recovery of gold from ores with chlorine in 1898

[bmgold2]

Check out Action Mining's System III leaching unit for ideas. Sounds like what you are building.

Here's a PDF with their instructions

http://www.actionmining.com/systemIII.PDF

bmgold2

 

[SMY_Met]

Hello Traveller11,

I will be doing a Research on Alternative less toxic lixiviants than CN & Hg (used in Cyanidation and Amalgamation practices by small scale mines here in my country) and is considering to investigate Chloride/Hypochlorite solutions as it is readily available and cheaper than Iodine. I am very much lucky to come across this forum and able to get very interesting points to begin with my study. If the result of my study would be convincing enough, then I will be presenting this alternative method/technology to the small scale gold mines group for them to adopt the new process.
Overview of my project proposal would be using gold oxide ores and will be comparing three ways of leaching the gold 1.) Drum-roll (same technique with bottle roll) and 2.) Electrolytic cell (same with your cell but no agitator, instead will have nitrogen blowing at the bottom) 3.) Enclosed tank with agitator, a pressure relief valve and 2 passages for nitrogen blowing 1 at the top & 1 at the bottom


Have read some studies related and you might also want to go through and might add to your compilation of great ideas..the pourbaix diagrams are really helpful here..
http://www.sciencedirect.com/science/article/pii/S030175160600192X
http://www.minproc.pwr.wroc.pl/journal/pdf/ppmp49-1.61-70.pdf


best regards,
SMY

 

[Reno Chris]

The problem with halogens is that even if less toxic than cyanide, they are toxic (Warfare using Chlorine is outlawed by the Geneva Convention), they are volatile, very corrosive to any pipe or tanks used in handling the solution and expensive to use. This why cyanide took over the industry when the process was fully developed.
Most small scale miners use gravity based processes - shaker tables, wheels, sluices, blue bowls, etc. Unless you have sulfide ores, gravity will usually recover the majority of the gold, and the rest is just too expensive for small miners to go after.

 

[Traveller11]

The problem with halogens is that even if less toxic than cyanide, they are toxic (Warfare using Chlorine is outlawed by the Geneva Convention), they are volatile, very corrosive to any pipe or tanks used in handling the solution and expensive to use. This why cyanide took over the industry when the process was fully developed.
Most small scale miners use gravity based processes - shaker tables, wheels, sluices, blue bowls, etc. Unless you have sulfide ores, gravity will usually recover the majority of the gold, and the rest is just too expensive for small miners to go after.

I don't believe you have read any part of this thread very carefully, or you would understand the reason I have been researching chlorine lixiviation methods from the 1890's. It is true that, at this period of time, chlorine solutions were made by first producing chlorine gas. However, this toxic gas was passed through water where it quickly dissolved; making hydrochloric and hypochlorous acid. Other methods at the time involved the electrolysis of brine, which produced a mixture of sodium hypochlorite (bleach) and sodium chlorate; an even more potent oxidizer than hypochlorite. It should be noted that none of the articles from the 1890's mention the need for making these solutions acidic.

While many on this forum subscribe exclusively to the chlorine method of first covering gold in HCl and then adding bleach (NaClO) until great clouds of chlorine gas are given off, this method is only useful to refiners and is totally useless to the miner wishing to extract fine gold from black sand. The reason for this is simple. The black sand, being mostly oxides of iron, is completely impervious to chlorine until the pH of the solution drops below 4-5; at which point chlorine gas comes out of solution. Below this pH, the acidity of the solution begins to break the bonds of the oxides and iron goes into solution, instead of gold. This is why the old timers roasted their ground ores in the presence of oxygen. It was not simply for the sake of making sulphide ores release their gold, it was also to oxidize base metals in the ore to make them impervious to lixiviation.

Suffice it to say, leaching with chlorine solution at a pH of 7-8, at which pH it exists as a mixture of hypochlorite/hypochlorous acid, is likely the safest method of dissolving gold. The only dangerous part of the process is taking bleach, normally at a pH of 12.2+, and gently adding an acid to it to lower its pH to between 7 and 8. Using hydrochloric acid, this is a rather delicate procedure with occasional violent reactions and a potential for adding too much acid. For this reason, a good pH tester is recommended, along with adding acetic acid (vinegar) instead of HCl. Acetic acid is a far milder acid that produces no reaction and allows far more flexibility.

