Thirsty Industries: Aluminium

The drinks can is the lightest thing on the table, and it began as the heaviest earth in the tropics. Bauxite is a red, clay like rock that forms where millions of years of warm rain have washed everything soluble out of the ground and left the aluminium and iron behind, and it is dug in open pits in the north of Australia, the hills of Guinea, the Amazon and the interior of Jamaica, in quantities that make it one of the largest mined materials in the world. From the pit it goes to a refinery, where it is cooked in caustic soda until the aluminium dissolves out of it; from the refinery, as a white powder, to a smelter, where electricity turns the powder into metal; and from the smelter, as an ingot, to the rolling mill and the can line. Every step has a water, and the largest is one that never appears on the site.

This article is about where aluminium's water goes, what the refinery leaves behind and what happened when a store of it broke in Hungary, why the smelter's real water is the reservoir behind a dam, and why the can that goes back into the recycling bin skips nearly all of it.

The pit

Bauxite is mined from the surface. The rock lies in a layer a few metres thick under a thin soil, over country that is, in nearly every case, tropical forest or savanna, and the mine strips the soil, digs the layer with excavators, and moves on, so that a bauxite mine is a slow front of red ground advancing across a landscape rather than a hole. Its water use is small, a wash for the ore and the dust suppression on the haul roads, and its water effect is the forest and the streams: the Boké region of Guinea, which has become the largest bauxite exporter on Earth in a decade, is a country of red dust, silted rivers and villages whose wells have gone dry or turned red as the pits cut the water table, documented by the lenders' own inspectors; the Cockpit Country of Jamaica, a karst landscape over the island's main aquifer, is the subject of the country's longest environmental argument because the bauxite lies on top of the water the island drinks. The mine is the smallest water user in the chain and the one that changes the most ground.

The red mud

The refinery is the Bayer process, invented in 1888 and unchanged in principle since. Crushed bauxite is mixed with hot caustic soda in pressure vessels, where the aluminium minerals dissolve and the rest, mostly iron oxide, does not; the slurry is settled and washed, the clear liquor is cooled and seeded so that pure aluminium hydroxide crystallises out, and the hydroxide is roasted in a kiln to alumina, a white sand. It takes about four tonnes of bauxite to make two tonnes of alumina and, at the smelter, one tonne of metal, and what does not dissolve is the residue: about one to one and a half tonnes of it per tonne of alumina, red with iron, caustic with the soda it has absorbed, at a pH of twelve or thirteen, and wet. The industry calls it bauxite residue. Everyone else calls it red mud.

The mud goes into ponds. Historically it was pumped as a slurry, with several tonnes of water for every tonne of solids, into lagoons behind earth dams next to the refinery, where the solids settled and the caustic water was drawn off and sent back round; the world's stock of it, in ponds from Jamaica to Jiaozuo, has passed four billion tonnes and grows by about a hundred and fifty million a year. The newer refineries thicken it to a paste or a dry cake and stack it, which uses less water and makes a smaller and safer pile, and the ponds are lined and the caustic is neutralised, in the countries where the regulator requires it. The tailings article on this site described the dams of the metal mines. The red mud pond is the same dam, holding a mud that burns.

Ajka

On 4 October 2010, at about midday, the corner of a red mud pond at the alumina refinery at Ajka, in western Hungary, gave way. The pond was one of ten at the site, a rectangle of earth walls thirty metres high holding decades of mud, and when the northwestern corner of it opened, about a million cubic metres of red slurry ran out at a height of two metres through the villages of Kolontár and Devecser, a kilometre and a half away. It knocked down walls, carried cars, filled the ground floors of the houses and burned whatever skin it touched, because it was caustic enough to strip paint. Ten people died, most of them drowned in the first minutes, and about a hundred and fifty were treated for chemical burns. The mud went on into the Torna stream and from it into the Marcal river, where the alkali killed everything in the water for the river's whole length, forty kilometres, to its meeting with the Rába and on toward the Danube, which the authorities saved by dumping gypsum and acid into the Marcal by the truckload to bring the pH down before the mud reached it. The villages were dug out over months; the fields along the stream were scraped and the soil taken away; the company's executives were tried and, after years of appeals, some were convicted. The investigation found that the pond's walls had been built on wet, weak ground and that the mud had been held wetter and higher than the design allowed, and the refinery went over to dry stacking afterward, which is what the industry says it should have been doing already.

Ajka is the aluminium industry's worst water accident in a rich country and it was, by the standards of the tailings dams on this site, a small pond. The mud in it was the mud every refinery makes. The difference between Ajka and the ponds in Jamaica, Guinea, India and China that have not failed is the design of the wall and the water content of what it holds, and those are the two things the industry has changed since.

The smelter and the dam

The smelter is the Hall and Héroult process: alumina is dissolved in a bath of molten cryolite at nearly a thousand degrees and a direct current of hundreds of thousands of amps is passed through it, so that the oxygen leaves at the carbon anodes and the aluminium collects, liquid, at the bottom of the cell, from which it is siphoned every day. A smelter is a hall a kilometre long of these cells in a line, the potline, and the electricity for it is thirteen to fifteen megawatt hours a tonne, which is the reason aluminium is sometimes called solid electricity. The smelter's own water is modest: cooling for the rectifiers and the casting, a few cubic metres a tonne, and the wet scrubbers that take the fluoride out of the cell gases, whose effluent is the one the regulator watches. The real water is the power.

For most of the twentieth century the cheapest large block of electricity on Earth was a hydro dam, and the smelters went to the dams: to the fjords of Norway, to Quebec and British Columbia, to the Tennessee valley, to Iceland, whose smelters use most of the country's electricity, and to the Snowy Mountains and Tasmania. A smelter beside a dam is, in the accounting of the power stations article on this site, a user of the reservoir: the water evaporated from the lake's surface and the river flow that the dam holds back and releases to its schedule, which is a share of the water in every tonne of metal. The newer smelters went to the Gulf, where the electricity is gas and the water is the desalination article's; and to China, which makes more than half the world's aluminium on coal, with the cooling water of a coal station behind every tonne and the carbon to match. The water in a can depends on where the metal was made, and the honest answer in Iceland is a lake and in Shandong a river warmed by a power station.

The can

The reason to end on the can is that it changes the whole account. Aluminium does not wear out: a can melted down is the same metal as one from the smelter, and remelting takes about five percent of the energy of smelting and none of the bauxite, the caustic, the mud or the dam. About three quarters of all the aluminium ever made is still in use, and the cans, the car parts and the window frames that come back through the recycling stream are the industry's largest and cleanest source. A can that goes into the bin and back to the furnace has, for its second life, a water footprint that is mostly the water of the furnace's cooling and the foundry's, which is small; a can made from bauxite carries the pit, the refinery, the pond and the reservoir. The difference is the largest for any common product on this site.

What it teaches

Aluminium is the industry on this site whose water is in the places the metal is not. The refinery leaves a mud that has to be held wet behind a wall, and Ajka is what happens when the wall gives way; the smelter leaves no mud and little effluent and takes its water from a dam a hundred kilometres away or a cooling tower in a coal station. The metal in the can is clean, light and endlessly reusable, and the first time it is made it costs four tonnes of red earth, a tonne and a half of caustic mud and a reservoir's share of electricity. The second time it costs almost nothing, which makes the recycling bin the industry's largest water measure.

One and a half tonnes of red mud a tonne, a million cubic metres through two villages, and a can that is clean the second time.