Assaying - Geevor Tin Mine

Geevor
Tin Mine Museum
2009
Assaying
Assaying is determining the amount of mineral/metal contained in an ore or mine
product or determining the purity of metal produced when smelting and refining etc.
Since man first exploited metals and minerals he has devised ways of quantifying
their value using mechanical, physical and chemical means. Today many
sophisticated techniques of chemical and instrumental analysis may be used to
accurately determine the composition of a whole variety of mineral products and
metals.
There is evidence to suggest that the ancient Greeks and later the Romans used
‘touchstones’ to determine the quality of gold and silver. The article to be tested was
rubbed across a black pebble or stone so that it left a streak. This was compared
with streaks produced by a series of needles made from different compositions of
gold, silver and copper, when the colour of the streaks matched the composition of
the article being tested was indicated. At this time there was also reference to “trial”
by fire but little evidence to suggest what methods, if any, might have been used.
Prior to the 14 th century Alchemists, particularly Geber, described methods of parting
gold and silver by acids and by sulphur, antimony or cementation, but it was not until
the early 16 th century that methods for the assaying of several metals including tin,
lead, copper and bismuth were recorded.
The early methods used to determine the quantity of metals in ores were often
scaled down versions of the methods used to smelt the ore. They normally involved
reducing, with carbon, a small pre-weighed amount of the (often pre-concentrated)
ore in a small furnace and collecting and weighing the resultant metal. These
methods gave approximate results but allowed good estimates to be made of the
amount of metal that could be recovered from a particular ore.
In Cornwall the method most widely used to determine the tin content of ores and
mill products was the Vanning Assay. This involved washing and separating a
weighed portion of the material to be tested on a specially shaped shovel. Here the
heavy cassiterite was collected whilst the lighter waste flowed away. Repeated
washing and subsequent roasting of the cassiterite concentrate to remove arsenic
produced a clean concentrate of ‘black tin’, and after removing any iron present with
a magnet, the concentrate was weighed and the amount of cassiterite in the original
sample calculated. Because the vanning assay emulated the milling processes it
was particularly useful to determine recoverable tin, and in the hands of a skilled
operator was quick and relatively accurate.
The vanning assay was used extensively for determining the cassiterite content of
ores, mill products and tailings but had limitations; to determine the metal content of
a concentrate or a particularly rich ore a different method was used. The Cornish Dry
Method involved mixing the concentrate with carbon (usually powdered anthracite or
‘culm’) transferring to a crucible and heating in a furnace (a variety of fluxes were
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Geevor
Tin Mine Museum
2009
often used to assist fusion) to produce tin metal. This was poured into a small ingot
mould, and when cool, freed from adhered slag and weighed. The slag was often retreated
to recover any entrained metal. A similar method was to fuse the sample with
cyanide to produce a bead of metal for weighing.
The procedure for copper ores was more complex and required several fusions to
eliminate sulphur and concentrate and reduce the copper. During the 17 th –19 th
centuries these ‘dry’ methods were widely used on Cornish tin and copper mines. By
the mid 19 th century ‘wet’ methods of chemical analysis were being increasingly used
to accurately determine the metal content of a whole range of mine products.
During the 19 th century a greater understanding of chemical processes and their
application for analysis saw many new assay methods being developed. By the end
of the 19 th century many mines had set up laboratories for the routine assay of
underground and mill samples, though the vanning shovel still remained an
important tool until the end of the 20 th century.
The ‘wet’ chemical analysis of tin became increasingly popular from about 1870.
Cassiterite (tin oxide) is unusual in that, under normal conditions, it is insoluble in
any combination of acids or alkali. To bring cassiterite into solution it must first be
reduced to metal. Heating the cassiterite in an atmosphere of hydrogen (usually as
coal gas) was very effective in bringing about this reduction. Once reduced the
sample was dissolved in hydrochloric acid to bring the tin into solution as stannous
chloride.
Tin like many metals can combine with other elements in different ratios. When tin
dissolves in hydrochloric acid one atom of tin combines with two atoms of chlorine to
produce one molecule of stannous chloride (SnCl2), thus the tin is said to be in the
lowest oxidation or stannous state. By oxidising the stannous chloride the tin can be
made to combine with more chlorine to produce stannic chloride with four atoms of
chlorine combined with one atom of tin (SnCl4). Reacting the stannous chloride with
an oxidising agent like iodine can bring about this oxidation. It always requires a
fixed amount of oxidising agent to effect this change.
Thus if a sample containing cassiterite is finely ground, accurately weighed, reduced
by heating in hydrogen, dissolved in hydrochloric acid and the solution reacted with a
measured amount of iodine solution of known strength, then the amount of iodine
used is equivalent to the amount of tin in the original sample. In practice the iodine
solution is slowly added to the solution of stannous chloride, from a burette, oxidising
it to stannic chloride. The end of the reaction is indicated by adding a few drops of
starch to the solution this changes from colourless to dark blue with the first drop of
excess iodine.
This method gave accurate and consistent results and by adjusting sample weights
and solution strengths could be used to assay tailing samples with less than 0.5% tin
to concentrates with over 70.0 % tin.
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Geevor
Tin Mine Museum
2009
Several other assay routines for tin were also developed around this time. The
Pearce-Low method used fused sodium hydroxide or better still sodium peroxide to
attack the sample and convert the cassiterite to soluble sodium stannate (this is the
preferred method for tin ores today) and iodine was often replaced by other oxidising
agents like potassium permanganate or potassium iodate /iodide solution.
