FILTER CLOTH WASHING
Filter cloths provided with the filter press have been selected specifically for use on each particular application. Proper care and maintenance of the filter cloths are important to the performance of the filter press. During filtration, the filter cloth is the initial barrier that separates solids from liquid. Therefore, the filter cloth must remain porous to provide high filtration rates.
During normal operation the filter cloth may gradually become plugged with microscopic particles. These particles penetrate the cloth and become lodged in the depth of the weave, which leads to decreased filterability. These particles must be removed periodically to maintain high filtration rates and drier cakes.
Filter cloth washing is required when one of the following factors indicate plugging has occurred.
- Initial high filtration pressure
- Long filtration cycles.
- Wet filter cakes
There are several methods used to wash cloths while they are still installed in the press. The most commonly used method with metal finishing sludge is acid washing, which requires the following:
1. Acid-Washing Filter Cloths
- Install acid storage tank of sufficient capacity to fill press and allow for re-circulation, approximately 1.5 x holding capacity of press (7.5 gallons per cubic foot).
- Use a 25% solution of hydrochloric (muriatic) acid. A lower or higher concentration may be necessary due to solubility levels of entrapped particles. Note: Extreme care must be taken when handling acid.
- Use a low-pressure (20-30 PSI max) acid resistant pump.
- Install necessary plumbing (hoses or rigid PVC pipe) to isolate the press from the sludge stream and allow for both re-circulation to the acid storage tank and final draining of the spent acid solution. A throttling valve installed in the return line to the acid tank may be necessary to insure complete top to bottom press filling and washing of the cloths.
- Clean all filter cloths of all sludge cake with nylon spatulas furnished
- Close filter press
- Disconnect center feed line from sludge pump
- Connect outlet of acid pump to center feed line to filter press
- Connect lower outlet of filter press to acid re-circulation tank.
- Open acid feed line to filter press
- Start acid feed pump. It will take considerable time to fill all of the chambers of the filter press before the acid will return to storage tank. Continually inspect filter press for leakage during filling and re-circulating.
- Allow pump to re-circulate for one to two hours
- Turn off acid feed pump
- Follow air blowdown sequence in operation instructions to purge acid from filter press (use maximum 15 PSI air)
- Disconnect acid feed system and reinstall sludge pump on outlet lines.
- Filter press is now ready for operation
Caution: Acid washing is not recommended on non-gasketed filters unless extra precautions are taken to contain the leakage between plates.
2. Acid Dip Washing
Customers can also acid “dip” wash the plates by immersing them in a tank of 3-5% acid and water mixture. The immersion method though is less efficient then through washing in the press, and will require at least an overnight soaking to clean out the depth of the weave. Polypropylene plates are lighter then water and will float, so they will need to be manually kept submerged.
3. Power Spray Washing
Another method used for cloth washing is a portable high-pressure (800-1200 PSI at 2-10 GPM) cold water spray unit. These units come with a hand held power wand with spray nozzle, which is slowly moved over the cloths. This method cleans by not only flushing off the cloth surfaces but by also penetrating the cloth to flush particles out of the depth of the weave. Contact Met-Chem for more information on availability of units.