Diatomite filter aid mainly intercepts the solid impurity particles suspended in the liquid on the surface and channel of the medium through the following three functions, so as to achieve the purpose of solid-liquid separation:
1、 Screening is a kind of surface filtration. When the fluid flows through diatomite, the pore of diatomite is smaller than the particle size of impurity particles, so the impurity particles cannot pass through and are intercepted. This effect is called screening. In fact, the surface of the filter cake can be regarded as a screen surface with equivalent average pore size. When the diameter of solid particles is not less than (or slightly less than) the pore diameter of diatomite, solid particles will be "screened" out of the suspension to play the role of surface filtration.
2、 Depth effect depth effect is the retention effect of deep filtration. In deep filtration, the separation process only occurs in the "interior" of the medium. Some relatively small impurity particles passing through the surface of the filter cake are blocked by the zigzag microporous channels in the diatomite and the finer pores in the filter cake. Such particles are often smaller than the micropores in the diatomite. When the particles hit the wall of the channel, it is possible to separate from the liquid flow, but can it achieve this, Determined by the balance of inertia force and resistance of particles, this interception and screening action is similar in nature, both of which belong to mechanical action. The ability to filter solid particles is basically only related to the relative size and shape of solid particles and pores.
3、 Adsorption adsorption is quite different from the above two filtration mechanisms. In fact, this effect can also be regarded as electrokinetic attraction, which mainly depends on the surface properties of solid particles and diatomite itself. When the particles with small pores in the diatomite collide with the internal surface of the porous diatomite, they are attracted by opposite charges. Another is that the particles attract each other to form chains and adhere to the diatomite. These belong to adsorption, which is more complex than the first two.
It is generally believed that the reason why solid particles smaller than the pore diameter are intercepted is mainly due to:
(1) Intermolecular forces (also known as van der Waals attraction), including permanent dipole effect, induced dipole effect and instantaneous dipole effect;
(2) The existence of zeta potential;
(3) Ion exchange process
From the above three functions, in the process of net pressure filtration of suspension, the loose granular diatomite filter aid is used as the filter medium, mainly to provide as many pores as possible for the filter medium layer, that is, the filter cake, and the spacing layer of the formed pores, so that the suspension passes through the pores of the barrier layer, and the solid impurities suspended in the liquid are intercepted on the surface and channels of the medium, So that the solid and liquid can be separated.
