Extracting micron gold from black sands

[ayeyou]

I am a placer gold miner from British Columbia. After recovering all visible gold from my concentrates I have sent three different samples of the concentrates in for assays. They came back at 83 , 188 and 306 grams per ton gold still left in the concentrates. In order to verify the existence of this gold I have ran several 20 pound batches through a mercury amalgamation test. I simply placed the twenty pounds of concentrate in a 5 gallon bucket added 2 ounces of Mercury and then mixed the material together for five minutes with a paint mixer attached to my drill.The amalgam ball was then cleaned of mercury using a retort. After 5 separate test runs I have recovered an average of 4.6 grams per 20 pound batch. This would indicate an average of 460 grams per ton of micron gold still left in these concentrates after removing all gold possible via conventional gravity methods.
The concentrates contain a high level of arseno pyrite which creates another problem in the recovery of the gold due the existence of the arsenic. While Mercury works well as a recovery method in small batches I produce a couple tons per day of these concentrates and since the use of Mercury is frowned upon or illegal depending where you are I am looking for another way to extract these gold values.
I am thinking that perhaps a leaching process with some type of chemical or bio leachant may be the way to go. Also I am curious as to why my actual recovery with Mercury is higher then my assays would indicate possible. I know my results are accurate as I weigh all concentrate prior to processing and weigh the resulting gold amalgam dore that is produced from each batch. I attach a picture of 5 grams extracted from one of my twenty pound batches. I also include some of the assays. I have one more assay still pending.
Any guidance that could be offered as to a safe economical leaching process would be appreciated. Thanks Guys

I realize now that I posted this in the improper category area. I apologize but am not aware as to how to move it now.

 

[qst42know]

Also I am curious as to why my actual recovery with Mercury is higher then my assays would indicate possible.

Does mercury also pick up silver?

Have you had the material assayed for silver?

 

[ayeyou]

Thank you for that. It just gave me an aha moment. I have not had my cons assayed for silver and yes Mercury does pick up silver. This would account for the added recovery weight. I will have the gold I recovered assayed and find out what the purity is of the amalgamated gold recovered. I know the gold I get in my sluice is 940 fine so any higher impurities then that will tell me how much silver the mercury is picking up.

 

[Gratilla]

You don't say, but the labs probably did fire assays, which notoriously under-report micron gold and below. Similarly for mercury, so you probably have more Au than is being reported. Chemical leaches will generally get this Au, but unfortunately the acid-side leaches also attack Fe; cyanide which targets Au and Ag is probably the way to go, although thiosulphate and thiourea could probably do the job too.

 

[ayeyou]

I received my most recent assay back from Acme Analytical. This assay was done on my concentrates that I recovered the 5 grams from twenty pounds of cons. All visible gold had been recovered on my Miller Table. I was shocked that it assayed at 79 ounces per ton gold and 6.5 ounces silver. Now I really need to find a way to recover these values. This assay tells me that ten 5 gallon pails of my concentrates are worth something in the $120,000. area . I may have to resort to a roaster and cyanide leach on a small 1 ton per day level. It will give me something to do all winter when I cant wash rocks.

 

[nickvc]

Nice problem to have, may I suggest you edit your post if the details of your address and location are correct, with the economy as it is I wouldn't leave that on there especially at the amounts you potentially have. Most members I'm sure will be fine but this is an open forum and who knows who's reading this thread, if you can't do it ask a moderator for help.

 

[Traveller11]

You don't say, but the labs probably did fire assays, which notoriously under-report micron gold and below. Similarly for mercury, so you probably have more Au than is being reported. Chemical leaches will generally get this Au, but unfortunately the acid-side leaches also attack Fe; cyanide which targets Au and Ag is probably the way to go, although thiosulphate and thiourea could probably do the job too.

Any idea by how much the fire assays under report micron gold ? Would anything else affect a fire assay, say, perhaps, a high salt content in the concentrates?

 

[solarsmith]

looks like you have direct smelt cons. just a matter of finding a good flux.
merry christmass ...Bryan

 

[Gratilla]

Any idea by how much the fire assays under report micron gold ? Would anything else affect a fire assay, say, perhaps, a high salt content in the concentrates?

The most accurate answer is probably, "It depends". And unfortunately I'm not the best person to ask what the various reasons it depends on are, although for one, size does matter. There are occasional reports both here and on MicronGold Yahoo Group of fire Assay under-reporting though and it's generally accepted as normal.

