Aluminum (Al) is the second most plentiful metallic element on earth. It has grown to be a great competitor for engineering applications since the end of 19th century, when it became economically viable. Aluminum is a light, conductive, and corrosion resistant metal with strong affinity for oxygen. This combination of properties has allowed aluminum to compete for a large number of applications. Currently, aluminum is the most widely used non-ferrous metal in the world, being present in different sectors such as transportation, packaging, construction, electricity, and medicine. The need for improved mechanical properties and new applications has led to the continuous development of new kinds of Al alloys with specific chemical composition.
Although aluminum is one of the most common elements on earth, it is too reactive with other elements to occur naturally. Bauxite ore is the primary source of alumina, i.e., aluminum oxide (Al2O3). The Bayer process, invented and patented in 1887, is the primary process by which alumina is extracted from bauxite ore and separated from red mud. The Hall–Héroult process, simultaneously discovered in 1886 by Charles Martin Hall and Paul Héroult, allows aluminum to be refined from alumina by means of electrolysis. Alumina is dissolved in a cryolite (Na3AlF6) bath with various fluoride salt additions in order to control the bath temperature, density, resistivity, and alumina solubility. Larger and more efficient plants have been developed and the process control has been improved, but the production concept remains basically unchanged
This is the primary route for Al production but a secondary route is available using Al scrap and recycling. It is claimed that recycling saves resources, decreases the need for landfill space and, in the case of non-renewable resources, such as metals, prolongs the necessary period to deplete them.
Compared to other high-volume materials, such as copper (Cu), zinc (Zn), magnesium (Mg), and steel, Al production has one of the widest energy differences between the primary and secondary routes, but not the main recycled fraction, that is the share of secondary production with respect to the total one below pic. The recycled Al fraction is about 35%, which is close to the values of recycled Mg and Zn (~30%). Nowadays, copper and steel remain the materials with the highest impact in terms of recycled amounts (~40%).
On the other side, Al recycling allows a reduction of 95% of the required energy, the highest value when compared to Mg, Cu, Zn and steel above pic and emits only 5% of the greenhouse gas. The production of secondary aluminum is estimated to consume an energy amount between 5 and 7 GJ/ton due to recent improvements. Furthermore, one ton of recycled aluminum saves up to 8 metric tons of bauxite, 14,000 kWh of energy, 6300 litres of oil, 7.6 m3 of landfill, and the average total exhaust emission is about 350 kg of CO2. Dust and air emissions from scrap processing are generally at a low level. However, the emission of hazardous air pollutants (e.g., dioxins and furans) may be generated during melting operations. Emissions from the furnace are being managed by means of suitable process control and special flue gas treatments.
The main waste of secondary Al production is the non-metallic residue coming from scrap smelting. It is often termed “salt cake” or “salt slag” and contains 5–7% of residual metallic aluminum, 15–30% aluminum oxide, 30–55% sodium chloride, 15–30% potassium chloride and, depending on the initial scrap type, carbides, nitrides, sulfides, and phosphides. The formation and recovery of salt slag must be considered from an environmental point of view because it is classified as toxic and hazardous waste; landfill disposal is forbidden in most of the European countries and it should be recycled and processed properly.
The chemical composition is the main challenge in Al recycling. Scrap originates from different Al alloys, with different alloying elements, in different amounts. This means that it is difficult to control the level of impurities but also difficult to obtain the targeted alloy composition. Both wrought and foundry alloys can be obtained by recycling, but they strongly differ. Casting alloys have higher alloying content than wrought ones. While the formers have a concentration of elements up to 20 wt %, the wrought alloys have up to 10 wt %. This difference distinguishes the recycling processes from the production process.
Remelters produce wrought alloys, usually in the form of extrusion billets and rolling ingots, from mainly clean and sorted wrought alloy scrap. Contrary, refiners are able to add alloying elements and to remove some undesired elements after the melting process, thus producing foundry alloys and de-oxidized metal from different types of scrap.
The feature of Al to absorb foreign and undesired elements, which are not normally described in the international standards, is sometimes a handicap. To remove impure elements from a molten bath is impractical or inconvenient. As a result, the scrap is usually recycled, which avoids the refinement stage. Two possible solutions are currently followed, i.e., downgrading and dilution. By downgrading, the low-alloyed scrap is used to obtain alloys with higher alloying contents, while, by dilution, the molten scrap is diluted with primary Al or low-alloyed scrap to reduce the concentration of elements below critical levels.
