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The Story Of Electrical Energy, Pt.22 The production of aluminium is the most energy intensive of all metal refining operations. But while most readers would realise that the conversion from alumina to aluminium .involves huge amounts of electricity, the mining & alumina refining stages are also very energy intensive. By BRYAN MAHER Over the next few issues, we will trace the aluminium story from the mining of the red bauxite ore, refining it to snow white alumina and then to . the electrolytic reduction to pure alu. minium. Every Australian purchases an average of 16 kilograms of aluminium each year, in various forms. Maybe you bought lots of "tinnies", or perhaps metal furniture or kitchen appliances, or even a new alloy head for 8 SILICON CHIP your car's engine. Australia produces well over a million tonnes of pure aluminium each year, two thirds of which is exported. Our main customers are .Japan, China and South East Asia . Over 125,000 Australians are employed in the overall production of aluminium, from the bauxite ore mines to the finished metal. We begin our story at ground level, where the red bauxite ore is found. It is ironic that although aluminium is the most abundant metal in the Earth's crust, it was one of the last to be isolated. Many rocks and clays contain aluminium in combination with other elements, usually iron, silicon, and oxygen, together with trace quantities of gallium, titanium, vanadium and phosphorus. The most useful source of aluminium oxide is a red pebbly ore known as bauxite. Australia leads the world in bauxite mining, with four large mining operations at widely separated locations. Alcoa (Aluminium Corporation of America) has perhaps the largest operation in the Darling Ranges of Western Australia. Next in annual tonnage is Comalco's mine at Wei pa, on the western side of Cape York in Queensland. Third is Nabalco's operation at Nhulunbuy on the north eastern tip of the Gove Peninsula in the Northern Territory. The Reynolds organisation mines at Worsley in Western Aus- ◄ Left: this front loader can fill these 80-tonne capacity trucks in just four bites. The ore is then taken to the crushers before being placed on a long conveyor for transportation to port. tralia, and there are others. Total bauxite won in Australia is around 36 million tonnes per year (1986 figures). Bauxite is called a residual ore. It is formed by the natural weathering of rocks wherein the non-valuable materials are leached away over millions of years, leaving the residual ore. Bauxite consists of two types of hydrous aluminium oxides combined with iron, silica and other trace metals. It is the strong presence of iron oxides that give bauxite its rich red colour. Two of the largest deposits of bauxite in the world are those found on opposite sides of Australia's Gulf of Carpentaria, at Wei pa on the eastern side and at Nhulunbuy on the northwestern extremity. Early explorers The Gulf was the most visited coastline of Australia in the very early days and coincidentally Weipa is the site of the first ever landing by white men on Australian soil. Willem Jansz in the Dutch ship Duyfken (Little Dove) sailed into the eastern side of the Gulf in 1606 and landed at Albatross Bay, the site of present day Weipa. He left an embossed metal plate nailed to a tree to prove his visit. The headland on which the lighthouse today stands, guiding giant bulk ore carrier ships out of the port, is named Duyfken Point. That other famous Dutch navigator, Abel Janszoon Tasman, in his ships Zeemeeun, Limmen and Bracq, charted the Gulf in detail in 1644, naming it "Carpentaria". He sailed around the Gulf from Weipa via Groote Eylandt (which he also named) and on past the site of present day Nhulunbuy. The first written record alluding to the existence of Australia's bauxite deposits are found in the 1803 log of Captain Mathew Flinders. In circumnavigating Australia, he traced Tasman's path in the Gulf, naming two mountains, Saunders and Dunas, and Melville Bay, today the site of Nhulunbuy's harbour. In his log, he commented on the stark red cliffs of Stretching off into the distance, this 19km conveyor takes bauxite from the crushers at Nhulunbuy to the stockpiles at Melville Bay in a journey which takes about 1½ hours. the Peninsula, not realising that he was looking at vast quantities ofbauxite. Identification of the red deposits as bauxite did not come until 1949 on offshore Marchinbar Island. Three years later, the first geological survey of the ore commenced but the first lease holders abandoned their claim. In 1965, the joint venture partners Swiss Aluminium and Gove Alumina obtained a special mining lease covering 20,000 hectares which was surveyed by air. Following a feasibility study, a $320 million prospecting and construction program commenced in 1966. By 1968, the known reserves confirmed by a massive core drilling program were found to be 254 million tonnes. Each tonne of bauxite contains 50% aluminium oxide (or alumina to give it its commercial name). Pilot shipments of crushed bauxite in bags were sent to a smelter in Venice in 196 7. These were followed by two bulk shipments totalling 3500 tonnes within eight months. Two years later, the venture partners formed a compan y named Nabalco to build a full-scale bauxite mine. Nabalco also built the conveyors and loading plant, a deep water harbour in Melville Bay, a modern town to house 4500 people and the alumina refinery plant. Nabalco completed the 1300 homes and infrastructure ofNhulunbuy, including the largest primary and secondary schools in the Northern Territory, by 1972. Mining operations The bauxite deposit at Gove exists as an extensive plateau 60 metres above sea level, overlaid by less than one metre of surface soil and eucalypt APRIL 1993 9 This map of the Gove Peninsula shows the extent of the bauxite reserves for the Nabalco mine. Known reserves are at least 250 million tonnes. vegetation. The mining sequence starts with sample drilling on a 50 metre x 50 metre square grid for analysis and planning. An Edison rotary vacuum extraction drill bores a 65mm diameter hole to the bottom of the ore body which averages 3.5 metres in depth. After surface vegetation is felled by two bulldozers hauling a heavy chain between them, the topsoil is removed and stockpiled for later use in rehabilitation. A loose cover