As far as being expensive, what could be less expensive to the small miner than a process that uses Clorox bleach (or calcium hypochlorite pool sanitizer), un-iodized salt and vinegar as its components?

 

[Gratilla]

Hi T, I had been considering bumping this thread to see what the latest is/was. I believe that you're very close to a successful system; what would be very useful though would be a brief overview/summary of all the msgs, links and pdf's pulling together the good stuff and filtering out all the dead ends. I had intended to do this myself, but you know what they say about good intentions and the road to hell. <g>

I was wondering whether a bump of the SALTEM guys might also be useful. It's been some time now since you last corresponded with them and they might have (or know of) a successful operational system. I'd volunteer to write (in exchange for an email address), if you like.

 

[Reno Chris]

I don't believe you have read any part of this thread very carefully, or you would understand the reason I have been researching chlorine lixiviation methods from the 1890's.

Actually you are not the only person to post in this thread, and I was responding principally to the person who posted just before I did. However, I will say to you that the use of chlorine to dissolve gold from black sands is not the best way to treat black sands. You will note that the papers you quote are about treating hard rock gold ores - not black sands. As I noted in my post, for most small miners the materials they recover are best treated using "gravity based processes - shaker tables, wheels, sluices, blue bowls, etc. Unless you have sulfide ores, gravity will usually recover the majority of the gold, and the rest is just too expensive for small miners to go after."

Its hard to convince folks of this, but there is nothing mysterious or magical about "Black Sands" most examples - like 99% - have only tiny traces of gold after proper removal of all the gold that can be recovered by gravity methods. Get assays done by a competent, qualified lab if you really believe your black sand holds unusual values. Get a fire assay done or have the test done by AA - use standard, well accepted methods. If the test shows just a trace or below detection - that's what you have. Almost nothing. Most back sands that do have gold, have free gold that can be best recovered with gravity based methods. Meaning it is best processed with jigs, vibrating tables, blue bowls, spiral wheels and the like.

I am not trying to give you a hard time - I am a mining engineer with many decades of experience and I have processed many thousands of tons of ore with cyanide and processed many tons of black sand. What I am telling you is not some stuff I made up off the cuff, but what competent gold miners have determined from many, many years of experience.

 

[oro-del-sur]

Gentlemen, this is a terrific subject, very relevant. Excellent work!! In particular my congratulations to Traveller11 for starting the conversation and to James122964 for sharing his hands on method. A modern version of what once was the primary method world wide to leach gold from whole ores prior to the discovery of the cyanide method is now once again relevant but as an alternative to cyanide. How ironic. The beauty being that the chlorine process can also be engineered to minimize / eliminate the hazards of toxic off gassing and resolve inherent issues with iron and base metal compounds in natural ores. To a small/medium scale miner this is nothing short of a breakthrough. Even more interesting is the method that employs a chlorine generator (salt cell) and simultaneously collects the values. How cool is that?

It's exciting to see the stage being set on this open forum which appears to be on the verge of developing/adapting the process for public interest…for the first time in history? The early processes in the late 1800's were all proprietary - the modern versions (iGoli and CETEM) also proprietary. This is an amazing space, thank you all!

A few months ago (March) member Gratilla asks Traveller11 if he could summarize his works up to this point. I was wondering the same even if only some starting reference points, for those that want to follow along with their own testing and share the results with the group. I have been working with SSN but iron and base metals are an issue. It is a good aggressive leach but you can't even test the ORP without ruining your meter :-(

Searching the forum turned up a few related topics which I am still reading through - much of it is working out the chemistry. I have complied the links and if anyone is interested I can post them.

