Sapphires
are well known gem stones. Their processing, grading, heat treatment and
eventual marketing are considered with a focus on the Australian sapphire
industry, in particular the industry of the New England Tablelands.
Processing
Sapphires are mechanically separated from the
clay and gravel in which they lie by virtue of the former’s greater true
density. Typically, the gravels are dumped into a feed box at the upper end
of the plant, where they are broken up by a high pressure water jet, 25 mm in
diameter and operating at a pressure of 7 kg/cm2. This material is
then gravity-fed over a grizzly with the undersized material (-1.5 mm) pumped
to a rotating-mesh scrubbing-and-sizing trommel. The oversized material (+25
mm) is transferred to the trommel with a small backhoe.
The trommel sizes the material by allowing it
to pass over progressively coarser meshes. The sized fractions pass to three
two-cell jigs fitted with 2.5 mm slotted screens, which effectively trap the
-5 mm, -12 mm, and -20 mm heavy minerals in trays or hutches. The
pulsating action causes liquefaction of the gravel, allowing the heavy grains
to sink rapidly. The undersized heavies
pass through a screen at the bases of the hutches and the lights are washed over the top. Due
to the clayey nature of the Strathdarr deposit, a twin-screw log washer was
included in the circuit to break up the clay-rich alluvial material. The
oversized material from the screen and jig tailings are deposited into a rock
bin, which is taken back to the cut as backfill. Recovery rates by this
process are estimated to be of the order of 90% or more. The hutches are
emptied by venturi extraction at the end of the day.
Grading and Sorting for
Sale
After collection of the concentrate, further
processing is required to retrieve the corundum and zircon from the remaining
material, which also is retained due to the nature of this processing. These
remnants generally comprise paramagnetic minerals, including ilmenite,
haematite, limonite, magnetite, and pleonaste. Various magnetic separators
are used, ranging from simple hand-held magnets to the high technology
three-pass Eriez RE-10 high intensity magnetic separator.
Following magnetic separation of the
concentrate, corundum and zircon remain. The concentrate initially is sieved
into different size fractions based on the pearl sieve size system, an
example of which is given in Table 1. This system has allowed the standards
for buying and selling to be established.
Table 1. Pearl sieve sizes.
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8
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2.06
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20
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4.22
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9
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2.32
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22
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4.66
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10
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2.62
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24
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5.10
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12
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3.00
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26
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5.32
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14
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3.24
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28
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5.82
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16
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3.68
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30
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6.18
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18
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3.98
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Subsequently, the low-value zircon is removed
by hand-picking, commonly known as chooking. Once the zircon is removed, the
corundum is graded according to its saleability. The five general grades of
corundum are given in Table 2.
Table 2. Corundum grades.
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Sapphire
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First
grade
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Requires
minimal heat treatment
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Sapphire
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Parti
coloured
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Requires
minimal heat treatment
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Sapphire
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Second
grade
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High
potential to be heat treated effectively
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Sapphire
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Low
quality
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Marginal
potential to be heat treated effectively
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Corundum
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Non-gem
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Minimal
potential to be heat treated effectively
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Four of the grades in Table 2 refer to blue
sapphires. Parti-coloured sapphires are stones that are homogeneous green or
yellow, or they may show combinations of blue, green, and/or yellow in a
single stone. The New England sapphire fields, however, produce an
overwhelming abundance of blue sapphires.
New technology has seen the modification of
diamond sorters into colour classifiers. These machines remove the
labour-intensive end of colour separation and grading of small inexpensive
stones. Further work is required to perfect such machines as they correctly
register only ~90% of the stones examined. The American company Gem
Technology Systems may provide advancement in this area with their highly
innovative machinery, which was introduced in 1992. They currently are
developing rough colour grading machines, which should surpass the existing
colour sorters.
Characteristics of
Sapphire
Table 3 lists some of the characteristics
that result in the desired wear and colour properties of sapphire as a gem
material.
Table 3. Characteristics of sapphire.
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Ti3+ Pink
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Ti4+ Colourless
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V2+ and V3+
Colourless
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Mn3+ Pink
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Fe2+ Blue
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Fe3+ Yellow
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Co2+ Light Pink
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Co3+ Green
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Ni3+ Yellow
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Cu2+ Blue/Green
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Plane
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HK (GPa)
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HV (GPa)
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KIC (MPa.m˝)
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Load (N)
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0001
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-
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-
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3.0
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0.490
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0001
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28.5
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21.8
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2.5
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0.920
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0001
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-
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-
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1.1
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1.690
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0001
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20.6
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-
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-
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2.940
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0001
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-
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-
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1.7
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9.800
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112-0
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31.9
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-
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-
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0.920
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112-0
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23.6
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-
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-
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2.940
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The hardness of sapphire results in its
being highly sought after for incorporation into jewellery. However, the main
attraction of sapphires, in particular those from the New England district,
are the desirable blue colours found. Colours are graded into categories
ranging from Type A to Type D.
