Gold Milling and Concentration

Nuggets taken from streams and free gold retrieved by panning, sluicing and dredging could go directly to the smelter.  Before the gold found in the ore taken from hard rock mines became gold ingots or gold coins, it went several complicated rounds of chemical and mechanical processing.  Men and machines at mills worked together to pulverize and break down the ore, discard the waste rock and prepare the concentrated ore for further processing.

A custom concentrating mill processed ore from numerous small mines that could not afford their own mills.  Integrated mills handled the ore from a group of mines all owned by the same mining company.

Generally companies built their mills on steeply sloping land, close to water, near the contributing mines and on major transportation routes.  Gravity helped carry the ore from upper levels to lower levels.  Steam engines powering the machinery; the milling equipment and concentrating processes required an adequate and constant supply of water.  Railroads and wagons brought ore, chemicals, and fuel.

At the upper level of the mill, the ore passed through rock crushers such as the “grizzley.”  Its iron bars, placed three inches apart to form a screen of sorts, allowed the finer pieces to pass through. The bars of the grizzley could be moved to increase or decrease the width between them, crushing the larger pieces until they, too, could pass through the bars.  Eli Blake of New Haven, Connecticut, in 1858, invented the cheap and efficient, cast iron “jaw crusher” that could crush ore into pieces ranging in size from one to three inches in diameter.

Stamp mills, first used in California, pulverized the ore into even smaller pieces.  Raised by a cam, the heavy stamps of the mill dropped about one foot onto an iron die below it.  Some stamps weighed up to one ton.  Workers kept the crushed ore flowing into the stamp mill.  Most stamp mills had five stamps that dropped in a prescribed order:  one, four, three, five, two.  A well-running stamp mill pushed the fines forward and retained the larger particles for further crushing.  Too little ore between the stamp and die could chip both of them.  Although noisy, the mills had much to recommend them:  cheap, efficient, easy to transport despite the weight of the stamps, and uncomplicated to repair.

After passing through the stamps, the pulverized ore mixed with water flowed over an amalgamation plate–a copper plate coated with mercury impregnated with gold.  If the slurry contained too much water, the gold would not adhere to the amalgamation plate or settle behind the riffles on the plate.  In some larger mills, the slurry crossed several plates, each lower than the last.  The drop from one plate to the next helped the gold adhere to the plate.

At clean-up time, workers heated the plates to retrieve the amalgam.  They then heated the amalgam in a retort where the mercury vaporized and condensed to be used again.  Although workers made every effort to recover the expensive mercury, some of it escaped the retort and some passed unrecovered with the slurry onto tailings piles and subsequently into ground water supplies.  The gold they melted and cast into ingots.

Alfred Redman Wilfley, a miner and entrepreneur who worked in mines in the Ten Mile Canyon, invented the Wilfley concentrating table and Wilfley pump, both playing an important role in Summit County’s mining history.  Powered by a King -Darraugh motor, the wooden table measured 5½ feet by sixteen feet and cost $450.  Linoleum with seven riffles covered the top.  A slurry of water and ore crushed to sand-size particles flowed across the transversely inclined table.  As the motor shook the table lengthwise, the riffles and beads of mercury behind the riffles trapped the gold.  Worthless ore and water exited the opposite side of the table.
The Wilfley table quickly earned its reputation as the best gold recovery method devised before mills began using chlorine, cyanide and glycerides to concentrate the gold.

The chlorination process, probably first used in New Zealand in 1889, might have been developed as early as the 1870s.  Mills in the United States adopted it in 1891.  Mill workers heated a mixture of crushed ore and chemical reagents including chlorine in a large tank.  The heat freed the chlorine gas and converted the gold to a soluble gold chloride.  Adding iron sulfate to the solution caused the gold to settle to the bottom of the tank.  After washing with water and drying, the gold could be melted and cast into bars.  Although the process captured 90 percent of the gold, mills quickly abandoned the process because of the expensive reagents, inefficient and clumsy process and deadly chlorine gas.

