Produces Al NaOH Al OH 3

Big Bang HTL 2, textbook

Basics of Chemistry (2nd year, 3rd semester) 143 Metals 10 Protective layer made of aluminum oxide, which makes it very resistant. For this reason, doors, windows and facade cladding are made of aluminum in the construction sector. Another important area of ​​application is the packaging industry. Small amounts of aluminum are also processed in mechanical engineering and household products. Primary production (manufacture from ore) Around 40 million tons of aluminum are produced annually. This production comprises four sub-steps: - Extraction of the ore bauxite - Purification of the ore and formation of aluminum oxide - Production of cryolite Na 3 AlF 3 (not explained here) - Electrolytic reduction of aluminum oxide to aluminum Bauxite is one of the most common ores in the world. It occurs in large quantities - especially in Jamaica, Brazil, China and India. The aluminum is mainly in the form of aluminum hydroxide Al (OH) 3. Most of the mined bauxite is contaminated with iron oxides, silicon, titanium oxides and other heavy metal oxides. The ore is cleaned using the so-called “Bayer process”. The ore is mixed with hot 10% sodium hydroxide solution (4atm, 150 ° C). Under these conditions the aluminum salts dissolve while the impurities remain insoluble. The insoluble residue is filtered off, washed and the excess sodium hydroxide solution is recovered. The impurities must be disposed of as “red mud”, as they are called due to the iron compounds it contains. Fig. 10.20: Aluminum windows and aluminum cans Fig. 10.21: Scheme Bayer process The solution is meanwhile cooled, whereby aluminum hydroxide crystallizes out again. This is filtered off and converted into aluminum oxide by heating to over 1000 ° C. Reaction equations of the Bayer process: Dissolve: Al (OH) 3 (s) + NaOH (aq) ® NaAlO 2 (aq) + 2H 2 O Precipitation : NaAlO 2 (aq) + H 2 O ® Al (OH) 3 (s) + NaOH (aq) Convert to oxide: Al (OH) 3 (s) ® Al 2 O 3 (s) + 3 H 2 O ( l) Since aluminum oxide has a high melting point (2000 ° C), it is used for the electrolysis dissolved in liquid cryolite (900-1000 ° C). Electrolysis in aqueous solution is not possible due to the insolubility of aluminum oxide and the high voltage of over 4V. At this voltage, the water would absorb the e - instead of the Al +3 ions. The fused-salt electrolysis takes place in a graphite tank (see Fig. 10.23) in which the aluminum oxide is dissolved to about 5% in the molten cryolite. The negative pole is applied to the graphite tub. As anodes (positive pole), several graphite blocks dip into the melt from above. At the cathode (negative pole) Al takes up +3 electrons and forms elemental aluminum. It collects at the bottom of the tub and acts there as the actual cathode. Liquid aluminum is continuously drawn off and new aluminum oxide is topped up. At the anodes, the oxide (O -2) is oxidized to form oxygen, which at the high temperatures immediately reacts with the graphite anodes to form carbon dioxide. These are used up and have to be replaced regularly. Electrolysis reaction equations: Cathode: 4Al +3 + 12 e - ® 4Al Anode: 6O -2 ® 3O 2 + 12e - C + O 2 ® CO 2 (g) Modern aluminum electrolysis cells work with 150-300 kA at 4-4.5 Volts and produce around 1-2 tons of aluminum per day. A large aluminum plant consists of around 400 such cells. (F16) Fig. 10.22: Scheme of fused-salt electrolysis, aluminum For testing purposes only - property of the publisher öbv

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