Secondary aluminium

The important role of recycled aluminum

One third of world consumption

Today, secondary aluminum accounts for about a third of the world's aluminum consumption.. Initial reasons for recycling were commercial, as well as environmental. Recycled aluminum provides significant energy savings, since it is used around 5% energy, necessary for the production of primary aluminum. In the Western world, recycling rates are high: from 60 % for used beverage cans (UBC) up to 85 % in construction and 95 % in transport. The quality of aluminum does not deteriorate during recycling - it can be recycled, therefore aluminum retains a high scrap value. Almost 100% scrap, formed during the production of aluminum products, recycled.

• Dominant use (approx. 70%) recycled aluminum accounts for the production of aluminum-silicon foundry alloys, which are mainly used in the automotive industry, and the rest is converted into wrought alloys for reuse as sheets or extruded products. A small proportion is converted into a deoxidizer for the steel industry.
• Remelters and refiners

In most countries, there is a well-established secondary aluminum market with well-defined distribution chains. There are two types of recycled aluminum producers – refiners and remelters [1]:

• Refiners produce foundry alloys and deoxidized aluminum from old and new scrap and supply them in ingots or molten form. In addition to alloying according to the required specification, refiners can flux metal to improve its quality by removing oxides and intermetallics, and sometimes removing such elements, as magnesium and calcium.
• Remelters produce wrought alloys primarily from clean and sorted wrought alloy scrap and supply them as rolled slabs, extrusion blanks or ligatures. Alloys are supplied in accordance with international standards and/or customer specifications.

Fig. 1 – Place of remelters and refiners in aluminum production [2]

Sources of scrap aluminum

In Fig. 2 shows the distribution of recycled scrap by source markets. The first significant boost to aluminum recycling came from the humble aluminum can.. The aluminum can has indeed been the face of aluminum recycling since the 1970s.. Intense can collection marketing led to real value of aluminum scrap, and it was continued by both governments, and local communities in their quest to reduce the number of landfills and create a sustainable environment [1].

Fig. 2 – Global recycling scrap by market [1]

• Packaging, which accounts for only about 12% aluminum consumption. It's connected, including, with a relatively short service life of the aluminum package (Tabl. 1).
• Transportation is the most important industry for the application of aluminum. So, c 2007 almost a year 30 % forged and cast alloys went to transport, cars, train, ships, planes, etc.. d. [1].
• In the past few decades, aluminum has been increasingly used in automotive transmission components.. Since the average life of a car is 13-20 (Tabl. 1), this source of aluminum scrap is very important for the production of recycled aluminum.
• The construction industry accounts for about 25% aluminum consumption, but with a relatively long service life of 35–40 years, it has only recently become an important source of aluminum scrap [1].

Table 1 – Product Life and Recycling Effectiveness for Market Types of Aluminum Scrap [3]

Raw materials for secondary aluminum

Categories of aluminum scrap

Sources of raw materials for recycled aluminum include:

• technological waste of aluminum production – so-called “new” scrap
• used aluminum products
• metal shavings and
• snap, formed during the smelting process.

The chemical composition of each category of scrap will determine the industry, in which it will eventually be used. Silicon-containing aluminum casting alloys can accommodate a greater variety of waste, because their impurity limits are wider, than wrought alloys. However, casting alloys have a lower added value., than wrought alloys, and therefore the use of some waste should be limited not because of the composition, and based on cost. for instance, all aluminum-silicon casting alloys can be produced exclusively from extrusion waste and alloying additives, such as silicon, copper and t. d., however, the lower cost of cast alloys makes this unprofitable.. Most of the scrap, which is processed by remelters, represents internal production scrap from the production of billets and slabs.

All collected scrap is sorted, processed and classified into different categories of scrap for sale to refiners or remelters. Aluminum scrap is sold in accordance with international specifications. There is about 40 grades of aluminum scrap, defined by the Scrap Recycling Industry Institute (ISRI).

“New” scrap

The advantage of new industrial waste is that, that they can be easily separated into different alloys, They, usually, do not contain impurities, which may be contaminants, and require minimal labor for pre-treatment before melting.
In many cases, the scrap is uncoated and, Consequently, will have a higher possible metal yield. Scrap pretreatment may include pressing, cutting and color removal. This category of scrap also includes waste and scrap from foundries..
An example of typical ISRI specifications for new scrap are:

• Tough - production waste in the form of mixed scraps and pieces of unpainted aluminum
• Tata - technological waste and rejection of aluminum profiles from one alloy (Fig. 3).

