Energy-efficient and cleaner aluminium

Aluminium production is extremely energy-intensive; improving its energy efficiency and cleaning up the process is essential

By Morand Fachot

Primary aluminium production relies entirely on electrical power, from the processing of ore (mainly bauxite) into the alumina (aluminium oxide) needed to make aluminium by electrolysis. Electrical energy costs represent a sizeable proportion of primary aluminium production expenses. The ever-expanding range of products that use or are made of aluminium means that global demand for this metal is constantly rising, pushing producers to seek ways to make its production as energy efficient as possible.

Aluminium scrap ready for recycling (Photo: © Norsk Hydro) Aluminium scrap ready for recycling (Photo: © Norsk Hydro)

A "young" metal

The processing of raw materials is at the source of any manufacturing activity. The invention of metals, starting with bronze in the 4th millennium BC and followed by iron around 1200 BC, marks a significant milestone in human history. The metals enabled tools and implements to be produced for agriculture and manufacturing

For its part, aluminium is a "young" metal. Although it is the third most abundant element in the Earth's crust, it is very difficult to produce in a pure form, a step that was only achieved in the mid-1820s. However, until American chemist Charles Martin Hall and French chemist Paul Héroult independently discovered the process for the industrial production of aluminium by electrochemical reduction of alumina in 1886, the global annual production of the metal amounted to just a few hundred kilograms, making it more valuable than gold at the time.

Now, this once rare metal is used for a broad range of applications in many sectors, including transportation, construction, electrical equipment, packaging and medicine.

Energy-intensive and costly production

Aluminium is still produced using the Hall-Héroult process. In this, alumina is dissolved into cryolite, a mineral, in "cells" or "pots" and pure aluminium is extracted by electrolysis using large carbon blocks as anodes, which allows the smelting process to take place. This procedure is very energy intensive: for older smelters, around 15-16 kWh (kilowatt-hours) of energy is needed per kilogram of aluminium produced; 13 kWh or under is required for more modern installations operating at higher current.

Aluminium production accounts for roughly 3.5 % of global electricity consumption. Energy (i.e. electricity) costs represent between 30 % and 40 % of aluminium production expenses. This is the main factor in determining where new smelters can be built: where energy is abundant and relatively inexpensive. Traditionally this is often close to hydroelectric power plants (as in Canada, the US, Norway or Russia) or sources of natural gas (such as in the United Arab Emirates). Easy access to waterways or ports is also important for bringing in ore or alumina to the plants.

The construction of new aluminium smelters, or alumina processing plants, is often accompanied by the parallel construction of dedicated power plants that will sustain their operations. Carbon anodes used in cells represent another significant cost, as they are eroded during the electrolysis process. 400 to 500 kg of carbon anodes are used for each tonne of aluminium produced, releasing CO2 (carbon dioxide) and requiring regular replacement.

IEC work covers the whole production chain

As aluminium production is entirely dependent on electrical power and systems, from ore processing to finished metal, many IEC TCs (Technical Committees) play a central role in preparing International Standards for the industry. They range from power generation and distribution to machinery, ventilation and control systems and many others.

The generation and distribution of electrical power relies on IEC International Standards prepared by TC 4: Hydraulic Turbines, TC 5: steam turbines, for power generation, TC 8: System aspects for electrical energy supply and TC 14: Power transformers, for regulating supply.

TC 65: Industrial process measurement, control and automation, and all its subcommittees, TC 44: Safety of machinery – Electrotechnical aspects, and TC 2: Rotating machinery, cover other aspects of the industry.

TC 2 prepares International Standards for electric drives used in all industrial sectors. Its work is essential for aluminium production since much of the machinery used is powered by electric drives. The initial processing of ore uses vibrating screens to sieve bauxite, conveyor belts to transport it or ball mills to grind it into powder for further handling into alumina, which itself requires yet more machines for the final production of aluminium.

Renewable sources for an environmentally-friendly metal

The IAI (International Aluminium Institute), the global forum of the world's aluminium producers, accounts for more than 80 % of world primary aluminium output. It indicates that nearly 55 % of primary aluminium is produced using renewable and environmentally-friendly hydropower. Coal-generated electricity makes up 28 % of the total and natural gas slightly over 13 %. Aluminium producers generate about a third of their electricity needs from hydropower, coal or natural gas.

