12.23 Powdered Activated Carbon (PAC)

PAC is normally delivered to site in bulk or in 500–1000 kg bags (FIBCs), to minimize manual handling on site, although it is also available in 25 kg bags. When delivered in bulk the chemical is blown across to vertical storage silos. PAC delivered in FIBCs is normally stored on site in the delivery packaging until required for use. PAC is prepared and dosed as a slurry in much the same way as lime powder (Section 12.10), using conveyors and batching tanks. If not wetted properly, PAC will tend to float in the batching tank. Wetting can be achieved by feeding PAC into the vortex of a mixer in the tank or via a wetting head consisting of a funnel with a continuous supply of water placed directly under the discharge of the screw feeder. Additional mixing is carried out in the batching tank. Batching tanks may be of rubber-lined or epoxy-coated steel, to resist abrasion. Plant for batching and dosing from FIBCs is commonly supplied as a skid-mounted system incorporating a frame from which the FIBC is suspended. PAC is drawn from the bottom of the FIBC and fed by a volumetric or gravimetric screw feeder into a water-driven eductor system. The resulting PAC slurry is either conveyed directly to the dosing point or else held in batching tanks from where progressive cavity or peristaltic pumps meter it to the dosing point.

PAC is a finely divided powder which can be difficult to contain. To minimize nuisance from escaping dust, storage and handling plant should be housed in fully segregated rooms. Any dust that escapes quickly forms a black covering on all adjacent plant and equipment. PAC is an electrical conductor and should not be allowed to accumulate as dust on open electrical circuits, to avoid potential for short-circuits. Some suppliers indicate that this dust should be considered as potentially explosive and appropriate precautions taken, although others state that this is not necessary. The lower explosive limit for activated carbon dust is variously reported as 50 (Cabot-Norit, 2014) and 140 g/m³ (Avantor, 2011). This suggests there is a small risk of dust explosions inside silos, conveyors, dust filters and other items of plant handling dry PAC, as well as in the area immediately adjacent to such equipment. In practice, most PAC is manufactured using steam activation. The resulting PAC contains only a very low level of volatile matter as a result of the high temperatures involved and is not considered to present an explosion risk. The so-called ‘Deflagration Index’ (K_St) for steam activated PAC places it in the lowest class of combustible dusts, Class ST1.

To minimize any risk of explosion, dust filters should be provided and sources of ignition should be avoided. Electrical equipment in the immediate vicinity of PAC plant should be protected to IP65 as a minimum and where possible should be located in a separate room. All equipment and piping used for dry handling of PAC should be earthed for static electricity.

Typically, carrier water is added downstream of the metering pump to obtain dilution down to 1% w/v (10 g/l), to maintain velocities in the dosing line and improve mixing at the point of application.