14.1. INTRODUCTION AND CLASSIFICATION OF MECHANICAL–PHYSICAL SEPARATION PROCESSES
14.1A. Introduction
In Chapters 10 and 11, gas–liquid and vapor–liquid separation processes were considered. These separation processes depended on molecules diffusing or vaporizing from one distinct phase to another phase. In Chapter 12, liquid–liquid separation processes were discussed, in which the two liquid phases are quite different chemically, leading to separation on a molecular scale according to physical–chemical properties. Also in Chapter 12, we considered liquid–solid leaching, ion-exchange, and adsorption separation processes. Again, differences in the physical–chemical properties of the molecules lead to separation on a molecular scale. In Chapter 13, we discussed membrane separation processes, where the separation also depends on physical–chemical properties.
All the separation processes considered so far have been based upon physical–chemical differences in the molecules themselves and on mass transfer of the molecules. In this way individual molecules are separated into two phases because of these molecular differences. In the present chapter, a group of separation processes will be considered where the separation is not accomplished on a molecular scale nor is it due to the differences among the various molecules. The separation will be accomplished using mechanical–physical forces and not molecular or chemical forces and diffusion. These mechanical–physical forces will be acting on particles, liquids, or mixtures of particles and liquids themselves and not necessarily on the individual molecules.
These mechanical–physical forces include gravitational and centrifugal, actual mechanical, and kinetic forces arising from flow. Particles and/or fluid streams are separated because of the different effects produced on them by these forces.
14.1B. Classification of Mechanical–Physical Separation Processes
These mechanical–physical separation processes are considered under the following classifications:
1. Filtration
The general problem of the separation of solid particles from liquids can be solved by using a wide variety of methods, depending on the type of solids, the proportion of solid to liquid in the mixture, viscosity of the solution, and other factors. In filtration, a pressure difference is set up that causes the fluid to flow through small holes in a screen or cloth which block the passage of the large solid particles; these, in turn, build up on the cloth as a porous cake.
2. Settling and sedimentation
In settling and sedimentation, the particles are separated from the fluid by gravitational forces acting on particles of various sizes and densities.
3. Centrifugal settling and sedimentation
In centrifugal separations, the particles are separated from the fluid by centrifugal forces acting on particles of various sizes and densities. Two general types of separation processes are used. In the first type of process, centrifugal settling or sedimentation occurs.
4. Centrifugal filtration
In the second type of centrifugal separation process, centrifugal filtration occurs, which is similar to ordinary filtration, where a bed or cake of solids builds up on a screen, but centrifugal force is used to cause the flow instead of a pressure difference.
5. Mechanical size reduction and separation
In mechanical size reduction, the solid particles are broken mechanically into smaller particles and separated according to size.