For many years now, aluminum has been successfully recycled on an industrial scale. The scrap metal is simply melted and used to make new products such as car bodies. Although recycling conserves natural resources, standard combustion processes offer room for improvement. Special burners and gas technologies from Linde (linde.com) raise energy efficiency levels and cut costs. They also significantly reduce harmful emissions.
Aluminum is a key part of everyday life, as common as the food we eat. We live in houses with aluminum window frames, for example, and drive cars with lighter aluminum bodies. We even wrap our sandwiches in it. Yet aluminum is a precious commodity, complex and expensive to manufacture. It is obtained from bauxite, an ore that is extracted from the earth in large mines, primarily in South America, Australia, and Africa. Huge bulk carriers then transport it across the sea to industrialized countries, where it is heated to temperatures of up to 1,300 degrees Celsius in aluminum smelters. Once melted, it is processed to aluminum oxide using special chemicals. This process consumes an enormous amount of energy. Around five tonnes of bauxite are required to produce one tonne of aluminum.
Recycling has therefore been a valuable option for aluminum manufacturers for a long time. Unlike plastics, recycling aluminum does not impact on quality. The recycled material is just as good as new aluminum. It can be remelted and used for new products any number of times, turning cans, for example, into engine blocks. In 2009, 37 million tonnes of new, or primary, aluminum were manufactured worldwide. Almost 13 million tonnes were recycled. “There is room to raise the recycling quota significantly,” says Thomas Niehoff, Head of Industry Segment Non-Ferrous Metals and Mining in Linde’s Gases Division. Recycling is not just about saving resources. It also saves huge amounts of energy. It takes almost 13,000 kilowatt hours to produce one tonne of primary aluminum. This falls to just 1,500 kilowatt hours for one tonne of recycled aluminum—a drop of almost ninety percent. Yet even this figure can be significantly improved, which could have a positive impact on price since up to 40 percent of the price of this much sought-after metal is attributable to energy costs.
Experts expect annual demand for aluminum to rise to 53 million tonnes by 2015. Recycling is the best way to meet this demand without overly depleting natural resources. Technical solutions for enhanced aluminum recycling have been one of Linde’s core competencies for some time now. The Group’s engineers have a wealth of experience in making combustion and melting processes more efficient and environmentally friendly. “Even in the most established facilities, it is still possible to tease out more efficiency,” explains Niehoff. “And not just in terms of energy consumption—emission levels can also be brought down.”
Experts expect annual demand for aluminum to rise to 53 million tonnes by 2015. Recycling is the best way to meet this demand without overly depleting natural resources.
Scrap aluminum is heated and remelted in large smelting furnaces, powered by natural gas. Earlier methods used air from the surrounding atmosphere for combustion. This was inefficient, however. Air comprises over 70 percent nitrogen, which means that a large amount of energy is wasted on heating the nitrogen ballast, only for it to be discharged into the air as flue gas. As a result, this method was replaced with oxyfuel combustion some years ago. Linde engineers were at the forefront of this new technology, which uses pure oxygen instead of air in smelting furnaces. It reduces flue gas volumes drastically and thus also the amount of wasted energy. Oxyfuel enables manufacturers to produce one tonne of recycled aluminum with just 500 kilowatt hours.
“However, the shift to our oxyfuel process brought its own challenges,” continues Niehoff. This is because aluminum is very reactive with oxygen. During combustion, the aluminum and oxygen react to create aluminum oxide. This white powder, known as dross, is an unwanted and unused by-product that accumulates in furnaces, reducing the aluminum melt. Oxygen and aluminum react particularly strongly in hotter parts of a furnace. “Many manufacturers were appalled at the prospect of using oxygen in aluminum smelting,” recalls Niehoff. This is because conventional oxyfuel furnaces use a light, hot, glaring oxygen flame, which, like a flamethrower, heats the furnace unevenly. This creates hot spots, where dross concentrates. It was a problem that Niehoff and his team were best suited to solve. Their idea was to distribute heat more evenly by enlarging the flame. To achieve this, the fuels have to be fed rapidly into the furnace, causing the furnace gases to circulate so strongly that the flame expands. “Increasing the size of the flame prevents hot spots from forming,” explains Niehoff. In contrast to the original hot glowing jet, the enlarged flame is hardly visible, which is why the process is also known as flameless combustion.
