Introduction
Zinc is a light, grey metal, which is relatively easy to extract. The density of zinc is 4,7 g/cm3. Zinc is naturally present in the environment and is the 24th most abundant element in the earth’s crust. The average concentration of zinc is calculated to be 70 mg/kg. At the mine, zinc is found in form of zinc blende (a sulphide of zinc).
When zinc is exposed to moist air, a thin film of hydroxide and carbonate is formed on the surface. The film has a low solubility in water and protects the zinc from further corrosion. If high amounts of sulphur dioxide are present in the atmosphere, zinc sulphates form on the surface. Zinc sulphate has higher solubility in water and is more easily washed away, so a fresh zinc surface is exposed to corrosion. However, the level of atmospheric sulphur dioxide has decreased considerably in both Europe and the Nordic countries during the last 20 years and today very low concentrations are measured.
Zinc in organisms
Zinc participates in numerous vital reactions, such as phosphorus metabolism, nucleic acid metabolism, protein and DNA synthesis, detoxification reactions etc. In higher animals and humans, zinc is important for vital brain functions, the immune system and reproduction. In presence of organic substances, zinc ions create complexes together with, for example, amino acids and proteins. This affinity to bio molecules is the reason for the important role zinc plays as an essential micronutrient for all types of organisms. There are 300 different enzymes, in which zinc is known to play a catalytic, structural or controlling role.
Humans need 12-15 mg zinc per day to maintain necessary biological functions and to avoid zinc deficiency. This means that, for example in Sweden, the inhabitants consume about 50 tonnes of zinc during a year. Unfortunately a lot of people in the world suffer from zinc deficiency, which for children leads to diarrhoea, stunted growth and other problems. Zinc is also used in wound creams, body lotions, baby powder, medicine, sun oils and so on.
For plants, zinc deficiency leads to significantly decreased growth. To counteract this, thousands of tonnes of zinc are added to fields in fertilizers. In animal breeding, for example, sucking-pigs get zinc in their food to boost immune defence, growth and skin condition. For this purpose alone, about 70 tonnes of zinc are consumed in Sweden each year.
Most systems for assessment of the environmental hazard of chemical substances include as a central criterion the bioaccumulation potential of the substance. This is normally expressed as the substance’s bio concentration factor (BCF) and is determined experimentally by exposing aquatic organisms to an aqueous solution of the substance in question. If the BCF value > 100 the substance is considered as having such a high bioaccumulation potential and it can be regarded as environmentally hazardous, according, for example, to the EU directives (26).
For substances, such as essential metals, whose uptake and accumulation in living organisms is controlled by sophisticated regulation systems, the use of the BCF is of very limited relevance for assessment of environmental hazard. This becomes evident when it is observed that the experimentally-determined BCF value for an essential metal such as zinc shows a very great variation, depending on how the bioaccumulation experiment is carried out. A test organism that is exposed to a low zinc concentration in water absorbs and accumulates more zinc to satisfy its zinc need, resulting in a higher BCF value (26).
• Series of experiments with molluscs showed that the BCFs for zinc varied between 600 and 55,000 in the common sea mussel and between 1,100 and 9,000 in a gastropod species after 8 days of exposure to zinc, despite the fact that the zinc content in the body tissue only varied by a factor of 2.
• For a type of shell, the BCFs for zinc varied between 1100 and 9000 after 8 days of exposure to zinc, despite the fact that the zinc content in the shell only varied by a factor of 2.
Fields of application
It is therefore important to take account of zinc’s role as an essential element when applying regulatory criteria.
The major application of zinc is for corrosion protection of steel in the form of a coating or as metallic powder in paint, in brass or in other zinc alloys. Zinc chemicals are used in a lot of different areas such as medicines, creams, wood protection chemicals, catalysts, food nutrients for humans and animals, rubber additives etc. Zinc oxide is the most common additive in rubber production, skin protection and wound creams.
In Nordic countries, 70 % of zinc is used for corrosion protection. A major part of that volume is used for galvanizing of steel sheets and in construction. The annual consumption of zinc in Sweden is 35 000 tonnes and most of this is imported from Norway or Finland.
Production and energy consumption
Zinc is mainly produced from the ore sphalerite, which after crushing, concentration and roasting is dissolved in an electrolytic solution. Recycled zinc from, for example, steel-making dust and other raw materials are dissolved directly into the electrolyte. The zinc is separated from the solution using an electric current. Metallic zinc is precipitated onto aluminium cathode plates. The cathodes are then removed from the solution and replaced with new cathodes. The zinc is stripped from the aluminium plates, smelted and cast into ingots. After all the stages in this process are complete, the zinc is a finished and saleable product. The zinc used for hot-dip galvanizing has a purity of 99.995 % of zinc. The remaining 0.005 % consists mainly of iron.
The Swedish Environmental Protection Agency has made a calculation of the relative energy need during primary production of zinc and concluded that its energy consumption is the lowest for all base metals, except iron, calculated both on weight and volume percent. According to one zinc-producing company, the energy consumption is 12-13 GJ per tonne of zinc. Recycling of zinc from galvanized steel sheet only consumes 5 % of the energy given above. Because of that, zinc is very advantageous compared to other base metals, regarding conservation of environmental resources.
Recycling
During hot dip galvanizing some process waste, or rather secondary raw materials, are generated, from which all the zinc can be recycled.
Of a total world annual zinc demand, about 35 % is met by recycled zinc. Zinc’s cycle life is on average 30 to 40 years – which means that almost 80 % of available zinc is recycled. Zinc in used chemicals and the small quantities of zinc which slowly weathers from coated steel is not recycled. This is the same for zinc pigments lost through degradation of paints.
Zinc can be recycled infinitely without deterioration in quality. This means that zinc fits well into a sustainable society. By protecting steel from corrosion for many decades, iron, energy, transport and emissions of mainly carbon dioxide, are reduced.
Emissions of zinc
a) Point sources
The emissions are principally of two different kinds – emissions to air and emissions to water. In Sweden, zinc emissions to water are largely from point sources, in particular from the waste and forestry industry. The main part of zinc emissions to air are created by waste-fed steel processes working without filters, and from fire wood and peat. The surface treatment industry has very effective cleaning equipment and the yearly emission of zinc (2002) in Sweden is only two tonnes. A large hot dip galvanizing plant in Sweden today has an annual emission of zinc of about 20-25 kg, which is four times lower than the limit set by the Swedish Environmental Protection Agency.
b) Diffuse emissions
Diffuse emissions of zinc are mainly from corrosion, traffic wear (tyres, asphalt, brake lining) and household drainage. A large part also comes from atmospheric deposition, caused by emissions in other countries. The trend is decreasing and the diffuse emissions have been lowered by 40 % during the last 10 years.