Wednesday, 24 July 2013

Extraction of Gold from the Ground


How Much Gold Is There in the World?

Gold Materials
There can only be an estimate of the total amount of gold on Earth. Those who have been bold enough to venture a figure suggest there may be about twenty billion tons of the precious metal in the outer crust of our planet, and perhaps another eight billion tons in solution in the oceans of the world. But that would not be all there is. In the Earth's mantle, beneath the outer crust, there are probably enormous quantities of gold in pure form and in tellurium and selenium compounds. Compared to such estimates, the amount of gold extracted from the Earth so far is extremely modest. only about 80,000 tons. Most of it, about 60,000 tons, has been mined during the twentieth century.

Dealing with such figures can be confusing. Even if the total amount of gold believed to be woven through the fabric of the Earth is enormous compared with the amount extracted so far, gold remains a rare metal. Known workable gold sources have a total estimated yield of some 30,000 tons. If gold extraction continues at the present rate, those sources are likely to be exhausted by the turn of the century. The implicit contradiction between enormous sources of gold some twenty eight billion tons in the Earth's crust and oceans and an extractable total of some 30,000 tons, can be misleading. Gold can really be found in just about all parts of the world, but usually only in very minute amounts. For example, gold equivalent in size only to about a speck of dust is contained in a cubic yard of sea water. Gold-bearing rock is deemed workable when it contains a minimum of five grams of gold per ton. Such a disproportion would seem to make extraction barely worthwhile. But even before the recent sharp rise in the price of gold, its value was sufficiently substantial for men to work every possible source. In California and Alaska, where the appropriate equipment is available, it is even profitable to work sand and gravel with a gold content of barely fifty milligrams to the ton, a hundred times less than the yield that used to be thought the very minimum for commercial consideration.

gold brickPerhaps the strangest gold extraction project was the one conducted after the First World War by the Nobel prize winning German chemist Fritz Haber, the man who .discovered the process for synthesizing ammonia. Haber set out to improve methods for extracting gold from sea water. His motive was not it personal gain; it was patriotism. He hoped to make a substantial contribution to paying off German war debts (following the Nazi rise to power in Germany in 1933, Haber fled to England). His elaborate calculations indicated that much more gold could be derived from the sea than had previously been possible. However, to his profound disappointment and considerable chagrin, his breakthrough foundered on a misplaced decimal point which had disguised the fact that the Haber method would have made costs infinitely higher than potential profits. The enterprise was abandoned and even now there is no satisfactory method to extract from the oceans of the world even a meager fraction of the huge amounts of gold they contain. Whether modern technology will long be inhibited by past and present limitations is, of course, for the future to decide.

Where Does Gold Come From?

Cosmologists, geologists and others who explore the origins of the Earth agree that billions of years ago, this planet was a rotating ball of hot gases and interstellar dust. Gradually that ball cooled and grew more dense. This stabilization process reached a crucial points some four and a half billion years ago at which time most of the chemical elements including gold were formed. Some scientists suggest that during that formative period, the Earth was subjected to radioactive fallout from the planet Uranus. That fallout, they say, created lead isotopes which are still in evidence. Traces of gold discovered in meteorites and on the moon probably date back to the same period.

The heavier elements including gold settled deeper than the lighter ones into the Earth's still molten interior during the period of rock formation. At the same time, centrifugal forces, produced by the Earth's rotation, pulled lighter elements toward the surface, and in many places carried other substances upward as well. The cooling of the magma, and the enormous pressures produced by that process, were responsible for the creation of the Earth's rock formations in their earliest forms. To say that the most ancient known surface rock formations on Earth date back billions of years is to conjure up numbers so immense as to be almost beyond comprehension.

The movement of the Earth's crust following the cooling process also lifted various rock formations to the surface. Included in this uplifting process were veins of precious ore. (It is, of course, well known that the face of the Earth has never ceased to change. Volcanoes, earthquakes, land subsidence and other upheavals and shifts are part of an endless transformation. Observance of terrestrial developments from space has, in recent years, expanded our understanding of the extent of the alterations continually taking place on the surface of our planet.)

Gold generally retained its chemical identity, unlike some other metals which became chemically interwoven with rock. The exceptions were gold and tellurium and gold and selenium compounds, and a few complex gold salts. However, gold is often found in minute amounts in rock. It can be found in copper pyrites and often in quartz deposits. Gold is usually Si found mixed with other elements, including silver, a copper, platinum, iron and lead and occasionally with mercury and bismuth. These mixtures however, arc not chemical compounds.

Gold mining procedures distinguish between primary it deposits so called mountain gold, with the mineral often found in quartz veins and secondary sources in alluvial and other surface deposits. The latter o consists mostly of grains, flakes or dust carried down erodes. A portion of this gold is always carried down into gravel or river beds in the surrounding rock 1 to the sea.

The German scientists Rabenau and Rau recently o formulated a new theory of the origin of .gold o which they confirmed through controlled experimentation. They established that gold, which is normally h not readily soluble, dissolves in certain liquids raised ct to very high temperatures. If there is a temperature difference within a hot bath of, say, between 480 and n 500 degrees centigrade, the gold in the solution will it shift from the colder to the hotter zone.

