Basic Instructions For Vacuum Filtration
Vacuum filter is the equipment for solid and liquid separation, which is formed by the pressure difference between the upstream and downstream sides of the filtration medium. The commonly used vacuum degree of a vacuum filter is 0.05~0.08MPa, but there are more than 0.09MPa. Vacuum filters can be intermittent and continuous two types. The continuous vacuum filter and the continuous vacuum filter have their own characteristics, but the continuous vacuum filter is more widely used. The common continuous vacuum filter has a drum type and rotary disc type.
The advantages of vacuum filter are as follows:
(1) Small labor intensity;
(2) Can directly observe the filtering situation, timely find out the problem, easy to check;
(3) Maintenance costs are low;
(4) The continuous vacuum filter has high working efficiency.
The disadvantages of vacuum filter are as follows:
(1) Cannot filter the material with low boiling point filtrate;
(2) The vacuum system needs regular maintenance;
(3) If the solid particle concentration and particle size distribution in the feed slurry change greatly in the filtration process of continuous vacuum filter, the filtration effect is poor.
Rotary vacuum filter
Its main body is a rotating horizontal cylinder, called a rotating drum, with a diameter of 0.3~4.5m and a length of 3~ 6m. The outer surface of the cylinder is composed of a porous plate or a special drainage member covered with a press cloth. The inside of the cylinder is divided into a number of fan-shaped compartments, each of which has a straw connected with a hole in the hollow shaft, and the hole in the hollow shaft leads axially to a rotating disk located at the end of the shaft and rotating with the shaft.
The rotating disc fits closely with the fixing disc to form a special rotating valve called the distribution head, as shown in figure (c) on the right. The fixed disk of the distribution head is divided into several arc-shaped voids, which are connected with the pressure relief tube, the washing liquid storage tank, and the compressed air pipeline respectively.
When the drum rotates, the fan can obtain vacuum and pressure respectively by using the function of the distribution head, so that the filtration, washing, and other operations can be controlled step by step.
Advantages of rotary vacuum filter:
(1) Continuous production, the higher degree of mechanization;
(2) According to the nature of the material, process requirements, using different materials to manufacture various types, in order to meet different filtration requirements (usually, for the suspension of medium particle size, viscosity is not too big material, rotary drum vacuum filter is applicable);
(3) Filter cake thickness and washing effect can be controlled by adjusting rotating drum speed; The loss of press cloth is smaller than that of other types of filters.
Disadvantages of rotary drum vacuum filter:
(1) Filter force is small, it is only the use of vacuum as the driving force. Due to the pipeline resistance loss, the maximum is no more than 80kPa, generally 26.7-66.7kpa, so it is not easy to drain, the final humidity of the filter cake is generally more than 20%;
(2) Equipment processing and manufacturing complex, the main equipment and auxiliary vacuum equipment investment is expensive, high energy consumption in the vacuum, at the same time, the larger the filter area manufacturing more difficult. The maximum filtration area of domestic production is about 50 square meters, generally 5 ~ 40 square meters.
Rotary vacuum filter
And rotating with the axis of the filter disk (or rotary disk). The structure and operating principle of the rotary disk vacuum filter are similar to that of a rotary drum vacuum filter. Each sector of the disk has its own outlet pipe leading to the central axis, and when a number of disks are joined together, the outlet of the sector of a rotary disk forms a continuous channel with the corresponding outlet of the other rotary plates in the same phase Angle.
Similar to a rotary drum vacuum filter, these continuous channels are connected to a shaft end rotary valve (distribution head). The operation of the rotary disk vacuum filter is very similar to that of a rotary drum vacuum filter. Each turntable is equivalent to a drum, and the operating cycle is also controlled by a rotary valve. Each rotary table has its own filter cake unloading device, but unloading is more difficult.
Rotary vacuum filter machine has a very large filtration area, can be as large as 85 square meters, its unit filtration area covers less land, a press cloth is easy to replace, less consumption, energy consumption is low. Its disadvantage is poor washing of filter cake, washing water and suspension are easy to mix in the filter tank.
The basic flow
Vacuum filter is at the side of the filter medium caused a certain degree of negative pressure (vacuum) and make the filtrate discharge of solid-liquid separation, thus the driving force is small, generally from 0.04 to 0.06 MPa, in some cases, up to 0.08 MPa, due to the low pressure drop on either side of the filter cake, thus filtering speed slower, superfine material of the filter cake moisture content is higher, the vacuum filter is one of its main deficiencies; However, its advantage is that it can operate continuously under relatively simple mechanical conditions, and in most cases can obtain satisfactory performance indicators.
Therefore, compared with other types of filters (such as filter press), the vacuum filter has been favored by users for a long time. Unless the filter press achieves a major breakthrough in continuous operation, mechanical properties, manufacturing, and operating costs, and can meet the comprehensive requirements of the application sector, vacuum filtration will be difficult to shake its important position in the solid-liquid separation field.
The working cycle of the vacuum filter can be generally divided into the following stages:
(1) Dehydration stage;
(2) Washing stage,
(3) Compaction stage;
(4) Drying stage;
(5) Cake unloading stage. Among them, washing, compaction, drying, and other stages are determined in disregard of the actual needs, while cake making, dehydration, and cake unloading are the basic working processes of most vacuum filters (in the filtration cycle of horizontal belt vacuum filters, the cake unloading stage can be excluded).In the filtering cycle, the timeshare of each operation varies with the filter.
Filter cake formation stage strictly processes (filter in terms of its significance is refers to the filtrate through continuous media the process of the separation of the solid materials), in the process of solid material by vacuum effect, lower feed or vacuum and upper (feed) by the joint action of gravity and adsorption on the surface of the filter medium, gradually formed a certain thickness of the filter cake; With the gradual thickening of the filter cake, the corresponding filtration resistance also increases gradually, so the filtration speed (that is, the filtrate volume per unit area of filter media per unit time) in this stage gradually decreases.
However, the main task of this phase is to form a certain thickness of filter cake to achieve the desired processing capacity. After the filter cake is formed, it leaves the feed tank and enters the dehydration stage. In this stage, the relative saturation (that is, the ratio of the water volume of the filter cake to the total pore volume of the filter cake) of the filter cake decreases from the original 100% to 10%~20%. Under vacuum suction, most of the pores occupied by water are replaced by air.