The filtration operation is to make the suspension pass through the porous filtration medium by gravity or artificial pressure difference, and the solid particles in the suspension are intercepted, while the filtrate flows out through the medium.
The principle of the filtration process is shown in Figure 6- 1. The suspension to be filtered is called filtrate. For mud filtration, mud is mud filtration. Porous materials with many pores to capture solid particles are called filter media. The liquid passing through the filter medium is called filtrate. The trapped substance is called filter cake.
In the device of Figure 6- 1, the filter slurry is introduced into the filter medium, and the water in the filter slurry flows out through the pores of the medium as filtrate, and the solid matter is intercepted by the medium and accumulated into a filter cake. The filter medium is usually a porous fabric. In fact, when the filter cake is formed, it itself becomes a filter medium.
Figure 6- 1 Schematic diagram of filtration principle
1- slurry filtration; 2- filter cake; 3- filter media; 4- filtrate
At the beginning of filtration, the filter cake has not yet formed, and the filtration resistance is moderate. However, with the increase of filtration time, the filter cake gradually formed, and the resistance of filtrate passing through the medium gradually increased. In order to maintain filtration, it is necessary to give some motivation. Therefore, in most cases, in order to make the filtrate overcome the resistance and flow out easily, it is necessary to use a pump or vacuum pump to maintain a certain pressure difference between the two sides of the filter medium.
During filtration, particles larger than the filter holes are intercepted by the medium, and small particles are also intercepted due to the phenomenon of "bridging". When filtering with fabric medium, there is only medium resistance at the beginning, and after the filter cake is formed, the filtration resistance is the sum of filter cake resistance and medium resistance; When the filter cake reaches a certain thickness, the medium resistance can be ignored, and the filter cake becomes the actual filter medium. Due to the different sizes of particles contained in the filter slurry, under normal circumstances, the medium can not completely prevent fine particles from passing through, so the filtrate is often turbid at the initial stage of filtration, and after a while, the filtrate becomes clear.
Mud filtration and dehydration is not carried out at a constant speed and pressure, but in stages. Because if the whole process is uniform filtration, the pressure requirement is very high at the end of the operation. Although constant pressure filtration is simple, the filtration speed is too fast because no filter cake is formed on the surface of the filter cloth at the beginning. Fine particles penetrate the filter cloth to make the filtrate turbid, and some particles block the pores of the filter cloth, and then form a dense initial filter cake on the surface of the filter cloth, which increases the resistance and brings difficulties to the subsequent filtration operation. Therefore, in the initial stage of operation, low pressure is usually used, and then the pressure is gradually increased. When the pressure meets the requirements, it is converted to constant pressure filtration. That is, the filtration is operated in stages, which is close to constant-speed filtration at the beginning and then changed to constant-pressure filtration.
In filtration operation, the volume of filtrate passing through unit filtration area in unit time is called filtration speed. Let the filtration area of the filtration equipment be a, and the amount of filtrate obtained when the filtration time is t be v, then the filtration speed v is
Non-metallic mineral processing machinery and equipment
Where is the filtrate volume per unit filtration area (m3/m2).
In the process of filtration, the thickness of filter cake is increasing. Under a certain pressure difference, the pass rate of filtrate decreases with the increase of filtration time and the thickening of filter cake. However, the cumulative filtrate q per unit area increases with time. Therefore, the calculation of filtration rate is to determine the amount of filtrate obtained per hour or the weight of mud cake produced.
Step 2, washing the filter cake.
In some filtration operations, in order to remove the residual soluble impurities in the filter cake to improve the purity of solids, or to recover the residual mother liquor in the filter cake, it is necessary to wash the filter cake before slag removal. Sometimes, in order to further improve the purity of solids, it is necessary to filter the slurry several times.
The expression of washing rate is similar to that of filtration rate. If v is the volume of washing liquid, the rate is. After filtration, the washing of filter cake is maintained, at this time, there is no filter residue deposition, and the filtration resistance remains unchanged. If the gauge pressure does not change, the washing speed does not change, and its value is about equal to the final filtration speed, or about a quarter of the final filtration speed, depending on the type of equipment. In the plate-and-frame filter press, the washing speed is about a quarter of the final filtration speed; In the continuous filtration of vane filter, the washing speed is approximately equal to the final filtration speed. It should also be pointed out that the washing speed is also affected by the viscosity of the washing liquid used. If the viscosity of the washing liquid is less than that of the filtrate, the value of the washing speed will increase accordingly.
