Liquid-solid separation involves the separation of two phases, solid and liquid, from a suspension. It is used in many processes for the: 1. recovery of valuable solid component (the liquid being discarded); 2. liquid recovery (the solids being discarded); 3. recovery of both solid and liquid; or 4. recovery of neither phase (e.g., when a liquid is being cleaned prior to discharge, as in the prevention of water pollution).
Any separation system design must consider all stages of pre-treatment, solids concentration, solids separation, and post-treatment. This encompasses a wide range of equipment and processes, summarized in Table 1. Pre-treatment is used primarily with difficult-to-filter slurries, enabling them to be filtered more easily. It usually involves changing the nature of the suspended solids by either chemical or physical means, or by adding a solid (filter aid) to the suspension to act as a bulking agent to increase the permeability of the cake formed during subsequent filtration. In solids concentration, part of the liquid may be removed by (gravity or centrifugal) thickening or hydrocycloning to reduce liquid volume throughput load on the filter.
A number of new 'assisted separation' techniques are making their way into the list of technical alternatives — these utilize magnetic, electrical or sonic force fields (or combinations) to provide more effective separation.
Solids separation involves a filter, types of which are classified in many different ways. For present purposes a division into those in which cakes are formed and those in which the particles are captured in the depth of the medium is adequate. Cake filters can be further divided into pressure, vacuum, centrifugal and gravity operations.
Post-treatment processes involve making improvements to the quality of the solid or liquid products. In the case of the filtrate, these operations are often referred to as polishing processes, and may involve micro or ultrafilters to remove finer substances. Further purification may involve removal of ionic and macromolecular species by, for example, reverse osmosis, ion exchange or electrodialysis. The relative position of these separation processes in the spectrum of the size of "particle" to be removed from the liquid is shown in Figure 1. Cake post-treatment processes include washing soluble impurities from the cake voids and removal of excess liquid from the voids. Thermal drying is often the final stage of liquid removal.
Figure 1. General techniques for contamination removal from liquids relative to the size of the species to be removed.
Solid/liquid separation is all too often designed as a 'stands alone' unit in a plant flowsheet. The performance of a solid/liquid separation device is sensitive to the history of the feed solution and, in particular, to the properties imparted to the suspension by its method of manufacture, e.g., on the shape, size and size distribution of the particles, which result from the operating conditions in the precipitator or crystallizer. A change in particle production conditions can affect the best choice of filter for a particular purpose. The economics and viability of producing the product is often affected by the amount of liquid removed in the post-treatment processes. For example, if the cake is to be transported, briquetted or pelletized, cake moisture content will need to be within a specified range; or if a bone dry product is required the thermal load on the dryers can be reduced by correct choice and operation of filter.
It is important, therefore, to consider simultaneously and in some detail those processes which are to feed suspension to the solid/liquid separations plant, and the subsequent processing of the solid or liquid products.
Three parameter types may be identified to fully describe a solid/liquid system. These are:
state of the system;
Primary properties are those which can be measured independently of the other components of the system; specifically, they are the solid and liquid physical properties, the size, size distribution and shape of the particles, and the surface properties of the particles in their solution environment. The way the particle interacts with its surrounding fluid becomes important for smaller particles (notably when the particle size <10−20 μm), since repulsive (surface) forces between the particles can become as significant as gravitationally- or hydrodynamically-induced forces. These factors decide whether the particles will, for example, settle slowly or quickly; whether they can be retained on some kind of septum or porous medium; or whether the resulting cake will be dry or sloppy.
The description of the state of the system (porosity or concentration, and the homogeneity and extent of dispersion of the particles) combines with primary properties to control the macroscopic properties, which are measured to investigate the application of a particular separation method. Such measurements may be the permeability or specific resistance of the filter bed or filter cake, the terminal settling velocity of the particles, or the bulk settling rate of the suspension.
Liquid particles almost always approximate a sphere due to the distribution of surface forces around the particle. In contrast, solid particles are rarely either spherical or uniform. Certain classes of materials are essentially crystalline and may be made up from fairly uniform particles, each of which is, for example, cubic or rhombohedral. But even crystalline materials may be a mixture of shapes, especially if, as often happens industrially, breakage of the crystals occurs due to handling. Indeed, breakage can be caused within the separator itself and is a common problem in, for example, pusher centrifuges. The great majority of particles are of irregular shape: fibrous particles are common, but they may possess a wide range of length to diameter ratios; they may have smooth surfaces; they may be fibrillated, and so on. It is rare that the shape of the particles to be handled can be defined precisely.
Particles may vary in size from very fine or colloidal matter to coarse granular solids. Sometimes all the solids may be of the same material, i.e., of homogeneous composition or, as is often the case with effluent suspensions, the individual particles may have very different compositions. In general terms particle size has a significant effect on solid/liquid separation behavior of the suspension. A knowledge of techniques for measuring size particles is therefore important to the process technologist.
