Editorial Feature

Aluminium (Aluminum) Recycling Process

Aluminium is a silvery white metal and is insoluble in water under normal circumstances. Totaling circa 8% of the earth's crust, Aluminium is the third most abundant element in Nature. Aluminum is a highly reactive element hence is found in more than 270 different minerals.

The key Aluminium ore is bauxite. Aluminium is light-weight and ductile, malleable, easily machined and cast and also has high durability and corrosion resistance.

Molecular formula - Al

Density - 2.7 g/cm3

Melting point – 660.32 °C (1220.58°F)

Chemical and Physical Properties of Aluminium

Lightweight: Aluminium is very lightweight, enabling a higher amount of metal to be used offering high rigidity.

Paramagnetic: Aluminium and its alloys show slight paramagnetism, as the magnetic permeability of Al is slightly greater than one.

Electrical conductivity: Aluminium has a high electrical conductivity, hence is used as electrical conductors especially in overhead cables. This however needs a high purity grade. The electrical conductivity of 99.99% pure Aluminium is 63.8% of the International Annealed Copper Standard (IACS) at 200°C (392°F).

Thermal conductivity: Aluminium has a high thermal conductivity, hence is used in cooking utensils and heat exchangers. The thermal conductivity of 99.99% pure Aluminium is 244 W/mK at a temperature range of 0 - 1000°C (32 - 1832°F).

Corrosion resistance: Aluminium is resistant to rust and corrosion, hence Aluminium paint is widely used. This is mainly due to the thin yet tenacious film of oxide. In oxygen atmospheres, this oxide layer is present on the Aluminium surface.

Affinity for oxygen: Aluminium has a high affinity for oxygen and can be used as a deoxidant for steels, in explosives, in welding reactions and manufacturing hardener alloys.

Manufacturing Process of Virgin Aluminium

During the commercial manufacture of Aluminium, first Aluminium oxide is separated from the iron oxide present in bauxite. Aluminium oxide is dissolved in a concentrated solution of sodium hydroxide. Aluminium ions and hydroxide ions form a soluble complex while iron ions do not

         Al2O3xH2O(s) + 2 OH¯(aq) ↔ 2 Al(OH)4¯(aq) + (x3) H2O(l)

Insoluble iron oxide is filtered from the solution precipitating Al(OH)3 by the addition of acid to bring down the pH to about 6. Dry Al2O3 (alumina) is then obtained by heating the precipitate

         2 Al(OH)3(s) → Al2O3(s) + 3 H2O(g)

Alumina is then subjected to electrolysis by the Hall-Héroult process resulting in the formation of Aluminium metal. The melting temperature of pure alumina is more than 2000°C. At a temperature of 1000°C, alumina is dissolved in molten cryolite so that an electrolyte is obtained at lower temperature. An iron vat lined with graphite houses the electrolyte. Carbon anodes are inserted and the vat acts as the cathode. Carbon dioxide and carbon monoxide are formed by the reaction of oxygen produced at the anodes with carbon. Hence the anodes must be periodically replaced. This results in the production of molten Aluminium that sinks to the vat bottom.

The mixture is maintained in a molten state by passing current through the cell causing resistive heating of the electrolyte. The current used is 100,000A and the potential is 4-5 V.

Applications of Virgin Aluminium

Some of the key applications of virgin Aluminium are:

  • Transportation material including airplanes, automobiles, trucks, marine vessels, etc.
  • Packaging such as cans, foils
  • Construction such as doors, windows, siding
  • Consumer durables such as cooking utensils, appliances
  • Machinery
  • Electrical transmission lines, wiring, sheathing
  • In connecting rods, pistons to provide less friction, better balance and lower bearing loads implying less energy is needed to overcome inertia

Environmental Impacts of Aluminium

Pollution caused by Aluminium smelters are a serious concern since it affects vegetation, humans, soils and animals. Emitted substances include particulate and gaseous fluorides, carbon and alumina dust, carbon dioxide, carbon monoxide and gaseous sulfur dioxide. Fluorides are of serious environmental concern resulting in damage to animal and plant life. CF4, C2F6 and PFCs are extremely harmful to the environment.

Aluminium can hazardous when it gets accumulated in landfills especially due to acidifying problems. It is possible that Aluminium gets accumulated in plants and causes health issues for animals consuming these plants. Aluminium concentration is quite high in acidified lakes resulting in a decrease in the number of amphibians and fish since Aluminium ions react with the proteins in the fish gills and frog’s embryos.

Birds consuming these contaminated fish also get affected causing thinning of the eggshell and chicks with low birth-weight. Animals breathing in Aluminium may have lung problems, weight loss and reduction in activity levels. High Aluminium concentration in groundwater of acidified soils may damage the roots of trees.

Recycling Process of Aluminium

Aluminium definitely needs to be recycled and currently used alumiun beverage cans (UBCs) are being recycled largely. Recycling of a single Aluminium beverage can is said to save sufficient amount of energy that can run a television set for 3 h. Most of the recycled Aluminium is used to make new cans, which can return to the grocer's shelf in less than 60 days of being collected. Aluminium can be recycled ‘n’ number of times and the recycling process does not in anyway diminish the quality or properties of Aluminium. The recycling process utilizes just 5% of the energy needed to produce new Aluminium and just 5% of greenhouse gases are emitted. Hence around 75% of Aluminium ever produced is still being used today.

Recycling of used beverage cans follow the following steps:

  • Firstly, bales of UBCs are shredded ensuring that smelters do not receive trapped liquid and easy separation of large-sized impurities is done.
  • Secondly magnetic separation is done wherein iron-containing contaminants are removed.
  • Thirdly mechanical separation is done wherein denser contaminants such as zinc, lead, stainless steel and high-nickel iron are removed.
  • Additional processing include delacquering wherein cans are brought to temperatures just below melting, which is required in order to remove organic contaminants.
  • Thermomechanical separation includes further breakdown of lid alloy pieces and different alloys are separated by a filter.
  • Separate processing of the body materials and lid is done by melting the UBCs into ingots, removing the impurities as skim, which is then treated to increase the recovery of Aluminium.

The recycling process can be improved by separating iron-zirconium particles based on density. The amount of new scrap remelted can be reduced. A more efficient shape such as square or rectangular prism can be adopted.

Applications of Recycled Aluminium

Recycled Aluminium can be used for the same applications that virgin Aluminium is used. Recycled Aluminium is used in transportation equipment, packaging and containers, durable goods and construction materials.

Sources

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