Filtration post-treatments

After the filtration operation, the deposited solid (the cake) is impregnated with a liquid of identical composition to that of the filtrate, which in most cases needs to be removed.

To achieve this, filtration post-treatments essentially involve washing and/or dewatering the cake. In some cases, the removal of very fine particles remaining in the filtrate must be continued by a finishing treatment; however, the present article will be limited to washing and dewatering operations.

• The purpose of the washing operation is to extract the dissolved substances present in the interstitial liquid contained in the pores of the filter cake, using a suitable solvent, the washing liquid. These operations are carried out in the same machine as the one used for filtration, or in other machines located downstream, depending on the performance to be achieved. In fact, the surface area of the filter dedicated to washing is large compared with that used during the filtration cycle. By way of example, the total surface fraction for vacuum belt filters is between 70 and 80% for gypsum or pigment purification. The mechanisms involved are both direct displacement and diffusion from stagnant areas of solutes by the washing liquor, so there are two main techniques for washing filter cakes:

  • washing by dilution ("repulping"), which consists of mixing the solid with the washing liquid in a separate tank and then refiltering the solute-free pulp, this operation being repeated as many times as the required degree of purity requires;

  • displacement washing, in which the washing liquor percolates through the cake deposited on the filter. In this way, the washing liquid displaces the interstitial liquid (in a plug flow at best), possibly eluting any solute adsorbed or absorbed within the solid.

The simplicity of direct displacement washing operations means that this technique is often preferred. However, repulping of the solid, followed by co-current or counter-current washing, is used when solute concentration is low and the diffusional transfer mechanism becomes predominant. The choice is also dictated by the filtration technology used: vacuum disc and drum filters, which do not achieve high washing performance, can be supplemented downstream, if necessary, by repulping tanks. When the particles making up the cake are coarse, a significant quantity of interstitial liquid can be removed by dewatering before washing.

• Dewatering operations ("deliquoring") are designed to remove part of the residual filtrate by reducing the moisture content of the cake. In many cases, this can be achieved by increasing...