Bulk Bag Unloading System contains Toxic Dust in Fluoridation Plants

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Flexicon (Europe) Ltd.

Stopping the Flow is a Cinch

A special feature of the unloader unit is a pneumatic Power Cincher flow control valve that can close the bag at any time, so that a partially empty bag can remain in place until more material is needed. This is important for the fluoridation plants, which use approximately 120 kg/d of Na₂SiF₆ per 100 million litres per day of water. The cincher also helps to keep moisture out of the bag and can isolate the bag in the case of an emergency.

Promoting flow are Flow Flexer bag activators – two pneumatically driven plates that rhythmically raise and lower opposing bottom edges of the bag to direct material to the outlet. As the bag empties, the stroke of the plates lengthens, forming the bag into a steep V-shape and promoting total evacuation. An adjustable timer controls the frequency of the strokes.

The dust tight system is vented to a Bag-Vac dust collector that removes residual powder and collapses the empty bag prior to tie off, preventing dust generated when empty bags are flattened manually.

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As mentioned earlier, a flexible screw conveyor transports the Na₂SiF₆ from the floor hopper to a storage hopper that feeds the mixing tank. The conveyor consists of a rotating, stainless steel spiral screw, housed in a 65 mm diametre polymer tube. The lower end of the roughly 5 m long tube passes through the wall of the floor hopper, near the bottom, and the top end discharges the powder through a chute into the top of the storage hopper some 4.5 m above the plant floor.

A Self-Centred Conveyor

As the screw rotates, it self-centres within the tube, providing ample clearance between the screw and the tube wall to prevent grinding of the product. A 4.0 kW electric motor, located above the discharge point, rotates the screw at a variable rate up to 6000 kg/h. The flexible screw conveyor is inherently enclosed throughout its length to avoid airborne dust.

As the bag's contents empty into the floor hopper, the conveyor is activated. The transfer of powder to the storage hopper continues until either the transition hopper is empty or the weight of the storage hopper reaches a preset high level, as indicated by four load cells underneath the hopper. The control system signals the conveyor to stop when the high level is reached.

From the storage hopper, a dry chemical feeder meters the fluoride powder into a mixing tank through a sealed unit that prevents the escape of dust. The flow of powder is automatically matched to the inflow of water to the tank in a ratio that results in a 0.2 percent Na₂SiF₆ saturated solution. The tank has a high-speed mixer and a retention time of 10 min.

Finally, the solution is carefully metered into to the flowing water supply by a peristaltic pump (or a progressive cavity standby pump). The dosage rate varies from 0.6 mg/l to 1 mg/l, depending on local requirements.

In rare cases a water treatment plant may have two independent pipelines, each with its own dosing system. This situation occurs, for example, when a town or city has grown and added more treatment capacity.

These cases require separate storage hoppers and dosing systems for each pipeline. However, a single bulk bag discharger and one transition hopper can feed two storage hoppers by incorporating two separate flexible screw conveyors into the single common transition hopper. Feeding two storage hoppers is well within the capacity of the system. McKee points out that a single conveyor delivers material at a rate of around 5000 kg/h, while the seven-day storage capacity of a large hopper is only about 8750 kg.

“The Flexicon system is the only one that ProMinent uses for fluoridation plants,” says McKee. “We have only ever promoted Flexicon bulk bag unloaders with double-wall telescoping tubes for fluoride, as we found it to be the best available to handle a toxic powder with minimum risk of dust,” he says. n

* * Alex Walton is General Sales Manager Flexicon (Europe) Ltd.

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