Industrial Filter press
An industrial filter press, is a tool used in separation processes, specifically to separate solids and liquids. The process uses the principle of pressure drive provided by a slurry pump.
Generally, the slurry that will be separated is injected into the centre of the press and each chamber of the press is filled. Optimal filling time will ensure the last chamber of the press is loaded before the mud in the first chamber begins to cake. As the chambers fill, pressure inside the system will increase due to the formation of thick sludge then, the liquid is strained through filter cloths by force using pressurized air.
Types of filter presses
There are three main basic types of filter presses: plate and frame filter presses, recessed plate and frame filter presses and automatic filter presses.
Plate and frame filter press
A plate and frame filter press is the most fundamental design, and many now refer it as a “membrane filter plate”. This type of filter press consists of many plates and frames assembled alternately with the supports of a pair of rails. The presence of a centrifugal pump ensures the remaining suspended solids do not settle in the system and its main function is to deliver the slurry into each of the separating chambers in the plate and frame filter. For each of the individual separating chambers, there is one hollow filter frame separated from two filter plates by filter cloths. The introduced slurry flows through a port in each individual frame, and the filter cakes are accumulated in each hollow frame. As the filter cake becomes thicker, the filter resistance increases as well. So when the separating chamber is full, the filtration process is stopped as the optimum pressure difference is reached. The filtrate that passes through filter cloth is collected through collection pipes and stored in the filter tank. Filter cake accumulation occurs at the hollow plate frame. The cakes then fall off from those plates and are discharged to the final collection point.
Cake discharge can be done in many ways. For example: Shaking the plates while they are being opened or shaking the cloths. A scraper can also be used, by moving from one chamber to another and scraping the cake off the cloth. At the end of each run, the cloths are cleaned using wash liquid and are ready to start the next cycle.
Automatic filter press
An automatic filter press has the same concept as the manual filter and frame filter, except that the whole process is fully automated. It consists of larger plate and frame filter presses with mechanical “plate shifters”. The function of the plate shifter is to move the plates and allow rapid discharge of the filter cakes accumulated in between the plates. It also contains a diaphragm compressor in the filter plates which aids in optimizing the operating condition by further drying the filter cakes.
Recessed plate filter press
A recessed plate filter press is made up of polypropylene squares at about 2 to 4 feet across with a concave depression and a hole in the center of each. Two plates join together to form a chamber to pressurize the slurry and squeeze the filtrate out through the filter cloth lining in the chamber. It is capable of holding 12 to 80 plates adjacent to each other, depending on the required capacity. When the filter press is closed, a series of chambers is formed. The differences with the plate and frame filter are that the plates are joined together in such a way that the cake forms in the recess on each plate, meaning that the cake thickness is restricted to 32mm unless extra frames are used as spacers. However, there are disadvantages to this method, such as longer cloth changing time, inability to accommodate filter papers and the possibility of forming an uneven cake.
Filter presses are used in a huge variety of different applications, from dewatering of mineral mining slurries to coal dewatering as well as filtrate recovery in coal preparation plants.
Other industrial uses for automatic membrane filter presses include municipal waste sludge dewatering, ready-mix concrete water recovery, metal concentrate recovery, and recycled aggregate washing.
A typical range of particle size and feed concentration are 1-100 μm and 1-30% by weight. The slurry feed concentration generally has large amount of ultrafine particles. The percentage of solids concentration in the slurry feed is normally more than 10% by weight.
Plate and frame filter press produce up to 99% of solids recovery, and the moisture left in the cake commonly ranges from 15% to 20%. During cake washing, 90% of the filtrate can be removed by the wash liquid.
Typical cloth areas can range from 1 m2 or less on a laboratory scale to 1000 m2 in a production environment, even though plates can provide filter areas up to 2000 m2. Normally, plate and frame filter press can form up to 50 mm of cake thickness, however, it can be push up to 200 mm for extreme cases. Recessed plate press can form up to 32 mm of cake thickness.
Pre-treatment of the slurries before filtration is required if the solid suspension has settled down. Coagulation as pre-treatment can improve the performance of filter press because it increases the porosity of the filter cake leading to faster filtration. Chemical treatment can control the size of the flocs.
In terms of cake handling, batch filter press requires large discharge tray size in order to contain a large amount of cake and the system is more expensive compared to continuous filter press with the same output.
Advantages and disadvantages compared to other competitive methods
Many debates have been discussed whether or not filter presses are sufficient to compete with modern equipment currently as well as in the future since filter presses were among one of the oldest machine-driven dewatering devices. Efficiency improvements are possible in many applications where modern filter presses have the best characteristics for the job, however, despite the fact that many mechanical improvements have been made, filter presses still remain to operate on the same concept as when first invented. A lack of progress in efficiency improvement as well as a lack of research on conquering associated issues surrounding filter presses have suggested a possibility of performance inadequacy. At the same time, many other types of filter could do the same or better job as press filters. In certain cases, it is crucial to compare characteristics and performances.
High-solids slurries: continuous pressure operation
Filter presses are commonly used to dewater high-solids slurries in metal processing plants, one of the press filter technology that could deliver the job is the Rotary Pressure Filter method, which provides continuous production in a single unit, where filtration is directed via pressure. However, in cases where solids concentration in high-solids slurries is too high (50%+), it is better to handle these slurries using vacuum filtration, such as a continuous Indexing Vacuum Belt Filter, since high concentration of solids in slurries will increase pressure and if pressure is too high, the equipment might be damaged and/or less efficient operation.
In the future, market demands for modern filtration industry are going to become finer and higher degree in separation, and particularly on the purpose of material recycling, energy saving, and green technology. In order to meet increasing demands for higher degree of dewatering from difficult-to-filter material, super-high pressure filters are required. Therefore, the trend in increasing the pressure for the automatic filter press will keep on developing in the future.
The conventional filter press mechanisms usually use mechanical compression and air to de-liquoring; however, the efficiency of producing low-moisture cake is limited. An alternative method has been introduced by using steam instead of air for cake dewatering. Steam dewatering technique can be a competitive method since it offers product of low-moisture cake.