Australia is a major producer of mineral sands containing titanium
minerals and zircon. A valuable by-product of this is monazite containing
thorium, which is radioactive.
Monazite is a minor constituent of many mineral sands deposits.
Appropriate occupational health provisions ensure safety in handling
materials containing thorium.
Heavy Minerals Mineral sands contain suites of minerals with high specific
gravity known as ‘heavy minerals’.
Mineral sand deposits are formed from the erosion and weathering of
pre-existing igneous rocks such as granite, pegmatite and basalt. Over 60
to 200 million years the combinations of wind and water from ancient
rivers and seas have leached the minerals from their past rocks and
concentrated them into beach and dune deposits. The result is that, today,
mineral sand deposits can occur at varying levels above the present sea
level. Some deposits have been located far inland from the present
coastline such as Horsham in Victoria. Generally, the minerals in the top
3-5 metres have a high titanium dioxide (TiO2) content and a concentrate
of zircon and monazite at the base of the deposit.
The typical mineral composition is: ilmenite 80%, zircon 10%, leucoxene
5%, rutile 1%, monazite 0.5%, others 3.5%.
Rutile (TiO2) is a red to black, naturally occurring titanium
dioxide with a theoretical TiO2 content of 100%, but impurities such
as Fe2O3 and Cr2O2 reduce this to 93–95%.
Ilmenite (FeTiO3) is black and opaque when fresh, but has
typically undergone some weathering and iron removal, so TiO2 contents
are between 45 and 65%.
Leucoxene is the name given to highly altered ilmenite.
Grains are brown or grey with a waxy lustre and TiO2 content of 68%.
Zircon (ZrSiO4), a colourless to off-white mineral, is the
world’s major source of zirconium products.
Monazite ([Ce,La,Th]PO4). - a rare earth phosphate containing
a variety of valuable rare earth oxides (particularly cerium) and
Xenotime (Y(PO4)) - a yttrium-bearing phosphate hosting 54 to
Geologists select areas for exploration by researching an area's
geology, topography, soil types and geological history. Areas are drilled
with a widely spaced grid to identify any occurrence and concentration of
minerals. If the results are promising, samples are taken from a more
closely spaced grid. When a favourable deposit has been identified, the
main exploration method is drilling. Usually small, four wheel drive
mounted reverse circulation (RC) drilling rigs are used.
The RC drilling method – where air or water is forced down an
annular tube and cuttings are returned up the central tube – produces a
clean uncontaminated sample at the surface. These are bagged at 1 or 1.5
metre intervals and, if heavy mineral is present, sent to a laboratory for
analysis. Samples are wet sieved and the amount of heavy minerals, clay
and sand determined. During drilling, attention is paid to recording the
presence of ground water and rock as these can substantially reduce the
profitability of a potentially economic deposit. After each hole is
drilled it is filled in or plugged using cuttings to prevent injury to
livestock or native animals. If drill samples contain significant heavy
minerals, further analysis determines the proportions of valuable minerals
and their suitability for commercial use. # Once a Mining Work
Authorisation has been obtained, mining can begin.
Mineral sands are mined by surface mining methods including open
cut mining, suction dredging and hydraulic mining. The first stage of the
mining process is to remove all timber and topsoil from the mine site. The
topsoil is stockpiled for later rehabilitation of the site after mining
has been completed.
There are three main mining methods;
1.Suction dredging - A dredge lifts the ore from the bottom of an
artificial pond, created over low grade deposits to allow rapid movement
of large amounts of sand, through a large suction pipe which carries it to
a separating plant. The dredge continues to slowly advance across the pond
while the clean sand tailings are spread behind the pond where they will
be revegetated at a later date.
2.Open cut mining - Higher grade deposits containing moderately
hard material or layers are mined using scrapers and bulldozers. The
scrapers mine the ore from the top of the face to the bottom, as well as
progressively mining across the whole face. This ensures that the ore
being mined is a constant blend on a day to day basis. The scraper carries
the ore to a screening plant where the ore is broken down into grains no
larger than 2mm. The screened ore then proceeds through an intricate
series of spirals to remove tailings and excess clay fines. The
concentrate is stockpiled for separation and treatment.
