Posts Tagged ‘chemical separation’

Science Research Class at Walden School on our second collection trip.

Science Research Class at Walden School on our second collection trip.

After our fall semester, my research science class ended and the two sections of chemistry were consolidated down to one, with me teaching a computer technology course third period instead of chemistry. Without the two classes that could support the Tintic soil analysis project, I had to put the project on hold until I could get some more students involved. We also had an unusually cold January and February, with snow staying on the ground. This hampered our ability to collect samples. Between 3rd and 4th terms we hold a two-week Intersession at Walden School of Liberal Arts that allows us to teach specialty courses, and I dedicated my course entirely to finishing the Tintic project.

Altogether five students took the course, including Jeffery, Indi, Sean, Jem, and Aaron. To finish collecting all the samples, we had to take three additional trips down to the Eureka area. We were fortunate that the weather cooperated and warmed up enough that the snow melted.

Our second collection trip was on March 5 to the area of the Knight Smelter, the cyanide leeching pile, and Silver City. We stopped at the Bullion Beck Headframe on the way to take a group shot.

Ruins of the Knight Smelter built by Jessie Knight to process silver ore.

Ruins of the Knight Smelter built by Jessie Knight to process silver ore.

The Knight Smelter was built by silver tycoon Jesse Knight, who made his initial fortune with the Humbug Mine, then expanded along the Iron Blossom lode. Eventually, Uncle Jesse needed a smelter to concentrate and refine the ores from his mines, and he built it south of Eureka near the Union Pacific line. To connect his mines with the smelter and the Union Pacific main line, he built a narrow gauge railroad so that the smaller engines could make the turns and the steeper grades. A fairly level grade was built around the hills into his mines, and the road I walked on to the Iron Blossom #2 last fall followed this old grade. Jesse Knight contributed quite a bit of money to what was then the fledgling Brigham Young Academy, now Brigham Young University. The Jesse Knight Building, where I had several classes, is named after him.

Tank foundations and kiln at the Knight Smelter

Tank foundations and kiln at the Knight Smelter

The technology for refining ore went through rapid change in the 1920s. The smelter only operated for about four years, at which point it became cheaper to ship the ore by rail to the more modern smelters in Murray. The same thing happened with the Tintic Standard Mine and the reduction mill near Goshen.

There isn’t much left of the Knight Smelter except crumbling foundations for the solution tanks, a few archways where the kilns stood, and a pile of slag. Just to the south is the leeching pile. During the 1980s the price of gold jumped up when we went off the gold standard and the price was allowed to rise. Investing gurus such as Warren Buffet were advising people to invest in gold, and that drove up the price even more. Now, all these old tailings and waste rock piles that hadn’t been economical to process suddenly were. A layer of thick plastic was laid down and the waste rock crushed and piled onto the plastic, then a solution of cyanide was pumped over the pile. The cyanide would chelate with the gold and silver and trickle down through the pile into its lowest area, where it was pumped out and transported for smelting. This same process is being used at the Cripple Creek and Victor gold mine in Colorado.

Collecting a sample inside the kiln at Knight Smelter

Collecting a sample inside the kiln at Knight Smelter

We walked into the old smelter ruins and identified spots where there would likely be contamination, such as inside the kiln and underneath the tanks. We saw that a layer of sand was laid down under the tanks over the original soil, which is now covered with new soil deposited since the 1920s. We also collected samples from the top of the leeching pile. I picked up some samples of slag as well.

This smelter took the original ore and concentrated it by crushing and chemical action, using both physical and chemical separations. Mercury was used to bind to the silver (amalgamation). The amalgam was then heated up in a kiln to drive off the mercury and leave silver and gold. Since the silver started out in a compound with a higher oxidation state (+1) and was now a metal with an oxidation state of 0, this process is also called reduction. There were several reduction mills in the Tintic District. The leftover ore, after heating, still contained appreciable amounts of iron and lead, and was dumped onto a heap in a molten state. This waste material is called slag.

Slag at the Knight Smelter.

Slag at the Knight Smelter.

Sample under the tank foundations. Notice the layering of the soil; a layer of sand was laid down under the tanks when they were first built which is now covered with new topsoil.

Sample under the tank foundations. Notice the layering of the soil; a layer of sand was laid down under the tanks when they were first built which is now covered with new topsoil.

We moved on to the waste rock pile at Silver City where the Swansea Consolidated mine was located. Here, water runoff since the pile was created in the 1980s has washed small gullies fanning out south of the pile, crossing the road, and going on down the valley. The asphalt on the road is stained red with the iron sulfides. We collected on the pile itself, and used a portable pH meter to test the soil at locations on and near the pile. It was still too muddy to walk around much, and we were getting short on time, so we packed back up and drove back to Provo. We collected ten samples from five sites on this trip.

