Selecting the right screen media and maintaining it can have an enormous effect on the costs and success of any operation.
The main objective when screening material, is to separate based on size, and having the stock piles meeting the required specifications. The ideal situation is to have this all achieved with the highest possible production rate.
When the material comes into contact with the screen it forms layers. The bigger particles are at the top, with the remaining particles sifting through so that the smaller particles are on the bottom.
The first stage of this screening process has an abundance of small particles trying to fit through the screen at the same time. The screen media in this section should be resistant to abrasion and impact. It is important that the amount of material going over the screen is not so much that it doesn’t give a chance for the small particles to feed through, resulting in them being pushed through to the end, producing fines overrun.
Other issues that can occur include:
- Similar size particles to the aperture opening get wedged, plugged or pegged in the opening.
- Smaller particles get wedged trying to fit though the opening at the same time.
- Blinding of the screen media happens when there is high humidity or moisture and clay in the material, making the small particles stick and block the screen surface.
The amount of production will be reduced when the screen is blinded or pegged. The amount of blinding or blockage on the screen will result in the same amount of lost production. If the screen efficiency is reduced by ten percent due to blinding then the production output will also be reduced by 10%. On a machine that is working at 300 tonnes/hour producing 12000 tonnes per week, a loss of 10% would be 1200 tonnes for the week. For product that is sold at $20/tonne that is lost revenue of $24000 for the week.
It will take more time to produce the same quantity of material through a blinded screen which may cause problems if you have time restraints and penalties, along with extra wages and machine costs.
Finally it may also produce a poor quality or out of spec product that is contaminated by fines overrun, a problem no contractor wants to have!
What you need to consider!
For maximum efficiency and capacity you need to consider the following when choosing the screen media:
- Screen inclination – this effects how fast the material flows over the screen media and consequently the capacity of the screen. If the material flows over the screen too quickly it may not have a chance to feed through the openings. The steepness the incline will also change how effective the aperture is. If the screen media is on an angle, when the screen is inclined, the opening is reduced. You may need to choose a larger aperture size to compensate for this, especially for near sized material. If the screen is horizontal the particles will fall straight through, when it is angled the area to fall through is reduced.
- If you have strict specifications on producing a particular shaped product you will need to consider using a screen media that helps sort this type of material. This will likely reduce production but will be much more selective in its sorting.
- If you have large particles of material you need to consider the strength and durability of the screen media you are using.
- Moisture content – If there is a high moisture and clay content in the material this will likely blind the screen media. This can be reduced with the type of screen media chosen.
- How the material is fed into the machine – The material should be consistently fed into the machine and quickly spread over the full width of the screen deck so the entire surface of the screen is utilised for maximum efficiency. It is important the screen is not overloaded which would make the smaller particles unable to fall through all the material. Feeding material from excessive heights will cause early wear on the screening media.
What Type of screening media should you choose?
Top Deck woven wire/mesh, bottom deck ripple screen!
The most popular screen media is a woven mesh as they provide a high screening area, are precise and are usually cheaper. They come in different weaving styles and geometries such as square or rectangular. If they are made of quality products they should withstand high abrasion, impact and tensioning. This type of mesh can however be subject to blinding and pegging.
Ripple screen and Piano wire
Self-cleaning screens help overcome the problem of blinding or pegging with crimped or straight wires that vibrate independently of each other when activated by the vibrating screen box and the weight of the material.
This helps increase production and the life of the screen mesh. This mesh is more expensive initially, however over time they show their value for money. Common names for this type of mesh are, ripple screen, Piano or harp wire.
Polyurethane screens are very good for high abrasion materials. They are very low maintenance and easy to install. The downside to this is there is less open area but it is resistant to blinding and has a long life.
Rubber screens can be extremely easy to install, which provides OHS benefits. They are suitable for high abrasive and high impact material and are great for pre-screening and scalping especially when there is a high percentage of large rocks or material being dropped on it from a significant height. They are also resistant to blinding due to the flexibility of the rubber. Often the apertures are cut conically to increase throughput - that is, the opening is smaller at the top and larger at the bottom.
The Grizzly bars as shown on the Portafill 3000ST are metal bars set on an incline in a stationary frame. They are very heavy duty and their best application is for separating unwanted materials before processing, scalping and for high tonnages.
Fingers screens are a great choice for wet sticky materials especially pre-screening and scalping. The vibration from the screen box and fingers spreads the material across the deck. There is no problem with blinding and plugging as the screens are self-cleaning.
Punch plate can handle high abrasion and high impact and is mainly used for scalping and pre-screening of material. They can handle bigger feed size and are very heavy duty.
PU Mesh is a steel wire mesh that is coated in a polyurethane material. This is more flexible then standard mesh and provides greater durability with a much longer life and a high open area. It also creates a secondary vibration due to it not needing as high a tension as steel mesh, helping keep the screen area clean. The polyurethane coating reduces the ability of the material to stick to the mesh and cause blinding. Although there is a higher initial cost it is worthwhile especially in sticky material.
These polyurethane rods are attached to the screen and sit on top of the screen media. As the screenbox vibrates they move and bounce dislodging and stopping any built up material on the screen media. They may not be suitable for smaller finer wire mesh screens as the rods could damage the wire.
If you are having issues with your screens breaking consider the following:
- Ensure material is fed evenly across the whole width of the screen bed ensuring full use of the screening surface
- Reduce feed height to minimize screen damage
- Check the screen media is fitted correctly and tensioned. If the screen media is able to move around vibrating and banging against the support bars this will cause damage to the screen media.
- Check the tension again after an hour or hour and a half of operating after initial installation as the screen media is likely to stretch with the first use.
- Make sure the stringer bars sit in line with the decks support bars
- Check that the clamp bars and rubber channel are not worn out, damaged or missing as they can allow material to wedge between the screen media and screen-box rails.
At the end of the day the screen media you choose can make a significant difference to how well your machines perform. They can determine if you meet specifications and how quickly you can produce the required quantities. It can also reduce OHS risks and effect your bottom line. Take the time to investigate thoroughly the best screen media for your application - it's worth the effort.