# Au and As Concetrate



## chawimac (Feb 1, 2015)

Hi All,

Long time away from the forum. I have a mineral processing question and would like to pick some brains. About me, I am capable of refining 99,98% Au consistently from scrap/bars but am a NOOB as far as mining/minerals goes.

I have a gold concentrate of over 1,000 grams per ton that I am producing using a centrifugal concentrator (iCon, Falcon, Knelson etc...). ICP of the ore this stuff comes from reads like this:

Au 3.5 g/t
Ag 3.4 g/t
Al 5.07%
S Total 0.05%
(Sulfide 0.02%)
(Sulfate 0.03%)
Ca 0.74%
Fe 3.55%
K 1.35%
Mg 0.18%
Ti 0.03%
As 19,722 PPM
Ba 272 PPM
Mn 543 PPM
Sr 125 PPM

Notice the bizarre amount of Arsenic?

If you leach this material with NaCn you can recover over 90% of the gold contained in about 24hrs. Unfortunately I am not setup for Cyanide leaching nor do I have the experience with it (Scared really  ). The mineral in question IS NOT Arsenopyrite, hence the low sulphur and the ease of recovery with NaCn at a relatively coarse particle size. I will know the mineral generating the Arsenic and thus what I believe are my problems in about 6 weeks when I get an MLA study back.

Since I am incapable of leaching this stuff with Cyanide I have 2 options; sell a concentrate or amalgamate the concentrate. To sell a concentrate requires many tons of this material as it is literally in the middle of nowhere and shipping and other expenses make in uneconomical to ship small quantities. To bankroll things I have been trying to amalgamate with of course Hg. We have only conducted laboratory tests, as I want a closed process that will not dump toxic mercury on to the environment. Unfortunately seems to me mercury is the least of my worries when I have such a massive amount of Arsenic in my cons.

My question really is if someone can reaffirm this chemistry: 

I want to get rid of the Arsenic in the mineral using sulfuric acid. From what I have read, I need to use concentrated sulfuric acid, as dilute acid may no yield the results I want. Sulfuric even in dilute proportions creates a significant reaction with the cons, creating fumes and heat. I have ventilation, proper gear, and fume scrubbers for all this work, so no worries. I am hoping the sulfuric will convert the arsenic into Arsenic Trioxide, thus removing it from the host rock. We rinse the cons with water and filter. I am assuming this Arsenic Trioxide is similar to Silver Chloride which I am very familiar with. Will this work in theory? Must I use concentrate acid, or will dilute do? Are the fumes produced deadly, or is the arsenic in a stable form?

To remove the Arsenic Trioxide, I plan to add Hydrochloric acid to create Arsenic Trichloride which is a liquid that can be rinsed and filtered from the cons. Waste waters can be treated and disposed of. Is this step necessary or is the Trioxide soluble in water? Will HCI disolve the Trichloride?

So in my mind, if I remove the arsenic this stuff will amalgamate well and I can achieve a high recovery. Lab tests have show recovery of over 90% using Hg, unfortunately we must use massive amounts of Hg as it seems to get contaminated quickly and loose its affinity to the Au. Hence why I want to remove the As in the first place.

Any thoughts? Advice? There is a ton more data if anyone wants to add questions.


----------



## butcher (Feb 1, 2015)

From what I understand about arsenic is that, it will be hard to get rid of this toxic poison, you may be able to remove it from the ore just to have the arsenic as a poison in another location, in the dust where you mill the ore, as dust where you roast the ore, polluting the area where this work is done, even leaching it can form water soluble poisons, either left in the toxic as a waste or reduced to another toxic form of arsenic or arsine's. Either way dealing with arsenic will not be easy to do safely and responsibly, without specialized equipment and a good understanding of its dangers, its chemistry and how to deal with it environmentally safely.

Roasting can form an air born dust contaminating an area with arsenic trioxide, giving off a smell of garlic, and purple flames, this dust is water soluble and can contaminate not only the surrounding ground, but can get into the waters of the area, spreading this poison down stream, or down river where people use the water for drinking.

Arsenic is pretty much unreactive element (like copper).
Arsenic will react with concentrated H2SO4 forming arsenic trioxide.
Arsenic will not dissolve in HCl, but will dissolve in concentrated nitric acid, to make an acidic arsenic solution.
With dilute nitric acid it can form arsenous acids.


Arsenic trioxide:
Water soluble forming arsenous acid. 
Will dissolve in a basic solution to form arsenites.
Will react with HCl to form arsenic trichloride.
Will react with strong oxidizers to make arsenic pentoxides. with reducing agents will form arsine's.
Metal reduction like Zinc will form zinc arsenide...
Will sublime with heating.

From just this little bit of info, what is the plan to safely deal with this kind of a poison when looking at mining on a small scale, without having the equipment or the knowledge and an understanding of how?

My thinking is concentrating and selling the ore is your best option.
even if you get past the arsenic problem you may have many more problems getting the gold out cost effectively, finding a method to do so and do it in a half ways environmentally safe manner and still make a profit...
If mining was easy I think we would all know a lot of successful professional miners. I know a lot of unsuccessful people who try mining but, not many successful miners, especially when it comes to ores or hard rock mining.


