Getting the gold out

[Gold Priisk]

Grind it up and leach it!

Easier said than done! .

Я не сторонник кучного выщелачивания, и предпочитаю работать с концентратами, когда содержание золота от 2г на килограмм материала.
Для этого, тщательно измельчаю рудный/шлиховой материал, пропускаю через центробежный концентратор и только тогда подвергаю выщелачиванию хлором.
Большая ошибка, работать с хлором, в открытой ёмкости, так как он при достижении концентрации в растворе, он становится летучим, и выщелачивание снижается, поэтому работаю в герметичных ёмкостях под давлением, где летучий хлор учавствует в постоянной рекурперации с образованием HOCl, HCl, Cl2.
Руду не обжигаю, так как при нагревании выше 300°, тонковкрапленное золото начинает окисляться продуктами распада, в основном это хлор из хлоритовых минералов, сера и мышьяк, и становится летучим.
К тому же, сам хлор хорошо разрушает сульфидные руды
Окисление пирита:
2FeS2+15Cl2+16Н2О=Fe2(SO4)3+H2SO4+30HCl
Окисление арсенопирита:
2FeAsS+14Cl2+16Н2О=2FeAsO4+2H2SO4+28HCl

Образовавшаяся свободная серная и соляная кислота, снова окисляет хлорную известь или дихлоризоцианурат натрия, для вырабатывания хлора и хоорноватистой кислоты.

Работу провожу в пластиковых бутылках 2л (2—2,5кг), а сейчас работаю над реактором в 50 литров, что позволит за раз, обрабатывать 65—70кг концентрата.

 

[g_axelsson]

Translated into English (google).

I'm not a supporter of heap leaching, and I prefer to work with concentrates, when the content of gold is from 2g per kilogram of material.
To do this, I thoroughly crush the ore / schlich material, pass it through a centrifugal concentrator and then only leach it with chlorine.
It's a big mistake to work with chlorine in an open tank, as it reaches the concentration in the solution, it becomes volatile, and the leaching is reduced, so I work in pressurized sealed containers where the volatile chlorine participates in a constant recurrence with the formation of HOC1, HCl, Cl2 .
Ore is not fired, because when heated above 300 °, finely divided gold begins to be oxidized by decomposition products, mainly chlorine from chlorite minerals, sulfur and arsenic, and becomes volatile.
In addition, chlorine itself destroys sulfide ores very well
Oxidation of pyrite:
2FeS2 + 15Cl2 + 16H2O = Fe2 (SO4) 3 + H2SO4 + 30HCl
Oxidation of arsenopyrite:
2FeAsS + 14Cl2 + 16H2O = 2FeAsO4 + 2H2SO4 + 28HCl

The resulting free sulfuric and hydrochloric acid, again oxidizes chloric lime or sodium dichloroisocyanurate, for the production of chlorine and hooronic acid.

I work in plastic bottles 2L (2-2.5kg), and now I work on a 50-liter reactor, which will allow to process 65-70kg of concentrate at a time.

 

[Liquidau]

Oh and as to thiourea...works great very fine gold. Better than cyanide. Easily recovered with zinc or borohydride. Of course one can always work with base hydrolysis and redissolve the sulfide in aqua regia, kill nitric, dilute and chill, filter and precipitate the gold with hydrazine salt. Much more expensive

Pregnant solutions may crystallize and are very orange red.

Hi Lou---
Just came across this thread, and it's right in line with my latest experiment in the lab, regarding Thiourea. I came across a technical paper proposing a concentration of 10 grams/liter, pH=1.5, Ferric ions (Fe+3)=0.3%, and temperature 22-27C, with 2 hours of stirring.
This does dissolve whatever gold plating of pins and fingers I added to the beaker, although seems much slower on thicker gold surfaces of ceramic chips.
My question is: what's the best way to drop the gold? You mention zinc or borohydride; can you please elaborate? Can you just drop zinc into the solution as you would with PGM precipitation of catalytic converter solutions?
Cheers!
Len

 

[Lou]

Well, I should mention that I was wrong. Gold thiourea complex seems to be colorless but Pd strongly colored.

