What is trommel screening
A trommel screen is a rotating circular mesh drum that can sort materials based on their size. The mesh allows small materials like soil or sand to fall through and be collected by a conveyer under the drum. The larger materials flow to the end of the trommel where they are gathered into another pile.
What is a trommel used for?
Trommels are perfect for screening a wide range of materials and they are the first choice for wet and sticky organic substances including.
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Soil
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Sand
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Mulch
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Wood pellets/chips
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Compost
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Domestic waste
How does a trommel work?
Load hopper
Mixed material is fed into the hopper. The hopper is attached to a conveyer that slowly moves the material into the top of the trommel drum. The speed that the hopper feeds material is important for the separation of fines. Adding material quickly into the drum might seem like the swiftest way to process it, but each trommel has a loading capacity. This is the maximum amount of material that can be in the drum at any one time - normally 40% of its width. If the drum is overfilled, there won’t be enough time for the valuable fines to be separated and they will be deposited alongside the larger materials. This means that the whole batch would need to be reprocessed. The speed of the feed hopper can be controlled on many machines.
Trommel screening
The hopper moves the mixed material into the top of the drum. As it is tumbled around, air is mixed in helping to dry any wet or sticky material - this improves separation. The dragging and dropping motion that is created by material being pulled up the side of the drum helps break up large lumps and allows the fines to pass through the mesh. If the trommel drum is not spinning fast enough, the material will sit on the bottom and form large clumps. This is caused by stones collecting wet materials and “balling” them together. Screened material is passed onto a lower conveyor which carries the fines away to a separate stockpile.
Sieving vs drag and drop screening.
Trommels are normally used at a 5 degree angle but this can be changed depending on the type and wetness of the material. If you were screening dry sand, you could have the trommel at a steeper angle - increasing the speed of the material as it passes through. Dry materials need less screening time as most of the fines can be removed near the top of the drum even at greater angles.
What size trommel screen should I choose?
Trommels come in a range of sizes from small, portable, compact screeners to large, tracked trommels. These large trommels are able to screen and separate 3 or 4 different sized materials as they have different mesh sizes along the length of the drum.
Trommel size: The length and width of the drum you need depends on a number of factors including.
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how wet your material is.
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type of material
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amount of material you want to screen.
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product you want.
For example, if you are screening a wet material, it will need a longer time in the trommel for good separation. In this case, a longer and wider trommel is best. Whilst drier materials that can be easily separated, can been screened in a small drum such as.
Mesh size: The size of the hole in the mesh controls the size of the material that will fall through. Hole sizes can range from 1/20’’ to 5’’ wide so you can choose the perfect sized mesh for your needs.
Space: Space can be an issue for companies that have smaller yards or if they are in an urban environment. In this case, a compact trommel is the best choice.
What are the benefits of screening?
Screening is used to separate mixed aggregates such as rock and soil into different sized products. Think of it just like a large sieve.
In the agricultural and farming industries, screening can be used to separate valuable products such as topsoil from unwanted larger materials. Topsoil contains the essential nutrients required for plants to grow and as the particle size is small, water is able to drain freely - helping to prevent root rot. Screened soil can be used on site or even sold for a profit. In general, the finer the topsoil is screened, the higher the price.
Construction sites that produce a lot of concrete waste can benefit from recycling and screening their concrete. Separated concrete sand - made up of granite and limestone, can be used for fill and bedding in structures including pavements and roads. Screened concrete sand is a perfect base as it promotes water drainage - stabilizing the structure. Recycling and screening your waste concrete aggregate can help reduce the number of trucks going to landfill and lower the costs associated with transport and disposal of waste. It can also increase your bottom line as you can sell the profitable fines.