A gold plated mascot, a royal crown, ingots stacked in the vaults of a bank – wherever it crops up, gold symbolizes wealth and power because of the huge efforts required to find it and extract it from the earth.
Gold is one of the most highly prized of all the metals and, for this reason, it has always been used to mint coins. Gold also gives value to paper notes – in theory, these can be swapped for gold. Unlike most metals, gold does not react with other common elements such as oxygen, sulphur or carbon. This is why it is known as a ‘noble’ metal. For the same reason, gold is found in nature as a pure metal rather than as a compound.
Most particles or nuggets are found in veins of quartz deep within hard, crystalline rocks, such as granite and solid quartz rock. They are also found in deposits of sand and silt washed out of quartz veins by running water. This is know as alluvial gold and is often found along the beds of streams and rivers.
Hard work mining
Gold is found in lots of places around the world, but only a few countries have enough to support a gold mining industry. The gold-bearing rock is usually extracted by underground mining in hard rock, often at great depths. Thus the Western Deep level and Goldfields mines at Carletonville, South Africa, are the deepest mines in the world. Western Deep level is the deeper of the two, extending 3,777 metres down and exceeding it everyday. At the lowest level of the mine, the temperature is 55 degrees.
Alluvial gold is extracted by dredging gold-bearing silt from the bottom of lakes, or by using high pressure water jets to wash deposits out of the sides of hills. In some countries – Brazil, for instance – individual prospectors and miners extract alluvial gold by washing and sieving gravel from the beds of rivers by hand. Gold miners in the old Wild West used this method of mining.
Today, gold is only used as everyday currency in remote parts of Arabia, where normal coins are unknown. Special golden guineas are minted by the Bank of England and exported directly to these areas. Large quantities of other gold coins, including the Kruggerrand from South Africa and the Maria Theresa thaler from Austria, are still minted for people who want to buy gold as an investment instead of, say, shares.
Silver is the only precious metal still used in ordinary coins but even this practice is restricted to a few rich countries, notably Germany and Switzerland. Most silver is now obtained as a by-product during the production of other, cheaper metals, such as copper, zinc and nickel. The ores of these metals contain small amounts of silver, so when electrolysis is used during copper or nickel refining, silver settles into the sludge of impurities that settle at the bottom of the tank. This sludge can then be further refined to recover the silver.
Chemical extraction
Like gold, silver may be found in its native state, but Argentite silver ore is mined in a few places. The ore is first dissolved in a sodium cyanide solution, which produces a compound called sodium argentocyanide. Zinc dust is added to this solution and the zinc combines with the sodium and cyanide because they are more reactive than the silver. This frees the silver from the compound and it sinks to the bottom of the tank.
A similar cyanide process is used to recover gold when the particles are too fine to be separated from silt or crushed rock by simple sifting or washing the unwanted material away. Lots of silver is used industrially, particularly for photographic and for making printing plates. When the films and plates are developed, a lot of silver is removed as waste. The waste is then passed through an electrical device that recovers the silver electrolytically. Silver recycled in this way is an important source of ‘new silver’.
