Separation of particles from liquids

In addition to vacuum and pressure, centrifugal forces are also used to increase driving force in separation of particles from liquids. 

Notwithstanding the fact that the filter fabric is used to effect the maximum separation of particles from liquids, absolute clarity is not always necessary. In certain gravity-or vacuum-assisted screening operations, the filter fabric is simply designed to capture particles greater than a specific size and in other filtration systems, a measure of solids in filtrate can be tolerated before cake filtration takes over and the necessary clarity is achieved. Recirculation of the slurry may also be possible in some applications until the same condition prevails. From this it will be appreciated that in some cases the solids are the more valuable component in the slurry whereas in others the process is concerned with clarification of the liquid, the solids thereafter being of little or no value. 

Centrifugation Imposition of larger gravitational forces aids separation of particles from liquid. Important if density difference small and/or particles small Widely used, particularly in pharmaceutical industry. Crystals often concentrated in centrifuges, for example, para-xylene and sugar... 

Consider a thin layer solid bowl centrifuge as shown in Figure 4.20. In this device, particles are flung to the wall of the vessel by centrifugal force while liquor either remains stationary in batch operation or overflows a weir in continuous operation. Separation of solid from liquid will be a function of several quantities including particle and fluid densities, particle size, flowrate of slurry, and machine size and design (speed, diameter, separation distance, etc.). A relationship between them can be derived using the transport equations that were derived in Chapter 3, as follows. 

The separation of solids from liquids forms an important part of almost all front-end and back-end operations in hydrometallurgy. This is due to several reasons, including removal of the gangue or unleached fraction from the leached liquor the need for clarified liquors for ion exchange, solvent extraction, precipitation or other appropriate processing and the post-precipitation or post-crystallization recovery of valuable solids. Solid-liquid separation is influenced by many factors such as the concentration of the suspended solids the particle size distribution the composition the strength and clarity of the leach liquor and the methods of precipitation used. Some important points of the common methods of solid-liquid separation have been dealt with in Chapter 2. 

FILTRATION. Filtration is the separation of two phases, particulate form. i.e.. sulid particles or liquid droplets, and continuous, i.c.. liquid or gas, from a mixture by passing the mixture through a porous medium. This article discusses Ihe more predominant separation of solids from liquids. 

In the filtration process, a liquid containing suspended solids is passed through a porous medium. The solids are trapped against the medium, and the separation of solids from liquids results. For large solid particles, a thick barrier such as sand may be used for smaller particles, a fine filter such as a filter cloth is preferable. Fluid passage may be induced by gravity, positive pressure, or a r acuum. A few of the more popular filter fypes are the plate and frame filter press, and shell and leaf and cartridge filters. 

Practical operation of a sludge separator, of course, requires that the solids be heavier than the liquid and not be resuspended by the action of the conveyor. A modification known as an axial-flow conveyor centrifuge (described later) is more effective in separating fine light solids than the full-scroll centrifuge shown in Fig. 30.44. Even so, the liquid effluent from these machines is usually not completely free from solids and may require subsequent clarification. Within these restrictions sludge separators solve a wide variety of problems. They separate fine particles from liquids, dewater and wash free-draining crystals, and are often used as classifiers. 

It was foimd that the degree of energy dissipation caused by viscous flow in the liquid interlayer approaches 100% at sufficiently small cone angle, which provides high rate of t.p.c. extension. Under these conditions, separation of particle from the bubble surface is prevented. The attachment is provided by first collision of particles with large diameter, i.e. 200pm. 

Filtering is a term for a process of separation of particles from the carrying liquid as the suspension passes through the permeable material called filter. The basic difference from process of filtration is in high permeability of the porous material (filter). A porous granulated material or fiber material can serve as a filter. The separated substance can form a solid deposit at a filter s surface or in the internal pores of material. Particles with sizes smaller than filter s pores, deposit on the surface of granules of which the filter consists, due to forces of hydrodynamic, molecular, and electrostatic interaction. Other kinds of interactions are also possible. 

For the purpose of gravimetric analysis, where it is necessary to effect an efficient separation of solid from liquid, it is generally accepted that precipitation should be carried out slowly from dilute solution. However, some substances, such as the hydroxides and basic salts of aluminium, iron and tin, demand extremely high dilutions and excessively long times for dense particles to be produced. The method known as precipitation from homogeneous solution (PFHS) allows coarse precipitates to be produced in relatively short times (Gordon, Salutsky and Willard, 1959). 

The field of filtration in its widest sense covers the capture of particles ranging in size from several millimetres down to the molecular scale. Table 71 provides an indication of the various particle size ranges, and where textile filter media fit into this picture. The industrial filtration processes on which the chapter will concentrate are separated into two broad sections, namely, the separation of solids from gases, more commonly referred to as dust collection, and the separation of solids from liquids, usually referred to as liquid filtration. 

Svarovsky, L. 2000. Efficiency of separation of particles from fluids. In Solid-Liquid Separation, Svarovsky, L. (ed.). London Butterworths-Heinemann. 

Filtration may be defined as the separation of solids from liquids by passing a suspension through a permeable medium which retains the particles. A filtration system can be shown schematically as in Figure 9.1. 

Thirdly, there are those chapters which only needed minor updating and amendments. These include Characterization of Particles Suspended in Liquids, Efficiency of Separation of Particles from Fluids, Hydrocyclones, Separation by Centrifugal Sedimentation, Filtration Fundamentals, Methods for Limiting Cake Growth, Pressure Filtration, Particle-Huid Interaction, Thermodynamics of Solid-Liquid Separation. 

Centrifugation. Separation of solids from liquids under the influence of centrifugal force is called centrifugation. The liquid containing various particles of different densities are rotated at a very high speed in a closed system, where these get settled at the base according to their densities. This is commonly used as a preliminary step before the use of an additional recycling method. 

Instead of considering only the separation of particles from one another, consider now the separation of all particles (species 2) from the fluid (species 1) feed or the elimination of liquid from the solid-rich underflow (region 2). Instead of the segregation fractions defined by (2.4.8), we will have... 

We will first provide a very brief illustration of the governing equations for mass transport and the operating line for a two-phase continuous cocurrent separation system in a conventional chemical engineering context. This will be followed by a brief treatment of the multi-component separation capability of such a system. Cocurrent chromatographic separation in a two-phase system, where both phases are mobile and in cocurrent flow, will be introduced next. The systems of interest are micellar electrokinetic chromatography (MEKC) chromatography with two mobile phases, a gas phase and a liquid phase capillary electrochromatography, with mobile nanoparticles in the mobile liquid phase. Continuous separation of particles from a gas phase to a cocurrent liquid phase in a scrubber will then be illustrated. Finally, cocurrent membrane separators will be introduced. 

The separation of solids from liquids requires a crisp, long, horizontal motion to shear the feed stream and quickly pass the liquid through the screen. A minimum amount of vertical motion is used to convey solids out of the separator. Dry separation, on the other hand, normally requires precisely the opposite types of motion - minimum horizontal motion to spread the material over the saeen in a controlled pattern, and a strong vertical motion to stratify the product so that particles can quickly pass through the screen openings. 

Many mechanical separations involve the movement of solid particles or liquid drops through a fluid. The fluid may be gas or liquid, and it may be flowing or static. As some assortment of examples mention may be made of the removal of dust and fumes from air or flue gas, the removal of solids from liquid wastes to permit discharge into public domain, and of the recovery of acid mists from an acid plant gaseous wastes. 

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