Department of Chemical EngineeringUniversity of Engineering & Technology

Peshawar, Pakistan

Many natural and manufactured materials occur in a disperse form, which means that they consist of differently shaped and sized particles. The particle size distribution, i.e. the number of particles of different sizes, is responsible for important physical and chemical properties such as:

mechanical bulk behavior

surface reaction

taste

miscibility

filtration properties

conductivity

This list could be continued at great length. The examples clearly show how important it is to have a knowledge of the particle distribution, particularly within the context of quality assurance in the production of bulk goods. If the particle distribution changes during the manufacturing process then the quality of the finished product will also change. Only a continuous monitoring of the particle size distribution can guarantee a constant product quality.

There are different methods for determining the particle distribution. The choice of a particular method depends primarily on the dispersion status, i.e. on the degree of fineness of the sample.

The oldest and best-known method is particle size determination by sieve analysis. The particle size distribution is defined via the mass or volume. Sieve analysis is used to divide the particulate material into size fractions and then to determine the weight of these fractions. In this way a relatively broad particle size spectrum can be analyzed quickly and reliably.

Screening is a method of separating particles according to size alonescreens are used on a large scale for the separationof particles according to their sizes, and on a smallscale for the production of closely graded materials incarrying out size analysesThe method is applicable for particles of a size assmall as about 50 μm

Undersize:

fines, pass through the screen openings

Oversize: tails:

Particles which do not pass through screen openings

Industrial screens are made from woven wire, silk or

plastic cloth, metal bars, perforated or slotted metal

plates.

Various metals are used, with steel and stainless steel

the most common.

Standard screens range in mesh size from 4 in. to 400-

mesh

A single screen can separate feed into two fractions

These are called un-sized fractions

Material passed through a series of screens of different sizes is separated into sized fractions, i.e. fractions in which both the maximum and minimum particle sizes are known

Sieve passage

The sieve passage is the fine material that passes through the sieve during a sieve analysis.

Sieve residue

The sieve residue is the coarse material that remains on the sieve after a sieve analysis.

Sieving loss

The sieving loss is the difference in weight between the original sample and the sum of the recovered fractions. According to DIN 66 165 part 1 it should not exceed 1% of the original sample weight.

Relative open sieve area

The relative open sieve area is the sum of all the open mesh areas of the sieve relative to the total area of the sieve.

Available open sieve area

The available open sieve area is the sum of the open mesh areas of the sieve that have not been blocked.

Cut sharpness

The cut sharpness characterizes the quality of the sieving into individual fractions.

Equivalent diameter

In sieving the equivalent diameter of a particle is the diameter of a sphere with the same mass or volume.

Undersize

Mass fraction below a defined cut point.

Oversize

Mass fraction above a defined cut point

Mesh

the number of openings in one inch of screen (The number of openings is the mesh size. So a 4-mesh screenmeans there are four little square =s across one linear inch of screen

Cutting diameter Dpc: marks the point ofseparation, usually Dpc is chosen to be themesh opening of the screen.

Actual screens do not give a perfectseparation about the cutting diameter. Theundersize can contain certain amount ofmaterial coarser than Dpc, and the oversizecan contain certain amount of material that issmaller than Dpc.

Volume-surface mean diameter, defined by

If the number of particles in each fraction Ni is known, then

where xi = mass fraction in a given increment, = average diameter, taken as arithmetic average of the smallest and largest particle diameters in incremen

If the number of particles in each fraction Ni is known, then

If NT = number of particles in the entire sample then

Arithmetic mean diameter is given by

Mass mean diameter

Volume mean diameter

The volume of any particle is proportional to its "diameter" cubed.

a = volume shape factorAssuming that a is independent of size and is inverse of spherecity

Stationary screens and grizzlies

Mechanically Vibrating screens

Gyrating screens

Centrifugal Screens

A grizzly has a plane screening surfacecomposed of longitudinal bars up to 3 m long,fixed in a rectangular framework

It is usually inclined at an angle to the horizontaland the greater the angle then the greater is thethroughput although the screening efficiency isreduced

