Filter presses are a type of liquid-solid separation equipment that consist of two fixed heads, an overhead beam or two side beams, and a moving head containing a pack of vertical plates that form a series of chamber when pressed together.
The filter plates have a central feed port where slurry material is pumped into the chambers and four filtrate ports (one in each corner of the plate) for the filtered liquid (called the filtrate) to exit.
Filter plates are squared or rectangular polypropylene (in most cases), and there are two main types: recessed plates and membrane plates.
Recessed filter plates feature a recessed area on both sides of the plate. The depth of the recess can vary. When the plates are pressed together, the recessed areas form chambers with the recessed areas on the adjoining plate. The feed slurry is then pumped into these chambers, where the filter cloths covering the plates trap the solids and allow the liquid to pass through. At the end of the filtration cycle, the plates open to release the solid cakes that formed in the chambers.
Membrane plates are designed to further reduce the moisture content in the filter cakes or just to keep the cake firm (avoiding cracks) during the process. This style of filter plate features a membrane on the surface of a recessed plate. Membranes can come in different materials according to the application.
The membrane can be movable, meaning it can be replaced when broken or worn, or the membrane can be integrated into the plate. In this instance, the plate is produced with a propylene membrane already inserted in its body and the membrane can't be replaced.
At the end of the first filtering cycle, compressed air or water is introduced in the space between the membrane and the surface of the plate at a certain pressure. This pressure squeezes the cake, resulting in further liquid removal from the cake and enhancing the performance of the process. It can also cut down on the filtering time.
Types of squeezing: water vs air
There are two types of membrane plate squeezing. The first involves pumping compressed air into the membrane plates. The second involves pumping water within the membrane plates.
Pumping water into the membrane plates has several advantages over compressed air. First, water allows for a more uniform pressure within the membranes and a more constant pressure within the membranes. Water also consumes less electricity for the cake squeezing than air. This is because an air compressor is required to pump air into the membrane plates, which can increase electrical costs.
Mixed plate pack
Filter presses can be designed with recessed chamber plates, membrane plates or a mix of both filter plate styles. In a mixed pack, the plates alternate between the fixed recessed chamber and the membrane plates. Mixed pack filter plates provide excellent filtration efficiency.
The surface of the filter plates is covered with a tissue called cloth. The cloth is susceptible to wearing and may be produced in different types, materials, weft and yarn to yield the best filtrating properties with reference to the characteristics of the sludge to be filtered.
The right style of filter plate (and right type of filter cloth) for an application depends on the characteristics of the feed material. In some cases, such as metal concentrated sludge applications, a mixed pack of recessed and membrane plates is the best choice.
It is important to test the feed material ahead of time to ensure not only the right type of filter press and plates are chosen, but also to ensure the best materials of construction are selected for the various components so the filter press will operate at optimum efficiency for that application.