Geevor:
The Preserved Tin Mill and Mine
See
also Geevor Mine - Underground in Wheal
Mexico and Geevor Mine
The Tin Mill is an excellent
place to learn more about tin processing, and the guides that work here now, not
only are very knowledgeable about their subject, but many actually worked here
when it was a working processing mill, and the whole experience of a visit to
Geevor Mine can only be thoroughly recommended - see the
Geevor Mine Website.
The preserved Tin Mill at
Geevor is attached to the Victory Shaft, and processed ore (Cassiterite) was
brought to the surface through this shaft, and transferred directly from the
kibbles to the mill via conveyor belts. The modern building and tin mill that
stands on this site date from as late as 1980, before its final closure in 1990
and the road to conservation, but some parts of the mill date back to the 19th
century and the days where tin processing techniques were very different. Tin
processing itself is also known as "tin dressing", and was carried out on the
"dressing floors". These were where the bal (mine) maidens used to work while
their men mined the ore underground. Techniques then would have involved a lot
of hand, and hard, labour to break the rocks by hand before the mined ore could
be further refined. This would have included "ragging" (with sledgehammers),
then "spalling"
and "cobbing" with smaller hammers as the pieces were broken smaller and some of
the waste was removed. The cobbed ore was then sent to the "stamps" - powerful
water-wheel driven (or later steam-driven) lifting stamping hammers that crushed the ore to "pulp" the
watery wash of crushed rock and ore that was sent for refining in the buddles.
The buddles were effectively gravitational separators to separate out the
heavier tin ore from the waste, and these techniques were further improved over
the years into the twentieth century using tables. All these processes were carried out -
usually in the open - on what were known as the dressing floors, which covered a
large proportion of the surface workings of a nineteenth century tin mine.
Tin processing and dressing at
Geevor, with the new tin mill, took place under cover, and the equipment
installed was more advanced, though it essentially did the same thing - to derive
the "black tin" - the refined cassiterite ore ready for sending for smelting.
The image below shows the top of the Tin Mill to the left of the Victory Shaft,
at the top of the slope down through the rest of the mill, where the broken rock
first enters the mill to be crushed.
Geevor Mine and Victory Shaft - the top end of the preserved Tin Mill is to the left
Before the ore entered the
mill, it is useful to see where it came from in the first place... Geevor Mine
was managed as were most mines, by the Mine Captain. His office is still
preserved at Geevor as perhaps it might have been seen with the mine still being
worked around it - though it is eerily quiet now. The board shows a stope where the
ore was mined from the "lode" or the vein of ore in the bedrock, or
the "country" rock as it was called.
The Mine Captain's Office
One of the boards in the
Geevor Museum shows schematically how this ore was won from the stopes and
various levels in the mine by drilling and blasting, and then "trammed" along to
the shaft for lifting to the surface. Some limited breaking of the ore also took
place underground, while the ore was still in the stopes and manually broken through
heavy grids or iron girders - large "grizzlies" - before being trammed out.
How the Tin Ore - Cassiterite - was mined by drilling and blasting
These two photos show a
reconstruction of an "overhand" stope, where a miner is drilling prior to
blasting...
A mock-up in the Hard Rock Museum of how the ore was won from the stopes
Looking up the mock-up stope with the blasted rock fallen down ready for breaking and tramming out
Once brought up the shaft, the
broken, mined ore was transferred to the mill, where one of the first tasks was
to pick out the inevitable rubbish that was brought up in the kibbles - whether
it be the odd glove(!) or iron contamination which from an old mine will appear from time to time. This was a manual inspection operation, but was helped by a large powerful electromagnet that could also pick out unwanted
heavier ferrous material or iron ore based rocks that were present.
The "Picking" Belt
After the mine closure, some
processing equipment including the more modern ore crushers that would have
taken the place of the original more manual crushing processes, were sold off to presumably offset losses or to settle the company books.
These crushers would have reduced the ore size down to the 10-20cm level for
passing on to the grinders.
Past the now gone ore crushers...
