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Floating diamond-recovery vessel performing well
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16th November 2001
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The South African designed and built processing plant on board the world's biggest floating diamond recovery vessel, Ya Toivo, is performing well off the Namibian coast.

The vessel, which is owned by the Namibian Minerals Corporation (Namco), began full-scale testing late last year and made its first diamond recoveries from the seabed last December.

Local company Dowding Reynard & Associates (DRA) was responsible for managing the construction and the design of the on-board processing plant.

Ya Toiva, previously known as the British Royal Navy nuclear submarine recovery vessel HMS Challenger, had to be converted and strengthened to accommodate the load.

Its width was increased from 18 m to 24 m by building an outer hull with full-depth tanks.

This work was carried out by Namco in Poland.

Internal decks and bulkheads were cut away by DRA, and what remained was stiffened to restore the ship's integrity and to enable it to support the 3 700 t load that was added.

Below the water's surface is the diamond-mining machine, a remote-controlled tracked vehicle with an integrated cutting and suction head, operated from the ship on surface.

Designed by Namco engineers and built in South Africa, the R48-million 185 t machine is targeted to recover diamonds from the seabed worth some R240-million in its first year of operation.

DRA process director Clive Hart reports that adapting the vessel to accommodate the diamond-recovery plant posed a number of technical challenges.

"Being a light ship, its decks and bulkheads did not have much capacity for support.

"We were obliged to span 18 m to land the loads on the original hull," he explains.

Construction work itself was carried out in sections by Dorbyl Marine at A Berth in Duncan Dock, Cape Town.

Fabrication began at the beginning of last year and on-board construction a few month later.

It was ready to sail at the end of November, with all work, including piping and electronics, complete.

The 9 000 t vessel is fitted with a global positioning satellite system to position the underwater mining machine within centimetres of accuracy on the ocean floor.

Once the crawler is lowered on to the seabed, transponders and electronic sensors inform the operator on board the vessel of the location of the machine at all times.

The machine operator uses a joystick to manoeuvre the machine and its suction arm on the seabed to remove all diamond-bearing material.

Sonar links give the operator real-time three-dimensional visualisation of the mining operations more than 70 m below the surface of the sea to ensure that no material is left behind.

A specially-developed dredge pump fitted inside the crawler provides large suction power at the ocean floor, while cutters break up harder material and assist in excavation.

On board, material from the seabed is fed to the communition circuit consisting of an attritioning mill and crusher to break down the clay and shell, which is removed and then washed overboard, leaving only the diamond-bearing gravel.

The dense-medium separation plant, designed to take 100 t of material an hour, performs repeated gravity separations and the result is fed to the X-ray sorters, where diamonds are identified by emitting fluorescence when treated with X-rays, and separated.

The tailings are returned to the seabed through four moon pools or disposal shafts.

Namco CEO Alistair Holberton says that Namibia's ocean diamond-mining industry has risen from 0,5% of Namibia's total diamond production ten years ago to over 60% in 1999.

"This growth is being driven by technological innovation.

"There will always be strong demand for Namibian production as about 95% of all our diamonds are gem quality," he concludes.

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