March 1, 2013
Filter presses have come a long way from the days of the old “plate and frame” presses.
By Cory Jenson
Three main factors have created a resurgence of interest in using filter presses to dewater fine wash plant effluent. First, it has become harder to get permits for new settling ponds due to changes in regulation and permitting processes. This has caused owners to seek closed-loop dewatering solutions. Second, many plants are running out of space for additional settling ponds, or settling ponds do not fit well with expansion plans by preventing access to, or use of, mineable reserves. Third, in order to reduce operating costs by achieving drier cakes or using less chemicals, operations are looking at replacing belt presses with newer style filter presses that can often achieve a drier product while using less chemicals.
Aggregate plant operators who may be familiar with older filter presses often refer to the filter presses on the market today as “plate and frame” presses. It is important to understand the difference between older style plate and frame presses and the newer style higher-pressure recessed plate and membrane plate filter presses more commonly used today. It is also helpful to know the primary styles of filter presses on the market and the frequently used options on these filter presses, as well as typical cake moistures achieved with recessed and membrane-style presses.
The filtration cycle of a filter press, at its most basic level, consists of covering two plates with filter media, applying enough force to create a seal between the two filter plates, and then using a feed pump to supply the pressure necessary to pump the slurry into the cavities formed between the two sealed plates. The filter cloth prevents the solids from escaping from the plates while allowing the filtrate to pass through the openings in the filter media. Once the chamber is packed full of solids, the feed pump stops, the pressure sealing the plates is released, and the filter plates are pulled apart from each other, allowing the dewatered cakes to discharge via gravity. A common misconception is that the press squeezes the plates in order to dewater the cakes. In actuality there is no movement of the press during the dewatering of the filter cakes. The plates are held together in order to form a seal, and the feed pump supplies the necessary pressure to dewater the fine solids. The only movements of the press during operation are, typically, for opening the press to discharge the cakes and closing the press to start another cycle.
The simplicity and limited movement of filter presses are what lends them to high levels of automation and reliability. The ability to select cycle times, cake thickness, feed pressures, and plate styles are what allow filter presses to achieve much drier filter cakes than many other competing technologies.
Plate- and frame-style plates
Plate and frame presses have been around for the longest time and are characterized by the use of essentially flat plates with accompanying frames. When the plates are pushed together (as shown in the filter plate styles diagram), the thickness of the frame determines the cake thickness. The filter cake forms inside the frame between the two plates. Common problems associated with this old plate design are the difficulty in removing the cake once it is formed inside of the frame. To discharge the cakes, the flat plates are pulled longitudinally away from the frame. Since the cake is formed inside the frame, operator intervention may be required in order to remove the cake with this older style plate. Newer presses typically incorporate recessed plates and/or membrane plates to overcome the disadvantages of the older style plate and frame designs. Another factor that limited the capabilities of the older plate and frame presses is that they were often restricted to much lower feed pressures of 80 to 125 psi.
Due to the wide use in past decades of the older style plate and frame design, it is still quite common to hear people call newer presses by the old name of “plate and frame” presses. This often creates confusion with people who are familiar with the problems associated with the older style plate and frame plates and do not yet understand the capabilities of the newer style plates and presses.
Recessed filter plates
In order to overcome the difficulties in cake discharge with older style plate- and frame-plate configurations, newer plates have been developed to improve the discharge of the cake from the filter press. These plates have recessed cavities on each side of the filter plate. When the plates come together, the two recessed plates form the chamber where the filter cake is formed. Once the cake is formed inside the chamber, the plates are pulled apart, and the cake is able to discharge freely, as it is no longer trapped inside of the frame as it was in the older “plate and frame” style plates.
Membrane filter plates
Like the recessed plate, the membrane plates are also characterized by having a recessed area on both sides of the plate. The key difference is that a flexible membrane is also added between the web of the plate and where the filter cake is formed. The membrane plate can then be placed next to a recessed plate in what is called a “mixed-pack” configuration, as shown in the filter plate styles diagram.
After the plates are closed and sealed, the feed pump fills the chamber formed between the recessed and membrane plates. Once the required amount of solids is introduced into the chamber, high-pressure water can be pumped behind the membrane to physically squeeze the cake. The addition of membranes adds some complexity to the operation when compared with recessed plates, but some materials allow for reduction in cycle times and drier cakes, which can justify the increased capital cost and maintenance.
