A starter’s guide to screens and screen media

By |  February 7, 2018

Photo courtesy of Mellott Company

When the general population thinks of screens or sieves, some people think of a small child at the beach using a screen to sift through sand to find seashells or other hidden treasures.

When it comes to rock screens for the aggregate industry, however, they really are quite technical and scientific.

The role of screens

In actuality, rock screens are vitally important to any company that deals with rock crushing. Crushers won’t crush rocks to a consistent size every time. Once rocks are crushed, they’re going to run through a screen for two reasons: First, you want to get the sizes you need. Second, they are used to either wash or dewater material.

The rock itself is filtered and isolated to the correct size to meet the customer’s specifications for base, asphalt or concrete. One quarry might make a lot of product specifically for concrete, while another quarry might focus heavily on making product of a smaller size for asphalt. At yet other times, a quarry might need the flexibility to make both.

Furthermore, customer product specifications are becoming even tighter (meaning smaller margins of error) and can vary widely – even across very geographically similar markets.

For these reasons, among others, aggregate producers must understand the significance of the rock screen to the success of their operation.

From crusher to screen

At their simplest, rock screens are made up of a screen box, a specific amount of screen decks, screen media and a vibrating mechanism that shakes and moves rocks through the screens.

The wearable layers that size the material and protect each deck inside the screen box are called media. Media is the screen metal cloth, rubber or polyurethane that have specified openings allowing stone to either be passed through or discharge over the decks. All of those openings will be set to a certain size that will achieve the dimensional threshold that the quarry needs per product.

The process starts when the rocks that pass through the crusher land on a conveyor belt that takes the material up to what’s called a screen tower. The rocks drop onto the screens, and the physical screening begins.

The most typical screens used in a quarry are inclined, set at a 20-degree angle. The rocks fall into the feedbox at the top of the screen and then cascade onto the first screen deck.

Many screens will have three decks. Aggregate producers will know what openings to put on each deck to make the specific products needed. For example, if a specified product is between 1.5 in. and 1 in., the top deck opening would be 1.5 in. to scalp off the oversize. Then, the material that falls through to the second deck will screen at 1 in. Everything smaller than 1 in. will fall through, and what is left off the second deck is the product required.

This process of passing and retaining material continues throughout the rest of the screen. And then material can be moved to a pile as a saleable product or be recirculated.

Generally, most quarries make similar products, such as rocks for base, concrete and asphalt. Though most states have their own names and specifications for what is required for those products, some quarries focus a little more on niche aggregate products. They’ll look to produce a more unique product spec depending on what the market dictates.

Quarries are able to switch screen media to different sizes when needed. In conjunction, quarries may need to change the settings on the crushers to make the feed size larger or smaller.

Screens haven’t changed much

Throughout the years, rock screens themselves really haven’t changed a whole lot. The mechanisms that shake the screens are relatively the same, as well. Basically, the mechanisms are either greased or in an oil bath.

There are different sizes of screens. If you were to go into a stationary quarry, most of the time what they’ll use is an incline screen, which is the screen set at 20 degrees. The screens themselves could be different sizes depending on the application. Some screens could be 8 ft. x 20 ft., and others could be 6 ft. x 16 ft. The size is dictated by the tonnage coming to the screen and the efficiency required at that station.

Then, there are flat screens. Flat screens are mostly used in portable applications owing to road height restrictions. Flat screens are generally efficient screens but will do less tonnages than an incline screen due to not having gravity on their side.

In addition, there are multi-sloped screens that have three differently sloped sections of each deck, commonly known as “banana screens.” These screens were designed to pass half-size particles quickly in the beginning while giving the near-sized particles more time to screen throughout the rest of the deck.

Different types of screens

Meanwhile, the screen media covering the decks on the inside will either be wire cloth or synthetic media. Most synthetic media is either made of rubber or urethane.
Rubber synthetic media, unlike the large sheets of wire cloth, are usually 1-ft. x 2-ft. panels that can be snapped onto the screen deck. The panels can be easily replaced, and it’s not required to replace all of them at the same time. This can save money in the long run.

Rubber panels are good for sound reduction and longer wear life in high-impact applications, making them great when dealing with larger feed sizes. The limitation on using rubber, however, is that smaller rocks have a sandpaper-like effect, which, in turn, thins out the rubber panel.

Urethane panels, while not as great with impact, are good at screening sand and wet material when water is sprayed across the screen. Urethane typically doesn’t wear down as quickly as other screen media.

Wire cloth has its benefits, as well. One benefit is upfront price. Wire cloth will have a higher open area, which could be critical if the screen is running at its peak capacity. Its shorter wear intervals should be taken into account when dealing with more abrasive materials.

Chris Andrews is an account manager for the Mellott Company in Columbia, South Carolina.

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