This is the companion piece to the one about gold from Friday night in Popular Culture. Obviously, we intend to get geekier tonight than we did Friday. Then we talked about gold being used primarily as money or other symbols of wealth.
Tonight we will discuss how gold is mined and purified, and the actual industrial uses for it as opposed to jewelry and investment purposes. The old picture that most folks have about the forty-niner with his gold pan is far from how gold is mined, and was not really very accurate even then, being mostly a product of Hollywood.
To mine gold, first you have to find it. This is sort of like the old recipe for rabbit pie that started, “first you catch the rabbit.” Gold is actually pretty widespread, but useful ores are not all that common. It would be useful to look at mining gold in a chronological progression.
Up until comparatively recent times, almost all gold was found in placer deposits, that is, concentrations of metallic gold in the bends of rivers and streams where eddies slowed the rushing water, allowing the much heavier gold to settle. Sometimes decent sized nuggets could be found, but lots of the gold was pretty fine, the so-called gold dust. To separate the gold from the other heavy minerals, people took advantage of the fact that gold is over twice as dense as even the densest common minerals. This was done hydraulically, the gold bearing sand being placed on sluice boxes and flooded with water. A sluice box is long tray with sides that has riffles on the bottom. The riffles are simply raised portions so that a series of ridges were formed on the bottom of the box. The sand was put at the top and water was run down it (the box is tilted at an angle so that the water will run out the bottom). The water picks up the sand (and some of the gold) and washes it down the box. The heavier gold gets caught in the riffles as the sand runs out the bottom.
Gold panning operates on the same principle, but on a much smaller scale. Some of the sluice boxes were many meters long and a meter or more wide, while a pan has to be small enough to hold. Panning was and is used mostly as a prospecting tool rather than a production tool. In the old days the prospectors would use to pan to find promising gold deposits, then build sluices to do the actual bulk separation. To get enough gold to be profitable from panning alone would require extremely rich deposits, although in a few places a meagre living could be made that way.
The old timers would use copper pans and rub mercury on the bottom of them. The mercury forms an alloy with the copper (alloys of mercury are called amalgams) and becomes solid. Mercury also forms an amalgam with gold, so a mercury coated pan was effective at trapping the very fine dust particles that otherwise might be lost. Around the campfire at night, they would put the pan in the coals after they had separated out all of the bigger gold and distill off the mercury, leaving a little gold button in the pan. This may be why legend has it that the old timers were not quite right, if you know what I mean. They were exposing themselves to mercury vapor on a regular basis, and of course mercury is a potent nerve toxin (I wrote about mercury a few weeks ago). I also suspect the the cheap whiskey might have also contributed to their problems.
Hydraulic concentration is still used today, with modern refinements for certain ores, but other ores are better concentrated using a floatation process where chemical compounds are added to water to make a froth when air is blown in from the bottom of the container. The chemicals are chosen so the froth preferentially binds to the gold containing minerals, so that they float to the top and are skimmed off of the container.
In any event, in the old days hydraulic concentration was the only game in town. Much of the gold was lost, since the particles were so fine that the operators could not see them. At times mercury was reverted to to try to get the really fine particles, but that involved a considerable expense.
It was found that gold will dissolve in a cyanide solution if oxygen is present. This revolutionized that gold recovery operation, because the water flow could be restricted so as not to wash the fine dust away. The sluice box contents, now enriched in gold, could then be treated with a solution of sodium cyanide (much, much cheaper than mercury) and then the solution with the dissolved gold in can be processed to recover the metallic gold. This is widely used to this day, and ores containing only about half a part per million can be recovered if they do not require a lot of preprocessing. Half a part per million is difficult to get your arms around if you are not technical, so let us look at it this way.