German scientists discovered cyanide’s ability to dissolve gold in 1846.  Using a weak alkaline potassium cyanide solution to dissolve gold received a patent in 1887 in Scotland.  The United States issued a patent for the process in 1889 and, eighteen months later, a mill in Utah used the process with gold ore.  With the introduction of the cyanide process, other methods became obsolete.

Mill workers mixed gold-bearing ore with a 0.5 percent by weight sodium cyanide solution and agitated the mixture in the presence of air, dissolving the gold.   Next, a vacuum chamber removed the air.  Powdered zinc added to the mixture returned the gold to a solid.  Next, a dilute sulfuric acid added to the mixture separated the zinc from the gold by dissolving the zinc.  After the gold had been washed and dried, it could be further purified by heating with a fluxing agent.  Although cyanide in higher concentration is poisonous, this process proved faster, easier and less dangerous than using chlorine.

Mills adopted the flotation process, first used successfully in New Zealand, after the turn of the century.  Replacing the cyanidation process, it relies on the fact that metallic sulfides, such as some forms of gold, stick to and float off with greases and oils.  Motors agitated the tank of ore, crushed to the size of fine sand and mixed with water and light oils or glycerides, aerating it and creating bubbles.  A bit of acid and sodium cyanide or other reagents added to the water bath etched the surface of the gold so that the oil would adhere to the gold.  Workers separated and dried the froth before shipping it to a smelter.  To help the process along, ore passed over a Wiflley table prior to undergoing the flotation process.

As miners dug deeper tunnels and recovered gold ore from greater depths, they found that the methods used to extract gold from the ore taken nearer the surface did not work.   At depth, sulfur mixed with the gold in a solid solution.  Mills lost up to 95 percent of the gold because the sulfur prevented the gold from adhering to mercury.

Nathan Hill, a chemistry professor at Brown University, who arrived in Colorado in 1864 to begin a career in mining, soon encountered the sulfur problem.  Up to that point in time, mining companies had been using copper and iron sulfides to collect the gold and silver in a mixture known as a “matte.” They shipped the matte to Swansea, Wales, for processing.  Hill visited Swansea to see the process in operation.  Back in the United States, he obtained financial backing to form the Boston & Colorado Smelting Company and opened a smelter in Black Hawk, Clear Creek County, where he introduced the Swansea process.  The process began with roasting (burning) the ore for six weeks in the open air.  This reduced the sulfur content but spewed sulfur dioxide into the air.  Then the ores were smelted in a furnace.  The gold, silver and copper settled to the bottom of the liquid.  Workers skimmed the worthless slag from the surface of the matte, the amalgam of minerals.  Slag went to the dump; the matte was crushed, assayed and shipped to a refinery where the copper, silver and gold were separated by heating.

After refining, the gold bullion went to a mint to be made into coins or to government vaults for safe keeping.  Because of a ready supply of refined gold and despite the lack of a fixed price for gold, which hindered its use as legal tender, the J.J. Conway & Company mint opened in Parkville in Georgia Gulch, east of Breckenridge.    Conway, a jeweler and banker, produced $2.50, $5 and $10 gold pieces in the summer of 1861.  When a newspaper article questioned the true value of the coins, Conway provided assays indicating that the coins equaled 97 percent of U.S. coins.  Nevertheless, the mint did not recover its reputation and closed.

Mining companies in the Breckenridge area suffered from the lack of efficient, large-scale smelters and refineries that could process the gold found at depth; concentrated ore transported to smelters in Black Hawk, Leadville, and Denver by wagons and trains cut into profits paid to investors.  But considering the impact on the environment of roasting the gold ore in the open air for weeks at a time and the piles of useless slag littering the ground around the smelters and refineries, Breckenridge residents escaped air pollution in the late 1800s and the question of what to do today with the piles of slag still littering the ground around long-quiet smelters.

written by Sandra F. Mather, PhD