Factors influencing the effect of horizontal screw centrifuge

In recent years, with the rapid development of sewage treatment in China, the popularization of sewage treatment and the adoption of advanced treatment, the amount of sludge produced is also increasing, and the status of sewage sludge treatment and disposal in urban sewage treatment is increasingly prominent Considering the post-treatment sections of sludge cake landfill, composting, incineration and so on after dehydration, the requirements for sludge dehydration are also increasing Therefore, horizontal screw discharge settling centrifuge (hereinafter referred to as centrifuge) is widely unknown in the water treatment industry due to its advantages of low moisture content of sludge cake after dehydration, high degree of automation, good operating environment and so on Its application has the potential to replace the belt filter press According to the investigation of the use of centrifuges, combined with the commissioning and use experience, the factors affecting the dewatering effect of centrifuges are discussed as follows 1 Structure and dehydration principle centrifuge is mainly composed of drum, screw, differential system, liquid level baffle, drive system and control system Centrifuge is to use the density difference of solid-liquid two-phase, under the action of centrifugal force, accelerate the settling speed of solid particles to achieve solid-liquid separation The specific separation process is that the sludge and flocculant solution are sent into the mixing chamber of the drum through the inlet pipe, where they are mixed and flocculated Due to the high-speed rotation and friction resistance of the rotor (screw and drum), the sludge is accelerated inside the rotor and forms a cylindrical liquid ring layer Under the effect of centrifugal force, the solid particles with large proportion settle to the inner wall of the drum, and then the phase of the screw and drum are utilized The solid phase is pushed to the cone end of the drum by the velocity difference After the liquid level is pushed out, the sludge can be dehydrated and dried, and then pushed to the slag discharge port for discharge The supernatant is discharged from the large end of the drum to realize the solid-liquid separation 2 Factors affecting the effect of centrifuge 2.1 mechanical factors that cannot be adjusted (1) drum diameter and effective length The larger the diameter of the drum, the longer the effective length, the larger the effective settlement area, the larger the processing capacity and the longer the retention time of the material in the drum At the same speed, the greater the separation factor and the better the separation effect However, due to the limitation of the material, the diameter of the drum of the centrifuge cannot be increased without limitation, because the allowable high speed of Zui will decrease with the decrease of the material firmness, so the centrifugal force will also decrease accordingly Generally, the diameter of the drum is between 200-1000MM and the length diameter ratio is between 3-4 In addition, under the same processing capacity, the centrifuge with large drum diameter can run at a lower differential speed, because the spiral slag conveying capacity with large drum diameter is larger, and the centrifuge with small drum diameter and the same slag conveying capacity must be realized by increasing the differential speed Reducing the differential velocity can increase the dewatering time of sediment, reduce the disturbance of spiral to the clarification area, and improve the dewatering effect (2) when the sediment on the inner side of the centrifugal drum is pushed to the discharge port along the cone end of the drum, it is affected by the reflux force due to the centrifugal force The larger the half cone angle is, the greater the centrifugal extrusion pressure on the sludge, the greater the torque of the screw thrust, and the greater the wear of the blades, so as to produce the phenomenon of sediment reflux and lead to the failure of the screw pusher to discharge the slag If the half cone angle is small, its effective settlement area will be greatly reduced, and then the performance of the centrifuge will be reduced It can be seen from the above analysis that the half cone angle of the drum is an important parameter in the design of the centrifuge In terms of clarification effect, the cone angle is required to be as large as possible, while in terms of slag transportation and dehydration effect, the cone angle is required to be as small as possible As slag conveying is a necessary condition for normal operation of centrifuge, Zui design must first meet the slag conveying conditions For materials that are difficult to separate, such as activated sludge, the half cone angle is generally within 6 degrees, so as to reduce the reflux rate of sediment For ordinary materials, the half cone angle is less than 10 degrees, which can ensure the smooth transportation of sediment (3) pitch pitch, that is, the distance between two adjacent spiral blades, is a very important structural parameter, which directly affects the success or failure of slag transportation When the screw diameter is fixed, the larger the pitch is, the larger the helix angle is, the greater the chance of material plugging between the screw blades is At the same time, the large pitch will reduce the number of turns of the spiral blades, resulting in the uneven distribution of the materials at the cone end of the drum and the increase of the vibration of the machine Therefore, it is difficult to transport the sludge from the refractory materials such as activated sludge The screw pitch should be smaller, generally 1 / 5-1 / 6 of the diameter of the drum, so as to facilitate the transportation For easy to separate materials, the screw pitch should be larger, generally 1 / 2~1 / 5 of the drum diameter, so as to improve the transport capacity of sediment (4) the screw type screw is the main component of the centrifuge Its function is to transport the sediment settled on the inner side of the drum and discharge the sediment smoothly It is not only a discharge device, but also determines the production capacity, service life and separation effect The type of screw can be divided into counter flow type and forward flow type according to the different relative movement mode of liquid and solid in the drum The feeding chamber of the counter flow centrifuge is located in the middle of the screw, i.e near the boundary between the drying area and the settling area, so as to ensure that the liquid phase has sufficient settling distance, but the solid phase can only stay for the time required for it to pass through the cone part, so it requires a higher centrifugal force; the material entering the drum from here will cause the settled solid particles in this area to float again due to disturbance, and will also produce There are turbulence and additional eddy current, which reduce the separation effect As the feed port of the forward flow centrifuge is at the end of the drum, the reverse flow turbulence is avoided, and the sediment is not disturbed The whole length of the centrifuge has played a role in settling, expanding the settling area and increasing the retention time of the suspension in the centrifuge, thus improving the separation effect Because prolonged and undisturbed settlement can effectively reduce the use of