Metals In Building 621 Iron and steel

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Iron was used in prehistoric times. Iron has been found in meteorites pure enough to be used without refining. Smelting iron from iron ore has been carried out for at least 5000 years. The first appearance of iron as a building material was in classical Greece, for reinforcing stone lintels and architraves. Uses in cramps, pivots, hinges and locks have been widespread. However, it is only from about the eighteenth century that we have seen more widespread uses for balustrades, balconies, furniture, stairs and various decorative items. The first structural iron girder was manufactured by Charles Bage in 1796 in England, and was used in a five-storey linen mill.

Whilst cast iron contains a large proportion of carbon, steel is an iron product with a carbon content of less than 2%. Towards the end of the nineteenth century steel became a rival to (and gradually replaced) the more brittle cast iron. Whole buildings with a steel structure started to appear then. Today, steel is the only iron-based material used in the building industry. It is possible to use about 20 different alloys in steel, and up to 10 can be used in the same steel. Particularly strong steel is formed by alloying it with small amounts of nitrogen, aluminium, niobium, titanium and vanadium. Normal construction steel such as reinforcing bars, structural elements, wall and roof sheeting does not usually contain alloys. Sheeting products are protected against corrosion by a protective layer of aluminium or zinc. Facing panels in aggressive environments are often made of stainless steel; which is 18% chrome alloy and 8% nickel. By adding 2% molybdenum alloy, an acid-resistant steel can be produced.

As a resource, iron ore is a fairly 'democratic' material, being spread quite evenly throughout the world; it is extracted in over 50 countries. Easily available reserves are, however, diminishing rapidly, and some of the alloy metals required (e.g. nickel and zinc) have very limited reserves.

Coal is an important element and generally a prerequisite for the production of cast iron from iron ore. The exception, where the reduction process uses natural gas, requires ore with a very high iron content.

Rock iron ore is extracted by mining; peat bog iron ore is much more accessible and generally the dominant source in earlier times. It lies in loose agglomerations in swamps or bogs. To find it, the bog was probed with a spear or pole. Where there is resistance to the spear, it can be assumed that there is ore. There may even be small traces of iron filings when the pole is removed.

The extraction of iron ore usually takes place in open quarries and extends over large areas, which means that the groundwater situation can change and local ecosystems can be damaged. One ton of iron ore usually produces 5 to 6 tons of waste materials. Extraction of coal in open quarries or deep mines causes similar environmental damage.

The conversion from iron ore to steel requires a series of processes. They begin with the breaking up of the ore, then cleaning, followed by sintering. The iron is smelted out and reduced in a blast furnace at 1700-1800 °C. A large, modern blast furnace can produce 1000 tons of pig iron every 24 hours. The amount of air needed is four million cubic metres, and the cooling water is equivalent to the amount a small town would use. It takes 440-600 tons of coal to produce 1 ton of iron. The total amount of coal needed can be halved if an oil spray is injected into the furnace. Coal is used in the process to remove oxygen from the ore by forming carbon dioxide, leaving the pig iron behind. Limestone is added to the blast furnace charge in order to bind ash, silica, manganese, phosphor, sulphur and other impurities. This slag can, in turn, be used as pozzolana in the production of cements, see page 186-187.

Steel can be made from pig iron and steel scrap. Most of the carbon in the iron is released through different methods, e.g. oxidizing. This is done in blast furnaces or electric arc furnaces. The latter is often used in smaller-scale facilities, consumes less energy, however mainly as electricity, and represents 30-40% of the world's production today. Steel production in electric arc furnaces can also be based on 100% steel scrap, while other production methods only permit a fraction of scrap. Finally, the steel is rolled out to produce stanchions, beams, pipes, sheeting and nails.

Large amounts of the greenhouse gas carbon dioxide, as well as sulphur dioxide, fluorine compounds, dust and a wide range of heavy metals are emitted in the production of pig iron. The carbon dioxide comes partly from the combustion of coal in the reduction process and also as chemical emissions from the burning of admixtures of limestone. The net emission of carbon dioxide as well as sulphur dioxide can be significantly reduced by replacing mineral coal with a maximum of charcoal in the reduction process (30-40%). This is done in Brazilian iron production. The charcoal can even be mixed in with wood chips to a certain extent.

In the production of steel emissions of carbon dioxide will be large, but in general significantly lower for electric arc furnaces than for blast furnace technology (Kram et al., 2001). Other available methods for reducing energy consumption and carbon dioxide emissions in steel production include direct reduction and iron bath smelting but these are not yet economical.

The toxic element arsenic is well bound in the iron ore, but with a second smelting from steel scrap a good deal is released. Production from scrap iron also causes emissions from other compounds in the scrap, and considerable release of polycyclic aromatic hydrocarbons (PAH) and dioxins can be expected. When producing from stainless steel, there will be a larger release of nickel and chrome.

Steel scrap is virtually inert, but ions from iron and other metal alloys can leak into water and soil. Serious pollutants can also include waste materials from surface treatments, paint or fire protection coatings.

Protection against corrosion

When ordinary steel is exposed to humid air, water, acids or salt solutions, it rusts. This is hindered by coating it with zinc, tin, aluminium, cadmium, chrome or nickel or a combination of these, usually through galvanizing.

For zinc coating, metal is dipped into molten zinc at a temperature of at least 450 °C. Zinc and iron bind with each other giving a solution that forms a hard alloy layer. Galvanizing is an electrolytic process. The metal to be coated acts as a cathode, and the material coating acts as anode.

