New gizmo could help placer miners snag lost gold

Randy Clarkson doesn't know how his homemade rod mill wasn't invented long ago. But he can predict with certainty it'll be found at every Yukon placer mine within the next five years.

Randy Clarkson doesn’t know how his homemade rod mill wasn’t invented long ago.

But he can predict with certainty it’ll be found at every Yukon placer mine within the next five years.

“It’s too cheap, simple and easy to use,” he said.

An engineer by trade, Clarkson has spent the past two years working on a prototype to allow placer miners to separate fine gold particles from difficult concentrates.

Clarkson, the executive director of the Klondike Placer Miners’ Association, gave a demonstration of his invention at last week’s Geoscience Forum.

He’s been in the field since 1985, working on upgrading sluice box efficiency and conducting research in the Amazonian jungle and elsewhere in South America.

Placer miners may pass many cubic metres of gravel through a sluice box, a process that usually recuperates between 70 to 80 per cent of the gold.

Sluice boxes help separate gold from dirt using barriers called riffles. The riffles catch the heavier gold, while lighter rocks are washed away.

Sluices in the Klondike are often pretty quick, Clarkson said, and can process 50 to 200 cubic metres of gravel per hour.

Once or twice a week, miners clean out the equipment and come up with about a cubic metre of sluice box concentrate, from which they can no longer separate the gold unless they do it by hand – a tedious and time consuming process.

There’s quite a bit of gold in that sluice box concentrate but miners can’t sell it, Clarkson said, because it’s not pure enough.

That’s why placer miners have been storing the concentrate away in coffee pots and peanut butter containers, waiting for the right equipment to come along to be able to separate the gold from it.

Although the gold weighs more than heavy minerals, the particles are flat, making it much harder to separate.

The particles vary in size but can be as fine as 50 microns, or 0.05 millimetres.

That’s where Clarkson’s simple invention comes in, to get the other 20 to 30 per cent of gold that was missed.

This time last year, Clarkson and a colleague at the University of Fairbanks in Alaska were ready to throw in the towel.

“We were at our wit’s end,” Clarkson said.

Then they hit a breakthrough and came up with the idea of using a rod mill, which has been around for hundreds of years.

In this case, the rod mill is a steel tumbler, small enough to be moved on a dolly, that’s partly filled with 20-cm steel rods. Concentrate is put inside, and as the tumbler turns, the rods inside grind any rock into dust and flatten any bits of gold.

Through trial and error, they eventually realized the mill had to contain approximately 1 to 1.5 kilograms of concentrate and rotate at 70 revolutions per minute for it to effectively work.

The minerals and gold are put into the rod mill with a small amount of water, and ground between six and 10 minutes.

“Then you just take this ground up paste and put it through a fine sieve and the gold sits on top.”

After that, it’s easy to recover the gold using a shaking table.

By the end of the process, about 99 per cent of the gold that was missed has been recovered.

The invention was tested over the summer by placer miners around the territory and yielded successful results.

“This technique recovered between five and 10 ounces of gold per day of testing this summer at a profit of $5,000 per day – from concentrates that would normally be left behind,” said Clarkson.

Gold is extremely malleable and can be flattened extensively, Clarkson said, referring to gold-plated ruins in the Middle East.

“They look impressive but there isn’t a heck of a lot of gold because it can be hammered so thin.”

It costs under $2,000 to put together Clarkson’s invention, using a cement mixer, pulleys and small steel rods.

He said he’s been asked to build four of them and is helping other placer miners build their own.

He has no plans to commercialize the design, however.

“You could, but that’s not the idea because all my life I’ve done research for placer mining and I’ve always made it available publicly,” he said.

“Usually there’s been some component of public funding involved so that’s the whole idea – if you spend public money it should be available to the public.”

The rod mill could work separating other minerals such as platinum and lead, Clarkson said, and might also be useful in the developing world where they’ve been using cyanide and mercury to separate gold.

He said he would be publishing a research paper on his findings, which will be published on the Yukon Geological Survey website, among other places.

The next step would be to improve the rod mill and turn it into a continuous, rather than batch, process.

“Someone else can look at that,” he said with a laugh.

Contact Myles Dolphin at

myles@yukon-news.com

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