tailieunhanh - Aluminium Design and Construction - Chapter 2

Tài liệu tham khảo giáo trình nhôm trong thiết kế xây dựng - Chương 2 Manufacture In this chapter we describe the manufacture of wrought aluminium products, such as plate, sheet, sections and tube, known in the trade as semi-fabricated products (or ‘semi-fabs’). The two main stages are production of pure aluminium ingot, and conversion of this into wrought material. | CHAPTER 2 Manufacture In this chapter we describe the manufacture of wrought aluminium products such as plate sheet sections and tube known in the trade as semi-fabricated products or semi-fabs . The two main stages are production of pure aluminium ingot and conversion of this into wrought material. PRODUCTION OF ALUMINIUM METAL Primary production The production of aluminium ingot comprises three steps 1 mining the bauxite ore 2 extracting alumina therefrom and 3 smelting. Bauxite is plentiful in many countries and is extracted by opencast mining. Alumina A12O3 a white powder is obtained from bauxite by the Bayer process which requires a large supply of coal and caustic soda. Roughly 2 kg of bauxite 2 kg of coal and kg of caustic soda are needed to produce 1 kg of alumina. In the smelting operation metallic aluminium is extracted from alumina by electolysis using the Hall-Héroult process. It takes place in a cell or pot comprising a bath of molten cryolite the electrolyte and carbon electrodes. A typical pot is around 4 or 5 m long. The cathode covers the floor of the bath while the anodes are in the form of massive carbon blocks which are gradually lowered into the cryolite as they burn away. Alumina is fed in at the top of the molten pool where it dissolves and molten aluminium is drawn off from the bottom. The metal emerges at a temperature of around 900 C and at a high purity . Copious fumes are produced. The electric supply is direct current at a typical potential of 5 V per pot the current requirement being high around 100-150 kA. Continual replacement of the carbon anodes is also a significant factor. The production of one kilo of aluminium consumes roughly 2 kg of alumina kg of carbon and 15 kWh of electrical energy. The cryolite Na3AlF6 is largely unconsumed. Copyright 1999 by Taylor Francis Group. All Rights Reserved. The main requirements for the production of aluminium are thus bauxite coal and cheap electricity. These are .