SedimentationAssignment 2

Classification of Solid ParticlesSeparation of different materials is based upon their inherent characteristics, physical or

chemical. Chemical exploitation being expensive and more complex is used only for

finishing steps, if required and more stress is laid on physical separation or separation

on the basis of variation in physical behavior of materials. Excessive use of separation

process is observed in mining industry, where separation of minerals from the adhering

material (gangue) is of prime importance to obtain the valuable ore before it goes to

chemical separation to metal or mineral. The exploiting property being density

difference demands the crushing of material to an extent so that individual particles

contains only one constituent. The gangue being less dense rises up and the mineral

settles down in a compatible fluid, liquid or gas.

Physical separations are much influenced by the degree of variation among different

materials on different parameters or physical properties. Operations include sieving

(size of particles), clarification (behavior in moving fluids) or the properties like electrical

or magnetic affiliations. Large particles are separated via screening but since the fine

particles tend to clog the screen apertures, separation via moving fluid is made in to

practice. Commonly used for separating a mixture of two materials, magnetic /

electrostatic separation and froth floatation are the other alternatives.

Classification is the separation of particles on the basis of their terminal velocity i.e.

velocity in a moving fluid, which in terms depends upon the size and density of the

particle. Classification is further divided in to gravity and centrifugal settling processes.

The first one caters larger particles, usually denser than the fluid employed while the

later deals with finest particles in solid-gas or solid-liquid separation.

Gravity Sedimentation EquipmentIt is the partial separation or concentration of suspended solid particles from a liquid by

gravity settling and may be divided into the functional operations of thickening and

clarification. Thickening increases the suspended solids concentration in a flowing

stream, while the clarification, as the name hints, removes a relatively small quantity of

suspended particles to produce a clear effluent.

Gravity Sedimentation applies the action of gravity to settle the solid particles in a gas or

liquid, flowing or at rest. The objectivity of the separation varies from removal of

particles from a stream (like water treatment) to the recovery of particles (like waste

water treatment). Few additives are deliberately suspended in fluids to achieve

optimum separation results. The settling is based upon two periods: initial acceleration

and terminal velocities. The first one being short, usually in order of tenths of a second

or less1 while the later ones can be maintained as long as particles is under treatment.

Continuous operations demands for low fluid velocity and ample settling time. Desired

separation and less labor being the process economics are equilibrated for optimizing

the sedimentation operation.

ThickenersThickeners concentrate the suspended solids via gravity settling and therefore reduce

their concentrations in the upper stocks. Solids are removed from the bottom of the

tank with the same rate as added to the upper part, thus maintaining a steady-state

material balance. The basic components include: the slurry tank, piping feed, feed well

(for tank entry of feed stream) and scraping mechanism (for moving the particles to

overflow launder). Thickeners can be categorized on the basis of operation.

Conventional Thickeners

High Rate Thickeners

1 McCabe Smith, Mechanical Separations, Vol 77, pg 1056

The difference is the use of flocculants to the later one which enhance the efficiency and

decrease the retention time. The more through put from the second type is due to the

use of flocculants, which bridge together the coagulated particles. Flocculants (e.g.

polyelectrolyte) gathers together floc particles in a net bridging from one surface to

another and binding the individual particles into larger flocs that could settle down

under gravity. It is favored by gentle mixing and a fast pace can destroy the flocs

formed. Flocculants work under the principle that a high molecular weight polymer and

can attach itself to many suspended particles creating a low density floc with an increase

in the overall size of suspended material. There’s a critical dosage of each flocculent for

a particular type of particle after which noticeable increase in capacity is observed.

Likewise, there is a maximum dosage quantity over which the efficient response

becomes constant. Flocculants are normally added in the feed line for effective mixing

with particulates.

Thickeners can be designed to produce underflows having a very high apparent

viscosity, permitting disposal of waste slurries at a concentration that avoids segregation

of fines and coarse particles or formation of a free-liquid pond on the surface of the

deposit2.

3

2 Perry’s Chemical Engineering Handbook, Liquid-Solid Operations and Equipment, Section 18, pg. 18-643 http://www.flsmidth.com/en-US/Products/Product+Index/All+Products/Sedimentation/Hi-RateThickeners/Hi-RateThickeners

ClarifiersClarifiers cater feed with dilute suspensions, like industrial water streams for pretreatment.

Water is fed to clarifier pre-decided flow rate, mixed in line with chemicals like ferrous sulfate

and chlorinated water, for enhanced mixing and oxidation of iron from ferrous to ferric.

Chemical (lime and polyelectrolyte) dosage is automatically adjusted according to the feed

water rate. The mixture enters the reaction zone of clarifier and is mixed with recycled sludge

and suspension of lime slurry. Mixing and recycling are ensured by a dual stirrer moving at 2 – 6

rpm. Through high activity of particles in reaction zone, suspended particles are held together to

make flocs and settle down to the bottom of clarifier. A bottom scrapper moving at 0.06 rpm

prevents building up of deposits and scales by conveying the sludge towards the extraction

cone, where it is withdrawn by gravity and recycled in some quantity to the reaction zone. The

main flow from the reaction zone to the upper portion passes to the upper flocculation area and

finally flows in to the outer clarification zone. During the final passage, it goes through the bed

of pre-formed sludge (also called sludge blanket), where it deposits both impurities and

suspended particles. Clarifier has a residence time of approximately 95 minutes and is usually

equipped with several sampling points for testing the concentration of the sludge at different

levels. Clarifier is set to maintain a particular sludge bed height at bottom, on exceeding; the

blow down will automatically start.

