Selecting & Sizing Process Screening Equipment

Screening is a mechanical process that separates material according to individual particle size. This is accomplished by transmitting a particular motion to a screen medium that is made of wire mesh or perforated plate. The motion causes particles smaller than the screen's openings to pass through as fines and particles larger than the openings to carry over as tailings. See sidebar for additional screening definitions.

SCREENING FUNCTIONS
There are basically three types of screening functions: scalping, grading, and fines removal.

Scalping
Scalping removes a small amount of oversize material (5% or less) from a feed that consists predominantly of fines (at least 50% half-size). Scalping capacities are the highest of all screening functions.

Grading
Grading separates material by particle size using single or multiple screen surfaces. These Sseparations can be coarse (larger than 4 mesh), medium (4 mesh to 48 mesh), or fine (smaller than 48 mesh). Grading capacities vary and depend on the characteristics of the feed material, the separations to be made, and the product specifications.

Fines Removal
Fines removal removes a small amount of fines material (10% or less) from a feed that consists predominantly of oversize particles. Relatively high capacities are attainable, particularly when the amount of fines to be removed is less than 5 % of the original feed.

MATERIAL CHARACTERISTICS

TABLE 1 Particle Size Distribution for Feed Material X
Test Sieve	Percentage Retained
+7	0.4
+8	1.6
+9	4.3
+24	51.1
+28	15.4
+32	17.6
-32	9.6

Sieve analysis also measures the amount of near-size particles-those particles close in size to the desired separation. The greater the amount of near-size particles, the less the capacity will be for a given efficiency. When a near-size cluster is present in excess of 25%, screen blinding can occur.

PARTICLE CONFIGURATION
Particles are described in many ways, such as granular, crystalline, flaky, spherical, fibrous, cylindrical, elongated, sliver-like, and irregular. The configuration of individual particles is a characteristic that distinctly affects the ease of screening a given material.

For instance, uniformly shaped granular particles (with no single, transverse dimensions) screen more readily, producing higher, capacities and efficiencies. Spherical particles also screen well if the separation does not take place in a concentration of near-size particles, which blind easily. Because spherical particles are near-size in every dimension, they can lodge into an opening regardless of the alignment at which they enter the opening.

By contrast, elongated sliver-like particles (with substantial differences between their maximum and minimum transverse dimensions) screen poorly. Identical sliver-like particles align to the screen openings so that some particles pass over the openings and some pass through them even though the particles are the same size. In effect, this is sizing by length and not a good application for screening. Removal of a single fraction by length is possible., however, with a screen motion that prevents up-ending of elongated particles.

BULK DENSITY
Another important material characteristic is bulk density. Since screening is a volumetric process, the bulk density of the feed material determines the bed depth, which, in turn, determines how easily stratification of fines will occur on the screen surface.

SURFACE MOISTURE
The presence of surface moisture adversely affects the screenability of most materials. Surface moisture causes particles to adhere to each other, making stratification difficult. The rate of travel is also reduces increasing the bed depth.

STATIC
Static charges also adversely affect screening operations. If the charge is sever enough to coat over the screen opening; screen blinding can occur. The use of static eliminating additives or the introduction chumidified air sometimes controls this condtion.

SCREENING MOTION
The type of screening motion employed also affects screening performance. See Fig. 2. There are a variety of terms to describe the array of screening motions, and combinations of motions, that are available to the user. Some common terms include inclined vibratory, horizontal gyratory, and circular vibratory.

Inadequate agitation dampens Too much screen action makes out particle stratification, particles airborne, reducing resulting in inferior separations. the frequency of screen contacts necessary for efficient particle pass-through.

Regardless of name, all screening motions involve a combination of a given amplitude and speed in a particular alignment or plane. The objective is to distribute the material over the full screen surface and simultaneously induce stratification of fines through the bed of material without violent agitation or vertical hop. This permits finer particles to achieve maximum exposure to the screen openings for efficient separation. Equipment that employs a horizotal gyratory motion is generally acknowledged to provide this type of action at high-production rates.

