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Wednesday, October 25, 2017

What Size Classifier Should I Buy?

There's a common misunderstanding about classifiers, and it's to be expected.

Most people pick up a classifier for the first time in the field, and use it to screen out larger gravel before panning or sluicing, and for that job most people will settle on one size that meets twin goals, speed of screening and efficiency of processing the screened material. For me, that size was a number four, or 4 mesh. #4 means four openings in a linear inch, so it's essentially a quarter inch minus the diameter of the wire.

So if your question is, "What size classifier should I take panning?" I have a ready answer for you, but, and this may surprise some people, that's not what they're made for.

Think about the root of the word, "classify," which is just another word for "sort." Classifiers are made for sorting. It just so happens that a #4 is great for sorting between 4+ (includes a lot of waste rock) and 4- (a good size for panning and sluicing), so they're handy in the field.

4 Mesh Classifier

Meanwhile, back at the ranch, you've just picked up a brand new Blue Bowl to process your cons and you automatically think, now what size do I need? Your sluice works great with a #4, so the Blue Bowl must have a magic number too, right?


This is where I break out my golf balls and bowling bowls example. You have a gold nugget the size of a golf ball and a hunk of magnetite the size of a bowling ball, and a big old valve to turn up the water. If you turn the water up high enough to wash away the bowling ball, what happens to the golf ball?

Yep, it washes away.

Gold is only about four times as heavy as your average black sands, so when there's a big enough size disparity, the gold can be lighter. If you had two golf balls, one gold and one iron, the iron would wash away first. That's why we classify.

That Blue Bowl, or any other piece of equipment that relies on gravity to separate the heavier gold particles from the rest of your concentrates, works best when all of the material is the same size, or as close to it as you can get, so we use classifiers to sort the material before we run it.

The common plastic and stainless steel mesh classifiers you find at most prospecting shops generally come in 2, 4, 8, 12, 20, 30, 50, 70, and 100, nines sizes to choose from for a total of ten classifications! That's a lot.

Thankfully, you don't need them all right away, but you do need a few. A selection of smaller sizes will improve your efficiency, say 30, 50, 70 for the Blue Bowl, or 12, 20, 30 for your spiral wheel. Either would give you four classifications to work with. Run each size separate, adjusting in between, and you'll see far better recovery than without.

As for which brand, I like these classifiers (the same pictured above) because they're durable and relatively inexpensive.