Decanter centrifuge | Sludge dehydrator | Centrifugal dehydrator | mud dehydrator | Jinhua horizontal screw centrifuge | Complete set of sludge dewatering equipment

Application of horizontal screw centrifugesludge dewatering

2021-05-28 11:22:03

1. Composition and working principle

1.1 unit composition

The horizontal screw centrifugal sludge dewatering unit is mainly composed of horizontal screw centrifuge, automatic flocculant preparation and dosing device, sludge crushing and cutting machine, sludge feed pump, dosing pump, single screw sludge transfer pump, flowmeter and automatic control system.

1.2 process flow

1.3 working principle

Horizontal screw centrifuge is the abbreviation of horizontal screw discharge centrifuge. It is mainly composed of high-speed rotating drum, screw and differential with the same rotation speed as the drum.

When the sludge enters the drum cavity of the centrifuge, the high-speed rotating drum generates strong centrifugal force. Due to the high density and centrifugal force of sludge particles, the sludge is thrown on the inner wall of the drum to form a solid ring layer; The density of water is small and the centrifugal force is small, so the liquid ring layer can only be formed on the inner side of the solid ring layer. Due to the different rotating speeds of the screw and the drum, there is relative movement between them (i.e. differential rotating speed), which pushes the sludge deposited on the inner wall of the drum to the outlet of the small end of the drum, and the separated water is discharged from the other end of the drum. The function of the differential is to form a certain slip between the drum and the screw. Flocculant is added to the sludge to produce flocculation, so that the dispersed sludge particles gather to produce large flocs and accelerate the separation of sludge and water.

2. Adjustment of operating parameters and treatment effect of horizontal screw centrifuge

2.1 selection of drum speed

The rotating speed of the drum can be adjusted between 1000 rpm and 2800 rpm. Increasing the rotating speed of the centrifuge will increase the centrifugal force acting on the sludge accordingly, which can further dehydrate the sludge. However, if the force is too large, it may lead to the decomposition and fragmentation of sludge flocs, which will affect the dehydration effect; Moreover, with the increase of rotating speed, the mechanical wear of the equipment also increases greatly. Considering the above factors, in practical application, the drum speed is set between 2200rpm and 2400rpm.

2.2 determination of dry solid load

Dry solid load refers to the weight of nonvolatile solid treated per hour, expressed in kgds (dry sludge) / h. Adjusting the dry solid load of the centrifuge has a great impact on the sludge dewatering effect. When the mud inlet flow (i.e. "hydraulic load") reaches a certain degree and the suspended solids content brought in exceeds the maximum dry solid load that the centrifuge can bear, it will increase the moisture content of the mud cake and increase the sludge carrying capacity of the supernatant. At this time, the mud inlet flow should be reduced to make the supernatant separated from the centrifuge clear. In actual operation, the hydraulic load must be adjusted to ensure that the load of dry solids entering the centrifuge does not exceed the maximum bearing capacity of the centrifuge. Otherwise, the excess dry solids will be discharged from the supernatant, and the suspended solids in the supernatant will increase sharply, but the output of cement cake will not increase. When the rotating speed of the brick drum of the centrifuge increases, the dry solid load will increase accordingly.

2.3 determination of flocculant dosing position

There are two dosing positions for flocculant of centrifuge, one is at the inlet of sludge screw pump and the other is at the inlet of centrifuge drum. Generally, it is added at the inlet of the centrifuge drum, and the reaction time is 7 ~ 10 seconds.

2.4 determination of liquid ring layer thickness (set the height of liquid level baffle)

When the horizontal screw centrifuge dehydrates the sludge, under the action of centrifugal force, the solid ring layer, liquid ring layer and shore area (Shore Area: refers to the distance from the sludge from the liquid ring layer to the discharge port) will be formed in the strand transfer, which is a part of the drum cone.

When the amount of sludge is certain, if the thickness of the liquid ring layer is large, the sludge will stay in the centrifuge for a long time, and the longer the sludge is separated in the liquid ring layer, more sludge will be separated, which can reduce the possibility of some small particles being disturbed and losing with the separation liquid. However, if the thickness of the liquid ring layer is too large, water will overflow from the sludge outlet with the dewatered sludge; If the thickness of the liquid ring layer in the centrifuge is small, the residence time of the sludge in the centrifuge is short, and the working pressure is not easy to increase, but the moisture content of the dewatered sludge is also low.