A cube of water one meter to a side contains 1000 kg (2240 pounds). 1 mL of this water contains 1 g (0.035 ounces). Thus, about 1/28th of an ounce is 1 part per million, so about 1/56 of an ounce is half a part per million. So for an ore at this grade, 56 metric tons would be required to get one ounce. However, it is not quite that simple. I used ounces avoirdupois where 28.35 grams make an ounce. Gold is weighed in ounces troy, at 31.1 grams per ounce. That is just about 10% more, so actually a little more than 61 metric tons of ore would be required to be processed to get an ounce troy (worth about $1400 now) of gold from it. That is a LOT of rock, about 138,000 pounds to get one ounce. This is almost as heavy as two fully loaded semi rigs.
Obviously, there is a considerable amount of environmental consequence for handling this bulk of material, since the tailings (worked out ore) have to be disposed of somewhere. Most folks do not ever think about the environment in connexion with gold, because we think of the old forty-niner gold panner. Before hydraulic mining (as opposed to hydraulic recovery) was banned in the United States, entire riverbanks and other gold bearing sands were destroyed. In hydraulic mining, huge, high pressure jets of water were used to disrupt the alluvial ore beds and direct them to the sluice boxes. It was extraordinary destructive, both to the land and the water. The rivers became loaded with silt, killing fish and everything else.
Some placer deposits are gotten to by dredging, where pumps pull in the silt in rivers and pass it over the sluices. This certainly disturbs the benthic ecology, but the damage is not visible since it is under water.
By far the largest source of gold now is hard rock mining. In the old westerns this is illustrated by the miner going underground and using a pick and shovel to get the rock, then loading it onto a cart and using a mule to bring it out of the mine. It actually was possible for a sourdough to make a pretty good living at this, IF his claim were extremely rich. A few struck it rich, and most died in poverty.
Now days hard rock mining is done with huge armies of workers and heavy machinery. Just like coal or other minerals, there are two ways to do it: open-pit mining or shaft mining. Where the gold tends to run in well-defined veins, shaft mining is usually preferred because less material has to be handles. Mining engineers can direct the digging to follow the veins, leaving the rock that does not have much gold in it in place. This the lesser environmentally damaging of the two, since only the high grade ore is used, so tailings are reduced. Crews of workers take power drills and drill holes around and into the vein, then blasters place explosives in the holes and blast the ore loose. Then mechanized loaders and transporters ferry the ore out of the mine, where it is loaded onto trucks and taken to the refinery. In a good vein, the gold may be rich enough that all that has to be done to refine it is to crush the ore and then melt the gold out of it. Otherwise, it is usually enriched by sluicing or floatation after crushing, they subjected to cyanide.
When the gold does not run in veins, but is more of less evenly dispersed in the ore, open-pit mining is used. Since gold, except for placer deposits, is rarely found in soft ground, a gold open pit gold mine is a great big hole in solid rock. Drillers bore holes into the rock, and blasters set and detonate explosive charges to break the rock loose. Huge loaders put the ore into equally huge trucks and it is taken the to refinery. This is by far the most environmentally damaging way to mine gold, because huge amounts of tailings are left since all the rock, not just the rich veins, is used. After crushing and enriching by sluicing or floatation, the ore is generally subjected to cyanide.
After the “raw” gold has been recovered, it has to undergo further processing to render it pure. Gold almost always has silver in it, and often other metals as well. Mercury is often present and presents a special challenge for the refinery because of its toxicity. There are several ways to refine the gold to a high state of purity, but only two are used on a large scale. If the purity of the gold is only required to be around 0.9995 or so, the Miller process is used. It is pretty cheap, and for jewelry is good enough. In this process, the gold is melted and chlorine gas is blown through it. The chlorine reacts with almost all of the other materials, “burning” them to their respective metallic chlorides. These form a slag and float on the gold, so the gold can be tapped from the bottom of the retort and separated. The slag is valuable, since it usually has quite a bit of silver in it, so it is not discarded.