In solid/liquid separation four reasons for measuring particles size can be identified [Scarlett and Ward (1986)]:
To measure and specify the quality of a liquid, which is the valuable product from a filtration process. In this case, the particles remaining in suspension are dilute in concentration, and are therefore difficult to filter. Often only the total concentration of solids is required as, for example, in water treatment processes. However, in operations such as the filtration of parental fluids or hydraulic fluids, the size (and occasionally shape) of the remaining solids is critical.
An extension of this requirement is to specify the performance of a filter medium in terms of its ability to retain particles of different sizes. This type of evaluation is usually associated with fluid polishing operations and the specification of a nominal pore size for a polishing medium, or with the performance assessment of separating devices such as sedimenting centrifuges.
In many operations the solid is the valuable product. It is rarely recovered in a completely dry state and is often processed further. Evaluation of this product is required for quality control and is not connected solely with the separation process. In this case, the method of evaluation is often dictated by the customer or by the standards accepted by the particular industry.
Occasionally, the requirement is to evaluate the solid in order to predict their probable behavior in a separation process. This may be to enable an initial choice between different separation methods; to select or test a suitable pretreatment process or filter medium; to improve the efficiency of an existing machine, or to estimate the size of a new one. In any of these, the objective is predictive in nature, and the measuring technique must be selected more carefully than for a quality control application. The measured size, for design purposes, should relate to the way the particles are handled in the process; but this is not essential for quality control purposes.
Interactions between the particle and the liquid in which it is suspended have the greatest influence when particles are smaller, particularly when their size is smaller than a few microns. Origins of interparticle repulsive forces lie in the distribution of solution ions around the charged surface of the particle, and the resultant electrical charge is dependent on the chemical species present at the surface. A potential energy of repulsion may extend appreciable distances from the particle surface, but its range may be compressed by increasing the electrolyte content of the solution. For practical purposes, the magnitude of net repulsive force between particles is represented by the zeta (ζ-) potential; and the following statements can be made about the influence of the ζ-potential in solid/ liquid separation [Wakeman et al. (1988)]:
The net repulsive force increases with increasing magnitude of the ζ-potential.
Reducing the magnitude of the repulsive force causes the dispersion to become unstable, and generally more easily separated.
Repulsive forces can be reduced by either (1) adding a non-adsorbing electrolyte to the liquid to change the distribution of solution ions around the particle, or (2) altering the electrical charge on the surface of the particle by the specific adsorption of certain ions.
Around the isoelectric point of the suspension, the process engineer can expect:
faster settling rates;
more rapid filter cake formation; and
slightly higher moisture content cakes and sediments
due to the aggregation of particles in the suspension, where interparticle repulsion forces are very small. At the maximum or minimum z-potential, the engineer can expect:
slower settling rates;
slower cake formation rates, and
slightly lower moisture content cakes and sediments
due to the existence of greater repulsive forces, which maintain the particles better dispersed throughout the liquid phase.
At pH's beyond those at which maximum or minimum z-potentials first occur, the process engineer can expect intermediate settling, filtration and expression rates and intermediate cake and sediment moisture contents. This is due to compression of the electrical double layer around the particle caused by high ionic strengths in the solution.
Apart from the effect of the fluid at the fluid/particle interface, the density and viscosity of the fluid are most important in industrial filtration. Density is generally only significant where separation depends on a difference in density between the fluid and the particles, e.g., in thickeners or centrifugal sedimenters. Whatever difference in density exists must usually be accepted and cannot often be controlled to any significant degree; occasionally the influence of temperature may be important, or it may be possible to alter the density to a very limited extent by varying the amount of dissolved matter.
Viscosity has a more widespread effect and, at the same time, is more amenable to control since it is usually sensitive to temperature changes. The rate of filtration of liquids can be greatly accelerated in many instances by a relatively small increase in temperature, which causes a drop in viscosity.
