GOLD-THE MAGIC METAL

By D. G. MAXWELL, B.Sc., A.R.S.M. 

Issued by the Publicity Department, Springbok Radio, Johannesburg. Presented In the Springbok Radio Programme on the 15th Nov. 1964.

Man has been susceptible to the magic of gold since the start of the Bronze Age, more than 5 000 years before Christ, when he had just learnt the art of melting and working metals. The early Egyptians were probably the first to acquire this skill in the art of simple metallurgy, and they produced gold Jewellery, ornaments and utensils of exquisite beauty and design.

This set the pattern for subsequent civilizations, most of which produced a great number and variety of gold articles of great beauty. Gold gradually assumed importance as a measure of the wealth of a nation and eventually came to be accepted as a medium of exchange. Thus, for over 7 000 years, gold has held a peculiar fascination for mankind and has exerted a tremendous influence in man's dealing with his fellows.

The point that I wish to make now is that this situation is by no means accidental. It is, in fact, the direct and logical result of the unique combination of physical and chemical properties of this beautiful: metal. The following are the more important of these properties:

First is its indestructibility, or, in more scientific terms, 'resistance to corrosion'. Gold is very reluctant to enter into combination with other elements and compounds and does not therefore easily corrode or even tarnish. This. accounts for the fact that nearly all the gold found in nature is in the metallic state.

Another great asset of gold is its colour and reflectivity. Reflectivity is a measure of the ability of a substance to throw back the light arriving on it. The high reflectivity of gold, in the yellow band of the visible spectrum, accounts for the distinctive colour that gives pure gold its. rare beauty. Gold is the only metal which is yellow in the massive and pure state.

A further advantage which gold possesses is malleability and ductility. Gold is malleable and ductile to a far higher degree than any other metal. Its malleability means that it can be beaten into leaves as thin as 4-millionths of an inch, while its ductility means that one ounce can be drawn into a wire over 40 miles long.

Then there is its conductivity. Gold yields only to silver and copper in its ability to conduct electricity. Finally, its density. Gold is very heavy. One cubic foot of water weighs 621 lb, while one cubic foot of gold weighs about 1,200 lb or well over half a ton. Use of this property is made in the recovery of gold from its ores.

In addition to its high density, there are two other properties of gold which are important for the recovery of the metal from its ores. These are its ability to form an amalgam with mercury and the fact that it is easily soluble in dilute cyanide solution.

The easiest deposits to work for gold recovery are the alluvial deposits. In these, the gold particles are disseminated through the sand and gravel of river beds. To recover the gold the sand is mixed with water and pumped over sloping tables on which and spread strips of ordinary cloth. The gold, by virtue of its weight, sinks to the bottom of the stream of slurry and is caught and held by the pile of the cloth, while the lighter rock minerals are washed over to be discarded in a suitable place. This is not only the simplest but also the oldest method of recovering gold.

Some 3 000 years ago the job of the corduroy cloth was done by rawhides. The legend of the Golden Fleece is no more than the description of a successful piratical expedition to Armenia about 1 200 B.C. to steal gold that was being recovered from streams with the help of sheepskins.

To recover gold by amalgamation the corduroy cloth is replaced by copper plates which are treated with mercury. The slurry is run over these plates in the same way as it runs over the corduroy, but this time the gold sinking to the bottom of the stream amalgamates with the mercury and is held there. The stream is periodically stopped and the pasty amalgam scraped off. This is heated to the boiling point of mercury, and the mercury vapour which is given off is condensed and re-used. The gold which remains behind L melted into bars and sent for refining.

Most gold nowadays is recovered from hard rock deposit and the first step is to reduce the rock to a fine enough state to liberate the gold particles from the other rock minerals. At the time of the discovery of the Witwatersrand in 1886, this was usually achieved in a stamp mill. This consists of a number of stamps, each weighing about three-quarter of a ton, which is successively lifted and dropped to crush the ore between the stamp and a fixed die. Stamp mills played a vital part in the recovery of gold in the early days of the Witwatersrand and this is given recognition in the coat-of-arms of the Johannesburg Municipality. which includes three golden stamps.