Testing the soil around the Swansea mine dump. The pH is very low, under 3.0.

Testing the soil around the Swansea mine dump. The pH is very low, under 3.0.

Sample at the Swansea Consolidated dump near Silver City

Sample at the Swansea Consolidated dump near Silver City

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Howardsville, Colorado on the Animas River.

So far on my tour through Colorado’s mining history, I have reported on how the ore was mined. Today, I got the chance to see how the ore was transported and processed at a mill. After completing my tour of the Old Hundred Mine near Silverton, I drove back down Stony Creek to where it joins the Animas River at a place called Howardsville, where some mining operations were still evident.

Arrastra Gulch

Google Earth view of Arrastra Gulch and Silver Lake. The Mayflower Mill is located at the bottom of the gulch in the upper left corner.

I stopped along the way toward Silverton at the base of Arrastra Gulch. This is the location of the main mining area around Silverton and one of the richest deposits in all of the San Juan Mountains. Before a proper mill could be built to process the ores, a Spanish-style arrastra was built here, which is a circular area with a flat stone floor and a central post with arms coming out. Each arm had a heavy stone or iron weight that hung from it and which would drag over the ore and crush it. Mules, donkeys, or even humans would be used to push the arms around in a circle. Once mills were built, the ore was transported to them from Arrastra Gulch and the high glacial circque above it (around Silver Lake) by tramlines or flumes. At one point as many as four separate overlapping trams were operating.

Arrastra Gulch marker panel a

Arrastra Gulch marker Part 1

The largest mill in the area was the Mayflower Mill (also known as the Shenandoah-Dives Mill) about two miles northeast of town. It was built in 1929 to process gold, silver, zinc, lead, and copper ores. Another large mill nearby was the Silver Lake Mill on the Animas River.

arrastra trams

Map of aerial trams in Arrastra Gulch near Silverton, Colorado.

Built of pre-framed Oregon fir and completed in six months for $373,000, the Mayflower Mill began processing ore in Feb., 1930 and continued in operation for 49 of the next 61 years, finally closing down in 1991. It is in fact still capable of operation, and all the original equipment is intact. The historical society allows self-guided tours that start in the machine shop, then move to the tram station, ore storage bins, ball mills, flotation cells, recovery system, assay office, etc.


A restored arrastra in Groveland, California. Heavy rocks were dragged around in a circle to crush ore.

It was an extensive operation, the biggest in the San Juan Mountains, and employed the latest technologies available in 1929, including the new techniques of ball mill crushers, froth flotation of sulfide ores, and recovery of base metals as well as gold and silver. These techniques are still used today in such places as the concentration plant at Utah’s Rio Tinto/Kennecott Copper operation, although the scale there is enormous.

Shenandoah-Dives mine

A sketch showing what the Shenandoah-Dives mine looked like during the 1930s. The aerial tramline connected with the Mayflower Mill.

For its 61 years of operation, it processed over 9,700,500 tons of ore to produce 1,940,100 ounces of gold, 30,000,000 ounces of silver, and over 1,000,000 tons of base metals.

Tramway in Arrastra Gulch

The aerial tramline connecting the Shenandoah-Dives Mine above Arrastra Gulch with the Mayflower Mill. The gulch is the canyon in the foreground, and the high circque is the basin around Silver Lake.

I used my camcorder to create a complete walkthrough of the mill, going in order from start to finish. At each stop I would stop the tape and take photos as well, and took my time to document everything. There were interpretive signs at each stop explaining what each piece of equipment did. Here is a rundown:

Mayflower Mill

The Mayflower Mill near Silverton, Colorado. A self-guided tour is available during the summer.

Processing Ore

The ore coming from the mines was about 5% metals and 95% waste rock (tailings). The metals have to be separated out, and this is done in stages so that all the metals (gold, silver, copper, lead, and zinc – the big five) could be individually removed and purified. This is done in three main steps: crushing, separation or reduction, and purification. The final step was done by a smelter off-site, but the first two steps were done at the mill.

tram station

Tram station at the Mayflower Mill. Full buckets descended from the mine by gravity, which also pulled the empty buckets back up.

The ore arrived in large open buckets by tramline. Gravity brought the ore down and allowed the empty buckets to move back up the loop. The ore was brought into the mill at the tram station and dumped, then transported by conveyor belt to the cone crushers. It was screened for size, and if too big would be returned to the crushers.

cone crusher

Cone crusher at the Mayflower Mill. It would crush the ore between rotating cones until it was pebble sized.

