Metallic Minerals

Some minerals have a chemical composition that makes it possible to extract metals from them. These are usually mixed with other minerals in the ores (see Table 6.1). The most common ore from which aluminium is extracted is bauxite, which contains iron as well as aluminium oxides.

In earlier times metals were worth a great deal because they were often inaccessible and required complicated working techniques. At

Table 6.1 Metals, their ores and their use in building

Metal

Ore

Use in building

Aluminium (Al)

Bauxite, nepheline, kaolin

Light structures; roof covering; wall cladding; heat-reflecting foils and vapour barriers; window and door construction and furniture; guttering; additive in aerated concrete

Antimony (Sb)

Stibnite

Pigment (yellow)

Arsenic (As)

Arsenopyrite

Ingredient in timber impregnation

Cadmium (Cd)

Sphalerite

Pigment (red and yellow); stabilizer in polyvinyl chloride; alloys

Chrome (Cr)

Chromite

Alloy in stainless steel; pigment (yellow and green); ingredient in timber impregnation

Cobalt (Co)

Cobaltite

Pigment (white); siccatives, colouring of glass

Copper (Cu)

Chalcocite, chalcopyrite

The most important constituent in bronze; roofcovering; door and window furniture; guttering; ingredient in timber impregnation

Gold (Au)

Gold ore

Colouring of glass; vapourized onto windows as reflective coating

Iron (Fe)

Hematite, magnetite

The most important constituent in alloy steels; balconies; industrial floors; pigment (red, yellow etc.); ingredient in timber impregnation

Lead (Pb)

Galena

Roof covering; flashing; pigment (white); siccatives; additive in concretes

Magnesium (Mg)

Saltwater, dolomite

Alloys, potentials as for aluminium

Manganese (Mn)

Braunite, mangenite, pyrolusite

Part of alloy steel; pigment (manganese blue); siccatives

Nickel (Ni)

Pentlandite

Alloy in stainless steel; galvanizing of steel; pigment (yellow, green and grey)

Tin (Sn)

Cassiterite

Stabilizer in polyvinyl chloride; colouring agent in glazing for ceramics; ingredient in timber impregnation; catalyst in the production of silicone and alkyd

Titanium (Ti)

Ilmenite, rutile

Pigment (white)

Zinc (Zn)

Sphalerite

Zincing/galvanizing of steel; roof covering; pigment (white); ingredient in timber impregnation; additive in concretes

Zirkonium (Zr)

Zircon

Siccatives

first they were used for weapons and tools. Through the great technological changes of the industrial revolution, metals have become essential in the building industry; in particular steel and aluminium, followed by copper and zinc. Uses are spread over a wide spectrum from roof-laying and window frames to structures, nails, fixtures, pipes, and colours in plastic, ceramics and paints.

In many cases, metals can be replaced with other materials such as timber, cement products, etc.

During the extraction of ore, the mountains of slag and dust produced causes environmental problems since they often contain problematic residues. Extraction, especially shallow open cast mining, also leaves huge scars in the landscape that require filling and planting to restore afterwards. Even after much work it can be difficult or even impossible to rehabilitate or re-establish the local flora and fauna and achieve an acceptable water table level.

Table 6.2 Important metal alloys

Alloy

Constituents

Use in building

Bronze

Copper (more than 75%)

Roof covering

DC

Tin (less than 25%)

< Û.

Steel

Iron (85-98%)

Structures, floors, walls and roofs; reinforcement in concrete; roof covering; wall cladding; guttering; door and window furniture; nails and bolts (galvanized or zinced)

Manganese (0.1-0.5%)

Nickel (1-10%)

Silicon (0.5-1.0%)

All industries that deal with metal extraction or smelting are environmental polluters (Table 6.3). This is partly through the common energy pollution from burning fossil fuels and partly through material pollution from the smelting process. Ores often contain sulphur, and during smelting huge amounts of sulphur dioxide are released. It is usual for this to be extracted and used in the production of sulphuric acid. Toxic heavy metal residues such as mercury, cadmium and lead are also common.

