Gold rush. As sheepskin cyanide and the bacteria help modern-day prospectors
Breathtaking stories of extraordinary luck prospectors and “gold fever” has created a common misconception that gold mining the main thing — in time to be in the right place. In fact, the battle for gold, which are always short and always need more — it’s always a test of observation, perseverance and ingenuity. N + 1 tells where and how to get the yellow metal.
Along with iron and copper, gold claims the title of first metal that man learned to extract and process. The average contents (Clarke) of iron in the earth’s crust about 50 kilograms per ton of copper is about 80 grams per ton, and gold is just 0.005 grams per ton. If the gold was distributed in the earth’s crust evenly, in order to score on the small ring, it would be necessary to divide and sort out grain by grain about three train car rocks.
Fortunately, the gold is distributed unevenly. And in certain anomalous zones — fields — the content can reach hundreds of grams per ton.
So, the famous placer Eldorado in the Klondike allowed experienced miners to receive one tray (5-6 liters or kilograms 14-17 Sands) about a kilogram of gold.
But such facilities in the world, and it is they who spawned the gold rush. In most placers with a tray you can get grams at best — tens of grams of gold.
At such low contents of gold had to have some truly amazing properties that already at the dawn of civilization people were able to find, extract and process. Today’s technology generally allows to get the gold of the poor (geologists say — poor) ore, where no more than 1-2 grams per ton of rock, and metal scattered on the atomic level and is not always visible even in electron microscope.
Water, wind and the Golden fleece
Due to the high density gold is poorly tolerated by wind and water. Therefore, the destruction of rocks containing even the smallest specks of gold, it stays in place while the lighter minerals are washed away by water or blown by the winds.
Gradually the gold content in the products of destruction of all increases and can reach tens or hundreds of grams per ton. So are placer — gold-rich Sands. They also became the first developed form of gold deposits.
In the bronze age people learned how to extract gold from ore. Then it was visible native gold, and took it most of the quartz veins, which blocks broke out with primitive tools or, where possible, warmed, spreading on the rocks in the fire and then suddenly cooled by pouring cold water.
Other available for the ancient people of the fields was the so-called “iron hats” — a characteristic rusty color of the topping over the grayish cliffs, the upper part of the sulfide deposits (copper, lead, zinc and other metals).
In the Ural tales by Pavel Bazhov there is such episode: “the Mistress, like, Copper mountain has ordered you, a stuffy asshole to you with the Krasnogorsk mine cleaned. If you’re still in my iron hat rack, so I’m all the copper in there Gamestah later, no way is it mine.”
The mechanism of accumulation of gold in the iron hat is fundamentally the same as in mineral deposits: rocks are destroyed under the action of water, wind and changes in temperature and turn into sand and clay. Part of them is carried by the wind, the remaining part is gold, and not “burdened” by other minerals.
Although placer or iron hat of gold in the hundreds of thousands of times more than the average in the earth’s crust, still it very little. This means that to obtain the coveted metal gold-bearing Sands that have accumulated along the bottom and banks of rivers, or clay from the iron hat, or fragmented quartz veins should be released from all superfluous. This process is called enrichment.
Want richer
The most simple option, suitable to anyone, even the arid areas, was the winnowing.
Natural (from mineral deposits or zones of oxidation) or artificial (crushed ore) gold sand tossed up and forward, while trying to throw sand in the direction of the wind. Lighter material was pushed aside, and the heavy gold fell to the feet of the concentrator. All good this way, except that it only works if the gold particles are large, a millimeter or more in diameter.
Slightly more complex but more effective method was washing.
On the way the gold thread has come up with different kinds of traps that help linger the heavy particles of gold. You all remember the story of the Golden fleece? Sheepskin pulled on Board and laid in the way of gold-bearing rivers or artificially organized flow, which sent the material placer or crushed ore. This method of extraction is described by the Greek geographer Strabo and the Roman historian Aptana. Individual (often illegal) artisanal method is applied so far.
Sometimes flushing used almost on an industrial scale. Ore mine of Las Médulas, located on the territory of modern Spain, in 1997 was listed as world heritage site by UNESCO ancient industrial culture.
For the ancient times of Las Médulas is a mining and processing plant: in the rocks carved channels with a length of several kilometers to redirect water from the surrounding rivers of gold-bearing Sands. Then the water fell into a special drainage ditches. Sometimes ditches were blocked special traps of brushwood. The power of water, sand and clay rushed away, and the gold particles stuck between the densely woven branches. After burning the branches from the ash scraped gold bullion.
