Introduction
Recessed plate and membrane plate Filter Presses are composed of a heavy-duty framework holding a series of plates lined with filter media.
The plates are held tightly together using hydraulic pressure, and a high pressure slurry feed pump is used to force slurry material into the void spaces between the plates.
The slurry solids are captured between the plates while the filtrate water passes through the filter cloths on the plate surfaces.
When no additional slurry can be pumped into the Filter Press, the slurry feed pump is stopped and the plates are separated, allowing the cakes to fall by gravity out of the press.
What is a Filter Press cycle?
By design, the dewatering technology used in recessed plate and membrane plate Filter Presses is a batch type process. A series of process steps occur to complete the batch process, and all of the steps combined are referred to as a Filter Press’s dewatering cycle.
A typical plate Filter Press cycle is made up of the following steps:
- Filter plate closing/clamping
- Filling
- Ramp/Filtration
- Core blow (optional)
- Cake dry (optional)
- Membrane squeeze (optional)
- Filter plate unclamping and opening
Filter plate closing/clamping step
The filter plates are closed together using a hydraulic cylinder and mobile plate. Lower pressure is required to first close the plate stack, then very high hydraulic pressure clamps the plates tightly together.
The clamping hydraulic pressure needs to be high enough to withstand the pressure created by the slurry feed pump pushing slurry into the void spaces between the filter plates.
Filling step
The feed pump is started so that it can begin filling the void spaces between the clamped filter plates.
During this fill step, the slurry feed rate can be very high and the feed pressure is very low.
To help reduce cloth wear around the filter plate feed hole areas, the fill flow rate is limited by controlling the pump speed. This limited flow rate is held constant until a minimum feed pump pressure is reached.
Ramp/Filtration step
Once the minimum feed pressure is reached in the fill stage, the feed pump speed is ramped up to increase the feed pressure over a period of time.
The feed pump speed is increased until the target filtration stage pressure is reached. The pressure is held constant for the balance of the filtration stage.
As the filtration step continues, the slurry flow rate drops off because the void spaces between the filter plates are becoming filled with dewatered slurry solids.
The filtration stage ends when the slurry flow rate reaches a preset minimum level and the feed pump is stopped.
Core blow step (optional)
After the feed pump is stopped, some amount of slurry remains in the core hole areas of the filter plates. A core blow step removes the remaining slurry using flush water and/or compressed air.
Flush water is first forced through the core to push the slurry out, then compressed air is forced through the core holes to push out the flush water and any remaining slurry.
The slurry and water are typically forced back to the slurry surge tank, where they can be fed back to the press in subsequent cycles.
Cake dry step (optional)
To remove additional water in the cakes, compressed air is forced through the filter cakes while the cakes are still being held in the filter plates.
This is typically used when the cake material is very porous and when very low cake moisture levels are required.
Membrane squeeze step (optional)
Similar to the cake dry step, the membrane squeeze step is used to remove additional water from the filter cakes after they have been formed.
Membrane plates have flexible surfaces that are expanded with water or compressed air. The expanded plate surfaces compress the cakes, forcing additional water to be removed.
Filter plate unclamping and opening step
Once all the dewatering steps have been completed, the high hydraulic clamping pressure is relieved.
The cylinder and mobile plate continue to open to make space for all the filter plates to be separated.
A plate opening system indexes each plate to open, allowing the dewatered cake material to fall by gravity out between the filter plates.
Once all of the cakes have been discharged, a new filter press cycle can begin.
Factors that affect cycle time
Each step of the filter press cycle will require some amount of time that will depend on the characteristics of the slurry material being dewatered as well as the design of the filter press.
Design factors
Key filter press design factors that can influence the cycle time include:
- Chamber thickness
- Slurry feed Pump pressure
- Filter plate opening and closing speed
- Filter cloth design
Chamber thickness
Ffilter plates with a larger chamber thickness will require longer fill and filtration time due to the higher volume of slurry that will be dewatered between the filter plates.
Slurry feed Pump pressure
Filtration time typically can be reduced with higher slurry feed pump pressures.
The time reduction achieved with higher feed pressures can only be determined by testing representative sample material at different feed pressures. Most modern Filter Presses are designed to operate between 100 psi and 225 psi maximum pressure.
For special dewatering applications, higher feed pressures are possible.
Filter plate opening and closing speed
The hydraulic power unit and hydraulic cylinder size will determine how fast the Filter Press can be opened and closed. Typical hydraulic cylinder opening and closing speeds are one to 10 minutes.
The plate opening speed is also dependent on the Filter Press plate shifting design, but it is typically one to five seconds per plate.
Filter cloth design
Filter Press cloths are typically woven synthetic fabrics made from fine nylon, polypropylene or polyester filaments. Many different types of cloth weaves and styles can be used for dewatering.
Filter cloths are designed to capture the solid particles and allow filtrate water to pass through. When a more tightly woven cloth is required to capture material composed of finer particle sizes, the flow of filtrate through the cloth can be negatively impacted, extending filtration time.
The optimum cloth design is chosen based on the dewatering characteristic of the material and the best cloth wear and cake release properties.
Key dewatering process factors that can influence the cycle time include:
- Slurry feed density (percent solids of the slurry)
- Composition of the slurry (clay, ash, coal, etc.)
- Slurry particle size distribution
- Dewatering aids
Slurry feed density
Slurry feeds with high densities will typically reduce the fill and filtration times. At higher percent slurry solids, the amount of solids being pumped into the Filter Press chambers occurs more quickly, resulting in reduced cycle times.
Composition of the slurry
The composition of the slurry material being dewatered will impact the rate of water removal.
Materials that tend to absorb water — or water-loving, hydrophilic materials — can be very difficult to dewater and may require extended filtration times or filter aids.
Materials that naturally repel water — or hydrophobic materials — will dewater much more easily, reducing the filtration time.
Slurry particle size distribution
Slurries with larger, rounded particles are more easily dewatered than slurries with finer particles that can be more easily packed together.
Fine clay materials can be very flat particles that pack tightly together, making the removal of water between them nearly impossible.
Dewatering aids
When slurry materials are very difficult to dewater, dewatering aids or chemicals can be added to decrease the filtration time.
The most common aids include perlite, diatomaceous earth (DE) and high calcium hydrated lime. Perlite and DE are naturally occurring minerals that provide larger particles in the slurry, allowing the water to be more easily removed.
Adding hydrated lime can chemically make the slurry material release water more easily.
All these factors are taken into consideration when designing a Filter Press to ensure optimum dewatering efficiency for each application.
