Dredging by m m of bucket W. suctionamer dredges is an important
m e t w of mining tin are, especially in South-East Asia, where it
is used fim recwering alluvial ore Emm river bgds and frwn beneath
artificial U s or paddocks; it is even used -to mine offshore
deposits fropn the sea bed, Dredging units have now reached a
considerable &gm of &velopment. The dredges themselves are
huge floating processing plan@ which move m 8 the W 8 r n digging
out tin-- sediments from the bed and elevating them tro the mineral
processing plant on board. The gross weights of dredging units an
in excess of 5000 t o m e s and the overaU length from the ladder
extremity to the end of the werburden stripping chum cm be as much
$s 200 metres,
The sizes of the tin dredges vary, depending on such fxtm as the
mount of material to be processed, the depth of operation and the
mount of barren overburden above the tin-rich strata, Dredges range
frum &hose handling abut 200 toms of matmid per hour from a
depth of about 10 metres to large plants which cm excavate and
~IWSSS abut loo0 t o ~ s per hour recovd from up to 50 metres below
the water level, The & g m of ccmanmatim un board the dredges
varies according to the amount of accessory'heavy mineral p e n t .
Initial tin concentrates usualIy have between 5 and 15% tin mined
but higher percentages result when little or nu accessory heavy
metal is present.
2
Tm are r e c o v d fiom gravel pump mining is
This is 8 dophg wooden trough about 2 metres wi& by 1 metre
deep and 30 metres long. As the sand and slurry from the &ravel
pump flow down the pdcmg, the cassiterite and other heavy minerals
Elettle m the bottom, Erom h e to time the feed ia diverted and the
impure concentrate mmoved for further treatment by jigs and tables
88 described already.
usually md first in 8, sluice box Ur palong,
In order to obtain tin metal fkom the concentrate, it is smelted
by reduction with carbon at high temperature using 8 fluxing agent+
The impure metal &rivd from primary smelting is purified by a
series of refining cqemtions until the required grade of m m m i a
l purity is obtahd. Befm smelting, however, it is often necessary
to treat the mncentmtm to move the impurities they may contain,
Generally speaking, the high-gtadt
preparation than the lm~r=grade concentrate# arising h m lode
mines.
lXmcernES from alluvial &@m fequitc less
4
molten metal usually by bubbling compressed air through it. When
both are necessary, liquating is carried out first. The refined
tin, whether obtained by fire refining or electrolysis is normally
re- melted and cast into ingots weighing 50 kg. The brand name is
generally cast into the ingot surface. There has been a rising
trend in recent years for tin to be smelted in its country of
origin and all major producers have their own smelters.
Secondary tin: recovery and recycling
In the case of tin, recycling can be broadly classified into
three categories: recovery of tin metal (secondary tin), recycling
of high-tin alloys and re-use of low-tin alloys. Secondary refined
tin metal is unwrought tin recovered from tin scrap, tinplate scrap
and residues arising from tinplating, timing, detinning and
alloying processes. An important source of secondary tin is the
process scrap generated by the can-making industry, for example
when stamping circles from strip or sheet tinplate. The material is
collected, usually as loose, unbaled scrap, and sent for recovery
of the tin and the steel. Tin is also increasingly recovered from
used tinplate containers in domestic refuse; these can be reclaimed
by magnetic recovery plants or by consumer deposit banks.
Current state of tin reserves
There has been much discussion and assessments made over many
years concerning the question of mineral reserves and resources.
There appear to be at least two common errors which have been
perpetuated in many of the attempts to predict future resource
availability. The fust relates to economic considerations and the
likely changes in the cost/price ratio which can quickly make
uneconomic deposits valuable; the second factor, so often
discounted, is technological innovation. Improvements in both
extraction and refining techniques are a continuing process and
there seems little reason to doubt that mining companies will
continue to develop more efficient ore recovery and smelting
practices. Earlier assess- ments of exploitable reserves have often
dis- counted the profitability of mining increasingly
lower grade ore by improved extraction methods. They would not
have deemed probable the recovery of tin from granite 750 metres
below the Earth's surface, or from the sea bed 30 metres below the
water line and up to 8 km out to sea, or from river beds opened up
to mining by river deviation schemes.
To illustrate the uncertain nature of reserves estimates, one
has only to cite the detailed forecasts published periodically by
the U.S. Bureau of Mines. If one goes back only a quarter of a
century the quoted reserves have ranged between thee and nine
million tonnes.
This illustrates the fact that such estimates are subject to
complex economic and geological factors, but generally represent a
minimum of 20 years' foward supply at average consumption levels.
As exploration continually uncovers further reserves, there will be
an assured supply to meet anticipated demand in the future.
Estimated Economically Recoverable World Tin Reserves
Year 1000s of tonnes
1965 4265' I970 3930' P 975 w601 I980 9100' 1985 3060' I990
711Oo2
( 1 ) Mineral Fats d Problem (1986) (2) Mineral Commodity
Swnmpries (1990) (Both publkkd by U.S. Bureau of Mines)
There seems little doubt that tin producing countries can look
to the future with little worry about meeting demand due to
heightened explora- tion activity and greater use of modern
technology.
n the demand side there are hopeful signs that lower tin prices
over the last few years have encouraged consumers to halt any
switch away from tin. The rise in the consumption of tin in
tinplate in the USA in recent years suggests that tin consumption
is recovering in this way.
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