Las Médulas
Ancient people knew and more sophisticated methods of extraction and enrichment of gold, many of which were subsequently lost, and rediscovered again in modern times. So, there is reason to believe that the method of cleaning gold concentrates by mixing them with mercury (amalgamation) was already known more than 2000 years ago. Amalgamation was used until the mid-twentieth century that led to the mercury contamination of gold-bearing valleys of the rivers. Now this method is used during laboratory tests of ores.
The ancient Egyptians, moreover, were able to make the heaviest of known metals float to the surface of the vessel with water along with oily film added to water oil (a primitive oil flotation). But in the ancient world the most common methods of gold extraction were the variety of gravity concentration, the benefit of easy aboutimage gold was still enough. According to the data given in the monograph by N. V. Peter “Native gold” for the first 5-6 thousand years of human history has produced approximately 31 thousand tons of gold.
Gold from scrap materials
By the beginning of the early middle Ages available sources of gold in Europe was practically exhausted. At this time, gold is never enough, and monarchs generously funded by those who promised them to turn to other, more accessible and cheaper substances of the alchemists.
The first candidate for the role of raw materials for the production of gold was the second-hardest of the famous people metals, lead.
Useful for alchemical transformation qualities of lead was its malleability, and fusible. With similar characteristics copper is used less frequently. Mankind was already well known Golden alloy of copper and zinc is brass, and this transformation is not considered a miracle. Therefore, the persistently alchemists sought to turn into gold grey metal.
Another candidate was liquid mercury.
The famous Arab alchemist of the VIII—IX centuries. Jabir believed that everything known at that time, seven metals — gold, silver, copper, lead, mercury, iron, tin, formed from a mixture of mercury and sulfur, which needs to ripen in the bowels of the earth. But the most perfect metal, gold, is brewing for the longest time, and to get the gold in the laboratory, it is necessary to accelerate the ripening of the above mixture, finding that extra special ingredient. Today we would call this substance a catalyst, the alchemists called it the elixir or philosopher’s stone.
As we know, the secret of turning gold into lead, and mercury were not found.
But alchemists have found something equally important: they’ve discovered a mineral acid, a substance capable of dissolving other substances.
In his “a Brief history of chemistry” Isaac Asimov says that “the mineral acids have given humanity more than they could give gold, if he learned how to make transmutation”.
The achievements of alchemy belongs to the preparation of Aqua Regis, of the whole, which, as established Catholic Saint Giovanni Fidanza (better known under the name of Bonaventure), dissolved gold. The composition of this — nothing like Aqua Regia, a mixture of hydrochloric and nitric acids. Caravodice decomposition is an important step in preparing gold ores for chemical analysis.
And during the Second world war, this corrosive mixture of hid, and so retained, the Nobel medals of German physicists max von Laue and James Franck: a Hungarian chemist györgy de Hevesy dissolved it “illegal” if the fascists medals, and after the war was isolated from the solution of gold, and the Royal Swedish Academy of Sciences made it again rewards.
Installation on enrichment
Over time, readily available and easily obogatili ore became less, and the needs of mankind in the gold, with population growth and General economic growth — more and more. The era of the great geographical discoveries of the XV—XVII centuries gave the Europeans a New world and new sources of gold. For some time the problem has been removed, but by the end of the eighteenth century that gold ended.
And Willy-nilly had to pay attention to more and more difficult to develop the field. Accordingly, it was necessary to improve the technique of gold mining. In the nineteenth century in the industrial everyday life became a series of enrichment plants, including, for example, Butare and dredge, which are used until now.
Butare is a drum of cylindrical or conical shape, the walls of which represent a sieve.
Inside Butare load of crushed ore, gold-containing sand or clay, pour water and start the month to rotate. Through the holes in the walls from Butare removes fine grains of sand and clay particles — mud, and obessilennaya ore is then ready for further enrichment.
Draga (to drag, “drag”) is a floating factory processor.
The bucket dredge to scoop the soil from the river bottom, after which the system flushing sluices gold washed from the gangue and unwanted soil and then released back into the river. Dredge completely destroys the river’s ecosystem, and site restoration after this method of extraction takes tens or even hundreds of years. But despite this dredging method of extraction is still used in many countries, including in Russia.
All of these tools is still possible to remove only relatively large, the size in millimeters, or at least in tenths of millimeters, gold. While in the waste, called tailings, was more subtle gold particles. The approach to this delicate gold searched long and hard, and significant discoveries made sometimes by accident.