Ore crushed down to the 10 - 20cm size for passing to the ball grinders
The grinding mill consisted of
essentially large rotating ball grinding machines, that tumbled and ground down
the ore much as a (highly!) glorified tumble drier might do! These grinders
replaced the old steam-driven stamps that would have originally ground down the
ore to the "pulp" watery-mix stage. The grinders shown below used heavy steel
balls (like the ones on top of the grinder), and the ore was washed in with water
until it was ground down to the size of sand, when it was discharged out -
anything too large was cycled back in again. These ball mills replaced the
stamps at Geevor in 1937.
Ball Grinding Mill (and our guide ghosting in!)
Another view of one of the Grinding Mills
With all this processing, only
about 1% total of all the ore material entering the mill actually emerged as
usable cassiterite grains for smelting as tin. The rest, excepting perhaps another 1%,
was waste. To get the precious tin ore further reduced to the best quality material, a
variety of refining techniques, or "classifying" were used. The most obvious and easiest to see
working in the Geevor tin mill, were the Shaking Tables. These used the flow of water
against gravity to settle out the heavier ore particles from the lighter waste
product, which was washed out and away. The lighter waste was kept mobile in the
water by the shaking, as the ore settled out against the shallow ridges on the
table, and this refined ore material could then be taken off
from the table surface when enough had accumulated. It could also be classified
or "graded".
An ore Shaking Table in operation - the separated out ore is the grey streak across from centre left
More Shaking Tables on the Dressing Floor
Some re-grinding was also
carried out in the re-grinding mills shown below. These would presumably be
used to further grind down some of the coarser material from the primary
grinding and classifying processes.
The Re-grind Mills below the Shaking Tables floor - not some strange spacecraft!
Besides the grinding mills and
shaking tables, other modern classifying methods were also employed at Geevor.
These included "Froth Flotation". This replaced the original Calciner that would
have been used to roast off the impurities such as Sulphur Di-oxide and
Arsenious Oxide, to be deposited up a long flue and Labyrinth, such as the one
at
Botallack. This very unsavoury method - for the workers - was thankfully
replaced when the Flotation process came into use. Flotation is a process by
which the unwanted minerals are separated using chemicals to alter the surface
of the mineral grains, to make them effectively repel water, thus "frothing"
them off the surface. The heavy cassiterite would then be harvested from the
bottom, and other minerals, even copper and gold could also be separated out
using this technique.
The Froth Flotation process - replacing the old Calciners
A close-up of the Froth Flotation description at Geevor
Another process was
Magnetic Separation. This involved separating the adventitious iron minerals
that were found in the refined ore material - either originating from the
original ore, or from being picked up during processing, for instance the
balls in the grinding mills. As tin is non magnetic (non-ferrous), the iron
can be separated out by electromagnets. The resulting high grade cassiterite
- the sought after "black tin" - could then be shipped out for smelting,
knowing that it was free of impurities, and that it would yield as much as
72% tin (see the description board below).
The High Intensity Magnetic Separator
The Magnetic Separation process
~
The Miners' Dry - All Quiet Now
The Miners' Dry is essentially
the miners' changing room. It was originally called the "Dry" back in the
nineteenth century, when a room or a space near the main boiler for the beam
engines was used for miners to hang their wet working clothes to dry over
steam pipes, after their shift down the mine, ready for next time. This evolved
into more modern changing and shower rooms, and personal lockers, that still
retained the name "The Dry". It is this part of Geevor Mine that is
the most poignant and moving
monument to the miners who worked here, and to this end, their are
photographs stand sentinel over all the walls. Their locker room stands as it
was left on the last day in 1990, just as if they might return the next day.
Boot cleaning coming off shift, and the miners who worked at Geevor all round the walls of the Dry
Lockers and changing areas
Some of the showers
Abandoned Lockers as they were left on the last day of 16 February 1990 - and the final message on the left
The now disused Miners' Dry Boiler House
No more will Geevor Miners sign on and off in the Shift Office
See
also Geevor Mine - Underground in Wheal
Mexico and Geevor Mine
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