Jul. 11, 2017
Chris Andrews is an account manager for the Mellott in Columbia, SC. He was awarded an engineering / physics degree from West Virginia Wesleyan College. Chris was an engineer in the automotive industry for more than a decade before working for the corporate office at Metso. Once Chris got into vibrating screening, he became a product specialist. He progressed to the product manager for North America for vibrating screen equipment. After several years, Chris moved to Mellott, and is now the account manager for eastern North Carolina, all of South Carolina, and northern Georgia. He is an expert when it comes to rock screens.
When the general population thinks of screens or sieves, most people might think of a small child at the beach, using a screen to sift through sand to find seashells or any other hidden treasures. When it comes to rock screens for the aggregate industry, however, it really is quite technical and scientific.
The Importance of Rock Screens
In actuality, rock screens are vitally important to any company that deals with rock crushing. The crushers won’t crush the rocks to a consistent size every time. Once the rocks are crushed, they’re going to run through a screen for two reasons. First, you want to get the sizes you need, and second they are used to either wash or dewater the 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 may focus heavily on making product of a smaller size for asphalt, and other times a quarry might need the flexibility to make both. Furthermore, customers’ product specifications are becoming even tighter (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 Rock Screen
Simply speaking, 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 that allows 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. Rock 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 inch and 1 inch, the top deck opening would be 1.5 inch to scalp off the oversize then what falls through to the second deck will screen at 1 inch. Everything smaller than 1 inch 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 could be moved to a product pile as a sellable product or recirculated.
Most Quarries Use Multiple Rock Screens
In general, 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.
Rock Screens Haven’t Changed Much
Throughout the years, the rock screens themselves haven’t changed much. 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. For example, some screens could be 8’ x 20’ and others could be 6’ x 16’. 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 due to road height restrictions. Flat screens in general are very efficient screens but will do less tonnages that an incline screen due to not having gravity on their side moving the material across the screen.
And finally there are also 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 Rock Screens
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 one foot by two feet 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 which can save you money in the long run.
Rubber panels are good for sound reduction and longer wear life in high impact applications which makes them great when dealing with larger feed sizes. The limitation on using rubber, however, is that the smaller rocks have a sandpaper like effect, which in turn thins out the rubber panel very rapidly. It’s in this application where urethane can shine.
Urethane panels, while not as great with impact, are good at screening sand and wet material, when water is sprayed across the screen. Urethane doesn’t wear down as quickly as the other screen media.
Wire cloth has its benefits as well. The obvious benefit is upfront price however its shorter wear intervals should be taken into account when dealing with more abrasive materials. Wire cloth will have a higher open area which could be critical if the screen is running at its peak capacity.
Mellott Helps You Find the Right Rock Screens
Picking the right screen box and its media is an intricate balance between many variables such as desired products, specifications required, wear life, ease of replacement, and tonnages. So whether your company needs a new or used rock screen, screen media, or replacement parts to help fix anything that might be damaged, you can lean on the screening expertise at Mellott for advice. Above all be sure that you realize the importance of the rock screen’s effect on your operations’ profitability.
Contact us today to find out just how Mellott can help you find the right rock screens.