Perlite filteraid
General operating notes on filtration



Filter septums.

In filteraid filtration the filter septum serves principally as the support for the "working" filteraid cake. However, the effect of the septum on performance is suffient to warrant careful selection. The main requirements are:

  • size of the openings should be fine enough to retain the filteraid particles and allow a firm cake to be formed quickly while at the same time giving a minimum resistance to flow. Openings too large may allow bleed-through of suspended solids and filteraid particles to cause poor clarity.
  • material of the element must be able to withstand chemical, pressure and temperature conditions existing during filtration.

Septa in general use today consist of cloths woven with a variety of natural and synthetic fibers; woven wire or slotted metal screens; tube or discs of porous powered metal, stone, carbon, aluminium oxide, and others.

The two most widely used septa are metal screens, and cloths woven of various materials. Metal screening is sometimes called wire cloth, with the Dutch weave the most common. No definite rule can be given for the screen mesh size, except that generally the openings should not be larger than those in a (0,14 mm) screen for direct precoating.

Excessive pressure

Pushing a filter beyond its normal pressure rating is not only dangerous but wastes time and process liquid, and may damage the equipment. Gaskets may blow, plates warp, screens or cloths tear. In addition, excessive pressures normally do not result in increased volume of filtrate.

Material or process changes

Variations from normal clarity, flowrate or cycle lenght sometimes occur. These variatons may be caused by; 1) a change in raw materials, 2) the process preceding filtration or 3) increased medium resistance (blinding). The filterability of liquid also may be affected by variations in viscosity or temperature, the quantity and nature of suspended solids, changes in pH, concentration and processing conditions.

Extreme agitation

In some instances when extreme agitation is used on unfiltered liquid, or when centrifugal pumps are used for recirculation as a means of agitation, various flocs, precipitates or gelatinous suspended solids are broke down. This has a decided harmful effect on the filtration rate.

The cure for the latter is to use independent agitation only sufficient to keep the filteraid in suspension. Do not use centrifugal pumps for recirculation.

Dirty cloths or screens

Septa that are blinded due to lack of proper care can greatly restrict flow by cutting down the effective filtering area. One of the best protections against this is faithful precoating with clean liquid which prevents slimes and gums from reaching the septum surface. Regular cleaning is also necessary.

Cleaning blinded filter cloths is usually a matter of thorough washing. Cleaning metal screens, however, seems to be more of a problem especially those with fine mesh openings. Boil or soak them in a cleaning solution (4% caustic water solution) then blow with WET steam at about 3,5 kg/m². The type of cleaning solution to use will depend on the liquid being filtered and the nature of the gums and accumulated deposit on the screens.

The manufacturer's directions for cleaning any type of filter element or septum are usually based on both thorough testing and long experience. It pays the user dividends to follow them carefully.



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