Screeners

Screening or sieving is a method of separating granular solids according to size. Screens are widely used in nearly every process that handles particulate materials. The picture below shows several flat, gyratory screeners.

(Copyright W. S. Tyler, St. Catharines, ON)

(Copyright Astec, Inc., Chattanooga, TN)

Types Of Screeners

Flat

Flat screens are usually inclined slightly and are categorized by the type of screen motion. The most common types are vibrating and gyrating.

(Copyright Astec, Inc., Chattanooga, TN)

General Information

The screener shown below is a single deck flat screener. The screen surface is held in a casing inclined at an angle typically between 16°-30°, but sometimes as steep as 45° for very fine materials.

(Copyright Astec, Inc., Chattanooga, TN)

The material is fed on the higher end of the deck. As the deck shakes, the material flows across the deck to the lower side. The undersize particles fall through the screen, while the oversize particles are discharged at the lower end.

Equipment Design

Gyratory screeners use a circular motion in the plane of the screen . The rate of gyration is usually between 600-1800 rpm. The screen surfaces, usually rectangular, range in size from 1' by 4' to 5' by 14' for large capacity applications. For increased efficiency, finer screens are gyrated at the feed end and reciprocated at the discharge end. This stratifies the material, causing the smaller particles to contact the screen surface.

When used with certain materials, gyratory screeners are prone to blinding, which reduces the efficiency of the machine. Often, a ball deck is used to prevent blinding. The rubber balls bounce against the screen and release lodged particles.

Vibrating screeners, which operate at 1800-3600 vibrations per minute, are much less likely to blind than gyrating screeners. The deck is usually inclined at an angle of 30-45°, although horizontal operation is also possible. Vibrating screeners have a greater capacity than gyrating ones, especially when separating fine material.

(Copyright W. S. Tyler, St. Catharines, ON)

The decks of most vibratory screeners vibrate in a vertical circle, as shown above. However, other methods of vibration are more effective for certain materials. Vibration is created either mechanically for coarse screening, or electrically for fine screening. The screener shown below is a mechanically vibrated screener.

(Copyright W. S. Tyler, St. Catharines, ON)

Often screeners with multiple decks are used to achieve better separation results. The vibrating screener shown here has two decks, the bottom one with smaller openings. This separates the feed into three fractions.

(Copyright W. S. Tyler, St. Catharines, ON)

Usage Examples

Flat screeners are used in numerous industries, including pharmaceuticals , chemical, food and candy, minerals, and fertilizers. The gyrating screener shown here separates wood chips into three fractions in a manufacturing plant.

(Copyright W. S. Tyler, St. Catharines, ON)

Advantages

Disadvantages

  • Can operate continuously.
  • High capacity.
  • Low maintenance costs.
  • Blinding may reduce efficiency in gyratory screeners.
  • Not effective for moist, sticky, or oddly shaped material.

Trommel

Trommel, or revolving, screens consist of slowly rotating perforated cylinders.

General Information

In a trommel screen, feed enters through a conveyor or hopper into the higher end of a slightly inclined rotating cylindrical screen. The tumbling action of the trommel mixes the material as it flows through the cylinder, so every particle contacts the screen surface. The undersize falls through the screen, while the oversize remains and is discharged at the lower end of the cylinder.

Equipment Design

The key to effective separation with trommel screens is in the cylinder design. If only two fractions are needed, a cylinder with uniform perforations is used.

For three or more fractions, perforations increase in size along the cylinder, allowing multiple fractions to be collected at different points along the screen.

Parallel operation is also possible, with several concentric cylinders rotating inside each other, but construction difficulties limit the practicality of these trommels.

Usage Examples

Trommel screens are often used to gravel and compost. They are also used in food processing and in recycling plants to separate waste and scraps.

Advantages

Disadvantages

  • Effective for wet and dry separations
  • Can wash and classify simultaneously
  • Low efficiency
  • Difficult to produce multiple fractions
  • Low capacity

Centrifugal

Centrifugal screeners operate similar to a centrifuge and allow separation of moist mixtures.

General Information

In a centrifugal screener, material is fed into a basket, with a perforated screen on the outside walls. Similar to a centrifuge, the basket spins, pushing the material to the outside screen. The larger material is left in the basket while the smaller material passes through the holes in the screen. This type of screening improves efficiency and allows for the screening of moist and sticky materials.

(Copyright Chemical Engineering, Access Intelligence, LLC)

Equipment Design

The two key components of this type of screener are the basket, which spins at high speeds, and the spoiler arm, which allows fine particles, 50 mesh and below, to be separated from the feed. Centrifugal screeners can accommodate capacities up to 20,000 lb/h.

Usage Examples

Centrifugal screeners are used to screen moist, sticky, and clumpy materials.

Advantages

Disadvantages

  • Effective for slurried materials
  • Effective for very fine particles
  • Compact
  • Not effective for large, bulky materials

Grizzly

General Information/Equipment Design

Grizzlies, the simplest screeners, have inclined and sometimes vibrating grates for very coarse screening. Often a grid of rotating gratings is used to keep the openings clear of stuck particles. Stationary grizzlies consist of sturdy parallel bars held apart by spacers. Manganese steel is frequently used for the bars because of its resistance to wear.