It also depends to which school of thought (David Hudson, Walter Lashley, Paul Welk, etc) on "unreported gold" you belong to. Until recently belief in the existence of undetectable gold (and PMs) was regarded as the "howlings of mad dogs" by the mainstream. But more recently with the findings of researchers such as Al Johnson it is believed that there are significant values (undetectable by traditional means) in certain ores encapsulated and tied up in amorphous silicon (and IMO chelation bonds). It's not high on my list of things to look into (before I die) though. :mrgreen:

<Where are we on Salt Cells, BTW? I'm disappointed that the Brazilians weren't very forthcoming; perhaps a search for the right person to talk to would be in order???>

 

[Traveller11]

Any idea by how much the fire assays under report micron gold ? Would anything else affect a fire assay, say, perhaps, a high salt content in the concentrates?

The most accurate answer is probably, "It depends". And unfortunately I'm not the best person to ask what the various reasons it depends on are, although for one, size does matter. There are occasional reports both here and on MicronGold Yahoo Group of fire Assay under-reporting though and it's generally accepted as normal.

It also depends to which school of thought (David Hudson, Walter Lashley, Paul Welk, etc) on "unreported gold" you belong to. Until recently belief in the existence of undetectable gold (and PMs) was regarded as the "howlings of mad dogs" by the mainstream. But more recently with the findings of researchers such as Al Johnson it is believed that there are significant values (undetectable by traditional means) in certain ores encapsulated and tied up in amorphous silicon (and IMO chelation bonds). It's not high on my list of things to look into (before I die) though. :mrgreen:

<Where are we on Salt Cells, BTW? I'm disappointed that the Brazilians weren't very forthcoming; perhaps a search for the right person to talk to would be in order???>

Still toying with the salt cell idea. At the moment, I am more interested in the actual solution produced by the salt cell and the ph of that solution. As you know, a 100 year old version of the electrolytic salt cell actually produces the chlorine solution in the cell, transports it to a revolving leaching chamber filled with ore and then brings the pregnant solution back to the electrolytic salt cell. There, gold is stripped from the chloride, deposited on the cathode and the liberated chlorine is recycled as fresh chlorine solution and sent back to leach more gold.

As far as I can tell, this saltwater electrolytic cell is producing either sodium hypochlorite (NaClO), sodium chlorate (NaClO-3) or both. NaClO in water becomes hypochlorous acid (HOCl) and NaClO-3 in water becomes chloric acid (HClO-3). Both are powerful oxidizers.

Seawater has a ph of about 8. Barring anything else taking place, I cannot see how the ph of this chlorine solution could be anything below 7-8. This is in direct contradiction to the HCl/Clorox leaching method where we are told copious amounts of HCl must be added to lower the ph to 1-2.

Sodium hypochlorite bleach exists only because sodium hydroxide (a base - NaOH) is added to bleach to raise the ph to over 12, preserving the NaClO and preventing it from becoming hypochlorous acid. When you add hydrochloric acid (HCl) to bleach, you get: HCl + NaOH = NaCl + H20. Acid neutralizes base and, once the ph falls below 11.86, sodium hypochlorite becomes hypochlorous acid and sodium hydroxide. NaClO + H20 = HOCl + NaOH, requiring more HCl to neutralize the additional NaOH and continue to lower the ph. In the interests of economy, it would probably be better to use dry calcium hypochlorite and mix it with water. This would start our solution at a ph of 8, requiring less acid, as this bleach solution would not have sodium hydroxide (NaOH) added to it as a preservative.

So, eventually, our HCl/Clorox arrives at the same place as the saltwater electrolytical cell; namely, a solution of hypochlorous acid at a ph of 7-8. If it was possible to put gold into solution using the chlorine solution from the saltwater electrolytic cell, is there any reason to continue lowering the ph of the HCl/Clorox solution, and risk making it so acidic that it begins to put base metals (iron) into solution, as well?

I am now building a unit that resembles a rock tumbler (ore agitator) and am awaiting mail delivery of a ph meter. First experiment, HCl/Clorox solution of 7.5 and black sand concentrate.

Here is some more interesting reading from the "Prospector's Paradise":

"Remember chlorine is very slow. You will have to maintain this system for several hours to a day or more.

Like all of the halides, if you allow the solution to go acid, the chlorine will rapidly boil off and if you are close by will be extremely uncomfortable. Bromine and iodine are not so bad. They are not nearly so volatile and will give you a little more time to rectify the situation by addition of a little lye water.

