The application, with the scrubbing process replicated within the chute is very unusual, said Weba. The chute design induces directional changes which create turbulence within the material being transferred. To further facilitate this replication of the scrubbing process, water is introduced at strategic points inside the chute. The increased capacity of the new chute is necessary to optimize the scrubbing process within the chute.

The new scrubber chute is bifurcated and allows 100:100 operation with a maximum lump size of 150 mm, as opposed to a standard operation with 50:50 split. The chute feeds product directly onto the primary screen. Specially designed flood boxes allow for absorption of 1,080 l3/h.

A new head pulley installation was repositioned to allow the incorporation of the return fines into the chute in order to eliminate any fines build up in the dribble chute that could result in blockage and excessive maintenance. The support structure and platforms were built accordingly.

Damtshaa has a projected life of 31 years, during which time the mine is expected to mine 39 million mt of ore, yielding 5 million carats. Some 228,000 carats were recovered in 2006.

A Weba chute system has also been engineered and manufactured for retrofitting at Debswana’s Letlhakane operation, the second mine developed by the company, which opened in 1975. This bifurcating chute feeds onto two screens and is designed for maximum lump size of minus 100 to plus 25 mm at a feed rate of 560 mt/h with a peak of 700 mt/h. The incoming belt is 1,050 mm wide, running at a speed of 1.63 m/sec. The primary objective of the redesign and replacement exercise was to guarantee a 50:50 feed split of 350 mt per leg at all times, onto screen. A shut-off arrangement was included to close off one leg when necessary. The chute is also capable of a 100:100 feed at a maximum feed rate of 700 mt/h.

An improvement on the previous water spray system was incorporated in the design to facilitate an effect similar to that being achieved with the Damtshaa chute; and an overbelt magnetic has been fitted to assist in the removal of any tramp steel within the ore being discharged into the chute and onto the screens. Letlhakane recovered 1,089 million carats in 2006.

Weba says that both these chute installations have advantages including minimum maintenance, a reduction in dust and noise pollution, reduced wear, easy access for checking, optimum material flow, a decrease in material degradation and elimination of uneven belt loading. These help the mines achieve considerable savings in manpower and related costs.