----------



## chawimac (Feb 2, 2015)

Butcher,

Thanx for the reply. :shock:

The plan really is to ship out concentrates to a facility that can appropriately process them. As it stands hundreds of miners are milling in arrastras to amalgam the gold. Once they are done the tails with As and Hg go straight to the river. I figure it must be better what I am doing than what has historically been done.

Gold extraction itself is not difficult, its the contaminants that are the problem.


----------



## chawimac (Feb 5, 2015)

So just in case anyone is reading, here is an update.

I had no luck whatsoever. The end result was a massive amount of iron appearing in my process.

I am not sure if the iron is the result of equipment corrosion or some how I chemically increased the Fe's affinity to Hg... Arsenic is still there in large proportions, perhaps I did not leach long enough or perhaps the As just changed forms. Will keep trying a couple things but am setting up for a thiosulfate leach instead, will post about that process elsewhere as there is already extensive posts on the subject.


----------



## solarsmith (Feb 26, 2015)

you might want to look at the floatation systems and see if your final results would be cleaner. floatation has a much better recovery rate and no fumes to wory about. If you get your concentrate over 20 oz per ton you can direct smelt. you will need to crush and mill your ore down to smooth as silk to be sure to get a good recovery rate. .There are a few labs that can do floatation test for you. I dont know there names. good luck Bryan In colorado


----------



## Gratilla (Feb 27, 2015)

There are a number of companies offering and operating a process for oxidizing and precipitating problematic substances such as arsenic inline with PGM recovery. Arsenic is generally oxidized to a ferric compound and precipitated as FeAsO4(s), an analogue of scorodite. This is inert and generally regarded as safe to dispose of (at least in Australia).

More information:

http://www.intecip.com.au/

Get in touch with Dave Sammut there; I found him helpful in the past.

As for mercury, I believe there is a ban on discussing Hg on the forum. If you can get a waiver, I'll let you in on how to minimize your contamination problem.


----------



## chawimac (Mar 9, 2015)

solarsmith said:


> you might want to look at the floatation systems and see if your final results would be cleaner. floatation has a much better recovery rate and no fumes to wory about. If you get your concentrate over 20 oz per ton you can direct smelt. you will need to crush and mill your ore down to smooth as silk to be sure to get a good recovery rate. .There are a few labs that can do floatation test for you. I dont know there names. good luck Bryan In colorado



Solarsmith, yes floatation yields a different product and well the question really wasn't about concentration but more about recovery. We get two products, a Knelson concentrate and then a floatation concentrate. The floatation concentrate is only about 100 grams per ton. Thnx for the input!

Great research labs in Colorado are RDI and of course Hazen Research.

Does anyone have any experience with smelting. Care to share good links?


----------



## chawimac (Mar 9, 2015)

Gratilla said:


> There are a number of companies offering and operating a process for oxidizing and precipitating problematic substances such as arsenic inline with PGM recovery. Arsenic is generally oxidized to a ferric compound and precipitated as FeAsO4(s), an analogue of scorodite. This is inert and generally regarded as safe to dispose of (at least in Australia).
> 
> More information:
> 
> ...



Gratila, Thnx for the feedback. Unfortunately most of pressure oxidation stuff costs a cagillion dollars and best be left off to the big boys. My problems turned out to be about liberation size instead of contaminants.

As per Hg, I hope I haven't stepped on any toes. I'll just leave it be.

Thnx all for comments.


----------



## g_axelsson (Mar 9, 2015)

Gratilla said:


> As for mercury, I believe there is a ban on discussing Hg on the forum. If you can get a waiver, I'll let you in on how to minimize your contamination problem.


After some discussion in another thread of the forum, the ban of discussing mercury is lifted.

Please, share how to minimize mercury contamination. We want to know.

Göran


----------



## chawimac (Mar 30, 2015)

g_axelsson said:


> Gratilla said:
> 
> 
> > As for mercury, I believe there is a ban on discussing Hg on the forum. If you can get a waiver, I'll let you in on how to minimize your contamination problem.
> ...




Axelsson,

Each situation is different and each pile of tails laden with mercury are different. What I ran into was gold tellurides, gold sulfides and free gold associated with pyrites, arsenopyrites and other sulfides. For us the first step was to achieve the proper liberation size for the ore/tails to be able to recover whatever valuables present. Durring that process we recover whatever gravity recoverable gold using a Knelson centrifugal concentrator or similar. All this will work in varying degrees depending on minerology and the only way to be shure without flushing money down the drain is to hire a proper research lab like RDI or Hazen. Afterwards we used floatation to recover the rest of the gold/Hg we could. By using this process we can get from 50-97% of all the gold/Hg in the tails, depending on the minerology. The Knelson or the floatation gets most of the mercury, which you can later recover. At this point you get a concentrate that is 2% or so of our original volume, containing gold, silver, mercury, etc...

Once you have a concentrate you can leach it with cyanide, thiourea, thiosulfate, or whatever else or you can smelt it if you like if the material allows it. You can also just sell it to a company that can appropriately process the material cost effectively.

The mercury problem in my experience is purely economic. Mercury amalgamation is cheap and it works. The solution to mercury is lots of money; money to buy research and money to buy equipment. Once you have money you can decontaminate lots of these tails or process the ores correctly, getting a better overall return.


----------