The share of hydropower usage in the aluminium industry is particularly high in Canada, Norway and Russia - 80% of Russia's smelters are run on electricity generated by Siberia's hydropower plants.

Aluminium is also an environmentally-friendly material. While requiring a lot of energy to produce (most of it from hydropower), it is also nearly 100 % recyclable, and can be recycled indefinitely. About 75 % of the aluminium that has ever been produced is still in use. Secondary (recycled) aluminium needs just 5 % of the energy required for producing the same amount of primary aluminium. In 2000, approximately 36 % of the aluminium supply in the US came from recycled aluminium, a sizeable percentage of it from beverage cans.

Because it is lightweight, aluminium can also help cut the fuel consumption of motor vehicles, aircraft and other means of transportation. Lighter cars or aircraft consume less fuel. It is estimated that every 100 kg of aluminium used in a car saves up to 1 000 litres of fuel per 200 000 km travelled.

Prospects for more efficient and sustainable production

Faced with energy costs that make up a third or more of aluminium production expenses and with the additional expenditure of replacing anodes, aluminium producers are constantly striving to achieve more efficient production methods. Their efforts are focused on the development and enhancement of energy-efficient production technologies and on reducing emissions. Major producers are developing their own processes.

Canadian aluminium producer Rio Tinto Alcan, claims that its AP Technology™ platform is the "world's cleanest aluminium production technology". It aims to "reduce and eventually eliminate emissions, and dramatically reduce energy consumption". The company adds that by operating cells above 400 kA (kiloamperes) – with 600 kA touted as "a realisable goal" – it has achieved a metal output per pot that is 40 % higher than that of existing technologies. Older processes typically use currents of 100 to 200 kA.

Russia’s largest producer, Rusal, makes similar assertions for its own proprietary smelter technologies that also operate at and above 400 kA.

The future is inert

The energy efficiency of aluminium cells has improved significantly as aluminium producers gradually introduce new technologies. A remaining obstacle to further improvement in aluminium reduction, and an additional cost, is the use of carbon anodes that have to be replaced (or constantly topped up), a lengthy and costly procedure as cells have to stop operating during the operation.

For many years the industry has been looking at the possibility of using "inert" anodes made of ceramics, metals or cermets (composites of ceramics and metals). Unlike carbon anodes, inert anodes are not corroded during the aluminium reduction process and will not release CO2 but pure oxygen.

The IEC is not only preparing International Standards for equipment used in aluminium production, it is also keeping abreast of future technologies, in particular the potential benefits offered by using inert anodes. Its MSB (Market Strategy Board) September 2010 White Paper, "Coping with the Energy Challenge – The IEC's role from 2010 to 2030", makes a series of recommendations regarding measures necessary for EEE (electrical energy efficiency). "Inert anodes for aluminium smelters" are clearly identified as a priority in the list of technologies that require input for further development.

The feasibility of producing inert anodes has long been disputed; however, it seems that this objective is now in sight. Rusal has said that it already has a material for inert anodes. It is currently improving it and retrofitting the cells' structure to suit the new technology. Rusal plans to start operating the first inert anode cells as early as 2015. It claims that inert anodes will allow it to cut operational costs by 10 % through reduced anode and energy consumption and will have considerable environmental benefits too: according to Rusal a single reduction cell will be able to generate the same amount of oxygen as 70 hectares of forest.

Owing to the growing demand for more energy-efficient and environmental-friendly products, aluminium has a bright future. The latest technologies will ensure the whole aluminium production chain will become also more cost effective.

Aluminium production at the Wenatchee smelter in Washington state, US (Photo © 2011 Alcoa) Aluminium production at the Wenatchee smelter in Washington state, US (Photo © 2011 Alcoa)
London: on his toes since 1893, Anteros was one of the first statues cast in aluminium London: on his toes since 1893, Anteros was one of the first statues cast in aluminium
Aluminium scrap ready for recycling (Photo: © Norsk Hydro) Aluminium scrap ready for recycling (Photo: © Norsk Hydro)