Differences among filter presses

Plate and frame filter press 1) Structure: Filter plates and filter frames are arranged alternately and assembled on the frame (as shown in the figure). There are two types of plates: those used for washing and those that are not used for washing. The number of filter plates and frames can be adjusted within the range determined by the length of the frame; it usually ranges from 10 to 60 pieces. Filtration and washing in a cross-flow plate and frame filter press The structure of the filter plates and frames is shown in Figure 4-12. Round holes are provided at the four corners of the plate and frame; once assembled, they form channels for the inflow and outflow of slurry, filtrate, and washing liquid (Figure 4-13) (undercurrent type). 2) Operating characteristics: Intermittent operation – one operating cycle consists of five stages: assembly, filtration, washing, residue removal, and cleanup. 3) Features: Advantages: Compact structure and large filtration area; mainly used for filtering suspensions with high solid content. Disadvantages: Most of the loading, unloading, and cleaning tasks must be done manually, resulting in high labor intensity. The emergence of various modern automatic plate and frame filter presses has helped to overcome this drawback to a certain extent. Box filter press: Compared with plate and frame filter presses, the box filter press has a similar appearance, but it consists only of filter plates. The two recessed surfaces of each filter plate, when pressed against other filter plates, form a filtration chamber. The slurry is added through the central hole, while the filtrate is discharged from the lower corner. A filter cloth with a central hole is placed over the filter plate; the feeding hole in the center of the filter cloth is pressed against the two wall surfaces, or the filter cloth on those wall surfaces is sewn together using a woven tube. In industry, automatic chamber filter presses have achieved a high level of automation.

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Basic structure of the plate and frame filter press: (1) Frame: The frame is the fundamental component of the filter press; at both ends are thrust plates and pressing heads, and the girders on either side connect these components together. The girders serve to support the filter plates, filter frames, and thrust plates. a. Thrust plate: It is connected to the supports, allowing one end of the filter press to be positioned on the foundation. In a chamber-type filter press, there is a feed hole in the middle of the thrust plate; there are also four holes at each of the four corners. The holes at the upper corners serve as inlets for washing liquid or pressing gas, while those at the lower corners serve as outlets (either for the underflow or for the filtrate). b. Compression plate: It is used to compress the filter plates and frames. Wheels on both sides enable the compression plate to roll along the tracks on the beam. c. Beams: These are load-bearing components; depending on the requirements for corrosion resistance in the operating environment, they can be coated with rigid polyvinyl chloride, polypropylene, stainless steel, or new types of anti-corrosion coatings. (2) Compression mechanism: manual compression, mechanical compression, hydraulic compression. a. Manual compression: This involves using a screw-type mechanical jack to push the compression plate in order to compress the filter plate. b. Mechanical compression: The compression mechanism consists of a motor (equipped with an advanced overload protector), a reducer, gear sets, a screw rod, and a fixing nut. During compression, the motor rotates in the forward direction, driving the reducer and gear set, which causes the screw rod to rotate within the fixed nut, thereby pushing the compression plate to press the filter plate and filter frame together. As the clamping force increases, the motor’s load current rises; when it reaches the current level set by the protector, the maximum clamping force is attained and the motor has its power cut off, causing it to stop rotating. Thanks to the reliable self-locking helix angle of the screw and the fixed nut, the clamping state is maintained throughout the operation. During retraction, the motor rotates in the opposite direction, and retraction stops when the clamping element on the clamping plate touches the travel switch. c. Hydraulic compression: The hydraulic compression mechanism consists of a hydraulic station, cylinders, pistons, piston rods, and a connection between the piston rod and the compression plate. (3) Filtration mechanism: The filtration mechanism consists of a filter plate, a filter frame, filter cloth, and a pressing diaphragm. The two sides of the filter plate are covered with filter cloth; when a pressing diaphragm is required, a set of filter plates is composed of a diaphragm plate and side plates. On both sides of the base plate of the diaphragm plate, rubber diaphragms are wrapped, and a filter cloth is wrapped around the outside of these diaphragms; the side plates are ordinary filter plates. The material enters each filter chamber through the feed holes on the thrust plate; solid particles are retained in the filter chambers because their size is larger than the pore size of the filtering medium, while the filtrate flows out through the outlet holes located below the filter plate. When the filter cake needs to be drained, in addition to membrane pressing, compressed air or steam can also be introduced through the washing port; the airflow removes the water from the filter cake, thereby reducing its moisture content. Features of plate and frame filter presses: (1) Due to the structural characteristics of the filter plates in existing plate and frame filter presses, these presses are composed of a series of filter plates stacked on top of each other, resulting in a large number of filter chambers. The operation process of the filter press takes place simultaneously in all these chambers, and there is interference between them; therefore, the overall efficiency of the machine depends on the efficiency of individual filter chambers. (2) The large number of filter plates results in a huge number of components in the entire machine, complex automated control, and difficulties in monitoring the operating conditions of the filter chambers. The unit has many sealing surfaces, the processing requirements for the filter plates are high, and it is difficult to control capillary leakage from the filter cloth. (3) The pressing of the filter cake achieves changes in the volume of the filter chamber through the deformation of the diaphragm or filter plate sealing ring. The range of variation in the chamber volume is limited by the diaphragm or filter plate seal rings themselves, resulting in limited volume changes. This limits the service life of the diaphragm or filter plate seal rings and increases the cost of replacement. (4) The filtration pressure and pressing pressure act in opposition to the sealing pressure of the filter chamber, resulting in significant kinetic energy consumption. (5) Filter cloth cleaning is carried out either through open cleaning while the filter plate is being pulled out or by disassembling it for centralized cleaning; this involves a large amount of work, and the cleaning solution adds to the workload of secondary filtration. (6) The processing requirements for filter fabrics are high; these fabrics are not suitable for direct use. It is difficult to replace filter fabrics in large-scale equipment, and the replacement process takes a long time. (7) It operates intermittently and cannot run continuously.

