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The Story Of Electrical Energy, Pt.24 The electrolytic smelting of alumina to pure aluminium is the most electricity intensive industry known to man. In fact, so much electric energy is required in the process that some commentators have referred to aluminium as “congealed electrici­ty”. By BRYAN MAHER Aluminium metal is electrolytically smelt­­ ed by the Hall-Heroult reduction process developed in 1886 and is now the metal of a thousand uses. Before this process was invented, the prohibitive cost of production made aluminium a rare substance. 88  Silicon Chip Today, well over a million tonnes of aluminium are produced in Australia each year. Aluminium is the sixth lightest of all metals and is exten­sively used in structural, decorative and functional applica­tions. When alloyed with other metals, aluminium is a major component in aircraft and transport vehicle construction. Being the fourth best electrical conductor (after silver, copper and gold), aluminium finds extensive use today in electric power lines at all levels from 240V street mains to 1.2MV DC systems. A critical property of any metal chosen for very large trunk power lines is its weight-resistance product. Although copper has the second lowest resistance, its high weight poses mechanical problems in the design of towers and hanging insulators. Compared with copper, aluminium has only two thirds the conductivity but on the credit side, it has only ▲ This overview of the Boyne Island aluminium smelter shows the two potline buildings, each almost 1km long. about one third the weight. To put it another way, if we have two equal conductor lengths of equal weight, one aluminium and the other copper, the aluminium could carry twice the current. Therefore, this metal is chosen for virtually all high voltage power lines, usually with a steel core for added strength. (For more on this subject, see Pt.4 of this series in the October 1990 issue). Major Australian smelters The three major aluminium smelters in Australia are Tomago Alumin­ ium’s plant near Newcastle, Alcoa’s Portland installation in Victoria, and the Boyne Island smelter in Queensland. They produce aluminium for Australian consumption as well as for export to Japan, the USA, Europe and other countries. Our story this month is based on the Boyne Island smelter. This island is at the mouth of the Boyne River, near Glad­stone, and is a joint venture managed on behalf of the partici­pating parties by Comalco Limited. In order of share holding, the participants are Comalco Limited, Austria Metal AG, Sumitomo Light Metal Industries, Kobe Steel, Mit­ subishi, Yoshida Kogyo, and Sumi­ tomo Chemical Corporation of Japan. These participants take the total production of the plant in proportion to their shareholding. The plant uses Comalco-modified Sumitomo Aluminium Ltd potroom technology. Almost 80% of the aluminium produced is in the form of 22kg ingots for overseas markets, for which the smelter earns Australia $240 million annually. The remainder of the product is in the form of cast billets or blocks for further processing by Comalco plants in Queensland, New South Wales and Victoria. The Boyne smelter is responsible for 18% of Australia’s aluminium production and 2% of the world’s total. The Hall-Heroult process During the smelting process, alumina is electrolytically reduced to pure This side view of one of the smelting pots shows nine of the 18 anode support rods. The workman is adjusting a gas collec­tion hood. Most of the aluminium produced at Boyne Island is produced in the form of 22kg ingots for export. The Boyne smelter is responsible for 18% of Australia’s aluminium production and 2% of the world’s total. aluminium in large rectangular carbon lined baths called pots. Because the oxide alumina is in a low energy state, vast quantities of electrical energy must be injected to achieve the high energy state of the pure metal. Each pot has a steel shell lined with a very thick layer of carbon, which is used as the bottom cathode and as the contain­ment for the molten contents. Alumina dissolved in molten cry­ olite (sodium aluminium fluoride, Na 3 AlF 6 ) forms the conducting electrolyte lying within the pot. A number of very large carbon blocks, used as the anodes, are immersed in the electrolyte. Enormous direct currents from a transformer and rectifier system are passed through the pot from the upper anode carbon blocks, through the molten electrolyte, and then out via the carbon lining at the bottom. Each pot has a voltage drop of 4V across it June 1993  89 This photo shows a pot being tapped. The molten aluminium is sy­phoned into the vacuum tank at right, after which is taken to a holding furnace