How the mill building worked

Processing copper in the mill building

by J.C.

Students and chaperones look up at the old copper mill in Kennecott AK.

Photo credit: M.G.

In September, Academy Charter School’s seventh grade class went to Kennecott, Alaska. While they were there, they visited the Kennecott mill near McCarthy and learned how it once processed copper from 1911-1938.

According to St. Elias Alpine Guide and National Parks service, the Kennecott mill is 14 stories high; while the mill was still in use, there were 27 workers in the entire mill. The floors were very cramped so it would be quite crowded in the mill. There were no bathrooms in the entire mill!

Each floor would have one to two people in it. It was very cold in the mill it would only be about 22 degrees except for the mechanics room that would be about 32 degrees.

The "crushing" floors

Copper was sent down from the mines in buckets to the top of the mill. The high grade copper ore would get sent down a giant chute that would go straight to the trains. Lower grade ore was dumped into the crusher. This crusher was two metal rollers spinning in opposite directions. Ore would get sent through chutes so workers could examine if the pieces were small enough. The rest would either get crushed to smaller pieces by the rest of the “crushing” floors or get sent down to the “sorting” floors. According to a St. Elias' Alpine Guide, the sound of ore going down the chutes would sound like explosion after explosion. It could be heard five miles away in McCarthy.

Above, students and chaperones look around inside of old copper mill at Kennecott, AK. Photo credit M.G.

Students stand near the Hancock jig in the mill, which was also the place where mill workers relieved themselves! Photo credit: M.G.

The "sorting" floors

Most of these sorting floors were made up of shaker tables. These shaker tables were tilted at an angle and had lots of ridges. The unsorted ore formed a slurry that would be poured onto the table while it was shaking. Since most of the ore was limestone, and the copper was heavier, the limestone would bounce all to the other side of the table while copper would stay at the front of the table. However, in the middle there would be a mix that would get sent through multiple floors of shaker tables, according to St. Elias Alpine Guides and National Parks Service.

The leaching plant

After sorting, ore was sent down to the leaching plant, where the mix was put in giant tanks. A St. Elias' guide said the tanks were very tall and smelly because they were filled with ammonia. This would dissolve the copper, leaving the limestone at the bottom of the tank. The copper would get sent down a tube connected to the bottom of the tank and get sent to the train for bagging after it dried.

Tanks inside the leaching plant. Photo credit: Wikipedia.org

Model of the train that once picked up copper from Kennecott.

Photo credit to M.G.

Bagging the copper

These bags were each 40 pounds. There would be 120 to 150 bags of copper put on each train car. The St. Elias' guide said that each layer of copper bags put on each train car would have a layer of salt in between them so the copper filled bags wouldn’t freeze together.

The entire process from the top of the mill to bagging would take between two to two hours, 45 minutes according to St. Elias guide.

By 1938, $200 million dollars worth of copper came out of Kennecott. In today’s money, that would be about $1 billion, according to CPI Inflation Calculator.

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