Mechanical Separation

A group of laboratory and production operations whereby the components of a polyphase mixture are separated by mechanical methods into two or more fractions of different mechanical characteristics. The separated fractions may be homogeneous or heterogeneous, particulate or nonparticulate.

 

Types of mechanical separator
Materials separated	Separators
Liquid from liquid	Settling tanks, liquid cyclones, centrifugal decanters,
	coalescers
Gas from liquid	Still tanks, deaerators, foam breakers
Liquid from gas	Settling chambers, cyclones, electrostatic precipitators,
	impingement separators
Solid from liquid	Filters, centrifugal filters, clarifiers, thickeners,
	sedimentation centrifuges, liquid cyclones, wet
	screens, magnetic separators
Liquid from solid	Presses, centrifugal extractors
Solid from gas	Settling chambers, air filters, bag filters, cyclones,
	impingement separators, electrostatic and high-
	tension precipitators
Solid from solid
By size	Screens, air and wet classifiers, centrifugal classifiers
By other	Air and wet classifiers, centrifugal classifiers, jigs, tables,
characteristics	spiral concentrators, flotation cells, dense-medium
	separators, magnetic separators, electrostatic
	separators

 

The techniques of mechanical separation are based on differences in phase density, in phase fluidity, and in such mechanical properties of particles-as size, shape, and density; and on such particle characteristics as wettability, surface charge, and magnetic susceptibility. Obviously, such techniques are applicable only to the separation of phases in a heterogeneous mixture. They may be applied, however, to all kinds of mixtures containing two or more phases, whether they are liquid-liquid, liquid-gas, liquid-solid, gas-solid, solid-solid, or gas-liquid-solid.

 

Methods of mechanical separations fall into four general classes:

(1) those employing a selective barrier such as a screen or filter cloth;

(2) those depending on difference in phase density alone (hydrostatic separators);

(3) those depending on fluid and particle mechanics;

(4) those depending on surface or electrical characteristics of particles.

 

A wide variety of separation devices have been devised and are in use. The more important kinds of equipment are listed in the table, grouped according to the phases involved.

Leaching

 

This is the process of removal of a soluble fraction, in the form of a solution, from an insoluble, permeable solid with which it is associated. The separation usually involves selective dissolving, with or without diffusion, but in the extreme case of simple washing it consists merely of the displacement (with some mixing) of one interstitial liquid by another with which it is miscible. The soluble constituent may be solid (as the metal leached from ore) or liquid (as the oil leached from soybeans).

 

Leaching is closely related to solvent extraction, in which a soluble substance is dissolved from one liquid by a second liquid immiscible with the first. Both leaching and solvent extraction are often called extraction. Because of its variety of applications and its importance to several ancient industries, leaching is known by a number of other names: solid-liquid extraction, lixiviation, percolation, infusion, washing, and decantation-settling. The liquid used to leach away the soluble material (the solute) is termed the solvent. The resulting solution is called the extract or sometimes the miscella.

 

Leaching processes fall into two principal classes: those in which the leaching is accomplished by percolation (seeping of solvent through a bed of solids), and those in which particulate solids are dispersed into the extracting liquid and subsequently separated from it. In either case, the operation may be a batch process or continuous.