Fig. 3 – Tata – new production aluminium extrusions.
Shall consist of one alloy (typically 6063).
May contain ‘butt ends’ from the extrusion process
but must be free of any foreign contamination.
Anodized material is acceptable [1]

“Old” scrap

This category will include products, which can last from a few weeks in the case of UBC to approx. 12 years for automotive sources or even 30 years or more in the case of building materials. They will be collected from a variety of sources and, usually, require sorting before any further pretreatment for melting. They will invariably be contaminated with one or more paints., varnish coatings, mud, plastic, oil, grease and various metallic and non-metallic nozzles.

Most Common ISRI Specifications “old man” breaks:

• Taint/tabor - pure mixed aluminum sheet, used (Fig.. 4)
• Tense - mixed aluminum castings (Fig. 5)
• Talon – aluminum scrap UBC in bags (Fig. 6)
• Telic - mixed aluminum shavings

Fig. 4 – Taint/tabor – clean mixed old alloy sheet aluminium.
Shall consist of clean old alloy aluminium sheet
of two or more alloys, free of oil, venetian blinds, castings,
hair wire, screen wire, food beverage containers,
radiator shells, airplane sheet, bottle caps,
plastic, dirt and other non-metallic items.
Oil and grease is not to total more than 1%.
Up to 10% of Tale permitted (painted siding) [1]

Fig. 5 – Tense – mixed aluminium castings.
Shall consist of clean aluminium castings
which may contain auto and airplane castings
but no ingots, and to be free of iron, brass,
dirt and other non-metallic items.
Oil and grease are not to total more than 2% [1]

Fig. 6 – Taldon – baled aluminium UBC scrap.
Shall have a minimum density of 225 kg/m³
for unflattened UBC and (353 kg/m³) for flattened UBC [1]

Production of secondary aluminum

Secondary aluminum melting furnace

Melting processes developed, which suppress the oxidation of aluminum during its melting. In the production of secondary aluminum is widely used kiln, in which scrap immediately immersed in a bath of liquid aluminum. there furnace, in which melting occurs at complete isolation of the melt from the surrounding air. Heavily contaminated aluminum scrap is melted under protective flux layer in special rotary kilns. Flux applied not only protects the aluminum melt from oxidation, but also helps to separate the oxide film from the liquid aluminum and purify it from the melt. In addition to use of flux in the aluminum melt, Additives used for melt purification or added small quantities of alloying elements.

More about furnaces, which are used in the production of secondary aluminum here.

Secondary aluminum smelting

Production of secondary aluminum alloys usually start with scrap, the chemical composition of which is close to the, that is specified in the order of the consumer. This can be easily, if it rejects a uniform scrap aluminum castings or scrap aluminum beer cans. And it is much more difficult, if it accidentally collected aluminum scrap.

In most cases, scrap, charged into the melter, It does not fully meet the requirements of the final chemical composition of a given aluminum alloy. It is therefore necessary to introduce into the melt more dopants. They may be from the existing scrap, pure elements or special alloys - ligatures. Typically the doping is performed in the part of the melting furnace, final doping - a casting furnace. Therefore, these furnaces must be adapted for such operations.

The aluminum melting furnaces using special additives for protecting molten metal from oxidation, removing impurities or grinding grains in the finished bars. Moreover, in itself produce the melt processing of such gases in the melting furnace or molten time of transmission to the casting equipment, how argon, chlorine or nitrogen, as well as mixtures of these gases.

Read more about recycled aluminum melt processing here.

Application of secondary aluminum

In the good old days of processing aluminum it was much easier. Scrap collecting good - and then there was little scrap - and just melted. Obtained in this secondary alloys are more or less arranged customers. They diluted casting alloys for casting castings at foundries. Problems with energy savings was not yet. With the growth of aluminum consumption and the use of its processing has become more complex. Now, customers want to get ready secondary aluminum alloys, each having a predetermined chemical composition, because many of these equipment allows only ready to melt the alloy and pouring it, and not to handle or to dope it.

Steel deoxidation

It is most convenient to use recycled aluminum in the production of ingots and granules - there is a lot of room for impurities, which are contained in the aluminum scrap. These ingots and pellets are used for deoxidation of steel and other needs of the steel industry.

Casting alloys

Casting alloys allow higher impurity content compared to wrought alloys, for example, gland. In die casting alloys, the maximum iron content can be up to 3,0 %. Therefore, the secondary aluminum is widely used in the preparation of foundry alloys.

Wrought alloys

wrought alloys contain much less impurities, including iron, than in cast alloys. Therefore their preparation from secondary aluminum - a difficult task. Usually, diluted secondary aluminum or pure primary aluminum production waste to limit the content of impurities, permitted by the requirements of the standard or order.

 

Sources:

1. Production of secondary aluminium /G. Wallace // Fundamentals of aluminium metallurgy – Ed. Roger Lumley – 2011
2. Aluminium Recycling in Europe The Road to High Quality Products – EAA/OEA Recycling Division
3. Aluminum Recycling /Mark E. Schlesinger – CRC Press, Taylor & Francis Group – 2007