SCREEN BLINDING
Screen blinding is any condition-such as plugging, coating, or bridging-that reduces the open area of the screen surface. See Fig. 3. Blinding can be a cumulative or noncumulative condition. Cumulative blinding eventually leads to total blockage of all screen openings and a complete loss of the screening function. Noncumulative blinding, however, is a stable condition; only a certain percentage of openings are blocked at any given time. As one set of openings are cleared another set become temporarily blocked, and the process continuously repeats itself.

To prevent blinding, it is sometimes possible to use a different size screen opening to shift the separation away from the concentration of near-size particles, but this can produce an unacceptable product spec. The best solution is a screen mesh cleaning system that ensures rated capacities without sacrificing separation accuracy. There are many antiblinding devices that can be used to control or prevent this condition, including bouncing balls, brushes, wiping rings, and screen heaters. The selection of an antiblinding device depends on the type of screen motion and the nature of the blinding to be controlled.

SCREEN CLOTHING SELECTION
For a given screen opening, the diameter of the wire screen can dramatically affect screening performance. The heavy wire screen shown in Fig. 4 lasts longer than a light screen with the same opening. The light wire screen blinds less easily, however, and handles more throughput since it has a greater percentage of open area. In practice, the selection of screen clothing is often a compromise between capacity, durability, and efficiency.

SCREENING PERFORMANCE
The proper selection of screening equipment begins with an accurate description of the application and the specific performance requirements. In general, there are three considerations: capacity, accuracy of separation, and product yield.

CAPACITY
Defined simply, capacity is the amount of material that a screener can handle under specified conditions. It is important to distinguish whether capacity is the amount of material fed into the screener or the desired output of a particular product. It is also necessary to account for any amount of material that can be recycled. Screening equipment should be sized to handle the maximum feed rate, even if it occurs for only a short period of time and not continuously. A screener should not be used as a surge-leveling device; it is not intended to handle surges of more than a few seconds in duration.

PRODUCT ACCURACY & YEILD
Product accuracy is a statement of the desired particle distribution of the finished product. Using the particle size distribution of feed material X given in Table. I, a product accuracy specification might call for a –9 mesh, + 28 mesh product to contain a maximum 5 % + 9 mesh material and a maximum 10°/> -28 mesh material.

To quantify product yield, it is important to specify the maximum amount of allowable, usable product in the rejects. For instance, using the particle size distribution of feed material X, the rejects can contain a maximum 5% -9 mesh in avers mid a maximum 5% + 28 mesh in fines. In other words, for each screen deck or separation to be made, the accuracy can be defined by the amount of fines permissible in the material passing over the screen and the amount of avers permissible in the material passing through the screen.

PERFORMANCE EVALUATION
Most screener manufacturers offer testing services in their application laboratories. Due to the many variables that affect a final screen area selection, it is advisable to take advantage of these testing services.

TABLE 3 Test Results for Feed Material X
Separation Analysis with Specifications on Reject Overs and Fines (% Retained)
Test Sieve	Overs	Product	Fines
+7	4.6	–	–
+8	18.7	–	–
+9	61.3	2.5	–
+24	15.4	62.0	–
+28	–	27.5	20.6
+32	–	5.9	52.0
-32	–	2.1	27.4

The desired product yield can be obtained by placing specifications on the reject avers rind fines. For example, the separation analysis in Table III shows a -9 mesh, + 28 mesh product with the same particle distribution as that in Table II; however, the reject fractions of the avers and fines contain large amounts of good -9 mesh (15.4%) and + 28 mesh (20.6%) product. This represents inefficient screening, perhaps achieved on less screen area but at the expense of lost product.

CONCLUSION
When specifying and selecting screening equipment, it is important to establish precise specifications for the desired product quality. It is also necessary to define the acceptable limits of good product in the reject material. This translates into an optimum product yield, the proceeds of which will easily pay for the premium of purchasing high-efficiency screening equipment.

ABOUT THE AUTHOR
Alex C. Young has more than twenty five years experience in process screening. He, has held various application engineering and sales positions with ROETX, Cincinnati, OH. Mr. Young holds a B. S. in mechanical engineering from Rensselaer Polytechnic Institute.