OH&S News

Mobile screens prove their mettle in alluvial diamond mines

In alluvial diamond mining, as with any other mining or quarrying operation, a lower cost per tonne of pay-dirt produced would find favour with those whose purse is affected.

Therefore, for one operation to save a massive ZAR40,000 ($AUD4000) per week in diesel fuel costs by employing a duo of Finlay screens instead of an electrically driven static screen run off a diesel generator surely makes a lot of sense (not just cents!).

There is fertile ground for alluvial diamond mining near the present run of the Orange River, south of the town of Douglas in the Northern Cape of South Africa.

In 2017 four experienced diamond miners formed a consortium under the name of Nelesco 318 to mine the elusive gemstones in the reddish earth known locally as Rooikoppie. The group mines two pits, at Remhoogte and Saxendrift.

“With the formation of the mining company Nelesco 318, each partner sold some of his equipment, especially articulated dump trucks (ADTs), into the mix and we took over a variety of wheeled loaders and excavators from the mining group which had previously mined this ground,” Remhoogte Mine manager Cobus Botha said.

“We have a fleet of 10 Bell B40D ADTs running here at Remhoogte and 10 Bell B50D ADTs at Saxendrift. All the machines have between 12,000 and 25,000 hours of service, which is testament to their longevity.”

Mining at the Remhoogte site is done in two shifts for 20 hours per day, five days per week. Topsoil is stripped and stockpiled for use in later rehabilitation.

{{image2-a:r-w:350}}According to Botha, the calcrete gravels and deeper basal gravels are relatively shallow, which means stripping ratios are less critical.

“We load the diamond-bearing gravel with an excavator and this is hauled to a stockpile, from where it is fed into a Finlay 883+ screen,” he said. “We bought the Finlay 883+ screen first, as it is used as a scalping screen and removes the oversized material out of the system at the start of the screening process. This oversized material is fed back into the mining area and forms the base of our later rehabilitation.

“We chose the Finlay 883+ screen for a number of reasons, but most important was that the brand is backed by Bell Equipment. We knew we could count on the technical and parts back-up that would ensure maximum productive uptime for us.”

Botha added some of the Nelesco 318 partners also had Finlay screens in their previous operations, so suggesting the Finlay brand as a first choice was an easy sell.

The operators at the Remhoogte mine have found that a high sand content makes processing the gravels difficult. For this reason, the mine recently took possession of a Finlay 694+ inclined screen.

Once through the Finlay 883+ screen, the material is fed directly into the Finlay 694+ inclined screen, where it is split into -34mm, +22mm and +4mm products. The separation protects the bottom deck of the 694+ against potentially too much weight.

“The way in which the 694+ separates out the sand is quite phenomenal, and we can honestly say that its purchase has only added value, as we are assured of feeding our four, 16-foot [5m] pans with diamond-bearing gravel of a far better quality,” Botha said. “Another noteworthy feature is the amount of fuel and subsequent money we’re saving. Before we used to feed all the material through a static screen that ran off a generator powered by a diesel motor, and this machine consumed 1000 litres of diesel per day. Running the two Finlay screens, we’re now saving up to ZAR40,000 per week.

“The feed to our four pans needs to be approximately 100 tonnes per hour, but our two Finlay screens running in tandem are easily pushing out up to 300 tonnes per hour, which means that we’re always ahead of our processing capacity in the pans.

“This just goes to show that the 883+ and 694+ are the correct tools for our application. With the huge saving in fuel costs, they really do add value to our mining operation by lowering our production costs.”


‘Flawless gems’

A separate quartet of Finlay screens are also proving to be “flawless gems” for mining giant Steyn Diamante at the Schutsekama diamond mine on the Riet River, near the village of Ritchie, south of Kimberley.

It is not a mine where the skyline is dominated by a shaft tower. In fact, the only raised portion shows four large, red mobile Finlay screens processing a constant stream of rock, gravel and sand that is separated onto different moving belts, which spew the material into heaps.

Archaeological data shows the narrow Riet River ran a lot wider some centuries ago.

Alluvial diamonds from as far as the present-day Jagersfontein and Koffiefontein were pushed into the surrounding areas as the river flooded the plain en route to joining the Vaal River further downstream.

This is where Schalk Steyn of Steyn Diamante, doyen of alluvial diamond mining in South Africa, established one of his mining operations in 2015.

“Compared to traditional alluvial mining methods, we’ve gone considerably bigger on this site, using a 240-tonne excavator for loading 100-tonne rigid dump trucks,” Schutsekama Mine manager Wikus de Winnaar said.

“They, in turn, deposit the diamond-bearing gravel onto stockpiles, from where huge 50-tonne excavators feed the material into Finlay 893 screens for separation.

“Anything larger than 125mm is returned to the mining pit to form the basis of rehabilitation, and finer material than that is sent to the processing plant.”

{{image4-a:r-w:350}}De Winnaar said that in April 2018, almost 160,000 tonnes of run-of-mine material was fed through three of its Finlay 893 screens.

This proved, in de Winnaar’s mind, that “we couldn’t mine so sustainably and on such a large scale if it weren’t for our fleet of Finlay 893 screens. We have four in a row here, but use only three actively, keeping the fourth in reserve for when any of the others need maintenance. Regular rotation also takes place so that the load is spread among the four machines.”

The Schutsekama mine works daytime shifts from Monday to Saturday, with month-end long weekends. Machine hours are clocked relentlessly, especially on the Finlay 893 screens. At the time of writing, the newest machine showed 340 hours and the oldest 5477 hours.

The 893 screens are placed as close to the mining areas as possible to shorten the haul distance. This works, as the rigid trucks run full in both directions, bringing mined material to the stockpiles beside the Finlay screens and returning with oversized material for rollover rehabilitation.

“Because of the area where we mine, we come across a lot of wet clay, sand and shale in the diamond-bearing gravels, but we can throw anything at these Finlays and they separate what we don’t want from that which goes to the plant. They are simply amazing,” de Winnaar said.

The four Finlay 893 screens were bought as replacements for four older Finlay machines that were subsequently sold on the pre-owned market.

“Schalk [Steyn] first used Finlay 883 screens back in 2004 when he started mining alluvial diamonds in South Africa,” de Winnaar said. “He knows the machines’ capabilities and while going on to the larger 893 machines has stayed with Finlay, as he has absolute faith in the brand.”

Fuel is a major cost factor for any miner and this site consumes 8500 litres of diesel per day. The Finlay 893 screens have been recording fuel burn of about 20 litres per hour and, given what they produce, this is considered low.

“What we appreciate, as the mining teams and those working with the Finlay screens daily, is that we have the backing of Bell Equipment in terms of technical support and parts supply,” de Winnaar said.

“Mining equipment always needs attention and can surprise you at any time, but knowing Bell Equipment is close by allows us to do what we do best – mine diamonds and rehabilitate the land afterwards.”

In Australia, Terex Finlay mobile crushers and screens are available through Finlay Crushing & Screening Systems (ACT, NSW, Queensland, SA, Victoria and Tasmania) and OPS Screening & Crushing Equipment (WA and the Northern Territory).


Source: Terex Finlay

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