If gold of a higher purity is required, the more expensive Wohlwill process is used. This is almost exactly the same one that is used to refine copper for electrical uses, since it gets most of the junk out of it. Properly conducted, the Wohlwill process will produce 0.99999 fine gold. In this process, Miller process gold is cast into a suitable shape and placed in a tank of electrolyte (chloroauric acid, HAuCl4 dissolved in water). Next to the Miller gold are placed very thin sheets of highly purified gold. An electric current is started, and if the voltage and current density are controlled precisely, the gold in the Miller casting will dissolve and literally become electroplated to the thin, highly pure gold sheets. The Miller casting (the anode, or positive electrode) gets smaller and smaller as the pure gold plates onto the originally very thin sheets of pure gold (the cathodes, or negative electrodes). The impurities fall to the bottom of the tank. Often these are valuable, since they can contain platinum or other valuable metals. Unless extremely high purity gold is required, this process is not used because it costs a lot in electricity, is slow, and ties up gold for the electrolyte.
There are a couple of older processes that are used on small scales, like the one that you may have seen on the TeeVee where the assayer takes the impure gold, puts it in a porous clay cup, and fires it in an oven for some time. In this process the impurities combine with oxygen and materials added as a flux (the gold does not) and sort of soak into the porous cup. This is rarely used these days.
In both the Miller process and the clay cup process a significant amount of gold is lost by evaporation. In all but the smallest scale operations, the flues from the operations are swept out on a regular basis to recover the gold that escaped from the process.
I almost forgot one other significant source for gold: copper refining. Just as gold almost always has some silver and copper in it, copper almost always has some gold and silver in it. Trace amounts of impurities reduce the electrical conductivity of copper to such a point that it is useless for electrical work, so it is purified in its final step by a process almost exactly like the Wohlwill process for gold. The impurities that fall to the bottom of the electrolytic cell in copper refining are quite valuable and are sold to gold producers. This significantly offsets the cost of refining the copper.
These are the major sources for new gold. Old gold is defined as gold recovered from previous uses. Old jewelery is a significant source, and recycling of electronic components is becoming more important.
For the sake of being complete, I must mention that seawater has gold in it. But it does not have much. If all of the oceans were stripped of gold, it would amount to only about 10% of what has been mined since antiquity. No scheme has ever been devised to recover gold from seawater economically. We were talking about half a part per million in marginal ores a little while ago. Seawater has been determined to contain about 20 parts per quadrillion. A quadrillion is is a thousand million million! Not very likely to be a good source for gold, in my estimate.
Now, on to the uses for the 10% of the gold that is mined annually that does not end up as jewelry or as coins or bullion. Gold actually has comparatively few industrial uses compared to silver, which has myriads of uses, and even platinum, also with many, many uses.
Quite a bit of gold is used to plate delicate electronic connexions. If you open up your desktop, you will see that the expansion cards have gold plated contacts, and the memory chips and CPU also have gold plated contacts. The reason for this is that gold is an excellent conductor of electricity (third only to silver and copper) and does not corrode in the atmosphere. Since both silver and copper form nonconductive oxides and/or sulfides in the atmosphere, gold is preferred for plating those delicate contacts so that they remain reliable. Gold has a few other uses in electronics, sometimes being used for extremely fine wires to make connexions. Silver of copper wires would eventually corrode and fail that the very fine diameters that gold wire is applied.
Gold is also used sometimes in electron microscopy. When deposited in a very thin film on objects to be observed, the image quality can be enhanced by a large factor in many cases. This is mainly because it is a good conductor of electricity (streams of electrons), so it forms a Faraday cage on the object to be observed, preventing the electrons from penetrating it and thus becoming defocused.
Gold used to be used in medicine quite a bit, and in the form of some chemical compounds was for many years the only effective (somewhat) treatment for rheumatoid arthritis (RA). The newer generation of RA drugs has pretty much supplanted gold therapy, but it did allow much relief from suffering for many people back in the day.
It is sort of making a comeback in medicine as a carrier for chemotherapeutic agents, but this is sort of new. Since metallic gold is biologically inert and nontoxic, it is well suited for these applications. We shall see what the future holds for it there.
Not exactly medical, but still related, is the age old use of gold for dental work. Pure gold is too soft for that, so special dental alloys have been developed for this use. Even in an alloy, the corrosion resistance of gold suits is well for crowns and even more intricate work. An extremely disturbing consequence of dental work is the documented historical record of the Nazis removing gold dental work from Holocaust victims during World War II to fund a little bit of the war effort.