Thinking ahead a bit…a medium scale production vessel for concentrates: from what I deduce there is a need for slight agitation / stirring in a sealed vessel. Excessive pressure and heat don't seem to be problematic issues, so a simple rubber lined steel drum rotating on a horizontal axis (use 3M 500 or 5400 series adhesive for rubber -steel bond). Natural rubber is much more abrasion resistant than plastic and for the most part is chemical resistant. The drum could be scaled to any size. Side plates could be rubber lined to create a seal and bolted, removable for maintenance - sealed ports/hatches for filling and draining….maybe a small liquid sampling port? The drum can be belt driven - single or multiple belts wrapped around the drum / reducer-electric motor below with tension adjuster - or gear/shaft driven casters. Experimenting with the drum speed might achieve sufficient mixing and minor abrasion but additional "folding action" could be easily achieved with rubber strips (lifters) bonded to the liner. Actual paddles may not be necessary but if so, easily done (think cement mixer) by bonding in some type of poly plastic material with supports. I assume the idea is basically to keep the slurry in motion and prevent settling. For concentrates with bonded values you could eliminate the lifters/paddles and instead add some river gravel and larger round river rock to enhance abrasion and further reduce (grind) bonded particles for additional liberation of Au particles (would require a heavier duty liner, maybe material used for conveyor belts?) Essentially this is very much like a rubber lined ball mill but acting as a mixer and light grinding / abrasion. Dual function - grind and leach. You wouldn't run it as fast as a ball mill, just fast enough to churn the rock and gravel.

Thanks again for hosting this forum and this topic. I have bookmarked this thread along with other related threads. Other than bookmarking the topic is there an auto-notification function that sends an email when threads are updated? I didn't see it in the user control panel.

Jeff

 

[FrugalRefiner]

I have bookmarked this thread along with other related threads. Other than bookmarking the topic is there an auto-notification function that sends an email when threads are updated? I didn't see it in the user control panel.

Unfortunately, the email notification does not work right now. You'll just have to check back on the thread from time to time.

Dave

 

[Gratilla]

Oro, thanks for the interesting post. I came across a pdf a couple of weeks ago that I think would be of interest to members following this topic. Titled, "Chloride-Hypochlorite Oxidation and Leaching of Refractory Sulfide Gold Concentrate", it can be found at:

http://www.minproc.pwr.wroc.pl/journal/pdf/ppmp49-1.61-70.pdf

Note particularly the Pourbaix Diagram on page 3/63, showing the pH ranges for the dominant species of Sodium/Calcium Hypochlorite, specifically:

pH < 3.5 - chlorine (Cl2)
pH > 3.5 and < 7.5 - hypochlorous acid (HOCl)
pH > 7.5 - hypochlorite (OCl)

with hypochlorous acid being reported to being the most effective. (This is pretty much in line with Traveller's findings.) Interestingly this would keep the pH above 3.5 and minimize/eliminate Cl off-gassing.

From personal experience I can't stress enough the importance of keeping track of ORP/Eh. Many reported problems can be solved with a good Eh meter and a Pourbaix Diagram. I use a Hanna HI 98121 and haven't had a problem measuring Eh/pH of my SSN so far (touch wood), but I do have a replacement platinum probe on order just in case.

 

[oro-del-sur]

Gratilla - Thank you for that. I don't have a background in chemistry but am gradually wrapping my mind around the concepts. The link you sent (study) was very interesting. It does seem to verify what Traveller11 has been searching for…the holy grail perhaps? The working pH is in the range he proposed - and seems to answer the question as to which derivative of chlorine is doing the most work. Dissolution and 82% recovery of Au after 2 hours seems fast but then again their sample was -400 mesh Au particles (which is like talcum powder, not seen with the naked eye). Even more interesting was their conclusion that the sulfides are simultaneous oxidized with subsequent recovery of Au from solution (I think Traveller also suspect this). If this is true, this skips the need for prior roasting with heat. This is a big deal. And so then it would add a third function to the vessel design I have in mind (for concentrates): grind, leach and oxidize sulfides in a single vessel. Gives me goosebumps. I ask myself is this really possible?

