Getting the pay out of pay dirt

This is an excerpt from a writing project I’m working on.

The impulse to find and extract gold and silver was one of the drivers of 19th century westward expansion in North America.  The discovery of gold in a California stream bed in 1849 and the subsequent discovery of gold and silver in other territories eastward to Pikes Peak and the Black Hills resulted in waves of migration of prospectors, merchants, investors, and swindlers from all directions, including Europe.

The staking of mineral claims in the American west by people who were engaged in the extraction of mineral wealth lead to an inevitable avalanche of settlers interested in tapping some of the wealth of the miners themselves. The open territory created a void that was filled by industrialists, merchants, government, and perhaps most importantly, the railroad. Miners needed supplies and their ore concentrates required transportation and beneficiation.

As claims were made on valuable mineral deposits, the outline of the geographical distribution of mineral value in a region eventually defined what came to be known as a district. The expansion of the railroad, sweetened by land grants, added permanence to the settlement of many regions around and en route to the mining districts.  The simple logistical requirement of frequent stops to fill the steam locomotive with water lead to the establishment of towns along the railway. This expanding transportation network, along with liberal access to land, lead to settlement by farmers and ranchers who then created a demand for goods exported from long distances by rail.

The history of man’s fascination with gold and other metals is well documented and there is no need to reiterate that saga in the present work. The mania for gold and silver in the west is legendary. Indeed, clues to the history of gold and silver mining in the American west are quite apparent even to the casual observer today. A drive to Cripple Creek or Central City in Colorado will take the motorist past a great many long abandoned mine dumps, prospect holes, adits, and antiquated mineshaft head works. These quiet features of the landscape mark the location of what was in times past a great and bustling industry.

Throughout the American west today there are many “tourist mines” and mining museums operated by individuals and organizations who recognize the importance of keeping this part of our cultural heritage alive. Through their efforts, visitors can view 19th century mining technology on site and experience the dark and eerily silent realm of the miner. Visitors can see for themselves the intense and sustained effort required in hard rock mining and the occupational hazards miners were exposed to.

The tourist mines and museums often focus on the activity of mining itself as well as the specialized equipment needed to blast the rock and muck it out of the mine. This is only natural. The gold and silver rushes left behind a large number of artifacts. These items are of general interest to all.

The technology that is often glossed over relates the matter of getting the pay out of the pay dirt. Indeed, this is a central challenge to gold and silver extraction. Once the streams have been depleted of placer gold and the vein or lode has been discovered somewhere up the mountainside, the business of extracting gold or silver from hard rock becomes technically much more challenging and capital intensive.

The panning and sluicing of placer or alluvial gold, while labor intensive, is conceptually easy to grasp. High density gold particles can be transported by suspension in a water slurry of the water is moving sufficiently fast. Gold particles will tend to settle at low points in a crevice or a gold pan where the stream velocity slows. A gold pan or the bend in a stream for that matter will have a flow gradient that will tend to collect the gold particles where the stream velocity slows.  A sluice or a Wilfley table are just devices designed to trip laminar fluid flow by inducing turbulence to encourage the denser gold particles to settle. Riffles or channels serve to concentrate the gold particles.

While gravity and clever tricks with fluid flow can be used to collect placer gold, isolating gold or silver from a hard rock ore body is quite a different challenge.  Gold and silver may exist in reduced form within the ore. They may also be found alloyed with one another or otherwise combined with other heavy elements. While gold tends to be inert even under oxygenated conditions near the surface, silver is subject to more facile oxidation and may be found in ionic form with several anionic species. Thus technology for the isolation of gold may not serve as an exact template for silver extraction and isolation.

Gold or silver may exist in the metallic form as bodies visible to the naked eye within the solid rock. Or they may be dispersed in microscopic elemental form throughout the ore body. Gold ore may be rich in elements that complicate its isolation even though the gold is in reduced form.  Silver ore is commonly found in ionic form and with numerous ionic base metals present.

Lode gold or lode silver, that is, gold and silver found dispersed in an ore body, were subject to considerable variation in mineral composition. As a result, differences in isolation techniques and process economics arose among the various operations.

In the 19th century a considerable body of chemical knowledge evolved as the gold and silver rushes progressed. This chemical knowledge was put into practice largely through the efforts of mining engineers.  It was not uncommon for the mining engineer to conceive of what today would be considered a process chemistry change, draw up plans, press the ownership for funding, and put the change into operation.

Twenty-first century chemists may recognize much of the nomenclature from this period as well as the intended inorganic transformations. However, the older literature is filled with obsolete nomenclature or that which is confined to the mining industry.  What should be apparent to the observant reader is the level of sophistication possessed by 19th century metallurgists and engineers in what chemists today might refer to as the “workup”.  That is, the series of isolation steps used to remove undesired components to afford a reasonably clean metal product. Mining engineers refer to this as beneficiation or as extractive metallurgy. Beneficiation of lode gold and lode silver involved chemical transformation in batch or continuous processing.

The story of the development of extractive metallurgy is in part the story of redox chemistry on complex compositions like rock. In the mid 16th century Europe, key individuals like Biringuccio, Agricola, and Ercker began to capture mining and extractive metallurgical technology in print. Vannoccio Biringuccio (1480-1539) published his De la pirotechnia in 1540, detailing economical methods of metallurgy and assaying. In 1556, the work of Georg Bauer (“Agricola”, 1494-1555) was published posthumously. His De re metallica is regarded as a classic of metallurgy. Agricola’s book describes the practical issues related to mining, smelting, and assay work and is illustrated with remarkable woodcuts.

By the year 1520, do-it-yourself books like Ein nützlich Bergbüchlein and Probierbüchlein were beginning to appear in Europe describing basic mining and metallurgy techniques.[1] By this time methods of cupellation and the separation of gold and silver were committed to print.

Cupellation is an assay technique wherein crucibles made of bone ash were used to fire prepared gold ore samples with an oxidizer, affording base metal oxides which then separated from the gold and absorbed into the crucible to afford an isolated button of gold.


[1] Aaron J. Ihde, The Development of Modern Chemistry, 1964, pp 22-24; Dover Reprint 1984, QD11.I44, ISBN 0-486-64235-6.

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About gaussling

Gaussling is a senior scientist in the chemical business. He occasionally breaks glassware and has been known to generate new forms of hazmats. Gaussling also digs aerospace, geology, and community theatre. View all posts by gaussling

3 responses to “Getting the pay out of pay dirt

  • John Spevacek

    While most chemists have a reasonably ok grasp of how the petrochemical industry supports the chemical industry (and was the driving force behind the creation of the chemical engineering field), they have little or no grasp of how important mining and mineral extraction is to their field and all of modern society.

    I’m not blaming or finger pointing. Until I took my current job, I was the just as guilty. But this job has been a real eye opener.

  • gaussling

    Did you recently take another job? What are you doing?

    • John Spevacek

      Yep, got out of the contract R & D business and back into good, old-fashioned product development for an unnamed Minnesota-based mining and manufacturing business. The division works on stuff that makes mining, oil and gas extraction easier/faster/cheaper…So I’ve been underground more than a few times this last year. Hoping to get offshore sometime too.

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