Recessed and Membrane Plate Filter Press equipment employs a filtration media to initiate the process of separating solids from liquid in slurries. The filtration media is typically referred to as a filter cloth.
Types of Filter Cloths
In the mineral and aggregate industries, the most common plate Filter Press filter cloths are woven synthetic materials such as polypropylenes, polyamides (nylons) and polyesters. The woven cloths are manufactured like other common cloth textile fabrics on large weaving looms, starting with individual strands of synthetic threads.
Mono-filament (single thread) or multi-filament (many single threads spun together) threads are used to produce a wide variety of filter cloths designed for specific dewatering applications.
Closeup views of mono-filament (left) and multi-filament (right) threads.
Slurries with very fine particle size distributions require filter cloths woven very tightly to capture the smallest particles while allowing the slurry liquid to pass through. More open weave filter cloths are effective when the slurry particle size distribution is larger.
Larger diameter mono-filaments are used to produce filter cloths that are stronger, more wear resistant and longer lasting, but they inherently cannot capture very fine slurry particles.
Smaller diameter mono-filaments are used to produce tightly woven filter cloths that can capture fine particles, but these cloths have lower strength and wear resistance. To overcome this disadvantage, fine-strand multi-filament filter cloths are used to capture very fine particle sizes as well as to provide strength, wear resistance and longevity.
What Filter Cloths Do
Filter cloths are used to separate the solid and liquid portions of the slurry once it is pumped into the void spaces between the filter plates.
Initially, some of the smallest slurry particles pass through the filter cloth mesh until a layer of dewatered solids accumulates on the cloth surface. The accumulated solids then begin to filter additional slurry material, capturing virtually all the solid particles and allowing only clear liquid (filtrate) to pass through the filter cloths.
Animation showing how a Filter Press works. Note how the filter cloths allow the liquid to pass through and the solids to build up on the surface.
Filter Cloth Wear
This initial filtering of the slurry material will cause wear on the filter cloth over an extended period of time. Normal wear also occurs on the cloth surface when the cakes are discharged from the Filter Press at the end of the dewatering cycle. This type of wear is very dependent on the abrasive characteristics of the cake material and the wear resistance of the cloth material. It is difficult to predict the normal cloth life cycle for any Filter Press application, but a good quality cloth material should last for hundreds if not thousands of cycles.
Inevitably, small amounts of slurry particles will become embedded in the filter cloth mesh during each filtration cycle. As more filtration cycles are completed, sufficient slurry particles will become lodged in the cloth mesh, inhibiting the filtration rate of the slurry and prolonging the filtration cycle time.
Typically, this process happens over long periods of time when an appropriately designed filter cloth has been chosen for the application.
To achieve maximum cloth life and high filtration rate efficiency, the filter cloths should be periodically inspected and cleaned, but eventually they will need to be replaced because they will wear out.
Routine Filter Cloth Cleaning
During normal Filter Press operation, small slurry solids will accumulate and build up on the surface of the filter cloths. The buildup on the filter cloths is dependent on many different factors including:
- Filter cloth design
- Slurry material make-up
- Material particle size distribution
- Material throughput rates
- Hours of operation
- Chemical additives utilised in the upstream processes
At a minimum, the filter cloths should be rinsed with process or clean water weekly to remove excess buildup from the cloth surfaces, especially near the perimeter sealing areas and on the stay bosses, see highlighted areas in photo below.
Filter plate with perimeter and stay bosses highlighted.
Cloth cleaning will prevent the accumulation of solid material buildup that will accelerate the cloth wear and may cause slurry blowouts during normal press operation.
In some extreme applications, it may be necessary to clean the filter cloths after each filtration cycle to sustain good filtration efficiency and cloth life.
The cloths should also be cleaned thoroughly if the Filter Press is shut down for extended time periods (several days or more) to prevent the material buildup from becoming dried on the cloth surface. Once dried, the contamination can become much more difficult to remove and will accelerate cloth wear.
Automated cloth washing systems are available when cloth cleaning is required on a frequent basis, from every few hours to every cycle. These automated systems can provide a simple flood wash to rinse the cloth surfaces with low-pressure, high-volume water sprays.