Zn and BH4- are the two reductants that will get the gold out of the solution. The ferric chloride might be annoying. Maybe something that won't form annoying hydroxide crap would be better, like ceric ammonium sulfate.

 

[Stibnut]

I used H2O2 as my oxidant in a small-scale test since I'd seen this mentioned in the literature. It went well except that much of the thiourea eventually ended up as sulfur powder, but it wasn't too hard to filter.

 

[Liquidau]

I used H2O2 as my oxidant in a small-scale test since I'd seen this mentioned in the literature. It went well except that much of the thiourea eventually ended up as sulfur powder, but it wasn't too hard to filter.

Interesting. The zinc works fine by dropping black powder (maybe including Pd; I'll check on the XRF).
When I added H2O2, nothing dropped but the solution turned orange-red clear (maybe Pd)?

 

[Liquidau]

Well, I should mention that I was wrong. Gold thiourea complex seems to be colorless but Pd strongly colored.

Zn and BH4- are the two reductants that will get the gold out of the solution. The ferric chloride might be annoying. Maybe something that won't form annoying hydroxide crap would be better, like ceric ammonium sulfate.

Lou, won’t tte zinc drop everything below it, including copper and iron? Maybe better to use copper instead to only drop gold? Or will copper not work in Thiourea?

 

[Lou]

Doesn’t work very well. Thiourea tenaciously holds the gold.

 

[Liquidau]

Doesn’t work very well. Thiourea tenaciously holds the gold.

Ok, so does that mean everything drops with zinc (I.e. gold, silver, copper, etc.)? Which means each element needs to be removed/refined separately?

 

[Lou]

Yep, it would go back to the front end. So thiourea is the recovery step then final refining.

 

[Liquidau]

Update:
So for ferric ions, ferric sulfate looks to be a lot more effective than ferric chloride in leaching the gold. But a lot of solid white residue/precipitate is forming as well. This white material Dissolves when a little nitric acid was added, turning the solution light green. So ferric nitrate? Stannous test is positive for gold, so it’s there, but proving elusive to isolate. Since the source was fingers, there must be silver and copper also leached out? Or isn’t Thiourea selective for gold only? This is not in Hoke so flying blind here; only a few tech papers on the subject, though I’m convinced this might be a really good approach for many of us boutique extractors/refiners.
So any details the mod experts can provide on Thiourea would be greatly appreciated, and I’ll keep the updates on my progress coming.

 

[Deano]

Most people who research thiourea leaching get to the part where they read that thiourea is a carcinogen and then they lose interest in using it.
If you make an informed decision that you want to use thiourea despite the known risks then that is your responsibility.
The chemistry of thiourea leaching is that thiourea itself will not leach gold, the thiourea must firstly be converted by oxidation to formamidine di sulfide, this is the active leaching agent.
The tricky part of the above is using an oxidising agent which has enough oxidising power to carry out the conversion but not so much oxidising power that it will keep oxidising the formamidine disulfide through to a mix of sulfates, nitrates and oxides which will just sit in solution without leaching gold.
The best practical oxidant used for bulk leaching is a ferric salt such as ferric sulfate, other oxidants work but not as well or as cheaply as ferric.
The above means that you have to keep a close check on the redox of the solution, a meter is essential. You also need to monitor pH in order to keep your ferric in solution but not to have the pH drop enough that the rate of oxidation of the formamidine disulfide is increased.
The biggest factor in deciding whether to use thiourea, apart from the OHSA aspect, is the cost of processing.
Firstly if you are using ferric as an oxidant you will have to have all wetted surfaces in your plant, including pumps and piping, made of non metallic material. This is because you will have to have the pH below 3 to keep the ferric in solution.
Gold thiourea complexes have very low loading levels on carbon and resin so you either have to get the gold in solution levels high enough for direct electrowinning or you have to filter and precipitate with zinc or other reducing agent.
Either way it will not be a cheap processing route for gold ores.
Thiourea has a role to play in working with concentrates where the high unit cost of leaching is balanced out by the high unit value of the recovered metal.
When you are looking at processes for leaching gold always start with what does the industry use. The answer is cyanide on a cost basis.
Next do some research on safe usage of cyanide, read lots of papers from pro and con advocates.
The fundamental safety aspects are pH and ventilation with a lot of care given to ecological concerns around dams and waterways etc.
You cannot always guard against stupidity but you can put in place working rules to minimise damage from stupidity.
Deano

