A lift-off for Dwarsrivier expansion

10th February 2006 By: andrew lanham

When Mining Weekly drove onto the Dwarsrivier mine property, this journalist was immediately struck by the orderly appearance of the operation.

Granted, the mine is one of the newest chrome operations but, nevertheless, someone has taken care with the surface layout.

The offices are the original farm buildings, which have been tastefully renovated.

The quality extends to the new underground operation, where the roadways are well maintained and, in spite of the diesel machines, the ventilation is good.

The mine is owned by Assmang, and forms part of Assmang’s chrome division. Assmang bought the Dwarsrivier property from Gold Fields in 1998, and soon after that started to explore the new acquisition.

The feasibility studies were completed and, in 1999, the Assmang board gave the go-ahead for thedesign of a new mine.

In January 2000, construction teams moved onto site, and in March the present general manager, Albie Hamman, took up office to bring the new mine into production.

Seven months later, the first ore was being fed into the new processing plant.

Now the mine is undergoing a sea-change as it converts from being an opencast operation to an underground mine.

The openpit operation had been producing 890 000 tons a year, so the new 1,2-million t/y underground operation is a significant step-up for this Assmang chrome operation.

The first phase of the operation, which lasted until the time ofwriting, consisted of a series of opencast pits, which exploited the outcropping ore down to a 40 m highwall.

While the contractor, MCC, carried out the mining contract, the overall design of the process plant was undertaken by Dowding Reynard & Associates (DRA).

DRA’s associated company, Minopex, was appointed tooperate the plant, an arrange-ment that lasted until three years ago when Dwarsrivier decided to operate its own plant.

“This has been very successful,” says Hamman.

The opencast mining has been carried to full term by MCC, which operated its own fleetduring this time.

When Mining Weekly visited the mine in November 2005, most of MCC’s fleet had moved off site, the site workshop had been removed, and the mining of the final openpit was being completed.

Rock from the openpit operations was fed into a mobile crusher from where it was taken by conveyor belt to the plant.

As far as the underground mine is concerned, the process of bringing the mine into production was begun about a year ago, explains Hamman.

The feasibility was carried outin-house with a certain amount of assistance from SRK Turgis.

TWP handled all the engineering, procurement, construction and management (EPCM) aspects of the job.

The underground operation is ramping up to full production at present with four underground sections.

“We will be establishing three or four more sections as required,” explains Hamman.

Making the change
“At this stage, the underground tons are fairly low, because we are feeding a stockpile of opencast material to the plant,” continues Hamman.

This was built up in anticipation of the changeover.

This is being done at a relativelyfast rate and the surface stockpile should be depleted in January 2006.

Asked if it was possible to feed the underground and surfacematerial to the plant simulta-neously, Hamman explains that though this could be done in theory, in practice it is not advisable.

The nature of the Dwarsrivier plant is a gravity separation and dense-medium separation (DMS) process.

“If you start mixing differentmaterials and the grade varies, then setting up the spiral separatorsfor optimum recovery becomes highly problematic,” Hammanexplains.

The underground material is being stockpiled for the day when the surface material is depleted.

This also gives the mine time to build up to full production, which should be reached in March next year.

The strike north-south is about 7,5 km, the full property being about 1 700 ha.

The underground area that the mine has on plan is about 3 800 ha, though most of the property isunderlain by chrome ore.

In terms of resources, the mine has in the region of 80-million tons.

“This excludes a substantialportion of the mine that we have not drilled as yet,” explains Hamman.

The resource figure is impressive. However, Hamman points out that not all of this is mineable.

At this stage the reserves are about 25-million tons, enough to give Dwarsrivier a life-of-mine of some 25 years.

“There is considerable upsidepotential,” continues Hamman.

The mine is blessed that, largely, the reef plane has limited geolog-ical interferences that might hinder mining.

In the north pit, there was an area that was quite faulted as a result of pegmatoid intrusions, explains Hamman.

A full geological survey, which included aeromagnetic surveys, was carried out before mining was started, so disturbances in the orebody in the north pit did not present a problem.

Geologically there were some major features the mine planners were very much aware of.

On the mine plan, an area is shown where major faults have displaced a block of ground upwards.

“We are aware that, in this area, we will have to carry out some off-reef development. However, this is all in the nature of mining,” says Hamman.

Tried-and-tested mining methods
The South pit has good consistent geology and consequently was the site the mine chose for its first foray underground.