A very old use for gold is making ruby red glass. It does not take a lot to color glass, and even though cheaper substitutes have been found, many aficionados of colored glass strongly opine that nothing else has the subtle color that gold glass has.
A really interesting application for gold is to make ice proof windows for low temperature applications. This works because it is possible to make deposits of gold on glass or Lexan that are so thin that light is transmitted, but that there is just a tad of electrical conductivity. When a current is passed over such a treated glass, enough heat is generated to keep it free from ice.
If you watch on the TeeVee anything to do with satellite launches, you will often notice that the insulation that is often used is gold colored. It is gold, in a very thin film. It turns out that gold is particularly reflective for infrared radiation, so this gold coated insulation is particularly effective. Besides, it costs more to lift payload than gold costs by orders of magnitude, so in these applications the cost of the gold is not important, but its efficiency is.
Actually, these few applications are just about all that there are for gold other than those that we discussed Friday. The bulk of the attraction for gold remains that it is a pretty, shiny thing. Even the insulation application mentioned is related to it being shiny. I am sure that I missed a couple, but to my knowledge this is about it. I should mention its use in photography, which is dwindling rapidly. It was used to soften the harsh contrast differences that silver alone could produce, sort of hinting at a more natural color, particularly in photographic portraits of people. With the advent of digital photography, it is ironic that the gold in the electrical contacts is now more important than the silver in the film emulsions!
Now for what I consider to be the most important use that gold ever had. Because gold is the most malleable (able to be hammered into extremely thin sheets) of all known substances, the brilliant physicist New Zealand born, but British citizen Ernest Rutherford used it to change our understanding of nature. He was curious about the models of the atom of his day, which were all over the place. He gets all of the credit but his graduate students actually did the work! By the way, one of them was Hans Geiger, forever remembered for his radiation detector. Anyway, Rutherford wanted to see if the “plum pudding” model of the atom was valid.
In that model, atoms were collections of protons and electrons (the neutron had not yet been discovered, this was 1909) that pretty much were a sea of positive charges with negative ones embedded in it to make it electrically neutral, giving essentially a homogeneous (at least by mass distribution) material. Rutherford reasoned that if he were able to shoot some bullets through it, they would all be deflected more or less and form a diffuse shadow on a target. He chose a screen of a phosphor that would give a flash of light when his bullet, an alpha particle from a bit of radium bromide in a lead can with a little hole in it to provide a narrow column. The alphas would make the phosphor glow.
He reasoned that he needed something only a couple of atoms thick, so he chose gold foil of the thinnest variety available, almost transparent to the eye. When Geiger and his fellow student put the alpha gun one one side of the foil and the screen of the other, all that they initially saw was a bright spot directly in front of the gun. That made no sense. They should have seen a half dollar, dim spot if the plum pudding model were correct. As they watched, they saw single flashes all over the screen, and various angles. The put up bigger screens, and saw that sometimes a flash would occur behind the gun!
Rutherford finally realized that the only way to explain this was to abandon the plum pudding, homogeneous model of the atom and replace it with what is much more like our current one: the atom is mostly empty space, with a very small, very dense positive nucleus in the center and a very diffuse cloud of electrons around it. The math worked out to indicate that the nucleus contained, in its very small volume, almost all of the mass of the atom.
That, my friends, was the genesis of quantum mechanics in many ways. Other giants stood on Rutherford’s shoulders, and we are still learning.
Well, you have done it again. You have wasted many more einsteins of perfectly good photons reading this metallic tasting drivel. And even though the Republicans pretend that they will be civil when they read me say this, I always learn much more than I could possibly hope to teach by writing this series, so please keep those comments, questions, corrections, and other feedback coming. I shall stay around for Comment Time tonight, and return tomorrow after Keith is over for Review Time.
Warmest regards,
Doc
Crossposted at Docudharma.com and at Dailykos.com
1 comments
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finally finding out something really useful for gold, other than being a pretty, shiny thing?
Warmest regards,
Doc