Two activated alumina defluoridation plants have recently been installed and commissioned at a mine in South Africa. A reduction in plant capacity was experienced as a result of fouling of the activated alumina by entrapped suspended solids, metal hydroxides, carbonates and adsorbed silicates. It was demonstrated, however, that plant capacity could be restored by using backwashing and air scouring with rinsing repeatedly, using acid treatment periodically for silicate and hardness removal and by performing an interrupted run. Plant performance would also have been improved by the use of smaller sized (0.5 to 1.0 mm) alumina particles. Chemical costs for fluoride removal should vary between 15 and 18c/m3. Periodic cleaning of the fouled alumina, for example every fifth cycle, would add 2c/m3 to production costs.

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