Course Project

I, Bushyman477 (cool name, I know), currently am a first year student at the University of Canterbury, New Zealand, studying and majoring in Geology. My course project I have been tasked with involves making a website/blog based on something in the natural world, and how it negatively affects several of the Earth's 'spheres' e.g. biosphere, hydrosphere, geosphere etc. For my topic, I have decided to look at the processes of gold mining using cyanide, and how it can affect the hydrosphere, biosphere and the anthroposphere. In this project I shall be looking at the gold-cyanidation process, the dangers of the chemical cyanide, and the disposal of the problematic byproducts produced.

Gold - The Mighty Ore

Fig. 1 - High-grade gold ore from a quartz vein near Alma, Colorado.
Small flecks of gold visible, but much more is hidden
microscopically within the rock requiring chemical extraction.

Gold; arguably one of the most valuable metals on Earth. This yellow metal is soft, dense, malleable and ductile. Common uses include as jewelry (can be buffed to give off a bright luster), great investment potential, electronics, and even medicine. However, despite its useful characteristics, gold is very rare, and in small quantities (fig. 1). Because of this, the lengths some companies go to to obtain it is very extreme. Other than striking lucky and panning for gold in a river, the only way to economically obtain large amounts of gold is via mining, and one method of this process is using gold cyanidation (e.g. Rees and Van Deventer, 1999). This process, while effective, can also be dangerous and affect the environment negatively.

While the methods we use to extract gold from ore is rather cost efficient and effective, it is not exactly the most environmentally friendly. The two biggest factors in this process that makes it harmful and potentially dangerous is the toxic nature of the chemical cyanide (e.g. Cooper and Brown, 2008), and how the disposal of the byproducts can cause acid mine drainage (e.g. Hoffert, 1947)

Gold Mining - Cyanidation

Scenario; You have a gold mine, and you want gold. You have extracted a rock from the mine which you know contains traces of gold. How do you get it out?
The particles of gold in the ore are so small, simply trying to manually extract them won't work, instead, a chemical process (the MacArthur-Dingus process) involving leaching via cyanide is used (e.g. Rubo et al., 2006).

The first step is to crush the rock into a fine material. The smaller the material, the faster the leaching can take effect. The rock powder is placed into a tank and mixed into a slurry by adding water. Then, sodium cyanide (NaCN) solution is added to the mix.

The reaction that occurs, the 'Elsner Equation', is as follows:

The water and the oxygen added to the solution during the slurry helps complete the reaction. The gold is dissolved and essentially 'leached' into a cyanide-complex solution. At this point, you now have an aqueous gold solution, with a sodium hydroxide byproduct.

You have your gold in a liquid solution. However, no one wants a watery necklace. The next stage of the cyanidation process is extracting the gold from solution. One of the processes used to remove the gold from the pregnant solution is the Carbon In Pulp (CIP) process (e.g. Cho, Dixon and Pitt, 1979).

This process involves adding activated carbon to the solution. The carbon acts as a sponge, adsorping the aurocyanide. The loaded carbon is removed and then undergoes elution; desorption of gold cyanide at high temperature and pH. The next process involves electrowinning, which uses electrolysis to separate the cyanide (remaining in solution) and gold (sediment at bottom) (e.g. van Deventer and van der Merwe, 1994). The gold sediment is finally mixed with a little Borax, and smelted, returning in the form of solid gold.


Here is a video on YouTube from NurdRage, showing him dissolving gold at home the same way it is done industrially, on a smaller scale.

Waste Rock and Tailings

Following the extraction of gold, there are two byproducts remaining;
  -Waste Rock: A rock containing little to no targeted minerals, economically useless.
  -Tailings: The slurry which remains after the gold has been extracted from the crushed ore.
These, like most chemical reaction byproducts, are unwanted, and need to be 'dumped'.

Waste Rock

Fig. 2 - Diavik Waste Rock Pile, mneuner

Depending on the current price of gold, any ore that has been mined gets tested to see its gold content. If it meets the requirement, it goes on to be processed and have the gold extracted. As this rock may still contain smaller traces of gold, it isn't destroyed, and could potentially be reprocessed if the price of gold increased making it economically viable to extract it (e.g. Von Ketelhodt, 2009). Therefore, waste rock needs to be kept in some place, most commonly close to the mines (see fig. 2).

Tailings

Fig. 3 - Geomembrane lined tailings storage, Jon Engels

Following the extraction of gold using cyanide, what remains in the slurry is known as tailings. This mixture is often around 30% solids, which are silt/sand size. These often dumped in tailing ponds or inpoundments. When dumped, the slurry will separate; the silt and sands will fall to the bottom. Prior to complete separation, the overall pond will be very porous, and the water will seep out into the ground. This will decrease over time as the silt settles decreasing permeability (e.g. Vicks, 1990; Vermeulen, 2001). This is a problem as this water is mixing with the local groundwater, and any chemicals present are being dispersed into the environment (see fig. 3).