Disposal and recycling of aerospace materials
The disposal and recycling of aerospace materials is an important issue in the whole-of-life management of aircraft. The use of sustainable materials is becoming more important as the aerospace industry moves towards a ‘cradle-to-beyond the grave’ approach in the management of aircraft. Until recently, the selection of materials for aircraft structures and engines was based on cost considerations and performance requirements. Materials are selected on economic considerations such as the costs of purchase, manufacturing, assembly, and in-service maintenance. Materials are also selected on performance requirements such as stiffness, strength, toughness, fatigue life, corrosion resistance, maximum operating temperature and so forth. The majority of the aerospace industry has previously given little consideration to the materials beyond the end-of-life of the aircraft. In the past, end-of-life meant the day the aircraft was taken out-of-service, never to fly again.
The meaning of end-of-life for aerospace materials has recently changed in an important way. There is a growing understanding in the aerospace industry that end-of-life no longer means when the aircraft is taken out-of-service, but extends beyond this point to include the management of the aircraft after end-of-life (or ‘beyond the grave’). Governments, environmental organisations, and the wider public are placing greater demands on the responsible management of products beyond the end-of-life when made using nonrenewable resources. There are growing expectations that products produced in large quantities can be recycled so their materials can be reused rather than being disposed via landfill.
Recycling reduces the demand for the production of new metals, which involves mining, extraction and refinement processes; all of which are environmentally harmful. Recycling may also reduce the need for new composite materials, which are produced using nonrenewable petroleum products and use energy-intensive manufacturing processes. The other benefit of recycling is the reduced demand on landfill and other hard waste disposal methods.
Until recently, the recycling of aircraft materials was not a major consideration for the aerospace industry. For decades, the majority of private, civil and military aircraft ended their days in graveyard sites, such as the Mojave Desert in California which stores many thousands of retired aircraft and helicopters (Fig. 24.1). Aircraft are too large to bury as landfill, and are left in remote locations such as the Mojave Desert where the dry environment slows the destruction of the airframe, engines and avionics systems. Some of these old aircraft are used for ground training purposes, others are cannibalised for spare parts, and others are dismantled for recycling.
The aircraft recycling rate is currently about 60%, with the remainder representing aircraft that are left to decay. However, the pressure on these graveyard sites intensifies as greater numbers of passenger aircraft reach their end-of-life in coming years. Figure 24.2 show the retirement of aircraft per year between the years 1990 and 2012; over this period the number of retirements per year has increased by more than 500%. The number of retirements per year typically accounts for 1–3% of the entire fleet. Airbus estimates about 6400 airliners will retire before 2026.
The majority of aircraft in graveyard sites are constructed mostly of aluminium alloy. Most of the fuselage and wings of old civil and military aircraft are made using aluminium, which can be sold as scrap for recycling. With the greater use of composite materials in aircraft over the past ten to twenty years it is expected that the recycling of carbon fibre–epoxy will become increasingly important.
Growing global concerns about the environmental impact of retired aircraft as well as economic efficiencies are beginning to drive the aerospace industry towards greater recycling. Several programmes and organisations have been established to manage the recycling of aircraft, such as the PAMELA project (Process for Advanced Management for End of Life Aircraft) by Airbus and the AFRA project (Aircraft Fleet Recycling Association) by Boeing. The aerospace industry is aiming to increase the recycling rate of aircraft materials from the current level of 60 to 80–90%. The increased recycling target is set in a future environment where greater numbers of aircraft are being retired. For this reason, an important factor in the design of new aircraft, which will eventually be taken out-of-service many years later, is the selection of sustainable materials that can be recycled. The aerospace industry has responsibility for considering the management of the aircraft beyond the end-of-life. This involves consideration in the design phase of the recycling of the entire aircraft, including the body, landing gear, cabin fittings, engines, avionic and hydraulic/control systems.
Sustainable engineering is a key issue in the design, manufacture and in-service support of aircraft and helicopters. An important component to sustainable engineering is the use of materials that can be recycled with little or no impact on the environment. As a minimum, recycling should be less harmful to the environment than the production of new material from nonrenewable resources. Table 24.1 shows the amount of energy consumed in the production of several metals used in aircraft from ore or recycled material, and the energy savings achieved with recycling are great. Table 24.2 shows the amount of carbon dioxide produced per kilogram of metal produced from the ore or recycled product. Producing metals from the processing of scrap consumes a lot less energy and generates less greenhouse gas than producing from the ore. Extraction of metal from ore requires a large amount of energy because the ore must be mined, shipped great distances, freed from tailings, and reduced or smelted. The aerospace industry is also concerned with the economic impact of sustainable engineering. The cost of the recycling process, which includes the costs of removing the material from the retired aircraft, cleaning the stripped material, cutting and grinding the material into small pieces, transporting the scrap to the recycling processing plant, and the recycling process itself, should be at most equivalent to the cost of using new material.
