Today’s Filter Presses
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.