Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction

Hydrometallurgy: Leaching in Heap, Vat, CIL, CIP, Merrill–Crowe, SX Solvent Extraction 2017-03-23T09:50:58+00:00
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Invisible Gold Identification (31 replies)

Ace Levy
1 year ago
Ace Levy 1 year ago

I’m provided two tests on refractory gold ore (sulfide & carbon). The result was surprising. I don't understand why in minerals processing I cannot find any papers or websites about it. I'm interested in the "invisible gold" as the size of the gold particles to determine the liberation size, to determine the grinding circuit. I understood from papers about 'invisible gold' (by geologists, but not experts of minerals processes), that the properties of the 'invisible gold' are different from normal (especially the melting temperature). But geologists should not determine the leaching, gravitation, flotation, etc. This is done by experts in mineral processes.

Invisible gold is a geologist's term for gold that cannot be seen with a hand lens. It is not un-measurable because its presence is known from fire assay.

This is at 75um grind. I spent the test: pulp passed through a hydrocyclone at different pressures. At one pressure, balance of gold was correct. In another pressure the gold after hydrocyclone doubled. Why? If this test will be minerals process expert he better than I will know the reason. He tells me that the reason, I make the right grinding circuit.
Now recovery of gold from refractory gold ore is low. No single process recovery for these ores. Someone takes roasting or bio-oxidation, someone takes autoclave. We do not know the roasting is useful or harmful to the invisible gold, or bio oxidation helpful? Or maybe better autoclave? Now it is known. Therefore it is necessary many tests for invisible gold. It is necessary to answer these questions. Then there will be one universal and simple flowsheet with high recovery.
For me now it is important what comminution flowsheet necessary for refractory gold ore.

Bill Rico
1 year ago
Bill Rico 1 year ago

So you have done some classification tests and some mass balances- and in some tests the mass balance varied significantly. Lots of reasons for this, as mots metallurgists would know - poor sampling, poor sample preparation, poor assaying,...and of course gold mineralogy...nuggety gold.

But I must admit I do have issues with the idea of 'invisible gold' - why does it not 'appear' in some assay techniques and yet 'appear' in hydrometallurgical or smelting processes that employ the same chemistry?

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

Just a note to set the record straight - "REFRACTORY GOLD" is NOT invisible gold. 

It refers to gold that is encapsulated within another matrix, could be sulphides or other minerals. Fine grinding to less than 10 microns has not proven effective for some 'REFRACTORY GOLD' ores. For this reason, processes have been developed (like POX - pressure oxidation) to expose the gold and subject it to cyanide leaching. This is an expensive method requiring costly pressure oxidation vessels.You mentioned 75 micron particle size - this is still not fine enough to free the gold.

Ace Levy
1 year ago
Ace Levy 1 year ago

I think, refractory gold show low leaching property, invisible gold show the fine size - 500nm and less. IG-gold is synonym withNano gold.

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

We have quite a range of target grinds from our gold customers. The finest grind being 56um and the coarsest being 150um. The greater majority have recovery percentages 96-98%.

It is important to note that finer grinds are now very achievable by blending compatible media grades and sizes to control the smaller ball population in any mill.
We have one customer targeting 18um for silver - is that a common thought for silver mines?

Ace Levy
1 year ago
Ace Levy 1 year ago

I think, the term (' invisible' gold) came from are geologists. For this term may be use short name - ' i-gold'. A follow-up term - 'i-gold leaching'. What is it? Conventional gold leaching process (by cyanide) long time - 12...24 hours. ati-leaching after 1...5 minutes gold recovery achieve 75...85%. For this process d' not have big leaching tanks, mixer with air. DissolveI-gold pass to fine screen for precipitate.

Dizzy Flores
1 year ago
Dizzy Flores 1 year ago

18 microns is not normal for Silver ores. Silver has usually been associated with Lead minerals like Galena - one does not want to fine-grind Galena as it will create "slime-sized" particles that will not be floated and reports to tailings.

I would be interested in finding out why your customer is trying to grind to 18 microns and what minerals contain the silver?

Zander Barcalow
1 year ago
Zander Barcalow 1 year ago

His lab results led him to this for recovery improvement - the question was - is it too costly to target this low grind with that additional recovery benefit? I do not have an assay of the ore type on file. If this is of particular interest I can contact him with the question - the mine is US Silver, from memory the site was in Montana.