Other considerations to keep in mind with membrane plates:
• Membranes have a much shorter life than recessed filter plates due to the flexing required of the membrane every cycle under high-pressures. Life of the membrane will depend on the number of cycles performed and materials used.
• Some membrane plates are available with replaceable membranes. Others require replacement of the entire filter plate when the membrane needs to be replaced.
• For pressures higher than 100 to 125 psi, it is generally recommended to pressurize the membranes with water, rather than air, for safety. Recommendations of the plate manufacturer should be followed.
• Some membrane plates require a minimum amount of solids to be introduced into the cavity before the membranes can be pressurized. Premature pressurization of these membrane plates can result in overextension and potential failure of these membrane plates. More expensive membrane plates are known as “empty chamber” membrane plates and can be pressurized without failure with any volume of solids.
Recessed chamber plates vs. membrane plates and cake thickness
For both recessed chamber and membrane plates, lab and/or pilot testing should be performed to determine cycle times and achievable cake moistures. Recessed chamber plates offer simplicity and reduced maintenance. Membrane plates may be able to achieve drier cakes or shorten cycle times. Depending on the clay content, recessed plates operating at 225 psi typically produce cakes of 70- to 85-percent solids on aggregate tailings. Membrane plates may increase the percent solids and reduce cycle times while treating the same slurry. The addition of membrane plates may be considered if higher solids content will be required to meet compaction requirements.
The selection of the proper cake thickness is also critical to the proper functioning of the press. Cake thicknesses typically range from 15 mm to 50 mm. As the filter cake becomes thicker, the permeability of the cake decreases. When large amounts of clay are present, even small increases in the cake thickness can result in large increases in the cycle time (i.e. increasing cake thickness from 25 mm to 35 mm can double or triple the cycle times on less permeable slurries). Testing should be performed to determine the proper cake thickness, and thinner cakes, such as 25 mm, are more commonly used when clays are a concern.
Overhead beam vs. side beam presses
Filter presses are frequently categorized into two main styles, each with their own advantages and disadvantages:
• Overhead beam — Denoting a press in which the filter plates hang from overhead beams.
– No parts on the sides of the press, allowing unobstructed access to change filter cloths on the press without plate removal.
– Much wider plate opening, often exceeding 3 feet between the plates. This creates more movement to aid cake release, while also allowing easier access for cloth washing and inspection.
– Dynamic loads are typically isolated from the beams of the press using tension shafts in various configurations.
• Side beam — Denoting a press in which the filter plates are supported by beams running along the side of the press.
– Easier to implement shaking systems for applications requiring shaking systems.
– Lower upfront cost.
– Less access to plates for cloth washing and inspection. Automatic wash systems should be considered more heavily with side beam style presses due to limited plate access.
Other options to consider when selecting a filter press
• Open filtrate discharge — Open filtrate discharge from each plate allows operators to easily identify damaged cloths, as the filtrate coming from that plate will be significantly dirtier than the filtrate coming from the other plates.
• Core wash/core blow — The feed core of the plates is lined with a non-permeable material and will remain wet compared to the rest of the cake. This feed core can be evacuated by pumping water through the core at the end of the cycle followed by air to remove the wash water. This can sometimes be limited to only using air to blow out the slurry core.
• Automatic wash system — Automatic wash systems should be considered more heavily when looking at side beam presses. Access for cloth washing is limited due to the minimal opening space between the cakes and the limited access due to the beams running along the side of the press. They can also be useful in overhead beam presses when operator availability for cloth washing is limited.
Newer style filter presses with recessed chamber plates and membrane plates are being installed at increasing rates to help aggregate plants deal with water/fines solids management issues. Filter presses have come a long way from the days of the old “plate and frame” presses and now incorporate newer style and much higher pressure filter plates than were offered in past decades. These technological advances have allowed all extractive industries to improve their effluent management using closed-loop solutions incorporating filter presses into their circuit in place of settling ponds or belt presses. Cake solids of 70 to 85 percent can often be achieved with no additional chemicals after the thickener, greatly reducing chemical and waste handling costs. Testing should be performed on any slurry under consideration to determine the plate style (recessed or membrane), operating pressure required, and cake thickness needed to achieve the desired level of moisture content.
Cory Jenson is general manager, Environmental Division of McLanahan Corp. He can be contacted at firstname.lastname@example.org.