3.Hydraulic mining - With this technique the ore body is washed
down using a water cannon. The ore is then pumped as a slurry to a wet
concentrator which separates the valuable minerals from the waste
Various methods which include magnetic, gravity and electrostatic
separation as well as chemical processes, are then used to separate the
sands into individual mineral species. The ore is put through a screening
plant which breaks it down into individual grains. The heavy mineral
grains are 0.05 to 0.3mm in size, material greater than 2mm is dumped back
in the mining area.
The heavy mineral concentrates are sent to a dry separation plant, and the
individual minerals are separated using their different magnetic and
electrical properties at various elevated temperatures. Separation
equipment includes high tension rolls (electrical), high intensity magnets
and electrostatic plate separators. Using electrostatic separation
techniques the conductors (rutile and ilmenite) are separated from the
non-conductors (zircon and monazite).
Magnetic separation is used to separate the magnetic minerals (ilmenite
and monazite) from the non-magnetic minerals (rutile and zircon). This
removes oxygen from the ore and produces metallic iron within the
ilmenite. Ilmenite grains are converted to porous synthetic rutile grains
with metallic iron and other impurity inclusions. Secondly, the iron is
drawn out as hydrated iron oxide from the synthetic rutile grains and a
mild acid treatment is used to dissolve the impurities and any residual
The synthetic rutile grains are washed, filtered, dried and transported to
white pigment manufacturing plants in Australia or exported for further
processing. Plants using the newer chlorination process produce white
pigment by heating a mixture of synthetic rutile, coke and chlorine to
form gaseous titanium tetrachloride (TiCl4). The titanium tetrachloride is
condensed to a liquid and most of the impurities separate as solids. It is
then reheated to a gas and mixed with hot oxygen to form very fine
crystalline rutile (raw white pigment). The displaced chlorine gas is
recycled to the start of the process. The properties of the raw pigment
produced from both pigment processes are enhanced for different uses by
coating the crystals with white hydrous oxides of silica, alumina, titania
Rutile, ilmenite, leucoxene (an alteration product of ilmenite) are used
predominantly in the production of titanium dioxide (TiO2) pigment. The
titanium-bearing minerals rutile and leucoxene are sometimes blended to
produce HiTi (High grade titanium with a TiO2 content of 70% to 95%) which
is used as a feedstock to produce titanium dioxide, make titanium metals
for the aerospace industry and in the manufacture of welding rods. Less
than 4% of total titanium mineral production, typically rutile, is used in
making titanium sponge metal. Zircon is used as an opacifier for glazes on
ceramic tiles, in refractories and for the foundry industry. Recently
there has been renewed interest in monazite as a source of thorium for
possible use to generate electricity in thorium nuclear reactors.
Televisions - for luminescence and colour
Electronics - for a variety of components including high-performance
Robots - electric stepping motors
Computers - monitor luminescence, electronic components and bubble memory
Lighting - energy efficient lanthanum lamps
Medicine - X-ray screens, fibre optics, pain-killing elements
Chemistry - catalysts
Ceramics - pigment.
Products from monazite are also used in metallurgy, flints, ferro-alloys,
glass polishing, jewellery, fuel cells, refractories, lamp mantles
(thorium) and welding electrodes.
Yttrium from xenotime has been used in the most effective superconductors.
The occupational health issue of specific relevance to the mineral sands
industry is radiation. Western Australian mineral sands deposits contain
up to 10% heavy minerals, of which 1-3% is monazite. This in turn
typically contains 5-7% of radioactive thorium and 0.1 - 0.3% of uranium,
which is barely radioactive. In ore, or general heavy mineral concentrate,
the radiation levels are too low for radioactive classifications.
However, when the radioactive material is concentrated in the process of
separation and production of monazite the radiation levels are increased,
creating the need for special controls to protect some "designated"
employees in dry separation plants. In the past, occupational
exposure to radiation levels of 50 mSv/yr, then the limit, were not
uncommon. Dust control is the most important objective in radiation safety
for the titanium minerals industry. The most significant potential
radiation problem is inhaled thorium in mineral sands dust.
This contrasts with other industries where the focus for radiation
protection has been direct gamma radiation from materials in rock.
Exposure to gamma radiation still needs to be controlled in the mineral
sands industry, due principally to uranium and thorium in zircon. However,
safety programs are targeting alpha radiation arising from airborne dust
which may be inhaled.