The amount of washing water is usually expressed as a percentage of the obtained filtrate. The amount of washing water divided by the washing speed is the time required for washing.
Third, the structure and operation of the filter press
The properties of suspensions formed by various production processes are very different, and the purpose of filtration and the capacity of raw materials are also very different. For a long time, in order to meet various requirements, various types of filters have been developed. According to the different ways of generating pressure difference, these filters can be divided into two categories:
1. Pressure filtration and suction filtration: such as vane filter, plate and frame filter press, rotary vacuum filter, etc. In the processing of nonmetallic minerals, the dehydration of mud requires low water content of mud cake, and generally a plate-and-frame filter press is used.
2. Centrifugal filtration: various centrifuges with intermittent discharge and continuous discharge.
plate and frame filter press
Plate-and-frame filter press is the simplest and most widely used of all filter presses. It is composed of alternating plates and frames. Both plates and frames can be made of various structural materials, such as cast iron, wood and polypropylene.
The surface of the filter plate can be made into a skeleton, or slotted, or drilled as a drainage channel. The filter frame is hollow, and the filter cake gathers in the filter frame during filtration, as shown in Figure 6-3. Generally, the filter medium of woven fabric covers two filter surfaces of each filter plate. The number of filter plates and filter frames can be adjusted within the range of frame length, generally between 10 ~ 60, and the filtering area is about 2 ~ 80m2.
The filter plate and the filter frame can be made into squares or circles. The filter plate and the filter frame are hung vertically on a pair of cross beams, and the push plate is fixed at one end, and the pressure plate at the other end moves the pressure plate forward with the help of a rotating screw and wrench, a rotating rod or a gear transmission device or a hydraulic pressing device, so that the filter frame is pressed between the two filter plates, so that the filter frame can be fastened without leakage, as shown in Figure 6-2.
Figure 6-2 Plate and Frame Filter Press
1- fixed head; 2- filter plate; 3- Filter frame; 4- filter cloth; 5- Compaction device
Figure 6-3 Filter Plate and Filter Frame
1- suspension channel; 2- washing liquid inlet channel; 3- filtrate channel; 4- Washing liquid outlet channel
The filter press supplies and discharges filtrate through channels located at the corners of the plate and the frame, or through lug channels extending from both sides of the plate and the frame. The filtrate channel runs through the whole length of the filter press, and the end is connected with the drainage pipeline, which is called subsurface flow type. If it flows to the open groove at the lower part of the filter press through the filtrate valve on each plate, it is called open flow. If the filtered substance is toxic and volatile, it must be an undercurrent.
The structure of filter plate and filter frame is shown in Figure 6-3. There are round holes at the four corners of the plate and the frame, which respectively form the channels for filtrate, filtrate and washing liquid to enter and exit when assembled and superimposed (Figure 6-4). Before the operation begins, the four corners of the interface between the board and the frame are covered with filter cloth with holes. After the plates are pressed against the frame, during the filtration operation, the suspension enters the filter frame from the channel 1, and the filtrate passes through the filter cloth on both sides of the frame and is discharged from the lower left corner of each filter plate through the channel 3. When the frame is full of filter cake, stop filtering. At this time, you can decide whether to clean the filter cake according to your needs. The filter plates of washable plate-and-frame filter press (washable plate-and-frame filter press) have two structures: washable plate and non-washable plate, which should be arranged alternately. The washing liquid enters both sides of the washing plate from the channel 2 (Figure 6-3c), passes through the filter cake of the whole frame, collects on the surface of the non-washing plate, and flows into the channel 4 from the small hole in the lower right corner to be discharged. After cleaning, stop and loosen the screws, unload the filter cake and clean the filter cloth to prepare for the next filtration. As shown in Figure 6-4.
Figure 6-4 Schematic diagram of filtration and washing of plate-and-frame filter press
As can be seen from the above figure, when washing, the path taken by washing liquid is the full thickness of filter cake, while when filtering, the path taken by filtrate is only about half of it; In addition, the washing liquid must pass through two layers of filter cloth, while the filtrate only needs to pass through one layer. In addition, the filtering area through which the washing liquid passes is only half of that of the filtrate. Therefore, the washing rate in the frame filter press is only about 1/4 of the final filtration rate.