Around 1912 a modification of the tin assay was demonstrated and rapidly gained
popularity. The Beringer assay used zinc metal instead of hydrogen to reduce the
cassiterite in the sample. The sample to be assayed was mixed with a small amount
of zinc oxide and powdered zinc metal, transferred to a small, covered crucible and
heated to a bright orange (about 950 degrees) for 10-15 minutes, after cooling, the
contents of the crucible were dissolved in hydrochloric acid and titrated with iodine
as before.
By the early 20 th century wet chemical methods for the assay of all of the different
metal encountered in the Cornish mines had been developed and refined so that
ores, mill products and concentrates could be accurately analysed and evaluated to
allow the efficiency and recovery of the milling processes to be determined.
From the 1940’s systems of instrumental analysis were being developed to
streamline and automate analysis procedures. Instruments to analyse the spectra
emitted and absorbed when metals are vaporised were developed into systems that
could routinely analyse samples of ores, feeds and tailings etc, though instrumental
analysis was not suitable for high grade concentrates and smelter returns.
By the 1960’s systems utilising the properties of X-ray fluorescence were being
developed and by the early 70’s were commercially available to analyse a variety of
metals including tin. These systems had the advantage of being able to analyse
samples without the need to bring them into solution. It was also possible to measure
material flowing through pipes so that the continuous and automated analysis of
metals in feed and tailing streams was possible.
At Geevor several systems of analysis were used to quantify the tin and other metals
present in the ore, mill products and concentrates produced for sale. A plan of the
mine from 1912 shows the Assay Office occupying the present sample house. Here
samples would have been prepared and assayed using both vanning and chemical
methods, with the upper of the two rooms being used for chemical analysis.
Just before the Second World War chemical analysis was moved to a small
laboratory (originally built in 1926 to support the Sulman and Picard experimental tin
fuming plant) sited on the lower part of the mine adjacent to the mill. Here mill feed,
tailing and concentrate samples were assayed on a daily basis using the Beringer
(zinc reduction) chemical assay. In addition the sulphides from the flotation circuits
were assayed for their copper and arsenic content and occasionally mill test work
required other metals like iron, tungsten and bismuth to be assayed using wet
chemical methods.
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Geevor
Tin Mine Museum
2009
The original assay office continued in use until 1986 (now called the sample house)
for the preparation and subsequent vanning of an increasing number of samples
from exploration, development and stoping operations underground.
In 1980 a new wet chemical laboratory, with improved fume extraction equipment
and storage facilities, was built in the New Table Plant to replace the now antiquated
1926 laboratory. Here routine chemical analysis was continued augmented by the
introduction of an Asoma X-ray fluorescence analyser.
X- ray fluorescence analysers were developed in the 1960’s and 70’s. When an
element is exposed to a beam of X-rays of suitable energy it emits secondary X-rays
whose energies are characteristic of that element. Thus if the energies and
intensities of the secondary X-rays emitted by the sample are measured the
elements present and their concentrations may be determined.
In 1972 an on stream XRF analyser was trailed at Geevor, the system known as
Mintek measured the tin content of the three tailings (waste) streams from the mill.
As tailing streams passed over the building housing the analyser a representative
sample of each stream was sequentially selected and passed through the analyser
where it was exposed to a beam X-rays generated by a radioisotope source/target
assembly, a gamma-ray solids gauge measured the solids contents of the streams
whilst a detector measured the secondary X-rays emitted from the sample. The
output from the detectors was electronically filtered and adjusted to compensate for
variations in the sample matrix. The whole system was automatic being controlled by
a small computer linked to a teletype. This printed a continuous record of the tin and
solids content of the tailing streams.
In 1975 a second Mintek system was installed to monitor and report the tin content of
the three mill-feed streams. An important feature of the system was to incorporate an
alarm to give warning when tin losses in the tailings exceeded predetermined levels.
Thus from 1975 to 1986 the amount of tin entering and leaving the mill was
automatically and continuously monitored doing away with the need to chemically
assay the samples.
During the 1970’s and 80’s XRF analysers were continually developed and refined.
At the Wheal Jane Mine a Mintek system was installed in 1975 to monitor tin levels
in flotation plant streams. A little later a Philips PW 1220C X-ray Spectrometer was
installed. This, multi element machine, was capable of analysing 180 samples per
day.
By 1980 at Geevor an Asoma 8010 six elements XRF analyser was in use to
analyse routine mill samples and to check underground samples that had been
previously vanned. Sample preparation for this analyser was minimal, samples only
needed to be dried and finely ground before being placed into a measuring cell and
positioned on the machine. Analysis took only a few minutes and could be performed
by persons with minimal training.
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Geevor
Tin Mine Museum
2009
By 1988 a second more advances Asoma was purchased, this machine was selfcalibrating
and programmable to allow a whole range of samples to be analysed.
At the time of the mines closure the Asoma XRF analyser was used for the daily
checking of mill samples etc (the Mintek system was not operable after 1986). The
tin concentrates sold for smelting continued to be assayed by wet chemical methods
along with various calibration and smelter exchange samples.
Irrespective of what method of assaying is used the sample being tested must be
truly representative of the whole. If it is not then the result will always be suspect.
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