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Introduction to Solid—Liquid Separation
1 Introduction to Solid-Liquid Separation L. Svarovsky School of Powder Technology, University of Bradford
As the title suggests, solid-liquid separation involves the separation of two phases, solid and liquid, from a suspension. It is used in many processes with the aim of \. recovering the valuable solids (the liquid being discarded); %. recovering the liquid (the solids being discarded); i. recovering both the solids and the liquid; 4. recovering neither (but for example to prevent water pollution). A perfect solid-liquid separation would result in a stream of liquid going one way and dry solids going another. Unfortunately, none of the separation devices work perfectly, all are imperfect in some way or other. Typically (see Figure 1.1) there may be some fine solids leaving in the liquid stream, Suspension
Solids (+ some liquid Figure 1.1. A schematic diagram of a separator
1
2
INTRODUCTION TO SOLID-LIQUID SEPARATION
and some of the liquid may leave with the bulk of the solids, the latter being a somewhat more common problem. This imperfection of separation can be characterized in two ways. The mass fraction of the solids recovered is often called the separation efficiency, and is expressed as a percentage (in filtration this is also known as 'retention') whilst the dryness of the solids recovered may be characterized by the moisture content (% by weight). The concepts of efficiency and dewatering are further studied in chapters 3 and 16. Sometimes, in order to compensate for the fact that the solids stream entrains some liquid, washing is used in order to replace the mother liquor with a wash liquid. In addition to solid-liquid separation it is often desirable to remove either the coarse or the fine particles from the product (de-gritting and de-sliming respectively). This process is referred to as classification or solid-solid separation and can be achieved in many types of solid-liquid separation equipment because of the particle-size-dependent nature of the principles employed in such equipment. Classification may also be made before separa tion in order that the material in each different size range may be treated by the type of equipment best suited to it. 1.1 SOLID-LIQUID SEPARATION PROCESSES Solid-liquid separation processes may be classified according to the principles involved (see Figure 1.2). If the liquid is constrained and particles can move freely within it (due to fields of acceleration) we have sedimentation and flotation. For sedimentation, a density difference between the solids and the liquid is necessary. If particles are constrained by a medium and the liquid can flow through we have filtration and screening, for which a density difference is not necessary. Further sub-division of these two main groups can be seen in Figure 1.2. Most of these processes are dealt with in some depth in this book; only a brief description is included here. 1.1.1 Flotation This process is based on the release of sufficient air from a pressurized liquid when the pressure is reduced to atmospheric. The gas is released as bubbles and these adhere to, or are absorbed into, the solids and transport them to the surface where they can be scraped off. Flotation has been used in the mineral separation field for some time and has become recognized as an effective means of solid-liquid separation in other applications such as paper-making, refineries or sewage treatment. 1.1.2 Gravity sedimentation The sedimentation of solids by means of gravity is employed in settling tanks; these are used either for thickening or for clarification. Thickeners are designed to produce a sludge, generally of the greatest possible density, while clarifiers
i
Flotation (dispersed air, dissolved air, electrolytic)
i Cake filtration (vacuum, pressure, centrifugal)
Figure 1.2. Classification of solid-liquid separation processes
centrifuges)
Centrifugal Gravity sedimentation sedimentation (thickeners, 1 clarifiers) Fixed wall Rotating wall (hydrocyclones) (sedimenting
Liquid constrained, particles free
SOLID-LIQUID SEPARATION
Deep bed filtration (sand and coke)
Particles constrained, liquid free Screening (dewatering, vibrating screens )
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INTRODUCTION TO SOLID-LIQUID SEPARATION
are used to produce an overflow containing a minimum of suspended solids. Flocculating agents are often used to enhance settling. 1.1.3 Centrifugal sedimentation 1.1.3.1 Hydrocyclones These have no rotating parts; a vortex is produced by feeding the suspension through a tangential inlet. High velocity gradients exist in a hydrocyclone thus causing shear; this may cause breakage of possible agglomerates of floes which is not desirable in separation but very suitable for classification. However, hydrocyclones are used extensively for both separation and classifi cation because of their reliability and low cost; in separation their primary use is for thickening. 1.1.3.2 Sedimenting centrifuges These have a bowl through which the suspension flows whilst rotating with the bowl. Lack of shear in the flow makes them most suitable for separation but they are often also used for classification. Of the five types available, the disc centrifuges (nozzle type) and the decanters (scroll type) are fully continuous in operation; the imperforate basket type are semi-continuous but their operation is usually fully automated. The moisture content of the separated solids can be relatively low, particularly with the basket type and also with the decanter type, and high separation efficiencies are usually achieved. 1.1.4 Cake filtration Equipment for cake filtration is commonly divided according to the driving force used for making the fluid flow through the porous medium: vacuum, pressure and centrifugal filters.
1.1.5 Screens and deep bed filters These usually rely on gravity to make the liquid flow through the medium. Screens are often used for dewatering: if multi-deck screens are employed they may also be used for classification. Vibration or some other type of motion is often employed to make sure that sooner or later all the material is introduced to the screen apertures. Deep bed filters (sand, coke etc.) are most frequently used for clarification. A number of considerations have to be taken into account when a decision is to be made about the best process for a given separation application— these are analysed in chapter 18. Detailed knowledge of individual separation equipment and its typical applications is necessary for such a selection, and it is hoped that this book will take the readers some way towards this knowledge.
