Fluoride Action Network

Bulk bag unloading system contains toxic dust in fluoridation plants

Source: Solids&BulkHandling | July 9th, 2014
Location: Australia

SYDNEY, AUSTRALIA — For many years various Australian authorities have been slowly installing water-fluoridation plants in a general endeavor to promote dental health. In the last few years, though, the pace of fluoridation has quickened as various states have introduced fluoridation programs, backed by state government subsidies. Queensland, for example, decreed in 2006 that 90% of Queenslanders would have access to fluoridated water by 2012.

ProMinent Fluid Controls Pty Ltd. has supplied more than 60 fluoridation systems over the past 25 years. Many of the earlier installations were for rural water supplies in relatively small water treatment plants. Initially, 25 kg bags of sodium silicofluoride powder (Na2SiF6) were manually loaded into a hopper; later installations used a vacuum loader designed by ProMinent. A dry chemical feeder meters the Na2SiF6 into a mixing tank of water, where it is dissolved before being added to the water supply. (Other chemicals used for fluoridation are sodium fluoride powder and hydrofluosilicic acid).

Toxic dust control

More recently, however, the company has supplied equipment for much larger plants, including five that started up in Queensland around the end of 2008. The plants range in size from approximately 125 MLD to 750 MLD and serve more than 50% of Queensland’s population of approximately 4.5 million, says Neville McKee, a ProMinent sales manager.

For plants of this size, ProMinent has designed a fully automated process in which the Na2SiF6 is completely contained in a sealed transfer system from the time it is received until the moment it is put into the mixing tank. Sealing is important for dust control, because the plants use up to 875 kg/d of Na2SiF6, which is toxic and subject to strict regulatory control. Bulk bags of Na2SiF6 are unloaded into a transition or floor hopper, from which a flexible screw conveyor transfers the material to a storage hopper that feeds the mixing tank.

The transition hopper is small, with a capacity of only 60 l. In contrast, storage hoppers may be as large as 8,750 l or more for a 750 MLD plant, and designed to hold up to seven days’ supply of Na2SiF6…

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