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popandsonminers
21:12:44 Sun
Jun 23 2013

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Hard rock liberation size, overgrinding, jaw crushers and ore processors


This up-date is mostly for the hard-rock miners, but could be of interest to all the miners on the forum. We’ve been progressing nicely with our development of a small scale ore processing and concentrating system, and now have a source for industrial jaw crushers. The goal is to throw the ore in at the front end of the system and recover the (usually very fine) gold out the back end. We use a jaw crusher, impactor and fine gold shaker table. You can see our progress at the MBMMLLC.com website and on YouTube at the MBMMLLC user name.

Small scale hard rock miners are at a disadvantage. We are usually limited by practicality, finances and permitting to a simple crushing and grinding circuit, with a gravity recovery system for the free milling values and value-containing sulfides.

We’ve gained some insights into the meanings of “overgrinding” and “commercially practical liberation size” when using gravity recovery. Where leaching with cyanide is used for final recovery, the liberation size is often in the 200-300 mesh range, in order to expose the maximum amount of gold for contact to the leach. Ball mills, rod mills and fine-grinding machines are excellent for this purpose.

However, if you are limited to a gravity recovery system such as a jig, sluice, shaker table or centrifugal bowl, ideally, you really want to recover the gold as soon as it is released from the quartz matrix. For instance, you don’t want to grind a 20 mesh piece of gold smaller and smaller, since most everything that is abraded from your gold particle will report to the tailings. The abraded pieces are too small for effective gravity recovery (but are perfect for leaching). So, one rule of hard rock gravity recovery is “thou shalt not overgrind your gold”. Basically, you want to run the discharge of your grinder (say average 20-50 mesh) through a gold recovery device(s), classify the barren tailings and recirculate the oversized ore particles back through the grinder.

Another consideration is “how fine to you grind your ore?” Deciding on your liberation size is a commercial matter, as it takes more time and power to get your ore finer and finer. We’ve found that a surprisingly high percentage of gold can be liberated at a fairly large mesh size. Even when our ore is ground to just 50 mesh, there is lots of minus 200 mesh gold. How can this be? Because quartz ore has a crystalline structure with the gold particles trapped between the crystals, much of the gold is freed when the ore is broken along these crystalline boundaries, even if the quartz remains relatively large.

So, if you recirculate 50% of your ore through your grinding circuit due to oversize, the net throughput of raw ore is cut by more than half (part of that 50% will have to go through more than once). If you can increase your liberation size to avoid such a significant recirculation load, even when it reduces your OPT gold recovery, it may be worth it due to increased through put and net gold at the end of the day. For a start up operations, run your high grade ore first to get some quick and easy cash flowing. Once you have a good feel for your ore and have some money in the bank, you can re-run the tailings to capture the remaining gold if it pays.

So, that’s where we’re at. If you visit the website and want to “talk shop” about hard rock milling, post on the forum or give us a call and I’m sure we can all learn something!

  
baub
03:24:43 Mon
Jun 24 2013

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Re: Hard rock liberation size, overgrinding, jaw crushers and ore processors

Archie Stutenroth once told me that his original pulverizers had a 20 or sometimes 30 mesh screen on the output. I asked him "Why" and he responded that going much finer, may net you less. I assume that the quartz or glass that encapsulates the gold does as you say, fracture easily, resulting in much finer material being created as well.
If I understand you correctly, then you could either rerun the classified tails thru the mill again or send them to the leach circuit, or both in what ever sequence you feel is best.

  
popandsonminers
16:06:17 Tue
Jun 25 2013

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Re: Hard rock liberation size, overgrinding, jaw crushers and ore processors

Hi Baub,

We generally get 70-85% of the gold in our ore with gravity recovery by grinding to -80 mesh (our effective liberation size). Additional grinding doesn’t increase recovery, as we speculate the gold is too fine. With ore from the same mine, Kinross was getting about 97% by grinding to 280 mesh and then leaching with cyanide. So yes, treating our tailings with cyanide would produce more gold, but the size reduction from -80 mesh to minus 200-300 mesh is essential to maximize recovery.

For gravity recovery, you really want the gold to have a minimum residence time in the grinding circuit to avoid overgrinding it. Here’s a picture of vein gold from ore we’ve processed. These are the infrequent “big gold” pieces we get.