Used for very coarse feed,as from a primary crusherThe spacing between thebars is 2 to 8 in. (50 –200 mm)

Stationary inclined woven-metal screensoperate in the same way like grizzlies

Separating particles 0.5 to 4 in. (12 to 100mm)

Effective only with very coarse free-flowingsolids containing few fine particles

Mechanically operated screens are vibrated by means

of an electromagnetic device or mechanically

In the former case the screen itself is vibrated, and in the latter, the whole assembly

Hummer electromagnetic screen

Tyrock mechanical screen

Because very rapid accelerations and retardationsare produced, the power consumption and thewear on the bearings are high

These screens are sometimes mounted in amulti-deck fashion with the coarsest screen ontop, either horizontally or inclined at angles up to45◦

With the horizontal machine, the vibratorymotion fulfils the additional function of movingthe particles across the screen

There is therefore a tendency for blockage of the apertures by thelarge material and for oversize particles to be forced through.

A very large mechanically operated screen

consists of a slowly rotating perforated cylinder with its axis at aslight angle to the horizontal.

The material to be screened is fed in at the top and graduallymoves down the screen and passes over apertures of graduallyincreasing size, with the result that all the material has to passover the finest screen.

Trommel

Rate of gyration is between 600 and 1800 r/min

Usually gyrated at the feed end in a horizontal plane

The discharge end reciprocates but does not gyrate

This combination stratifies the feed, so that fineparticles travel downward to the screen surface,where they are pushed through by the largerparticles on top

This phenomenon occurs as vibration ispassed through a bed of material. This causescoarse (larger) material to rise and finer(smaller) material to descend within the bed.The material in contact with screen clotheither falls through a slot or blinds the slot orcontacts the cloth material and is thrownfrom the cloth to fall to the next lower level.

Material is fed into the feed inlet andredirected into the cylindrical siftingchamber by means of a feed screw. Rotating, helical paddles within thechamber continuously propel the materialagainst the screen, while the resultant,centrifugal force on the particlesaccelerates them through the apertures. These rotating paddles, which nevermake contact with the screen, also serveto breakup soft agglomerates.Over-sized particles and trash areejected via the oversize discharge spout.

Let F, D, and B be the mass flow rates of feed, overflow, and underflow, respectively,

and xF, xD, and xB be the mass fractions of material A in the streams.

The mass fractions of material B in the feed, overflow, and underflow are 1- xF, 1- xD, and 1- xB.

F = D + B

FxF = DxD + BxB

F = D + B

FxF = DxD + BxB

Elimination of B from the equations gives

Elimination of D gives

A common measure of screen effectiveness is the ratio of oversize material A that is actually in the overflow to the amount of A entering with the feed. These quantities are DxD and FxF respectively. Thus

where EA is the screen effectiveness based on the oversize

Similarly, an effectiveness EB based on the undersize materials is given by

A combined overall effectiveness can be defined as the product of the two individual ratios

The capacity of a screen is measured by the mass of material that can be fed per unit time to a unit area of the screen.

Capacity and effectiveness are opposing factors.

To obtain maximum effectiveness, the capacity must be small,

Large capacity is obtainable only at the expense of a reduction in effectiveness.

A quartz mixture is screened through a 10-mesh screen. The cumulative screen analysisof feed, overflow and underfolw are given inthe table.

Calculate the mass ratios of the overflow andunderflow to feed and the overalleffectiveness of the screen.

Mesh Dp (mm) Feed Overflow Underflow

4 4.699 0 0 0

6 3.327 0.025 0.071 0

8 2.362 0.15 0.43 0

10 1.651 0.47 0.85 0.195

14 1.168 0.73 0.97 0.58

20 0.833 0.885 0.99 0.83

28 0.589 0.94 1.0 0.91

35 0.417 0.96 0.94

65 0.208 0.98 0.975

Pan 1.0 1.0

From the table:xF= 0.47xD= 0.85xB= 0.195

Hummer electromagnetic screen

Tyrock mechanical screen