Chemical Dosage to a clarifier could be divided into three types: coagulants (ferrous sulfate and

chlorine), flocculants (polyelectrolyte) and softeners (lime). Sometimes, natural iron present in

raw water is used to supply the part of coagulant. When iron salts are used, the best flocs are

formed when the pH value is 10.2 to 10.4. Therefore, if dissolved iron content exceeds 4 to 5

ppm, it is not necessary to add ferrous sulfate. Chlorination may be considered as a coagulant

aid since it reduces many of the organic substances present in water which inhibit floc

formation4. Chlorinated organic compounds are more readily removable by the floc and

therefore, final quality of effluent is lower in organics. Polyelectrolyte is anionic polymer that

attracts the neutralized suspended particles through its positive charge and provides them with

a nucleus to deposit on. This leads to floc formation and settling. Lime reacts with soluble

4 M. Yaqoob Ch. (1987). Process and Operating Manual for Utilities Plant. Mirpur Mathelo: PakSaudi Fertilizers Limited.

hardness molecules and reduces them to insoluble. Lime dose is a function of pH of raw water

and is regularly adjusted.

TABLE 1 CHEMICAL DOSAGE IN A CLARIFIER5

Chemicals % by weight Mass Flow RateFerrous sulfate 25 1200 kg/hrsChlorine 99.5 7.8 kg/hrPolyelectrolyte 0.3 0.6 kg/hrLime 5 675 kg/hrs

Following modifications are achieved to the quality of water in a clarifier:

Turbidity reduction Color and organic matter reduction Lime softening

o Calcium reductiono Magnesium reduction

Alkalinity reduction Partial demineralization Free carbon dioxide reduction (up to zero level) Iron reduction (up to zero level) Silica reduction

TABLE 1 CLARIFIED WATER PARAMETERS5

Parameter QuantityTurbidity Lss than 5 NTUpH 9.9 – 10.2Free chlorine Less than 0.2 ppmIron Less than 0.2 ppmAlkalinity 2p=m

6

5 Osama Hasan, Internship Report (Production Unit), FFC MM, pg 41-426 Osama Hasan, Internship Report (Production Unit), FFC MM, App. III, Page E

Sorting ClassifiersThese devices separate particles by virtue of their difference of densities. It uses a liquid

sorting medium with an intermediate density between the heavier and lighter materials.

This results in the settling of the denser particles and floating of the lighter ones. The

separation is advantageous as it is accomplished due to density difference, being

independent of particle size. It is often termed as Sink-and-Float Method. The process is

used to deal particles heavier than 10 mesh. Selection of liquid medium and specific

gravity are of prime importance and are achieved via mixing different samples and

achieving a desired quality of it.

The other approach, which is less often adopted for this type of gravity sedimentation

equipments, is application of settling methods of different particles based on the density

difference. The prime issue is the settling of the particles with same terminal velocities

and giving a mixture of particulates rather than separation. This technique is applied in

cases, where particulates are just needed to be separated from liquid phase and solid-

solid separation is not the motive behind.

Centrifugal Sedimentation EquipmentTwo types of centrifuges are employed for liquid-solid separation, Sedimenting

Centrifuges and Filtering Centrifuges. The first type demands the density difference

between the two phases while the later employed a filtration medium retain the solid

phase on it.

CyclonesCentrifugal separators for solid-gas separations employ simple

mechanisms and no moving parts. The simple construction is a vertical

cylinder which slopes down to a cone shaped bottom. A tangential

inlet of stream near top and outlet for dust at bottom of the cone is

made. The incoming air containing particles that travels in a spiral path

round the structure and moves down the cylindrical-cone body. The

centrifugal force developed in the vortex tends to move the particles

radially toward the wall and the particles reach the wall slide down

into the cone shaped bottom and then to the collection bin. Cyclones

encounter centrifugal force against a weak gravitational force.

The ratio of the two forces is termed as Separation Factor. The factor decreases with

increase in cyclone diameter and vice versa. Due to more efficient operation by small

cyclones, these are grouped with same entrance and exit points for larger gas flow

handling.

The force on the particle throughout its trajectory in a cyclone varies because of change

in diameter. Physical interpretation being difficulty, empirical relations are more

stressed upon. The efficiency of a cyclone is directly proportional with particle size and

inversely proportional with carrier gas temperature or viscosity.

HydroclonesCyclones when employed for solid-liquid separation are termed as Hydroclones.

Hydrocyclones is another synonym. Feed is introduced at an angle near the top and

follows a spiral path near the vessel wall, forming a strong downward vertex. Large

particles separate to the wall and are dragged downward in the form of slurry. The

liquid returns through an inner vortex and leaves upwards through the central discharge

pipe, called Vortex Finder.

Often used to separate denser particles from a liquid mixture originating at a centrifugal

pump or some other continuous source of pressurized liquid, it employed where lighter

particles are in high quantity and heavier particles settles down easily. They are usually

used in continuous operations. Their applications include:

Pulp and paper mills

Drilling industry

Separate oil from water or vice versa.

Metal working

Food Processing

Mineral processing

Sludge SeparatorsSludge Separators release their discharge downward, with some of liquid in it. The discharge could be dried by application of a drying mechanism on the sludge, coming out from the separators.

A cylindrical bowl with conical end is rotated along horizontal axis. Feed is entered through a stationary axial pipe and clarified liquid flows over the ports in the plate covering the non conical end of the bowl. Solids in the liquid are settled to the inner surface of the bowls.

A helical conveyor is employed which turns slightly slower than the bowl and separates out the solids out of the pond, to the discharge openings in the small end of the cone. Wash liquid may be sprayed as they move up the beach to remove solid impurities. Drained sludge and clarified liquor are thrown out from the bowl into different parts of the casting, from which they leave they leave through suitable openings.

Solids in the slurry should be denser than the liquid and should not suspended by the action of conveying