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Comment PDF Separation Processes

Factors Affecting Screening Performance

By William E. Lower, Rotex, Inc. |

Consider these points when selecting and specifying equipment that separates material according to particle size

Screening is an integral part of many dry material processes, ensuring an end product that is free of fine or oversized material, or in some cases achieving a specified particle distribution. In any case, the screening process is critical to the quality of the end product.

screening

Screening equipment uses screening medium (usually wire mesh or perforated plate) and a screening motion to convey the material across the screening medium, separating the material by particle size.

Screening performance, measured as either the product yielded in the operation, or the quality of product yielded, is affected by many factors. In the context of equipment selection, screening machines are evaluated on their ability to optimize product quality and screening efficiency.

In any screening application, one or more of the fractions or flows produced by the screener, is the “product.” The product has a quality specification defining its acceptable particle-size distribution. Typically, this specification consists simply of particle size limits and tolerances.

Screening efficiency, or product recovery efficiency, is the ratio of (1) the amount of on-size product separated out by the screener to (2) the amount of on-size material available in the feed to the screener. Screening efficiency will determine the process yield, which in turn determines production rates, and overall system efficiencies. Yield is the amount of material separated as product and is expressed as a percentage of the rate at which material is fed to the screener.

Product quality is an absolute requirement, but equally important is the screening efficiency that determines production rates and overall system efficiencies. Clearly, the more efficiently a screener extracts on-size product from the feed material, the higher the production rate.

Material Characteristics

Particle size distribution

Particle size distribution is the most important characteristic impacting screening performance.

It is not practical to measure the size of each particle in a given batch of material, the material’s overall particle-size distribution is determined by taking a small, representative sample and determining the weight of the particles that fall into standardized size ranges. This is usually accomplished by sieve analysis.

Sieve analysis involves using standardized test sieves with precisely controlled screen apertures. These sieves are arranged in a stack from coarsest to finest with a collection pan below the bottom sieve. The material to be analyzed is introduced into the top sieve, and the stack is exposed to a motion causing the material to stratify by particle size through the sieves. After a specified time period, the stack is disassembled and the material retained on each sieve is weighed, and expressed as a percentage of the total (Table 1).

screening

Mechanical sieve analysis is the most common method for determining particle size distribution, but other measurement techniques, such as laser diffraction and video imaging systems, are also available. Regardless of the method used, results will show that for practically all granular materials, particle size distribution resembles a bell shaped curve (Figure 1). By locating the nominal separation points for a screening application on the X-axis of the particle-size distribution curve, the user can determine which one of the following mechanisms will be required:

Scalping — separation and removal of materials that are considerably larger than the average particle size

Fines removal — separation and removal of materials that are considerably smaller than the average particle size

Grading — separations based on the size of material on single or multiple screen surfaces, into one or more grades of the material

Performance, specifically screening-machine capacity, will be greatly influenced by where the desired separation point falls in particle size distribution. This relationship is shown by the capacity curve superimposed on the particle distribution curve in Figure 1.

screening

In scalping applications at the coarse end of the particle distribution (right side of curve), very high screening capacities are achievable. This is because most of the particles are much smaller than the screen opening and pass through easily, with multiple particles passing simultaneously. The capacities are also quite high for fines removal (left side of curve), because again, the fine particles that should pass through the screen are much smaller than the screen opening. Note that capacities are not quite as high as with scalping because these fine particles must first work their way through the bed of material that is being screened.

Capacities are lower in grading applications because the screen openings fall near the middle of the particle size distribution curve, where there are numerous particles approximately the same size as the screen opening. The “near-size” particles that should pass through the screen are competing for a limited number of screen openings, and often must pass through one at a time. For this reason, accurate gradations require relatively lower feed rates and a highly effective screening motion.

Particle shape

Particle shape also affects screening performance; particles can be granular, spherical, cylindrical and so on. Particle shapes, such as spherical and granular, tend to produce relatively sharp separations, while, screening of irregular shapes, such as elongated, sliver-like or plate-like, can lead to inaccurate separations. Such variability results from how the particles present themselves to the screen opening.

Certain particle shapes, including crystalline and spherical particles, are prone to screen blinding, a situation where particles slightly larger than the screen opening become caught in the openings, effectively plugging the screen. Blinding can significantly affect screening performance and production rates.

Bulk density

In general, the higher a material’s bulk density, the higher the screening capacity of the application will be. Gravity is the driving force in screening operations. So, where there is higher mass, or higher density material, gravity simply applies a higher screening force, thereby promoting particle separation.

