Strategy to Improve Recycling Yield of Aluminium Cans

Millions of canned drinks are consumed everyday globally and their wastes create an enviromental issue. Fortunately, the cans are made from aluminium (Al) so that it can be recycled. There are two main keypoints existing during the recycling process of Al cans, i.e. the aluminium loss or low Al-yield and low recycling yield. This work outlines the strategies to improve the recycling perfomance for Al beverage cans, i.e. by adding drossing flux, applying improved melting strategy, and cans decoating prior to melting. Drossing flux was added to assist the detachment of Al from the slag. Another improved melting strategy was worked out by decreasing exposure time cans to the furnace atmosphere during melting. All those above strategies result in an increase of recycle yield in a range of 4 % to 5 %.


Introduction
Aluminium is the 3 rd common used metal in the world and it is important metal for construction, automotive, airplane, and packaging industries. The construction of doors, windows, and facades, followed by walls and roofs is the main uses of aluminium in building. In automotive, aluminium is used as engine blocks, wheels, cylinder heads, gearboxes and many other automotive and engineering components. Aluminium also fits to be used as packaging of food and beverages due to its unique barrier and physical properties. Aluminium can effectively protects food and drink against the quality-reducing effects of oxygen, light, moisture, micro-organisms and unwanted aromas even in its thinnest form.
Consumption of soda and beer in aluminium cans reaches 2 × 10 11 cans every year [1]. If a can weighs 30 × 10 -3 kg, thus 2 × 10 11 cans provide 6 × 10 9 kg waste every year. This leads to growing environmental concerns of which is for the aluminium industry to be in a position to continue its growth while optimising its environmental performance. Recycling activity has to be boosted in order to conserve resources and to avoid littering.
From the enviromental point of view, aluminium recycling is not only beneficial due to reduction of aluminium waste but also due to its less energy consumption and greenhouse gas emission. Moreover, primary production of aluminium from bauxite is very energy intensive with the estimated energy consumption. Secondary aluminium production through recycling consumes much less energy, i.e. only 6 % of the energy consumption of primary aluminium production [2]. At present, around 1 % of the man made greenhouse comes from the aluminium industry. Recycling is an essential activity of sustained aluminium use, as more than a third of all the aluminium currently produced globally originates from old, traded and new scrap [3]. The use of old scrap, i.e. scrap from end of life products, is approximately 50 % of the scrap. If scrap is pre treated and/or sorted appropriately, the recycled aluminium can be utilised for almost all aluminium applications, thereby preserving raw materials and making considerable energy savings.
However, aluminium recycling experiences some challanges, i.e. aluminium oxidation which leads to aluminium loss as well as problem with purity of recycled aluminium. Carbon, silicon, and magnesium in steelmaking can be removed from liquid steel through oxygen blowing since oxidation of silicon and magnesium has more negative energy than ferrous oxidation. As a result, carbon goes to off-gas as CO or CO 2 , silicon and magnesium go to slag as SiO 2 and MgO, respectively. In contrary, alloying elements in aluminium alloy, e.g. silicon and magnesium, are difficult to be removed from aluminium alloy. If oxygen is injected into liquid aluminium alloy, aluminium oxidizes prior to oxidation of silicon and magnesium since aluminium oxidation has more negative Gibbs free energy. Aluminium loss thus occurs. Due to this circumstance, recycled aluminium tends to be used as similar products in the process. For example, recycled aluminium cans will be reused as aluminium cans as well.
To avoid oxidation of aluminium, fluxing is one of the solutions. There are several flux types are commonly used in aluminium recycling, i.e. covering flux, drossing flux, and cleaning flux [4,5]. Type of flux: (i) Cover flux is used to protect surface of liquid aluminium from oxidation and hydrogen absorption. (ii) Drossing flux is to separate trapped aluminium from oxides layer (iii) Cleaning flux is used to clean oxides from liquid aluminium. (iv) Furnace wall-cleaning flux prevents excessively formation of alumina on furnace wall. In this study, drossing flux was used and its function to increase the recycling performance was evaluated.

Recycling procedure 2.1 Chemical composition
The beverage cans used in this study were from Sprite drink cans. The chemical composition of the can body and lid is different ( Table 1). The body is made from alloy series 3 104 and the lid is from 5 052 [2,6]. The magnesium content in the body is higher than in recycled cans lid can be reused as cans lid material while cans body can be reused as cans body material.

Melting preparation
Prior to melting, cans are decoated since decoated can since preliminary research work shows that decoated cans were predicted to deliver higher recycling yield higher compared to coated cans [7]. After decoating, the cans body and lid are separated. The cans body were prepared into two different forms; first one was by squashing them into thin products (approximately 0.03 m high) and second form was sectioning them into small pieces of 0.02 m × 0.02 m. Those forms were chosen to accelerate the melting process by shortening the contact duration between cans body and furnace atmosphere which results in less aluminum oxidation.

Crucible coating
To prevent iron diffusion to the aluminium melt, the inner wall of the steel crucible used in this work was coated prior to melting process ( Figure 1). Iron diffusion to the aluminium should be prevented since iron deteriorates mechanical properties of aluminium alloys [3].

Melting process
Melting of aluminium cans was worked out in an electric resistance furnace.  Meanwhile, the use of flux followed a particular procedure: 1 × 10 -3 kg of flux containing NaCl and KCl was charged into the molten aluminium in three consecutive stages. The first flux was charged soon after the batch completely melted. The second charge was given after the last aluminum batch melted. Finally, the last one was charged afterward and the molten aluminium was put in hold for 6 × 10 2 s. From the experiment 1 and experiment 3, it was observed from the slag without drossing flux that some aluminium were adhered to the slag (Figure 2). The presence of this aluminium in slag leads to reduction of aluminium content in the melt. It can be seen on the photograph of the slag that the metallic (shiny) parts on the slag surface representing quite significant amount the free aluminium in the slag. On the other hand, slag formation in experiment 2, exp. 4, and exp. 5 (with flux addition) shows almost no free aluminium sticking on the slag. Additionally, pouring of molten aluminium to cast the samples was easier when drossing flux was used. This leads to shorten tap to tap time which improves recycling productivity.

Recycling yield
Recycling yield (Yield Recycle ) in this work is defined as the ratio of total metal mass after melting (m tap ) compared with the input mass of aluminium cans (m Cans,total ) (Eq. 1). Cans recycling with drossing flux shows higher recycling yield (55.2 % to 59.1 %) than recycling without flux use (52.5 % to 54.7 %) ( Table 3). Drossing flux increases the recycling yield of cans body 5 % higher than the yield obtained without drossing flux. The recycling yield of lid using drossing flux was also improved by 4.4 % higher compared with the process without drossing flux. Pirker et al. [9] also reported that NaCl and KCl increase recycle yield.
m Al,body = x Al,i m i (8) m Al,lid = x Al,i m i (9) Where i = cans body or cans lid Aluminium concentration after recycling of cans lid is higher than initial aluminium of cans lid (Table 5). Similar finding was identified with the aluminium content of recycling the cans body (Table 6). On the other hand, magnesium concentration after recycling of cans lid is slightly lower than the initial magnesium concentration. This condition occurs also on the recycling of cans body. In the Ellingham Diagram, magnesium oxidation shows more negative Gibbs free energy than aluminium oxidation. Thus, magnesium is more easily oxidized by the furnace atmosphere and also by oxides of other elements including Al 2 O 3 . Thus, the oxidized aluminium, both by furnace atmosphere and other oxides, may transfer its oxygen to magnesium resulting in magnesium loss. Exemplary reactions resulting magnesium loss are presented in Eq. 10 to Eq. 13.