The basics of screening

Screening is one process that cannot be overlooked when it comes to running an efficient aggregate operation.

As McLanahan Corp.’s Mark Krause describes, the screen is the cashbox of every plant. In this article, Krause, the managing director of North America at McLanahan, details screening basics and offers a number of tips to ensure your operation’s running efficiently.

Screens

Generally, there are two types of screens in the aggregate industry: inclined/sloped screens and horizontal screens.

Inclined/slope screens require lower machine stroke and g-force, relying mostly on gravity to convey material down the sloped screen.
Horizontal screens differ because, without the aid of gravity to move material down the screen, they rely on large machine stroke and high g-forces to effectively move and separate material.

Screen media

The more screen openings a particle can see as it travels down the screen, the better chance it has to fall through the opening. Because of this, choosing the right screen media is crucial to the efficiency of the screen.

One area to focus on is the open area of the media selected. If the open area is decreased, the chance of making the separation falls and you’ll need more screening surface due to fewer holes in the media.

There are a few types of screen media that producers can choose from, including:

■ Standard woven wire cloth. This is the most widely used and accepted media, according to Krause. The tradeoff with woven wire cloth is that there is more open area for throughput but also less wear life than other media.

■ Rubber panels. These are most commonly used for dry applications and abrasive materials. The tradeoff with rubber panel media is less open area or throughput in exchange for extended wear life. Be cautious with the weight of material on rubber panel media, Krause says, as decks and stroke are affected by the weight of the media installed.

■ Polyurethane. This is normally used in wet applications and for abrasive material. Similar to rubber panels, polyurethane screen media trades extended wear life for less open area while cautioning the weight of the material.

Key issues

No matter the screen or screen media, there are two common problems producers face with screening: plugging and blinding.

Plugging occurs when near-size particles become lodged in screen openings, blocking the open surface area and decreasing the amount of material being screened. Suggested solutions for plugging include:

• Increasing the stroke
• Changing the hole shape of the surface area or using a different surface media
• Adjusting the crusher setting to allow material to pass through the surface area
• Choosing a different surface media

Blinding occurs when fine particles become stuck to the surface area due to moisture, gradually covering the open area and creating a layer of blockage. Blinding is a more difficult problem to solve, Krause says, but he suggests several solutions:

• Increasing the speed or changing the stroke
• Choosing different surface media
• Heating screen decks to prevent material sticking
• Adding water to allow material to pass through the screen

Types of motions

Another key ingredient to effective screening is motion. There are three types of motions screens make:

1. Circle throw. This motion is commonly associated with inclined screens. This is a lower-energy motion that uses gravity to move material down the screen. It is also resistant to plugging. Still, blinding can become a problem due to lower stroke.

2. Straight line (reciprocating). This motion is usually used in horizontal screens. While this motion can be more efficient and accurate, plugging can become a problem.

3. Oval stroke (elliptical). This motion produces the highest efficiency and accuracy, Krause says, and is common in horizontal mounting. The high-energy motion helps loosen fines, creating a high resistance to pegging and blinding.

Separation and stratification

Two actions must happen for screening: separation and stratification.

First, stratification must occur before any separation can happen. Stratification is the finer particles sifting, with the help of vibration, through the bed of material to find the screen media. Separation then is the probability of the material falling through the opening or not.
For efficient stratification, Krause suggests following these rules of thumb for maximum bed depth: No more than four times the surface opening, at the discharge end of the screen, for aggregate.

To effectively separate materials, two operating parameters also need to be considered: speed and stroke.

Speed is an important parameter as, together with stroke, it creates material acceleration. The speed of the machine is measured in revolutions per minute and is a function of the motor speed and the sheaves. Periodic testing of the screen should be taken to ensure the screen stays running at its designed speed.

According to Krause, some advantages of increased speed are improved screen carrying capacity, increased material acceleration and increased material travel rate. Disadvantages of increasing speed are added operating stresses, decreased bearing life and the possibility of operating at a critical frequency.

Stroke, or throw, measures the distance a screen moves vertically and is controlled by the counterweight placement on the shaft of the screen. This is important because if the stroke is too long or great, the material will move too far and miss much of the open screen area, Krause says. If the stroke is too short, material will plug the screen openings and create additional problems.

To effectively measure the stroke of a screen and avoid these issues, operators should use a stroke card in all four corners of the screen.
Advantages of increasing the stroke are improved screen carrying capacity, higher material acceleration, increased material travel rate, reduced plugging and blinding, and improved material stratification.

Disadvantages of increasing the stroke are more stress on the screen, decreased bearing life and the possibility of inefficiency due to bouncing.