If you are extracting with halides, in particular, chlorine where you have no visual reference as to what is happening, you should have some hydrochloric (muriatic) or sulfuric acid at hand. If your reaction should start to slow down and you are sure you have an excess of halide in solution you might have to add a little acid in order to liberate the halide from it’s salt form in order to keep sufficient free halide to ensure a good extraction. If you can keep the pH at say 8.5 you will be about right.

If chlorine production should get out of hand, you probably should have some solution of sodium thiosulfate on hand. It can be bought from any chemical supply. This is the stuff that tropical fish freaks use to treat tap water to destroy chlorine. You will need much more than they use however. This is a good way to neutralize any solution you wish to dump. Your neighbors will probably appreciate your thoughtfulness."

 

[Gratilla]

I'm guessing that SSN will also be a reasonable chemical parallel to the salt cell, in which case the Pourbaix diagrams in the SSN PDF show ClO4 an ClO3 predominating (ClO3 at lower Eh and/or Ph) at 80 deg C.

For mixed halide systems (Cl/Br/I), this quote from the Intec site looks relevant:

"An alternative system to cyanide is to use halides. Gold‐halide complexes become more stable as the halide changes from chloride to bromide to iodide. Gold recovery processes using only a single halide are difficult, and no commercial processes are available (Marsden and House 2006). This is due to chloride complexes being insufficiently stable to prevent preg‐robbing, and circuits using just bromide or iodide struggle to limit the loss of the expensive halide. In contrast, Intec has developed and patented the use of mixed halide systems for the recovery of base and precious metals from sulphide and oxide feedstock. The key advantage of a mixed system is that the major halide can be chloride, and only trace amounts of the gold‐stabiliser halide (bromide or iodide) need to be used. In this manner, the circuit is economically viable and achieves excellent recovery of gold."

All very interesting, but how does it help with your black sands?

 

[Traveller11]

I'm guessing that SSN will also be a reasonable chemical parallel to the salt cell, in which case the Pourbaix diagrams in the SSN PDF show ClO4 an ClO3 predominating (ClO3 at lower Eh and/or Ph) at 80 deg C.

For mixed halide systems (Cl/Br/I), this quote from the Intec site looks relevant:

"An alternative system to cyanide is to use halides. Gold‐halide complexes become more stable as the halide changes from chloride to bromide to iodide. Gold recovery processes using only a single halide are difficult, and no commercial processes are available (Marsden and House 2006). This is due to chloride complexes being insufficiently stable to prevent preg‐robbing, and circuits using just bromide or iodide struggle to limit the loss of the expensive halide. In contrast, Intec has developed and patented the use of mixed halide systems for the recovery of base and precious metals from sulphide and oxide feedstock. The key advantage of a mixed system is that the major halide can be chloride, and only trace amounts of the gold‐stabiliser halide (bromide or iodide) need to be used. In this manner, the circuit is economically viable and achieves excellent recovery of gold."

All very interesting, but how does it help with your black sands?

Are you quite sure that the Saturated Salt Nitric Acid leach produces perchlorate (ClO-4)? This is a very risky compound to have around as it becomes perchloric acid in water (HClO-4). For a real eye opener, look up perchloric acid on Wikipedia. I was quite concerned about the Salt Water Electrolytic Cell making the progression from NaCl (salt) to NaClO, NaClO-2, NaClO-3 and then to NaClO-4 (hypochlorite, chlorite, chlorate and perchlorate) but further reading revealed that, while it is a simple thing to make chlorate, a number of changes must be made in the cell to make perchlorate and it is not something that would occur accidentally.

About black sands. Further reading from older articles revealed that roasting of ores not only liberated gold from sulphides by making SO2 from the sulphur and FeO2 from the iron, it also aided in making the iron and other base metals (except copper) more resistant to chlorine leaching by making them into oxides. From what I have been able to garner, breaking the bonds of the black sand iron oxides, such as magnetite and haematite (Fe3O4 and Fe2O3), requires an extremely acidic environment to accomplish; say between 1-2.

Leaching with chlorine at a ph of 6-8 may very well put gold into solution without being able to break the iron oxide bond and putting iron into solution. In other words, chlorine quite possibly can be made into a selective leach for gold by controlling ph, contrary to popular belief.

 

[Gratilla]

Are you quite sure that the Saturated Salt Nitric Acid leach produces perchlorate (ClO-4)? ...

Take a look at Pourbaix diags 1 and 3 on pp 3 and 5 of the SSN PDF, respectively. While a number of conditions show ClO3 and ClO4 ligands, for HClO4 a pH of less than -0.8 is needed.

