In order to maximise the benefits obtained from flocculating agents when used as settling aids in high rate and conventional thickeners, it is often necessary to dilute the thickener feed prior to flocculant addition. In some cases, no flocculation at all is possible unless dilution water is used. The most common method of diluting thickener feed has been to recirculate some of the thickener overflow by- means of a centrifugal pumping system. This represents a significant capital and operating cost.

Supaflo Technologies of Australia has developed and patented an internal recirculation method known as Auto Dilution (or “Autodil”) which is based on the principle that an hydraulic head differential exists between the inside and outside of the feedwell because of the higher S.G. of the pulp inside the feedwell. By installing non-return ports in the feedwell wall at the appropriate level, supernatant solution flows into the feedwell at a calculated rate and dilutes the feed without any need for external recirculation of overflow.

These ports are fitted with a flap which is designed to float on the surface of the slurry in the feedwell. They prevent over dilution of the feed slurry and stop the flow of slurry outwards into the overflow region of the thickener if the level inside the feedwell is equal to or greater than that outside.

Supaflo “Flocmiser” feedwells fitted with Autodilution are designed such that the energy of the incoming feed stream is used to mix the dilution water, slurry and flocculant. The intensity of this agitation is critical to good floc formation and thus to minimising flocculant usage. Adjustable baffles are fitted to the inside of the feedwell to enable the mixing regime to be easily varied.

In many instances it has been found that the floc structure is extremely fragile and will break down if it remains in the feedwell for too long. To avoid this, adjustable flocculant spargers are provided so that the flocculant can be dispersed into the feedwell at the optimum level. This level is often very close to the outlet from the feedwell.

Autodilution is now operating on many thickeners in Australia and a number of thickeners are operating with this system in other parts of the world. A few case histories have been selected to illustrate some typical applications and are described below.