Vibrating equipment and solutions original-equipment manufacturer Kwatani, previously called Joest Kwatani, is capitalising on the recent drive by South African mines to increase the efficiency of their water use through innovation and technology by providing customised vibrating dewatering screens.

Kwatani CEO Kim Schoepflin notes that the series of droughts in South Africa over the past two years, coupled with the general scarcity of water, has influenced the mining industry’s drive towards improving the efficiency of water use.

Mines also operate in highly water-stressed regions and are in competition with surrounding communities for the commodity, resulting in the industry searching for alternative water sources to reduce its dependence on relatively fresh groundwater from existing wellfields.

The water-intensive process of mining, especially in processing plants, requires mines to reduce their overall water use through the introduction of innovation and technology, instead of relying on the discovery of new water sources.

Once used for processing, waste water is pumped into a tailings dam. Schoepflin says that the primary consumptive use of this waste water is in slimes dams and associated return water. However, she laments that much of this water is lost to evaporation, owing to the size and surface area of the dams. Moreover, they are potentially dangerous, as they can collapse if their design capacities are exceeded. This environmental risk and the drive to conserve water have promoted the use of alternative tailings storage methods such as dry stacking or backfilling the mining pit with suitably dry material.

While water is a much-needed part of the mining process, this is completely reversed when dealing with the production of the desired end-product and tailings, as, in these instances, water is an unwanted component.

The presence of unwanted water impacts on the quality of some products, which necessitates dewatering, says Schoepflin. This process prepares the product to the quality desired for sale to customers, she adds, noting that wet coal, for example, is subject to financial penalties as it lowers the product’s calorific value.

In the case of tailings, a certain level of dryness is also needed to allow for transport on a conveyor and for storage or stacking.

Dewatering is also used to recover water for reuse in the processing system, which Kwatani facilitates through its vibrating dewatering screens (VDSes), says Schoepflin.

Innovative Design

Schoepflin tells Mining Weekly that the design of Kwatani’s VDSes is focused on the incline of the dewatering screen’s deck, the G-forces that power the machine and the vertical stroke required to generate this power to deliver VDSes that meet customers’ specific requirements regarding tonnage and moisture content. She says customer requirements may vary, depending on the mining application, the size, particle distribution and the type of ore being processed.

Kwatani’s VDSes are designed to have a slightly inclined deck, sometimes up to 5º, depending on the application. Schoepflin explains that the feed that comes in forms a pool in the angle between the back of the screen and the inclined deck. A large amount of drainage occurs at this stage, after which particles from the feed form a cake on the bed of the screen. This compacted cake acts as a filtration bed, allowing for only limited fine particles to pass through.

Through the vibration action, this cake is conveyed along the dewatering screen, during which water is squeezed from this bed of particles off the panels. Depending on the type of ore and/or material being dewatered, this bed can be as high as 300 mm at the discharge end, Schoepflin says.

She indicates that Kwatani’s VDSes apply higher G-forces, compared with conventional vibrating screens, to generate power for the dewatering process using the ideal combination of stroke/amplitude and speed.

Another advantage of the screens is that they allow for enough retention time on the deck of the dewatering screen, which is facilitated by a more vertical stroke-angle alignment of the exciter drive gearbox, while still ensuring that the required tonnage can be processed across the deck.

Moreover, Schoepflin highlights that using tepee-shaped polyurethane screen panels allows for a dramatic increase of the available panel area (compared with conventional panels), which allows for the improved drainage of water, thereby requiring a smaller VDS.

Schoepflin points out that the efficiency of dewatering is usually predicated on particle size distribution, with larger materials being easier to dewater. A mixture of fine and coarse particles is also favourable, owing to the large voids between the different particle sizes.

Kwatani’s VDSes operate successfully, however, in even the most arduous applications, such as mineral sands, adds Schoepflin. For example, the company’s exciter gearbox-driven single-deck dewatering screens can receive incoming heavy mineral sands feed at 300 t/h, consisting of 1-mm- to 0-mm-sized particles and 24% moisture. Through a cut size of 300 μm and an optimised combination of G-force, stroke angle and deck incline, Schoepflin says, the VDSes can successfully dewater and reduce moisture content on discharge to 13% to 14%, resulting in material dry enough to be conveyed and stacked, with the recovered water returned to the circuit.