of pisolitic low grade ore is also scraped off. To break up the ore itself, a Caterpillar D11N dozer/ripper is used. It breaks up the bauxite into large chunks measuring up to 2 x 1.5 metres. This 100-tonne bulldozer, the largest in Australia, is powered by a 575kW diesel engine and was delivered by a roll-on/roll-off barge from Darwin on 9th March 1990. The arrival of this machine rendered the previously used explosives redundant, hence reducing the impact of the mine on the environment. Other large bulldozers assist in the clearing operations. Caterpillar front end loaders of 12.2 cubic metres capacity then load the ore into 80-tonne dump trucks. These front-end loaders lift 20 tonnes at a bite, thus filling each dump truck in four passes. The ore is then carried 4.6km to the primary crusher. The dump trucks use an onboard 10 SILICON CHIP computer to manage their 850hp (634kW) turbocharged VB diesel engines and to control the sequential operation of · their massive electrohydraulically operated clutches. With 85 tonnes unloaded weight, each truck tips the scales at 165 tonnes when fully loaded. Mine rehabilitation Each worked out area of the mine is rehabilitated as quickly as possible by returning the stored topsoil and planting native flora . Great care is taken to return the area to its natural state by restoring the original contours, planting ground cover and so on. The return of native fauna soon follows. A crushing power load The ore is dumped from the trucks to the primary crusher, after which it passes via selective screens to the secondary crusher. Each crusher stage is powered by a 500kW 3-phase motor. To start these wound-rotor slipring motors , resistances are introduced into the rotor circuit to minimise the initial current drag on the power station. These resistors arethen progressively reduced as the motors accelerate and, finally, shorted out so that the motors reach full speed. The power for these very large 6.6kV motors comes via the partly under- ground high voltage powerline from Nhulunbuy power station. The bauxite ore, now reduced to chunks of 25mm or less; feeds via a 1500-tonne surge bin to an overland conveyor which takes it to the port site at Melville Bay. The first section of the conveyor is a 50-metre long variable speed transporter which matches the exit rate from the surge bin to the carrying capacity of the overland conveyor. This short transporter section is powered by a 400V DC motor, hence its ability to run at virtually any speed. The main overland conveyor is 18. 7km long and consists of four separate flights. Each flight is powered by a 147kW 6.6kV 3-phase motor. Nhulunbuy is an isolated power station, without much "spinning reserve", therefore the power station must give permission before any of these very large motors are started. After it is loaded onto the continuously running conveyor, the bauxite takes 1½ hours to arrive at Melville Bay. Approximately 44% of the bauxite mined at Gove is shipped overseas to alumina refineries in every continent except Africa. The remainder is converted to alumina at the Nhulunbuy refinery. Currently, 1.4 million tonnes of alumina is exported each year. From the overland conveyor, the bauxite is stored in four stockpiles by an electrically powered jib stacker. The four stockpiles are arranged in two pairs of rows, with the belt conveyor passing down the centre of each pair. This gigantic machine runs on rails, propelled by low-geared traction motors. The feed belt of the stacker is driven by a 22kW 415V 3phase constant speed motor, while the supply comes via a 6600V trailing cable. Two of the stockpiles supply the shipping wharf, while the other two are for the Nhulunbuy alumina refinery. The stacking area measures 1. 2km long by 80 metres wide. Each of the four stockpiles can hold 100,000 tonnes of bauxite. Ore is retrieved from the stockpiles by two electric barrel reclaimers. These have a barrel 39 metres long and 4 metres in diameter and as it rotates, it swallows the crushed ore and feeds it out at the other end to another conveyor belt. The reclaimer on the export shipping stockpile is capable of moving 2200 tonnes of bauxite per hour and the other, for the refinery, 600 tonnes per hour. The bauxite ore from the export stockpiles is transferred by a shipping conveyor over the 982-metre long bridge to the loading wharf in Melville Bay which can handle ships of up to 100,000 tonnes capacity. Nhulunhuy power station All electric power for the mine, stacker, reclaimers, ship loader, the town, alumina refinery, the water supply and Yirrkala Aboriginal town are provided by Nhulunbuy power. station. This oilfired installation has three boilers, each evaporating 226 tonnes of water to steam per hour at 520°C. The steam flows through three 47,000 hp back pressure turbines, each of which drives a 35MW alternator. Exhaust steam from these turbines flows to the alumina refinery to satisfy its massive heating requirements. In addition, a 7.5MW condensing turboalternator can be run from the back pressure ~steam of the larger machines. The complete power station was purchased as a turnkey contract from Brown Boveri et Cie of Germany. Because Nhulunbuy is completely isolated, a separate emergency diesel power station was also installed adjacent to the steam plant. This power The alumina refinery & bauxite stockpiles occupy a big area. Note the barrel reclaimers which straddle the stockpiles & can move ore at up to 2200 tonnes per hour. · station houses five Allen diesel engines, each direct coupled to a 6.6kV 3-phase alternator. Each of these supercharged diesel engines is just under four metres high and is almost 10 metres long. Emergency power is essential for the alumina plant because any interruption to the flow of alumina solutions through the pipes results in solids crystallising out. Pipes and vessels must then be either scrapped or blasted out to remove the rock hard alumina. In the event of a power failure at the steam power station, the emergency diesel plant starts up automatically, restoring power within two minutes 30 seconds. Next month we will look in detail at the bauxite-to-alumina separation process at the world's largest alumina refinery. Acknowledgement Grateful thanks to Larry Davies and the staff of Nabalco, The Aluminator, QAL, Ken Cowd, C. A. Kneipp, H. L. Levin, M. McEwan and the people of Nhulunbuy. SC APRIL 1993 11