 

[kurtak]

You cannot always guard against stupidity but you can put in place working rules to minimise damage from stupidity.

 

[Biom]

Easier said than done!
Here's what I've tried so far: Into a five gallon bucket with a lid I added one gallon of 10% sodium hypochlorite solution, 1 cup of NaCl and twenty pounds of crushed ore (minus 20). I slowly added diluted hydrochloric acid stirred tested with litmus paper and added more acid until reading a Ph about 6. I checked on it hourly for the first 8 hours stirring and adding water and acid if necessary to keep the Ph at around 6 and sprinkling in a little calcium hypochlorite (from shock treatment MSDS for which listed 48%calcium hypochlorite and the remaining % NaCl) from time to time to maintain some chlorine scent in solution. Enough to smell wafting off the leach. After two days of maintaining the Ph best I could and adding fresh hypochlorite I let it settle out and I siphoned the liquid through a pvc column (1.5"x 16") filled with washed activated carbon. I burned the carbon to ashes and smelted about 25 grams of ashes with sodium carbonate borax litharge. Cupeled the lead and got about 2 milligrams of a silvery colored bead.

The second time I basically did all of the above but when I filtered the liquid through the pvc column filled with activated carbon it started reacting strongly bubbling heating up and giving off chlorine gas.

I was very careful to exclude any sources of carbon in the leach. Probably be a lot better if I had an ORP/Ph meter but I didn't at the time (still don't).

I am at a stage where I am not at all committed to any one course of extraction method. It's all small scale experimental phase at this point. I've had some limited success with amalgamation. Whatever I decide to scale up to a production level I'm really wanting to keep it as simple as possible.

Cyanide-easy,simple,cheap, but if you get it wrong it's gonna break eggs. know what I'm saying?

Mercury- a little out dated this day in age. It has a place as long as the gold particle size isn't too large or too small. Potentially environmentally deleterious if it escapes its metallic phase and combines with something in the ore and goes water soluble organic compound mercury.

Chlorine- it works but it tough to tailor/ pretreat the ore to a condition where that chlorine doesn't get used up eating all the iron and base metals in sight, that and maintaining Ph for a hypochlorous state where its active but not off gassing.

Thiourea-short leach life. Don't know much about it really.

Iodine- not economically viable for treatment of raw ores

Bromine- like chlorine but more expensive and potentially more dangerous

That's where I am at. I feel kinda stuck. I don't know which direction to go on this.

Hello please you have to be very carefully with all chimicalls espacly cyanide any ore you find it's not easy to be done by small scall especially gold the best way if you find gold ore get it assayed and cleam the place and sale it to big company's to have big success is to be done by big scale they use for most part cyanide if you find high grade ore it will be easy to sale refining metal it's not that simple and it can be dengerous you can work with some electronic scrap at small scall but ores to compacted for small scall

 

[kurtak]

Since the source was fingers, there must be silver and copper also leached out?

Per the bold print (in above quote) - there is NO silver in/on the fingers from circuit boards

If they are clean cut fingers (no solder) you have a layer of copper - then there is a layer of nickel plated on top of the copper - then there is a layer of gold plated on top of the nickel plating

If there is solder on the fingers - then besides the copper/nickel/gold - you have tin & antimony (in the solder) if the fingers are from new boards - &/or lead (in the solder) if the fingers come from OLD boards

If you are processing the "silvery looking" fingers - they are more then likely tin plating over the copper

There are some very rare exceptions where fingers were plated with silver - BUT - that is VERY RARE - & will most certainly never be found in everyday common consumer electronics

Kurt

 

[Liquidau]

Thank you Lads. I do work in an industrial lab and always under a hood. The carcinogenic aspects of Thiourea are well noted and the material is being handled carefully. I won’t ever touch cyanide or use it in processing.