In this chapter, we examine the recycling of aerospace materials. The recycling of metals and fibre–polymer composites used in aircraft structures and engine components is reviewed. The recycling methods described in this chapter are used not only to treat materials reclaimed from retired aircraft, but are also used to recycle waste material generated during the production of aircraft structures and engine components. The trimmings, swarf and other material removed in the casting, milling, machining and drilling of metal components can be recycled using the same methods as metal components removed from retired aircraft. The production of intricate metal components can require the removal of a large percentage of the original material (in some cases up to 90%), which should be recycled as part of the sustainable engineering approach to aircraft production. The manufacture of composite components also results in trimmed and dust materials, which can also be recycled using the same processing methods as used for composite parts removed from old aircraft.
Aluminium is the most common metal used in civil and military aircraft and, therefore, the ability to recycle this material using an economically viable and environmentally friendly process is essential to sustainable aerospace engineering. Aluminium is the second most recycled metal (after steel), and about one-third of all aluminium is extracted from scrap products. The most common source of scrap aluminium is general purpose items, such as beverage cans and household products. Aluminium reclaimed from aircraft is a small but growing source of scrap material.
There are several important benefits in the recycling of aluminium. Firstly, the quality of aluminium is not impaired by recycling; the metal can be recycled repeatedly without any adverse affect on the properties. Secondly, aluminium recycling is less expensive than the production of new aluminium from ore. Recycling of aluminium generally results in significant cost savings over the production of new aluminium, even when the costs of collection, separation and recycling are taken into account. As a result, it is financially viable to recycle scrap aluminium from aircraft, and based on current price the estimated value for reclaimed aluminium from the skin and airframe of a Boeing 747 is in the range $200 000 – $250 000. The process of aluminium recycling simply involves re-melting the metal, which is much cheaper and less energy intensive than producing new aluminium by electrolytic extraction (via the Bayer process) from bauxite ore. Recycling scrap aluminium requires about 5% of the energy needed to produce new aluminium. One of the challenges with recycling aluminium or any other metal is the unstable price of the recycled material. For instance, Fig. 24.3 shows the fluctuations in the price of recycled aluminium from 1990 to 2005, with the price adjusted for inflation. The price can vary greatly over relatively short periods of time, and this affects the financial profitability of the recycling process. Although the price of new metal also changes, it is almost always more expensive than recycled metal.
The third key benefit of aluminium recycling is that it is less polluting than producing new material. The environmental benefits of recycling aluminium are enormous. The electrolytic extraction of aluminium from bauxite is an energy-intensive process requiring large amounts of electricity. For example, the production of new aluminium in the United States consumes about 3% of the national energy requirements. The amount of nonrenewable resources (e.g. coal, liquefied gas) needed to generate the electricity for the melting and refinement of scrap aluminium is much less than the production of new aluminium. Therefore, scrap aluminium has less environmental impact because less energy is needed to power the recycling process and therefore fewer greenhouse gases and other pollutants are generated. It is estimated that recycling 1 kg of aluminium saves up to 6 kg of bauxite, 4 kg of chemical products used in the electrolytic refinement process, and 14 kWh of electricity. Therefore, in the recycling of a mid-sized passenger aircraft, which contains about 20 tonne of aluminium, over 130 tonne of bauxite is saved along with 300 MWh of electricity needed to extract the metal from the ore. If brown coal is used to generate the electricity, then recycling also saves over 100 tonne of CO2 and other pollutants.
Recycling of scrap aluminium from aircraft is a simple process. The aluminium components are removed from the aircraft, cut and shredded into small pieces, and then chemically treated to remove paint, oils, fuels and other contaminants. The aluminium pieces are compressed into blocks and then melted inside a furnace at 750 ± 100 °C. Refining chemicals (e.g. hexachloroethane, ammonium perchlorate) are added to the molten aluminium. The furnace is tapped to cast the refined aluminium into ingots, billets, rods or other product forms. The cast aluminium can be reused for any application, including the production of new aircraft parts.
The use of magnesium in modern aircraft and helicopters is very small (typically under 1–2% of the airframe weight), although its high value makes recycling economically viable. The process of recycling magnesium is relatively simple. Scrap magnesium removed from the aircraft is stripped of paints and coatings, cleaned and then melted inside a steel crucible at 675–700 °C. As magnesium melts, there is the risk of ignition and burning owing to high-temperature oxidation reactions with air. To suppress burning, the melt is protected from oxidation by a covering of flux agents or an inertgas stream. Once the scrap has completely melted and purified, the molten magnesium is cast into ingot moulds for reuse.