Once it was pebble sized, it would be transported to the Fine Ore Bin, which would hold 1200 tons of ore, enough for one full day of operation. The ore was then transported out of the bottom of the bin and mixed with water to form a slurry, then passed through a rod mill (which used long iron rods rolling around) where the ore was further crushed to a fine powder and sorted by a spiral classifier, an auger-like device that pushed the ore upward. If the ore was fine enough, it was pushed all the way to the top – if not, it would fall back down and be returned to the rod mill for further crushing.

rod mill

Rod mill at Mayflower Mill. Iron rods were fed into the mill, then allowed to roll around inside to crush the ore to the size of sand grains.

The powder, now the consistency of sand, was passed through a ball mill, with 2-3 inch diameter iron balls rolling around to crush the ore even finer. These balls were added frequently during the day through pipes from a ball bin. Now the ore was now the consistency of talc and fine enough to start to separate.

Spiral classifier

Spiral classifier at the Mayflower Mill. Ore slurry from the rod mill would be pushed up the spiral. If it was fine enough, it would be pushed over the top. If not, it would return to the rod mill.

The first metal to be separated was gold, using a system of settling jigs that pumped the ore through, allowing the heavier gold particles to settle out through vibration and suction. The lighter remaining material was passed on to flotation cells, where reagents and flocculents were added that would float the desired metals to the top of the tank solution while depressing or sinking the other metals. Lead was removed first, then copper, and finally silver and zinc removed in large tanks. The soapy bubbles would simply be skimmed off the top of the cells.

Ball mill

Ball mill at the Mayflower Mill. Ore crushed to the size of sand grains would enter the rotating drum and be crushed to powder by 2-3 inch iron balls.

The flotation cell solutions were then passed through filters with pumps that pushed the water through, drying out the solution to a damp cake-like material that was then shipped to a smelter for final refining, where it would be heated to drive off the sulfides. Each day, samples were removed and filtered through a squeeze press, then sent away to an assayer to determine the percentage of metals in each day’s run.

gold jigs

Gold jigs at the Mayflower Mill. Using air pressure, the lighter ore powder was suctioned away from the heavier gold particles.

Meanwhile, the gold filtered out by the jigs was sent through a concentration process. It would be passed over a shaking Deister table where the gold would be caught by riffles and formed a streak to be collected. It was mixed or amalgamated with mercury to remove the gold from the remaining waste ore. The amalgam was then formed into rounded boats or cakes and heated in a retort at 1200 ° F for 12 hours to evaporate the mercury, which was bubbled through water to condense it for reuse. The remaining gold was now called “sponge” and was about 80% pure. It would be sent off to a foundery for final purification. Four to five sponges would be produced each week. Each sponge weighed about 22 pounds. During the last year of the  mill’s operation (1991), a new process was developed that eliminated the need for mercury (which was highly toxic).

Lead cleaner cells

Lead flotation tanks at the Mayflower Mill. Reagents were added that would float the various metals, such as copper or lead, to the top of the liquid on soap bubbles which were skimmed off into the trough in front. The remaining metals were depressed to the bottom. Impellers would keep the solution agitated while blowing air through it.

Once processed, the waste material is called tailings and was made up of water and sandy ground rock. It was pumped down to settling ponds, where the solid tailings would settle out. This was an innovation of the Mayflower Mill, as previously the tailings would simply be allowed to flow into the Animas River. The high sulfur and iron content in the tailings would travel down the river and created the reddish stains on the rocks that I noted on my train trip up the gorge several days ago. At the Mayflower Mill, the ponds were shifted so that the solid tailings would build up a series of mounds downhill from the mill. These have now been collected into a large tailings pile near the mill.

Deister table

Deister table at the Mayflower Mill. It would shake, causing the gold particles to separate out against the riffles.

I found this self-guided tour to be fascinating from a chemistry perspective. The mill used a system of physical separations to crush, concentrate, and amalgamate the ore. The final smelting used a system of chemical separations. It is a perfect example of a chemical engineering process, and was continually upgraded and improved during its 61 years in operation. The mill could be run, during the night shift, with only three people. During the day there were additional people to do repairs and take samples, to run the gold process, and to run the machine shop. Shift supervisors oversaw the operation from the dog house, one man ran the crusher facility, and one man ran the flotation cells. This was the biggest operation of its kind in southwest Colorado and processed more ore than any other mill in the area.

gold sponge

A model of what gold sponge looked like after being removed from the retort furnace. The holes in it are caused by mercury vapor bubbling out.

Retort furnace

Retort furnace and gold button mold at the Mayflower Mill. The gold particles would be amalgamated with mercury, then heated in this retort furnace to drive the mercury off.

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