Marshal Meru
1 year ago
Marshal Meru 1 year ago

There are two aspects - firstly, the sample preparation and the sampling methods of the separation products (issues can arise here) and secondly the assaying method and sub-sampling (issues can also occur here).But as I noted previously, why does this 'extra, unaccounted for' gold turn up in a less chemically and physically aggressive processing environment and not in the more chemically and physically aggressive assaying?All rather bizarre and does make any sense expect for poor sampling and assaying methods.

So what is missed by these aggressive assaying methods? I have heard some strange explanations about forms, but then in the rather gentle chemistry of practical leaching, it suddenly appears. Sounds like a quantum mechanical problem!
No doubt this is what you, as a comminution expert, did find - but until a thorough, independent audit by someone who is across all aspects is required, both quantum mechanics and the parallel universe explanations are looking better all the time.

Jean Rasczak
1 year ago
Jean Rasczak 1 year ago

Invisible gold is the complex gold associated with pyrites and arsenopyrites.

Sugar Watkins
1 year ago
Sugar Watkins 1 year ago

We tried vapour metallurgical method on refractory gold that did not work with cyanide and it seems works very well. Do you have samples to try? Gold moves together with Ag and Cu as volatile complexes. This is vapour refining, gold removed as volatile gold chloride complex. Usually refractory gold reacts in minutes and removed from feed material. If sample is big enough, we should be able to see gold in the scrubber even if concentration is low. The lowest level we worked was 2.4 g/t.

Ace Levy
1 year ago
Ace Levy 1 year ago

What do you think about my comment of gold leaching during 1...5 minutes? Is it possible?

Marshal Dienes
1 year ago
Marshal Dienes 1 year ago

There are some definitions being forgotten here. Invisible gold is a geologist's term for gold that can be detected by fire assay but never seen with a hand lens or microscope when observing a length of core. Reasons include being very small but liberated particles, as in the need for at least 600 power magnification in some Nevada ores. A second reason is that the gold is found as either very small (<10 micron) inclusions in another mineral or that it is in solid solution with a sulphide like arsenopyrite. Yet another but rarer possible reason is that the gold is not free but in a true mineral, particularly the gold tellurides like Calaverite, AuTe2.

Never mix up refractory gold with the term invisible gold, they are too different paths that only cross sometimes.

At no time have I heard someone say that invisible gold cannot be detected by fire assay. By definition, the only reason that the geo knows it is there is the assay result. That direction enters the pseudo-world of mono-atomic gold and the charlatans that claim they have gold in a deposit but that certified fire assay labs are unable or too incompetent to detect it.

Dizzy Flores
1 year ago
Dizzy Flores 1 year ago

Invisible gold is a geologist's term for gold that cannot be seen with a hand lens. It is not un-measurable because its presence is known from fire assay.

Some deposits just have very small gold grains, in some parts of Nevada a 600 power microscope cannot see it. This does not mean that it cannot be recovered. A moderately porous rock can leach well. Refractory gold is not invisible gold although sometimes they overlap. Refractory gold does not leach which means that it is not exposed at the mineral surfaces. It can be present as fine grains in another mineral or in solid solution in the mineral. By definition it is not chemically bound to the host mineral. On rarer occasions the gold is in a true mineral, often a telluride like Calverite or AuTe2. All of these examples would fit the term invisible gold.

Recovery is not a function of invisibility. Gold in arsenopyrite can be concentrated by floating the arsenopyrite and sending it to a specialized facility.

Be wary of the pseudo-science of "mono-atomic" gold or non-assayable gold, which is also invisible. The story is that "the gold is in my deposit but most fire assay labs do not know how to measure it". "I must go to a special lab in *****" as one prospector put it to me. There are many ways to insert measurable gold in a blank rock and all have been used by these charlatans.

Ace Levy
1 year ago
Ace Levy 1 year ago

After 1...2 years in industry will be apply fast gold leaching based on knowledge invisible gold, them i "like" you comment. Anyone has work with refractory gold ore?

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

At present I'm working on a uranium project but worked on a doubly refractory gold ore, very high carbon content and gold locked in both pyrite and arsenopyrite a couple of years ago.

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

There is a lot of interest in finding methods to increase recovery from refractory gold. Many of the free milling gold deposits have been found and are in production or are past producers. There are a lot of low grade refractory deposits which require economic processing methods.

The term invisible gold can be confusing. The traditional meaning is gold usually found in solid solution inside of pyrite or arsenopyrite. With current SEM mineralogical equipment, micron sized gold can be spotted and its images captured.

As stated in earlier comments, there are some deposits where the owners claim the gold cannot be assayed using traditional fire assay techniques. They claim there is gold and that some proprietary recovery or assay methods are required to liberate the gold. In general these are shown to be frauds.

