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Vacuum filtration and vacuum filter

(I. Overview
The so-called filtration is to cause a pressure difference between both sides of the porous filter medium such as a filter cloth, and the solid particles in the filter slurry are trapped by the filter medium to separate the liquid-solid phase. Vacuum filtration is carried out by using a pressure difference between the vacuum source on one side of the filter medium and the atmospheric pressure on the side of the filter (typically 4.9*104 Pa-6.68*104 Pa). Most of the mechanisms of vacuum filtration belong to filter cake filtration, and some belong to deep filtration. Vacuum filters and other attachments are used to achieve vacuum filtration.
Vacuum filters are divided into two types: continuous type and gap type. The attachments are: in the washing machine, the purpose is to reduce the soluble salt in the filter cake; the filter cake drying device is designed to reduce the moisture content of the non-compressible filter cake; the pump type, the vacuum pump mostly uses the water ring vacuum pump and Roots The pump and the steam jet pump can also be used as a vacuum source; if the air of the vacuum pump contains corrosive or toxic gas, it is absorbed by a packed tower, a tray tower, etc.; and a smoke separator, a blower, and the like are included. Compressor, etc.
(2) Vacuum filter
1. Multi-chamber drum vacuum filter
The common feature of this type of filter is that the drum is immersed in the filter tank for rotation, and the outer surface of the drum has filter medium for continuous filtration. The circumference of the drum is divided into a plurality of filter chambers, which are connected to the dispensing head mounted on the drum main shaft via respective lines. The dispensing head is in turn continuous with the vacuum source and the compressed air source such that each filter chamber is connected to the vacuum source during the swing or to the source of compressed air. Through the rotation of the drum, each filter chamber continuously achieves filtration, washing, drying and unloading of the cake. In order to prevent solid particles from settling, a stirring device is provided on the filter.
According to the difference in the separation and discharge mode of the filter cake, the drum vacuum filter can be classified into the following types.
(1) scraper discharge drum vacuum filter
When the drum is turned to the discharge zone, compressed air is initially supplied to the zone due to the action of the dispensing head. Under compressed air backflushing, the filter cake was peeled off from the filter cloth and scraped off by a doctor blade. Air backflushing also helps prevent clogging of the filter cloth. This machine is not suitable for unloading the filter cake. It is also not suitable for the filter and the sedimentation fast filter which cannot maintain a constant concentration and particle size distribution. In addition, the filter cake has good air permeability and is easy to fall off from the drum surface, and it is not suitable to use this machine. Since the filter cake is removed by the doctor blade, at least a filter cake having a thickness of 6 mm to 10 mm is formed on the filter surface. Since the filter cloth is tied to the drum with a wire to prevent bulging during backflushing, it is inconvenient to change the filter cloth. Since the air piping and the filtrate piping are used in combination, the filtrate remaining in the piping is blown onto the filter cake during back-blowing, thereby increasing the moisture content of the filter cake.
(2) Rope unloading drum vacuum filter
The machine is wound on a part of the filter surface of the drum and the guide roller at a certain interval by a plurality of endless ropes. The applicable filter is roughly the same as the filter used for the scraper drum vacuum filter. In addition, the viscous filter cake can be removed. It is not suitable for the filter slurry and the cracking filter which are easy to cause the filter cloth to clog. Since the filter is discharged by the rope, the filter cake having a thickness as thin as 1.6 mm can be removed, but the thickness of the filter cake is preferably 5 mm or more.
(3) filter cloth walking unloading drum vacuum filter
The filter cloth is wound endlessly on the partial drum surface and the filter cloth guide roller and the tension roller. The filter cloth is driven by the rotating drum, and the thin filter cake is placed away from the drum surface, and the filter cake is removed at the turn of the filter cloth. The unloaded filter cloth is washed by the nozzles on both sides. There is also a filter cloth anti-bias device. This machine is suitable for filtering the filter slurry which is easy to block the filter cloth. Since the filter cake is removed at the turn of the filter cloth, the curvature of the roller is large, so that the filter cake having a thickness as thin as 2 mm can be removed. Since the filter cloth is not fixed to the drum, it is easy to replace. There is no anti-blowing when unloading the cake, so there is no problem of humidification of the filter cake. The shortcoming is that the filter cloth washing water needs to be treated before discharge, and a little filter slurry will leak from the two ends of the filter cloth to the drum, which affects the orange clearness of the filtrate.
(4) Roller discharge type drum vacuum filter
Near the drum, a filter cake roll is placed, which rotates in the opposite direction to the drum. When the filter cake on the surface of the drum contacts the roll, it is taken up and then scraped off the roll by a doctor blade. Even a filter cake as thin as 1 mm can be removed. The machine is suitable for filtering the filter cake which has a dryness and a large filtration resistance. However, the filter cloth is easily clogged, and the filter cake having a thickness of more than 5 mm is difficult to remove.
(5) Pre-coated drum vacuum filter
First, a filter aid such as diatomaceous earth is prepared into a slurry, which is then filtered by a rotating drum equipped with a filter cloth, so that a filter aid precoat layer, that is, a filter medium is formed on the filter cloth. Since the officially filtered filter cake is formed on the surface of the precoat layer, the filter cake is cut off with the same thin layer of precoat layer by a thin scraper. Generally, the thickness of the precoat layer is 40 mm to 90 mm, and if the drum is cut 15 μm to 150 μm thick per revolution, it can be continuously operated for half a day to one week. When the precoat is cut to a certain extent, it is necessary to reform the precoat. A relatively clear filtrate can be obtained with this machine, which is suitable for filtering a slurry containing less than 1% of solid discs, a slurry containing fine solid particles, and a colloid-containing filter. Since the filter cake is mixed with the filter aid, the machine should not be used in the case of the filter cake.

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