5.2: SCREENING NOOR MUHAMMAD SYAHRIN BIN YAHYA

Transcription

1 5.2: SCREENING NOOR MUHAMMAD SYAHRIN BIN YAHYA

2 INTRODUCTION Screening is a method of separating particles according to size alone. In industrial screening the solid are dropped on, or thrown against, a screening surface. The undersize or fines, pass through the screen opening; oversize or tails, do not. A single screen can make but a single separation into two fractions. These are called unsized fractions, because although either the upper or lower limit of the particle sizes they contain is known, the other limit is unknown.

3 INTRODUCTION Material passed through a series of screen of different sizes is separated into sized fraction, i.e. fractions in which both the maximum and minimum particle sizes are known, Screening is occasionally done wet but much more commonly dry. Industrial screen are made from woven wire, silk or plastic cloth, metal bars, perforated or slotted metal plates, or wires that are wedge shaped in cross section. Various metals are used, with steel and stainless steel the most common. Standard screen range in mesh size from 4 in to 400mesh and woven metal screen with opening as small as 1um are commercially available.

4 5.2.1: FACTORS AFFECTING SCREEN PERFORMANCE Particles in dry bulk materials are found in a variety of shapes, sizes, surfaces, densities, and moisture content. Each condition must be taken into account when attempting to predict screen performance, through its effect on capacity in terms of weight passing a given screen opening per unit area. The combined effects on screen performance, or screen ability, of particle shape, surface texture, and surface or internal moisture, are beyond the reach of empirical solutions based only on size and density, independent of these variables. More exact information on their influence has to be gained from actual laboratory testing.

5 5.2.1: FACTORS AFFECTING SCREEN PERFORMANCE 1. MATERIAL FACTOR I. SIZE AND SHAPE II. DENSITY III. MOISTURE IV. SIZE DISTRIBUTION

6 i. SIZE AND SHAPE The shape of an individual granule may be: 1. Angular 2. Spherical 3. Acicular 4. Ovaloid 5. Flaky 6. Slabby They can be mixed in the same material, as sawdust in wood flakes. Separation cut point sizes in most screening applications range downward from 4 to 325 mesh (.0018 ). The cut point defines the minimum particle size retained on the screen, and the maximum undersize particle passing. Unless the particle is acicular, platy, ovaloid or a perfect sphere, it will probably (but not necessarily) be sized by its largest dimension.

7 ii. DENSITY For any given shape and size distribution, bulk density in lb./cu. ft. (PCF) for any material will be directly proportional to its specific gravity. Screening is essentially a volumetric measurement, but capacity, or the rate of passage through the screen, is typically charted in units of weight per unit time, based on a standard bulk density of 100 PCF. The actual rate for a material of different bulk density then has to be adjusted by the ratio PCF:100. Tables of bulk density for various materials can be found in most material handling publications. *PCF = Pound Cubic Feet

8 iii. MOISTURE Moisture in granular particles may be absorbed, adsorbed, or both. Either condition can impair screen ability, but tolerance is much greater for internally absorbed than for external surface moisture. Surface moisture causes particles to stick together, resisting stratification. Allowable surface moisture for unimpaired dry screening of inorganic granular or pelletized particles ranges from bone dry for screen openings below 20 mesh, to 3% for 1/4 openings. Absorbed moisture in permeable soils such as ground clay can block the screen openings with cumulative build ups of extreme fines attached to the screen wires. Absorbent grains such as corn, soybeans, wheat etc. will screen freely after drying to 13-15% internal moisture. Screening of wood chips, flakes and sawdust is unimpaired up to about 30% internal moisture; however, in laboratory tests with sawdust, efficiency was reduced by almost 60% when moisture was increased to 68%.

9 iv. SIZE DISTRIBUTION The size distribution of particles in a granular bulk material is the primary characteristic that governs the rate of undersize passage through a screen opening that is larger than the smallest particle and smaller than the largest particle in a representative sample of the material. Size distribution is measured by sieve analysis, using a series of standardized wire mesh sieves with square openings that progress, in the commonly used Tyler standard scale, at the fixed rate of from 1.05 to.0029 (200 mesh). The size distribution is expressed as the weight percent of each fraction between successive sieves in a series. If the weight is plotted on the y-axis against the mean size of each fraction on the x -axis, the result will resemble a frequency distribution curve.

10 LOG PROBABILITY GRAPH COORDINATES Communited limestone size distribution

11 5.2.1: FACTORS AFFECTING SCREEN PERFORMANCE 2. MACHINE FACTORS I. THE SCREENING MEDIA II. MOTION I. MOTION IN THE HORIZONTAL PLANE (SHAKING SCREENS) II. MOTION IN THE VERTICAL PLANE (VIBRATING SCREENS)

12 i: THE SCREENING MEDIA There are many varieties of screening media. The most common, available in carbon steel, stainless or other metal alloys, is woven wire screen, made with openings that may be either square or rectangular. Others include profile bars, perforated plates, polyurethane and rubber. The importance of making the best selection of media for any screening application cannot be overstated. In any screening machine, the media will affect performance in terms of capacity, efficiency and cost. Manufacturers of screening equipment will offer their recommendations. Much has been written on the subject, but often the best results are achieved through trial and error.

13 ii: MOTION Screening requires relative motion between the sieve and the particle mass. In a few specialized cases the sieve is stationary, but in most commercial screening applications, the particle mass flows over a sieve to which some kind of motion is mechanically applied. It s velocity determines the volumetric flow rate of the particle mass over the sieve, whose motion is intended to enhance both the flow and the passage of undersize through the sieve. This motion takes several different forms, depending on the design of the screening machine. It may be circular in the horizontal plane; gyratory, with a vertical rocking oscillation superimposed on the circular motion; oscillating in a straight-line, simple harmonic motion; vibrating with a circular motion in the vertical plane; vibrating with a linear pitching motion on a horizontal sieve having both vertical and horizontal components; or vibrating only in the vertical direction. In each case, the surface is sloped as required to obtain the desired mass flow, usually at velocities between 40 and 100 fpm.

14 PICTURE OF SCREENING MACHINES

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