Screening

SCREENING pp. 19-18 to 19-13

Screening • Separates a mixture of various sizes of particles into two or more portions, each of which is more uniform in size of particle than the original mixture – Dry screening – the treatment of a material containing a natural amount of moisture or a material that has been dried before screening – Wet screening – an operation when water is added to the material being treated for the purpose of washing the fine material through the screen.

Importance of screening • Means of preparing a product for sale or subsequent operation • Means of analysis, either to control or gauge the effectiveness of another operation such as crushing or grinding • To determine the value of a product for some specific application

Methods of reporting particle size distribution • Oversize: 20 mesh (or ¼ in) → +20 (+¼ in) • Through 10 mesh on 20 mesh → -10+20 or 10/20 • Undersize: 200 mesh → -200

Screen analyses • Screens – used for control of particle size analytical work. They are constructed of wire mesh cloth, the diameter and the spacing of the wires are specified • Aperture – clear spaces between individual wires of the screens • Mesh – number of apertures per linear inch; i.e., 10 mesh: 10 openings per inch and aperture of 0.1 inch minus the diameter of wire • Screen interval – relationship between successive sizes of screen openings in a series

Types of screens • Tyler standard sieve screens – series of screens based on a 200-mesh screen with wire diameter 0.0021 in. thick and with an opening of 0.0029 in. Other sizes vary by a fixed ratio of . The mesh is up to 400 mesh. Table 19-6 list properties of Tyler screens. • United States Sieve Series – introduced by National Bureau of Standards differ slightly from Tyler series being based on a 1-mm opening and varying . • International Test Sieve Series – similar to the other two but the wires have different gauges.

Methods of Reporting Screen Analyses • Differential screen analysis or fractional plot

• Cumulative analysis or cumulative plot

Screen Effectiveness

Industrial Screening Equipment • • • • • • •

Grizzlies Stationary screens Vibrated screens Oscillating screens Reciprocating screens Trommels or Revolving Screens Gyratory Screens

Testing Sieve shakers • Ro-Tap testing sieve – equipped with 1-13 screens at a time • End-Strak Testing Sieve Shaker • Dynamic Sieve Shaker

Capacity of screens • Indicates how much material a screen can handle • Expressions:

Problem 1 It is desired to separate a mixture of sugar crystals into two fractions, a coarse fraction retained on an 8-mesh screen, and a fine fraction passing through it. Screen analysis of feed, coarse and fine fractions show Mass fraction of +8 particles in feed = 0.46 Mass fraction of +8 particles in coarse fraction = 0.88 Mass fraction of +8 particles in fine fraction = 0.32

Calculate the overall effectiveness of the screen used for the separation purpose per 100 kg of feed.

OMB: 100=R+P 100(0.46)=R(0.88)+P(0.32) R=25 kg P=75 kg

Problem • If the total percentage of particles larger that the screen opening in the feed, product, and undersize are 36%, 89% and 3%, respectively, calculate the effectiveness of the screen.

Problem • Limestone is crushed by six units operating in parallel and the products separated by six 35-mesh screens also in parallel, into two fractions. The effective dimensions of each screen is 6' by 20'. The common undersize from the screen comes out at the rate of 50 tons/hour. Assume no losses.

calculate the efficiency and the capacity of each screen

Problem • Fine silica is fed at 1500 lbs/hour to a double-deck vibrating screen combination to obtain a 48/65 mesh (Tyler) product. The silica feed is introduced into the upper screen, of the 48 mesh and the product is discharged off the surface of the lower screen of 65 mesh. During the screening operation, the ratio of oversize to product to undersize is 2: 1.5 :1 . Laboratory analysis of the different fractions:

• Calculate the effectiveness of the screening equipment and if the screens measure 5 ft x 8 ft each, calculate the capacity in MT/day-ft2mm of the 65 mesh screen on the basis of a perfectly functioning 48 mesh screen.

Desired material : -48+65

Problem • 1800 Ibs of dolomite per hour is produced by crushing and then screening through a 14-mesh screen. The screen analysis is as follows:

• Calculate the total load to the crusher