Lower volume, high-pressure systems are also available for the most challenging applications.
Routine cleaning will extend the filter cloth life and prevent unplanned cloth cleaning due to blowouts or unplanned cloth changes due to wear failures.
Repairing/Replacing Damaged or Worn Filter Cloths
As cloths become older, the risk of developing small holes increases due to localised cloth wear. Most often, when a small cloth hole occurs, the filtrate water will remain cloudy coming out of the Filter Press near the damaged cloth, so the filtrate should be inspected regularly to check for potential cloth damage.
Once a small hole occurs the cloth surface, the slurry will pass through the cloth rapidly in that area, causing further damage to the cloth and potentially severe damage to the filter plate surface.
Filter cloths should be inspected once a week for tears or folds. Any time a cloth hole is found, it should be repaired or replaced immediately to prevent further damage.
If the cloth hole is small enough, less than a couple inches, it is possible to temporarily patch the hole using duct tape or similar following the procedure below:
Cloth Temporary Patch Procedure
- Locate the cloth damage area and inspect both sides of the cloth and filter plate surface.
- If the damage is not severe, thoroughly clean both sides of the filter cloth.
- Dry off the filter cloth as much as possible. Common compatible solvents can be used to aid the drying process.
- Attach tape to both sides of the filter cloth, ensuring the tape remains attached.
- Note the location of the patched cloth and inspect it routinely until the cloth is replaced due to normal wear.
The cloths can also be damaged when a slurry blowout occurs between two plate sealing surfaces while the Filter Press is being filled with slurry or during the high-pressure filtration stage.
If the blowout is allowed to continue for several minutes or more, the cloth is easily damaged due to the high velocity of slurry passing across the cloth surfaces.
Any time a slurry blowout occurs, the press cycle should be stopped and the cloth should be inspected to find the possible cause of the blowout and to check the amount of cloth damage. If the cloth is severely damaged, it should be replaced immediately to prevent the risk of another blowout, which will further damage the cloth and eventually damage the filter plate surface.
How To Replace Filter Cloths
McLanahan Filter Press filter cloths can be replaced without removing the plate from the press. To do this, first de-energise and lock-out all equipment and components before performing maintenance on the Filter Press. In addition, follow all required fall protection procedures and all site-specific safety requirements.
To remove the filter cloth:
- Fully open the press, leaving as large a gap as possible between the filter plates.
- Center the filter plate for the cloth change in the center of the gap.
- Cut the zip ties and open the Velcro® strips to release the filter cloth from the perimeter of the filter plate.
- Roll up the filter cloth on one side of the filter plate and slide it through the filter plate feed hole opening.
- If the backer cloth is being replaced, cut the zip ties and remove the individual backer cloth from each side of the filter plate. It is not necessary to replace the backer cloth every time the outer cloth is replaced.
- If the backer cloth is not being replaced, carefully inspect it and clean it thoroughly with water. Also, inspect and clean any slurry buildup on the filter plate surface.
To install the filter cloth:
- If the backer cloth is being replaced, position the two backer cloths on each side of the plate and attach them with zip ties across the top of the plate, with the cloth properly oriented to line up with the filter plate feed hole opening and the upper feed filtrate holes. Once in position, attach the backer cloths to each other with zip ties along the sides and bottom of the filter plate.
- Roll up one side of the new filter cloth and slide it through the filter plate feed hole opening.
- Unroll the filter cloth and position the filter cloth on both sides of the plates, with the appropriate orientation to line up the filter plate feed hole opening and the upper filtrate holes.
- Attach the filter cloth at the top of the filter plate with zip ties.
- Attach the filter cloth Velcro® strips on the sides and bottom to the filter plate to secure the filter cloth properly, ensuring there are no wrinkles or creases in the cloth surfaces.
- Wash down the new filter cloth with water to remove any dirt or debris.
Good filter cloth inspections and routine filter cloth cleaning are an essential part of efficient plate Filter Press operation. Routine inspections and cleaning will prolong filter cloth life and maintain the highest dewatering efficiencies.