The structure of the foot-type upper artificial discharge centrifuge is as shown in the figure; the system composed of the drum body 9, main shaft 13, bearing housing 14, casing 16, motor 17, V-belt pulley 1, and chassis 5 is suspended on the spherical seats of three pillars 6 by three swing rods 8. A cushioning spring 7 is installed on the swing rod; both ends of the swing rod are connected to the chassis 5 and the pillar 6 via spherical surfaces, allowing the entire chassis to swing. As a result, the natural frequency of the entire suspension system is much lower than the rotation frequency of the drum. This vibration isolation measure **effectively isolates the vibrations caused by uneven distribution of materials inside the drum. The spindle 13, which carries the drum, is vertically mounted within a pair of rolling bearings. The drum consists mainly of three parts: the drum body 9 with filter holes, the liquid retaining plate 11, and the drum bottom 10. When processing suspensions, a substrate mesh and a filter mesh are required inside the drum. The drum is relatively short, with its bottom bulging upward; the drum shaft is short and thick, in order to reduce the overall height of the machine, thereby facilitating operation and minimizing vibration. To ensure uniform feeding, the suspension is gradually added to the drum after the centrifuge is started (the feeding speed depends on the properties of the material). When handling paste-like materials or finished products, the material should be evenly distributed inside the drum before starting the centrifuge. In the centrifugal force field, the liquid contained within the material is flung against the inner wall of the shell through the filter screen and the holes in the drum; it collects on the bottom plate and is then continuously discharged via the filtrate outlet pipe 3. The solid phase remains trapped inside the drum; after shutdown, it is removed manually from the upper part of the drum. During the separation process, the filter residue can be washed with a washing solution. The main advantages of the three-legged upper-type manual discharge centrifuge are its strong adaptability to different materials, the ability to control the filtration and washing times arbitrarily, which enables the production of a drier filter cake along with thorough washing, while the solid particles remain almost undamaged. In addition, the machine operates smoothly, has a simple structure, is compact, and has a low cost. Its disadvantages are intermittent operation, long auxiliary time during production, low production capacity, and high labor intensity. Therefore, the development trend of three-legged centrifuges is mainly the mechanization and automation of unloading.

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