Marvin Woodie explains how getting the best performance from a vibrating screen starts with the way material is fed to the equipment
Let’s start at the beginning. A common problem seen throughout the industry, both in dry and wet screening, is how material is presented to the vibrating screen. You can’t dump material directly from a chute, pipe or belt onto a vibrating screen and expect optimum efficiency. A vibrating screen is a separation device and trying to make it do something it’s not designed to do will only cause problems. To increase the wear-life of vibrating screen components and screen media, in addition to maximising screening efficiency, begins with how material is fed to the machine. This is accomplished by introducing a customised feed system between the feed point and the vibrating screen. Proper design of a feed box, flat or curved sieve, or configurations of both will take into consideration the surrounding area and the source of feed to get the most out of your vibrating screen.
Brad Parker, a process engineer for Rosebud Mining Company responsible for optimising metallurgical performance at its coal preparation plants, reports great results from the sieve flumes that have been incorporated for many years in all five of the company’s heavy media coal preparation plants. “Conn-Weld sieve flumes ahead of banana desliming screens makes fines removal more efficient, increases the capacity of the banana screens and increases screen media life on the screens,” he states. “Likewise, the sieve flumes ahead of our heavy media drain and rinse screens do the same thing with the added benefit of increased magnetite recovery. Using the sieve flumes ahead of desliming and heavy media drain and rinse screens also ensures proper introduction of material to the screen it is feeding to insure longer screen media life.”
Feed system solutions are as countless as the challenges presented by mining applications. Here, we assess solutions in dry and wet applications incorporating a feed box and fixed flat sieve.
A key factor when it comes to dry screening is uniform distribution. A properly designed feed box will successfully use all the screening area, maximising throughput, thus making the screening more efficient.
The design of a feed box will vary depending upon what is deemed necessary for the application and the available space there is to work with. Additionally, the right liner (AR, ceramic tile, urethane, rubber, etc.) plays an instrumental role in not only preventing premature wear but designed correctly, will also distribute the flow properly as it transfers onto the screen surface.
There have been cases where the material is not distributing across the width of the machine until a third of the way down the screening area. This causes uneven wear and loading, resulting in inefficient screening. Material flow can be managed by customising the liner to the vibrating screen feed box.
In one example, a conveyor was dropping material fed through a chute onto a Conn-Weld 8ft x 20ft TD Circular Motion Incline. Out of 160ft2 of screen area, only 120ft2 was being used, with screening efficiency worsening even more on the lower decks. Challenged with a very abrasive aggregate application, a conventional feedbox would wear within a couple of weeks.
The solution? Conn-Weld designed a feed box with rubber liners to include end-dams moulded onto a steel liner. The dams created an evenly distributed flow across the entire screen. The final outcome is that the customer uses 100% of the screen area while providing long wear-life.
Considerations in wet screening applications are when material is being pumped to a screen or slurry fed to a screen. This is a crucial factor in designing the correct static separation device in combination with components to help manage the rate of flow and moisture content.
A feed box with a fixed or curved sieve that has appropriate screen media placed ahead of a vibrating screen in a wet application will provide uniform distribution. Additionally, it offers benefits from a process standpoint by taking out some of the material before it gets to the vibrating screen thus increasing the throughput of the combination of static screen and vibrating screen. Conn-Weld’s engineering sieves specialist, Robert Riley, explains, “One must understand how the material is being sent to the feed box. Many times, depending on the piping arrangement, it may become necessary to use a manifold to achieve proper distribution. Also, depending on point of entry (front feed, back feed, top feed, etc.), a determination is made as to the positioning of baffles and/or deflectors in the feed box. Once baffles and/or deflectors are located, we then concentrate on slowing the velocity of the material.
“Slowing the velocity of material aides in better distribution across the width of the screening surface of the sieve where it is pre-screened before traveling towards the vibrating screen.”
Installing a rubber skirt at the feed section of the sieve will aid in slowing the rate of material flow the full length of the screen media, allowing for better screening. Sometimes material will begin to build-up towards the discharge end of the sieve. This is fine because while on the discharge end of the screening surface, the material will drain more liquid.
Cutter blades ahead of static sieves can assist with removing water before material is presented to a static sieve. Designed with a linear actuator enables the blade to be used dependent upon the moisture content. If the moisture content is higher than 50%, the blade can be raised to cut into slurry whereas the blade can be lowered when there is less water present.
A recent case in point saw an underground operation having a water issue on its main production shaft belt, which carried 3,400 TPH of coal with an extremely high moisture content. The water was causing the operator’s load cells to fail. As a result, the belts were overloaded with material.
The solution? Conn-Weld engineered a 5ft6in x 48in x 25° flat fixed sieve with a cutter blade. The blade extended into the bottom of the flow of material diverting the smaller material onto a CG 5/32in sieve with 5mm OPG.
The end result was that the flat fixed sieve eliminated up to 90% of the water that once went into the operator’s storage bins.
Conn-Weld provides various types of feed systems with the knowledge of how to get the most out of vibrating screen equipment, therefore bringing value to the overall operation. Introducing an interface between the source of feed and the vibrating screen will result in extended wear life and maximum efficiency.
Marvin Woodie is president of Conn-Weld
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