The fingers are well trimmed from 10-15 year old boards. So the white precipitate is likely to be some complex of nickel, copper, tin or lead, and can be filtered out. But no point in reinventing the wheel here—-AP is the proper way to process fingers. I am attempting to extract the gold from incinerated, washed, magnetically separated, milled and centrifugally separated blue bowl concentrate. This has been leached with HCl to remove tin, lead and nickel, then washed thoroughly, then leached in nitric acid to remove copper and silver. Only PGMs should remain, along with some silicon residue. It is here that I’m investigating Thiourea rather than AR.
Opinions are welcome and eagerly awaited. Cheers.

 

[madscientistatlarge]

Would that be formamadine di sulphide

Most people who research thiourea leaching get to the part where they read that thiourea is a carcinogen and then they lose interest in using it.
If you make an informed decision that you want to use thiourea despite the known risks then that is your responsibility.
The chemistry of thiourea leaching is that thiourea itself will not leach gold, the thiourea must firstly be converted by oxidation to formamidine di sulfide, this is the active leaching agent.
The tricky part of the above is using an oxidising agent which has enough oxidising power to carry out the conversion but not so much oxidising power that it will keep oxidising the formamidine disulfide through to a mix of sulfates, nitrates and oxides which will just sit in solution without leaching gold.
The best practical oxidant used for bulk leaching is a ferric salt such as ferric sulfate, other oxidants work but not as well or as cheaply as ferric.
The above means that you have to keep a close check on the redox of the solution, a meter is essential. You also need to monitor pH in order to keep your ferric in solution but not to have the pH drop enough that the rate of oxidation of the formamidine disulfide is increased.
The biggest factor in deciding whether to use thiourea, apart from the OHSA aspect, is the cost of processing.
Firstly if you are using ferric as an oxidant you will have to have all wetted surfaces in your plant, including pumps and piping, made of non metallic material. This is because you will have to have the pH below 3 to keep the ferric in solution.
Gold thiourea complexes have very low loading levels on carbon and resin so you either have to get the gold in solution levels high enough for direct electrowinning or you have to filter and precipitate with zinc or other reducing agent.
Either way it will not be a cheap processing route for gold ores.
Thiourea has a role to play in working with concentrates where the high unit cost of leaching is balanced out by the high unit value of the recovered metal.
When you are looking at processes for leaching gold always start with what does the industry use. The answer is cyanide on a cost basis.
Next do some research on safe usage of cyanide, read lots of papers from pro and con advocates.
The fundamental safety aspects are pH and ventilation with a lot of care given to ecological concerns around dams and waterways etc.
You cannot always guard against stupidity but you can put in place working rules to minimise damage from stupidity.
Deano

Would that be formamidine di sulfide dichloride or?

 

[Deano]

Formamidine di sulfide can be purchased as a chloride salt, the exact form is dependent on the supplier. The chloride salt is only stable in solution at pH values <1. If an oxidant is not present in the solution then the chloride salt form can be reduced to thiourea. This reduction is not clean and substantial losses of thiourea occur. After all this you now have to add an oxidant to get the thiourea to formamidine disulphide.
Much cheaper and simpler to just run an oxidant with thiourea solution and form formamidine di sulfide in situ.
Apart from the health aspects there two main reasons not to use thiourea leaching.
Firstly the pH must be acid, usually between 2 and 3, this means that the leaching circuit must be made of non-metallic components.
Secondly the gold thiourea complex will load only poorly onto collectors such as carbon and resin. This means that you have to recover the gold directly from the leach solution. Not easily done if you want relatively clean gold.
Deano