I cyanide, cyanide
About the Swedish chemist and pharmacist Carl Scheele said that he could not touch any substance so in order to make discoveries.
In 1782, Scheele “touched” to ferrocyanide and isolated from her prussic acid. Rumor has it that he also noticed that the salts of hydrocyanic acid, the gold goes into solution.
In 1843, the Russian chemist and engineer Peter Romanovich Bagration (the nephew of the hero of the battle of Borodino) studied the conditions for the dissolution of gold in cyanide solutions and found that this requires the presence of oxygen.
Later this conclusion was confirmed by L. Elsner and Michael Faraday. However, the industrial use of cyanide leaching began after 1887 and 1888 years, the British John Stewart MacArthur and the brothers Robert and William forest not only managed to dissolve gold and precipitate it from a solution of metallic zinc. Already in 1889 they implemented their method on the field, the Witwatersrand in South Africa. Originally used potassium cyanide, it was later replaced with sodium cyanide, which is used today.
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Famous jewelry, from which alone problematically associated with these decorations, can be attributed to the troubled times in the history, mental problems of their owners, and it is possible to believe in mysticism. Whatever it was, we would not dare to try these ornaments, in spite of their beauty.
Method of gold extraction by translating it into solutions (in the jargon — liquid phase) is called hydrometallurgy.
Modern mining and processing plants it is found in three varieties: heap, underground and Canova leaching. During heap leaching cyanide solutions pour laid on a special film stacks (actually a heap) of gold ore. With underground leaching solutions are injected directly into the bowels, and when Canova — build cascades of huge vats.
The choice of a particular method depends on the composition of the ores and enclosing rocks and even the climate.
For example, only recently heap leaching began to try to use in the Northern areas, where summer frosts — the solution does not freeze, the pile should be warm.
Interacting with solutions of salts of hydrocyanic acid, the gold forms a liquid dicyanoaurate. The gold-bearing solution is easily separated from the solid phase — the actual crushed ore, which after processing by solutions of lost gold and moved into the category of “tails.” Now, since our ultimate goal is not the solution, and the ingot, it is necessary to return the gold from solution to the solid state.
Accomplish this by sorption in liquid phase active carbon is added, zinc shavings or pass it through an ion exchange resin.
The sorbent is expensive, so aim to use it repeatedly. So the next stage is desorption. We have again to dissolve gold, but a small amount ten times stronger cyanide solution. Then through the solution to pass electric current. In this case, the cathode is deposited metallic gold, but still “dirty”, containing impurities of other metals, primarily silver. After affilirovany — decontamination — metal finally gets the coveted coded “9999” (which means that 99.99 percent gold, and only 0.01 per cent of impurities) and becomes suitable for casting high-grade bullions.
But all of these technologies allow us to extract only the gold atoms which may come in contact with solutions, and it’s still native gold, no matter thin or large. In the second half of the XX century — you guessed it — fit for the industrial development of deposits of native gold was not enough, and the man took up the ore, which are called resistant.
Competition in perseverance
Probably it is intuitively clear that we are talking about ores that puzzle and to persevere in search of a way to take out of them the precious metal. In these ores, most of the gold is hiding in other minerals and is not available to leaching solutions. Quite often in such secluded places, where you can hide gold chooses sulphur compounds with iron or sulfur in iron and arsenic minerals pyrite and arsenopyrite. Such ore in which gold forms the smallest, down to the atomic, inclusions in sulfide minerals, are gold-sulfide ore.
A Golden-yellow iron sulfide — pyrite — one of the most common ore minerals on earth.
Due to the very rough resemblance to gold, cheat is that the uneducated and inattentive hunters for easy money, pyrite sustained popular nickname of “fool’s gold”.
But centuries passed, and people became convinced that pyrite might be the real gold.
And that by mixing ground ore with water (the language of the dressers this process is called raspolojenie) small grains of pyrite, arsenopyrite and some other minerals, which hides the gold surface, forming a thin surface film. This happens because the surface of these minerals are poorly wetted by water. To better understand this process, imagine a drop of water on the oily surface of the glass. It will not be spread and will remain a round droplet. This phenomenon of hydrophobicity based method of flotation.
The fact that if a mixture of water and crushed ore (pulp) blow air, it is poorly wetted (hydrophobic) grains adhere to the air bubbles and float with it on the surface of the vessel to form mineralized foam. And to enhance the natural hydrophobicity of sulfide minerals in the pulp add a specially selected reagent. So, the bubble beyond the bubble, grain by grain, collect the gold and sulphide concentrates.
