrolytically deposited, which makes it impossible to obtain pure aluminum. Bauxite cleaning is carried out worldwide using the Bayer process, which was developed in 1892 by the Austrian Carl Bayer (1847–1904). Bayer process Bauxite is finely ground and digested under pressure with hot, approximately 40% sodium hydroxide solution. In the process, aluminum oxide and aluminum hydroxide change into a soluble hydroxo complex, sodium aluminate. The impurities remain undissolved and are filtered off from the hot aluminum hydroxide solution. They are colored red by iron hydroxide, which is why they are called red mud. They are a landfill product for which there is hardly any use. About one ton of red mud is produced per ton of Al 2 O 3. In order to recover aluminum hydroxide from the purified aluminate liquor, it is cooled, slightly diluted and inoculated with aluminum hydroxide to accelerate crystallization. As a result of the dilution, the pH value drops and the equilibrium shifts to the side of poorly soluble aluminum hydroxide (reversal of the reaction during bauxite digestion). This is filtered off and the resulting sodium hydroxide solution is returned to the digestion process. The aluminum hydroxide is dehydrated (= calcined) to aluminum oxide in a rotary kiln (Fig. 124.2). Electrolysis The aluminum oxide ("alumina") purified in this way cannot be electrolyzed directly because its melting point is over 2000 ° C. During electrolysis, it is dissolved in a melt of a lower melting salt. Cryolite Na 3 [AlF 6] is used for this. This melts at around 1000 ° C, a mixture of cryolite with around 8% Al 2 O 3 at 950 ° C. Cryolite does not interfere with the electrolysis because both the Na + ion on the cathode and the fluoride ion on the anode require a higher deposition potential than aluminum and oxygen. The melt is electrolyzed in a tub made of graphite bricks, which also acts as a cathode. Graphite anodes dip into the melt from above. A supply of the specifically lighter Al 2 O 3 floats on the melt and dissolves to the extent that it is consumed in the electrolysis (Fig. 125.1). The electrolysis does not produce oxygen, but carbon dioxide and the graphite anode is consumed. A mixture of tar and petroleum coke is therefore used as the anode material, which sintered to graphite at the temperature of the melt, with the tar decomposing. In this way, the anode can be pushed in automatically and the electrolysis does not have to be interrupted to change the electrode (Sø derberg electrode). Aluminum collects in liquid form at the bottom of the tub and is extracted from there at regular intervals. It has a purity of 99.5-99.8%, which is sufficient for most uses. The electrolysis is operated with a voltage of around 5 V and the current is up to 150,000 A. This creates so much heat that the melt remains liquid by itself. The production of aluminum is therefore very energy-intensive. For 1 t of aluminum, about 4 t of bauxite, 0.5 t of electrode material, 20 GJ of thermal energy for bauxite cleaning and 13.5 MWh (corresponding to 49 GJ) of electrical energy for electrolysis are required (Fig. 125.2). Mp: 660 ° C density: 2.7 g / cm 3 hardness (Mohs): 2.75 Fig. 124.1: aluminum: data aluminum 27.0 13 EN 1.5 Al digestion of bauxite: Al (OH) 3 + Na + + OH - → Na + + [Al (OH) 4] - calcining: 2 Al (OH) 3 → Al 2 O 3 + 3 H 2 O cathode: 4 Al 3+ + 12 e - → 4 Al anode: 3 C + 6 O 2– → 3 CO 2 ↑ + 12 e - C + CO 2 → 2 CO ↑ Fig. 124.4: Electrode reactions in electrolysis Fig. 124.3: Reactions in the Bayer process Fig. 124.2: The Bayer process calcining washing Water Al (OH) 3 Heating Drying Red Mud Bauxite Alumina Grinding NaOH F Filter For testing purposes only - property of the publisher öbv

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