The iron pyrites in the gold-bearing reefs of the Witwatersrand was weathered for some 200 feet below the surface and as a result, in the early days, no major problems were encountered with the recovery of gold.

By the beginning of 1890, less than four years after the discovery of the field, Johannesburg was a prosperous town of 17 000 inhabitants, and there was unbounded optimism about the future. Only a few months later, however, the mines started to get to the end of the zone of oxidised ore, and as more and more pyritic ore arrived in the stamp mills the recovery of gold started to fall drastically. There was a slump, which turned into a near panic; but the panic was quite unjustified. For during the previous three or four years a Scottish chemist, by the name of John Stewart MacArthur, in collaboration with two doctor brothers, Robert and William Forrest, had patented and was working on the commercial development of a process to recover gold from its ores with cyanide.

A successful demonstration plant was erected at the Salisbury Mine in 1890, and subsequently, a larger plant started operations at the Robinson Mine. This plant immediately made profits for the company, and the cyanide process has never looked back from that time.

The ability to treat pyritic ore opened up vast new possibilities of deep-level mining and within a very short period, Johannesburg's slump had changed to a boom. By 1892 the boom was in full swing, the new capital was pouring into the Rand. champagne flowed, and extravagant parties were the order of the day. The prosperity then is closely linked with the prosperity that we are enjoying today. South Africa owes a great debt of gratitude to MacArthur and the Forrest brothers.

In order to expose the gold in the pyritic ore to the action of the cyanide solution, it is necessary to rind the rock fine-almost as fine as face powder. Nowadays this is done by mixing the crushed rock with water and tumbling it with biz lumps of rock, or steel rods, or balls in rotating cylindrical mills, known as tube mills, rod mills, or ball mills. The finely ground ore is then ready for cyanide treatment. If you were to mix a few grains of sugar with a handful of sea sand and stir the mixture up with water the sugar would clearly dissolve in the water and you would be able to recover it by putting the mixture on a cloth that would retain the sand but allow the water to run through. It would then merely be a matter of recovering the sugar from the water.

The cyanide process for gold recovery is similar to this simple sand-sugar separation, and we end up with a clear solution of gold in cyanide. The huge tonnages of ore treated by the mining industry these days consume 1 400 tons of cyanide every month, which, incidentally, is more than enough to kill every living human being on earth.

The gold is re-precipitated from the cyanide solution with small quantities of zinc dust, and the zinc-gold precipitate is smelted to produce gold bullion, which is sent to the Rand Refinery for final refining.

The impure gold bullion from the mines contains about seven per cent of silver and three per cent of base metals such as copper, lead, and zinc. At the refinery, the molten bullion is treated with chlorine gas, which turns the silver and base metals into chlorides which are easily separated from the pure gold. The Rand Refinery produces four tons of gold every working day.

Although most of this gold finds its way back underground into the vaults of the central banks some are also used in the industry. The uses of gold in the industry are manifold and expanding, but are not yet a major outlet. Gold-plating is a process which enables the special properties of gold to be used at an acceptable cost. Gold may be alloyed with other metals to impart special properties to it. The alloys with copper and silver, for instance, have a wide range of colours, including various hues of red, yellow, white and green.

Apart from jewellery gold is used in dentistry, for laboratory ware, for the decoration of pottery and glass, in the electronics industry, and in architecture. Gold chloride is used in colour films for photography and gold leaf has a wide application. Gold is playing its role in space research as a coating for space capsules. This coating reflects heat by virtue of its reflectivity and cosmic rays by virtue of its density.

Gold also plays an important part in medicine. One example is the use of radioactive gold as a convenient method of getting radiation to the heart of a tumour. The radio-activity of gold decays rapidly and the inert gold left in the tumour will do no damage.

This list of industrial uses of gold is by no means complete. Nevertheless, it is impressive in variety and potential.

This potential, together with the already entrenched position of gold as a medium of exchange, should give us quiet confidence for the future. We may be sure that gold's unique combination of physical and chemical properties will not change and that for this reason, it will continue to be sought after by mankind for the next 7 000 years, just as it has for the last 7 000.