Characterize aluminium using your words and word-combinations written out

ALUMINIUM

Read and translate the text. Write out all the words concerning the properties and characteristics of this metal.

Aluminium is a silvery-white metal. It has a molecular weight of 27 and atomic number of 13. The specific weight is about 2.7. The melting point of pure metal is about 660°C.

Although aluminium is the most abundant metal in the world, comprising some 8 per cent of the earth's crust, it is now produced commercially only from bauxite. Large deposits of bauxite occur in many parts of the world.

Unlike some of the other common metals, aluminium cannot be produced directly from its ore because the metals present as impurities would be carried through the proceedings and become alloyed with aluminium.

For this reason the ore must be first refined to exclude the impurities. This refining operation produces alumina (aluminium oxide) of high purity, from which pure metallic aluminium is obtained.

Chemically, aluminium ordinarily has a valence of 3+, and may be either acid-forming or base-forming. Thus, with the common acids it forms salts, such as chloride, nitrate and sulphate. With strong bases it forms aluminates. In aluminates, the aluminium oxide forms the acid part (NaAlO₂, KAIO₂).

The most important compound is the oxide Al ₂ O ₃. This exists in several crystalline forms. The most common and most important of them is corundum. Its extreme hardness makes it useful as an abrasive, for grinding wheels and emery powders. Most of the corundum used in emery wheels is made synthetically by smelting bauxite with carbon in an electric furnace. Transparent red or blue corundum crystals are precious stones known as ruby and sapphire.

Rubies are now obtained artificially by fusing alumina in an electric furnace. They are used not only for decoration but for technical purposes, such as for the manufacture ofprecision instrument parts, for the jewels in watches, etc. Non-transparent corundum crystals containing considerable impurities are commonly known as emery.

The principal elements used in making aluminium-base alloys are copper, zinc, silicon, magnesium, manganese, and nickel. Minor amounts of chromium, lead, titanium, zirconium and vanadium are sometimes used for special purposes.

Iron is always present as an impurity in commercial aluminium and its alloys, and has some effect on their properties. In some cases it may be considered as an alloying ingredient which is added for special purposes in amounts of 1 per cent or more.

Дата добавления: 2014-11-07