Sieves & Screens
which is very similar to that of a Rare Earth Roll Separator. Looking at a case history where the objective was achieving maximum Fe2O3 removal at 330 kg/hr with a maximum product loss of 1%, milled zirconium powder processed on the Rare Earth Roll Separator resulted in a high product loss, mainly due to electrostatic attraction on the belt. Eriez laboratory technicians then ran the material over the RRS Drum. After extensive test work, two key issues arose: 1. The electromagnetic Vibratory Feeder, critical to the separation, was having difficulty feeding an even layer of the fine material onto the drum surface.
2. There was still too much product loss with the normal rotation speed of the Drum.
The initial results, with the standard Vibratory Feeder and Drum speed, showed a 47% reduction in magnetics. However, once the Vibratory Feeder was redesigned with a ‘Airflow’ tray and the RRS Drum was rotated at around 100m/sec, the reduction in magnetics increased to 75% with only 0.6% of the feed ending up in the separated fraction. The ‘Airflow’ tray aerates the material, improving the flow characteristics and ensuring an even feed of material onto the drum.
Dynamic Drum Separator (DDS)
An intriguing development for finer powder processing came in the form of the Dynamic Drum Separator or DDS. Even though only suitable for the removal of free iron, the DDS has proven very successful. The most common applications have come from hard mineral processing, such as refractory minerals including alumina, corundum and silicon carbide.
material with a feed of 20000ppm magnetics over the DDS it was possible to reduce the magnetics content to below 190ppm. This equates to a 99% magnetics removal. Additionally, the amount of feed discharged into the magnetics was only 5%, constituting an acceptable loss of good product. This was achieved with a feedrate of just under 3 tonnes per hour per metre.
Dry Vibrating Magnetic Filter (DVMF)
For the finest powder processing of industrial minerals such as talc, the Dry Vibrating Magnetic Filter is employed. This is an electromagnet with a background field of up to 5000 gauss. The electromagnetic coil of the DVMF generates a magnetic field, focused in on its centre. A magnetic matrix is positioned in this central core. The DVMF operates on a batch processing basis. Product is fed in from the top and magnetics captured on the matrix. The magnet is then turned off and the matrix vibrated, releasing the captured magnetics.
With a DDS, there is a conveyor with a hollow head pulley. Inside the head pulley is a strong magnetic rotor that rotates at high speeds. Material is conveyed into the magnetic field with free iron attracted to the rotating magnetic field and deposited away from the clean product under the belt. The magic of the separation occurs immediately when the material enters the zone. Examining a close-up of the separation zone illustrates why the DDS being so well suited to fine particle separation. As the magnetic particles enter the rotating field, the free iron particle spins, colliding with neighbouring particles. This collision helps liberates them from the non-magnetic powder, enabling a clean separation of iron.
The following case history looks at a silicon carbide sample with a free iron contents of 2.7% where the objective was to reduce to <0.1% free iron at 500kg/hr with a minimum product loss. Previous tests conducted with Rare Earth Roll Separators and Magnetic Drums had separated the magnetics, but with an unacceptable high product loss. Minimising this loss was crucial to the economics of the project. Tests in the laboratory and, ultimately, production results at site showed that by passing a
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One successful installation is processing talc in South America. The separation objectives were similar to most other projects, requiring maximum iron removal with a minimum loss of product. Handling talc on traditional magnetic separators such as the RE Roll or Magnetic Drum has previously proven difficult. The results exactly met the customer’s objectives. The Fe2O3 content of a product up to 3.5% iron could be reduced to less than 0.5%. Also, and equally important, the talc recovery was between 96% and 98%.
On a project in the UK, the objective was to reduce the iron content of a magnesia alumina spinel. Once again, the material was very fine and unsuitable for other types of magnetic separator. Tests had shown that the product loss and separation performance were simply not good enough. In this particular test, a 50kg feed with 14ppm magnetics was processed, removing over 100 g of magnetics with some entrapped product. However, when fired, only 2 iron spots were found in the end product. This met all the separation objectives and the customer ordered a production unit.
In conclusion, selecting the best magnetic separator for specific separation objectives is best achieved by conducting controlled tests in an Eriez laboratory. Advice can then be given on what process would enable the best separation and, ultimately, give process guarantees.
For more information contact Eriez on tel: +44 (0) 29 2086 8501 or visit:
www.eriez.com
May 2010 • Solids & Bulk Handling 31
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