化学化工专业英语(杨定乔 )15 Aluminium.doc

15 Aluminium15.1 Introduction This is probably the most abundant metal on the earth, as the oxide ,alumina Al2O3,makes up the chief part all clays and many other rocks. The metal was first isolated as a powder by Wohler. It was not until about 1854 that it was obtained on a manufacturing scale. Aluminum is a very light metal. Weighing only about 2.7 times that of water. And it is therefore only about one-third the weight of iron. It is good conductor of electricity ,and although inferior to copper for the long distance transmission of electric power owing to its lightness and lower cost.It forms a valuable alloy with copper, known as aluminum bronze. To obtain the bronze a mixture of corundum(alumina), charcoal and granulated copper is heated in an electric furnace. The carbon takes up the oxygen of the alumina, while the copper unites with the aluminum to form a golden-colored alloy which has great strength and elasticity. The development of the motor and aircraft industries led to the introduction of a number of aluminum alloys of great value owing to their lightness and strength. The most important of these are magnalium,containing magnesium, and duralumin ,which is aluminum with about 4percent of copper and a litter magnesium. Other alloys with copper, nickel and zinc are used in the construction of motor-car and aeroplane parts. Alloys of aluminum and silicon, with small amounts of other substances, are used in shipbuilding ,as after heat treatment they are very strong and resist the corrosive action of sea water. Aluminum has been made for many years by the electrolysis of alumina (prepared bauxite, Al2O32H2O)dissolved in fused cryolite (sodium aluminum fluoride the cathode. The current is brought in by thick carbon rods forming the anode which while carbon monoxide gas escapes. As the metal is removed the supply of alumina is kept up by adding more bauxite to the molten mixture. In the production of aluminum,as in so many other processes, the source of power is the energy of falling water. Factories have been built near many of the great waterfalls of the world such as Niagara. The Falls of the Rhine at Schaffhausen and in England at Kinlochleven in Argyllshire, and in North Wales. The bauxite used is chiefly from the south of France,but useful deposits occurring in the southern part of the United States, and Antrim, Northern Ireland. The remarkable increase in the use of aluminum during the past half century is shown by the following figures for the total world output of the metal.World output of aluminum1907 30，000 tons1919 136,000 tons1924 171,000 tons1929 270, 000 tons 1934 171,000, 000 tons 1950 1,417,000 tons The increasing use of aluminum is well shown by the above figures. Of this amount of nearly one and a half million tons produced in 1950s,the American output was 718,000 tons,while British production has increased from 15, 000 tons in 1936 to 33, 000 tons in 1959. Aluminum is valuable not only for its lightness but on account of its peculiar behavior towards acids and alkalis. It dissolves rapidly in dilute hydrochloric acid, but is only slowly attacked by sulphuric and nitric acids, and even less by alkaline solutions. Hence, although aluminum is used for making cooking vessels and for storage of many foodstuffs, it is well to remember that saucepans or pots of aluminum must not be cleaned by soda. In the air aluminum soon becomes covered with a thin, almost invisible, film of oxide, which protects the metal from further corrosion. This process of self-protection can be hastened and made more effective by anodic treatment. The metal is made the positive pole in the electrolysis of a suitable solution, and when oxidation occurs the resulting surface is immune to atmospheric corrosion. This anodic process is used for the treatment of aeroplane parts. Aluminum melts below a red head, and when heated in air it oxidizes readily. A thin piece of aluminum foil in a bottle of oxygen gas if touched with a red-hot wire disappears instantly with a brilliant flash, leaving a white oxide behind. When aluminum combines with oxygen more heat is given off than by any other burning metal. The readiness of aluminum to take up oxygen and the heat thus given off during the combination is used commercially to obtain metals which are difficult to get by other means. A mixture of aluminum heated with the oxide of another metal gives a violent chemical reaction. The aluminum is converted into oxide while the other metal is left in the metallic state. The action is so violent in some cases, copper oxide for example; that a kind of explosion occurs and part of the metal is volatilized. Chromium is obtained in this way in a pure state, as also manganese which previously had been known only in combination with carbon or iron. An ingenious application of this property of aluminum is found in the thermit process. A mixture of ferric oxide with aluminum powder is put in a crucible with a removable bottom. When a fuse is lit the whole .mass glows and in few minutes a layer of molten iron sinks to the bottom of the pot and can be run off into a mould. The method is used for the repair of broken castings or to join the ends of rails without removing them. A mould around the rail-end holds the melted metal, and after solidifying the excess iron can be cut or ground away. The temperature produced in the mixture is about 3, 500C, and is sufficiently high to melt every known metal. The thermit process is used in many incendiary bombs, as once the reaction is started it cannot be stopped, as many fire-fighters discovered during the air-raids on London and other cities in the last war.15. 2 Preparation of Aluminum In 1827 Friedrich Wohler of Germany secured aluminum in sufficient quantity by reduction with potassium to study its properties. By 1852 aluminum sold for $ 545 a pound. In 1859, Deville of France had perfected a method for extracting aluminum which reduced its price to $ 17 a pound. But it remained for Charles Martin Hall, in 1886, to perfect a satisfactory commercial method for extracting aluminum from its ore. Just nine months after graduating from Oberlin College, Hall obtained metallic aluminum by the electrolysis of a solution of alumina, Al2 03 , in molten cryolite, Na3 AlF6. Most of you already know that young Hall worked in a crude laboratory set up in his father s woodshed. At about the same time Paul Heroult in France perfected an identical process for producing metallic aluminum. Today, using refinements of the process, aluminum is produced at somewhat less than twenty cent a pound.The ore used in the electrolytic process is bauxite, a mixture of Al203 H20 and Al203 3H20. Its main impurities are iron oxide, silicon oxide, and titanium oxide, each of which must be removed before the electrolytic process. These oxides are removed by screening and washing, by magnetic devices and recently by froth floatation. Even this concentration or ore dressing does not yield aluminum oxide sufficiently pure for use in the electrolytic cells. The aluminum oxide used must contain neither iron nor titanium compounds when it is placed in the electrolytic cell. If either of these metals is present, it is reduced along with the aluminum, and an alloy results. The remaining impurities must be removed chemically. Sodium hydroxide, which reacts with neither iron nor titanium oxides, is added. It reacts with the aluminum oxide, forming soluble sodium aluminum. The impurities are filtered out and the solution is cooled. Aluminum hydroxide crystals are added. Under these conditions the sodium aluminate decomposes to Al(OH)3 and NaOH. The .sodium hydroxide is removed to be used again, and the aluminum hydroxide, Al(0H)3, is then heated in a rotary kiln. Heating decomposes it to aluminum oxide and water. Thus purified, the aluminum oxide, also called alumina, is ready for use in the electrolytic cell. The metallurgy of aluminum required large quantities of electric current, so the factories are generally located near sources of hydroelectric power. The cell used for preparing aluminum consists of a rectangular iron box lined with carbon which becomes the cathode. The anode consists of carbon rods which are suspended from bus bars above the iron box. Long rows of these cells are used, each cell producing about 500 pounds of the metal a day. Cryolite, Na3AlF6, is melted in the cell by the heat from an arc between the anode rods and the carbonlined box. The molten cryolite then dissolves aluminum oxide, which is added in amounts to make about 5 percent of the weight of the cryolite. Additional batches of the oxide are added at regular intervals. Theoretically, the cryolite serves only as a solvent