In addition to the melting process, magnesium is recycled by grinding the scrap into powder for steel production where it is used to remove sulfur impurities in molten iron. This use is limited to relatively pure magnesium scrap with a low alloy content, otherwise the alloying elements may contaminate the steel.
The aerospace industry is a large supplier of scrap titanium, which is removed from engines and structural components on aircraft and helicopters. Titanium is a valuable metal and therefore the strong economic incentive exists to recycle. The environmental impact of recycling titanium is less than producing new metal from the ore and, therefore, an environmental incentive for recycling also exists.
Recycling of titanium is more complicated than for aluminium because the metal is reactive at high temperature. Titanium is recycled by melting the scrap at high temperature (above 1700 °C). Liquid titanium reacts with nearly all refractory furnace linings as well as with oxygen and nitrogen in the atmosphere. Titanium cannot be melted in an open-air furnace because of oxidation. For this reason, the melting process is performed in a vacuum or inert atmosphere inside a furnace lined with nonreactive refractory material.
Vacuum-arc remelting (VAR), as the name implies, is a process whereby metal is melted under vacuum inside an electric arc furnace. The recycling process begins by forming the scrap titanium into a cylinder which serves as an electrode inside the arc furnace. The titanium electrode is placed just above a small amount of titanium resting at the bottom of a large crucible. An electric current flows into the upper electrode and creates an arc with the underlying titanium. The temperature generated by the electric arc causes the base of the titanium electrode to melt, with the liquid metal dripping into the crucible. The low pressure inside the furnace suppresses oxidation of the molten titanium and inhibits the formation of brittle titanium nitride precipitates by the reaction with atmospheric nitrogen. The vacuum also removes dissolved gases from the melt. After the electrode has completely melted, the titanium is cast into vacuum-sealed moulds for re-use in new aircraft components or nonaerospace applications (e.g. medical devices such as joint replacements and armour plates for military vehicles).
Steel used in landing gear and other highly-loaded structures can be recycled. Steel is the most recycled of all the metals, with about 60% of steel products being recycled. The most common products for scrap steel are automobiles, food cans and appliances; scrap from aircraft represents a tiny fraction of the total amount of recycled steel. The recycling of steel involves melting the scrap metal at high temperature (1600–1700 °C) inside a furnace. Chemicals are added to the molten steel to remove carbon and other alloying elements as dross. The purified iron is then cast into ingots, billets or some other product for reuse.
There are strong environmental reasons for recycling the steel in aircraft. Every tonne of steel that is recycled makes the following environmental savings: 1.5 tonne of iron ore; 0.5 tonne of coal; about 250% energy saving compared with making new steel from iron ore; reduction of carbon dioxide and other gas emissions into the atmosphere by over 80%; reduction of slag and other solid waste products of 1.28 tonne. The only major problem with steel recycling for the aerospace industry is it is not profitable. The cost of removing steel components, cutting them into small pieces, transporting them to the recycling plant, and then recycling and casting the metal is greater than the sale value of the recycled steel. Table 24.3 gives the relative prices of scrap metals from aircraft, and steel is much less valuable than the other materials. The main incentive for the aerospace industry to recycle steel is environmental rather than economic.
The recycling of superalloys from jet engines is attractive because of the relatively high price of nickel scrap. The recycling of superalloy can be divided into two categories: air-melted and vacuum-melted processes. Air melting, as the name implies, involves melting the scrap superalloy at about 1600 °C inside a furnace under normal atmospheric conditions. The molten scrap is refined by the removal of alloying elements, some of which are extremely valuable (e.g. W, Nb, Ta, Hf, V) and undergo further recycling and recovery. The purified nickel is then cast into ingots for reuse. Vacuum melting involves the melting and refinement of nickel scrap in a low-pressure furnace to eliminate dissolved gases and impurities. The low-pressure atmosphere is needed to produce high-purity nickel, which is virtually free of detrimental impurities. This process is used to recycle scrap in the production of superalloys for jet engine components, such as blades and discs.
The disposal of composites in an environmentally friendly way is emerging as one of the most daunting challenges facing the aerospace industry. Carbon–epoxy composites are not sustainable materials because the thermoset polymer matrix cannot be recycled. The cross-linking of thermoset polymers is an irreversible process that cannot be undone when the material is ready for recycling. Another problem is that the cost of recycling composite material is not competitive with the price of using new material. The cost of recycling carbon fibre–epoxy is greater than the cost of new material. Furthermore, the mechanical properties of reprocessed composite are lower than the original material, and are usually too low to find application in high-performance structures requiring high stiffness and strength. For these reasons, the current practice for disposing of most composite products is landfill. Not only does this pose an environmental problem because of the many thousands of tonnes of waste material that occupy landfill, but the polymers and fibres are extremely durable and take many decades (or centuries) to break down in soil. There is approximately one million tonnes of composites manufactured each year (including materials for aerospace applications). Europe has introduced new regulations on the control of waste organic materials such as polymer composites. It is illegal to dispose of composites by landfill in many European countries whereas other countries have specified maximum limits which are well below the current amounts that need to be disposed. Other countries outside Europe are also enforcing stringent regulations on the disposal of waste composite.
