Instrumentation Inflight particle size analysis

Hello Group,
Does anyone have suggestions re determining the -6.2mm fraction of a stream of iron ore?
This ore is sourced at an inland facility where it's already screened
to provide +6.2, and then railed to the port and stacked onto chevron ply stockpiles at the port.
The reclaimed ore is sent to the ship via a screenhouse, and the return fines fraction is sent back to "fines" stockpiles. We are seeing unexpected variations in the amount of fines returned. Not only is there "too much", but also this amount slews from "normal" to "excessive" and back again over a minute or two. Two things immediately spring to mind:
a) We need to see if we are getting spec material coming in on the trains, and b) We need to see if the ore is being subjected to too much attrition in out process
To this end, we would like to be able to see what the -6.2mm fraction is, of the ore as it is in transit. First Prize would be a (semi)portable unit that could analyse the iron ore pre- and post- stockpile.
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Pete wrote:

This seems so obvious to me that I must be naive -- if you're talking semi-portable that implies a guy going out there to do the measurement. So what about a shovel and a screen, or a shovel, a scale, and a screen?
There may be some higher-tech way of doing this (vision systems come to mind, followed almost immediately of mental pictures of lenses caked with dust). But it's not my field, so I can't help you there.
Somewhere in between, what about a sampler in there someplace with a screen? If you were really clever you could put a little trommel with the right sized holes in-line between point A and point B and measure the rate of fall-through. If you're lucky you may even be able to pour the measured residue back into the screen, if it's not convenient to put it into the waste pile.
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Tim Wescott
Wescott Design Services
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Tim Wescott wrote:

Getting a representative sample of the stuff on the train could be tricky. I'd expect much more fines near the bottom of a carload that has jostled down the tracks than at the top. In fact, the variation could arise from that.
Jerry
P.S. The number 6.2 mm is interesting. Assuming that the wire in the screen is about 1.5 mm diameter (about 15 AWG) that would make the center-to-center spacing of the wires 6.35 mm, in other words, quarter-inch mesh.
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Thanks Tim and Jerry:
Tim, the only downside of the shovel and screen approach is having to stop the belt. But if that is what it takes to get the results, then so be it. I was hoping for an automated solution based on what I once saw for an oversize determination - that was indeed a visual approach consisting of a camera mounted vertically above the belt, pointing down, and it scanned the top layer of the ore in real time. It kept up with the belt running at full tilt, and digitised the image, drew borders around the individual stones on the belt, determined their sizes, and gave a percentage of oversized material on the belt. I guess they made some kind of compensating assumptions about the vertical distribution of the ore on the belt, but that's another story. I do like your "sampler with a screen" - it would require some mechanical modifications inside a chute to take a representative cut, but not impossible.
Jerry, the profile of how the ore ends up in the orecar (after travelling 500km) will undoubtedly show more fines on the bottom, but then the car gets tipped upside down in the dumper anyway, and that will mix the lot up again. I have noticed that, upon being stacked onto the stockpiles, larger lumps of ore tend to roll down the sides of the stockpile. The reclaimer would probably be capturing more fines at the top of the stockpiles, and that is where our long sweeps in feed to the screenhouse comes from.
Thanks again, and regards,
Pete
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Tim, the only downside of the shovel and screen approach is having to stop the belt. But if that is what it takes to get the results, then so be it. I was hoping for an automated solution based on what I once saw for an oversize determination - that was indeed a visual approach consisting of a camera mounted vertically above the belt, pointing down, and it scanned the top layer of the ore in real time. It kept up with the belt running at full tilt, and digitised the image, drew borders around the individual stones on the belt, determined their sizes, and gave a percentage of oversized material on the belt. I guess they made some kind of compensating assumptions about the vertical distribution of the ore on the belt, but that's another story. I do like your "sampler with a screen" - it would require some mechanical modifications inside a chute to take a representative cut, but not impossible.
Jerry, the profile of how the ore ends up in the orecar (after travelling 500km) will undoubtedly show more fines on the bottom, but then the car gets tipped upside down in the dumper anyway, and that will mix the lot up again. I have noticed that, upon being stacked onto the stockpiles, larger lumps of ore tend to roll down the sides of the stockpile. The reclaimer would probably be capturing more fines at the top of the stockpiles, and that is where our long sweeps in feed to the screenhouse comes from.
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http://www.google.ca/search?num=20&hl=en&q=sample+from+gravel+crusher
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