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Setting up a small gold ore processing pilot operation in ga ... (3 replies and 2 comments)

1 month ago
SaltwaterServr 1 month ago

First, hello from Phoenix, Arizona where we have all of the beach, but none of the water.  My real name is Jeff

I've got several claims and about 6 more prospective areas near here that have promising looking gold ores on them, all refractory.  We have arsenopyrite, pyrite, chalcopyrite, and limonite, but not all are present depending on the location.  

What I have planned, so far, is to crush/grind down to 75% or so 75µm and try a simple gravity separation.  After that I'm going to bioleach the arsenic and sulfur out in aerated 55 gallon plastic drums at 20% solids by volume at a pH favoring the growth of T. ferrooxidans and L. ferrooxidans for 7-10 days.  

Once the bioleach is done, I'll transfer the solution to a settling tank for removal of the solids.  First question then here.  How many times can I recycle the acidic solution from the bioleach tanks onto fresh ore based on arsenic loading?   If there's a high enough iron concentration, will the arsenic react and lock up with it, or would it be better to put it in a holding tank, add iron, let it set for a few days, then recycle the solution, and dispose of the insoluble ferro-arsenic compounds?  Is there another limitation to the recycling that I might be missing?  

Step two, leaching.  I've seen someone using 2M HCl as a lixiviant. My minor in college was polymer chemistry so I'm comfortable with using chemicals, but I sure don't want to get involved with cyanide.  I was talking to another gentleman online about thiourea but that stuff isn't exactly inexpensive, unless I was looking at too pure a grade for leaching purposes.  

I ran across an article on processing some gold in Nigeria that did leaching with HCl at 2M at slightly elevated over ambient temperatures and they were getting good enough recoveries, 80-83% after about 120 minutes of contact time.  I'm not pressed for time and my contact time can be longer, as necessary. What makes HCl attractive to me is that as I understand it, you can recycle the HCl again and again by adding in more to adjust the pH as necessary for the next leach.  

So, I'm going to attempt to leach with HCl, unless you fine folks have a better suggestion.  

Next step up, if I go with HCl, am I pretty much limited to Merrill-Crowe precipitation for simplicity's sake?  Can I used activated carbon, burn it once it's loaded and dried, and then smelt the ashes?  Will I lose a significant amount of gold due to volatilization?  Where can I buy the proper type of carbon to begin with?

I apologize if any, or all, of these questions are below the expertise level of the members here. To be perfectly candid, I feel a bit like a kid with a one piece bent Walmart pool cue walking into a New Orleans pool hall asking one of the old bluesmen if he can teach me 9 ball.  

No assays have been done yet, btw.  It's been too dang hot to go out and grab samples until the last week or so. 

1 month ago
minex 1 month ago

Hi Jeff,

Before you go to chemical processing/extraction, you really need to do the economics and have good grade estimates.  Perhaps gravity concentration only is needed/economically viable.

I would be glad to assist and advise for free on this - skype stevecanby


1 month ago

I greatly appreciate it Mr. Canby, I'll be picking your brain for certain as we move forward. One of our claims that we purchased from a guy who threw out his back said he had an assay of...he couldn't remember. He did mention that he thought it was wrong because he crushed, panned, and didn't get squat. I took a 5 gallon bucket sample to a friend and he got the same result, no appreciable amount of free mill at all. Break a sample of it in half, and it smells like a fine Italian dinner, all the lovely garlic As you can chew on. That's one spot we need to assay yet along with the 5 other spots.

1 month ago
Colette 1 month ago
1 like by David

Hey Jeff

In addition to seconding Minex's suggestion - with any luck you just need to go with gravity concentration - unless if you have a source of active bugs, getting them to do your oxidation may well be a bit tricky. In most bacterial oxidation processes we take a few months to adapt bacteria to a specific ore, and you need to treat them with great care during this process. You also need to know what your sulphide grades are, and ideally have a reasonable idea of mineralogy. I'd rather give a less biological route a go in terms of oxidising them (which is a big statement, as I do love bio-ox!) - is the ore particularly refractory to begin with? Would a fine grind and oxygen not possibly get you where you want to go? Or maybe even good old aqua regia, as much as I hate suggesting that usually? If you are thinking HCl then it isn't a huge step to go to AR.

Circulating the  liquor will work up to a point - at some stage the iron may well poison the bugs, as may other metals in the solution. You can adapt the bugs to higher concentrations, but if I remember correctly, not much more than 60 gpl (could be wrong, it has been a while!).

You can ash your carbon, I know a few folk who have done this - do it low and slow and you should be fine, don't just set it alight, you are aiming more for a quiet smoulder. You will lose some gold, how much is really not possible to answer - a fair bit. Activated carbon - you probably aren't too fussed with the quality of this if you are going to torch it, you can get it pretty much anywhere they sell water filters, although that is a bit of an expensive source. Maybe get hold of Jacobi in the region and ask if you can buy a few sample sizes? 

I heartily recommend getting hold of Marsden and House's "Extractive Chemistry of Gold" - it is worth the SCRIBD subscription - if you have done polymers then it will be easy to read, and has loads of ideas and references for the back end refining side of things.

Have fun, and please do stay safe!


1 month ago

Colette, I appreciate your reply as well.

ATCC.org carries live strains of L. ferriphilum, T. caldus, and T. ferrooxidans for about $350 a tube I believe. From what I've read, the first two are the most beneficial with T. ferrooxidans being the third wheel in the bio-ox lunch date. You do bring up a valid point; don't change their diet. Get an ore type and roll with it, don't start slinging new food on the table every other day or you'll end up with less than optimal growth and bacteria adapting rather than thriving.

I could possibly start initially dosing with H202 for the oxidation route and then leach with HCl for Merrill-Crowe. I can't see AR being scalable and HCl is fairly inexpensive by the drum or 275 gallon tote.

Here's a thought on an alternate path for carbon. Could I load it in the HCl, wash it, grind it in a slurry, and then amalgamate it with Hg followed by retort? This would be only for bench sized tests. I wouldn't consider mercury for anything but the smallest of tests with the carbon.

I have that book, second edition. Read it cover to cover, which is why I'm looking at thiourea on a pilot scale, in time. There are some interesting kinetics going on with pregnant thiourea solution being able to deposit gold on steel wool or using powered aluminum.

Oh, and the safety comment, I'll pass that along to my mining partner. We brought back some samples and he was cleaning them at his house. He sends me a few pictures via text as he's doing so, brushing off the pyrite with a pick and his bare hands. In the background? Bottle of hydrogen peroxide. He's cleaning arsenopyrite with hydrogen peroxide. He spent 4 days down and then 3 weeks on a special diet while his kidneys recuperated from the arsenic. Damn lucky he didn't end up killing himself.

1 month ago
Bhaduri 1 month ago

A fine garlicky Italian dinner in the Pit reminds one of Arsenic & Old Lace...so be careful out there. Precious metals in their elemental state require an oxidizer...hence peroxide/HCl or Aqua Regia (Nitrosyl chloride formed from 3:1 HCl:HNO3); strongly recommend you obtain a copy of "Chemistry of Au Extraction" by John Marsden & Ian House before venturing into a toxic & verboten territory to the uninitiated:


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