There are many solid/liquid separation techniques which have established practical value for general application in the processing industries, and there are a few which are in their early stages of industrial exploitation. The dividing line between these two categories is arguable, particularly as the field is noted for innovation and rapid development. The area of solid/liquid separation techniques is broad; the following list indicates the diversity of equipment available to the engineer:
Gravity settlers, e.g., clarifiers, deep thickeners, lamella separators, settling tanks, lagoons, thickeners;
Sedimenting centrifuges, e.g., tubular bowl, skimmer pipe, disc, scroll discharge;
Hydrocyclones, e.g., conical, circulating bed;
Classifiers, e.g., hydraulic, mechanical, screens, sieve bends;
Gravity filters, e.g., deep bed, Nutsche;
Line filters, e.g., cartridges, strainers;
Pressure filters, e.g., continuous pressure, diatomaceous earth, fibre bed, filter press, horizontal element, pressure Nutsche, vertical element, sand, sheet filter, tubular element;
Filters with compression, e.g., belt press, membrane plate and frame, screw press, variable volume filter (e.g., tube);
Vacuum filters, e.g., top/bottom fed drum, disc, leaf, belt, pan, table, precoat drum;
Filter thickeners or crossflow filters, e.g., delayed cake, dynamic or high shear microfilters, low shear microfilters/ ultrafilters;
Filtering centrifuges, e.g., basket, pendulum, oscillating, tumbling, plough/peeler, pusher, worm screen;
Magnetic filters, e.g., low gradient (drum, grid or belt), high gradient.
Computer software is available [Tarleton and Wakeman (1991)] to aid in the selection of appropriate equipment for specific solid/ liquid separation problems.
Compressibility of particulate structures is a key factor in the behavior of all solid/liquid separation equipment. Particulate aggregation in suspensions determines the degree of compressibility, which is controlled by suitable pretreatment processes. As increasing thicknesses of deposits cover the separation surfaces, developing stresses continually compress the particulate bed. The principal sources of stress are (1) the unbuoyed weight in gravity thickeners, (2) centrifugal forces, (3) pump pressure converted into Darcian drag at the particle surfaces, and (4) surface forces developed by pressure-actuated impermeable membranes.
When stress is applied to a bed of flocculated particles, the bed is compressed by particle movement into open pores until no further movement into the interstices can occur. Any further deformation of the bed results in particle deformation or breakage. The porosity of the compressed floc is a function of the initial structure and particle shape — spherical particles generally have a compressed packing density (solidosity) of about 0.65. The difference between the initial solidosity resulting from flocculation and 0.65 represents the measure of compressibility that is used for correlation, design and scale-up purposes.
In sedimentation processes the solids are subjected to low compressive pressures of the order of 0.1 to 0.2 bar. At the other extreme involving high pressure expression the effective pressure can be up to hundreds of atmospheres. At low pressures, compression arises from squeezing of particles into unoccupied voids whilst at high pressures, particle deformation and crushing (where the coefficients of elasticity and ultimate strength are important) determine cake compaction. Separations equipment can be categorized according to the typical effective pressure involved in the process, as shown by Figure 2.
Figure 3. Relations between null stress volume fraction of solid and particle size, shape and degree of flocculation.
Particle size, size distribution, shape and degree of flocculation determine the solids packing density in solid/liquid mixtures, and hence the compressibility of the mixture. This is shown schematically in Figure 3. Particles larger than about 20 μm form beds which are essentially incompressible, the solidosity of which depends primarily on particle shape. Irregular particles form beds with larger porosities than those associated with spheres. Filter aids such as diatomaceous earths and expanded perlite are so irregular that they form beds whose solidosities range from 0.1 to 0.2, even though the primary particle sizes may be below 10 μm.
Stresses developed in the matrix of large particles during separation processes do not generally reach sufficient magnitude to disturb the structure.
As the characteristic dimension of the particle decreases, the effects ofinterparticle forces increase relative to gravitational force. When attractive London-van der Waals forces predominate in comparison with electrostatic and gravitational forces, the particles tend to form aggregates. As the number of particles grows in an aggregate (or in a polymer-flocculated system) in suspension, internal porosity of the overall suspension increases. Although a highly-porous floc is distorted upon deposition, the unstressed solidosity of the cake or sediment may be as low as 0.1. On the right-hand side of Figure 3, beds formed from large particles are incompressible and have a packing density dependent on particle shape. On the left of the diagram, the solids volume fraction depends mainly on the degree on aggregation (or the state of dispersion) of the suspension.
Scarlett, B. and Ward, A. S. (1986)In Solid/Liquid Separation Equipment Scale-up, Eds. D. B.
Purchas and R. J. Wakeman. Uplands Press & Filtration Specialists.
Wakeman, R. J., Thuraisingham, S. T., and Tarleton, E. S. (1988) Proc. 391st EFCE Event Particle Technology in Relation to Solid/Liquid Separation. Technologisch Instituut-Koninklijke Vlaamse Ingenieursvereniging — The Filtration Society. Antwerp.
Tiller, F. M. and Yeh, C. S. (1986) In Advances in Solid/Liquid Separation. (Ed. H. S. Muralidhara). Battelle Press.
Tarleton, E. S. and Wakeman, R. J. (1991) Solid/liquid separation equipment simulation and design: pc-SELECT — Personal computer software for the analysis of filtration and sedimentation experimental data and the selection of solid/liquid separation equipment. Separation Technology Associates. Exeter.