Tin-From ore to ingot

TIN:
FROM ORE TO INGOT
INTERNATIONAL TIN RESEARCH INSTITUTE
724
Tin: From Ore To Ingot
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 a b u t 200
t o m s of matmid per hour from a depth of about
10 metres to large plants which cm excavate and
a b u t loo0 t o ~ per
s
hour r e c o v d
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.
~IWSSS
2
The combination of the vibration and the flowing
film of water causes the partides to become
smtim, with the heavier particles of cwsiterite
being carried between thc ripples to the ghm end
of the table, whilst the lighter waste sands am
washed down the table and off the long edge. Most
tables produce three products, 8 concentrate,
W g a and a ‘rniddlings’ oi mixed product,
which is =-tabled.
3
Tm are r e c o v d fiom gravel pump mining is
usually md first in 8, sluice b
o
x Ur ‘palong’,
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.
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 & r i v d 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
lXmcernES from alluvial &@m
fequitc less
preparation than the lm~r=gradeconcentrate#
arising h m lode mines.
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 remelted 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.
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
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 assessments of exploitable reserves have often discounted the profitability of mining increasingly
1000s of tonnes
1965
4265'
I970
P 975
I980
1985
I990
3930'
w601
9100'
3060'
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 exploration 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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