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;
- Wood pellets/chips
- Domestic waste
How does a trommel work?
Mixed material is fed into the hopper - just like the one below. 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.
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.
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 - just like the one below.
Trommel size: The length and width of the drum you need depends on a number of factors including;
- how wet your material is
- type of material
- amount of material you want to screen
- 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 this one.
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 a 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.
Not sure which trommel is right for you? Machinery Partner have a team of material experts that can help match you with the perfect machine to suit your needs and budget.
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. You can learn more about the benefits of topsoil screening here.
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. Want to find out how you could make money recycling concrete? Check out our blog post here.
Different trommel designs and modifications
Trommels can be customized depending on your needs. Below are a few examples of how trommels can be modified;
Trommels can have built in conveyers such as the Barford 5' ✕ 16' tracked trommel screen. Some trommel machines have 3 built in conveyors depending on the number of fines it is separating. By using conveyors you can increase your stockpile height and collect more material before having to move the trommel - reducing downtime.
It is also possible to add in separate long stacking conveyors. These can be easily moved to create new stockpiles without the need for you to move the trommel.
Mesh can be replaced easily on most trommel drums - making it possible for you to change the size of product you produce.
Lift and cut bars
These can be added to in the inside of the drum to stop the “balling” of wet and sticky materials. These bars cause a dragging and dropping motion and keeping the flow of fines consistent.
Tracks or Tires
Most smaller trommels will come with rubber tires as standard. This keeps the machine lightweight and easily transportable. On certain jobsites, for example if the ground is rough or very muddy, tires would get easily stuck or damaged. Tracks or crawlers can be installed on most machines and are perfect for adding traction and providing even weight distribution on soft and slippery ground.
These are a set of large bars placed across the infeed of the hopper to remove unwanted heavy materials. Typically grizzlies are used in rock and gravel screening to sort out large rocks before they enter and damage the trommel mesh. Grizzlies are angled so the large materials simply falls off the side of the machine. They can also come equipped with hydraulic cylinders to lift and dump any material that has piled on top.
During the screening process, materials that are too wet can get clogged inside the mesh. Adding a stiff bristled brush to the top of the drum will clean the mesh as it rotates around - keeping it free from dirt and debris. Trommel boom brushes are really important to maintain screening quality.
These trommels feature a large spiral that is able to control the flow of material through the drum. This design also improves screening and leads to a more even product. Addition of the spiral however increases the cost of these drums and it is not possible to replace parts of the drum if it gets damaged.
Water jets and dust suppression
High pressured water jets can be added to the drum for certain uses such as gold mining. Jets can help break apart large lumps of clay and mud to remove them from the valuable gold. Dust suppression systems can also be added to the inlet and outlets of trommels to help meet health and safety requirements.
These can be added to the trommel drum to remove unwanted ferrous or magnetic materials from the screened product. This is a perfect addition for those who screen on construction or demolition sites.
Where can I buy a trommel?
Machinery Partner carries an entire range of screeners including trommels for sale. Our team of highly experienced screening experts can help you chose the best trommel for your needs and budget - reach out today to speak directly with one of our material experts.