THE ELEMENT CADMIUM
Cadmium is a naturally occurring metallic element, one of the components of the
earth's crust and present everywhere in our environment. Its presence results
mainly from gradual phenomena, such as rock erosion and abrasion, and of
singular occurrences such as volcanic eruptions. The element's existence was
revealed in 1817 and it owes its name to "cadmia fornacum", the "zinc flowers"
which formed on the walls of zinc distillation furnaces.
Its industrial applications were developed particularly during the first half of
the 20th century, based on its unique chemical and physical properties.
Its potential risks to human health have been well studied and are now well
controlled.
Cadmium emissions into the environment resulting from its use have been
continually decreasing since the 1960's. Today these are insignificant and will
approach zero in a decade's time, when cadmium products will be almost totally
recycled, thereby eliminating any perceived problems.
Cd
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white, soft metal
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malleable, ductile and flexible
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321 °C
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48
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112.41
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8.64 g/cm³
0.1 - 5 ng/m³
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0.1 - 0.5 µg/g
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~ 1 µg/g
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~0.1 µg/l
CADMIUM IS USEFUL
Cadmium is a byproduct of the primary non-ferrous metal industry. Rather than
disposing of it as a waste, engineers have been able to utilise its unique
properties for many important industrial applications.
The most remarkable characteristics of cadmium are its great resistance to
corrosion, its low melting-point and excellent electrical conductivity. Cadmium
compounds exhibit excellent resistance to chemicals and to high temperatures.
Finally, cadmium pigments produce intense colourings such as yellow, orange and
red and are well known in artists' colours, plastics, glasses, ceramics and
enamels.
For all these reasons, cadmium is widely used in special alloys, pigments,
coatings, stabilisers and, above all, in rechargeable nickel-cadmium batteries
(almost 70% of
its use).
CADMIUM IN THE ENVIRONMENT
- 40 YEARS OF STEADY DECREASE
Cadmium levels in the environment reached a peak in the 1960's. Since then,
these levels have been constantly decreasing due to improved technology for the
production, use and disposal of cadmium and cadmium-containing products.
Industrial emissions are now tightly controlled due to the significant
improvement in pollution control technology and to strict regulation and
legislation, particularly in the metals industry.
The problems of incinerating waste containing cadmium can be solved using
existing best available technology to capture more than 99 % of incinerator
fume emissions.
With regard to end-of-life disposal of products containing cadmium, it should be
emphasised that, in many of its applications, cadmium is embedded in a product
matrix and hence not directly bioavailable. In the very long term, the limited
traces of cadmium eventually released from waste products will transform to a
stable chemical form (oxide or sulphide) and so return to the original state
found in nature.
Cadmium-containing batteries, coatings and alloys are totally recyclable, the
techniques have been mastered and existing capacities are available throughout
Europe. The costs involved are economically acceptable and hence viable.
Increasing recycling of cadmium products using modern technologies will further
decrease its dispersion into the environment resulting from human activity.
THE HEALTH EFFECTS OF CADMIUM
ARE WELL KNOWN AND CONTROLLED
Beginning in the 1950's, the scientific community has drawn its attention to the
potential toxicity of cadmium and to the risks presented by its accumulation in
man.
Cadmium mainly accumulates in one specific organ - the kidney. At high levels it
can reach a critical threshold and can lead to serious kidney failure. The most
recent studies, however, (Buchet/Bernard, 1998) have shown that these effects
are reversible, at least at low exposures, once exposure to cadmium is
reduced.).
The major route for cadmium intake is ingestion (for non-smokers). This is
largely due to the presence of trace levels of cadmium in foodstuffs of natural
origin or of the use of phosphate fertilisers and sludges on agricultural
soils.
The diagram above presents the relative importance of the various cadmium
sources to human exposure. It clearly indicates that the total of all cadmium
applications represents a very low proportion, less than 3 %, of the total
cadmium sources to human exposure.
Investigations conducted around the world show that, for the general population,
the average daily cadmium intake from all sources is very low and is at the
lower end of the total range of 10 - 25 µg/day. Human exposure has continued to decrease over the past 20 years and is
still doing so today. In comparison the tolerable daily cadmium intake
established by the WHO is 60 µg/day for women and 70 µg/day for men.
In industrial environments, workers could be exposed to cadmium through
inhalation. In modern enterprises, workers' exposure to cadmium is controlled
by rigorous industrial hygiene practice and continuously monitored by medical
tests and follow up, thus preventing any health risks for the workers.
The carcinogenic risk of cadmium by inhalation is increasingly being contested
by newer studies (Soroha, 1995-97) demonstrating the importance of confounding
factor
CADMIUM AND THE FUTURE
Nickel-cadmium rechargeable batteries are vital in
daily life
These batteries are essential in many portable and industrial applications.
These include such consumer applications as power tools, personal computers,
cellular phones and household appliances etc. for which demand is constantly
increasing and industrial uses as in aircraft and railways.
They are safe and recyclable and, alongside the development of other battery
couples, NiCd batteries continue to provide unique benefits for specific
applications.
It is thanks to NiCd batteries that the electric car has developed significantly
in Europe and that many other types of electric vehicles will continue to be
developed in the future. This is simply because nickel-cadmium battery
technology offers the best combination of power, energy, cycle life and cost of
all existing electric vehicle batteries.
The battery industry has committed itself to a pro-active policy directed
towards a "zero cadmium input" into the environment. By promoting the
collection and recycling of end-of-life products and waste containing cadmium,
the European Directives should facilitate these developments.
Pigments are useful and safe
Cadmium pigments are insoluble in water and in organic solvents and have a very
good resistance to detergents and the action of most corrosive alkaline
chemicals. The most important property of Cd pigments and stabilisers is their
heat and ultraviolet light stability.
Plastics such as ABS, nylon, fluorocarbons and polycarbonates, and some other
thermoplastics, such as polystyrene, polypropylene and high-density
polyethylene (HDPE) are processed at temperatures ranging up to 400ºC.
Their dispersion, non-migration and non-bleeding properties make cadmium
pigments useful in plastic applications where uniform colouring is important.
These superior characteristics together with many others make cadmium pigments
useful in all plastics applications.
Cadmium plating ensures high safety and reliability
Cadmium plating provides outstanding corrosion protection to metallic structures
together with low friction coefficients and low electrical resistivity. The
combination of these properties along with others such as good brazability and
solderability and galvanic compatibility make it the preferred coating when
critical or safety-related applications such as fasteners or equipment in the
aerospace, electrical, defense, mining, nuclear and off-shore industries.