Pure sapphire is colourless. In the presence
of the common impurities iron and titanium, the blue colour is generated by
the Fe-Ti charge-transfer. Chemical analyses of New England blue sapphires
are given in Table 4. These data were obtained using an electron microprobe
(Cameca SX-50) on stones from Reddestone Creek No. 1 Mine.
Table 4. Quantitative analysis (weight%)
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1
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51.272
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47.855
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0.851
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0.022
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2
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51.464
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47.718
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0.804
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0.014
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3
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51.301
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47.855
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0.834
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0.010
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4
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51.498
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47.643
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0.843
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0.016
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5
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50.989
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48.075
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0.913
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0.023
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Average
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51.305
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47.829
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0.849
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0.017
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Heat Treatment
Today, heat treatment is an integral part of
the sapphire trade, with this process being widely practised and accepted
throughout the world. In fact, the practice is so widespread that sapphires
that are indicated not to have been heat treated are questioned. The use of
heat treatment has been responsible for many of the lower-quality sapphires
being sold for higher prices than would otherwise be possible, thus
prolonging the economic life of many mines. Many of the New England sapphires
possess a characteristic trait, which decreases their value, in the form of
needle-like rutile (TiO2) precipitates, commonly known as silk.
Heat treatment involves two aspects of
enhancement, these being colour modification and clarity improvement. Both
shortcomings can be improved concurrently by high-temperature (~1600°C)
reduction. The blue colour can be intensified and homogenised by reduction of
Fe3+, solution of Ti4+, and the associated
charge-transfer. The clarity is improved by reduction of the TiO2
to Ti2O3, the latter of which is more soluble in the Al2O3
lattice. The relevant reaction is:
4TiO2
+ Fe203 + 3H2 --> 2Ti203
+ 2FeO + 3H20
Marketing
Today, correct marketing of the entire
run-of-mine material is extremely important. The miner must sell the lower
quality material as well as the premium grade sapphire to make the mining of
any deposit viable. At the turn of the century, the best Australian sapphires
were sold to Germany, where the large cutting, polishing, and jewellery
industries were based. The 1960s brought a significant shift in the
perception of business people from Southeast Asia, particularly those from
Thailand, who saw the benefits of cutting the lower quality material for the
cheaper end of the jewellery market. The considerably lower labour costs
within their own countries in comparison to those in Europe saw the growth of
the industry within Asian countries, This shift, further aided by government
support and the discovery of new sapphire and ruby deposits, has allowed
Thailand to develop fully while consolidating its present virtual monopoly in
the gem and jewellery industries. The actual cutting and polishing of the
rough sapphire are highly labour intensive. Thailand claims to employ 600,000
cutters, and the labour costs of cutting and polishing in this country are
estimated to be between 5 to 10% of those required in Australia. To date,
automatic cutting machines waste up to 90% of the rough stone whereas the
average lapidary cutter would lose only 65 %. Table 5 summarises the current
labour costs of lapidaries from various countries.
Table 5. Lapidary labour costs per person
per day.
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Thailand
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Bangkok
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6.60
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Laos
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-
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4.30
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Thailand
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Chiang Mai
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5.70
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Vietnam
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-
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2.90
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Thailand
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Korat
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4.70
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Sri Lanka
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Colombo
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2.90
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Thailand
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Ayutthya
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5.40
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Indonesia
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-
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2.10
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China
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4.30
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India
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-
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1.40
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The sapphires are sold in two main ways,
these being through contracts and by spot sales. Contracts, the more
desirable method, are usually six monthly, whereby an agreement is drawn up
for the miner to sell all run-of-mine production to one buyer at an agreed
price. At the end of each six months, new contracts are established. The less
favourable way is to sell the production to travelling buyers who pass
through the New England district, negotiating the best price.
Summary and Conclusions
For the future of the New England sapphire
industry to advance, a constructive blend of the working miners’ knowledge
and an acceptance of new technology must be found if they are to obtain the
maximum benefit. There are four main difficulties facing Australian sapphire
miners: new finds of Australian alluvial deposits are becoming rarer, current
heat treatment technology is not sufficiently developed, third world labour
is very inexpensive, and new overseas sapphire deposits have been located
recently. In order to maintain its dominance of the world trade in sapphire
rough, New England miners must make advances in detailed exploration and highly
economical downstream processing. The latter will become increasingly
important if a higher percentage of the final added value is to be retained.
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