Friday, July 15, 2011

What can we say about a really big hole in the ground?

When I was visiting the Salt Lake City metro area last month I didn’t have time to see the amazingly large Bingham Canyon Copper Mine. People have been digging there for over a century, and now it is 2.5 miles wide by 0.7 mile deep. It’s a National Historic Landmark that even is visible from earth orbit. There is a visitors center where you can look at the mine, and even see them blasting. That mine is a good example for examining how we can clearly describe something really big in a presentation.

Kennecott Utah Copper has a six-minute From Ore to More video about it. Each day they mine 500,000 tons of ore, which is described as being 10,000 50-ton humpback whales. Most of us don’t see humpback whales in everyday life. They could equally well convert the 500,000 tons to a billion pounds per day.

The video also mentions that each giant truck they use for ore hauling has a capacity of 320 tons. How does that compare with a railroad hopper car or a semi-trailer truck? A ore hopper car holds 110 tons, so it would take three of them to equal a giant truck. A semi-trailer rig can only carry about 25 tons, so it would take 13 of them to equal one giant truck.

If we divide 500,000 tons by 320 we see that they fill 1563 giant trucks per day. There are 1440 minutes in a day, so if we waited by the rock crusher we would see slightly more (1.085) than a truck per minute drive up and dump. That's quite a procession!

They also said that their crusher has a capacity of 10,000 tons per hour. I think that is low by a factor of two. It would mean they only could crush 240,000 tons of ore per day. If they mine 500,000 tons per day, then half of it would just pile up beside the crusher.

How much volume is 500,000 tons per day? The ore probably is about 2.8 times as dense as water, which weighs 62 pounds per cubic foot. Then the volume per day is 5, 760,000 cubic feet. If we dumped that into a mold like a giant ice cube, it would be 179 feet on a side.

A football field is 300 feet long by 160 feet wide. If we could pile that ore up into a big block covering just the field, then it would be 120 feet high. We really couldn’t do that because the pile would keep sliding out into the stands and form a gigantic cone. It eventually would bury a college or pro football stadium. Pick your favorite, and then estimate how many days that would take.


cafedave said...

Thanks for the ideas - you're right: picturing humpback whales isn't easy to do!

One minor correction - there are 1440 minutes in a day, not seconds. So that's one truck per minute.

Richard I. Garber said...


You are right. It should be 1440 minutes per day, So it's about a truck per minute not per second. I went back and corrected the text.

Roughly one giant truck per minute is still a pretty impressive procession, almost a Truck-mageddon.