黄金:将垃圾转化为财富

With the rocks collected and hoisted to the surface, it was time to go back up to ground level to see how the gold is processed because, unfortunately, this isn’t like you see it in the films. There are no shiny hunks of gold waiting to be pulled out of the walls of a tunnel. Instead, tiny particles of gold are locked up with the rocks alongside other useful materials, like uranium and copper, and there’s also a huge amount of waste in there, things like pyrite or fool’s gold. The first stage in getting the gold out is to break up - or mill - the material to liberate the gold particles, as Ricardo Barker Cooke explained. Chris Smith then caught up with Grant Stewart and James Wellsted about further gold processing.

Ricardo - The gold is situated in your finer particles and you need to actually grind it to get to the extra gold.

Chris - And how do you smash up the particles?

Ricardo - Currently we've got six mills. So basically what happens is your feed goes into your mill and it's basically the steel balls that is crushing your material.

Chris - So the steel balls bouncing around inside the milling machine, and as the slime goes between the balls it gets crushed?

Ricardo - That's correct, yeah. It's basically just crushing but on a much smaller scale.

Chris - And how much stuff is going through these mills every minute or two?

Ricardo - Okay. We pushing 400,000 tonnes a month. We're talking about doing about 550 tonnes per hour.

Chris - That's a lot isn't it?

Ricardo - It is a lot.

Chris - Next, the excess water is removed, leaving a pulp. And then stage three involved something I wasn’t expecting…

Ricardo - After thickening we get to a section which we call the leaching section. And in the leaching section is where we add cyanide.

Chris - Say that again, you use cyanide?

Ricardo - Cyanide. Sodium cyanide which gets used for gold dissolution.

Chris - The milled ore is mixed with cyanide, and that pulls the gold out of the rock and into solution, and then you add carbon to soak it up...

Ricardo - Carbon is a reagent which has got pores, and that will absorb all the gold onto the carbon. The gold is not in its purest form yet because it still has all the other impurities which is within the slurry. Then the carbon gets taken to the elution process where we expand the pores of the carbon and then we take the gold out back in to the solution form.

Chris - So having now separated the gold from the carbon, which is done using sodium hydroxide and a very high temperature, they use electricity to pull out the gold in a system called an electrowinning cell…

Ricardo - And that solution gets circulated through the final stage which we call the smelt stage whereby it circulates the electrowinning cell and that's where we produce our final product. Still not in its purest form and that gets dispatched to rand refinery three times a week.

Chris - What they then send off by helicopter is about 88% purity. And after the final clean up at the refinery, you’re left with about a 99% pure gold bar.

The current process tries to minimise the waste that it produces. Indeed, once the gold has been extracted, the mine is backfilled with that waste. But it hasn’t always been like that. Across South Africa there are huge piles of waste material - known as tailings - which have built up since gold was first discovered in the country in the late 1800s. But techniques have improved a lot since those days, meaning that the clean up process can now actually pay for itself. Grant Stewart heads up the gold retreatment facility that’s handling the old mine waste from years gone by…

格兰特——的基本目标之一programme is an environmental and socially responsible objective, and that's to take all the nasty tailings dams which exist over 40/50 kilometre radius to a central deposition site, thereby taking away the dust pollution, freeing up available land for development and obviously there's a benefit from a gold and uranium and a sulphuric acid perspective that would make it economically viable.

There's around 795 million tons of material that we have available for immediate processing and that would obviously be processed over a life of around 30/35 years. But that, essentially, contains about 150 odd million pounds of uranium, and I think it was about 7.1 million ounces of gold. So there's a substantial amount of economic benefit from being able to extract and process these tailings.

Chris - So literally a case of trash turning into treasure.

And with my day in the mines nearly over, it was time to catch up once again with James Wellsted and we considered the scale of South Africa’s contribution to the world’s gold supply…

James - I think I've heard numbers of around 40% of all the gold in circulation, of which most of it is still in circulation because gold doesn't tend to get used up in any processes. In terms of reserves, if you look at resources that still lie underground and that potentially could be unlocked through new technologies, there's still probably as much as what we've mined in the past, so significant amounts. Much more than, I think, any other country in the world. But the problem is it’s depth and the cost of extraction which makes it difficult too, so that's why it's likely to possibly lie there for quite some time.

Chris - So next time you go walking past a jewellers, you can safely assume that over a third of the shiny stuff in the window probably came from here, near Johannesburg but a very long way down. I’m not sure if this programme qualifies us for the world’s deepest radio programme or podcast but it should certainly make us all think deeply about how we reach and how we use the world’s resources; and, as I drove back to Johannesburg away from the deafening drilling, mining and processing noises, I experienced something else golden - silence...