About black sands. Further reading from older articles revealed that roasting of ores not only liberated gold from sulphides by making SO2 from the sulphur and FeO2 from the iron, it also aided in making the iron and other base metals (except copper) more resistant to chlorine leaching by making them into oxides. From what I have been able to garner, breaking the bonds of the black sand iron oxides, such as magnetite and haematite (Fe3O4 and Fe2O3), requires an extremely acidic environment to accomplish; say between 1-2.

Leaching with chlorine at a ph of 6-8 may very well put gold into solution without being able to break the iron oxide bond and putting iron into solution. In other words, chlorine quite possibly can be made into a selective leach for gold by controlling ph, contrary to popular belief.

Commercial nitric acid @ 68% is about 0.5 molar. Standard SSN is 1/8 nitric acid, so the pH should be well above 0. Perhaps straight (or dilute) SSN could solve the problem???

<Warning! Speculation ahead!> I've been wondering for some time whether a "resin in pulp" method would be effective for separating AuCl3 directly from SSN leaching of black sands. Normally any Au leached/solubulized would be re-cemented back to metallic Au by the large excess of Fe. But if an organic solvent (eg butyl diglyme) were in the mix, some of the AuCl3 would be taken up by the solvent before Fe could get to it and the gold would be (slowly?) accumulated in the solvent???

 

[ayeyou]

Nice problem to have, may I suggest you edit your post if the details of your address and location are correct, with the economy as it is I wouldn't leave that on there especially at the amounts you potentially have. Most members I'm sure will be fine but this is an open forum and who knows who's reading this thread, if you can't do it ask a moderator for help.

Thanks for the tip Nick. Assays edited.
Since my last post I have done one further attempt with Mercury amalgamation to see if I could extract more then the 500 grams per ton then I achieved on previous attempts. This time I crushed the black sand concentrates as best I could without a proper crusher and then I roasted them to break down the sulphides. After the cons quit smoking and stinking I plunged them into ice water to "crack" them and further release micron gold from its matrix. I then did my standard mercury amalgamation process and the results were impressive. My recovery jumped from 500 grams per ton to 1080 grams per ton.
Although that is a great number it is still a far cry from the assayed 2449 grams per ton the cons are supposed to contain. As a result I have decided that mercury amalgamation is simply not sufficient to process my concentrates. After much research into the many different leaching methods out there I have decided to try the Iodine leach with Luguls solution 7% iodine. I have already found and purchased the Iodine and am currently looking for a couple other items I need. Lye is one of them and the other is sodium nitrate. My plan is to do the same crushing and roasting and then leach the cons with the Iodine. Although Iodine is much more costly then chlorine as a leach agent it seems to be a much more stable process as long as one watches his PH levels carefully.
I have chosen this method as it is the most enviro friendly and cost effective means of leaching since the Iodine can be recovered and recycled which makes the initial high cost of the Iodine bearable. If I can get in the neighborhood of 80 to 85 % recovery using Iodine I think I will commit to using this method and build myself a lab designed to process 1 to 2 tons per day. Reports I have read put Iodine in at around 90% recovery of fine gold particles and that will work for me.
Maybe I am barking up the wrong tree here as I have read that Thiourea is a better all round leach especially for sulphide ores. Does anyone here have experience using thiourea on ores?

 

[Palladium]

in certain ores encapsulated and tied up in amorphous silicon

I once done a whole research study based just on that.

 

[Gratilla]

in certain ores encapsulated and tied up in amorphous silicon

I once done a whole research study based just on that.

Care to share?

I think you'll be interested in click here, if you haven't already seen the site. There's a link to one of Johnson's patents, which has some pretty interesting experimental results.

 

[Traveller11]

Any idea by how much the fire assays under report micron gold ? Would anything else affect a fire assay, say, perhaps, a high salt content in the concentrates?

The most accurate answer is probably, "It depends". And unfortunately I'm not the best person to ask what the various reasons it depends on are, although for one, size does matter. There are occasional reports both here and on MicronGold Yahoo Group of fire Assay under-reporting though and it's generally accepted as normal.

