Safe, deep level mining at record-breaking levels

3rd April 2009 By: Fatima Gabru

Gold miner AngloGold Ashanti is pioneering safe, deep level mining, with the announcement of its record breaking, below 3 777-m working depth at Mponeng mine in early February.

AngloGold vice-president: Southern Africa Johan Viljoen confirms with Mining Weekly that Mponeng is its deepest operation yet, and that no other current, global operation is as deep. The company will be applying to the Guinness Book of World Records to record this feat.

Viljoen outlines three significant factors which the company is working on improving. These improvements are contributing to its deep level success. The first is to reduce the incidences and effects of fatigue on workers. Secondly, to control underground heat and its effects on people, specifically at these depths, and thirdly, the reduction of the effects of seismic events on workers at depth.

AngloGold studied the effects of fatigue on miners by looking into their living conditions, as well as issues such as nutrition and sleep conditions. In addition, factors such as the amount of time spent underground and their working environments were also studied, he says.

While these are factors the company constantly looks into across its operations, they become highly significant for deep level mining. He says that, from a people perspective, the company has been working extensively to improve the overall living and working conditions of all its employees.

Ice Cooling
To create a comfortable work environment, the company employs the use of ice-technology for underground cooling.

"Mponeng is the only deep level mine in the word using ice-technology for underground cooling, with great success," comments Viljoen.

Ice is generated at a rate of about 150 t/h on surface. Further, with the energy limitations the country is facing, the company has an initiative called ice-thermal storage. This enables the mine to produce ice in the low energy demand periods, store it and then melt the ice and send the cool water down the mine during high energy demand periods. In addition, the use of ice-technology requires only a third of the water used for conventional refrigeration pumps.

Seismic Events
AngloGold has been implementing the scattered mining method in its intermediate depth mines in the Vaal river region, and the longwall mining method in the deep level WesWits region mines.

Over the past 10 to 15 years, the company has been converting its deepest underground operations from longwall mining to the sequential grid mining method. Successful sequential grid mining has a positive effect on countering seismic events.

Viljoen comments that the company has been successful with this method of mining, which has made it easy for the company to take the next step into true deep mining.

An advantage of sequential grid mining is an improved understanding of the ore-body and the geological structure ahead of the advancing mining activities.

The design parameters in a sequential grid are managed significantly differently from either scattered or longwall mining. In sequential grid mining, the energy release rate (ERR) associated with the mining front is initially relatively low, owing to the small mining spans, and gradually increases with the increasing mining span.

By limiting the mining span to a designed maximum, the ERR is also capped. The average pillar stress of a reef pillar left on site can be kept to an acceptable level, by designing the pillar size in relation to the mining span.

Viljoen explains that seismic activity is caused by stress changes on structures at depth, as well as by mining volumes. He says that the understanding and implementation of sequential grid mining emphasises that the balance between seismicity, structure at depth and mining volumes is critical, and is underplayed in the mining ratios, extraction ratios and other related issues.

He states that the company has adopted the sequential grid mining method at Mponeng and will be extending this to its other operations.

Further, he outlines the microtechnologies at Mponeng to counter seismicity effects. "These steps cover the widest scope of available measures to reduce the impact of seismicity," he adds.

He states that Mponeng has been successful with the preconditioning of stope faces. Preconditioning is the drilling and blasting of longer holes, which are longer than, and detonated just before, production shotholes.

These holes do not add to the production advance. Preconditioning is primarily aimed at mobilising the fracture zone in front of the advancing stope panel to force it to dissipate its energy, as well as reduce its load carrying capability.

This effectively moves the high stress zone further away from the mining front. When a seismic event occurs, the energy of the seismic wave is dissipated in the fracture zone, significantly reducing the risk to workers, he adds.

A further step to counter the effects of seismic activity is in the control and management of systems design through the planning process, which is critical, states Viljoen.

For instance, in AngloGold's deep mining operations the placement of mine-wide backfill, which fills in over 75% of the area mined, has significantly reduced the occurrence and effect of large magnitude seismic events. The backfill is kept about four or five metres from the advancing panel face.

This is combined with very good elongate support and rockbolting in the main corridors. Going deeper, the company has studied mine designs and will adapt the size and position of stability pillars to accommodate the increase in depth. Consideration is being given to leave dip and strike pillars for increased stability.

"The company has changed the key criteria for the design of the mine to fit with deep level mining. Further, the company is looking at mining advancement rates and other technologies to reduce any exposure of workers to the rockface," he adds.

Safety Considerations
Viljoen states emphatically that AngloGold would not go deeper if it was unsafe. He emphasises the company's first value of safety, and that its current deep level mining is no different to the risk the company is taking across its operations.

The company's safety programme, Separating People and Risk, is exploring the reduction of personal exposure of workers to the rockface when mining at depth.

"The company is spending a substantial amount of money on, as well as learning from, the deep mine studies project being conducted by the Safety in Mines Research Advisory Committee (Simrac) since the early 2000s.

AngloGold is using these studies as a base to continue and expand its work into deep level mining. Through Simrac there are some good ideas being explored to enable even deeper levels of mining," adds Viljoen.

He says that, for deep level mining, the company uses the same miners as its other operations, who have been made aware of the risks of deep level mining.

Future Depths
Viljoen states that AngloGold is currently going 300 m down from the current base of 3 777 m at the Mponeng gold mine. "The project is proceeding successfully, and stoping operations will begin in about three years, with the first gold from this project expected in 2011," he says.

The company has completed the feasibility work and is busy with the financial assessment of the Carbon Leader project. "On this project AngloGold will be going down to the 144 level, which will be 4 390 m below datum, and 4 450 m below collar. The first gold from this project is expected around 2018 and will extend the life of the operation until 2040," concludes Viljoen.