The size range of the filter plate of the plate-and-frame filter press is (100×100 ~1550×1550) mm2, the thickness of the filter frame is 25 ~ 200mm, and the working pressure is 0.7MPa For the plate-and-frame filter press made of general metal materials, it is 460×. Reliable design data must be obtained through material experiments.
Plate-and-frame filter press has simple and compact structure and large filtering area, and is mainly used for filtering suspension with high solid content. Because it can withstand high pressure difference, it can be used to filter fine particles or materials with high liquid viscosity. The water content of filter cake is lower than that of general filter, and it occupies less land per unit output. However, due to the wear of filter media in the process of slag discharge and flushing, the service life of filter media is short, the labor intensity of manual frame cleaning is high, and the working conditions are poor. The appearance of various automatic plate and frame filter presses in modern times has overcome this shortcoming to some extent.
Plate-and-frame filter presses produced in China mainly include BAS, BMS, BM, BA and other models, which are widely used in chemical, petroleum, pharmaceutical, food, ceramics and non-metallic mineral processing departments. There are three types of compaction methods: manual spiral compaction, mechanical spiral compaction and hydraulic compaction.
The working pressure of the plate and frame filter press is 0.6 ~ 0.8 MPa, the fastening pressure of the handle is 5MPa, and the hydraulic pressure is 30MPa. Equipment is divided into washing and non-washing two kinds. The model significance of plate-frame filter press is as follows: b- plate-frame, m- open flow; A- undercurrent; s-manual； J- machinery; Y- hydraulic; For example, the manual spiral compaction plate-frame filter press BMS60-8 10/25 represents a filtration area of 60m2, a filter frame size of 8 10× 8 10m2 and a plate frame thickness of 25mm.
Automatic plate and frame filter press is a continuous operation and intermittent process. This machine is equipped with a special mechanism to complete the operation steps of automatic compaction, automatic frame opening, automatic cake unloading and automatic cleaning of filter cloth. Controlled by electrical appliances, each operation step can be completed automatically according to the pre-arranged program, which makes the whole production process realize semi-automatic control and remote control, thus overcoming various disadvantages brought by manual operation of ancient plate-and-frame filter press. China has produced three kinds of automatic plate-and-frame filter presses Bajz 30/ 1000 ~ 60, and their structural shapes are shown in Figure 6-5. The area of automatic frame filter press is 15, 20 and 30m2. The filling volume of the filter frame is 0.3, 0.4 and 0.75m3；; The maximum filtration pressure is ≤0.6MPa, and the maximum compressed air pressure in the cavity is 0.5 ~ 0.6 MPa.
Figure 6-5 Structural Shape of Automatic Plate-and-Frame Filter Press
When the automatic plate-and-frame filter press works, compressed air of 0.5 ~ 0.6 MPa can be introduced into the inner cavity, and the rubber film is blown to squeeze out the water in the filter residue, and the filter cake is automatically dried. When the plate and frame are automatically pulled open, the rubber film returns to its original state and the filter cake is discharged. Through the driving mechanism of the filter cloth, the filter cloth of the filter press receives the water spray from the water spray pipe in the process of passing through the washing box, and the brush roller and the filter cloth rotate reversely, thus realizing the purpose of automatically cleaning the filter cloth. The whole operation process realizes semi-automatic remote control.
Table 6- 1 lists the specifications and technical performance of some domestic plate and frame filter presses.
IV. Installation and use
1. Install the filter press. Put the left and right frames on the cement foundation, install the crossbeams, level them well, tighten the nuts, pour concrete into the reserved holes until they are full, and then tighten the anchor bolts and nuts, and install the filters, compaction devices and other components.
2. The filter press is used with mud pump or diaphragm pump. Mud is sucked out of the mud pit, and the mud cake is discharged in pieces. Therefore, in the process layout, the height difference between the slurry tank and the filter press should be fully considered, such as small lines, small bends, easy transportation of mud cakes to designated places, and convenient discharge of clean water.
Table 6- 1 Technical Performance of Plate and Frame Filter Press
3. According to the filtration principle, it is best to start with low pressure at the beginning, maintain constant speed filtration, and maintain constant pressure filtration after a period of time, which can be controlled by pressure regulating valve.
4. In order to prevent mud leakage accidents, the sealing surface should be smooth, the pressing force should be uniform, and the load should not be unbalanced.
5. Lock the male and female nuts, the tightness of the filter cloth should be moderate, and it should be replaced in time after being damaged.