As the title suggests, solid-liquid separation involves the separation of two phases, solid and liquid, from a suspension. It is used in many processes with the aim of \. recovering the valuable solids (the liquid being discarded); %. recovering the liquid (the solids being discarded); i. recovering both the solids and the liquid; 4. recovering neither (but for example to prevent water pollution). A perfect solid-liquid separation would result in a stream of liquid going one way and dry solids going another. Unfortunately, none of the separation devices work perfectly, all are imperfect in some way or other. Typically (see Figure 1.1) there may be some fine solids leaving in the liquid stream, Suspension
Solids (+ some liquid Figure 1.1. A schematic diagram of a separator
1
2
INTRODUCTION TO SOLID-LIQUID SEPARATION
and some of the liquid may leave with the bulk of the solids, the latter being a somewhat more common problem. This imperfection of separation can be characterized in two ways. The mass fraction of the solids recovered is often called the separation efficiency, and is expressed as a percentage (in filtration this is also known as 'retention') whilst the dryness of the solids recovered may be characterized by the moisture content (% by weight). The concepts of efficiency and dewatering are further studied in chapters 3 and 16. Sometimes, in order to compensate for the fact that the solids stream entrains some liquid, washing is used in order to replace the mother liquor with a wash liquid. In addition to solid-liquid separation it is often desirable to remove either the coarse or the fine particles from the product (de-gritting and de-sliming respectively). This process is referred to as classification or solid-solid separation and can be achieved in many types of solid-liquid separation equipment because of the particle-size-dependent nature of the principles employed in such equipment. Classification may also be made before separa tion in order that the material in each different size range may be treated by the type of equipment best suited to it. 1.1 SOLID-LIQUID SEPARATION PROCESSES Solid-liquid separation processes may be classified according to the principles involved (see Figure 1.2). If the liquid is constrained and particles can move freely within it (due to fields of acceleration) we have sedimentation and flotation. For sedimentation, a density difference between the solids and the liquid is necessary. If particles are constrained by a medium and the liquid can flow through we have filtration and screening, for which a density difference is not necessary. Further sub-division of these two main groups can be seen in Figure 1.2. Most of these processes are dealt with in some depth in this book; only a brief description is included here. 1.1.1 Flotation This process is based on the release of sufficient air from a pressurized liquid when the pressure is reduced to atmospheric. The gas is released as bubbles and these adhere to, or are absorbed into, the solids and transport them to the surface where they can be scraped off. Flotation has been used in the mineral separation field for some time and has become recognized as an effective means of solid-liquid separation in other applications such as paper-making, refineries or sewage treatment. 1.1.2 Gravity sedimentation The sedimentation of solids by means of gravity is employed in settling tanks; these are used either for thickening or for clarification. Thickeners are designed to produce a sludge, generally of the greatest possible density, while clarifiers
i
Flotation (dispersed air, dissolved air, electrolytic)
i Cake filtration (vacuum, pressure, centrifugal)
Figure 1.2. Classification of solid-liquid separation processes
centrifuges)
Centrifugal Gravity sedimentation sedimentation (thickeners, 1 clarifiers) Fixed wall Rotating wall (hydrocyclones) (sedimenting
Liquid constrained, particles free
SOLID-LIQUID SEPARATION
Deep bed filtration (sand and coke)
Particles constrained, liquid free Screening (dewatering, vibrating screens )
4
INTRODUCTION TO SOLID-LIQUID SEPARATION
are used to produce an overflow containing a minimum of suspended solids. Flocculating agents are often used to enhance settling. 1.1.3 Centrifugal sedimentation 1.1.3.1 Hydrocyclones These have no rotating parts; a vortex is produced by feeding the suspension through a tangential inlet. High velocity gradients exist in a hydrocyclone thus causing shear; this may cause breakage of possible agglomerates of floes which is not desirable in separation but very suitable for classification. However, hydrocyclones are used extensively for both separation and classifi cation because of their reliability and low cost; in separation their primary use is for thickening. 1.1.3.2 Sedimenting centrifuges These have a bowl through which the suspension flows whilst rotating with the bowl. Lack of shear in the flow makes them most suitable for separation but they are often also used for classification. Of the five types available, the disc centrifuges (nozzle type) and the decanters (scroll type) are fully continuous in operation; the imperforate basket type are semi-continuous but their operation is usually fully automated. The moisture content of the separated solids can be relatively low, particularly with the basket type and also with the decanter type, and high separation efficiencies are usually achieved. 1.1.4 Cake filtration Equipment for cake filtration is commonly divided according to the driving force used for making the fluid flow through the porous medium: vacuum, pressure and centrifugal filters.
1.1.5 Screens and deep bed filters These usually rely on gravity to make the liquid flow through the medium. Screens are often used for dewatering: if multi-deck screens are employed they may also be used for classification. Vibration or some other type of motion is often employed to make sure that sooner or later all the material is introduced to the screen apertures. Deep bed filters (sand, coke etc.) are most frequently used for clarification. A number of considerations have to be taken into account when a decision is to be made about the best process for a given separation application— these are analysed in chapter 18. Detailed knowledge of individual separation equipment and its typical applications is necessary for such a selection, and it is hoped that this book will take the readers some way towards this knowledge.