Extracting Gold from Rock

by Mike Leahy Crushing the rock

Powdering the rock

Mercury

Where could we find mercury?

The amalgamation

Baked potatoes

The Challenge

Unlike alluvial gold, which is found as very small flakes on riverbanks, gold-bearing rocks have to be dug out of the ground. We're after pure gold, but rock that contains gold also includes lots of other materials such as quartz, calcite, and the sulfides of other metals. Since we need to separate the gold from the rest of the stuff in the rock, we need to treat the rock in some way. To start the challenge, we chiseled some promising rocks from an outcrop near a small creek. We were pretty sure our rocks contained some gold, because there were disused gold mines dating back from the 1870s in the area. Crushing the rock

Crushing the gold-bearing rock was not as simple as it sounds. We couldn't just hit it with a hammer because pieces would fly everywhere and we might lose some of our gold. What we needed was a very strong container and bash plate. With those tools we could crush the rock without losing any of the precious gold particles. We also needed a special kind of hammer - one that would fit into the container without leaving too much room for the rock to escape. Rather than use a regular hammer, we used pulleys and a rope to lift and drop a heavy weight onto the rock. Using a pulley system meant we needed a longer piece of rope in order to raise the weight, but at the same time pulleys reduce resistance. Using a two-pulley system, we had to pull the rope twice as far than with one pulley or no pulleys at all, but we were also able to lift twice as much weight. This was useful because for the same amount of effort, we could smash twice as much rock! Back to top Powdering the rock

The next step was to take the crushed rock pieces and make them even smaller. We took a heavy metal rod (a broken half shaft from a four-wheel drive vehicle) and used it to smash our rock in a saucepan. The rock needed to be powdered so we could get to all the gold inside it. This was a time-consuming job because we needed a very fine powder  much finer than even sand. (We wore out several pairs of gloves in the process.) Mercury

We used mercury to actually extract the gold from the powdered rock. Gold, like most minerals, can be 'dissolved' in mercury to create what's called an amalgam. An amalgam is a physical mixture of metals, or a type of alloy. Using mercury is a pretty straightforward way of separating the two materials, since the mercury only affects the gold and leaves the rock behind. However, the use of mercury is very hazardous. In fact, recent gold rushes have flooded South American rivers with mercury and poisoned many indigenous inhabitants of the Amazon rainforest, as well as the environment as a whole. But since we'd picked that method, we needed to get our hands on some mercury. Back to top Where could we find mercury?

The most common ore of mercury is called cinnabar. Its scientific name is mercury sulfide (HgS). In its pure, manmade form it's known as vermilion, which is a red pigment used in artist's paint. The good news is that we were given a huge chunk of cinnabar for this challenge (even though New Zealand has large natural cinnabar deposits, there wasn't any around where we were based). The bad news was that it was pretty low-grade stuff. Good cinnabar ore has such high concentrations of mercury sulfide that you can actually see little beads of metallic mercury in the ore. Sadly, that was certainly not the case with our batch. Still, that was what we had available, so we had to make it work. In theory, at least, all we needed to do was grind the cinnabar to a fine powder and gently heat it. The mercury sulfide would easily decompose and produce mercury vapor (plus sulfur). The mercury vapor could be condensed on the walls of a cold test tube as metallic beads, making it easy to collect. Back to top The amalgamation

So how do you actually use mercury to extract gold? When you mix the finely powdered gold-bearing rock with some mercury, the gold within the rock dissolves and forms what's called a gold amalgam, while the rest of the rock is left behind. The next step in the process was to separate the gold amalgam from any 'unreacted' (leftover) 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, and left the gold amalgam behind. Baked potatoes

We now had some gold amalgam, but our challenge was to obtain pure gold and not some weird alloy. So how do you recover the gold from an 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 (breaks apart) into mercury vapor plus purified gold. The pieces of potato would absorb the mercury vapor, preventing its escape into the atmosphere. It sounds absurd, but our resident alchemist, Mikey B. got it to work. The gold we mined this way came out as tiny dark bits, 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. Back to top





Published January 2005. Copyright © 2005 The Open University and WETA . All rights reserved.Published January 2005.