For example, heavy materials such as metal powders can be screened quite readily, even through fine screens. By contrast, lightweight materials such as sawdust are more difficult to screen and must be screened at very-low mass flowrates.

Bulk density determines the volumetric flowrates and therefore the bed depth of material on the screen. Lower bed depths typically result in more efficient screening.

Flowability

Flowability of a material can affect screening performance. In materials that do not flow well, the particles do not spread over the screen surface nor present themselves properly to the screen openings.

Friability

Friability — the tendency of a particle to crumble or break — is also a concern because the screening action, itself, can cause particle degradation and change the particle-size distribution.

Surface moisture

Surface moisture can have a negative impact on screening performance. High levels of moisture can reduce screening accuracy by causing particle bridging, effectively blinding the screen opening and decreasing screening performance.

Moisture also promotes particle agglomeration — formation of a rounded mass of particles — thereby preventing the particles from passing through the screen.

Static charge

Bridging of an opening can be caused by static charge in the materials. Static charge is most likely to occur in materials that have been excessively dried or are very fine particles in which the static force of attraction overcomes the gravitational screening force.

Screen selection

Selection of the screen media has a significant impact on screening performance. The most common screening media is woven wire cloth screen. Typically, production screens are specified by at least two of the following four parameters: mesh count, wire diameter, opening, and percentage of open area.

Screening media

Screen cloth can be woven from several types of material. Steel wire is the most common, with Type 304 stainless steel being the standard for chemical applications. Synthetic material, such as nylon or polyester can also be used to weave screens. The apertures in synthetic screens have a slightly different shape than those of steel-wire screens, and are more flexible. As a result, in some applications, synthetic screens are less prone to blinding. This is particularly the case when crystalline materials are involved.

screening

Another common screen media is perforated plate, with rectangular or circular openings, these plates are particularly effective at making particle separations based on length; however, they have a lower percentage of open area and as a result have lower capacity for a given screen area, compared with a woven wire mesh. This is by no means an exhaustive selection, there are many other alternative media each with their own characteristics. Generally screening suppliers can help you with these selections.

Mesh count

Mesh count is the number of openings, per lineal inch. Subtracting the wire diameter from the inverse of the mesh count gives the actual opening dimension. The percentage of open area is the ratio of the area of the openings to the total surface area of the screen. The relationship of these wire cloth specifications is show in Figure 2. (It is important to avoid confusion between sieve numbers used to identify test sieves and mesh counts that are used to specify production screen.)

screening

Wire diameter

Typically, multiple wire diameters are available for a given opening, resulting in several different mesh counts and screen characteristics. Therefore, screen selection involves more than simply choosing a desired screen opening and wire material. Often, several commercial grades of screens are available with the same opening.

Opening and percent open area

Square-mesh screens are by far the most common. Meanwhile, screens woven with a rectangular opening can be particularly effective in increasing screen capacity or making separations based on particle length.

Screen Motion and Configuration

Vibratory versus gyratory

While there are many methods of separation, vibratory and gyratory are the two most common types of screen motion used in production screening machines.

Vibratory is characterized by a short stroke, high frequency motion, and is utilized in many dry and wet screening applications. The short-stroke, high-frequency motion is good for conveying material and breaking the surface tension in liquid solid applications. These screeners are well suited for scalping and fines removal but the motion is not conducive to very-high efficiency grading applications. They do not typically have sufficient stroke to properly spread and stratify material as it is fed onto the screen deck. This prevents the finer particles from achieving maximum exposure to the screen openings. Also, the short stroke does not promote effective deblinding of the screens.

Gyratory is characterized by a longer stroke and lower frequency motion, and is most suitable for applications involving finer separations or separation where accuracy is a priority. Also, this motion has a more positive conveying characteristic, good for higher capacity applications. The long stroke of gyratory machines spreads the material across the full width of the machine and stratifies the bed of material so the finer particles will work their way down to the screen surface. They also use a sifting motion that keeps the material in contact with the screen surface, maximizing opportunities for fine particles to pass through the screen. Gyratory machines are not well suited for wet applications because there is very little vertical motion to break the surface tension.

The performance of a screening machine is determined by its combination of stroke, frequency, slope and screen shape.

Stroke

The stroke is the peak-to-peak distance traveled by the screen in one cycle. For vibrating screeners, the stroke is typically 3–10 mm, while for gyratory screeners, it is 35–90 mm. Gyratory screeners’ longer stroke promotes better material conveying and more aggressive ball mesh cleaning, a common method of deblinding. The better a screener conveys material and deblinds the screens, the lower slope it can employ, thereby improving accuracy. See slope below.

