The Rotary Drum Precoat Filter

Description

The Rotary Drum Precoat Filter is used to polish solutions having traces of contaminating insolubles. To polish the solution the drum deck is precoated with a medium of a known permeability and particle size that retains the fines and produces a clear filtrate.

The following materials are used to form the precoat bed:

• Diatomaceous Earth (or Diatomite) consisting of silicaceous skeletal remains of tiny aquatic unicellular plants.
• Perlite consisting of glassy crushed and heat-expanded rock from volcanic origin.
• Cellulose consisting of fibrous light weight and ash less paper like medium.
• Special ground wood is becoming popular in recent years since it is combustible and reduces the high cost of disposal. There are nowadays manufacturers that grind, wash and classify special timber to permeabilities which can suit a wide range of applications.

These materials when related to precoating are wrongly called filter-aids since they do not aid filtration but serve as a filter medium in an analogy to the filter cloth on a conventional drum filter.

The Precoat Filter is similiar in appearance to a conventional Drum Filter but its construction is very different:

•  The scraper blade on conventional drum filters is stationary and serves mainly to deflect the cake while it is back-blown at the point of discharge. The scraper on a precoat filter, which is also called "Doctor Blade", moves slowly towards the drum and shaves-off the blinding layer of the contaminants together with a thin layer of the precoating material. This movement exposes continuously a fresh layer of the precoat surface so that when the drum submerges into the tank it is ready to polish the solution. The blade movement mechanism is equipped with a precision drive having an adjustable advance rate of 1-10 mm/hr. The selected rate is determined by the penetration of fines into the precoat bed which, in turn, depends on the permeability of the filter aid. Once the entire precoat is consumed the blade retracts at a fast rate so that the filter is ready for a new precoating cycle.

• The cake discharges on conventional drum filters by blow-back hence a section of the main valve's bridge setting is allocated for this purpose. On precoat filters the entire drum deck is subjected to vacuum therefore there are two design options:
  • A conventional valve that is piped, including its blow-back section, to be open to vacuum during polishing. When the precoat is consumed its blow-back section is turned on to remove the remaining precoat heel over the doctor blade.
  • A valveless design in which there is no bridge setting and the sealing between the rotating drum and the stationary outlet is by circumferential "o" rings rather than by a face seal used on conventional valves.
• The clearance between the drum deck and the agitator on conventional drum filters is sufficient to accommodate cakes of up to about 50 mm while on modern precoat filters higher clearances to allow the formation of 150 mm precoats are quite common.
• The internal drum piping on conventional drum filters is sized for relatively low hydraulic loadings. On precoat filters, as opposed to conventional filters, the filtrate rates during the precoating stage are high so often more cross section of internal piping is available to reduce head loss.

The flowscheme of a Precoat Filter Station will generally look like this:

Selection Criteria

Precoat filters are generally used in the following cases:

• Solutions that require a very high degree of clarity.
• The solids content is very low and less than 2-3%.
• The solids are sticky and would otherwise clog the filter cloth.

Operational Sequence

The operation of a precoat filter is done in two stages:

• Precoating stage
• Polishing stage

The two stages are closely related since the efficient and economical performance of the filter in the polishing stage depends entirely on the selection of a suitable filter aid and the care taken during the precoating stage.

Precoating

An 8-10% precoat slurry is prepared by feeding the selected filter aid to a tank filled with water and equipped with a low speed mixer, such as a sweep-arm agitator, that rotates at 30-50 rpm. The low speed is necessary in order to avoid the breakage of the fine skeletal or expanded rock structure.

A precoat cake, depending on its permeability, may contain about 50% moisture so it is sometime advisable to prepare the slurry with polished filtrate rather than with water. Two arguments exist in favor of the filtrate option:

• The amount of dry diatomite in the wet cake is 310 Kg/m3 (for coarse diatomite) and 370 Kg/m3 (for fine diatomite) so, if we assume a 150 mm precoat then it may contain some 50 Kg/m2 water. Therefore, on a middle sized filter of 20 m2 the amount of water for downstream evaporation is 1 ton and this must be taken in consideration.
• With filtrate the transition from precoating to polishing does not necessitate the emptying of the tank whilst with water the tank must be drained in order not to dilute the solution.

When the alternative to water is preferred attention must be given to the specific gravity of the filtrate since difficulty may arise while attempting to wet and mix the light filter aid in a solution of say 52% CaBr2 having a specific gravity of 1.7. In such a case the solution of using a high speed mixer is not desirable since, as mentioned above, particle breakage may occur so using an eductor to enhance wetting is a better method .

There are some basic rules to observe during the precoating of the drum in order to ensure that the filter aid is evenly distributed and the formed bed is tight and crack free:

• When precoating commences the cake resistance is low therefore the vacuum pump should be capable to deliver a very high air flow and in some installations a separate vacuum pump is dedicated just for the precoating stage.
• The precoat filtrate pump should be capable of pumping the slurry at a pipe velocity of 1.5-2.0 m/sec to avoid segregation and the settling of the heavier filter aid particles.
• For the same reason the drum should be rotated at a high speed.
• The weir in the overflow box should be lowered so that the drum is barely submerged and the bed does not form too fast. A fast forming precoat is generally uneven in thickness and requires substantial "shaving" prior to feeding the contaminated solution in the polishing stage. Likewise, a precoat that forms too fast tends to crack so filtrate clarity may deteriorate and the cloth under such cracks may blind .
• The doctor blade must be retracted to the maximum clearance between drum deck and agitator and then advanced slowly to smoothen the bed's surface in preparation for the polishing stage.
• Vacuum should never be cut-off during precoating (or polishing for that matter) since the heavy cake will most certainly fall off the drum into the tank with all the consequences. A rule of thumb suggests that to hold a 100 mm wet precoat to the drum a vacuum of 180-200 mmHg is required.
• The filter cloth acts merely as a support for the precoat that forms when precoating commences and then the already formed thin layer takes over. Therefore, the cloth should be sufficiently dense to retain the forming bed depending to a large extent on the particle distribution of the selected filter aid.

Polishing

The insoluble contaminants, which are often slimy and gelatinous in nature, cause fast media blinding and soon reduce filtrate rate to a trickle. Hence, the operating principle of polishing is to avoid formation of a thick layer but rather form a very thin layer that is removed continuously by shaving it off.

This requires careful optimization since polishing is an expensive operation. The optimum conditions are:

• Select a precoat material that is permeable enough to yield a high filtrate rate but dense enough to avoid the penetration of the contaminants into the precoat bed.
• Set the Doctor Blade to the lowest advance speed so that it barely cuts into the precoat bed since deep penetration of the insoluble matter increases diatomite consumption and shortens the polishing cycle.

Maintenance

All the points mentioned in the section related to conventional Drum Filters also apply to Precoat Filters but some additional requirements should be observed:

• Diatomacueos earth or perlite are abrasive materials and the Doctor Blade's edge must be kept sharp otherwise the cut is not smooth and vibrations to the fine discharge mechanism may be experienced. A ground and polished seam of stellite is often welded along the edge to prolong the life of the blade.
• Good practice is to manufacture the blade from short segments since practice shows that the wear is not equal across the drum's face. Preferably, these segments should be manufactured with two sharp edges so that when one edge wears out it may be turned 180 degrees thus doubling the time required for replacement.
• The moving parts of a rake type reciprocating agitator have many bearings that once they wear out the entire filter is subjected to shocks and shakes. This situation is dangerous since during the precoating stage the vacuum is relatively low and the forming bed may drop-off so special precautions should be taken to inspect the bearings as part of the preventive maintenance schedule.