An industrial filter press is a tool used in separation processes, specifically to separate solids and liquids. The machine stacks many filter elements and allows the filter to be easily opened to remove the filtered solids, and allows easy cleaning or replacement of the filter media.
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Filter presses cannot be operated in a continuous process but can offer very high performance, particularly when low residual liquid in the solid is desired. Among other uses, filter presses are utilised in marble factories in order to separate water from mud in order to reuse the water during the marble cutting process.
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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, but the use of water could be more cost-efficient in certain cases, such as if water was re-used from a previous process.
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The first form of filter press was invented in the United Kingdom in , used in obtaining seed oil through the use of pressure cells. However, there were many disadvantages associated with them, such as high labour requirement and discontinuous process. Major developments in filter press technology started in the middle of 20th century. In Japan in , Kenichiro Kurita and Seiichi Suwa succeeded in developing the world's first automatic horizontal-type filter press to improve the cake removal efficiency and moisture absorption. Nine years later, Kurita Company began developing flexible diaphragms to decrease moisture in filter cakes. The device enables optimisation of the automatic filtration cycle, cake compression, cake discharge and filter-cloth washing leading to the increment in opportunities for various industrial applications. A detailed historical review, dating back to when the Shang Dynasty used presses to extract tea from camellia the leaves and oil from the hips in BC, was compiled by K. McGrew.[1]
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There are four main basic types of filter presses: plate and frame filter presses, recessed plate and frame filter presses, membrane filter presses and (fully) automatic filter presses.
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A plate and frame filter press is the most fundamental design, and may be referred to as a "membrane plate filter." This type of filter press consists of many alternating plates and frames assembled with the supports of a pair of rails, with filter membranes inserted between each plate-frame pair.[citation needed]
The stack is compressed with sufficient force to provide a liquid-tight seal between each plate and frame, the filter membrane may have an integrated seal around the edge or the filter material itself may act as a gasket when compressed.
As the slurry is pumped through the membranes, the filter cake accumulates and becomes thicker. The filter resistance increases as well, and the process is stopped when the pressure differential reaches a point where the plates are considered full enough.
To remove the filter cake and clear the filters, the stack of plates and frames are separated and the cake either falls off or is scraped from the membranes to be collected in a tray below.[2] The filter membranes are then cleaned using wash liquid and the stack is re-compressed ready to start the next cycle.[3]
M.W. Watermark Plate and Frame Filter PressAn early example of this is the Dehne filter press, developed by A L G Dehne (&#;) of Halle, Germany, and commonly used in the late 19th and early 20th century for extracting sugar from sugar beet and from sugar cane, and for drying ore slurries. Its great disadvantage was the amount of labor involved in its operation.[4]
(Fully) Automatic filter press[
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An automatic filter press has the same concept as the manual filter and frame filter, except that the whole process is fully automated.[5] It consists of larger plate and frame filter presses with mechanical "plate shifters".[citation needed] 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.[citation needed]
Fully automatic filter presses provide a high degree of automation while providing uninterrupted operation at the same time. The option of the simultaneous filter plate opening system, for example, helps to realise a particularly fast cake release reducing the cycle time to a minimum. The result is a high-speed filter press that allows increased production per unit area of filter. For this reason, these machines are used in applications with highly filterable products where high filtration speeds are required. These include, e.g. mining concentrates and residues. There are different systems for fully automatic operation. These include, e.g. the vibration/shaking devices, spreader clamp/spreader cloth version or scraping devices. The unmanned operating time of a fully automatic filter press is 24/7.[citation needed]
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A recessed plate filter press does not use frames and instead has a recess in each plate with sloping edges in which the filter cloths lie, the filter cake builds up in the recess directly between two plates and when the plates are separated the sloping edges allow the cake to fall out with minimal effort.[6] To simplify construction and usage the plates typically have a hole through the centre, passing through the filter cloth and around which it is sealed so that the slurry flows through the centre of each plate down the stack rather than inward from the edge of each plate. Although easier to clean, there are disadvantages to this method, such as longer cloth changing time, inability to accommodate filter media that cannot conform to the curved recess such as paper, and the possibility of forming uneven cake.[7]
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Membrane filter presses have a great influence on the dryness of the solid by using an inflatable membrane in the filter plates to compress remaining liquid from the filter cake before the plates are opened. Compared to conventional filtration processes, it achieves the lowest residual moisture values in the filter cake. This makes the membrane filter press a powerful and widely used system. Depending on the degree of dewatering, different dry matter contents (dry matter content &#; percentage by weight of dry material in the filter cake) can be achieved in the filter cake by squeezing with membrane plates. The range of achievable dry matter contents extends from 30 to over 80 percent. Membrane filter presses not only offer the advantage of an extremely high degree of dewatering; they also reduce the filtration cycle time by more than 50 percent on average, depending on the suspension. This results in faster cycle and turnaround times, which lead to an increase in productivity. The membrane inflation medium consists either of compressed air or a liquid medium (e.g. water).
