Compared with conventional mineral flotation, the special flotation process is relatively simple, most of which are single-slot operations without flotation circuits, and the flotation cell has a large volume.
(1) Inflatable flotation When using a lighter substance such as surfactant, protein, organic matter, oil, etc. and concentrated activated sludge, an inflatable flotation machine is often used. The gas is passed through a porous device, and bubbles dispersed to a diameter of several millimeters are bubbled into the liquid. When a very concentrated liquid is encountered, a diffuser of medium-sized bubbles is required to generate local turbulence to cause the bubbles to break. The size of the bubbles must be appropriate to ensure a high adhesion rate on the particles to be floated.
In the apparatus for removing light substances (grease, fiber, paper, organic matter), two zones are generally provided, one of which is a mixing and emulsification zone, and the other is a relatively calm flotation zone. Figure 1 is the operating principle. In the emulsion zone, the spiral flow generated by the agitation and mixing of suspended solids by air lengthens the path of bubble movement; in the flotation separation zone, the lateral flow is very slow, thus reducing the flow of water. Turbulence.
Inflatable flotation machines are further divided into flotation columns and trough foam separators. See Figure 2 and Figure 3.
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The flotation column is mainly composed of a cylindrical container and a foam receiver. The cylindrical container contains the selected material, and the bottom of the cylinder is provided with a porous gas disperser. Figure 4 is a schematic illustration of several modes of operation of a flotation column wherein "concentrated" is the return of a portion of the foam to the foam zone of the column to provide a higher concentration foam product. The "purification type" is to add a collector to the bottom stream to be discharged, so that the bottom liquid is further purified. In order to obtain a thick foam product, a foamed discharge surface in a contracted form should be used, which can squeeze the foam and promote foam coalescence. In contrast, expanding the foam discharge surface allows the foam to be discharged better, resulting in a substantial reduction in foam in the tank.
In addition to the single-tank operation, the flotation column can also arrange a plurality of flotation columns in series like mineral coarsening and sweeping to achieve high purification; the foam product can also be inflated and bubbling to obtain a high-grade foam product.
The defoaming methods are:
(1) Chemical defoaming method Injecting antifoaming agent.
(2) Physical defoaming method The temperature is suddenly defoamed (heated or quenched), the ultrasonic vibration is defoamed, the cyclone is defoamed, and the jet is defoamed.
(3) Mechanical defoaming method Mechanical stirring defoaming, centrifugal defoaming.
(4) Combined defoaming method Combination of various thermal devices and mechanical devices.
(2) Gas analysis flotation The gas analysis flotation process is divided into three types: pressurized flotation, reduced pressure flotation (or vacuum flotation), and electrolytic flotation. Among them, the pressurized flotation process has been widely used due to its easy adjustment and control of foaming amount and flexible flow. The vacuum flotation needs to be operated under sealed conditions and only used for flotation separation of toxic gases or odors. Process; electrolytic flotation is still in the research stage due to its high energy consumption.
A Pressurized flotation The pressurized flotation device is mainly composed of a pressure pump, an air compressor, a dissolved gas tank, a pressure reducing valve, a flotation tank and the like. The working procedure is as follows: the pressure pump introduces the raw water or part of the treated water together with the compressed air of a pressure of 0.196 to 0.49 MPa (2 to 5 kgf/cm 2 ) into the closed dissolved gas tank, and the water stays in the dissolved gas tank for 1 to 5 minutes. After that, the pressure reducing valve and the unpressurized waste water are introduced into a flotation tank which is opened at normal pressure, and the air is precipitated in the flotation tank, and foam or scum is formed with the target object, which is scraped off by the scraper; the water is in the flotation tank. After staying for 10 to 30 minutes, the treated water is discharged from the bottom of the flotation cell or the other end of the tank. [next]
a Pressurized flotation process (1) All raw water pressurization process is shown in Figure 5. It is suitable for the flotation process with high suspended solid content in raw water, large amount of foaming and no damage to flocculation.
(2) Part of the raw water pressurization process is shown in Figure 5b. Part of the raw water is pressurized and dissolved, and then mixed with the unpressurized raw water to enter the flotation tank. The process has reduced power consumption and is suitable for the water quality that can be flocculated again once mixed with unpressurized raw water after the flocculation is destroyed by pressure. Mostly for the purpose of clarification and purification of water.
(3) Process water circulation pressurization process as shown in Figure 5c. According to the amount of foaming required for the raw water, 10 to 30% of the treated water is pressurized and dissolved, and then mixed with the raw water to enter the flotation tank. The process does not destroy the flocs, and the amount of foaming can be flexibly adjusted according to the nature of the raw water. However, the corresponding flotation tank has a larger volume and the power cost is slightly higher. Suitable for sludge concentration.
b Dissolved gas method The pressure of the dissolved gas tank and the solubility of the gas are important factors in the pressure flotation. The solubility of a gas in water is in accordance with Henry's law and is proportional to the partial pressure of the gas. As shown in Figure 6. Solubility of various gases in water (101.33 kPa, 20 ° C).
Practice has proved that the dissolution efficiency of air in a dissolved gas tank is related to the pressure, the way water and compressed air enter the dissolved gas tank, the gas supply speed, the stagnation time, and the flow stirring conditions. [next]
Figure 7 shows several ways of dissolving gas. Among them, the dissolved gas method of Fig. 7a and b has a low dissolved gas efficiency. When the pressure of the dissolved gas tank is 690 kPa and the gas supply rate is 70 liters/min, it takes more than 45 minutes for the water to remain in the tank to be saturated. If the intake speed is increased or decreased, the residence time required for saturation is longer.
If air enters the pressure pump from the suction end of the pressure pump and water at the same time, as shown in Fig. 7c, the dissolved gas efficiency is improved due to the shear agitation of the pressure pump blades. However, since air and water simultaneously enter the pressure pump, the pumping speed and pressure of the pump are lowered, and the amount of intake air is limited. At a pressure of 690 kPa, the air intake can only achieve an air saturation rate of 23%.
If a branch pipe is provided at the drain end of the pressure pump, as shown in Fig. 7d, the compressed air is formed into a branching circulation, which can greatly improve the gas dissolution efficiency, and can make the air solubility reach 75 to 85% of the saturated solubility.
In addition, the dissolved gas tank is filled with a porous material of a proper depth, and water is supplied from the water pipe buried in the center of the filling filler to the dissolved gas tank to increase the contact area of ​​the air and the water, and the saturation rate can be greatly improved. As shown in Figure 7e.
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