Based on the above two aspects, the thickening of the liquid ring layer will generally improve the solid recovery rate of dehydration (supernatant and clear liquid), but the thickening of the liquid ring layer will shorten the shore area accordingly. As shown in the above figure, the sludge separated from the liquid ring layer will not have enough time to be dumped dry, so the solid content of the mud cake will decrease. When controlling the thickness of liquid ring layer, the time should be balanced between high solid recovery and solid content of mud cake. In addition to incineration after sludge dewatering, it is generally unnecessary to pursue too high solid content of sludge cake, and the higher the solid recovery rate, the better. Therefore, the thickness of liquid ring layer should be increased as much as possible.

Adjust the thickness of liquid ring layer of centrifuge by changing the position of liquid level baffle. The adjustment of the liquid level baffle of the centrifuge is very important, which directly affects the dehydration effect and the vibration degree of the centrifuge. When adjusting the height of the liquid level baffle, pay attention to ensure that all liquid level baffles are at the same height, otherwise it will cause great imbalance and violent vibration of the centrifuge, and ensure that the height tolerance of the liquid level baffle is ± 0.25 mm.

2.5 differential

"Differential speed" refers to the difference between the rotating speed of the drum and the screw speed, that is, the relative speed between the two. Increasing or decreasing the "differential speed" will change the residence time of sludge in the drum, which has a very important impact on the treatment effect.

"Differential" shall be selected according to the following principles: when the amount of sludge is certain, if the differential is relatively low, the sludge will stay in the centrifuge for a long time, the dewatered sludge will be drier, the supernatant will be turbid and the treatment capacity will be relatively low; If the differential speed is high, the sludge will stay in the centrifuge for a short time, and the dewatered sludge will be wetter, but the treatment capacity is also high; At the same time, the sludge dried by the centrifuge is pushed out by the screw in time, and will not return to the supernatant due to the long residence time. The solid recovery rate is also greatly increased, and the supernatant is clear. However, if the differential speed is too large, the greater the relative movement between the drum and the spiral will increase the disturbance to the liquid ring layer, and the sludge separated from the solid ring layer will be flooded to the liquid ring layer again, and may be lost with the separation liquid (the supernatant is turbid).

If the solid content of the supernatant is large, it indicates that the dry solid load of the differential centrifuge is large, so the differential speed should be increased accordingly. After the differential speed is increased, reduce the residence time of the sludge in the centrifugal concentration, and quickly push the dehydrated dry sludge out of the centrifuge, so that it has no chance to return to the liquid phase, which will increase the output of dry sludge and reduce the solid content of the supernatant. Some types of equipment have the function of automatically accelerating slag discharge, that is, when the set torque reaches a certain limit value, the equipment will automatically reduce the amount of mud and medicine, increase the differential speed, and quickly push out the accumulated mud ring layer. After the torque is reduced to a certain value, the flow and differential will automatically return to normal. Therefore, the differential speed should be determined according to the material properties, processing capacity, processing requirements and centrifuge structural parameters. In other words, the best processing capacity and the differential value range required by the processing effect shall be found on the site according to the situation, so as to achieve the highest processing capacity as far as possible under the condition of meeting the degree of mud biscuit. The differential adjustment range is generally changed by increasing or decreasing by 1 each time, and the range should not be too large.

In short, there is a contradiction between processing capacity and processing effect. To improve processing capacity, it is necessary to increase the differential ratio, but it may reduce the degree of mud biscuit; In order to improve the degree of sludge biscuit, it is necessary to reduce the degree of difference, so as to reduce the treatment capacity. Therefore, the on-site commissioning work is to find the best equipment operating condition parameters that meet the actual sludge property conditions on site, so as to achieve the dual purpose of maximum equipment operating efficiency and best treatment effect. There is no simple data to calculate. We can only rely on long-term actual debugging to accumulate experience and adjust in time according to changes.

2.6 aspect ratio

When the drum speed is constant, the greater the length diameter ratio, the greater the processing capacity, the longer the relative settlement time of solid materials in the centrifuge drum, the lower the moisture content in the solid materials of the banking office, and the better the separation effect; On the contrary, the worse the separation effect.