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Filter presses are used in a huge variety of different applications, from dewatering of mineral mining slurries to blood plasma purification.[3] At the same time, filter press technology is widely established for ultrafine coal dewatering as well as filtrate recovery in coal preparation plants. According to G.Prat, the "filter press is proven to be the most effective and reliable technique to meet today's requirement".[8] One of the examples is Pilot scale plate filter press, which is specialized in dewatering coal slurries. In the Society for Mining, Metallurgy and Exploration published an article highlighting this specific application.[9] It was mentioned that the use of the filter press is very beneficial to plant operations, since it offers dewatering ultraclean coal as product, as well as improving quality of water removed to be available for equipment cleaning.[10]
Other industrial uses for automatic membrane filter presses include municipal waste sludge dewatering,[11] ready mix concrete water recovery,[12] metal concentrate recovery, and large-scale fly ash pond dewatering.[13]
Many specialized applications are associated with different types of filter press that are currently used in various industries. Plate filter press is extensively used in sugaring operations such as the production of maple syrup in Canada, since it offers very high efficiency and reliability. According to M.Isselhardt, "appearance can affect the value of maple syrup and customer's perception of quality".[14] This makes the raw syrup filtration process extremely crucial in achieving desired product with high quality and appealing form, which again suggested how highly appreciated filter press methods are in industry.
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Here are some typical filter press calculation used for handling operation applied in waste water treatment:
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S=(B x 8.34 lb/gal x s)&#;A
Where,
S is the solid loadings rate inlb
+
h
&#;ft2
.<r /> B is biosolids ingal
&#;h
s is the % solids/ 100.
A is the plate area in ft2.
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N F Y = S × P T C T {\displaystyle NFY={\frac {S\times P}{TCT}}}
Where:
(S × P) gives the filter run time.[15]
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u = 1 A d V d t = Δ P μ × ( R c + R f ) {\displaystyle u={\frac {1}{A}}{\frac {dV}{dt}}={\frac {\Delta P}{\mu \times (R_{c}+R_{f})}}}
Where:
Those are the most important factors that affect the rate of filtration. When filtrate pass through the filter plate, deposition of solids are formed and increases the cake thickness, which also increase Rc while Rf is assumed to be constant.[16] The flow resistance from cake and filter medium can be studied by calculating the flow rate of filtration through them.
If the flow rate is constant, the relationship between pressure and time can be obtained. The filtration must be operated by increasing pressure difference to cope with the increase in flow resistance resulting from pore clogging.[16] The filtration rate is mainly affected by viscosity of the filtrate as well as resistance of the filter plate and cake.
