Extracting gold from rock

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The Challenge
Unlike alluvial gold, which is found as very small flakes on river banks, gold-bearing rocks have to be dug out of the ground. We want pure gold, but gold-bearing rocks include lots of other materials such as quartz, calcite and the sulfides of other metals. We need to separate the gold from everything else in the rock so the rock needs to be treated in some way.

In New Zealand we chiselled some rocks from an outcrop near a small creek. We were pretty sure our rocks contained some gold as there were disused gold mines dating back from the 1870s close by.

Crushing the rock
Crushing the gold bearing rock is not as simple as it sounds. We can't just hit it with a hammer because pieces will fly everywhere and we may lose some of our gold. What we need is a very strong container and bash plate. That way, we can crush the rock without losing any of the precious gold particles.

We also need a special kind of hammer, which fits into the container without leaving too much room for the rock to escape. Rather than use a conventional hammer, we used pulleys and a rope to raise and drop a heavy weight. Pulleys increase the length of rope that needs to be pulled in order to raise the heavy weight but at the same time they reduce resistance. If we use a two-pulley system we will have to pull the rope twice as far as if we had one pulley or no pulleys at all, but we will be able to lift twice as much weight. This would be useful because, for the same effort, we can use twice the weight to crush our rocks.

Powdering the rock
The next step was to take the crushed pieces of rock and make them even smaller. We used a heavy metal rod (a broken half shaft from a four wheel drive) and smashed the small bits of rock in a saucepan. The rock needed to be powdered for us to get to all the gold within the rock. This is a time-consuming job because we needed a very fine powder - far finer than even sand. We wore out several pairs of gloves in the process.

Mercury
We used mercury to get the gold out of the powdered rock. Gold, like most minerals, can be 'dissolved' in mercury to form what's called an amalgam. An amalgam is a physical mixture, a type of alloy. This is a comparatively straightforward way of separating the gold from the rock, as the rock remains untouched by the mercury.

However, the use of mercury is very hazardous and mercury pollution in South American rivers following recent gold rushes has poisoned many indigenous inhabitants of the Amazon rainforest as well as the environment.

But how could we get our hands on some mercury?

Where could we find mercury?
The most common ore of mercury is called cinnabar. Its scientific name is mercury sulfide (HgS). In its pure man-made form it's called vermillion, a pigment much valued by painters. Luckily, we were given a huge chunk of cinnabar for this challenge - even though there are large deposits of cinnabar in New Zealand, there were none where we were based. Unfortunately, the cinnabar we were given was of a pretty low grade. But needs must and we had to use it. In some cinnabar ores, where the mercury sulfide concentration is high, you can actually see small beads of metallic mercury in the ore. It certainly wasn't the case with our cinnabar, unfortunately.

It theory at least all we needed to do was grind the cinnabar to a fine powder, and heat it gently, whereupon the mercury sulfide readily decomposes, producing mercury vapour (and sulfur). The mercury vapour can be condensed on the walls of a cold test tube as metallic beads, making them quite easy to collect.

The amalgamation
So how do you use mercury to extract the gold from gold-bearing rock? When you mix the finely powdered gold-bearing rock with some mercury, the gold within the rock dissolves, to form what's called a gold amalgam.

The next step in the process is to separate the gold amalgam from any 'unreacted' mercury. We did this by squeezing the crude amalgam through a small piece of chamois leather. Shiny metallic beads of mercury oozed through the leather, leaving the gold amalgam behind.

Baked potatoes
We now had some gold amalgam, but the challenge was to obtain gold, not some weird alloy. So how do you recover the gold from the amalgam?

Believe it or not, that's where a potato comes in handy. Mercury has a melting temperature well below that of gold, and, when gold amalgam is heated gently, it decomposes to give mercury vapour and purified gold. The flesh of the potato absorbs the mercury vapour, preventing its escape into the atmosphere.

It sounds absurd, but our resident alchemist, Mikey B got it to work. The gold that we got this way looked a bit black and bitty, but it was definitely gold. I don't know how efficient the process was, but we probably got a couple of grams of gold from several large sacks of our gold-bearing rock. And all it took was a lot of crushing and a bit of chemistry wizardry.