Inhalation and Retention of Thorium Dusts by Mineral Sands Workers

Abstract

Heavy mineral sands consisting of ilmenite, rutile, zircon and monazite, have been mined and processed in Western Australia since 1956. The industry is a significant one; in 1995 it mined 37.4 million tonnes of sand and processed 2.6 million tonnes of heavy mineral concentrate to produce 2.1 million tonnes of the individual mineral sands, at a value of $550 million. A specific occupational health concern associated with this industry is radiation exposure arising out of the presence of thorium and, to a lesser extent, uranium with all the heavy minerals. Monazite, a rare earth phosphate, is radiologically the most significant mineral, containing typically between 5 and 7% thorium and 0.1 and 0.3% uranium. Although monazite is a low volume product, comprising only about 0.5% of total mineral sand production, it tends to preferentially concentrate in airborne dust because it is softer than the titanium and zirconium bearing minerals. This is of particular concern during the processing of mineral sands because the minerals are subjected to a variety of vigorous physical treatment processes, such as screening and magnetic, electrostatic and gravity separation. Without the application of appropriate dust control technology, considerable airborne dust (and consequently radioactivity) concentrations may be experienced by workers who operate and maintain the separation plant. While the presence of radioactivity in this industry has been long appreciated, early protective and regulatory measures were focussed on control of external radiation exposure in circuits where monazite was being concentrated and bagged. Intake of radioactive dust was only recognised as a potentially significant source of exposure in the early 1980s, following national review and acceptance of ICRP Publication 30 (ICRP, 1979). This resulted in derived air concentration (DAC) values for thorium an order of magnitude lower than those previously applied to the industry (Hewson and Terry, 1995). The issue of intake of radioactive dust prior to the mid 1980s is important because there are many long-term workers in the industry and it is well recognised that each of the five dry separation plants were very dusty. In addition, early work practices, such as use of compressed air to blow down equipment, and sweeping, banging and brushing of floors and plant equipment, coupled with limited use of respiratory protective equipment, exacerbated the potential for dust inhalation by workers operating and maintaining the separation plants. Industry management