The new flame technology is now being successfully deployed in several aluminum smelters. One facility in Sweden has seen melting performance increase by ten percent compared with the conventional oxyfuel process as a result of homogeneous heat distribution. Energy consumption also fell by ten percent. And dross formation dropped dramatically. The flame can be controlled more easily thanks to the flue gas stream. “Every aluminum plant and every furnace is different, which is why we offer individual solutions and fine-tune the combustion process to exact customer requirements,” reports Niehoff. “Our service doesn’t stop on delivery.”
One of the main reasons for this heterogeneous process landscape is that different aluminum producers handle very different kinds of secondary aluminum. A medium-sized smelting furnace can melt around 30 tonnes of aluminum. The furnace is gradually filled in several batches and the scrap aluminum is added to the melt. Some manufacturers use old engine blocks; others use empty beer cans together with plastic wrap and labels. Products with short lifespans soon return to the smelter, whereas an aluminum car body will be on the road for at least ten years. Niehoff recalls a plant that feeds tonnes of shredded drinking cartons into its furnaces. “The cartons are made of a mixture of cardboard, plastic, and wafer-thin aluminum foil,” explains Niehoff. “Recovering aluminum is still a viable option, even with this small ratio of metal in the feedstock.”
However, Niehoff was also concerned about the plastic, paint, and engine oil residues released during secondary melting. Recent changes to emissions regulations reinforced the need for tighter control. Hydrocarbons are the main substances released when residue vaporizes in molten aluminum baths at temperatures of 750 degrees Celsius and higher. Niehoff and his colleagues therefore developed a technology that burns off rising substances while they are still in the furnace. The engineers designed a lance that extends into the furnace from above and feeds in oxygen for additional combustion. Loading fresh scrap into a furnace triggers particularly intensive reactions, with large amounts of hydrocarbons released in just a few minutes. The lance can be ignited at this point to burn off these unwanted substances. “This process turns hydrocarbons into fuel, helping to heat the furnace and reduce natural gas consumption,” says Niehoff. When the lance ignites and destroys the hydrocarbons, it can relieve the burner—accounting for up to 15 percent of combustion performance.
This intelligent secondary combustion technology is called WASTOX®. It kills two birds with one stone by using emissions to heat the furnace while at the same time reducing the amount of hydrocarbons in the flue gas, according to Niehoff. “Compared with conventional oxyfuel processes, WASTOX results in 10 to 50 times lower volumes of hydrocarbons,” says Niehoff.
To ensure that the WASTOX lance can be activated and deactivated at the right time, Linde engineers install sensors that continually measure hydrocarbon levels in the furnace. As always, every solution has to be tailored to individual plant requirements. In some plants, technicians install laser scanners; in others, they use optical sensors. The gases are usually measured by a light beam which changes when it encounters different gases, as these absorb different wavelengths. The pattern of light absorption delivers a detailed profile of the various gases in the furnace. In addition, the light signal’s strength can be used to determine the concentration of each gas, thus enabling the lance to be controlled with a high degree of precision. One Linde customer uses optoacoustic sensors, which also monitor the sound of the gas flame to detect whether hydrocarbons are rising from the melt. “These kinds of sensors are particularly challenging,” explains Niehoff, “as the aluminum recycling environment is extremely dirty and extremely hot.” Sensors therefore have to withstand steam, heat, and sprays of molten metal. The combustion process is still primarily controlled by hand and so the Linde engineer and his colleagues are currently focusing on further automating the WASTOX technology. In the near future, the sensor, burner, and lance will be working on auto-pilot.
Linde engineers have matured the processes sufficiently, however, to enable resourcefriendly aluminum recycling. But this doesn’t mean that Niehoff’s work is over. “Oxyfuel is an established process. However, there is still need for further optimization in many regions, above all in Asia, but also in Eastern Europe and the US,” continues the metal expert. “Many of the plants in these areas can be retrofitted with flameless combustion and WASTOX technologies.” Niehoff has no doubt that aluminum recycling is growing in importance. After all, as demand for primary aluminum rises, so too will the amount of secondary aluminum. “And recycling is the key to sustainability,” he concludes.