(Copyright Astec, Inc., Chattanooga, TN)

Usage Examples

Grizzlies are effective at separating particles 2" in diameter and coarser. They are often used before crushers in rock or ore crushing plants to remove fines before the feed enters the crusher. They are also used on top of coal and ore bins to remove unwanted materials.

Advantages

Disadvantages

  • Simple construction
  • Inexpensive to install and maintain
  • Difficult to adjust opening size
  • Ineffective for moist or sticky materials

Sieve Stacks

General Information

Sieves stacks are used to separate solid feed material for particle size analysis and weighing. Sieve stacks are used in the processes of dry and wet sieving, which is one of the most commonly used methods to size particles and has a fairly simple design.

Equipment Design

A typical sieve is a shallow plan with a wire-mesh bottom or an electroformed grid. In both designs, the sieves contain uniform openings that can filter particles as small as 5 micrometers. Sieves are available in various particle openings and the size of the opening determines which sizes of particles are able to pass through. An image of a typical sieve pan is shown below.

Sieve.png

(Copyright W.S. Tyler, Mentor, OH)

Wire-mesh sieves have square openings for particles to pass through. These sieves are used more frequently as they tend to be sturdier and less expensive. Electroformed grids have openings that can be round, square, or rectangular depending on the process need. These sieves are more expensive, and their use is limited to processes that require very fine particle ranges that wire-mesh sieves cannot attain.

Sieves are typically stacked with particle openings of diminishing opening sizes from top to bottom, as shown below. Dry sieving contains a solid feed, typically powder, that enters the sieve stack at the top sieve. The sieves are then shaken, vibrated or pulsed with air. With the help of gravity, solid particles pass through the sieves until they fall onto the sieve with openings they cannot pass through. Finally the material on each sieve is recorded and material with the desired particle size range is retained. Other material can be recycled and re-processed until the correct particle size is achieved.

Sieve Stack.png

(Copyright W.S. Tyler, Mentor, OH)

Wet sieving is similar to dry sieving in that a stack of sieves is used with decreasing particle size openings. However, in wet sieving a non-dissolving liquid is used to help pass particles between the descending sieves. A surfactant can also be added to further promote particle dispersion and passage through the stack.

Usage Examples

Sieves are used heavily in industries involving solids processing ranging from mining and construction to agriculture and pharmaceuticals. In mining processes, sieves are used to assess mineral release from ore crushing. In construction, sieves are vital to assessing the foundation stability of soils, sand and gravel. They are also used in the agricultural industry to measure seed size uniformity. In the pharmaceutical industry, sieving is used to achieve uniform particle size of powdered medicine to ensure accurate dosage is included in pharmaceutical capsules .

Advantages

Disadvantages

  • Fairly Simple Design
  • Relatively inexpensive process
  • Requires manual feed

Types Of Materials

Screen Materials

General Information

Screens, the most important part of any screening device, are classified by mesh, the number of openings per linear inch of cloth. There are three main types of screens: woven-wire cloth, synthetic cloth, and punched plate.

Equipment Design

Woven-wire cloth is the most used type of screen surface, with sizes available from 4" openings to as small as 500 mesh. Usually a square weave is used, although other types, such as twill square weave, are also available for specific applications.

(Copyright W.S. Tyler, Mentor, OH)

Punched plates, as in the schematic below, are stamped from sheets of metal, usually steel alloys. Depending on the application, the openings may be round, square, rectangular, or slotlike. Punched plates are more rigid and wear resistant than woven screens. They are frequently used for coarse separations.

Synthetic cloth, made of nylon, silk or other fibers is also frequently used for screening. These screens are especially resistant to abrasion and can be manufactured with openings as small as 100 mm.

Acknowledgements

Astec, Inc. , Chattanooga, TN

Chemical Engineering, Access Intelligence, LLC

W.S. Tyler , St. Catharines, ON

W.S. Tyler , Mentor, OH

References

LePree, Joy. "A Look At Screeners." Chemical Engineering . March 2012: 22-27 Print.

McCabe, Warren L., Julian C. Smith, and Peter Harriott. Unit Operations of Chemical Engineering . 5th ed. New York: McGraw-Hill, 1993, 994-1002. Print.

McKetta, John J. "Size Separation." Encyclopedia of Chemical Processing and Design . 1995.      Print.

Niazi, Sarfaraz K. Handbook of Pharmaceutical Formulations. Vol. 2. CRC LLC, 2004. Print.

Perry, Robert H. and Don W. Green, Perry's Chemical Engineers' Handbook . 7th ed. New      York: McGraw-Hill, 1997: 19-18 - 19-23. Print.

Orr, Clyde. Particulate Technology . New York, The MacMillan Company, 1966. Print.

Schmidt, Paul. "Screening." Ullmann's Encyclopedia of Industrial Chemistry . 5th ed. 1985.      Print.

Trottier, Remi. "Size Measurement of Particles." Kirk-Othmer Encyclopedia of Chemical Technology. Wiley, 2007. Print.

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