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Application of decanter centrifuge in gypsum dehydration

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Application of  decanter centrifuge in gypsum dehydration 

 

Desulfurization usually adopts limestone-gypsum wet flue gas desulfurization process. And decanter centrifuge is an important part in the process.

FGD uses a furnace and tower desulfurization device, 100% flue gas desulfurization. The desulfurization rate is not less than 95%.

The waste water treatment capacity of the waste water treatment device is designed according to 4 furnaces (4 × 300MW) of the whole plant.

1. Wastewater generation

The flue gas desulfurization project uses the limestone-gypsum wet desulfurization process. The limestone is crushed and ground into a powder, which is mixed with water and stirred to make an absorption slurry. In the absorption tower, the absorption slurry is contacted and mixed with the flue gas. The SO2 in the flue gas is removed by chemical reaction with the calcium carbonate in the slurry and the oxidized air blown in. The final reaction product is gypsum.

The gypsum slurry with a concentration of 15% in the slurry tank of the absorption tower is transported by the discharge pump to the primary dehydration device-gypsum hydrocyclone. The gypsum hydrocyclone has the dual functions of pre-dehydration and classification of gypsum crystals. The gypsum suspended tangential flow entering the hydrocyclone produces centrifugal motion, and the heavy solid particles are thrown toward the wall of the cyclone and flow down to form an underflow. The fine particles flow upward from the center of the cyclone to form a top flow. The solids content of the slurry leaving the cyclone underflow slurry to the gypsum slurry buffer tank is 40% to 50%. The slurry in the gypsum slurry buffer tank is pumped to the vacuum belt dewatering system through the gypsum slurry pump. Part of the slurry overflowing from the top of the gypsum hydrocyclone flows into the hydrocyclone tank, and is driven into the waste water cyclone by the hydrocyclone pump; partly flows into the slurry tank. The underflow of the waste water cyclone enters the slurry tank and returns to the tower through the slurry pump. The overflow part of the waste water cyclone is divided into two ways, one way enters the filtrate tank, one part enters the slurry tank return tower, part returns to the grinding system, and the other way enters the waste water tank, and the waste water is pumped to the waste water treatment system.

2. Wastewater treatment system

Waste water treatment process:

Wastewater pump—neutralization tank—reaction tank—flocculation tank—clarifier—sludge hopper—centrifugal dehydrator—slurry pump—sludge buffer tank—discharge—outlet pump—outlet tank    

In the neutralization tank, add Ca (OH) 2 to adjust the PH value, so that the heavy metal ions in the desulfurization wastewater form hydroxide precipitation. Add polymerized iron to produce flocculation reaction, add coagulant in the flocculation tank to enhance the flocculation effect. Organic sulfur is added to the flocculation tank to further remove heavy metal ions, especially mercury (Hg2 +).

The clarifier has the functions of coagulation, clarification and sludge concentration at the same time. After entering the clarification tank, the wastewater passing through the flocculation tank is further flocculated and precipitated. The clarified liquid overflows to the outlet tank and is discharged to the outside of the desulfurization island by the outlet pump discharge pump.

3. Sludge treatment system

 

The sludge at the bottom of the clarifier is discharged to the sludge buffer tank, and then pumped into decanter centrifuge by the sludge transfer pump for dehydration. The dehydrated sludge first enters the mud bucket and is then transported away by truck.