But the gold-sulphide concentrate — this is not the ingot. Even to the uninitiated it is difficult to believe that this unsightly dark grey powder is the final product concentrator, gold ore processing. To pull hidden in the sulphide impurities of gold, you need to destroy the mineral-owner. This can be done in several ways, for example in an autoclave using high temperatures and pressures. And you can achieve the same result by using bacteria that eat containing gold sulphides.
However, exposed sulfide minerals, after their destruction of material, as in milling of ores with free gold, treated with cyanide to dissolve and collect became available, freed from the shackles of alien crystal lattice of gold.
There are also more refractory ore.
In the second half of the twentieth century, people paid attention to the fact that sometimes gold is found in the strata of rocks, humus resulting from organic residues — the so-called black shales.
In the black-shale ores of gold may be present in the form of tiny nuggets, and in the form of sulphides. The second of these two cases is the most difficult to enrich such ores the ore is called the dual of the hard-work. First resilience is a sulfide mineral, locking the gold in its crystal lattice. And the second refractory creates the “evil” carbon. As we have seen, from cyanide solutions gold Sorb on the coal and black shale, though not quite coal, but can also adsorb the gold. As a result, in the process, the share won from double refractory gold ore is falling. The phenomenon of sorption of gold present in the ore sorbent is called preg-robbing (predatory capture).
It is interesting that by learning to extract from ore, fine gold, people better understand how nature works. After all, the majority of deposits — product giant natural hydrometallurgical factory, which is a very similar process.
Gilded life
In the endless search for the gold man can help wildlife. If, say, the birch grows on the “blank” quartz, and over a gold mine, it is almost certainly in its bark and leaves of gold there will be more. This ability of plants to “give” secrets of the subsoil based biogeochemical method of prospecting for deposits.
Moreover, gold can accumulate in living organisms, but also to replace them completely or partially, just as the calcium carbonate replaces the shells of mollusks, skeletons of arthropods, and even dinosaur bones. In 1979, Dieter Hallbauer from Johannesburg in the ore deposits of the Witwatersrand described filamentous forms of native gold and suggested that they formed in place of the covers of filamentous bacteria.
But the possibility of the replacement of bacteria of gold was only a guess — until then, until you went to mass findings in the Amur region, on Kamchatka and in the Khabarovsk territory. In 1991 John Watterson in the samples of gold from the placers of Alaska has allocated garland hollow gold beads, which in size and form exactly corresponded to the size of soil bacteria. In 2005 Emil a Schoolboy found in the same field, the Witwatersrand gold replacement cyanobacteria, maintaining all of the sculptural details.
These and similar observations have prompted geologists to the need to rethink the role of living matter in the formation of gold deposits.
Probably, the bacteria played a significant role in the history of migration and the redistribution of gold in the world.
The ability of bacteria to dissolve the gold into the hands of the miners. Mechanisms of bio-oxidation is still poorly studied, industrial technology literally “grope” empirically. To use bio-oxidation in an industrial scale, it is important to choose species that are most resistant to changes in the environment and at the same time, effectively decomposing sulphides. Usually one of two types: Thiobacillus ferrooxidans and Thiobacillus thiooxidans. Gennady Mineev and Adolf Panchenko in the book “Solvents of gold and silver in hydrometallurgy” people have suggested that the thionic bacteria “scrunch” the crystal lattice of sulphides in two ways: using enzymes and oxidizing sulphides contained in the iron.
Bright Golden future
It is unlikely that in the future will open a new Eldorado. Most likely we will have to settle for the fields with poor content and resistant to processing ores, technogenic deposits — that is, the “tails” of their own processing plants, and “mine” gold from old electronics. Apple, for example, in 2015, produced nearly a ton of gold from the equipment, which its owners surrendered in exchange for new devices.
There is, of course, interesting discoveries of rich deposits of gold-bearing sulphides on the seafloor. And in every cubic kilometre of sea water contains about five pounds of gold. However, cost-effective way, these five pounds out again there is no humanity.
Is fantastic to look at ideas to learn how to “grow” gold, understanding the mechanisms of its accumulation in living organisms.
From asteroids, say, gold will not be mined, the average content of his there’s a comparable in order of magnitude with an average content in the earth’s crust. In short, from the sky rain gold on us will not be shed, to think and search will have — as, indeed, always was.
Nadezhda Guseva
Olga Dobrovicova