flocculant, greatly improve the flow state of the fluid in the machine, and increase the diameter of the drum to increase the centrifugal force, so it can significantly reduce the speed, save power consumption, reduce noise, and extend the life of the machine The forward flow spiral structure centrifuge is especially suitable for the difficult separation materials with small solid-liquid density difference, poor solid-phase settling performance and low solid content However, the filtrate of the forward flow centrifuge is discharged by the skimming pipe The solid particles that are not separated when the filtrate passes through the skimming pipe will be separated and deposited in the skimming pipe again, and the passage of the skimming pipe will be blocked for a long time, so it needs to be washed regularly In recent years, with the increasing requirement of sludge dewatering, a type of spiral structure has appeared For example, the BD baffle technology of Alfa Laval company in Sweden, that is, a special baffle is set at the screw discharge end of the cone section of the centrifuge, which can make the centrifuge in the state of ultra deep liquid pool, so as to increase the slag pressing force on the mud cake, and only transport the lower sediment, while the sludge with high water content in the upper part is retained on the outer side of the pressing cone section, so as to realize the pressing dehydration and make the mud dry Adopting lamella technology based on inclined plate sedimentation principle, Sweden's noxon company has designed the centrifugal screw pusher blade into Zui preferred inclined state The parameters such as blade inclination, pitch and blade spacing have been optimized and designed to improve the processing capacity, reduce the consumption of flocculant and moisture content of mud cake 2.2 adjustable mechanical factors (1) the adjustment of drum speed and drum speed is usually realized by frequency conversion motor or hydraulic motor The higher the rotating speed is, the greater the centrifugal force is, which is helpful to improve the solid content of mud cake However, if the rotating speed is too high, the sludge flocs will be destroyed, and the dewatering effect will be reduced At the same time, high speed requires high materials, increases the wear of the machine, and correspondingly increases the power consumption, vibration and noise level (2) the differential velocity directly affects the slag discharge capacity, biscuit degree and filtrate quality Increasing the differential speed is conducive to improving the slag discharge capacity, but the dewatering time of the sediment will be shortened, and the moisture content of the mud cake after dehydration will be large At the same time, the excessive differential speed will increase the disturbance of the spiral to the liquid pool in the clarification area, and the quality of the filtrate will be relatively poor Reducing the differential speed will increase the sediment thickness, increase the sediment dehydration time, and reduce the moisture content of the mud cake after dehydration At the same time, the screw has little disturbance on the materials in the clarification area, and the filtrate quality is relatively better, but it will increase the load of the screw pushing material It is necessary to prevent the blocking phenomenon caused by the decrease of the slag discharge amount, which can not discharge the sediment in the centrifuge in time, and prevent the filtrate from carrying a large amount of mud Reduce the amount of feed or increase the differential speed Therefore, the differential speed should be determined according to the material properties, handling capacity, handling requirements and structural parameters of the centrifuge It is generally believed that the dehydration time of mud cake in the drying area should be controlled between 4-6s seconds, and the effect will not change obviously no matter how long the time is (3) thickness of liquid ring layer thickness of liquid ring layer is an important parameter of process optimization, which directly affects the effective settling volume and the length of drying area of centrifuge, and then affects the treatment effect of centrifuge Generally, it is realized by manually adjusting the height of the liquid level baffle in the shutdown state During the adjustment, it must be ensured that the height of each liquid level baffle is the same, otherwise it will cause severe vibration during the operation of the centrifuge Some products of foreign manufacturers can also realize the automatic adjustment of the thickness of the liquid ring layer With the increase of the thickness of liquid ring layer, the settlement area will increase, the retention time of materials in the machine will increase correspondingly, the quality of filtrate will improve, but at the same time, the length of drying area in the machine will be shortened, which will lead to the decrease of mud biscuit degree On the contrary, a higher solid content of mud cake can be obtained by adjusting the thickness of liquid ring layer, but at the expense of filtrate quality Therefore, it is necessary to adjust the height of the liquid level baffle reasonably to make the biscuit degree and filtrate quality reach Zui optimal combination 2.3 process factors as the centrifuge uses the density difference between solid and liquid phases to achieve solid-liquid separation, the larger the proportion of sludge particles, the easier the separation The primary sludge is easy to be dewatered and the surplus sludge is difficult to dewatered, while the dewatering performance of mixed sludge is between them In order to improve the dewatering performance of sludge, an appropriate amount of organic polymer flocculant, such as polyacrylamide (PAM), should be added evenly before mechanical dewatering to reduce the specific resistance of sludge and make it easy to dewater The type of flocculant must adapt to the characteristics of sludge There are three types of polyacrylamide: cationic type, anionic type and non-ionic type The selection should be comprehensively measured from the technical and economic aspects The appropriate flocculant type and brand should be selected through experiments, and the molecular weight of flocculant should also meet certain requirements According to the actual operation, the amount of flocculant has little effect on the solid content of the mud cake in the centrifugal dehydration, and has a great effect on the quality of the filtrate Therefore, centrifugal dewatering can be used without adding or less adding flocculant, so as to reduce the operating cost, but adding a proper amount of flocculant can significantly improve the solid recovery rate If the dosage and concentration of flocculant are insufficient, it is difficult to form flocs, which will affect the quality of filtrate; if the concentration of flocculant is too large, the total amount of gold mutual shielding will decrease, and the redispersion will also destroy the stability of flocs, so the flocculation effect is not good; if the dosage of flocculant is too large, it will not only cause waste, but also the treatment effect is not significantly improved In municipal sludge treatment, the concentration of organic polymer flocculant solution is generally 1 ‰ ~ 5 ‰, and the dosage of flocculant is generally 3 ~ 5kg / TDS Conclusion there are many factors that affect the dewatering effect of centrifuge