The trick will be to keep the solution within parameters over a several hour period as the pH and ORP fluctuate. Any thoughts on that?…a method average Joe miner can manage? If the pH is adjusted does the ORP pretty much take care of itself? Adjusting the pH also maintains adequate production of hypochlorous acid? Is it all about the pH?

I just bought a new Hanna meter and quickly rinse it in water after dipping in the SSN, haven't had a problem yet. How much do the platinum probes cost?

Thanks again!

Jeff

 

[Gratilla]

If the pH is adjusted does the ORP pretty much take care of itself? ... Is it all about the pH?

I see you've been reading the SSN.pdf, but this is one assumption that Lashley definitely got WRONG. Eh changes pretty much independently of pH and usually more dramatically.

Some time ago I leached four ore samples (3 gold, 1 galena) with fresh SSN, keeping track of the Eh/pH at each step. What I found was that while pH changed only marginally (around pH 0), Eh dropped from around 870 mV to between 480 and 360 (most dramatically for the galena), as chemical reactions "consumed" various components of the ores. Now, most importantly, as gold needs an ORP of at least 800 mV to convert to AuCl3, the SSN was no longer able to dissolve the Au.

Also SSN and stannous chloride (to a lesser degree) "go off", so it's recommended you start with freshly made samples.

These are also the reasons why a number of members have had bad luck with Gold Test Kits based on SSN and stannous chloride. And why I believe:

Many reported problems can be solved with a good Eh meter and a Pourbaix Diagram.

IMPORTANT:
There are a number of different standards for measuring ORP, although only two (SHE: Standard Hydrogen Element and Ag/AgCl) are now widely used. (SHE is about 200 mV higher than Ag/AgCl.) Most depressingly, most authors just ASSume you know which one they're using!!!???!!! The Hanna uses Ag/AgCl (and so do I.)

Dissolution and 82% recovery of Au after 2 hours seems fast but ...

Lashley states that SSN reactivity (ie gold dissolution) DOUBLES for every 11-13^oC increase in temperature. Think about that for a moment. If you're working at 25^oC and increase that to 50^oC then the speed of your reaction will increase by a factor of four! Increase temp to 75^oC and the reaction increases by a factor of 16! etc.

Even more interesting was their conclusion that the sulfides are simultaneous oxidized with subsequent recovery of Au from solution ...

Sulfide oxidation is an important feature of most/all of the halide/chloride leaching systems developed over the past decade or so by systems such as Intec, Nikko, Sumitomo, Outotec, etc. All operate at temps between 85 and 110^oC.

 

[oro-del-sur]

Gratilla, yes, you picked up on that. Lashley said if you're not able to measure the ORP just to keep an eye on the Ph and assume the ORP stays within range. Yes, you are right, hot SSN is aggressive as hell - I had dissolution of -200 mesh particles in less than 30 minutes. I like the performance but didn't like the hassle of removing iron et al prior to leaching. The rest of the message I was referring to the Iranian study (Iran?)

So you are a fan of SSN - also use Halides. For ore concentrates what is more practical / effective from your experience? Or are you a switch hitter, go back and forth?

That's an interesting detail you point out about measuring ORP - will make not of that.

I was just checking the concentration of the solution / dosage the Iranians used for their study (optimum result), not sure I got it right - 400g of chemicals / 200 grams of solids / liter or water (2:1 chemicals / solids - 1 dm3 = 1 liter). So, to treat 25 gallons of concentrate requires 340 kilos of chemicals and 225 gallons of water? Wow! Did I get that right?? So, strictly a lab procedure, question being what part is useful in the field knowing the optimum dosage is not feasible?

 

[oro-del-sur]

…correction, should be "will make note of that"

 

[james122964]

Hi all, seems someone found some of my work, well, actually like most things with gold, it is based on modern use of old technology.

I can tell you that you want the hcl and bleach to be balanced based on expected gold content, the reason to do this and why I mix them separately is that when
the chlorine initially forms it is CL- not CL2, the single CL atom is what directly chlorinates the gold, if other free metals are there it will chlorinate them also, but since the materials are not in a solution when the chlorination takes place, very little, metal ion exchange takes place.

