Membrane plate and recessed plate Filter presses are made out of a heavy-duty framework that holds a number of filter media-lined plates.
Hydraulic pressure is utilized to hold the plates together, and a high pressure slurry feed pump is used to shove slurry material into the void spaces between the plates.
While the filtrate water flows through the filter cloths on the plate surfaces, the slurry solids are collected between the plates.
When no more slurry can be poured into the Filter Press, the slurry feed pump is turned off and the plates are separated, enabling the cakes to fall out of the press by gravity.
What is a Filter Press cycle?
The dewatering technology utilized in recessed plate and membrane plate Filter Presses is a batch procedure by design. To complete the batch process, a succession of process steps occur, and the sum of the processes is referred to as the dewatering cycle of a Filter Press.
The following steps comprise a normal plate Filter Press cycle:
- Closing/clamping of the filter plate
- Filling \sRamp/Filtration
- The core blow (optional)
- The cake is dry (optional)
- Membrane compression (optional)
- Unclamping and opening the filter plate
Filter plate closing/clamping step
A hydraulic cylinder and moveable plate are used to close the filter plates together. Lower pressure is necessary to close the plate stack first, followed by extremely high hydraulic pressure clamping the plates closely together.
The clamping hydraulic pressure must be sufficient to withstand the pressure generated by the slurry feed pump as it pushes slurry into the void spaces between the filter plates.
The feed pump is started, and the void areas between the clamped filter plates are filled.
The slurry feed rate might be quite high and the feed pressure can be very low during this fill step.
The fill flow rate is restricted by adjusting the pump speed to help decrease cloth wear around the filter plate feed hole locations. This restricted flow rate is maintained until the minimum feed pump pressure is reached.
Step of ramp/filtration
Once the minimum feed pressure is attained during the fill stage, the feed pump speed is gradually increased to gradually increase the feed pressure.
The speed of the feed pump is increased until the desired filtration stage pressure is obtained. For the duration of the filtration stage, the pressure is remained constant.
As the filtration phase progresses, the slurry flow rate decreases as the void areas between the filter plates fill with dewatered slurry solids.
The filtration step is completed when the slurry flow rate reaches a predetermined minimum level and the feed pump is turned off.
Step of the main blow (optional)
When the feed pump is turned off, some slurry remains in the filter plates’ core hole locations. A core blow step uses flush water and/or compressed air to remove the remaining slurry.
Flush water is pumped into the core holes first to drive the slurry out, followed by compressed air to push out the flush water and any remaining slurry.
In most cases, the slurry and water are driven back into the slurry surge tank, where they can be fed back into the press in consecutive cycles.
Stage of cake drying (optional)
Compressed air is pumped through the filter cakes while the cakes are still held in the filter plates to remove any remaining water.
When the cake material is exceedingly porous and very low cake moisture levels are required, this is often used.
Step of Membrane Squeeze (optional)
The membrane squeeze stage, like the cake dry step, is used to remove further water from the filter cakes after they have been created.
Membrane plates feature flexible surfaces that expand when compressed air or water is applied. Because the increased plate surfaces compress the cakes, additional water must be removed.
Filter plate unclamping and opening step
The high hydraulic clamping pressure is reduced once all of the dewatering stages have been accomplished.
The cylinder and mobile plate continue to open in order to separate all of the filter plates.
A plate opening mechanism indexes each plate to open, allowing the dewatered cake material to fall out of the filter plates by gravity.
A fresh filter press cycle can commence once all of the cakes have been discharged.
Factors that affect cycle time
Each step of the filter press cycle will take some time, which will be determined by the properties of the slurry material being dewatered as well as the design of the filter press.
FACTORS IN DESIGN
The following are key filter press design parameters that can influence cycle time:
- thickness of the chamber
- Feed Pressure of the Slurry Pump
- Speed of filter plate opening and shutting
- Filter cloth layout
thickness of the chamber
Because of the greater volume of slurry that will be dewatered between the filter plates, larger chamber thickness Ffilter plates will require more fill and filtration time.
Feed Pressure of a Slurry Pump
Higher slurry feed pump pressures often minimize filtration time.
Only by testing representative sample material at various feed pressures can the time savings gained with increased feed pressures be estimated. Most current Filter Presses are intended to operate at maximum pressures ranging from 100 to 225 psi.
Higher feed pressures are feasible for particular dewatering applications.
Speed of filter plate opening and shutting
The speed with which the Filter Press can be opened and closed is determined by the hydraulic power unit and hydraulic cylinder size. The typical hydraulic cylinder opening and shutting times range from one to ten minutes.
The plate opening speed is also determined by the plate shifting design of the Filter Press, but it is normally one to five seconds per plate.
Design of a filter cloth
Filter Press cloths are woven synthetic fabrics composed of fine nylon, polypropylene, or polyester filaments. For dewatering, a variety of textile weaves and designs can be utilized.
Filter cloths are intended to catch solid particles while allowing filtrate water to pass through. When a more densely woven fabric is required to collect material with finer particle sizes, the passage of filtrate through the cloth can be hampered, resulting in increased filtration time.
The optimal cloth design is determined based on the material’s dewatering characteristics as well as the best cloth wear and cake release features.
The following are key dewatering process parameters that can influence cycle time:
- Density of slurry feed (percent solids of the slurry)
- The slurry’s composition (clay, ash, coal, etc.)
- Size distribution of slurry particles
- Dewatering agents
Density of slurry feed
High-density slurry feeds often minimize fill and filtration times. At larger slurry solids percentages, the amount of solids forced into the Filter Press chambers occurs more quickly, resulting in shorter cycle times.
The slurry’s composition
The rate of water removal is affected by the content of the slurry material being dewatered.
Absorbing materials — or water-loving, hydrophilic materials — can be difficult to dewater and may necessitate extended filtering durations or filter aids.
Hydrophobic materials, or those that naturally repel water, will dewater significantly more easily, lowering filtration time.
Size distribution of slurry particles
Slurries with larger, rounder particles dewater more easily than slurries with finer particles that can be packed together more easily.
Fine clay materials can be very flat particles that pack firmly together, making it practically impossible to remove water between them.
Dewatering aids or chemicals can be applied to reduce filtration time when slurry ingredients are difficult to dewater.
Perlite, diatomaceous earth (DE), and high calcium hydrated lime are the most commonly used aids. Perlite and DE are minerals that exist naturally and provide bigger particles in the slurry, allowing the water to be extracted more easily.
Adding hydrated lime can help the slurry material release water more freely chemically.
All of these variables are considered while constructing a Filter Press to achieve maximum dewatering efficiency for each application.