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The Story Of Electrical Energy, Pt.23 This month, we present the second story on the production of aluminium. We look at how alumina is separated from the bauxite ore in a chemical process which takes lots of energy, in the form of electricity, coal & natural gas. By BRYAN MAHER The Bayer process to extract alumina from bauxite ore and the HallHeroult process of electrolytic smelting of alumina to obtain the pure metal have remained the backbone of the aluminium production industry for over 100 years. However, many advances have been made in these process and the amount of energy needed has been significantly reduced. Australia is the biggest producer of alumina in the world and much of our production is shipped to smelters in other countries. Alumina and bauxite together rank fourth (after coal, wheat and wool) in Australia's annual exports. Worldwide, nearly 100 million tonnes of bauxite are refined annually by the Bayer chemical process. These are the digesters & flash tanks. The digesters dissolve the bauxite in caustic soda under conditions of high pressure & temperature. The flash tanks then produce considerable amounts of steam as the resultant slurry is reduced to atmospheric pressure in several stages. 86 SILICON CHIP And the world's largest refinery happens to be the Queensland Alumina Ltd plant at Gladstone in Central Queensland. This plant is run by the consorti um QAL (Queensland Alumina Ltd), formed in 1967. Initially, CRA joined with Kaiser Aluminium of the USA to form the Commonwealth Aluminium Corporation (later renamed Comalco) in 1956 to develop the Weipa bauxite deposits. In choosing a site for the alumina refinery, Weipa was considered first but rejected as unsuitable. The present site at Parson's Point, Gladstone was purchased in 1963, leaving room for future expansion. Alcan of Canada and Pechiney of France then joined the consortium and construction of the alumina refinery comrn'enced. The original Gladstone plant was designed to produce 360,000 tonnes of alumina a year but immediately plans were put in hand to upgrade it to 600,000 tonnes. From 1967 to 1973, three further expansions of the plant took place. The third expansion increased the production of alumina to 2,400,000 tonnes per year, a sevenfold increase on the original design. Today the plant produces 3,325,000 tonnes of alumina annually, employs a workforce of 1200 and receives eight million tonnes ofbauxite by ship from Weipa each year. To refine this ore, 630,000 tonnes of caustic soda and 140,000 tonnes of limestone are required. The alumina produced is shipped to aluminium smelters in Tomago, NSW; in Bell Bay, Tasmania; and in New Zealand, the USA and Canada. Alumina ore is also sent by conveyor belt to the giant smelter at nearby Boyne Island. The Bayer process In 1888, the Austrian chemist Karl Bayer discovered that alumina could be dissolved in a solution of sodium hydroxide (caustic soda), given sufficiently high temperature and pressure. Thus, the Bayer refinement process is essentially one of dissolving alumina in caustic soda to separate it from the other chemicals in bauxite, and then crystallising the alumina from the solution after the red mud has been settled out. A typical analysis of bauxite is 50% aluminium oxiqe, 12% ferric oxide, 5% silicon dioxide, 3% titanium oxide, 0.03% alkalis and 0.01 % chlorides, plus small quantities of gallium and rare earth metals. The alumina contained in bauxite may be either the monohydrate form Boehmite (Al 2 0 3 .H 2 0), or the trihydrate type Gibbsite (Alz0 3 .3H 2 0). The deposits at Nhulunbuy are 98% Gibbsite, while the Weipa field has separate deposits of both types, the Boehmite predominating. The refining plant at Gladstone has been designed to accept both types of ore, but separately. Grinding mills The bauxite ore is first ground in seven two-compartment combined rod/ball mills. These mills are the largest electro-mechanical units in the plant, with motors ranging from 1.ZMW to 1.6MW. Because the grinding mills run continuously, synchronous motors are preferred. These motors draw a leading power factor from the electricity mains. This is used to compensate for the lagging power factor drawn by induction motors elsewhere in the plant. Thus, the overall system power factor is improved which means better utilisation of the Gladstone power plant and the overall distribution system. A 10% solution of caustic soda in water is added in the mills to form a thick slurry which is pumped out to the pre-treatment tanks. These are heated to allow silication to take place. The silica products pass out through the system as sodium aluminium silicate waste compounds. High-temperature digesters From the pre-treatment tanks, the caustic/bauxite slurry is pumped into high temperature digesters where steam is injected to heat the mixture to above 240°C. To prevent boiling, the digester tanks are pressurised vessels , operating above 3500kPa (about These two aerial photographs show the extent of the world's largest alumina refinery at Gladstone in Queensland. It produces over three million tonnes of alumina each year. 500 psi) . Under these conditions and with agitation , the bauxite breaks up. The alumina content dissolves in the caustic solution, while the iron oxide and other niinor compounds remain as solids in suspension. After digestion, 70% of the bauxite mass is in solution in the caustic liquor, while