The mining method is bord-and-pillar on an apparent dip of eight degrees, the true dip being ten degrees in a westerly direction.

The strike sections, in turn, are slightly updip.

The operation is fully mechanised, load-haul dumpers (LHDs) being used to convey ore from the face to strike conveyors, which, in turn, feed the main decline conveyor taking ore out of the mine.

The stopes are 12 m wide with pillars being five or six metres square, depending on the ground conditions and the depth at which mining is being conducted.

The mine is blessed in that the chrome deposit gives the minealmost an ideal mining height of 1,8 m.

Hamman explains that, depending on the seam thickness, stope height can be, on occasions, as high as 2,5 m.

The mining fleet of Sandvik equipment can operate at a stope height of 1,6 m.

The mine operates nine EJC 115 LHDs with their 4,8 t payload, as well as a seven-ton EJC 777.

The drillrig fleet consists of four single-boom Sandvik Axera LPs.

Two utility vehicles, a Fermel and a Boart machine, complete the fleet.

There are minor areas of the orebody which go below the 1,6 mcutoff height. However, these would be mined using conventional methods.

In terms of hangingwall support, the normal pattern is on a one-metre grid, using 1,8 m roofbolts.

There were areas near the faulted section of the mine which needed extensive support using long cable anchors and shotcreting.

“We are establishing sections and opening up the mine, so, until we feel we know the underground mining conditions more fully, we will maintain a stringent support pattern,” explains Hamman.

Depending on the rock mechanics at depth, the level of support will be reviewed.

Although the mine is not divided into discrete production levels, typically, one section will consist of nine bords, which the mining team follows.

Currently, the declines have been sunk to a distance on dip of some 600 m.

The declines are projected to go to a final length of some two kilometres, which will take mining to a vertical depth of 350 m.

The south pit decline provides the first access to the underground mine. However, it is anticipated that further declines may be needed in future.

To this end, part of the northern pits has not been backfilled, and this may be used as a boxcut from which to start a future set of declines.

“We may not need these declines. However, as the current declines are in the centre of our orebody, we have made provision in our con-veyor design to mine Dwarsrivier’s deposit from one set of declines,” says Hamman.

Limiting the number of declines is an attractive option in terms of future capital expenditure.

The only support infrastructure needed would be raise-bored ventilation holes of which the mine has two in place at present.

Although the mine has not reached this stage yet, it is envisaged that the strike belts will beextended every 45 m to keep the cycle times of the LHDs withineconomical limits.

The belts will be limited to one kilometre in length “but that is way in the future”, adds Hamman.

Opting for contractor mining
Murray & Roberts Cementation has a two-year contract both to develop and to produce ore.

Chrome-mining is governed by the parameters of cost, quality and volume and, so far, the contractor has been delivering on all three,explains Hamman.

What has been the practice at Dwarsrivier from the outset has been to include the management team from all the contractors in the Assmang Chrome management team.

As such, the contractors formed part of the drawing up of the vision and mission for the mine, and are integral to the realisation of these objectives.

What is desired from any chrome-mine is lumpy ore that will createinterstices in the resulting arcfurnace charge.

Here, Dwarsrivier keeps tight control on its blasting patterns to limit the creation of fines.

“We are careful to avoid double handling of the ore and we have done much work in the plant toensure better and softer handling of the material,” continues Hamman.

“For example, if you look at the lumpy stockpile, you will see that it is fed by a luffing conveyor, which can be raised and lowered so that the drop is minimised,” he adds.

Most of the mine’s metallurgical grade ore is supplied to Assmang’s Machadodorp smelter, althoughthe mine also sells a substantial amount of its chrome ore on theinternational market.

The plant was erected in 2000 and since then a foundry-sand circuit has been incorporated into the plant in 2002.

Foundry sand is manufactured from naturally-occurring fines from the mine. What makes this very neat and orderly mine competitive is that it has the ability to contain costs very effectively.

“At the phase we are in at present, the expenses are high, however, we will work through this,” he adds.

Murray & Roberts is actively contributing to the cost-control process through proactively exploringinnovations of the mining methods, explains Hamman.

In terms of safety, in October 2005, the mine achieved its1 000 fatality-free shift award from the Department of Minerals and Energy.

In the current financial year, the mine operated without a single lost-time injury and with three dressing cases only.