Dewatering Screens

Some applications have experienced a moisture content as low as 7% with McLanahan Dewatering Screens, but final moisture content is dependent on both surface moisture and inherent moisture. When reporting moisture by weight (kg water/kg pulp), the specific gravity of the solids also plays a factor, with higher specific gravity materials showing lower moisture contents. If you remove all the water from the gaps between the particles, the only water left is that which is adsorbed as a thin film on the outside surface or as inherent moisture within the material itself (some brown coals can contain up to 20% inherent moisture). Therefore, the finer the materials being handled and the more irregular the surface, the higher the moisture content. Certain materials are also more hydrophobic or hydrophilic, which also affects the residual water. It is also important to understand that a Dewatering Screen’s action is critical to creating droplet formation on the underside of the bed of material. This combination of amplitude and frequency was studied by the University of Karlsruhe, which developed recommendations for this action.

In the early days of Dewatering Screens, urethane pin and sleeve design media was common. Since then, parallel developments occurred, rated capacities increased (doubling in some cases) and more robust media designs came about. Today, pin and sleeve style designs are perfectly adequate and appropriate for sizing screen applications; however, with the heavy loads and aggressive action on Dewatering Screens, few media manufacturers can match the mechanical integrity required for the application. McLanahan spent a long time researching and working on designs to match the need and the result is a longitudinal rail style system with urethane-to-urethane connections between the media and the stringers -- this type of interface doesn’t fail. The frequency of replacement of media in Dewatering Screens is such that no manufacturer or representative is going to lose much downstream revenue, but the end user always benefits from a better system.

Several factors need to be taken into consideration when designing a support structure for a Dewatering Screen, including forces, natural frequencies and multiple unit installations. McLanahan can provide design criteria and assistance; call our offices for support or contact us now. As a quick guide, Dewatering Screens should never be underestimated in their ability to shake a structure with their mass and the 900 rpm and 10 mm amplitude. Multiple Dewatering Screen installations can be a challenge with harmonics; occasionally a beat will be experienced. In some cases, vibration transference through the ground can be experienced in other structures. Good design practices include placing columns directly under the support points and substantial foundations to provide mass. Always allow 3” (75 mm) of clearance to other components in the direction of action (longitudinal to the screen) so no impact occurs when the vibratory motors or exciters go out of phase during shut down. Electric cables should always be supported/suspended from above directly to the junction box and never draped over vibrating components. McLanahan’s Dewatering Screen motors come with sealed junction boxes with long extended cables to connect into the electrical circuit.

McLanahan’s personnel pioneered the use of deck-dividers to allow two or even three products to be dewatered at one time. Producing a fine product on one side and a coarse product on the other with the facility to blend is part of the standard offering from McLanahan. Process engineers will assist with the application to ensure all factors are considered. Extensive out-of-balance loading (one or more of the products not being fed for long periods of time) can decrease the lifespan of the equipment.

The smallest aperture on a Dewatering Screen is typically 50 mesh (0.3 mm), so you will expect to see some of that size fraction and finer in the underflow of the screen. Not all of that fraction will pass through due to the very nature of caking style screens; fines are captured in the bed formed on top of the media. The usable fraction that does pass through the screen can be handled in several ways. The easiest is to recycle back to the device feeding the screen. In cases of greater flows, a dedicated scavenger circuit could be used.