What Is a Colloidal Suspension?

In some mixtures, the substances distributed in the liquid material are not dissolved, but only dispersed in it. Once the mixture stops oscillating, it will precipitate. This heterogeneous and heterogeneous mixture is called Suspension.

Solid particles dispersed in

(1) Polydispersity of the suspension: The particle size distribution of the solid particles in the suspension determines that it has the distinctive characteristics of colloids and is different from true colloids. Therefore, Williams refers to such suspensions as colloidal-non-colloidal mixed suspensions.

All particles in the suspension, without reducing their size, are subject to diffusional displacement caused by the disordered collision of thermal movement of liquid molecules, also known as

on

Filtration is the operation of treating a suspension with a certain porous substance as the medium. Under the action of external force, the liquid in the suspension passes through the pores of the medium and the solid particles are trapped, thereby achieving solid and liquid separation.

Suspension filtration method

(1) Deep filtration-also known as clarification filtration When the particles contained in the suspension are small and the content is small (the volume of particles in the liquid is <0.1%), a thicker granular bed filter medium ( (For example, a sand layer for tap water purification). Because the size of the particles in the suspension is smaller than the diameter of the pores of the filter medium, when the particles enter the slender and curved pores in the bed with the liquid, they are attached to the walls of the pores by static electricity and molecular forces. No filter cake was formed on the filter media bed. Therefore, this filtering is called deep filtering. Because it is used to obtain a clear liquid from a dilute suspension, it is also called clear filtration, such as purification of tap water and sewage treatment. ^[3]

Conditions: When the suspension contains small particles, but the content is small (the volume of particles in the liquid is <0.1%), it can be filtered with a thick bed filter medium.

(2) Filter cake filtration When the suspension is filtered, the liquid passes through the filter medium and particles are deposited on the surface of the filter medium to form a filter cake. When the particle size is larger than the pore size of the filter medium, a filter cake is formed. However, when the particle size is smaller than the pore size of the filter medium, some particles will enter the pores of the filter medium at the beginning of filtration, and a "bridge phenomenon" will occur quickly, as shown in the right figure. However, a small amount of particles will pass through the filter medium and flow away with the filtrate.

Filter cake filtration

With the gradual accumulation of filter residue, a filter cake layer will form on the filter medium. After that, the filter cake layer becomes an effective filter medium and a clear filtrate is obtained. This type of filtration is called filter cake filtration, and it is suitable for suspensions with a large amount of particles (> 1% of the volume of particles in the liquid).

Conditions: d _p > media pore diameter, a filter cake is formed on the bed; d _p <media pore diameter, but there is a "bridge phenomenon" in the bed.

Before the filter cake layer is completely formed, some particles will flow away with the filtrate. After the filter cake layer is formed into an effective filter medium, a clear liquid is obtained.

Applicable when there are many particles (volume of particles in the liquid> 1%).

Suspension classification

Atmospheric filtration-by position difference;

Pressure filtration-using a pump or compressed air;

Vacuum filtration-vacuum on one side of the medium;

Centrifugal filtration-use centrifugal force as the driving force.

Suspension step

Filtration washing dehumidification unloading.

Suspension filter media

Effect-Allows liquid to pass through while retaining solid particles.

Selection-according to the nature of the liquid (acid, alkali), particle content and size, operation p, T, mechanical strength of the medium, etc.

(1) Weaving media-filter cloth made of cotton, linen, silk, wool, wire mesh filter cloth, can retain 5 ~ 65 m particles.

(2) Accumulated granular media-sand, charcoal, etc., for deep filtration.

(3) Porous mediaPorous plate-like and tubular media that are sintered and formed by powders such as ceramics, plastics, and metals, can trap fine particles of 1 to 3 m.

Suspension filter aid

Premise-fine particles, easy to block the pores of the filter medium; the filter cake formed under the effect of pressure difference, the pores become smaller, the resistance increases, making filtration difficult.

Function-Mixing another solid with a hard texture into the suspension or pre-coating on the filter medium to form a sparse muffin layer and make the filtrate flow smoothly. This consortium is called a filter aid.

5% Common filter aids-diatomaceous earth, perlite, asbestos, charcoal, etc .; the added amount is 0.5% or less.

There are two ways to use filter aids. One is to first formulate the filter aid separately into a suspension to filter it, and form a layer of filter aid on the surface of the filter medium, and then formally filter. The second is to add a filter aid to the suspension and filter together. The filter cake obtained in this way is relatively loose, the compressibility is reduced, and the filtrate is easy to pass. Because the filter residue and the filter aid are not easy to separate, if the purpose of filtration is to recover the filter residue, the filter aid and the suspension cannot be mixed together. The amount of filter aid added is generally less than 0.5% of the mass of the homogeneous particles. ^[3]