Frequency

The frequency of a screening machine is the number of cycles of motion completed by the screen in a given time period — typically one minute. Vibrating screeners generally operate at 1,000–3,600 rpm, while gyratory screeners typically operate at 200–300 rpm.

Slope, or plane of action

To produce accurate separations, a near-horizontal screening surface is preferred over one that is steeply inclined. Gravity-induced flow on a steeply inclined screen may cause material to travel too rapidly across the screening surface, resulting in reduced screening accuracy. Furthermore, a steeply inclined screening surface presents two distinctly different openings to the material: the actual opening of the inclined screen, and the horizontal projection of the inclined opening (Figure 3).

screening

A screen with a 2.0-mm opening inclined at 40 deg. will have an effective screen opening of 1.5 mm (2.0 mm multiplied by the cosine of 40 deg.). Vibrating screeners, which often do not have a sufficient stroke to convey material across a horizontal screen surface, typically require a steeply inclined screen so that gravity can be used to boost conveying rate. However, this steep slope reduces the sharpness of the separation.

Consider, for example, the case mentioned above, where the screen has 2.0-mm openings. While most of the particles will be separated by the 1.5-mm projected opening, some of the particles, as a result of the random motion of the vibrating screen, will actually move perpendicular to the screen surface; and as they do, they will be exposed to — and pass through — the actual 2.0-mm screen opening. Thus, while the bulk of the particles passing through the screen will be 1.5 mm dia., the final separated product will also contain particles as coarse as 2.0 mm dia.

Screen shape

Screen shape will also affect performance. Relatively small machines that handle lower capacities use round, horizontal screens. These machines are typically fed at the center of the screen and material conveys out toward the edges. The resulting short, effective screen-length limits horizontal, round-screen application when accurate grading is required.

Rectangular screen decks are used for higher capacity applications and when greater separation accuracy is needed. These rectangular decks can be inclined to improve conveying and increase capacity. By feeding the material to one end of a rectangular screen, very accurate separation can be achieved because the material must travel the full length of the screen, thereby maximizing exposure to the screen openings.

Screen blinding

Screen blinding occurs when material obstructs the screen, reducing the open area of the screen surface. Blinding results in inconsistent screening performance due to loss of screening capacity. Screen blinding results from plugging or bridging of the material being screened.

Plugging occurs when particles that are very close to the size of the screen openings (near-sized) become trapped. Spherical particles and certain types of crystalline particles are more prone to plugging than other particles shapes.

Bridging occurs when groups of undersize particles bind together to block screen openings. Particle bridging tends to occur when surface moisture or electrostatic charges are present. These often occur in materials that have been excessively dried, and those with extremely small particle sizes. Fibrous materials can also cause blinding when the fibers become wrapped around the screen wires.

Preventing blinding: In screening applications where blinding is a concern, preventive measures must be used. The most common method of preventing blinding is to use bouncing balls that are trapped in pockets below the screen surface. The motion of the screening machine causes the balls to bounce randomly within their pockets, effectively loosening any bound material and continuously cleaning the screening media. Similarly, vibrating rings, compressed air, rotating brushes, and ultrasonic vibrations are also used to control blinding.

Capacity

The capacity of any screener is the rate of material it can handle while providing a product to a specification.

The performance of a screening machine can be significantly affected by the material feed rate. A screener has an ultimate volumetric capacity, which is a function of internal construction and material handling capabilities. However, the true capacity at which it will produce acceptable separation performance is typically a much lower feed rate. This balance between capacity and screening performance must be evaluated when selecting screening equipment.

While screen-opening selection has the greatest effect on product quality, screening-machine motion has the greatest effect on screening efficiency. At the same time, changing the size of the screen opening can certainly increase screening efficiency. For example, in a scalping operation, using a screen opening larger than the desired separation point can impact screening efficiency. However, this will have a negative impact on product quality, as the larger opening will permit oversize particles to pass through the screen producing an off-spec product.

Using a larger screen opening in a fines removal operation, allows on-size particles to pass through with the fines, thereby wasting product. By using a more-efficient screening motion instead of increasing the screen size, one can yield a quality product without needlessly rejecting product to the waste stream.

In general, screens with a relatively heavy wire for a given opening will be the most durable. However, such a screen will be more likely to blind and will also have lower capacities due to a lower open area percentage. Conversely, screens with a lighter wire will tend to blind less and can generally handle more throughput since they have a greater percentage of open area.

Conclusion

As you can see, there are many factors that affect screening performance, and all of them need to be taken into account when specifying a screener. Screener manufacturers can help you put together a solution that will work for your particular application.
 

screening

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