What really possesses men to dig a hole to 4.5 km beneath the surface of the earth? How do you dig a hole that big, and more over how many people does it take to make a hole that big? And what is so important that we have got to risk the rewards of such a dangerous adventure? The simple answer is gold.
ANGLOGOLD have embarked on a project to deepen the flagship Mponeng gold mine in South Africa and extend the life of the mine to beyond 2040. The current life of mine runs to 2017 and a project is underway to deepen the mine to further exploit the Ventersdorp Contact Reef (VCR), pushing the life out by another nine years via declines.
In September 2007 the Mponeng gold mine retook the record to become the deepest point in the world, pushing down through 3,770 metres. Their extension project, which is thought to cost R9bn, would push the mine down to between 4,300 and 4,600 metres.
The project is designed to maintain feed at the nearby gold plant steady at 160,000 tonnes a month. Mponeng has been exceeding feed to the plant and there is a 40,000 tonne stockpile sitting in the plant which makes you wonder how we can really be in a pickle financially around the world when gold is at an all time high price wise.
The project will entail the construction of three vertical tertiary shafts to access the VCR. Experiance from other mines which attack the same ore deposit give the Mponeng team a good idea of what to expect from the reef. It is considered a very uniform body with a steady volume of gold ore extraction as they follow the seam.
One of the key issues management has to consider in advancing the project is the safety of its workers, who will be dealing with rocks at a temperature of 70 degrees Celsius, and in an area where seismicity is an issue, particularly at depth. Over 5900 people are employed in the warren of tunnels under the city and safety is of paramount importance to the company who have set safety records in many ways over the last 10 years. The likely option is that the work will be as mechanised as possible, keeping as large a gap between workers and danger as possible.
The Mponeng main shaft, which reaches to 85 level (some 2,300 metres below surface) was commissioned in 1986 and the sub-shaft in 1993. More recently, the shaft deepening project down to 123 level was undertaken to access the Ventersdorp Contact Reef (VCR) to extend the life of the mine by three years.
The mine extension consists of four parallel decline shafts that are to be sunk from the 120 level to gain access to the Ventersdorp Contact Reef on levels 123 and 126, some 3,600 meters below surface. The declines will be equipped with a conveyor belt, monorail and chairlift to service the new mining area.
The project, from which production will start in 2013, is expected to produce 2.5Moz of gold over a period of 10 years, at a capital cost of $252m, and will extend the life of the mine by approximately eight years to approximately 2024. It also creates further opportunities to access additional potential projects in the Mponeng area, including Western Ultra Deep Levels (WUDLS), further to the south which ultimately will take the mine beyond the 4km zone as it pushes ever deeper.
Since the commission of the main shaft in 1986 some 422,011 ounces of gold have been recovered from the Mponeng mine operation at an average cost of $236 per ounce. This works out at approximately $9 billion in costs to operate the mine, but gives a profit in excess of $34 billion at today’s gold prices. The volume of earth removed from the core of the mine is staggering when you put into scope the 160,000 tonne target that the mine aims to produce each month. Conversely the total of square metres mined has increased from 20,000 m2 a year to 27,500 m2 a year since 2003.
Ventilation is critical for this project and extending the mine further has presented a huge engineering problem. However Mponeng is one of AngloGold Ashanti’s showcase mines, as reflected in its use of technology. It boasts the world’s only ice plant of its kind, used for ventilation cooling in a mine where the rock temperature can reach close to 70°C. The ice plant consists of six units, each capable of producing 800 tonnes of ice a day. With one of these acting as a contingency unit Mponeng produces 4,000 tonnes of ice a day, and this is done by using vacuum conditions to induce the formation of ice at the triple point of water. The ice is mixed with brine to prevent it forming blocks, and is poured down the ventilation shaft to 84 level (some 2,300 metres below surface). With the ice having five times less specific gravity than water, the mine reduces by that factor the amount it is required to pump to achieve the same cooling.
When you think about it, there is very little that man is incapable of, and our demand for minerals and raw materials is insatiable. I sometimes sit and think to myself, we must have removed millions of tonnes of gold from the earth below us already. Where has it all gone. Man has been panning, mining and speculating for gold as long as history can remember yet our thirst for more drives us to dig many miles below the crust we trust as being safe, just to ensure that somewhere someone is in power over the rest of us, by having massive stock piles of the world’s most precious resource.