Settling – Filtration

Decantation and filtration are two phase separation techniques used either in processes or in analytical methods. As this article is aimed at anyone looking for an analytical separation method or a method for preparing samples for analysis, the techniques are only discussed from the analytical point of view, i.e. at laboratory level.

The aim is to describe the phenomena involved, so as to situate these techniques within the analytical process, and to present the various elements that enable them to be implemented.

Decanting is a process for separating :

• or a solid phase of suspended matter in a liquid of lower density;

• or two immiscible liquid phases of different densities.

In both cases, the action consists in letting the phases in contact rest and waiting long enough for them to separate under the action of gravity. It's a simple but time-consuming operation, requiring little equipment, and therefore inexpensive, but not very selective. All that's involved is a constant external force - gravity - and all that's required is to avoid any agitation or remixing action once separation has taken place.

Filtration is a process for separating an initially mixed continuous phase (liquid or gas) and dispersed phase (solid or liquid). Separation is achieved by passing the mixture through a filter medium, a porous medium adapted to the characteristics of the suspension to be filtered, under the action of a pressure force providing the suspension with the necessary energy to pass through the porous medium. It therefore involves defining the appropriate filter medium, as well as the conditions under which it is used, i.e. the filter and its environment.

In practice, the application of filtration to analytical methods most often concerns suspensions (solids dispersed in a liquid) or fumes (solids dispersed in a gas), which use the same filtration media. The case of mists (liquid dispersed in a gas) or emulsions (dispersion of a liquid in another immiscible liquid) will not be discussed, as only solid-liquid separation will be considered. Applications in fields as varied as food processing, biology, pharmaceuticals and water treatment will be developed.