Metal Recycling

(Smelter of the VOEST-Alpine, Austria)

Flashback: Metals are chemical elements displaying certain properties, notably metallic lustre, the capacity to lose electrons to become cationic (a positive ion), and the ability to conduct heat and electricity, by which it is normally distinguished from a non-metal. This group of materials comprises about two thirds of the known elements. Some elements, e.g. arsenic and antimony, exhibit both metallic and non-metallic properties, and are called metalloids. Metals are categorised in the periodic table according to their electron orbital arrangements which correspond in similar chemical properties. Such groups include the alkali metals (group Ia in the periodic table), the alkaline earth metals (group IIa), and the rare earth metals (lanthanide and actinide series). Most metals other than the alkali metals and the alkaline-earth metals are called transition metals. The oxidation states, or valence, of the metal ions vary from +1 for the alkali metals to +7 for some transition metals.

The periodic table of elements

Chemically, the metals differ from the non-metals in that they form positive ions, basic oxides and hydroxides. Upon exposure to moist air, a great many metals undergo corrosion, i.e. enter into a chemical reaction, in that the oxygen of the atmosphere unites with the metal to form the oxide of the metal, e.g. rust on exposed iron.
Inevitably, trends of recent years have shown that consumer habits shifted from bottled drinks to canned products that use aluminium as the primary raw material. Such beverage cans were virtually unknown in the 1960's; yet by the early 1970's over 650 thousand tons of Aluminium (Al) had been used for these containers. There are several reason why Al became so popular in the beverage industry; the most important one is its low density (light weighted). Covering the food compartment with an additional PE-layer, makes it "nontoxic", odorless, and tasteless. Furthermore, it is thermally conducting, so the fluid inside the container can be chilled rapidly.
Even though Al is the most abundant metallic element in the earth's crust, its content in most minerals is low, and the commercial source of it is bauxite, is a hydrated and very impure oxide. The bauxite ore is processed to obtain aluminium-oxide (Al2O3). Al metal is obtained by the Hall process that operates at a much more economical temperature of 950C, instead of the 2050C to melt the ore. Nevertheless, the processing of bauxite to generate aluminium requires an enormous quantity of energy: An average of 15E6MWh is required to obtain 1 ton of Al. Before recycling of Al was widespread, each day the aluminium industry consumed the electricity used by 100 thousand people in one year! Thus, recycled Al significantly reduces the energetic demand to a level that is sustainable; i.e. just to as much energy which is required to heat the metal to its melting point of 660C. Thus, by recycling Al-cans, about 91% of the energy that otherwise is required to make new metal from bauxite, can be saved.

Al is not the only metal worth recycling; e.g. the nonferrous metal like Copper (Cu), or ordinary tins (usually made of steel) can contribute enormously when speaking about cutting energy costs and boosting efficiency. Ferromagnetic metals like steel and iron (Fe) can easily be separated from other dia- or paramegnetic materials. Non-magnetic materials require more sophisticated techniques to successfully separate them into their pure fractions.

Collection: With the available modern separation technologies, metal collection does not require to be split into its respective fraction at the collection site (Fe, Al, etc.). Though it has to be said that only those "household-metals" should be collected that are safe to use in the food industry (cans, foil, etc). These are more or less pure in their composition (no alloys and other compound materials) and therefore do not interfere with the smelting process).

Separation: Automated separation devices enable almost ideal separation efficiency. Powerful electromagnets separate the ferromagnetic fraction from the non-magnetic components, while high-frequency eddy current inducers enable separation of the para- and diamagnetic fractions.

Processing: The separated fractions are delivered to suitable smelters where they are reprocessed to new materials. Being more or less pure, these materials do not require energy-demanding extraction processes such as a blast-furnace. Mineral ore, on the other hand must be processed in such a furnace in order to remove the impurities, which trap the metal oxides. In the case of copper (Cu) for example, the blast furnace is chiefly used in smelting, i.e. extracting metal from their ores by a series of induction steps. The basic principle is the reduction of the ores by the action of carbon monoxide, i.e. the removal of oxygen from the metal oxide in order to obtain elemental copper (aquisition of negative charges to obtain elemental Cu0). A blast furnace is kept operational for as long as the internal insulating sheath protects the reaction chamber. Cu-ore treated in a blast furnace yields a copper sulfide mixture, which requires further refined by electrolytic means.
Aluminum (Al) instead is extracted from the ore by the Hall process. To separate the very conductive aluminium (redox-reaction) from the non-metallic fraction, an electric current is pass through a non-metallic conductor (e.g. molten sodium fluoride).
Pig iron (Fe) is obtained in the blast furnace and converted to steel by the Bessemer process. The reduction of Fe (metallic form), requires a mixture of ore with limestone and coke. Different reactions occur in different zones when the blast of air and oxygen is admitted. The ore, an oxide, is reduced to the metal via carbon monoxide (CO) produced in the furnace. At this stage, the recycled metallic fraction is added to the raw material. It is quit obvious that recycling does indeed cut short on both greenhouse gas emissions and the energy needed to produce an equivalent amount of prime metal. Output of the molten metal is primarily regulated via the gravitational pull of the molten column in the oven itself.
In Cu and Al refinery, the molten impure metals are poured into moulds and purified by electrolysis, while Fe is further purified in the "basic oxygen process".
For metal extraction and processing, refer to the Google web-site.

The slag generated during the melting process is disposed off in appropriate landfills.
Þ For final deposition, refer to the landfill section.

Metal recycling

Eddy inducer

Blast furnace

Metal processing (from left to right: iron, copper, aluminum)

References: Chang R. (1994); Chemistry 5th ed.; McGraw-Hill; New York - USA

For additional information visit one of the following web-sites
http://the Hall process
http:// / periodic table

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