Inclined Dewatering Screens
The material is usually introduced as slurry. On an inclined dewatering screen the accelerations along with a portion of the gravitational force will cause the material to travel towards the discharge end while the water is being screened out by means of proper screening media.
The resulting force for material travel is indicated as the yellow arrow. The force causing the water to separate from the solids is gravity enhanced by the vertical vector of the G-forces produced by the screen.
The problem in this design is that the gravitational forces are only partially used towards dewatering but also towards material transport. The later will ultimately drag water into the product.
In order to improve dewatering one would have to decrease the inclination of the screen which in return will also decrease the material travel rate drastically. In other words the “dry” product will end up quite wet.
This can raise a couple of problems. A customer could be quite hesitant to pay for a high amount of water in the product. A product w ith high water content w ill flow back on a conveyer belt and will cause severe damage to the rollers of the belt by washing fine solid product into the bearings. Finally the amount of w ater in the product is a loss if the customer doesn’t pay for it just as well as it is a loss if the plant runs a closed water circuit.
Horizontal Dewatering Screens
The horizontal screens used for dewatering are actually not exactly set at 0°. It has proven to be very beneficial to set them up at a negative incline of about 3°.
As for the applications discussed above the material is usually introduced as slurry. Other then on inclined machines the only force resulting in material travel is the g-force produced by the screen. This g-force is aligned a 45° and transports the material uphill. The gravitational forces enhanced by the vertical portion of the acceleration of the machine arc fully utilized towards dcwatcring.
In operation those forces w ill build a material layer on the screen that is pushed out of the wet zone towards the discharge end. A back dewatering field is used to reduce the amount of water right after feeding the screen. The thick layer of material acts as a filter cake and not only presses water out but traps fine particles that would be lost in a thin layer screening process.
Furthermore a dam at the discharge end is normally used to further enhance product quality.
The main advantages of that technology can be summarized as follows:
- Large angle between material transport and gravity = Good separation
- Filter-cake bridges openings and “traps” fine particles = Minimal material loss
- Water won’t run uphill + Filter-cake presses water out = Excellent dewatering.