Discussion on the factors influencing the dewatering effect of horizontal screw centrifuge

Wang Sizhe Liang Yunping introduced the structure and working principle of the horizontal screw discharge settling centrifuge, analyzed the main factors affecting the dewatering effect of the centrifuge: the non adjustable mechanical factors such as the diameter of the drum, the half cone angle and the pitch of the drum; the adjustable mechanical factors such as the drum speed, the speed difference and the thickness of the annular liquid layer The technological factors affecting the dewatering effect, such as the addition of flocculant, are described In recent years, with the rapid development of sewage treatment in China, the problem of sludge treatment and disposal has become increasingly prominent Considering the requirements of sludge dewatering in subsequent treatment sections such as mud cake landfill, composting, incineration, etc., horizontal screw discharge settling centrifuge (hereinafter referred to as centrifuge) is widely used in water treatment industry due to its advantages of low moisture content of mud cake after dehydration, high automation and good operating environment 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 Its structure is shown in Figure 1 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 (spiral and drum), the sludge is accelerated inside the rotor and forms a cylindrical liquid ring layer; under the effect of centrifugal force, the dense solid particles settle to the inner wall of the drum; The solid phase is pushed to the cone end of the drum by the relative velocity difference between the screw and the drum After the liquid surface 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 2.1.1 the larger the diameter and effective length of the drum, the longer the effective length, the larger the effective settlement area, the larger the processing capacity, the longer the residence time of the material in the drum, and at the same speed, the greater the separation factor, the better the separation effect; limited by the material, It is impossible to increase the diameter of the drum of the centrifuge without limitation, because the allowable high speed of Zui with the increase of the diameter will decrease due to the decrease of the material firmness, so the centrifugal force will also decrease accordingly Generally, the diameter D of the drum is 200-1000 mm, and the length diameter ratio L / D is 3-4 In addition, the centrifuge with large drum diameter can operate at a lower speed difference under the same treatment capacity The reason is that the large drum diameter of the spiral slag transport capacity is larger, to achieve the same slag transport capacity of the small drum diameter of the centrifuge must increase the speed difference to achieve From the discussion of the following velocity difference, it is known that reducing the velocity difference can increase the dewatering time of sediment and reduce the disturbance of spiral to the clarification area, so as to improve the dewatering effect 2.1.2 when the sediment on the inner side of the centrifuge drum is pushed to the discharge port along the cone end of the drum, it is affected by the reflux force of downward sliding due to the centrifugal force The larger the half cone angle is, the larger the sludge is squeezed by centrifugal force, the greater the torque of the screw thrust is, the greater the abrasion of the blades is, and even the phenomenon of sediment reflux occurs, which results in the failure of the screw pusher to discharge 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 When the liquid pool depth h is fixed (see Figure 2), the cone angle is large (α 1 > α 2), the length of the drying area (i.e the distance between the sludge separating from the liquid level and the slag discharge port) is short (L 12 < l 22), the dehydration time is short, and the moisture content of the sediment is high, but the length of the settlement area L 11 is greater than l 21, the settlement area volume increases, and the quality of the filtrate is relatively better On the contrary, if the cone angle is small, the length of drying zone is long, the dehydration time is long, the moisture content of sediment is low, and the quality of filtrate is poor 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 ° in order to reduce the reflux rate of sediment If the half cone angle of common materials is less than 10 °, the sediment can be transported smoothly [1,2] 2.1.3 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 From Tan λ = s / π D, it can be seen that the larger the pitch s is, the larger the helix angle λ is, the greater the chance of material plugging between the helix blades is At the same time, the large pitch will reduce the number of turns of the spiral blades, which will cause the uneven distribution of the materials at the cone end of the drum and increase the vibration of the machine Therefore, it is difficult to transport the slag for the materials that are difficult to separate, such as activated sludge Smaller pitch, generally 1 / 5-1 / 6 of the drum diameter, is conducive to transportation For easily separated materials, the pitch is larger, generally 1 / 2-1 / 5 of the drum diameter, so as to improve the transport capacity of sediment [3] 2.1.4 screw type screw is the main component of centrifuge Its function is to transport sediment settled on the inside of drum and discharge sediment smoothly It is not only a unloading 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 relative movement of liquid and solid in the drum The feeding chamber of the countercurrent 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 enough 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 high centrifugal force; the material entering