The consumption of energy for the extraction and refining of metals from ore is very high (Table 6.4). All metals can, in principle, be recycled. Through material recycling of steel, copper, zinc and lead from waste, the energy consumption can be reduced by 20 to 40% and for aluminium by 40 to 90%. The metal industry also has a high potential to increase efficiency by using excess heat, which can be distributed as district heating.

Once in the buildings themselves, metals cause relatively few environmental problems, except for particles that are washed off the

Table 6.3 Material pollution from the production of metals

Metals

Boiling point (b C)

Emissions

Aluminium

2057

PFCs, PAHs, Al, F, CO2, SO2, dust

Cadmium

767

Cd, SO2

Cast iron

up to 3000

CO2, SO2, dust, Ar (when smelting scrap iron)

Chrome

2200

Cr

Copper

2310

SO2, Cd

Lead

1620

Pb, Cd, SO2

Nickel

2900

Ni, SO2

Steel

1535

CO2, Pb, Hg, Cd

Zinc

907

Pb, Hg, Cd, SO2

Zincing Galvanizing

Cr, Fl, phosphates, cyanides, organic solvents Cr, Fl, phosphates, cyanides, organic solvents

Note: The boiling point indicates the risk of vapourizing through different processes, such as when making alloys.

jTable 6.4 Embodied energy in important metals

I

Metal

From ore (MJ/kg)

Recyciing 50% (MJ/kg)

Recyciing 10% (MJ/kg)

DC

Aluminium

165-260

89-95

13-30

< Û.

Steel

21-25

18

6-10

Copper

80-125

55

Zinc

47-87

surface when exposed to different weather conditions. Roofing, flashings and metallic salts used in the impregnation of timber can pollute ground water and soil. Large amounts of metal, as in reinforcement for example, can lead to a stronger electromagnetic field in the building.

As waste products, metals exposed to water release particles into soil and water, which can then harm many different organisms. It is important to note that pollution due to metals is irreversible. Metals left in the natural environment will always be there - they do not decompose. Even if the present discharges are reduced, the total amount of metals ending up in the environment will still be increasing. Recycling metals, however much, only postpones the inevitable pollution.

Iron, aluminium, magnesium and titanium can be considered relatively 'benign' metals. Even though the environmental consequences of their extraction and production are still quite severe, they are not particularly toxic. Their recycling potential is high and they are based on relatively rich reserves (see Table 1.1 in Chapter 1).

Chrome, nickel, copper and zinc, however, should be used very sparingly or notatall. Cadmium and lead should be avoided completely.

Extensive extraction of minerals can cause damage to local biotopes and ground water.

6.1.1 Raw materials

All metals should, in the long-term, be used within closed cycles in order to maximize recycling and minimize losses during production or the life of the building.

Metals have limited reserves and demand exceeds the maximum possible supply of scrap, for steel by a factor around two. On current statistical predictions, iron reserves will last 95 years, aluminium 141 years, copper 31 years and zinc 22 years (US Geological Survey, 2007). These statistics do not take into account an expected increase in consumption. The growth rate in use of aluminium for example is now 6% per year, and of iron 10%.

The production of aluminium is based on bauxite, which contains 4060% aluminium oxide. Ninety per cent of the bauxite reserves are in countries with low and medium levels of industrialization, whilst the same proportion of extracted aluminium is used in highly industrialized countries. There are also other sources of aluminium such as kaolin, nephelin and ordinary clay. In the former Soviet Republics bauxite is scarce, so aluminium oxide is extracted from nephelin, although it is much more expensive to extract aluminium from these minerals than from bauxite.

Probes are now being made to find new sources of iron ore, and have resulted in the discovery of interesting sources on the ocean floor - the so-called iron nodules. These also contain a large amount of manganese. Extraction of iron from peat bogs is also being considered. More recycling of scrap steel would help and it is also possible to use alternative metals. There are alternatives for most metals and alloys except chrome, which is essential in stainless steel.

Was this article helpful?

0 0

Post a comment