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High filtration rate can be obtained from producing thin cake. However, a conventional filter press is a batch system and the process must be stopped to discharge the filter cake and reassemble the press, which is time-consuming. Practically, maximum filtration rate is obtained when the filtration time is greater than the time taken to discharge the cake and reassemble the press to allow for cloth's resistance.[16] Properties of the filter cake affect the filtration rate, and it is desirable for the particle's size to be as large as possible to prevent pore blockage by using a coagulant. From experimental work, flow rate of liquid through the filter medium is proportional to the pressure difference.[17] As the cake layer forms, pressure applies to the system increases and the flow rate of filtrate decreases.[7] If the solid is desired, the purity of the solid can be increased by cake washing and air drying.[18] Sample of filter cake can be taken from different locations and weighed to determine the moisture content by using overall material balance.[9]
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The selecting of filter press type depends on the value of liquid phase or the solid phase. If extracting liquid phase is desired, then filter press is among the most appropriate methods to be used.[19]
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Nowadays, filter plates are made from polymers or steel coated with polymer. They give good drainage surface for filter cloths. The plate sizes are ranged from 10 by 10 cm to 2.4 by 2.4 m and 0.3 to 20 cm for the frame thickness.[18]
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Typical cloth areas can range from 1 m2 or less on laboratory scale to m2 in a production environment, even though plates can provide filter areas up to 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.[7]
In the early days of press use in the municipal waste biosolids treatment industry, issues with cake sticking to the cloth was problematic and many treatment plants adopted less effective centrifuge or belt filter press technologies. Since then, there have been great enhancements in fabric quality and manufacturing technology that have made this issue obsolete.[20] Unlike the US, automatic membrane filter technology is the most common method to dewater municipal waste biosolids in Asia. Moisture is typically 10-15% lower and less polymer is required&#;which saves on trucking and overall disposal cost.
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The operating pressure is commonly up to 7 bars for metal.[18] The improvement of the technology makes it possible to remove large amount of moisture at 16 bar of pressure and operate at 30 bars.[3] However, the pressure is 4-5 bars for wood or plastic frames.[18] If the concentration of solids in the feed tank increase until the solid particles are attached to each other. It is possible to install moving blades in the filter press to reduce resistance to flow of liquid through the slurry.[21] For the process prior to cake discharge, air blowing is used for cakes that have permeability of 10&#;11 to 10&#;15 m2.[9]
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. Varying the temperature, concentration and pH can control the size of the flocs. Moreover, if the filter cake is impermeable and difficult for the flow of filtrate, filter aid chemical can be added to the pre-treatment process to increase the porosity of the cake, reduce the cake resistance and obtain thicker cake. However, filter aids need to be able to remove from the filter cake either by physical or chemical treatment. A common filter aid is Kieselguhr, which give 0.85 voidage.[21]
In terms of cake handling, batch filter press requires large discharge tray size in order to contain large amount of cake and the system is more expensive compared to continuous filter press with the same output.[3]
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There are two possible methods of washing that are being employed, the "simple washing" and the "thorough washing". For simple washing, the wash liquor flows through the same channel as the slurry with high velocity, causing erosion of the cakes near the point of entry. Thus the channels formed are constantly enlarged and therefore uneven cleaning is normally obtained. A better technique is by thorough washing in which the wash liquor is introduced through a different channel behind the filter cloth called washing plates. It flows through the whole thickness of the cakes in opposite direction first and then with the same direction as the filtrate. The wash liquor is normally discharged through the same channel as the filtrate. After washing, the cakes can be easily removed by supplying compressed air to remove the excess liquid.[18]
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Nowadays filter presses are widely used in many industries, they would also produce different types of wastes. Harmful wastes such as toxic chemical from dye industries, as well as pathogen from waste stream might accumulate in the waste cakes; hence the requirement for treating those wastes would be different. Therefore, before discharge waste stream into the environment, application of post-treatment would be an important disinfection stage. It is to prevent health risks to the local population and the workers that are dealing with the waste (filter cakes) as well as preventing negative impacts to our ecosystem. Since filter press would produce large amount of waste, if it was to be disposed by land reclamation, it is recommended to dispose to the areas that are drastically altered like mining areas where development and fixation of vegetation are not possible. Another method is by incineration, which would destroy the organic pollutants and decrease the mass of the waste. It is usually done in a closed device by using a controlled flame.[2]
