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Practicality of Leaching Gold Ore with Acids (5 replies and 4 comments)

2 months ago
kenyaAurefiner 2 months ago

Hi everyone. Recently my friend and I came across some 30 year old tailings - about 1000 tons - which was analyzed to contain mainly 0.002% gold and 3.35% ferrous materials. I have no experience in gold processing, but my friend has some. He seems to think that we could leach the gold using Aqua Regia (Nitric and Hydrochloric acids), but I have my doubts about the practicality. Could somebody please help with information as follows:

  1. How practical/economical is it to use aqua regia
  2. If the above is not feasible, we intend to leach using Sodium Cyanide
  3. Does the presence of ferrous metals interfere with the leaching?
  4. What is preferable to use for precipitation: activated carbon or zinc?
  5. Whether it will be aqua regia or cyanidation, we intend to use the heap method

Thank you


2 months ago
Colette 2 months ago

Hi there Adam

As a preface to my response - 0.002% Au is 20 g/t, which is a rather high gold grade, especially for tailings. Gold has some rather nasty behaviours when it comes to being sampled and assayed, and I would recommend that you have a few more samples taken and duplicate assays done (If you haven't already).

  1. The practicality of aqua regia - the fact that it is rarely utilised outside of a laboratory should indicate that it rarely isn't easy to work with (see https://www.911metallurgist.com/blog/how-to-use-aqua-regia-to-purify-gold). It is a pretty indiscriminate leach, and will dissolve a lot of elements that you don't necessarily want to have to deal with in downstream processes. This links through to the question of economics - firstly, it isn't possible to give you a cost comparison without testing. Using $300/tonne for HCl and $375/tonne for HNO3, a tonne (or 1200 L) of the reagent will cost around $320/tonne. What your consumption rate will be depends on what else is in the tailings to consume the acid. Your equipment is going to have to handle the acid - which, with chloride included, can be a real pain in the proverbial. Also - on tailing recoveries tend to be on the lower end. With 1000 tonnes (or is it tons?) you have around 20 kg of gold in the material - if you get reasonable recovery of 85%, this gives you 17 kg to finance your activities - a lower recovery of 60% will give you 12 kg. Assuming you can sell for around $1300/oz,you have between 385 and 550 troy oz - so between $715 000 and $500 500 to finance your activities. I am assuming that you would like to make a profit on this - say a modest 10%. this means that you have between say $450 000 and 650 000 to invest in capital and life of operation costs. Most producers operate around the $600 - %700/oz mark (http://www.kitco.com/ind/fulp/2015-02-04-The-Real-Cost-of-Mining-Gold.html) lets assume because you don't need to mill or mine you are looking at the lower end of things - around $400/oz. So for life of operating your operating costs are around $220 000 to $155 000. This leaves you with around $350 000 to 500 000 to invest in capital. I've made some sweeping assumptions here - you need to relook them in your context. 
  2. Fair enough - loads of people do, and it is most likely the easiest route for you to take if you want to keep things low cost.
  3. Yes iron consumes cyanide, this will drive your operating cost up a bit - generally this is relatively insignificant versus the cost of labour and power, but with no mill, it may be a bigger chunk of operating costs. You will need to do a test to see what the cyanide consumption is for your feed material.
  4. Carbon doesn't precipitate gold, it adsorbs it, and you then need to desorb and electrowin it. On conventional sites with low silver to gold ratios, carbon is less expensive. In your case however things get interesting, as to electrowin you need electrowinning cells, while zinc cementation can be done in a more manual manner with less specialised equipment - you need a vacuum pump and a tower to get the dissolved oxygen out, but that can be put together out of bits.
  5. Is this material tailings or waste? You can't heap leach material that is fine, and you will need to move the current pile to stack it and agglomerate it. Heap leaching is great for big low grade ore bodies - you have the exact opposite. Your recovery will be on the lower end of the above, and if you are in an area with high rainfall things can get very interesting. You might also want to do a small test heap to see what happens - depending on what the iron is like in the material you could find it actually capturing some of the gold in a cementation process, which could reduce your recoveries.

Unfortunately you need to do a bit of test work and give the whole concept a good hard look in terms of economics - it is going to be quite marginal, despite the excellent grades, and you are unlikely to come out of the deal millionaires. Could be fun as a side project though!


2 months ago

Another thing - Aqua regia gives off some nasty fumes, but these are nothing like the stuff that will boil off the sample when you try to remove the nitrate. Be SURE that you vent your apparatus carefully to remove the possibility that anyone could breathe the fumes. Not as deadly as hydrogen cyanide, but the nitrogen oxides are toxic nevertheless. If you're going to do a lot of experiments like this, you should have a fume hood of some kind or a backyard shed ala Marie Curie.