Using the correct terminology helps everyone to understand the nature of the gold in the project

Zander Barcalow
1 year ago
Zander Barcalow 1 year ago

You applied the term - "refractory gold". It’s a technologist term. "Invisible gold" the term of geologist. The meaning of both terms is one. You know the recovery of refractory gold is low - Around. 80%. What is the name of non-recovered gold- 20%? We (technologist) don't have the term for this gold.

Jean Rasczak
1 year ago
Jean Rasczak 1 year ago

Terminology is the key to solving any of the problem. Well-stated problem is 50% of the success in its solution. 

There are two terms for Au discussed here: 1) invisible and 2) refractory. They are like apples and oranges: one is physical size of the gold grains, another – technological term meaning that it cannot be recovered with conventional grinding-gravity/leaching procedure. Also one need to remember other mineralogical terms that relate to these – forms of Au – mineral form and elemental form.
Invisible gold is gold invisible by human eye with a lens and optical microscope. The size is less than ~0.4 µm due to optical resolution, limited by the physics - light wavelength. Visible gold have other classification terms according to size, recovery methods and association with other minerals and liberation. I’ll leave it out for now.
The invisible gold can be simplified to the following: 1) colloidal, 2) in solid solution, 3) adsorbed on the surface of carbon, clay, sulfide etc. Invisible gold can be extracted well e.g. Au on clay, sulfides or Mn/Fe oxyhydrites can be leached.
Refractory gold can be visible, but hard to get. It can be caused by: I – mineral native gold: 1) fine encapsulated native gold in mineral-carrier (quartz, pyrite, bornite), 2) colloidal native gold in sulfides, II – elemental gold: 3) solid solution in sulfides (arsenopyrite, pyrite, fahlores etc.), 4) surface bound Au on carbon, Mnoxyhydroxides, III – other gold minerals: 5) gold tellurides/selenides/antimonides (petzite, calaverite, fischesserite, aurostibiteect.), 6) gold intermetallic compounds (maldonite).
So, according to presence of gold you can have the following: 1) just refractory ore (needs fine grinding/oxidation), 2) double refractory ore (need oxidation + prevention of preg-robbing). 

Bill Rico
1 year ago
Bill Rico 1 year ago

We get instructions from our own customers about final grind targets, these as we all know come from site laboratories based on final grind P80 to recovery percentage determinations. 

We "tune" the media to do this without overgrinding which is a difficult task as you all will agree but if the site laboratories are not detecting the full targeted element quantity - then the final grind target may be incorrect?
I am asking this question because perhaps I should be asking our R&D team to focus more on finer grinding within ball mills? They tell me that this is very possible, but it would require some more design engineering in our grade/size blending scenarios.At this time we are capable of targeting in the vicinity of 50um - do we need to work on a finer grind to liberate these smaller product particles? OR at least have this in our product range to meet this need?

Marshal Dienes
1 year ago
Marshal Dienes 1 year ago

Some sulphide deposits require that rougher concentrates be reground to the 10-20 micron range, finer than you indicate can be done with your equipment. Such grinding might also be required for a gold leach but it is irrelevant for refractory gold and, as noted above, "invisible" gold particles might be 0.4 microns, many times smaller than required for sulphide flotation.

Ace Levy
1 year ago
Ace Levy 1 year ago

It looks like "invisible gold" is the gold particles less 0.4 micron or 400 nanometres. Is itrecoverable or not? That is a question.

Marshal Meru
1 year ago
Marshal Meru 1 year ago

With a traditional 2 stage milling process....maybe, perhaps, subject to quite a few factors....there is a "possibility" that we could manage 30um? One of those things that is very hard to estimate as this priority trades off a great deal of throughput - however, if you have the head grade and recovery benefits outweigh the time/cost negatives - not a problem.

I am happy if anyone wants to look at this on a production basis, as I would be glad to have our R&D team give this a serious shot.Ultra-fine grinding with ceramic media, maybe the way to go.

Sugar Watkins
1 year ago
Sugar Watkins 1 year ago

Ball mills can deliver 20 micron products and have done for years. The problem is that the efficiency is grim at low sizes and grinding is inefficient to begin with. Today there are new types of mills that routinely reach into the teens. They do not work the same way (rotating drum, falling balls) and are now winning territory in coarser ranges over the conventional ball mill. Look up "Isa mill" if you do not know what that is.

The size of gold particles has no bearing on recovery, rather it is the way that they are found in the rock. If all of the gold is in 0.4 micron thin layers or blobs lining fractures that are exposed by crushing the 0.4 micron gold could be recovered by heap leaching. If 10 micron gold is sealed in silicified rock, a 40 micron grind can only be expected to liberate a small percentage despite grinding and tank leaching.