These strategies will gradually lead to a non-recyclable scrap surplus if no other solutions are considered. Alternatives may come from a greater standardization of the commercial alloys and, where possible, a wider range of accepted impurities. However, this is possible only with the definition of new regulations.
A sustainable solution is the improvement of the efficiency of Al recycling in the production chain, which includes the melting process but also several preliminary treatments of the scrap, such as sorting, commination, and thermal treatments, as shown in the flow chart of next pic. All of these stages lead to an increase in cost but, on the other hand, they improve the scrap quality in terms of metal yield and recyclability.
The selection of the melting furnace is a critical aspect and it depends on the quality and quantity of the scrap. Each solution has the same objective: generate the highest melting capacity per unit volume while maximizing the thermal efficiency in order to reduce the fuel cost and cycle time.
In order to minimize the dissolution and amount of hydrogen in the bath and the metal loss, and to remove metallic and non-metallic impurities, fluxing and refining are generally used. These techniques refer to the addition of chemical compounds to clean the molten bath and prevent the formation of oxide.
In the next sections, the main treatments used to upgrade the scrap are critically reviewed and discussed, considering both the consolidated technologies and the innovative solutions. In addition, the melting phase is critically analysed in terms of technological evolution and furnace selection, as it is the most important choice to optimize the melting rate. Fluxing and slag treatments have also been considered to complete the production chain.
The treatment of Al scrap to produce new Al metal and alloys is an alternative to primary Al production. The chemical composition of the alloys is strictly related to the scrap quality. Therefore, recycled aluminum presents a certain amount of impurities, generally not present in primary alloys, and the alloying elements are more difficult to manage.
Nowadays, this distinction is not completely exhausting. By properly selecting high quality scrap, a purity level close to primary alloys can be achieved in secondary alloys too.
Iron (Fe) plays an important role in distinguishing between primary and secondary Al alloys. This element cannot be easily removed from the molten metal and it forms generally brittle intermetallic compounds that influence the final mechanical properties of the components.
Primary Al alloys present low Fe content and so they are used for applications where the best exploitation of some specific properties is required (mechanical strength, ductility, corrosion resistance, workability, weld ability, electrical conductivity); secondary alloys show good cast ability, which, combined with the natural low volume density of Al alloys, makes them suitable in high-pressure die casting.
Different types of furnaces for melting Al scrap are used, depending on the initial metal content in the scrap, type and content of impurities, geometry of the scrap, frequency of change in the alloy composition, operating conditions, energy cost, and desired product quality.
Where energy cost is high as in Europe, the energy efficiency has been an operating priority for many years. For this reason, rotary furnaces are more common than reverberatory furnaces in Europe. In contrast, in the United States 95% of Al scrap is melted in gas reverberatory furnaces, which operate with a lower energy efficiency (20–30%) and require lower capital cost. They are easier to operate and maintain than rotary furnaces.
Two important criteria to be considered during furnace selection are the metal content in the scrap (metal yield) and the production volume. Below pic reports the main available solutions considering these features.
A brief description of the different furnace types is hereafter reported, from the consolidated technologies till their actual evolution.
The main difference is between electric and fossil-fuel furnaces. Most of the secondary aluminum is produced in furnaces fired with fossil-fuels, commonly natural gas, where reverberatory and rotary furnaces are the main technologies.
Electric furnaces, typically used in small processing operations, have some advantages over fossil-fuel furnaces for melting Al scrap. Firstly, the exhaust gas is much lower because no combustion products exist. Therefore, dross generation is much less and the metal purity is improved. Electric furnaces have 0.5% to 3% metal loss compared to 5% to 8% loss in fossil-fuel furnaces.
A side well is provided for charging scrap, thus removing the need to open the furnace door. This prevents great convective heat loss. The electric furnaces are generally more efficient than gas furnaces, especially for small sized scrap and they are less noisy. Energy losses are typically 0.49 to 0.81 kWh/kg of aluminum. Induction furnaces are typically more than 90% energy efficient, while gas-fired crucibles are 15% to 28%, and electrically heated crucibles are 83% efficient in terms of energy use.
The stirring motion of an induction furnace minimizes temperature gradients within the melt, improving consistency.
