South Africa – Southern African diamond development and exploration company DiamondCorp has proudly achieved the two major milestones that it had set for itself 2015 at its 74%-owned Lace diamond mine in the Free State province of South Africa. Written by Chantelle Kotze.

While still in the process of steadily ramping up to achieve commercial production in the first half of 2016 at the first mining block at Lace – the high-grade Upper K4 (UK4) block, DiamondCorp has been able to simultaneously commission a 400 tph underground conveyor system from the first production level – the 310 m level – to surface.

One of the last dump truck hauls at the Lace mine prior to the conveyor installation
One of the last dump truck hauls at the Lace mine prior to the conveyor installation

DiamondCorp CEO Paul Loudon says that the commissioning of the 1.6 km long conveyor system is a game changer with respect to the development of the Lace mine, as it will enable an effective ten-fold increase in the capacity to haul ore out from the mine – a task previously undertaken by five dump trucks hauling only 15 t of ore on a two hour round trip.

He says that the conveyor belt capacity has been sized at double the current front end capacity of the processing plant so that future opportunities for increasing the underground mining rate are not constrained by an inability to transport ore to surface, as well as providing an ability to handle both kimberlite and development waste.

“Not only does the conveyor belt provide Lace with the ability to increase development rates, the cost of future development will decrease in the absence of the need for trucks for waste hauling, as the underground loaders will now empty their buckets directly onto the end of the conveyor belt,” Loudon explains.

Continuous mine development

The old workings at the Lace mine
The old workings at the Lace mine

DiamondCorp’s second milestone has been the successful development of twin declines down to the 310 m production level, where the company has since started establishing the infrastructure required to mine the UK4 block, which it will do by means of long hole open stoping, using a Sandvik421 long hole drill rig.

Loudon explains that the reason the kimberlite pipe is initially being mined using a long hole stoping method to ensure that the project generates early high cash flow ahead of the block cave development.

DiamondCorp will subsequently develop the twin declines down to the second production level – the 470 m level – where the first block cave will be established and allowed to propagate.

Next major milestone – waste sorting

The company’s next milestone to be achieved is the implementation of an optical/X-ray waste sorting system to optimise the processing of the Lace kimberlites.

Positive test results have been received from initial 3 t bulk tests undertaken on an optical/X-ray waste sorting system for the processing plant to optimise the processing of the Lace kimberlites and demonstrated that the majority of the waste can be ejected ahead of the DMS processing plant.

Loudon explains that waste sorting is being investigated owing to the constraint of water at the mine.

“While not a unique constraint to Lace, the current drought has meant that there is a general lack of available water across most of South Africa. This is the reason for the investigation into waste sorting,” says Loudon.

The Lace mine produces both high grade and  lower grade kimberlite
The Lace mine produces both high grade and lower grade kimberlite

A conventional dense media separation (DMS) plant, such as the 220 tph DMS plant at Lace, uses an average of 1 m3 of water for each ton of kimberlite processed – a significant amount of water.

Without the installation of a waste sorter ahead of the DMS plant, Lace will require about 45 m3  of water to keep the plant operating at capacity.

Because the kimberlite pipe at Lace only produces about 25 m3ph of water, with the balance of water coming from surface rainfall which is stored in the water storage dam on surface, Lace has evaluated ways of using less water.

One of the measures implemented to tackle the water issue at Lace was installing a de-grit circuit within the plant and  increasing the bottom screen size in the processing plant from a 1 mm screen size to 1.25 mm. This enables the plant to effectively reject the small sand fraction size diamonds, which use the largest amount of water to process.

While this has meant that DiamondCorp will lower its recoverable grade by up to 20%, the small diamonds being discarded in the sand fraction are the least valuable, meaning those that are recovered are worth on average 20% more per carat.

“By doing this, we are relatively revenue neutral, but now only need to use 0.6 m3 of water per ton as opposed to over 1 m3 of water per ton,” says Loudon.

Meanwhile, Loudon says the addition of a high-volume optical and X-ray waste sorter will have the knock on benefit of optimising the processing plant even further by reducing the volume of internal waste rock from the kimberlite ore prior to processing.

“Because the Lace kimberlite contains between 45% and 85% of internal waste, the removal of this will not only ensure significant water consumption savings, it will also increasing processing rates, says Loudon.

Loudon says that the company’s rock engineers believe that Lace can be mined up to 60% faster than the 1.2Mtpa initially proposed, without over-pulling the cave.

“By taking out the waste, this means we can get to that faster mining rate within the existing plant and water constraints. And having the belts sized to double the plant front end means we will never be constrained by ground handling. If successful, this could see Lace’s peak production increase from 500 000 carats per annum to more than 700 000 carats per annum,” Loudon explains.

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