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Many debates have been discussed about whether or not filter presses are sufficient to compete with modern equipment currently as well as in the future, since filter presses were 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 improvements 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.[22]
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Filter presses offer a wide range of application, one of its main propositions is the ability to provide a large filter area in a relatively small footprint. Surface area available is one of the most important dimensions in any filtering process, since it maximises filter flow rate and capacity. A standard size filter press offers a filter area of 216 m2, whereas a standard belt filter only offers approximately 15 m2.[22]
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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.[22]
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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.[23]
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Filter presses are commonly used in the mining industry to dewater tailings and recover process water. These machines feature plates that create chambers when closed, which a feed pump then fills with slurry. The pressure from the feed pump dewaters the slurry, and the water either passes through a manifold created by the holes in the corners of the filter plates or through individual spigots on each plate, leaving behind solid cakes in the chambers. The cakes are then discharged onto either a conveyor or directly onto the ground before the cycle repeats.
While mechanical filter presses as we know them today can be thought of as relatively recent, filter presses themselves are not a new invention.
Early filter presses date back thousands of years and have roots in several cultures all over the world. During the Shang Dynasty of B.C.,a type of wooden filter press was developed to produce tea from camellia, a genus of flowering plants native to eastern and southern Asia. Egypt&#;s 18th Dynasty from - B.C. extracted juice from grapes using a sack press that was squeezed by a large tourniquet. The Romans shared a similar history. Cato the Elder described how the Roman Empire made wine with wooden presses between100 BC and 400 A.D.
The design of the filter press remained unchanged for the most part until the Industrial Revolution in the 18th century. In , the United Kingdom introduced the first mechanical filter press to the world. This machine utilized pressurized cells to obtain oil from the seeds of plants and was a precursor to the Johnson Press filter press. It was the first steel plate and frame filter press in history and was mostly used for food processing and industrial applications.
The filter press made its mining debut in the 20th century. The Eimco plate and frame filter press was used to recover zinc dust from gold cyanide solutions for more than a century.
Like the Johnson Press filter press and Eimco filter press, plate and frame style filter presses dewater material between two flat plates that close against a frame to form a chamber. Slurry is pumped into and filled the chambers. The pressure from the Feed Pump expels the water from the slurry, while the solids are retained inside the chambers. The number of solid cakes discharged from the filter press is determined by the number of frames, and the thickness of the cakes depends on the thickness of the frame.
One issue with the Plate and Frame style filter press was the removal of the cakes when the plates were pulled apart. The cakes would often stick to the frame, and an operator would have to go in and manually dislodge the cakes from the frame. This led to more innovative thinking to solve the problem, and the machine was again updated.
In , K. Kurita and S. Suwa modified the filter press for improved cake removal and moisture absorption with Recessed Chamber style filter presses. Recessed Chamber Filter Presses feature recessed plates, hence their name.
When two Recessed Chamber filter plates are closed next to each other, they form a cavity. The resulting cavity determines the chamber depth, or cake thickness. The cake thickness equals the summation of the two adjoining plates. For example, if the right side of Plate #1 is 20mm and the left side of Plate #2 is 20mm, the chamber formed when the two plates close together is 40mm, which means the cake will be 40mm as well.
The first and last plates are only recessed on one side so that a partial cake does not form between the plate and the frame of the filter press. This means that number of cakes formed in a Recessed Chamber Filter Press will always be one less than the number of plates. For example, if the Recessed Chamber Filter Press has 150 plates, it will form 149 cakes.