the remaining 30% undissolved waste takes the form of a thin red mud suspension. This liquor is now flash-cooled to atmospheric boiling temperature and pressure. Considerable amounts of steam are generated as the pressure is reduced in stages down to atmosphere and this steam is used later in the process to preheat the liquor coming from the settling tanks . The final steam condensate is returned to the Gladstone power station boilers as feed water and some is used MAY 1993 87 These are the huge rotary kilns which dry the alumina slurry after crystallisation has taken place. The kilns are continuously rotated by 200kW induction motors through a multistage gearbox & are gas fired to 1100°c. for washing the red mud waste from the settling tanks. Sweetening The more soluble trihydrate type bauxite lode at Weipa is separately mined and shipped to Gladstone. Passing through its own grinding and pretreatment facility, the Gibbsite is easily digested at lower temperatures and the additional bauxite slurry is injected into the main stream in the flash tanks to maximise the alumina content in the liquor. This mixing, called sweetening, occurs at a point where the stream temperature is below 200°c. The red mud waste is extracted from the liquor stream by holding it in large h orizontal settling tanks. The clarity of the liquor is improved by adding flocculants to accelerate settling. 88 SILICON CHIP The overflow from the mud settling tanks, after further filtering, becomes the wanted clear liquor. This is now a hot super-saturated solution of alumina in caustic soda and water. It is cooled by flash evaporation, by subjecting it to a partial vacuum. This also generates lots of steam and this is recycled to heat the spent liquor stream returning to the digesters. Crystallisation of alumina The dissolved alumina in the form of sodium aluminate is recovered from the clear filtered liquor by seeded crystallisation. This occurs in a series of vertical tanks 30 metres in diameter. Sodium aluminate crystallises out to give trihydrated alumina plus caustic soda. The cooled pregnant liquor flows to rows of agitated precipitation tanks which are seeded by intro- ducing crystals of solid trihydrate alumina. The liquor is held in each tank for about three hours before passing on to the next. The whole crystallisation process takes 25-30 hours, producing crystals of various sizes along the way. The sizing of the crystals is a carefully controlled process. The liquor entry temperature into the crystallisation tanks , temperature gradient across each tank, seeding rate, caustic soda/water concentration and holding time are all vital in the control of crystal size. The crystalline alumina trihydrate is removed from the tanks by an auger feed and the crystals are separated into three size ranges in gravity classifiers. The major coarse fraction is the wanted product to be smelted later while the smaller crystals are used as seeds in the crystallisation tanks. The overflow from the classifiers is the spent caustic soda solution. This is re-concentrated by evaporating the water, heated and then recycled back to the digesters, to begin the whole process cycle again. After the gravity classifiers , the alumina trihydrate slurry is treated to remove both combined and free water. This is done by passing it through a series of calcinating units. First in the line is a circulating fluidised bed calciner and then nine rotary kilns, each four metres in diameter and 100 metres long. These huge kilns are rotated by zookW induction motors, driving through a multistage gearbox. The kilns are gas fired to 1100°c, to remove all free and combined water from the alumina. After cooling to below 90°C, the finished product - dry, sandy, white alumina - is stockpiled in enclosed storage buildings. Conveyor belts then carry the product either to the overseas shipping wharf on South Trees Island, or overland to the Boyne Island Smelter. Naturally, the whole process is subject to computer control. Electronic sensors monitor all temperatures, pressures, flow speeds, bauxite input rates, liquor concentrations and other parameters at hundreds of points in the plant, and send all data on-line to one large central computer. This optimises output and minimises energy requirements. Next month we shall look at the most energy intensive stage in the production of aluminium - the smelting of the alumina. SC Acknowledgements Special thanks for photographs and data to Queensland Alumina Ltd; Noel Wootton and Eric King ; and to C. A. 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Wulf antennas & aim to provide the same quality service and antennas that you have come to expect, so please call us and discuss your antenna needs. Call Andy Coman, VK3WH Lot 6, Websters Road, Clarkefield 3429 (054) 28 5134 Silicon Chip Binders ; ',, · \ 'ih.; ' ' n I : .. ...,,~ . . . These beautifully-made binders wi ll protect your copies of SILICON CHIP. They are made from a distinctive 2-tone green vinyl & will look great on your bookshelf. Price: $A 11 .95 plus $3 p&p each (NZ $6 p&p). Send your order to: Silicon Chip Publications PO Box 139 Collaroy Beach 2097 As in any modern chemical plant, there are countless large tanks & hundreds of kilometres of piping. The 40-metre diameter tanks in the foreground are settling tanks, used to separate out the red mud from the caustic soda solution. Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. MAY 1993 89