These two processes, zinc coating and galvanizing, are considered serious environmental polluters. In both cases there is an emission of organic solvents, cyanides, chrome, phosphates, fluorides, etc., mainly in the cleaning water. These pollutants can be precipitated in sludge form, but then still have to be handled as hazardous waste. Many galvanizing industries do not do this. In the largest Norwegian zinc plant, the toxic sludge, named Jarosite, is pumped into mountain waste caverns for permanent disposal.

One method for relatively pollution-free galvanizing is a process making use of the natural occurrence of magnesium and calcium in seawater. The technique was patented in 1936 and quite simply involves dropping the iron into the seawater and switching on an electric current to give it a negative charge. This method has proved effective for underwater sea structures. It is, however, not known to what degree this technique gives lasting protection from corrosion for metal components that are exposed to conditions on land.

Zinc ions are washed off roof surfaces by rain. This can be halved by having small amounts of aluminium added in the galvanizing process. Treating surfaces of steel and metals with a ceramic coating could give even better results environmentally. These methods are currently only used on materials in specialized structures.

Steel reinforcement is not galvanized. Concrete provides adequate protection against corrosion. But even concrete disintegrates over time, and the reinforcement is then exposed. Correct casting of concrete should give a functional lifespan of at least 50 years.

The most corrosive environment for galvanized iron and reinforced concrete structures is sea air, the air surrounding industrial plants, and car traffic. As a result of climate change, chemical damage to steel is expected to increase in the northern part of Europe and to decrease in the southern part. On the other hand, chloride-induced zinc corrosion within some hundred of metres from the sea is expected to increase in all coastal areas (Noah's Ark, 2006).

6.2.2 Aluminium

Aluminium is a relative newcomer amongst metals and was first produced in 1850. Since an aluminium structure weighs only about one third of a corresponding steel structure, it is used in light constructions and as roof and wall cladding. Use in the building industry is increasing rapidly.

Aluminium is usually extracted from the ore bauxite. A large part of global reserves is situated in rainforest areas of Brazil, Surinam and Venezuela. Extraction occurs mainly in open cast quarries. Production entails a highly technological process of which electrolysis is an integral part. Efficient production plants require particularly high capital investment; countries with large reserves of bauxite but low levels of industrialization have mostly had to export the ore rather than refine themselves. This is also because of the very large amount of energy that is required to produce aluminium. Still, it is probable that today's large aluminium-producing plants in the USA, Canada and Northern Europe will, like other industries, progressively relocate to developing countries.

Aluminium is produced in two stages after extraction of the bauxite ore. Aluminium oxide is first extracted by heating it to between 1100 °C and 1300 °C in the presence of sodium hydroxide and lime. This is known as 'calcination'. The oxide is then broken down in an electrolytic bath at around 950 °C with sodium and fluorides. The pure aluminium is deposited on the negative pole, (cathode) and on the positive anode, oxygen is released which combines with carbon monoxide and carbon dioxide. The anode consists of a paste mixture of powdered coal and tar - for every kilo of aluminium, half a kilo of paste is required. A huge amount of water is used too.

The aluminium is then formed into sheets. The surface of aluminium oxidizes naturally, providing it with protection. However, this thin layer is attacked by chlorides when in the proximity of salt water and by sulphur dioxide in urban environments, so it is common to add an extra layer by the anodizing process. This can include a colour and often gives a metallic sheen to the surface. An alternative plastic coating is termed powdercoating.

The processes in the aluminium industry release large amounts of the greenhouse gases carbon dioxide and perfluorocarbons (PFCs). These latter are especially potent and generate the equivalent of 2.2 metric tons of carbon dioxide for every ton of primary aluminium. In addition, the process releases large emissions of sulphur dioxide, polyaromatic hydrocarbons (PAHs), fluorides and dust. PAH substances, fluorine and aluminium ions remain in the sludge and slag from the production processes. This especially tends to cause problems in ground water where deposits have to be stored in landfills.

Recycled aluminium can be used a great deal in cast products. Aluminium waste is recycled by smelting in a chloride salt bath at 650 °C, which at best only requires 7% of the energy needed for the production from ore. The waste aluminium has to be pure; this is often complicated by residue from coatings such as powdering -and by ongoing efforts to increase the material quality by developing new aluminium alloys (Azar et al., 2002). Recycling of aluminium requires a great deal of transport however because of centralized production.

6.2.3 Copper

Copper was probably the first metal used by mankind. The oldest copper artefacts we have were made about 7000 years ago in Mesopotamia. The earliest known use in buildings is as cramps to fix stone blocks in the Valley Temple of Chephren in Egypt from 2500 BC (Wright, 2005). An early development was the invention of bronze, produced by adding tin to create a harder metal.

There are many examples of bronze being used in ancient buildings. The roof on the Pantheon in Rome was covered in bronze sheeting. Copper has always been an expensive material and is found mainly in churches and larger buildings.

The most important alloy, brass, consists of 55% copper and up to 45% zinc, occasionally combined with other metals. It is commonly used in armatures. Copper compounds are also used in a variety of timber impregnation treatments.

Copper ore is extracted in the Congo, Zimbabwe, Canada, USA and Chile and entails a heavy burden on the natural environment. Reserves are very limited. Large quantities of sulphur dioxide are emitted during copper smelting. Modern plants resolve this problem by dissolving the ore in sulphuric acid, then extracting pure copper by electrolysis.

Copper is washed off roofs by rain, especially where the rain is acid. Copper ions bind quickly in the soil and is thus not transported for longer distances.

Copper is toxic and can accumulate in animals and aquatic plants; however, unlike many other heavy metals, it does not accumulate in the food chain. Copper has a very high durability. Most copper in Western Europe is now recycled. Some, however, is re-used locally, such as thick copper sheeting.

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