The dewatering process is similar to dewatering with Plate and Frame style presses. The slurry is pumped into the chambers formed by the plates, and the pressure from the feed pump forces the chambers to pack full of solids, expelling the water as the solids are packed in. The time, pressure, feed or filtrate flow rates determine when the cakes are fully formed. When the cakes are fully formed, the pump shuts off, the filter plates are pulled apart and the cakes easily fall out from between the plates since there is no frame for the cakes to stick to.
Neither the Recessed Chamber style nor Plate and the Frame style filter press squeeze the cakes to dewater the slurry &#; a common misconception. The plates are held closed with hydraulic rams, and the fluid pressure created by the pump dewaters the slurry. The filter press only opens and closes to separate the plates and release the cakes at the end of each cycle.
After the invention of the Recessed Chamber Filter Press, a variation of this style was created with the advent of Membrane Filter Presses. Like the Recessed Chamber style, Membrane style filter presses feature recessed plates, but they also have a diaphragm, or membrane, that lines each side of the plate. At the end of each cycle, the membrane physically squeezes each cake formed in the cavities between the plates.
Whereas the Recessed Chamber style and Plate and Frame style filter presses relied on the pressure of the pump feeding the machine for dewatering, slurry is pumped into the Membrane Filter Press at pressures of no more than 100 psi. The membranes are pressurized using water or air behind the membranes to push the membrane into the cavity and physically squeeze each individual cake. Using water for pressures more than 125 psi is recommended due to the compressive and explosive nature of compressed air. Once the desired squeeze time has been reached, the membrane water is discharged to depressurize the membranes, the press is unlocked, and the filter plates open to discharge the cakes.
Membrane style filter presses can benefit operations dewatering highly compressible solids. As a hypothetical example to illustrate this point, if you squeeze a slurry full of ball bearings, which are not very compressible, the membrane would only succeed in rearranging those ball bearings, offering a slightly drier cake. However, if you squeeze a slurry full of sponges, which are very compressible, you&#;ll notice a significant difference.
Membrane plates can come in two major configurations: all of the plates can feature membranes, or every other plate can feature a membrane. In this configuration, called a Mixed Pack Membrane, the alternate plates are composed of companion plates, which are similar to recessed chamber plates. The Mixed Pack Membrane is more frequently supplied, as it results in lower upfront capital costs and lower long-term operating costs.
Membrane plates also come in two main styles: welded membranes, and replaceable membranes. Welded membranes are heat welded to the body of the plate. When these membranes fail due to fatigue from flexing, the entire plate needs to be replaced. Replaceable membranes attach to the plates, so only the membranes need to be replaced when they fail.
A comparison of filter plate styles.
Producers have two options when it comes to supporting the plates of their filter press: Overhead Beam and Side Beam. Overhead Beam style filter presses support the weight of the plates with overhead beams. Because the plates are suspended in the Overhead Beam style, the dynamic loads created from holding the plates closed are isolated from the beams, which is good, because forces can be as high as 1.5 million pounds in a 2,000mm x 2,000mm press operating at 225psi.
Overhead Beam style filter presses are more suitable for high-pressure cloth washing systems due to increased space between the plates. The wide opening between the plates allows for less abrasion as the cakes are released, more movement to release cakes, fast plate opening to keep cycle times down and improved cake release. Overhead Beam Filter Presses also allow easier access for cleaning and for changing out the filter cloths.
Side Beam Filter Presses support the weight of the plates with beams running along the sides of the machine. This style features a fast opening system, easier-to-implement plate shaking systems and are shorter to fit small or portable applications. They allow for faster, low-pressure wash systems used commonly in mineral concentration applications.
Many mining, aggregate and coal customers are using Recessed Chamber or Membrane Plate Filter Presses to dewater their tailings due to increasing environmental regulations and the cost savings compared to alternative technologies like tailings dams and belt presses.
It&#;s up to you to determine the best Filter Press for your operation. Based on the type and amount of material you&#;re dewatering, consider the plate style, size and number, as well as the desired cake thickness and the filter media appropriate for the material in your slurry.
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