Sorry to not be around much and I will help anyone that sends me a email, but I am in the country as in boonies and only have signal when the stars align just right

jim

 

[Kbob]

Hi

I have some gold (fine) concentrate that has a lot of galena (60%) and some chalcocite (5%). Normal gold smelting of the material results in a product that has way too much lead. Is there a way of using hypochlorous acid (HOCl) to dissolve the gold and later precipitate with sodium metabisulphite. i assume the galena will not react much with the HOCl (since lead chloride is largely insoluble)

thank you

Kbob

 

[Deano]

Chloride leaching of your concentrate is a difficult undertaking at pH 7.

To maintain the precious metals in solution you will need to have 20% sodium chloride in the solution, this will also solubilise the lead as lead chloride in stable solution.

Oxidising sulfides with hypochlorous acid/ hypochlorite will consume large quantities of your oxidant, you will be shovelling hypochlorite in by the bucketfull.

Another problem area is that the pH will drop rapidly as the sulfides oxidise, maintaining pH 7 now becomes a difficult undertaking with frequent caustic additions.

This is not a system that you can run unattended for several hours and you will still have the real problem of having a toxic lead chloride solution at the end.

I recommend either using cyanide on fine milled cons or finding a smelter which will buy the cons from you for the precious metals value.


Deano

 

[fishaholic5]

After running rotating "pressurised" HCl/ Hypochlorite leaches on Ore in bottle rolls, buckets and barrels over a period of time I have a few thoughts.

Most of my leaches were between pH 3 and 5 measured at the completion of the leach during filtration. This will vary depending on the type of ore, some will give a higher pH
The vessels were never more than half full of ore, mostly around a third, enough water was used to just cover the milled ore and then the chemicals added and the vessel sealed.

The leach vessel initially pressurises as chlorine gas is produced, then depressurizes as the chlorine goes into solution. I believe that the pressurisation of the vessels in the 1890's was to force as much chlorine back into solution as possible and offset the risk of vessel failure under negative pressure during the leach.

The salt produced in the reaction helps keep the gold in solution. If there isn't enough salt gold can start dropping from solution during filtering.
Newberry and Vautin mention this as being from the formation of excess HCl, I think it has more to do with salt levels and the pressure change when the vessel is opened. I also believe its why they used relatively coarse filters that allowed the passage of the small gold particles as they dropped from solution.
It can be a little frustrating at times but can be overcome by trial and error.

Lower pH and higher pressure leaches work faster. Higher pH >4 leaches don't attack Sulfides and Iron as quickly, leach times on fine gold are relatively quick. Leach times can be as short as 30min and the longest leaches I run now are 4hrs depending on the ore type and gold content.
Gold will start to adsorb back onto any silica in the leached ore in longer leaches.

Quantities of Chlorine will be gassed off when the vessel is opened and during filtering as well as any other gases created during the leach. Take precautions.
There is always a risk of vessel failure in a system subject to rapid changes in pressure.

Open agitated or rotated leaches at a higher pH take longer but much less Chlorine gas is produced. The same issues with adsorption of gold onto Silica occur with longer leach times.
They are much more user friendly but material may need to be leached several times.

Cheers Wal

 

[fishaholic5]

I've been revisiting my old experiments. This is the "Purple Mud" mentioned in the Newberry Vautin process.


As mentioned in my last post here it can start dropping from solution within hours or even minutes of a pressurised leach being opened.
The speed of filtration in the old processes was critical in allowing it to pass through the filters before the particle size got too big.

My experience is that it can be reverse gravity seperated then processed seperately while the rest of the ore residue is re leached or put back into solution in the next leach.

Cheers Wal

 

[fishaholic5]

further testing shows that it can be dissolved with a HCl solution then precipitated normally with Iron Sulfate or Sodium Metabisulfite

I'm assuming from the reactions at this stage that it drops as Gold Oxide

Cheers Wal

 

[fishaholic5]

This is the Solution obtained by dissolving 1kg of the purple mud with a HCl solution. 5L of solution obtained was diluted to 20L with water before precipitation


And the start of precipitation with Iron Sulfate


Cheers Wal