On the other side, there are important disadvantages: electricity is often more expensive than fossil fuels, which eliminates the cost advantage. Further, electric furnaces cannot compete in terms of melting capacity with large-scale fossil-fuel furnaces.
As a result, electric furnaces are mainly found in small-volume operating systems where Al scrap is usually a home-made rather than purchased material.
Reverberatory furnaces are brick-lined and constructed with a curved roof. Furnace design is simple, rectangular or round, depending on the specific application. The rectangular design with the front door across the full furnace width allows for maximum access during charging and skimming. The molten metal is held inside the furnace at the required temperature before tapping.
Typical reverberatory furnaces present energy efficiency, i.e., the ratio between the amount of heat absorbed by the raw material and the amount of heat from the total consumed fuel, in the range of 15–39%. The main advantages provided by reverberatory furnaces are the high volume processing rate, and the low operating and maintenance costs. The disadvantages refer to the high metal oxidation rate, low energy efficiency, and large space requirements.
The earliest and simplest type of reverberatory furnace is the wet-heart single chamber furnace, where scrap is simply loaded into the furnace, the door is closed and melting begins. Usually, a heel of molten metal is left inside the chamber bottom after tapping in order to facilitate the melting process of the new charge.
The dry-heart furnace was the evolution of the wet-heart furnace. A sloping hearth is present before the melting zone onto which solid scrap is placed for initial heating. Metal remains on the dry hearth until the bath temperature has recovered the set point. At this time, the preheated and semi-molten charge is pushed into the bath and cold metal is placed again onto the dry hearth. This solution allows different advantages: the average melting rate is improved, the energy consumption is reduced, and the operating time of the furnace increases.
The stack furnaces can be considered as modified reverberatory furnaces where the efficiency is improved by better sealing of the furnace and the use of the flue gases to preheat the charge. Here, the scrap is charged directly into the exhaust stack, forcing the exhaust gas to pass through. As the heated scrap descends to the sloping hearth, additional burners melt it, causing it to flow into the molten bath. This in turn allows more scrap to descend to the hearth, creating a semi-continuous melting operation. The use of the hot exhaust gases to preheat the incoming charge improves the energy efficiency of the furnace by 40% to 50%. The height of the stack furnace is not less than six meters due to the charging mechanism, and this is clearly a constructive disadvantage. Consequently, the refractory at the bottom of the charging door is greatly stressed by repeated impact and wear.
An important evolution in the furnace design is represented by the multi-chamber furnaces which are generally based on integrated scrap preheating/delaquering and submerged melting process. They are designed for remelting scrap with impurities such as oil, paint, and plastic. In the preheat/gasification compartment, the scrap load is exposed to an intense hot gas flow and the organic compounds are transformed into combustible gases. The combustion and post-combustion take place in all the furnace chambers. The melting takes place not from direct flame impingement but from the heat coming from the molten metal (submerged). This reduces drastically the metal loss without using any flux.
Several improvements on this type of furnace have been implemented with two objectives: increasing the energy efficient without affecting the metal recovery, and, if possible, increasing both. Exhaust gas recirculation, regenerative burners, and molten metal pumps have obtained important results.
A rotary furnace consists of a cylindrical steel drum internally covered with refractory below pic. The scrap feed is charged into the rotary furnace, which is heated by a burner of natural gas. Rotary furnaces are faster and more efficient than ordinary reverberatory furnaces. Higher melting rates, reduced emissions, consistent metal composition, and lower fuel consumption are achieved by this type of furnace. This is partially due to the rotation of the hot internal refractory, which transfers more heat to the charge via direct contact.
Rotary furnaces are more expensive to install and more difficult to maintain. As a result, they are generally best suited for melting dross and other oxidized scrap. The furnace fume is collected in a chamber where it is extracted by a gas cleaning system.
The melting process inside the rotary furnace is very complex and difficult to be experimentally studied. This is due to several reasons: random distribution of scrap and void, heterogeneity of scrap (type, size, shape), turbulence, gas combustion, mass, and energy transport. The melting involves mainly thermo-hydrodynamic processes, but also other mechanisms such chemical reactions, mass transfer, phase change, surface reactions, porous media flow, free surface flow, combustion, radiative transfer, and fluid-solid interaction exist.
