DEWATERING SCREEN

What is a dewatering screen?

Any material that comes into contact with water will get wet. Material needs to be washed or transported as concentrate and waste pulp in the enrichment of the ore for the production of the aggregate and sand necessary for ready-mixed concrete production in ore preparation facilities and in the construction sector.

Finely ground sand or concentrates that are to be transported by ship should have 5% water content. Otherwise, the material in the ship's hold will liquefy due to the movement of the ship over waves, making it behave like water, and may cause the ship to sink. Furthermore, transportation costs are increased by the water content of the ore. For this reason, moist materials must be dehumidified.

It is possible to reduce the humidity of wet material to below 1% in a furnace at a temperature of 105 degrees. This is a costly operation, and it is more economical to dry the remaining surface moisture when the free water in wet or washed materials is removed through special treatments.

After a certain stage, washed sand must be purged of free water prior to use in aqueous concentrate, for transportation or for the following process. Dewatering processes remove the free water from liquid-solid mixtures through the use of screens or centrifugal force.

For which processes/applications are dewatering screens used?

The simplest and least costly dewatering process involves leaving the material in a pile and waiting for the water in the material to filter out. This is a time consuming process and requires a large storage area, and is applied in businesses have no time or space constraints.

Dewatering screens quickly remove free water from the material and reduce its moisture to a level that can be transported, used, or dried on belt conveyors.

After washing the salt, clay, lime and waste from concentrate, waste, coal, sea or river sand, dewatering screens are used to remove the free water from the material.

Dewatering screens are also used for the dry storage of waste produced by ore preparation and enrichment facilities, or before dehydration.

What are the main criteria to follow when selecting a dewatering screen?

Dewatering screens are usually horizontal or very close to horizontal. The screen opening and the direction, amplitude and frequency of the vibration are selected based on the size of the solid to be separated from the water and the expected outcome of the process.

If the flow rate of the liquid+solid to be dehydrated is too high, water will flow over the screen alongside the material without passing through sieve during dewatering process, defeating the object of the exercise. Using multiple dewatering screens side by side can overcome this problem, but in such cases, the dewatering process will be uneconomical and so the density of the pulp should be increased first by subjecting the solid+liquid mixture to a cyclone treatment before treating with a dewatering screen.

What are the main criteria to be followed when selecting a dewatering screen?

·        The inclination of dewatering screen is between 0 and 5 degrees, and these screens operate at a frequency of 1000–1500 rpm. If necessary, the screen sieve can be moved in the opposite direction to its inclination.

·        The screen opening should be selected based on the size of the material to be dewatered. As the material size increases, so should the screen opening, and can be as small as 45 microns for small-sized materials, and up to up to 500 microns for larger materials.

·        For example, screens with a 250–500 micron screen opening are used for dewatering coal in sizes larger than 6.3 mm. If the part under the screen contains coal, it is evaluated separately – the screen opening for 2 mm waste iron ore is around 100 microns.

·        Motion is provided by two linear motion vibrators operating in tandem. These vibrators can be connected to the screen sieve in the same direction or vertically. By changing the direction and weight angles of the vibrators, the screen vibration can be optimized to suit the operating environment.

·        Unbalanced weighted vibration motors are used in dewatering screens. Each of the unbalanced weights attached to either side of the shaft have two parts that can slide over the other. The vibration characteristics can be changed by changing the position of the unbalanced weights placed on top of each other on either side of the shaft in symmetry.

·        The weights on the vibration motors of the screen should be located symmetrically. The imbalance created by the asymmetrical weights on the screen body prevent the screen from running.

·        Dewatering screens generally feature a single-layer moving screen system with an adjustable slope, and contain linear channels that collect the water as it filters through to below.

·        If the screen vibration is insufficient, the material will not accumulate on the screen and the water-solid separation will be inefficient. If screen vibration is too high, on the other hand, the water will flow over the screen with the material and will not pass through the sieve.

·        Dewatering screens are made of modular polyurethane materials. The modules have a very long useful life and are easy to replace when necessary. Polyurethane screen sieves, and pumps and cyclones covered with polyurethane material are protected from abrasion.

·        In solid+water mixtures, the material density is high and there is a tendency to collapse. During dewatering processes, solid-water separation occurs in the layer formed on the screen sieve, and the material close to the screen opening forms a layer on the sieve due to the vibration. This layer naturally prevents water from passing through the screen. Water flowing over this layer formed by material creates also a water layer on the top that prevents the material from passing through. In order to prevent these layers from forming, “V” shaped or “--------“ lath-shaped sets are placed on the screen at certain intervals. When it is necessary to obtain very clean material from the dewatering screen, the washing water supplied to the screen sieve through the crowbar jets for washing also prevents stratification.

·        In properly operated dewatering screens, the layer formed by the material at the bottom acts as a filter.

·        A dewatering screen with holes should never be used.

·        In ore preparation plants, very fine material may enter the concentrate or waste, which is undesirable for the following process, and these can be removed using dewatering screens.

·        In dewatering screen processes, unwanted fine particles are removed as slime by dewatering cyclones.

·        Single cyclones are used in ore preparation and sand facilities, while 1st or 2nd grade construction sand is produced using double cyclone dewatering screens, especially in sand washing facilities.

·        In double cyclone dewatering screens, sieves with two different opening sizes can be used with a separating plate on the screen surface. This dewatering system can be used to produce washed sand measuring 0–5 mm.

·        Dewatering screens are used for ore preparation, and for sand and gravel production, as well as for cleaning stagnant water, and sludge from the beds of rivers and ports.

·        Through the use of polyurethane screens attached to high-frequency motion system, the moisture of the treated material can be reduced to 10–15%, bringing it to a state in which it can be transported by belt conveyors.