It also depends to which school of thought (David Hudson, Walter Lashley, Paul Welk, etc) on "unreported gold" you belong to. Until recently belief in the existence of undetectable gold (and PMs) was regarded as the "howlings of mad dogs" by the mainstream. But more recently with the findings of researchers such as Al Johnson it is believed that there are significant values (undetectable by traditional means) in certain ores encapsulated and tied up in amorphous silicon (and IMO chelation bonds). It's not high on my list of things to look into (before I die) though. :mrgreen:

<Where are we on Salt Cells, BTW? I'm disappointed that the Brazilians weren't very forthcoming; perhaps a search for the right person to talk to would be in order???>

Still toying with the salt cell idea. At the moment, I am more interested in the actual solution produced by the salt cell and the ph of that solution. As you know, a 100 year old version of the electrolytic salt cell actually produces the chlorine solution in the cell, transports it to a revolving leaching chamber filled with ore and then brings the pregnant solution back to the electrolytic salt cell. There, gold is stripped from the chloride, deposited on the cathode and the liberated chlorine is recycled as fresh chlorine solution and sent back to leach more gold.

The most important thing I have discovered about the saltwater electrolytic cell is that it is not a true cell at all. It is actually a chlorine solution generator and the chlorine solution can be used as a leach in a totally separate vessel. Once the solution is pregnant with gold chloride, it can be brought back to the electrolytic cell and the gold stripped from the chloride and deposited on the cathode. Once the gold is stripped from the chlorine, it instantly combines with other elements in the cell to make fresh chlorine solution. In other words, it is a leach solution generator, stripping cell and chlorine solution recycler in one. Amazing or what?

As far as I can tell, this saltwater electrolytic cell is producing either sodium hypochlorite (NaClO), sodium chlorate (NaClO-3) or both. NaClO in water becomes hypochlorous acid (HOCl) and NaClO-3 in water becomes chloric acid (HClO-3). Both are powerful oxidizers.

Seawater has a ph of about 8. Barring anything else taking place, I cannot see how the ph of this chlorine solution could be anything below 7-8. This is in direct contradiction to the HCl/Clorox leaching method where we are told copious amounts of HCl must be added to lower the ph to 1-2.

So, let's do some comparisons between the saltwater electrolytic cell and the HCl/Clorox leaching method.

Sodium hypochlorite bleach (NaClO) exists only because sodium hydroxide (a base - NaOH) is added to bleach to raise the ph to over 12, preserving the NaClO and preventing it from becoming hypochlorous acid. When you add hydrochloric acid (HCl) to bleach, you get: HCl + NaOH = NaCl + H20. Acid neutralizes base and, once the ph falls below 11.86, sodium hypochlorite becomes hypochlorous acid and sodium hydroxide. NaClO + H20 = HOCl + NaOH, requiring more HCl to neutralize the additional NaOH and continue to lower the ph. In the interests of economy, it would probably be better to use dry calcium hypochlorite and mix it with water. This would start our solution at a ph of 8, requiring less acid, as this bleach solution would not have sodium hydroxide (NaOH) added to it as a preservative.

So, eventually, our HCl/Clorox arrives at the same place as the saltwater electrolytical cell; namely, a solution of hypochlorous acid at a ph of 7-8. If it is possible to put gold into solution using the chlorine solution from the saltwater electrolytic cell, is there any reason to continue lowering the ph of the HCl/Clorox solution, and risk making it so acidic that it begins to put base metals (iron) into solution, as well?

I am now building a unit that resembles a rock tumbler (ore agitator) and am awaiting mail delivery of a ph meter. First experiment, HCl/Clorox solution of 7.5 and black sand concentrate.

Here is some more interesting reading from the "Prospector's Paradise":

"Remember chlorine is very slow. You will have to maintain this system for several hours to a day or more.

Like all of the halides, if you allow the solution to go acid, the chlorine will rapidly boil off and if you are close by will be extremely uncomfortable. Bromine and iodine are not so bad. They are not nearly so volatile and will give you a little more time to rectify the situation by addition of a little lye water.

If you are extracting with halides, in particular, chlorine where you have no visual reference as to what is happening, you should have some hydrochloric (muriatic) or sulfuric acid at hand. If your reaction should start to slow down and you are sure you have an excess of halide in solution you might have to add a little acid in order to liberate the halide from it’s salt form in order to keep sufficient free halide to ensure a good extraction. If you can keep the pH at say 8.5 you will be about right.

If chlorine production should get out of hand, you probably should have some solution of sodium thiosulfate on hand. It can be bought from any chemical supply. This is the stuff that tropical fish freaks use to treat tap water to destroy chlorine. You will need much more than they use however. This is a good way to neutralize any solution you wish to dump. Your neighbors will probably appreciate your thoughtfulness."