prior to casting. Note the huge con­ductors in the foreground. Total current is 180,000 amps. gen is oxygen just formed from the breakdown of alumina and is in a very reac­tive state, probably in the atomic form). Each pot produces approximately 1.25 tonnes of aluminium per day. To keep the contents of the pots up to temperature, the process must be continuously maintained, 24 hours of the day, every day of the year. A hopper feeds dry granular alumina into the pot and the molten aluminium lies at the bottom. Floating on this is a layer of molten alumina dissolved in cryolite, while on top of this a cake of unmelted cryolite forms. Approximately every two minutes, hydraulic rams punch four 150mm diameter holes down through the cryolite crust and these pass a charge of 1.5kg of alumina into the melt below. Periodically, a pipe is pushed down through the cryolite cake into the molten aluminium at the bottom of the pot. This allows the molten aluminium to be syphoned up into a vacuum vessel. This is then carried by cranes to a holding furnace before being cast into ingots, rods or blocks for shipment. Up to 20% of Comalco’s share of the aluminium produced is alloyed with other metals to enhance properties such as hardness, strength and toughness. Various metals such as magnesium, sili­con, manganese and copper may be added to the melt to produce special product characteristics. Alumina supply The carbon anode blocks are continually burnt away by the reduc­tion process in the smelting pots & so they need to be replaced at frequent intervals. Here a new anode is being swung into place. from anode to cathode when 180,000 amps DC is passed through it. As well as providing the energy necessary to reduce the alumina to pure aluminium, this huge electric current also heats the contents of the pot, keeping the cryolite, alumina and pro­duced aluminium all in a molten state. Electrochemical reduction The passage of electric current through molten aluminium oxide 90  Silicon Chip releases the pure aluminium from the compound. This is the crux of the Hall-Heroult electrochemical reduction process. The simple equation is: 2Al2O3 + 3C + electrical energy ➝ 4Al + CO2 Molten aluminium appears from the electrolyte at the bottom cathode, while oxygen is produced at the anodes. At the operating temperature of 965°C, the evolved nascent oxygen burns the carbon anodes, forming carbon dioxide. (Note: nascent oxy- Granular alumina is carried to the Boyne smelter by a 9km overland conveyor belt from the Queensland Alumina refinery at Gladstone. For each tonne of aluminium produced, the Boyne smelt­ er consumes two tonnes of alumina. Each pot is electrically a very low impedance device (4V drop at 180,000 amps is equivalent to a pot resistance of 22 micro-ohms). To make this a more manageable load, a large num­ber of pots are connected into series groups. Thus, the Boyne smelter consists of two separate potlines. Each consists of 240 pots in two rows, all connected in series. Gigantic aluminium conductors, 600 x 600mm in cross sec­ tion, are used to carry the huge current through all the pots in one line. The circuit K ALEX The UV People ETCH TANKS ● Bubble Etch ● Circulating LIGHT BOXES ● Portuvee 4 ● Portuvee 6 ● Dual Level TRIMMER ● Ideal PCB DRILL ● Toyo HiSpeed MATERIALS ● PC Board: Riston, Dynachem ● 3M Label/Panel Stock ● Dynamark: Metal, Plastic These are connecting rods for the carbon anode blocks. They are attached to the carbon blocks using cast iron as the joining medium.The manufacture of new anodes is a never-ending process length for these 180,000 amp currents is almost 2km! Because all pots are part of the electrical circuit, they are insulated by their concrete foundations from each other and the building. As well, care must be taken when overhead travell­ ing cranes service the pots. Crane runway support columns and girders must be kept electrically isolated from the potline. The overall dimensions of each potline are enormous. They are 870 metres (almost a kilometre) in length, reputedly the longest in the world. During the construction phase in 1980, the company was spending $1 million dollars per day, 80% of which went to Australian industries and subcontractors. Total cost of the plant was more than $750 million. Power supply The total voltage applied across one whole potline is ap­proximately 1000V DC. This is obtained from “recti­ formers”; ie, transformers incorporating banks of huge silicon rectifiers mounted within an oil-filled tank. The primary supply is via twin 132kV 3-phase AC lines plus one 275kV line from Gladstone power station 18km away to the northeast. Because of the proximity of the plant to the sea, the entire 132kV switchgear is enclosed within gas (sulphur hexafluo­ ride (SF6) filled pipes and vessels. This gas acts as both an excellent insulator and a flame retardant for the circuit breaker contacts. The smelter consumes 385MW from the state grid continuously on a “take or pay” basis, making it the power station’s largest single load. Control of the current through the pots is achieved in two stages. The 132kV/1kV transformers for each potline are equipped with off-load tap changing switches. Regulation of the high voltage supply is by a 275kV/132kV on-load tap-changing auto­trans­­former in the main supply. This huge 500MVA oil-immersed transformer weighs close to 300 tonnes and has a separate fan-cooled heat exchanger and breathing tank. ✸ AUSTRALIA’S NO.1 STOCKIST ✸ K ALEX 40 Wallis Ave, East Ivanhoe 3079. Phone (03) 9497 3422, Fax (03) 9499 2381 Silicon Chip Binders Carbon electrodes Each of the 480 pots in the two pot lines contains 18 carbon anodes. These anode blocks, each weighing 1.4 tonnes, are consumed by the burning effect of the hot oxygen gases released in the smelting reduction process, as noted above. Therefore, a vital function of the plant is the continuous production of new carbon blocks for the periodic replacement of the 8640 anodes in service. The carbon section of the Boyne plant produces 130,000 tonnes of anodes annually. Petroleum coke imported from the USA, coal tar pitch from Newcastle, and recycled anode butts from the These beautifully-made binders will protect your copies of SILICON CHIP. They are made from a dis­tinctive 2-tone green vinyl & will look great on your bookshelf. Price: $A14.95 (incl. postage in Australia). NZ & PNG orders add $5 each for postage. Not available elsewhere. Send your order to: Silicon Chip Publications PO Box 139 Collaroy Beach 2097. Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. June 1993  91 This view of one of the pot lines shows the huge scale of the plant. Note that since each smelting pot is connected in series with 239 others in the plant, they must each be insulated from each other & from the building. potrooms form the ingredients of the anodes. The coke and butts are crush­ed and ground and mixed with coal tar pitch, and then heated to 160°C. The resulting hot paste is vibrated into the shape of the anode blocks. These are then immersed in 4.9-metre deep refractory lined baking pits and progressively heated to 1150°C over a period of 18 days. Oxygen is excluded to prevent burning and the volatile gases given off are used as supplementary fuels in the heating process. This baking imbues the anodes with the necessary elec­trical conductivity and mechanical strength. After baking, the anode blocks have large aluminium rods attached. These provide both mechanical support and electrical connection for the anodes while in the pots. The metal rods are bonded to the carbon blocks using molten cast iron as the joining med­ium. The manufacture of new anodes is a never-ending process, with thousands of units in various stages of assembly 92  Silicon Chip on the overhead conveyors at any one time. Before use in the potlines, each new anode is sprayed with aluminium to establish initial conductivity. precipitators. Total emissions from the plant are monitored by both the company and the Department of Environment and Conservation. Measuring equipment is installed within the plant, in the surrounding buffer zone and out in the community. Environmental protection Future developments Being electrically powered, aluminium smelting is basically a clean operation. However the exhaust carbon dioxide from the pots also contains traces of fluorides due to reactions with the cryolite. To keep these toxic gasses from the atmosphere, the potline exhausts are drawn off and the fluorides are absorbed by a process known as dry scrubbing. In this process, the exhausts are passed over hanging alumina-coated bag filters to catch the fluoride emissions, either in gaseous or solid particle form. Residues collected by the filters are ultimately returned to the pots for reprocessing. The four giant dry scrubbing plants operate at better than 99.7% efficiency. Emissions from the carbon baking furnace are cleaned by electrostatic A feasibility study is presently being carried out to investigate the possible building of a third potline to almost double the present plant capacity. This would make Boyne Island the largest smelter in Australia and possibly in the world. Such a plant enlargement also depends on the proposed purchase by the consortium of the existing Gladstone power station from the Queensland SC Government. Acknowledgements Special thanks to Trudy Habner and the engineering staff of Boyne Smelters for photos and data; also thanks to ABB (Aust), IE (Aust) and Alcoa.