An industrial filter press
is a tool for separation processes, especially for separating solids and liquids. The machine can be stacked with many filter elements and allows easy opening of the filter to remove the filtered solids and easy cleaning or replacement of the filter media. The filter press cannot be operated in a continuous process, but can provide very high performance, especially where little liquid is required to remain in the solids. Amongst other uses, filter presses are used in marble plants to separate water from slurry in order to reuse the water in the marble cutting process. – The concept behind filter press technology Typically, the slurry to be separated is injected into the centre of the press and fills each chamber of the press. Optimal filling time will ensure that the last chamber of the press is loaded before the mud in the first chamber starts to clump. As the chambers fill, the pressure inside the system will increase due to the formation of sludge. Compressed air is then used to filter the liquid through the filter cloth by forced action, but in some cases it may be more cost effective to use water, for example if it is re-used from a previous process. History The first filter presses were invented in England in 1853 for obtaining seed oil through the use of pressure sensors. However, there were many disadvantages associated with them, such as higher labour requirements and discontinuous processes. A major development in filter press technology began in the middle of the 20th century. In 1958 in Japan, Kenichiro Kurita and Seiichi Suwa successfully developed the world’s first automatic horizontal filter press to improve the removal efficiency and moisture absorption of the filter cake. Nine years later, Kurita began developing a flexible diaphragm to reduce the moisture in the filter cake. This machine optimises the automatic filtration cycle, cake compression, cake discharge and filter cloth cleaning, thus increasing the opportunities for various industrial applications. A detailed historical review can be traced back to the Shang dynasty in 1600 B.C. when presses were used to extract the oil from the leaves and hips of Camellia sinensis from tea by K. McGrew. Types of Filter Presses There are four basic types of filter presses: plate and frame filter presses, concave plate and frame filter presses, membrane filter presses and (fully automatic) filter presses. Plate and frame filter presses The plate and frame filter press is the most basic design and can be referred to as a “membrane plate filter press”. This type of filter press consists of a number of alternating plates and frames which are assembled together with a pair of rail supports and the filter membrane is inserted between each pair of plate frames. The plates provide support for the filter membranes under pressure and have narrow slits to allow filtrate to flow through the membranes into the plates and then out into the collection system. The frame provides a chamber between the membrane and the plates into which the slurry is pumped and the filter cake accumulates. Compressing the stack with sufficient force to provide a liquid-tight seal between each plate and the frame, the filter membranes can have an integral seal around the edges, or the filter material itself can act as a gasket when compressed. As the slurry is pumped through the membrane, the filter cake builds up and thickens. The filter resistance also increases and the process stops when the differential pressure reaches the point where the plate is considered full enough. To remove the cake and clear the filter, the plates and frames are stacked apart and the cake is then dropped or scraped off the membrane and collected in the tray below. The filter membrane is then cleaned using a washing solution and the stack is recompressed in preparation for starting the next cycle. M.W. Watermark plate and frame filter press An early example is the Dehne filter press, developed by ALG Dehne (1832-1906) of Halle, Germany, and commonly used in the late 19th and early 20th centuries for extracting sugar from sugar beet and sugar cane and for drying mineral slurries.industrial filter press Its greatest disadvantage was the amount of labour involved in its operation. (Fully) automatic filter press An automatic filter press has the same concept as a manual filter and a frame filter, with the difference that the whole process is fully automated. It consists of a larger plate and frame filter press and a mechanical “plate shifter”. The function of the plate shifter is to move the plates and allow rapid discharge of the filter cake that accumulates between the plates. It is also equipped with a diaphragm compressor on the filter plates to help optimise operating conditions by further drying the filter cake. Fully automatic filter presses offer a high degree of automation while providing uninterrupted operation. For example, the option to open the filter plate opening system simultaneously helps to achieve particularly fast cake release and thus minimise cycle times. The result is the acquisition of a high-speed filter press, which increases the output per unit area of filter. As a result, these machines are used in applications requiring highly filterable products with high filtration speeds. These include, for example, mining concentrates and residues. Different systems are available for fully automatic operation. These include for example vibration/vibration units, spreader/cloths or scrapers. Fully automatic filter presses have an unmanned operation time of 24/7. Plate Filter Presses Instead of using a frame, the concave plate filter presses have a recess in each plate with a sloping edge in which the filter cloth is located and the cake accumulates directly in the recess between the two plates, the sloping edge allowing the cake to fall out with minimal effort when the two plates are separated. To simplify construction and use, these plates usually have a hole through the centre which passes through the filter cloth and is sealed, sealing it around the hole to allow the slurry to flow down the centre of each plate through the stack rather than inwards from the edge of each plate. Although easier to clean, this method has a number of disadvantages such as longer cloth change times, the inability to accommodate filter media (e.g. paper) that does not fit into the curved notches and the possibility of uneven cake formation. Membrane filter presses Membrane filter presses have a significant impact on the dryness of the solids by using an expandable membrane in the filter plate to compress the residual liquid in the cake before opening the plate. It achieves the lowest residual moisture values in the filter cake compared to conventional filtration processes. 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 – weight percentage of dry matter in the filter cake) can be obtained in the filter cake by pressing with the membrane plates. The achievable dry matter content ranges from 30 to over 80 %. Membrane filter presses not only have the advantage of a very high degree of dewatering, but also of a high dewatering rate. Depending on the suspension, they can also reduce the filtration cycle time by an average of more than 50%. This results in shorter cycle times and turnaround times and thus higher productivity. The membrane expansion medium consists of compressed air or a liquid medium (e.g. water). Areas of application Filter presses are used in a variety of different applications ranging from mineral slurry dewatering to blood plasma purification. At the same time, filter press technology has been widely established for the dewatering of ultrafine coal and filtrate recovery in coal processing plants. According to G. Prat, “filter presses are proving to be the most efficient and reliable technology for today’s needs”. One example is the medium-scale plate filter press, which is specifically designed for the dewatering of coal slurries. In 2013, the Institute of Mining, Metallurgy and Exploration published an article focusing on this particular application. It was mentioned that the use of the filter press is very beneficial to plant operations as it provides dewatering of ultra-clean coal as a product and improves the quality of the water available for plant cleaning. Other industrial uses for membrane automatic filter presses include municipal waste sludge dewatering, ready-mixed concrete water recycling, metal concentrate recovery and large fly ash pond dewatering. Many specialist applications are associated with the different types of filter presses currently in use in various industries. Flatbed filter presses are widely used in sugaring operations, such as the production of maple syrup in Canada, due to their very high efficiency and reliability. According to M. Isselhardt, “appearance affects the value of maple syrup and the customer’s perception of quality”. This makes the raw syrup filtration process extremely important for obtaining the desired product in a high quality and attractive form, which again demonstrates the high appreciation of the filter press method in industry.