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Appendix to NORM Information Paper
(March 2008)
Australia and Africa have extensive depositsxenotime - yttrium phosphate with traces of uranium and thorium. of mineral sands which comprise
These mineral sands are in placer deposits which have been naturally concentrated by gravity. They have been mined since 1934 and Australia has a major share of the world market for both titanium minerals and zircon. In the mining plant they are concentrated by gravity (in spiral sluices) and magnetically (for ilmenite).
Most of Australia's mineral sands occur on the east coast of Australia between Sydney and Fraser Island or on the southern section of the west coast.
While the main products of mineral sands mining are titanium oxide and zircon, monazite is also a significant component. In some deposits xenotime also occurs. Monazite and xenotime may be processed to recover rare earth oxides which are used in electronics and other specialist fields, but the presence of thorium makes them commercially unattractive. (Lanthanum and cerium now come from ionic clays in China, which do not have thorium present and are thus more valuable.) Thorium oxide is used in refractories, lamp mantles, specialised glass and welding electrodes. However, the potential supply as a by-product of mineral sands mining vastly exceeds demand.
Monazite is thus normally returned to the mine with the tailings. "Rare earths" while valuable are not particularly rare and preferred sources do not have thorium present.
Radioactivity
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. However, if decay products of either are present in the minerals, the radioactivity levels may be significant when the monazite is concentrated.
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.
The more precise identification of airborne radiation in mineral sands dry separation plants led to the introduction of voluntary codes of practice in 1980. These codes were incorporated into protective legislation in 1982. The method of calculating permissible exposure levels was changed in 1984 and again in 1986. The result was an effective six-fold reduction in radiation exposure limits.
The industry responded with two major initiatives:
Collectively, the WA companies have spent more than $30 million on engineering programs to improve dust control measures. As a result, average radiation levels have been reduced by more than 70 per cent. Protective masks are no longer required for most plant operators. All new plant is designed to incorporate efficient dust control equipment.
Titanium minerals production is managed under the Code of Practice and Safety Guide for Radiation Protection and Radioactive Waste Management in Mining and Mineral Processing. The current radiation levels are well below the Commonwealth Radiation Protection Code limit of 20 millisieverts per year occupational exposure. Current performance data indicates that Australian producers have no difficulty meeting the new standards. Australia was the first titanium minerals producing country to adopt the new standard of 20 millisieverts per year.
New South Wales and Queensland producers are required to meet the same standards as Western Australian miners. However, the limited monazite content of most east coast deposits means that radiation levels in New South Wales and Queensland dry plants have always been well below occupational health limits.
The main focus of research relating to occupational health in the mineral sands industry is thorium, particularly its biological behaviour. Research will aim to discover more about the amount of thorium which is retained in the body and its potential effects.
Prepared from Australian Titanium Minerals Industry and company information.