Numerical modeling and simulation play a key role for improving the process. By means of these tools, it is now possible to study the melting rate and the energy distribution, which increases the efficiency of the combustion process, to optimize the furnace design (location of the fuel burners, consumption of the refractory walls, etc.), to reduce the pollutant emissions, and to assure the product quality.
At the end of the melting stage, the furnace is stopped and the molten metal is discharged and tapped into a holding furnace, further refined and directly transported to the industrial partners or cast into ingot molds. The liquid melting flux used in rotary furnaces floats over the molten bath and is removed as salt slag.
The most important innovation for rotary furnaces is the transition from stationary drums to tilting drums. The ability of the furnace to tilt minimizes the amount of time spent on non-melting operations such as charging, tapping, drossing off, and cleaning. Tilting rotary furnaces can melt high quality scrap without the use of fluxes.
Pre-treated scrap is generally charged into the melting furnace mixed with fluxes. Scrap may be charged as high density bales, loosely packed bales, or as dry shredded scrap from a conveyor. In order to minimize the aluminum oxidation, and consequently the melt loss, the scrap is mechanically submerged into the liquid bath as quickly as possible. The melting process is aimed to maximize the metal recovery, i.e., the ratio between the aluminum present in the scrap and the secondary aluminum obtained. The energy consumption and the harmful gas emission are considered too.
Fluxing indicates the addition of chemical compounds in the scrap feed to improve the recovery of aluminum and the quality too. Fluxes are usually classified depending on their application. Four categories can be individuated: cover and drossing fluxes, cleaning fluxes, and furnace wall cleaning fluxes.
Most salt fluxes are made from sodium and potassium chlorides. They present a melting point of 801 and 771 °C respectively, but they form a lower temperature eutectic at 657 °C and a high fluidity mixture if the flux is based on an equimolar ratio of these chlorides.
Fluxes based on chloride do not react with the molten metal. To increase the reactivity of the flux and the removing efficiency of inclusions from the melts, cryolite or other fluorides may be added, such as Na3AlF6, CaF2, Na2SiF6, that accelerate the wettability with oxides and inclusions. In this way, the magnesium removal from the aluminum scrap is enhanced and the aluminum recovered from the dross is increased.
The addition of fluorides to the equimolar chloride mixture decreases the interfacial tension between the salt flux and the aluminum, and the viscosity of the final mixture favors the coalescence of the Al droplets within the salt flux cover. The salt also promotes the stripping of the Al oxide layer according to a mechanism similar to the hot corrosion process.
Fluxing is temperature dependent. Temperature must be appropriately selected to provide for good contact and reactivity, and for achieving a good physical separation. Excessive temperature increases energy loss and causes fume and gas formation. It causes the fluxing treatment to make skimming more difficult and it reduces the accuracy and efficiency of the refining process.
Depending on the specific situation, refining treatments can be also carried out, such as degassing and damaging. The first is the simplest method to remove dissolved hydrogen and sodium, and it can be achieved by purging gas with inert as well as reactive gases, the application of a vacuum, tableted flux degassing, or mechanical stirring. On the other side, damaging fluxes are used when the melt contains excessive amounts of magnesium. The flux helps to reduce the magnesium content by burning (oxidizing) it from the melt.
Molten aluminum, both from primary and secondary production, generates residues containing Al, Al oxides, and other impurities.
The red mud produced during the Bayer process is the most important waste generated in the primary Al route. Depending upon the quality of the ore, between 1.9 and 3.6 tons of bauxite is required to produce 1 ton of alumina. The major components of red mud are Fe, Si and titanium (Ti) oxides, but also Zn, phosphorous (P), nickel (Ni) and vanadium (V) oxides. These are acidic oxides which are not dissolved in the Bayer process. Waste management of red mud is usually carried out by means of controlled landfill disposal.
Various types of waste can be generated during the second Al melting process. Residues with more than 45% Al are called “skimming”, while material containing less than 45% Al is called “dross”.
Dross is classified according to the metal content into white dross, generated from primary smelter, and black dross, from secondary refiners. White dross may contain from 15% to 70% recoverable metallic aluminum and it comprises a fine powder from skimming the molten aluminum. Black dross typically contains a mixture of Al oxides and slag, with recoverable Al content ranging between 12% and 18%, and a much higher salt content than the white dross, typically greater than 40%. The non-metallic residues generated from the dross smelting operations contain metal beads, crystallized salt, and solid non-metallic particles.
