South32 began an ambitious project to limit the amount of hydrogen fluoride (HF) being emitted from its gas treatment center (GTC) at its Hillside smelter in South Africa to 0.5 mg/Nm³ by April 2025. This will enable the smelter to comply with new environmental regulations.
In order to achieve this goal, South32 is collaborating with Fives to update its GTC, following Fives’ recommendations made by Fives during previous audits. Having successfully implemented a first alumina cascade feeding system on its GTC at the beginning of the year, South32 now plans to deploy the solution on four additional GTCs. In addition, it will convert its Vibrair pocket filters to extended surface bags, which increases capacity for pollutant treatment, reduces system pressure drop, and reduces HF stack emissions.
To install the alumina cascade feeding system, the companies needed to rearrange and modify the existing conventional alumina distribution system. With the new system, a pair of filters installed in parallel, with each processing an equivalent amount of raw gas. During operation, twice the normal quantity of fresh alumina is injected into the first filter, with the overflow of partially fluorinated alumina from the first filter being redirected to the second filter. The material then flows from the second filter into the fluorinated alumina silo and then to pots. The operator from the control room will automatically carry out the cascade mode during hot season or when emissions are above normal values. This solution allows for a 30–50% reduction in fluoride gas emissions from GTC during summer time and a reduced formation of scales. It is also easy to retrofit on existing GTCs.
The extended surface bags provide a lot of advantages when applied to a GTC, including an increase in the filtration surface, an increase in the pollutants treatment capacity, a reduction of pressure drops in the system, and lower HF stack emissions. At the beginning of 2023, Fives signed a partnership agreement with SOLAFT, an Australian innovation company, to market and install this technology. SOLAFT has recently developed the Primaflow™, Starbag™, and Starcage™ products, which are de-bottlenecking the original design of the extended surface bag.