The experiment demonstrated that ideal solvents were iodocholoride or hydrobromic acid together with an oxidizing agent, such as oxygen. At the lower temperatures, gold combined with the iodine chlorine or bromine to form a relatively stable compound that shifted into the higher temperature zone. There the compound gradually broke up again and released the gold. The gold molecules could attach themselves to precious metal crystals already present and these crystals grew.

If the temperature differential was accompanied by high pressure, the experiment also worked in weak solutions of cooking salt. It would seem that such high temperature solutions have played an important part in nature. At the appropriate temperature, gold dissolves and moves into the warmer zone to crystallize there. The German scientists dissolved ten grams of gold wire and were able within a few days to grow gold crystals measuring a centimeter across. It is now believed that this process explains the origin of nuggets that are to be found in great quantity in many parts of the world.

From Rock to Gold Ornaments

No one knows when, in the vast regions of prehistory, man first took an interest in gold. Probably gold was first worked during the fifth or sixth millennium BC. The earliest evidence of gold working comes from ancient Egypt. Hieroglyphics from the period between 4100 to 3900 BC tell of shining, yellow "workable rock" which was apparently used for ornamentation.

Gold is known to have been extracted from alluvial deposits in the' Nile around 3500 BC. Large pieces were hammered out in metal or wooden molds to form elegant artifacts. These were fairly pale in color, since Nilotic gold had a pronounced silver content.

It was probably around the same time that the Egyptians discovered the smelting process. At first T they applied this skill to copper. But goldsmiths were soon concocting alloys by mixing various metals in a smelter. No doubt they were surprised to discover h that various gold alloys had a much higher melting e point than gold itself.

High temperatures were produced by the first metallurgist’s wit h the use of blowpipes, not much different than those still employed by tradition oriented gold-smiths. From that breakthrough in the evolution of the technology of gold processing, it was a simple step to join together pieces of pure gold with high melting points by using alloys with lower melting points. Thus did the Egyptians invent the art of soldering. Their skills developed to such a high level that when gold ornaments are examined even today, the joins arc I often invisible to the naked eye.

It seems likely that the Egyptians soon discovered how to do their smelting in crucibles fire resistant containers to melt down gold dust and flakes to produce larger pieces of pure gold. However, they never cast jewelry; they molded their gold by working it over a sturdy underlay, made of clay, tar or other such substances. They then used a small, blunt chisel for embossing.

The First Gold Mines

gold brickProspectors in ancient Egypt systematically washed the sands of the Nile. They are said to have extracted about eighty grams of gold from a ton of sand, a remarkably high yield. They must have noticed that this alluvial treasure was often to be found with quartz, so plentiful in the mountains skirting the Nile Valley. About 3000 BC, Egyptians drove adits up to thirty feet deep into those mountains, to tap the often twisting quartz veins to be found there. These were the first gold mines in history.

The ancient prospectors and mine engineers were resourceful and thorough. No source of gold has ever been unearthed in Egypt which had not at least been investigated by them. But they were highly selective in the sources they exploited. They only tapped workings in which the yield ranged from one hundred to five hundred grams a ton.

The process was arduous. The hard, quartz bearing rock was first made brittle through heating with fire. It was then broken away layer by layer from gallery walls by slaves. Then the flakes of gold embedded in the quartz had to be extracted. The ore was pulverized manually in mortars and the resulting powder was sprinkled over inclined boards to be washed down. The process was repeated until all the light rock particles had been washed away and only pure, heavier gold dust remained.

The invention of the bellows, around 2000 BC, helped the Egyptians remove undesirable impurities from gold. With the use of bellows, operated by slaves in shifts, a temperature of 1000 degrees centigrade, required for the smelting process, could be maintained. Raw material was placed in a Crucible between layers of porous stone or slate dust, mixed with rock salt and iron sulphate. The crucible was then heated until it was red hot, just short of the 1063 degrees centigrade melting point of gold. The process, continued over a period of days, eliminated impurities and alloyed metals, leaving virtually pure gold behind.

Gold Working from Antiquity to the middle Ages

Gold Shoes
The first known stone molds for gold bars and rings date back to the twelfth century BC. This is surprisingly recent, considering that casting techniques for copper, bronze and, later, iron were known as early as the sixth millenium BC. In the eleventh century BC, a way of testing gold was discovered that is still sometimes used today: the touchstone method. The touchstone is a flint like black stone. When gold is rubbed across it, it leaves a telltale streak. The color can reveal how much silver or copper the gold contains. Silver colors the gold white; copper colors it red. A drinking song dating back to 550 BC says: "Gold is tested by the touchstone and can be clearly recognized. In time, a man's character, good or evil, will be recognized too."

The Egyptians devised a method for covering less valuable metals with gold gilding with fire. Across the Mediterranean, in Italy, the Etruscans developed this technique to perfection in the following way:  one portion of gold and one of lead were reduced to finest dust and made into a thin solution by means of gum arabic dissolved in water. The metal to be gilded, perhaps copper, was dipped in the solution and then heated in a fire. After several repetitions of this procedure, the copper began to acquire an increasingly pronounced golden color. The heat dispersed the lead in the solution, but not the gold.

The Etruscans also mastered the techniques of granulation and filigree. For granulation, the molten metal was quenched the Etruscans let it fall drop by drop into water and the resulting granules were soldered to a gold surface. For filigree, wire was hammered flat and was then bent into intricate patterns and soldered together. 



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