Here’s the approach we’ve settled on for our gravity processing strategy:

1) We tune the impactor/hammer mill so the discharge is fairly coarse. There is a natural range of sizes that will exit any device, so the particles (gold or gangue) could be as large as 1/8” or more from an unscreened impactor. We don’t have a screen on the impactor because any “big gold” would have to be reduced in size before it would pass the screen, and this is exactly what we want to avoid.

2) We screen the slurry after it is discharged from the impactor through a 20 mesh screen.

3) We send the +20 mesh overs from the screened slurry, including any “big gold”, thru a P&S sluice to capture the gold, and recirculate the barren +20 mesh overs back to the impactor for further reduction.

4) We send the -20 mesh unders to our fine gold shaker table and get three distinct cuts: 1) gold with a bit of black sand, 2) sulfide concentrates (save for later refining) and 3) barren quartz tailings which are classified through a hydrocyclone. The +80 mesh (our effective liberation size) is sent back to the impactor for further reduction. We’re successfully getting 325 mesh gold and smaller. The smaller pieces in the picture are about 400 mesh (35 microns).



5) The -80 mesh table tailings are sent to waste, or could be leached for additional recovery. If leached, grinding to -200 to 300 mesh will improve recovery even more.

I should draw up a flow sheet for this, as I’ve never seen it done just this way by anyone else.

How about comments from others doing hard rock ore processing? Or just thinking about it? I know you're out there! What works and doesn’t work? What have your challenges been?

  
shaftsinkerawc
20:06:32 Tue
Jun 25 2013

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Re: Hard rock liberation size, overgrinding, jaw crushers and ore processors

Do you get a froth on the surface of your water after grinding and how do you deal with it in your system? I get a muddy froth of sulphides from my Keene bucket crusher. Curious to hear more about your talbes, went to your website but I didn't find any info. & I don't do you-tube. Have a great day. awc

ps- nice looking Au

  
baub
22:31:56 Tue
Jun 25 2013

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Re: Hard rock liberation size, overgrinding, jaw crushers and ore processors

Hi Shaft,

I assume you've tried a few things like soap, alcohol etc to reduce the foam? Perhaps adding more water would do the trick.
I'm new to this so I would suggest doing some surfactant research.
U-tech tables used to have some suggestions on its website too. At the time I bought mine, Darvin P. Wade was the owner, but he's since sold out. He had some ideas on surfactants that were on the site that I hope might point you in the right direction.

P/S

Thanks for the info. Makes sense to me.
I have crushers here, but not an effective transfer system to move from one crusher to the other. This is further complicated by the usual capitol limitations of small mining.
This might get remedied if I get my house sold.
In the meantime I pick up rox and murder them by hand if it's just one or two and massacre a mess if there's a lot.
Did find one place that had an old pile of rox and fines mixed. I classified and ran the larger and got no gold. Then I pulverized the finer stuff, say -20 and panned it, getting the only au so far. Will try the unpulverized -20 again and just pan it for comparison.

Ok, I did and got some very fine au but less than pulverized -20.
More research to be done I guess.

baub

  
popandsonminers
05:44:41 Wed
Jun 26 2013

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Re: Hard rock liberation size, overgrinding, jaw crushers and ore processors

Shaftsinker,

Yes, we sometimes get the same oily/shiny sulfide scum and just process it along with the rest of the slurry. We run our impactor wet, so everything is thoroughly wetted before it goes to the table. We found that a sheet of clear visqeen, used as a dampener right where the slurry hits the table, can break some of the surface tension and release more fine gold.

The website is a work-in-progress and we have to get some good pictures and descriptions up there. Here are links to some of the videos that include good shots of our table in action. They are on YouTube, but I hope you will watch them. If you'd rather, send me a PM and I'll put them on a CD and send it to you.

http://www.youtube.com/watch?v=q2JuzVRo6Vc

http://www.youtube.com/watch?v=kSuQG3pHpxg

Here's a video of an earlier prototype table that best explains the principles of how it produces three distinct cuts.

http://www.youtube.com/watch?v=pNbL1hOKHmE


Baub,

On the website, we offer to run some of a miner's ore thru our system, which is the one in the video link on the homepage. Call or e-mail Jason to discuss if this is of interest to you. I'd be interested to see what you've got in your ore, and it might help you with some of your decision making.

  

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