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SOLID-TO-SOLID SEPARATION: Screening Mechanism

Fundamental function of screening:

• Pass the undersize particles through the screen and reject the oversize particles

• Particles must be brought to the screen openings at such a velocity and direction that the undersize particles is not affected by the edges or walls of the openings.

Stratification

• Lets the large particles (oversize) rise to the top of the material bed while the smaller particles sift through the voids and find a way to the bottom of the screen surface (bed) to reach the screen apertures (openings)

• Factors affecting stratification

  • MATERIAL TRAVEL FLOW

    • A function of material stratification, bed thickness, and screen slope.

  • STROKE CHARACTERISTICS

    • Such as amplitude, direction, rotation, type of motion, and frequency

  • SURFACE PARTICLE MOISTURE

    • High surface moisture content makes stratification difficult

  
  

Separation Probability

• Pertains to how frequent undersize particles pass through the screen and how frequent oversize particles retain in the screen

• It is a function of the particle size and the screen opening.

  • Ideal particle size for screens

    • Particles that have dimensions of 1.5 times the screen opening (w) are less important for the screening result as these will not all pass through

    • Particles that have lesser than the 0.5 size of the screen opening have lesser importance since they pass through easily

Screen Surfaces

1. Parallel Bars/rods

   1. Usually made of steel bars, discarded rails, or cast iron bars

   2. Ideal screening surface for primary screening

2. Punched Plate

   1. Made of metal or plastic sheets punched by dies of various patterns

   2. Circular, square, hexagonal, slot-like

3. Woven wires

   1. Used in finer screens 

   2. Woven wires are crimped so as to avoid distortion upon impact of feed.

   3. Woven wires have two sub-types depending on purpose:

      1. Rectangular Opening

         1. These are ideal for materials that have needle-like shapes

      2. Square Opening

         1. Offers distributed loads throughout the screen allowing for coarser work

   4. Must be

      1. Strong enough to carry its load

      2. Wear and abrasion resistant (carbon steel and alloy steel)

      3. Corrosion resistant (iron, bronze, copper, monel, stainless steel)

      4. Economical

Screening Equipment

1. Grizzly Screens

   1. widely used for screening large percentage of coarse particles with sizes 20 to 300 mm and more 

   2. Can be stationary at an angle (20-50) or moving

2. Vibrating Screens

   1. widely used screen equipment

   2. standard equipment for screening coarse, medium, and fine materials

   3. high capacity and efficiency

3. Gyratory Screens

   1. Consists of a series of decks with its own discharge spout (up to four decks)

   2. Discharge Spout

      1. Undersize Particles

         1. discharged through gravity at the bottom of the screen

      2. Oversize Particles

         1. discharged through centrifugal force generated by the gyratory vibration into the outer end of the screen

4. Trommel Screens

   1. frames may include: cylindrical, conical, or prismatic frames surrounded by woven wires or punch plate screens that acts as the screening surface

   2. Usually open at both ends at a slight angle to initiate material flow

   3. Arrangement

      1. Single Trommel with Different Screen Opening Sizes

      2. Trommels in Series wherein the first one have the largest screen aperture size

      3. Trommels in Series: wherein the first one have the smallest screen aperture

      4. Compound Trommels: concentric trommels with the same shaft, different screen aperture size

5. Banana Screens

   1. Recent innovations in the 21st century

   2. Key Feature: Changing slope sections (declinations) increases material flow

   3. Steeper Slope: high material flow rates and rapid fines removal

   4. Lower Slope Angles: reduced material flow rates higher efficiency

   5. Stages in a Banana Screen

      1.