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Favorable factors of decanter centrifuge to biogas digester

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Favorable factors of decanter centrifuge to biogas digester

 

Analysis of the use effect of decanter centrifuge:

1) In the past, the volume of the biogas tank built by the farmer was too small, and the drainage time after the sewage entered the tank was too short, resulting in excessive discharge of sewage concentration. Use the decanter centrifuge to process the sewage before it sinks into the pool, which can effectively reduce the COD and BOD content of the sewage, reduce the operating load of the biogas digester, and thus achieve the reduction of sewage discharge;

2) The use of decanter centrifuge in advance can transform the feces in the fresh sewage into valuable fish feed or organic fertilizer. The separated sewage intermittently enters the biogas digester, so that the anaerobic digester can have a full digestion time, so that the active bacteria in the digester can reproduce more vigorously, the gas production is higher, and the content of gas methane is naturally higher High purity will be overcome naturally to solve the problem of abnormal operation of biogas generators;

3) The sewage treated by the decanter centrifuge enters the biogas digester, which can effectively solve the crust phenomenon of the biogas digester and eliminate the work of cleaning the biogas residue;

 

4) The use of decanter centrifuge can reduce the investment in the construction of biogas digesters, for example: a breeding or slaughterhouse that discharges 100 tons of sewage per day. If a matching decanter centrifuge is used for pre-treatment, it can reduce the 300 cubic anaerobic tank Construction investment, because the concentration of manure and sewage mixed into the pond is too high, the anaerobic time will need to be extended accordingly, so if a farm that discharges 100 tons of sewage per day, without decanter centrifuge treatment, then an anaerobic pond 1000 is needed -1200 cubic meters, if using decanter centrifuge can reduce the construction of 300-400 cubic meters.

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Explosion-proof performance of decanter centrifuge

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 If the materials processed by the decanter centrifuge contain flammable and explosive substances such as organic solvents, the centrifuge should have explosion-proof performance. The explosion-proof performance is determined according to the explosion-proof requirements of the process. The explosion-proof occasions involved in the past have only been proposed to be equipped with explosion-proof motors. In the past two years, most of the occasions where the processed materials contain organic solvents such as solvents have been required by nitrogen protection. In fact, to realize the explosion-proof in the true sense, centrifuges must take various measures in mechanical, electrical control, accessory configuration and other aspects.

For instance: electric control system: explosion-proof isolation barrier, non-contact energy consumption braking system, electrostatic grounding, automatic control of explosion-proof.

Mechanical system: explosion-proof motor, explosion-proof lighting, anti-collision measures, anti-static transmission belt.

Explosion-proof accessories: nitrogen protection device, nitrogen online detection, on-site explosion-proof control button.

 

GN Explosion proof electrical control panels or electrical control stations are used in hazardous Location. The Ex control panel is designed and built to operate in environments where concentrations of flammable gases, vapors, liquids or combustible particles pose risks of ignition and explosion. GN Solids Control provide explosion proof control panel with ATEX approval, IECEX Approval, China National explosion proof approval. The protection degree is up to IP65 for weather and dust proof.

GN explosion proof control panel enclosures are cast from heavy duty aluminum so that they cannot create a spark that could possibly ignite the vapors in a hazardous location. These heavy duty panels come with seals keep explosive fumes from entering the control panel. Moreover the GN explosion proof control panels are designed to contain an internal explosion without igniting the explosive vapors present in the hazardous area to make sure it is safe for the hazardous Zone 1 or Zone 2 areas.

GN Explosion Proof Pressurized Control Panel is popular used for VFD controland soft starter of motor, especially in high temperature ambient. The explosion proof pressurized control panel consists of main cavity and auxiliary cavity. The main cavity is pressurized cavity which includes electrical components, pressure check sensor system, ventilation air distribution system, and air conditioning system. The purging system works automatically to maintain a certain positive pressure inside the main cavity.

GN Solids Control is capable to provide you with IEC Ex, ATEX or CNEx Standard Positive Pressurized Control Panel control panels with Zone 1 or Zone 2 for hazardous locations. This will also suit the North America Class I Division 1 or Division 2 hazardous locations application.

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Three Main Factors That Affect the Decanter Centrifuge Effect

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Three Main Factors That Affect the Decanter Centrifuge Effect

 

The production capacity of the decanter centrifuge depends on the density difference between the solid and liquid phases and the length of the sedimentation zone. The closer the density difference between the solid and liquid phases and the higher the viscosity of the slurry, the more difficult to separate and settle. In actual production, there are three main factors that affect the separation effect of the decanter centrifuge: the feed temperature, feed rate, and abnormal process conditions.

