(4.2)Screening

By stratification

By freefall

Basic processes taking place during screening

the vibration of the screen

The process of the particle stratification and probability are caused by ______________

By stratification

screening is carried out by building up a material bed on a screen deck

the motion of the screen reduces the internal friction in the material

the material gets stratified when __________

Vibration passes through the bed

coarse material to rise, finer material to descendB

By free fall

double inclination is used from 10-15 to 20-30

during free fall

no particle layer can build up on the screen deck and the particles are sized directly via screening media, giving higher capacity during _________

Screening Performance Parameters

measured by screen efficiency:

motion

inclination

screening media

Cumulative weight of material that has passed the screen

Most commonly used measure of screen efficiency

Screen Performance variables

Capacity (tons/hr)

Effectiveness (efficiency)

Capacity (tons/hr)

measured by the mass of material that acn be fed per unit time to a unit area of screen

Effectiveness (efficiency)

measure of success of the screen in closely separating undersize and oversize materials

inversely

efficiency varies with loading ______

Screen Performance Prediction

Particles in dry bulk materials are found in variety of shape size surface densities moisture content

need to predict performance, through its effect on capacity

screen performance and screenability

the combined effects on ________________ of particle shape —> mosture are beyond the reach of empirical solutions

size and density

the only variable that screen performance and screenability is based on

Woven wire screen

the most commonly available screen media (made in carbon steel, stainless, or other metal alloys)

may be square or rectangular

capacity, efficiency, and cost

the media affects the performance of a screen in terms of ____________

Industrial screens

made from woven wire, silk, or plastic cloth, metal bars, perforated or slotted metal plates

single inclination stratification screens

80% worldwide being used

circular 15deg or linear 0-5deg

Double inclination screens

free fall compact screens with high capacity

typical in circuit screening

Triple inclination screens

combine capacity and selectivity

typical control screen for advance product fractions

Multiple inclination screens

Banana screens, effective thin-layer screens

popular in coal and metallic ore mining

Selection of screening media

equally important as the screen sizing selection

¼ of maximum feed size

minimum thickness of the screen panel is that it should be _________

Thicker panel

reduced capacity, accuracy, and service life

increased blinding tendency

Types of screen panels

Tension mats with hooks

Bolt down panels

Wire mesh panels

Self-supporting panels

Modular systems

Tension mats with hooks

Fits all screens designed with cambered decks and tensioning rails

Bolt down panels

Pre-tensioned for easy installation and guaranteed screening performance

Wire mesh panels

Offers superior open area and are quickly available

Self-supporting panels

Used for screens of open frame design for tough applications

Modular systems

Provide flexibility in wear material/hole configuration combinations

Square hole in screen panels

Standard choice

Round hole in screen panels

improved service life and coarse screening

Rectangle aligned to length hole in screen panels

improved capacity

Rectangle aligned to width hole in screen panels

improved accuracy and dewatering

Types of screening

Dry screening

Wet Screening

Dry Screening

treatment of a material containing natural amount of moisture or a material that has been dried before screening

Wet Screening

An operation where water is added to the material being treated for the purpose of washing the fine material through the screen

Relative motion

Screening requires _________relative motion between the sieve and the particle mass

Velocity

determines volumetric flow rate of the particle mass over the sieve

intended to enhance both flow and passage of undersize through the sieve

Inclined Screen motion

The vibration lifts the particles up, causing stratification

will cascade the particles down the slope and introduce the probability of particle passage through the screen

Horizontal Screen Motion

Used when height restrictions prohibit inclined screens

lifts particles up from the screen surface and moves the particles toward the discharge end

Forms of motion (dependent on screen design)

Circular in horizontal plane

gyratory (spiral) with rocking oscillation

Oscillating in a straight line, simple harmonic

Vibrating circularly in a vertical plane

Vibrating linear pitching motion on horizontal sieve

vibrating only in the vertical direction