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# End of chapter exercises

Textbook Exercise 14.2

Give one word/term for each of the following descriptions:

The part of the Earth that includes the crust and in which all minerals are found.

Lithosphere

The process in which minerals are extracted from the ores.

Minerals processing or extraction

An age in which humans discovered the use of fire to improve the properties of stone.

Middle Stone Age

More profits, more poisons

Since the last three decades gold miners have made use of cyanidation to recover gold from the ore. Over $$\text{99}\%$$ of gold from ore can be extracted in this way. It allows miners to obtain gold flakes – too small for the eye to see. Gold can also be extracted from the waste of old operations which sometimes left as much as a third of the gold behind.

The left-over cyanide can be re-used, but is more often stored in a pond behind a dam or even dumped directly into a local river. A teaspoonful of two-percent solution of cyanide can kill a human adult.

Mining companies insist that cyanide breaks down when exposed to sunlight and oxygen which render it harmless. They also point to scientific studies that show that cyanide swallowed by fish will not 'bio-accumulate', which means it does not pose a risk to anyone who eats the fish. In practise, cyanide solution that seeps into the ground will not break down because of the absence of sunlight. If the cyanide solution is very acidic, it could turn into cyanide gas which is toxic to fish. On the other hand, if the solution is alkaline, the cyanide does not break down.

There are no reported cases of human death from cyanide spills. If you don't see corpses, everything is okay.

What is cyanidation?

Cyanidation is the addition of cyanide to the gold ore.

What type of chemical reaction takes place during this process: precipitation; acid-base; redox?

Redox reaction

Is the solution after cyanidation acidic, basic or neutral?

acidic

How is the solid gold recovered from the solution?

By addition of zinc metal and then heating in a furnace.

Refer to cyanidation and discuss the meaning of the heading of the extract: More profits, more poisons.

Cyanidation extracts over $$\text{99}\%$$ of gold from the ore, this leads to more (i.e. process is very effective) profits. However, left over cyanide can end up in rivers and aquifers that can be (i.e. negative effect of process on environment) harmful to life.