Willoughby: Floating right along
Legends & Legacies
The Aspen Times, under an article headline “Floating Right Along” in 1925, heralded what it hoped would be a major milestone for Aspen. A group of floatation-mill experts and builders was scheduled to arrive in Aspen with the goal of adding a floatation-milling system to the Hunter Creek Mill at the base of Smuggler Mountain. The mill, which primarily processed ore that was brought out through the Covenhoven Tunnel, had been closed for a couple of years.
Floatation mills had been around at mining sites beginning in 1905 in Australia — but not in Aspen. The closest one was in Utah, operated by the United States Smelting Co. The visitors were representatives of that company. Floatation milling was improving, and mines — like Aspen’s that needed to have a better way to process lower-grade ores — were contemplating trying one.
Milling minerals had taken the same form for centuries but with refinements tested and patented escalating in the 1880s and 1890s. Ore is extracted, and it is mixed with material that has no value. In addition, several kinds of minerals would be in the same ore, and they needed to be separated. The most basic system was hand-sorting, common in the early days of Aspen. A miner broke off material that had little value and threw it on the mine dump. The rest went to the smelter.
Only the very best ore was profitable without some milling because it cost too much to ship the waste along with the mineral. Milling used a logical system, gravity. Silver is heavier than shale, so if you ground up the ore the silver, being heavier, would work its way to the bottom. You could clear off the waste and what remained was mineral.
That process was refined, adding various ways to shake the material to make the sort faster and better. A popular one, used in Aspen, was the Wilfley table. Think of a pool table with longitudinal ridges every few inches that could be tilted and vigorously shook. The mineral would slowly slide down the groves, and the lighter non-ore would slip off the table.
That technology with other forms of shakers worked until lower-grade ores were mined. The process was too slow for them and could not differentiate when there was a mix of ore quality. You can get some idea thinking about these numbers. Lower-grade ore would have 20 ounces of silver in a ton — that works out to only 1%, so you can imagine how important the milling process was. Looking for a better way resulted in two ideas: better ore grinding and the floatation system.
Even using the “shaking” method, it would work better if the ore was ground to a finer grain. One of the solutions was a ball mill, where ore was put into a container with steal balls then rotated rapidly and vigorously with the balls pounding the ore into a fine powder.
Floatation was not so obvious as it is the exact opposite of the gravity system. It is just as the name implies, ore is floated. After being ground to a powder, it is put in a large vat of water. Reagents are added. Air is injected to form bubbles. The air bubbles float the ore producing a froth on the surface of the vat. That is skimmed off and dried.
That system also worked for separating different kinds of minerals. In Aspen that meant separating silver, lead, and zinc.
The visitors in 1925 did not pan out. They began making improvements to the Hunter Creek Mill but stopped at the end of the year.
There is a reference that there was another attempt in 1927 and a reference to a floatation mill at the Hope mine — but not much evidence. The Midnight Mine built its mill in 1933 featuring the floatation process. It was ideal for its ores and was used until it closed in 1950. In 1937, the Hunger Creek Mill was revitalized, again, and employed the floatation method. The 300 ton/day mill was used to process the Smuggler Mine dumps that had enough mineral that using the floatation process made it profitable to run it thorough the mill.
Tim Willoughby’s family story parallels Aspen’s. He began sharing folklore while teaching at Aspen Country Day School and Colorado Mountain College. Now a tourist in his native town, he views it with historical perspective. Reach him at firstname.lastname@example.org.