Despite the challenges with recycling, various reprocessing techniques are available which are classified as regrinding, thermal or chemical processes. Regrinding is the simplest and cheapest recycling process; it involves cutting, grinding or chipping the waste composite down to a suitable size to be used as filler material in new moulded composite products. The maximum particle size for most products is under several millimetres. Whereas regrinding is a simple process, the problem with using ground material in new products is that the continuous fibres have been broken down into small fragments, and thereby lost their ability to provide high stiffness and strength.
Thermal recycling involves the incineration of waste composite to burn off the polymer matrix and reclaim the fibres for reuse. Waste composite is incinerated above 500 °C in the absence of oxygen to break down the polymer matrix into oil/wax, char and gas. The process generates a large amount of greenhouse gas. The fibres are recovered for reuse after the matrix has been removed, but their mechanical strength is reduced by the high temperature needed to decompose the polymer. The strengths of both carbon and glass fibre decrease rapidly with increasing temperature above 300–400 °C, and the temperatures used to incinerate epoxy matrix composites (500–600 °C) result in a fibre strength loss of 80–95%. The large strength reduction means that recycled fibres are not suitable for use in high-performance structures. An added problem is that the cost of recycling composites by high-temperature incineration is often greater than the original cost of the material, and there is no financial incentive to reclaim fibres. Recycling by low-temperature incineration is currently under development to minimise the loss in fibre strength, but the process is not ready for large-scale processing.
Although reclaimed fibres cannot be used in aerospace applications, there is a potentially large market for low-grade carbon fibre in other industries. It is believed that the net profit from reclaiming carbon fibres from pyrolysis is about $5 k− 1. This translates to $275 000 in combined scrap value for a B787-8, with similar figures for a A350. Considering both B737 and A320 may be superseded by high-composite replacements within the next ten years, the potential reclamation value is several billions of dollars. Clearly, there is a commercial case for advancing composite recycling technology.
Chemical processing is another approach to reclaim the fibres in composite materials. The process involves using strong acid (e.g. nitric acid, sulfuric acid) or base solvent (e.g. hydrogen peroxide) to dissolve the polymer matrix, leaving the fibres for recovery and reuse. Acid or base digestion processes are less harmful to carbon fibres than thermal recycling, with only a 5–10% loss in strength. However, the chemical dissolution of the polymer matrix is slow, much slower than incineration, and therefore large digestion facilities are required for commercial-scale recycling.
Problems exist with the regrinding, thermal and chemical processes for recycling composite waste. The composite industry is investing in the development of new, more environmentally friendly and cost-effective processes. At the moment, however, the recycling of composite components from aircraft is not environmentally friendly or economically viable.
Materials selection must consider whole-of-life management issues, including the selection of sustainable materials that can be recycled using processes that are cheaper and more environmentally friendly than the processes used to make new materials. The recycling of structural and engine materials is becoming more important to the aerospace industry with the increasing rate of aircraft retirements.
The aerospace industry is moving towards high targets (above 80%) in the recycling of structural materials. About 60% of the airframe is currently recycled, but the industry is aiming to increase this to 80%. Careful consideration of the selection of sustainable materials in the design phase of aircraft is essential to ensure high levels of recycling.
Recycling of metals is possible without any loss in mechanical performance, and these materials can be recycled and reused an infinite number of times without any detrimental effect on properties. The energy consumed in the recycling of metals is much less than the energy needed to extract metal from ore. The commercial incentive to recycle metals such as titanium, nickel, aluminium and magnesium is strong because of the high sale value of the scrap, whereas the value of steel is much less.
The recycling of aluminium and steel components is performed using standard melting, refinement and casting processes. More specialist recycling processes are needed for the other metals, such as vacuum melting of titanium and nickel alloys to avoid excessive oxidation and to eliminate trapped gases in the molten metal.
Recycling of fibre–polymer composites is difficult, particularly with thermoset matrix materials. Composites are recycled using grinding, incineration or chemical processes, although the cost of recycling is not competitive against the cost of new material. Furthermore, the fibres are weakened by grinding and thermal recovery processes, thus limiting their reuse in structural products requiring high strength.