Reynolds Metals

How is aluminium produced by electrolysis?

That's all we need to know. How is aluminium produced by electrolysis?

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1. In chemistry and manufacturing, electrolysis is a method of separating chemically bonded elements and compounds by passing an electric current through them. Electrolysis involves the passage of an electric current through, in general, an ionic substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at the electrodes. The positive electrode is called the anode, and the negative electrode is the cathode. To be useful for electrolysis, the electrodes need to be able to conduct electricity, and metal electrodes are generally used. Graphite electrodes and semiconductor electrodes are also used. (Both of the electrodes used in the electrolysis of aluminium oxide are carbon.) In electrolysis, an ionic compound, or a compound that reacts with the solvent to produce ions (such as an acid) is dissolved in an appropriate solvent, or an ionic compound is melted by heat. Then some free ions exist in the liquid. An electrical potential is applied between the pair of electrodes immersed in the liquid. Each electrode attracts ions that are of the opposite charge. Therefore, positively-charged ions (called cations) move towards the electron-emitting (negative) cathode, whereas negatively-charged ions (termed anions) move toward the positive anode. The energy required to separate the ions, and cause them to gather at the respective electrodes, is provided by an electrical power supply. At the electrodes, electrons are absorbed or released by the ions, forming a collection of the desired element or compound, in this case Aluminium hydroxide. Aluminium is a strongly reactive metal that forms a high-energy chemical bond with oxygen. It is difficult to extract from ore, such as bauxite, due to the energy required to reduce aluminium oxide. Because aluminium oxide has a melting point of about 2,000 °C, it must be extracted by electrolysis. In this process, the aluminium oxide is dissolved in molten cryolite and then reduced to the pure metal. The aluminium oxide (a white powder) is obtained by refining bauxite in the Bayer process. In the Bayer process, bauxite is digested by washing with a hot solution of sodium hydroxide at 175 °C. This converts the alumina to aluminium hydroxide, which dissolves in the hydroxide solution. The other components of bauxite do not dissolve. The solution is clarified by filtering off the solid impurities. Next, the hydroxide solution is cooled, and the dissolved aluminium hydroxide precipitates as a white, fluffy solid. When then heated to 1050°C (calcined), the aluminium hydroxide decomposes to alumina, giving off water vapor in the process. Once the refined alumina is dissolved in the electrolyte, its ions are free to move around. At the anode (positive electrode), oxygen is formed. This carbon anode is then oxidized by the oxygen, releasing carbon dioxide. At the cathode (negative electrode) the aluminium ion is being reduced (electrons are added). The aluminium metal then sinks to the bottom and is tapped off.