the drum from here will cause the settled solid particles in this area to float again due to disturbance, Turbulence and additional eddy current will also be generated, which will 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 Due to the extension of settling time and no interference, it 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 the rotating speed can be significantly reduced, save power consumption, reduce noise, and extend the life of the machine [4] The forward flow spiral structure centrifuge is especially suitable for hard to separate materials with small solid-liquid density difference, poor solid-phase settling performance and low solid-phase content [5] 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 re separated and deposited in the skimming pipe, which will block the passage of the skimming pipe for a long time, so it needs to be washed regularly In recent years, with the increasing requirements of sludge dewatering, there are various types of spiral structures For example, the BD baffle technology of a 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, increase the pressing force on the mud cake, and only transport the lower sediment, while the sludge with high water content in the upper part is intercepted at the outside of the pressing cone section, so as to realize the pressing dehydration and make the mud dry A Swedish company adopts lamella technology based on inclined plate sedimentation principle to design the blade of centrifugal screw pusher into Zui preferred inclined state The parameters such as blade inclination, pitch and blade spacing are optimized to improve the treatment capacity, reduce the consumption of flocculant and moisture content of mud cake 2.2 adjustable mechanical factor 2.2.1 the speed of the drum is usually adjusted by the frequency conversion motor or the hydraulic motor The higher the rotating speed is, the greater the centrifugal force is, which is helpful to improve the solid content of sludge cake, but when the rotating speed is too high, it will destroy the sludge floc and reduce the dehydration effect 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 2.2.2 the speed difference has a direct impact on slag discharge capacity, mud biscuit degree and filtrate quality Increasing the speed difference is conducive to improving the slag discharge capacity, but the dewatering time of sediment is shortened, and the moisture content of sludge cake after dewatering is large At the same time, the spiral disturbance to the liquid pool in the clarification area is large, and the quality of filtrate is relatively poor If the speed difference is reduced, the sediment thickness is increased, and the sediment dewatering time is increased, the moisture content of the mud cake will be reduced after dehydration At the same time, the smaller the disturbance of the screw to the materials in the clarification area is, and the better the filtrate quality is, but the load of the screw pushing material will be increased It should be prevented that the decrease of the slag discharging amount will cause the blocking phenomenon caused by the timely discharge of the sediment in the centrifugal machine Otherwise, it is easy to cause a large amount of mud in the filtrate, At this time, the feed quantity must be reduced or the speed difference must be increased Therefore, the speed difference 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 4-6 s, and the effect of longer time is not obvious [1] 2.2.3 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 foreign manufacturers can also realize the automatic adjustment of the thickness of the liquid ring layer As the thickness of liquid ring layer increases, the settling area increases, and the retention time of materials in the machine increases accordingly The result is that the quality of filtrate is improved, but at the same time, the length of drying area in the machine is shortened, which results in the decrease of mud biscuit degree On the contrary, reducing the thickness of liquid ring layer can obtain higher solid content of mud biscuit at the expense of filtrate quality Therefore, the liquid level baffle should be adjusted reasonably to make the mud biscuit degree and filtrate quality reach the best combination of Zui 2.3 process factors as the centrifuge uses the density difference between solid and liquid phases to achieve solid-liquid separation, the larger the density of sludge particles, the easier the separation It is easy to dewater the primary sludge and difficult to dewater the surplus sludge in the municipal sewage treatment plant The dewatering performance of the mixed sludge is between the two 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 be suitable for the characteristics of sludge There are three types of polyacrylamide: cationic type, anionic type and non-ionic type The selection should be based on the comprehensive measurement of technology and economy The proper 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 adding less flocculant, so as to reduce the operating cost, but adding a proper amount of flocculant can significantly improve the solid recovery rate The dosage and concentration of flocculant are insufficient, so it is difficult to form flocs and affect the quality of filtrate; the concentration of flocculant is too large, but the quality of active groups decreases due to mutual shielding, and the redispersion will also destroy the stability of flocs, the flocculation effect is also not good, and the dosage of flocculant is too large, which not only causes waste, but also does not significantly improve the treatment effect In municipal sludge treatment, the proportion of organic polymer flocculant solution is generally 1 ‰ ~ 5 ‰, and the dosage of flocculant is generally 1 ‰ ~ 5 ‰