The amount and composition of the salt depend on the initial scrap mix and the melting furnace used. The non-metallic compounds consist mainly of Al oxides, oxides of alloying elements (Si, Cu, Fe, Zn, etc.), spinels, Al4C3, AlN, and AlP.
Salt slag is classified as toxic and a hazardous waste according to the European catalogue for hazardous wastes. below pic shows the main properties of the salt slag.
There are two possibilities for the management of the salt cake: the separation of its components for possible recovery and application, or the storage in controlled landfills. The main problem for the storage is the leachability and the high reactivity with water or humidity of air.
Since salt slag contains a large amount of NaCl and KCl, it would release the chlorides into water when in contact with waterfall or groundwater. The gaseous emissions resulting from the contact of the salt slag with water could also have a great environmental impact due to the presence of toxic, harmful, explosive, poisonous, and unpleasant odorous gases, such as NH3, CH4, PH3, H2, and H2S.
Aluminum metal, salt, and compounds containing alumina are recoverable as high quality products for the recycling process, or sold as non-toxic material. Today, the treatment of the salt slag is generally done in the US, Canada, and Europe, where landfill is prohibited by law. The treatment of the salt cake also has a considerable economic impact. Actually, the recovery of the residual metallic aluminum and the salt fraction (halite, NaCl, and sylvite, KCl) is permitted and this justifies, for large refining companies, the investment in an on-site salt cake recycling facility.
It is shown that the generated residues can be considered non-toxic and the alumina-containing compounds become a new raw material for other processes and applications, such as refractory materials, aluminum composites, and a high temperature additive for de-sulphurizing steel.
Secondary Al production is continuously increasing because it offers economic and environmental advantages. Different types of Al scrap exist according to the amount of alloying elements and impurities, even if the most essential input material for Al recycling remains metal recovered from fabrication aluminum scrap. Large amounts of Al scrap are currently recycled by downgrading and dilution, due to difficulties in refining. These two strategies induce a surplus of Al scrap that can’t be used in the recycling chain.
The innovations in recycling processes are focused on increasing the Al scrap value and extending the capacity of melting different types of scrap, both of high and low quality.
Though the new integrated design processing of products should be developed and introduced from the beginning to optimize the comminution process, several efforts have been made to improve the comminution process through a reduction of energy consumption and an increase in the lives of the shredders. The increase of the angular speed in the swing-hammer shredders leads to a reduction of whole energy consumption, however with greater dust and noise emission.
Other solutions may help to separate Al scrap from other materials, such as high-pressure water-jets, which have been recently applied to the disassembly of washing machines, car seats, and computers.
Sorting of solid scrap streams is a key stage to optimize the final quality of recycled Al alloys. Different types of scrap require various sorting methods and several results can be obtained. Innovative sorting methods distinguish the Al scrap by analyzing the concentration of main alloying elements in the Al fragment; XRF, LIBS, PGNAA techniques have already demonstrated to be potential and valid solutions in the field.
Color, shape, and the apparent density of scrap can also support the sorting decision. By means of a 3D imaging camera, equipped with a linear laser, and an optical CCD, each Al fragment may be distinguished and sorted. The aim is not only to remove endogenous materials as in the past, but to get the cleanest possible Al scrap and to classify it in the different alloy groups. A close loop recycling would allow the targeting of an Al alloy from a scrap of the same alloy, thus reducing the refining problem. Evolutions in the melting furnaces are nowadays focused on reducing the consumed energy and increasing the metal recovery.
The selection of the melting furnace is a critical aspect and it depends on the quality and quantity of scrap. Each solution has the same objective: generate the highest melting capacity per unit volume while maximizing the thermal efficiency to reduce the energy cost and cycle time. Electric furnaces are typically used in small processing operations, i.e., where Al scrap is usually home-made rather than purchased material. The electric furnaces cannot compete in terms of melting capacity with the large-scale fossil-fuel furnaces, such as reverberatory and rotary furnaces, which remain the main technologies. Several improvements on these furnaces have been implemented. Important results have been achieved by exhaust gas recirculation, regenerative burners, and molten metal pumps.
Many technologies and innovations from scrap upgrading to the melting process appear to be robust and warrant further research and development in the Al recycling process.