2 months ago

Hi Colette and thank you very much for the detailed comments.

I wish to clarify the following:

The material is described as tailings. It is very fine. A company used to do commercial gold extraction from the material using mercury amalgamation, and the milled remains were discharged into a valley.

Aqua Regia method is obviously too expensive. We will do cyanidation.

Our method:

1. Dig a wide pit deep enough to extract about 200 tonnes of the material, line the pit with a leak-proof tarpaulin, connect leach flow piping and return the tailings into the pit for heap leaching extraction.
2. Lime is available and we could use it for agglomeration. Any ideas about the proper ratios?
3. If you could blow by blow describe your comment: "You can't heap leach material that is fine, and you will need to move the current pile to stack it and agglomerate it".
3. Will the presence of mercury in the tailings affect our processing, and how?
4. Yes, the area has high rainfall especially in September-December season. What's the implication beside the obvious dilution of the cyanide solution. Should we cover the pit?

I would appreciate any other pertinent info

Thank you for your time


2 months ago
inOr 2 months ago
1 like by David

Thanks for including a photocopy of the analysis. It helps formulate some questions. Specifically, what is the assay method used to produce these results? It looks like it might be x-ray fluorescence. If I remember correctly, XRF is a semi-quantitative method. It gives you a yes/no answer as to the presence of gold in your sample and a rough estimate of its concentration. In this case, the margin of error is given as +/- .001%, which is 50% of the listed gold concentration. That's a lot. I would interpret these results as saying there is detectable gold in your sample at a concentration about 2x the error. How reliable is the instrumentation and the staff using it? Were this my only analysis, I would want to test its reliability. You could submit at least one other sample as a standard comparable to yours. How accurately do they measure the concentration of the standard (known only to you). Another test would be to 'dope' one of your samples with a known amount of gold and submit that together with another sample identical to your initial assay. Naturally, the identity of the doped sample would be known only to you. Do the 2 analyses differ by the expected amount? Or, you could perform an analysis by a more accurate method. I'd like to see error margins more like 10% of the indicated concentration of my sample before I'd count on the method's quantitative reliability.

Another thing... From what I've read, if you will purify the gold from the aqua regia solution by chemical means, you will need to remove all nitrate from the dissolved sample before you can process further.  You are trying to create a solution of the aurochloride complex ion for further purification, and the nitric acid is a powerful oxidizer and its presence in the solution changes the chemistry significantly.  The method recommended (for small samples) is to add another volume of HCl to the aqua regia solution and boil to evaporate all but a small volume of sample.  Then repeat this at least 2x.  If you can assay the nitrate content of the sample throughout this process, so much the better.  But, you can see that the procedure is quite laborious and requires extra energy to carry out the evaporations.  Chlorine gas can also be used to extract gold from ore.  I don't know the details, and it requires the use of chlorine which is very toxic.  


2 months ago

Thank you inOr for your time and sharing.
You are right the report used XRF method which has accuracy issues. How about 200 tonnes as a test run?


2 months ago
David 2 months ago

Hi kenyaAurefiner and welcome to 911Met.

Before you go all-in chemical leach, have you testing to see if you can recover that gold with a good gravity machine Falcon/Knelson?

Run a GRG test https://www.911metallurgist.com/blog/gravity-recoverable-gold-grg-test

Yes, take that  0.002% (20 ppm) gold with a grain of salt. A GRG with give you a true gold content reading too.

Have you considered pre-concentration by froth flotation?

max skinner
2 months ago
max skinner 2 months ago

Agglomeration followed by vat leaching on tailings works very well.     

2 months ago
cyanid188 2 months ago
1 like by David

Hi Adam

The best thing you can do for starters is to concentrate the gold. At 20 grams per ton, it would not be feasible to try and extract the gold as it is. Depending on the composition of the tailings, you could either use flotation or centrifuge and concentrate. Fine gold is very difficult to work with. Centrifuging is a very good option. Higher G's are a must for fine gold. 100 - 150 G's will get you down to 1 micron or thereabouts. You could get anywhere between 100g/t to 2000g/t in upgraded material. Depending on the tails, if majority of the gold exists as free gold then centrifuging and upgrading would not be a problem. A Nelson concentrator works very well for very fine free gold and gold bearing pyrites as well (just to point out the efficiency). If you want to try other proven methods, do small scale tests first. Tainted gold reduces cyanidation efficiency and it is advisable to do small scale cyanidation tests as well as check out the gold surfaces under 40x magnification. What you need is nice shiny particles which will be amenable to cyanidation. 

1 week ago

hello all.
What about the option of using hypo chloride or thiosulphate to leach what ever concentrate he would have come up with.
can any body shed some light on the two suggested processes"

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