2.7 comprehensive adjustment of centrifuge operating conditions

The adjustment principle of centrifuge is to adjust the differential speed and another working parameter (flocculant dosage or sludge inflow) when one parameter (sludge inflow or flocculant dosage) is fixed. During the specific operation, at the initial stage, add 6 grams of PAM per kilogram of dry sludge. On the premise of keeping the dosage of flocculant unchanged, adjust the speed of the sludge screw pump and feed the sludge according to the maximum flow, and gradually increase the differential speed from low to high until the supernatant is completely clear. If the differential speed has been increased to more than 7 R / min, the suspended matter content of the supernatant is still high, It indicates that the sludge inflow has exceeded the maximum dry solid load of the centrifuge. At this time, gradually reduce the sludge inflow according to the gradient of 100%, 90%, 80%, 70%, 60%, 50% and 40% until a good supernatant night appears; After the above adjustment, after the centrifuge runs stably, gradually reduce the dosage of flocculant until good solid content of mud cake and supernatant can be obtained with the minimum dosage of PAM.

3. Problems encountered in unit operation

(1) The sludge and drainage are not smooth, causing the separated sludge and water to collude with each other between the drum and the casing. Due to the high-speed rotation of the drum, the mud and water separated by the horizontal screw centrifuge are also "sprayed" outward from the sludge discharge port and discharge port at a relatively high flow rate. Due to the gap between the drum and the casing, the sludge discharge port and the drainage port are connected. If the sludge discharge and drainage are not smooth, it will cause the mud and water separated by the centrifuge to "collude" with each other, making the mud thin or mud in the water, which will seriously affect the separation effect. In particular, the sludge discharge is not smooth, and the mud is blocked between the drum and the casing, which will overload the main motor and make the centrifugal unit unable to operate normally. Therefore, the sludge discharge equipment and drainage system of the centrifuge must have sufficient capacity to ensure the normal operation of the centrifuge.

(2) When the sludge contains organic particles with lower density than water, the centrifugal force generated by its high-speed rotation is also small. These organic particles cannot be deposited on the drum wall, but can only be suspended in water and discharged out of the machine with water. Horizontal screw centrifugal sludge dehydrator cannot separate organic sludge particles with low density.

(3) Sludge should not be flocculated in advance. At the feed inlet of the centrifuge, the sludge and flocculant enter the drum cavity at the same time, flocculate instantaneously, and quickly separate the sludge and water through the action of centrifugal force. If flocculant is added into the sludge in advance, it will flocculate before entering the drum cavity to form large flocs. After entering the centrifuge, the flocs will be broken, making it difficult for the mud and water to be separated and the separation effect worse.

4. Others

4.1 is there a thickener for sludge moisture content and dosage

In the case of integrated concentration, the dosage is slightly less, because in the concentration process of the concentration tank, the dosage needs to be added. On the whole, the design of integrated concentration is lower than that in the case of setting secondary sedimentation tank concentration tank.

4.2 sludge moisture content

For different sludge, the moisture content is generally different after centrifugal dehydration. Generally speaking, due to the large amount of inorganic substances such as sediment in the sludge of the primary sedimentation tank, the moisture content is generally 70% after centrifugal dehydration; The sludge of secondary sedimentation tank contains more organic components, and the moisture content after dehydration is generally 80%.

For sludge containing more organic components, such as sludge from secondary sedimentation tank, it is difficult to reduce the moisture content to less than 80% after dosing and dehydration by centrifuge.

The principle of the rigid dehydration machine under study is to further reduce the water content of the sludge after centrifugal dehydration by heating, and the water content can be reduced to 60%.

Water content distribution of sludge in secondary sedimentation tank:

(1) Interstitial water, accounting for 70%, can be concentrated and separated by gravity.

(2) Capillary water, accounting for 20%, applies centrifugal force and negative pressure to destroy the tension and cohesion of capillary surface.

(3) Surface adsorbed water, accounting for 7%, coagulates and flocculates with each other through colloidal particles to remove the water attached to the surface.

(4) Internal water, accounting for 3%, exists in the interior of sludge particles or microbial cells. Biological method is used to destroy the cell membrane to remove the water in the cells, or high-temperature heating method is used.