Our Environment - Mining and the Environment: From Ore to Metal - page 295

containing trace metals, potentially causing long-term health problems. Pyrometallurgical
processing and the combustion of sulphur-bearing fossil fuels such as coal, are arguably the
main contributors of air pollution in the mineral cycle, predominantly emitting particulate
matter, carbon dioxide and sulphur dioxide ( Case 7.1 ). Sulphur dioxide, of course, is one
of the main sources of acid deposition. Acid rain is caused by emissions of sulphur dioxide
and nitrogen oxides, which are converted to sulphuric and nitric acids in the atmosphere.
Dilute forms of these acids fall to the Earth as precipitation or are deposited as acid gas or
particles, leading to the degradation of forests.

Noise

Noise, defined as any loud, discordant or disagreeable sound or sounds, counts as an air
pollutant. In an environmental context, noise is defined simply as unwanted sound. Mining
and mineral processing activities inherently produce sound levels and sound characteristics
that have the potential to create noise. Sound generated by mining becomes noise due to
adjacent land use, i.e. if adjacent land is used by people for residential or other purposes.
Close to the source of sound itself, sound pressure levels are a matter of occupational health.
The environmental effects of sound and human perceptions of sound can be described
in terms of four characteristics, pressure, frequency, duration, and pure tone. Sound pres-
sure level (SPL, also identified by the symbol Lp) or perceived loudness is expressed in
decibels (dB) or A-weighted decibel scale dB(A) which is weighted towards those portions
of the frequency spectrum, between 20 and 20,000 Hertz, to which the human ear is most
sensitive. Both measure sound pressure in the atmosphere. Frequency (perceived as pitch)
represents the rate at which a sound source vibrates or makes the air vibrate. Duration
represents recurring fluctuation in sound pressure or tone at an interval; sharp or startling
noise at recurring interval; the temporal nature (continuous vs. intermittent) of sound.
Pure tone is sound comprising a single frequency, relatively rare in nature but, if pure tone
does occur, it can be extremely annoying.
Another term, related to the average of the sound energy over time, is the Equivalent
Sound Level or Leq. The Leq integrates fluctuating sound levels over a period of time to
express them as a steady state sound level. As an example, if two sounds are measured and

Noise, defi ned as any loud,
discordant or disagreeable sound
or sounds, counts as an air
pollutant.

The environmental effects of
sound and human perceptions of
sound can be described in terms
of four characteristics, pressure,
frequency, duration, and pure
tone.

CASE 7.1
Lead Smelting at Mount Isa

Built in 1978 with a stack height of 270 m, the Mount Isa
lead smelter in Inland Queensland, Australia operates the
tallest fl ue gas stack of any pyrometallurgical operation on
Earth. Mining in Mount Isa commenced in 1923 following
the discovery of substantial deposits of lead. Today Mount
Isa is one of the top ten producers of copper, silver, lead,
and zinc. Until 2000, the lead smelter operated without
a desulphurization unit dispersing signifi cant amounts of
SO 2 through the stack. Depending on the prevailing wind
direction it was possible to detect the plume in Perth
some 2,000 km away.

320 m 270 m

EIFFEL TOWER MT ISA SMELTER STACK