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Failure not option as tech champions push forward through rocky R&D terrain

MECHANICAL MINING Gold reef being bored at AngloGold Ashanti's TauTona gold mine, west of Johannesburg

MINING THE REEF Gold ore has been extracted from these holes without blasting

SHAUN NEWBERRYHuge number of gold ounces cannot be mined conventionally

19th September 2014

By: Martin Creamer

Creamer Media Editor

  

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Only 60% of the gold ore in South Africa’s rich Witwatersrand basin is accessed; 40% is left behind in pillars to protect against seismicity.

The mining method then loses a further 25% to 30% of that 60% because blasting breaks the ore into so many pieces that not all of them can be recovered, with fractured sizes ranging from fine powder to large boulders.

Once blasted, the product is moved up to seven times, which results in more losses.

On average, there is 200% more waste than gold in the mix moved and dilution can be as high as 1 500%, which means that huge volumes of nonpay material has to be transported out of ever-deepening cavities.

One hundred grams of gold in the ground becomes 30 g or 25 g of gold on surface, after getting out only 60% of what was there in the first place.

Asset use is hamstrung by mining taking place only on 275 days of the 365 days a year.

Moreover, in the drill-and-blast environment, only 66% of each day is worked, leaving net physical asset use at 50%.

What is far worse is the death and injury rate, which is unacceptably high.

During the twentieth century alone, the Leon Commission calculated that 69 000 people were killed in South Africa’s mines and a million seriously injured, maimed or physically damaged.

Currently, safety has plateaued and, unless significant change is made to what creates this plateau, death and injury in mines will continue.

There is thus an absolute need to change for deep-level gold mining to be sustainable – and AngloGold Ashanti is at the forefront of a move under senior VP: technology and projects Shaun Newberry to ensure sustain- ability. (Also see page 54)

The current mining method is clearly not the one that should take South Africa into the future.

In fact, if it continues to be pursued, gold mining will end long before the Witwatersrand basin’s three-billion ounces of gold are extracted.

Some 1.7-billion ounces have been mined, only 200-million more ounces are in reserve and 1.1-billion ounces stand to be left in the ground unless a new mining technology is introduced.

Since 2010, AngloGold Ashanti has been going hammer and tongs to introduce through research and development (R&D) a safer and a more efficient way of mining narrow-reef orebodies.

It has developed and manufactured proto- types in a relatively short space of time and targeted the slogan “safely mining all the gold, only the gold, all the time”.

The technology has produced 2 000 oz of high-grade gold using newly prototyped equipment.

The proposed mining method is to bore holes of the required reef-channel width in order to mine all the gold and only the gold.

All the bored chips containing the gold are collected and transferred in an enclosed system for further processing.

Backfill stabilises the underground environment and reduces the incidence of seismic events.

In order to reach depths of 5 km and beyond, the rock mass removed needs to be replaced with similar material of 170 MPa compressive strength to prevent movement.

Each hole is sealed at the bottom access point and a high-viscosity mixed product is pumped to the top access point.

All this is already happening.

The focus is on reef boring, geological drilling, backfill and mechanical development plus peripheral activities that enable any of these.

Steps are being taken to enable the reef- boring machines already in use to move and align themselves automatically on the basis of geological data.

In two years, AngloGold Ashanti has managed to develop three reef-boring machines, have them manufactured and put them to the test.

Near-term work is also being undertaken to evolve towards a form of gold liberation using chemicals, thermal energy or other methods to free the gold for hydraulic transport back to surface and achieve the original vision of having gold on tap 24 hours a day, 365 days a year.

Knowledge of rock fragmentation is also an enabler for reef boring, as less power will be needed if better rock fragmentation methodology can be developed.

There are many such projects under way in the background because of their relationship with the programme’s first two stages.

Exponentially faster geological information will be needed to deploy the new technology.

To be able to bore the reef over distance, the location of the gold-bearing rock will have to be determined far more quickly and accurately.

Currently, there is nothing available that can provide the information at the pace needed and something suitable will have to be developed.

In the meantime, the quick win of shaft pillar mining is being seized upon, where such information is already available.

What is being proposed is a mining method that leaves only half of what sequential mining currently leaves behind.

If the pillars are backfilled, what is left behind can be halved.

Innovation Consortium

The technology innovation consortium was established to bring about fundamental safety improvement, stop production decline and release massive revenue potential.

The three-stage strategy, which uses innovation and technology to realise step changes, selected reverse-circulation (RC) drilling as the preferred technology to obtain information ten times faster than the current methods.

RC drilling uses a pneumatic percussion hammer to penetrate the rock.

High-speed haulage-boring machines are needed to create the main infrastructure for logistics and geological drilling platforms as well as smaller-diameter access for on-reef infrastructure.

Once the on-reef grid has been established, the idea is for ventilation to be achieved by drilling holes between the drives using a raisebore machine.

It is envisaged that reef boring and ultra- high-strength backfilling can then take place from the drives.

Reef boring begins from the bottom reef drive using a machine set up on the lower of the two levels to be connected.

Line boring is used to drill a hole from the lower level to the upper level and is then backfilled on completion.

The completed hole has smooth walls and does not require support.

The vision is of mechanical mining being remotely controlled from safe areas and extraction ratios being boosted by a new mining methodology and backfilling of cavities with material as hard as the rock itself.

Targeted are significant portions of orebody currently left behind and the elimination of gold loss down microcracks.

Instead, the plan is to transport consistent smaller chips from the rockface to the mill in one enclosed system.

Much of this is being developed and tested underground right now.

An automated method of providing far greater volumes of backfill at TauTona’s three-rig production site is being tested.

Stage one has three critical areas: geological exploration drilling, mechanical mining and ultrahigh-strength backfill.

Geological exploration drilling provides a far more detailed and accurate geological model from which a mine plan for mechanical mining can be developed to cope with the ore-bearing reef being a complex interwoven series of terraces, all at different angles and heights.

The reef-boring machine is equipped with a cutter to match the variations in width and the inclination of the reef and thus keeps dilution by waste rock to a minimum.

Reef boring produces rock chips of consistent particle size, which are collected in a fully enclosed system for transport to surface.

Once the mining operation is complete, the ultrahigh-strength backfill reduces the risk of seismic events.

The technology is said to have the potential to increase the extraction ratio from the current 60% to 70% to at least 80% to 90% and will help solve the high closure challenge expected at mining depths of 5 000 m and beyond.

Skin-to-skin drilling leaves a wedge of gold that may be recovered by lancing or micro- waving.

The second stage is targeting intelligent mining machines and the third stage a visionary gold-on-tap mimicking of the oil and gas industry.

The three cohesive stages of the roadmap aim to bring sustainability to gold mining at ever-increasing depth, with safety, extraction ratio, dilution, mine call factor and asset use coming under the microscope.

If the country succeeds in finding a viable new alternative, it would not only be able to mine all remaining pillars and go to the ultradepths, but it could also reopen a large number of closed mine shafts and revive mining ghost towns, which is why that national shoulder should be put to the wheel with far more vigour than is currently the case.

Edited by Martin Zhuwakinyu
Creamer Media Senior Deputy Editor

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