Filtration principle

Fundamental Principles of Filtration

Filtration

Under the action of external force, the liquid in suspension passes through the porous channel, and the solid particles in suspension are intercepted on the medium, thus realizing the operation of solid-liquid separation.

Explanation: (1) Porous medium is called filter medium; suspension treated is called filter slurry; solid particles intercepted by filter medium in filter slurry are called filter cake or filter residue; liquid after filter medium is called filter liquid.

(2) The driving forces that drive the liquid through the filter media can be gravity, pressure (or differential pressure) and centrifugal force.

(3) The purpose of filtration may be to obtain clean liquid products or solid products.

(4) The function of washing: recovering the residual filtrate or removing the soluble salt in the filter cake.

Filter media

The filter medium plays the role of supporting the cake and allowing the filtrate to pass through. Its basic requirement is that it has enough mechanical strength and minimum flow resistance, at the same time, it should also have the corresponding corrosion resistance and heat resistance. Common filter media in industry:

Fabric medium: also known as filter cloth, is a fabric made of natural fibers such as cotton, wool, silk, hemp and synthetic fibers, as well as a net made of glass or metal wire. The minimum diameter of particles that can be intercepted by this kind of medium is 1. Fabric media are widely used in industry.

(2) Accumulation medium: It is composed of various solid particles (sand, wood carbon, asbestos, diatomite) or non-textile fibers, which are mostly used in deep bed filtration.

(3) Porous solid media: solid materials with many micro-channels, such as porous ceramics, porous plastics and porous metal tubes or plates, can intercept fine particles.

(4) Porous membranes: various organic polymer membranes and inorganic material membranes for membrane filtration. Cellulose acetate and aromatic amide are two kinds of organic polymer membranes widely used. It can be used to intercept the following small particles.

3. Deep Filtration and Cake Filtration

(1) Cake filtration: The size of particles in suspension is larger than that of the channel of medium. When the suspension is placed on the side of the filter medium, at the beginning of the filtration operation, some small particles will enter the medium channel, and may pass through the channel without being intercepted, so that the filtrate is still turbid. As the process progresses, particles gradually accumulate on the medium, forming a granular layer, called filter cake. After the formation of the filter cake, it becomes the main interception medium for the subsequent particles. Therefore, the ever-increasing filter cake is the real effective filter medium, and the liquid passing through the filter cake becomes the clarified liquid.

(2) Deep filtration: At this time, the particle size is much smaller than that of the medium channel, and the particle is easy to enter the medium channel. However, due to the elongation and bending of the channel, particles flow through the zigzag channel and adhere to the wall of the channel under the action of surface force and electrostatic force. Therefore, filter cake is not formed on the medium during deep filtration, and solid particles are deposited inside the filter medium. This kind of filtration is suitable for treating suspension with very little solid particle content.

Compressibility of filter cake and filter aid

The compressibility of filter cake refers to the phenomenon that the void fraction of filter cake decreases obviously after being compressed. It makes the filtration resistance increase obviously when the filtration pressure increases. The larger the filtration pressure, the more serious the situation will be.

In addition, the particles in the suspension are very fine. When these fine particles enter the channel of the medium at the beginning of filtration, the channel will be blocked. Even if they are not serious enough, the cake formed by these very fine particles has poor permeability to the liquid, that is, the resistance is high, which makes the filtration difficult.

In order to solve these two problems, filter aids are often used in industrial filtration.