     A. Feeding temperature: The temperature of the slurry can directly affect the viscosity of the mother liquor. Generally speaking, the higher the solution temperature, the lower the viscosity, the thinner the liquid film on the solid phase, and the easier the fine particles to settle. And the liquid in the pores will be less.

     B. Feeding rate: Sometimes, large feeding amount will lead to poor separation effect, mainly because the sedimentation time of the particles in the decanter centrifuge drum is not enough. The premise of achieving the separation conditions designed by the decanter centrifuge is that the time for the solid particles to settle on the drum wall must be less than the residence time of the particles in the drum. The effective residence time of the slurry to be separated in the drum must be guaranteed, So that the solid particles have enough time to settle out. In our practical experience, the separation effect is not good when the feed volume is 1m3 / H for the same material, but when the feed volume is 0.5m3/h, the separation effect will be ideal.

C. Abnormal process conditions: mainly refers to the insufficient crystal content in the feed slurry or the crystals are not crystallized but flocculated, which is very difficult for the decanter centrifuge to obtain the ideal separation effect.   

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Unregulated Mechanical Factors of Decanter Centrifuge

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Unregulated Mechanical Factors of Decanter Centrifuge

 A. Drum diameter and effective length

The larger the diameter of the drum and the longer the effective length, the larger the effective settlement area will become, the larger the processing capacity. And the longer the residence time of the material in the drum. At the same speed, the larger the separation factor, the better the separation effect. However, due to the limitation of the material, the diameter of the drum of the decanter centrifuge cannot be increased indefinitely, because the maximum allowable speed will decrease as the solidity of the material decreases, and the centrifugal force will correspondingly decrease. Generally, the diameter of the drum is between 200 and 1000 mm, and the aspect ratio is between 3 and 4. The current development of decanter centrifuges tends to have a large aspect ratio with a high rotation speed. This Decanter centrifuge is more suitable for the treatment of low-concentration sludge, and the degree of mud biscuit is better.

In addition, in the case of the same processing capacity, the centrifuge with a large drum diameter can run at a lower differential speed, because the spiral slag conveying capacity of the large drum diameter is larger. To achieve the same slag conveying capacity, a centrifuge with a small drum diameter must be achieved by increasing the differential speed.

 

B. Drum's half cone-angle

When the sediment settled inside the drum is pushed to the discharge port along the end of the drum cone, it is subjected to the downward flow of the backflow force due to the centrifugal force. The half cone angle of the drum is an important parameter in the decanter centrifuge design. In terms of clarification effect, the cone angle is required to be as large as possible; and in terms of slag conveying and dehydration effects, the cone angle is required to be as small as possible. Since the slag transfer is a necessary condition for the normal operation of the centrifuge, the optimal design must first meet the slag transfer conditions. For materials that are difficult to separate, such as activated sludge, the half cone angle is generally within 6 degrees, in order to reduce the reflux speed of sediment. For ordinary general materials, the half cone angle is within 10 degrees to ensure the smooth transportation of sediment.

C. Pitch

The pitch, which is the distance between two adjacent spiral blades, is an important structural parameter that directly affects the success or failure of slag transport. When the spiral diameter is constant, the larger the pitch and the larger the spiral rise angle, the greater the chance that the material will be blocked between the spiral blades. At the same time, a large pitch will reduce the number of turns of the spiral blades, resulting in uneven material distribution at the cone end of the drum and causing increased machine vibration. Therefore, for difficult-to-separate materials such as activated sludge, it is difficult to convey slag, and the pitch should be smaller. Generally, the diameter is 1/5 to 1/6 of the drum diameter to facilitate transportation. For easy-to-separate materials, the pitch should be larger, generally 1/2 to 1/5 of the diameter of the drum, in order to improve the sediment transportation capacity.