On a recent rare earth project we were studying a 20 micron grind using a ball mill in the lab. While recovery increased, the predicted extra cost of finer grinding pointed towards a grind in the 40 micron range below which there was no further economic gain. We are paid to make money, not to grind rock.

On the subject of the size of invisible gold, the original discovery at Carlin Nevada had ore and waste that looked identical even under a hand lens yet the ore contained >10 g/t. A 20x hand lens has a resolution of perhaps 100 microns so this could be considered a threshold for invisible gold. Further mineralogy showed that the gold became visible with a magnification of 600 power. Depending on some factors that suggests that the gold was in the <20 micron range. Some was much smaller, yet the recovery by agitated leaching was good.

Bill Rico
1 year ago
Bill Rico 1 year ago

To liberate and recover colloidal gold and gold in solid solution one need to break the structure of mineral - oxidize sulfides for example. There are number of ways to do it: POX, bacterial oxidation, atmospheric oxidation, Albion process, chlorination, etc. The final method depends on individual circumstances: grade, mineralogy, climate, infrastructure, staff and other criteria.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

Isamill's are accepted as the finer grind tools for sure, fully agree.However, if you are working with a F80 of 80mm, a +20WI and have invested heavily in a SAG/ball mill circuit, the last thing you would want to hear a suggestion to buy more equipment. To get a P80+ at 20-30um is by no means an easy task with throughputs in excess of 3,000 m/t hour. 

These are existing plants with no want to spend additional funds, so we have to work with what we have - this is not a circuit design scenario. There are many facilities that struggle to get better than 100um as the final grind. This is one of our own main tasks to improve - and it is going well, without buying new equipment.

Marshal Dienes
1 year ago
Marshal Dienes 1 year ago

Isa-mill can be utilized for micron gold in sulfides. It is not effective for colloidal unless combined with oxidation.

Jean Rasczak
1 year ago
Jean Rasczak 1 year ago

Rare earth is another set of elements (REE), most of them more valuable than gold.

A great need for ultrafine grinding there.

Ace Levy
1 year ago
Ace Levy 1 year ago

In ultrafine grinding is one problem: agglomeration of fine particles.

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

I have to admit that I have never had much experience in ultrafine grinding, and so I do appreciate your comments - these issues are new to me. My work is only in the 300mm - 30um.

Ace Levy
1 year ago
Ace Levy 1 year ago

In the industry of refractory gold ore (RGO) big problem. How to recovering refractory gold? At better PGO plants the recovery is nearly 70 %( feed grade- 2...3 g/t or 0.07...0.1 oz/t). The others 30% gold is unknown. Therefore now it is important question. 

Unfortunately, the investigations in minerals processing and metallurgy at this theme is very few.
If we will be speak, that investigation of "invisible gold" is charlatans, we don't solve the problem of "30%".
Now we have only geologists’ knowledge about "invisible gold":
1. At refractory gold ore the size of gold particle less 1 micron. Many gold have size 10...100 nanometers and even 1...2 nanometers. Geologists called this gold- "invisible gold".
2. This gold particles including in pyrite and arsenopyrite particles 10...1000 microns. In arsenopyrite more.
3. The melting temperature of visible gold is 1063 C. The melting temperature of "invisible gold" is less 1063C (for 1...2 nm particle - 300C!).
4. At heating pyrite matrix to 650C the small invisible gold coagulated to visible gold. Other small invisible gold is fly.
5. At fire assay the small invisible gold is fly.

The technologist's knowledge is less:
1. The leaching time of free "invisible gold" is short- 1...5 minutes.

Many thanks for geologists. And we, technologists, must be investigate the "invisible gold" for increase gold recovery.
We have blinkers. We must remove it.
We must ask at follow questions:
1. What is percentage of size "invisible gold"?
2. How many "invisible gold" is fly at fire assay?
3. Behavior "invisible gold" at minerals processing (grinding, ultrafine grinding, high G gravitation, flotation, magnetic separation etc.).
4. Behavior “invisible gold" at metallurgy (roasting, bioxidation, autoclave oxidation etc.).

The question is new generation on gold industry.

Thanks of all participants of this discussion. I'm sure will be discussion at this theme yet.

Dizzy Flores
1 year ago
Dizzy Flores 1 year ago

This discussion seems to be bogged down in semantics i.e. what is "invisible gold"- a term used by geologists (generally not noted for their mineralogical or process prowess) to describe fine micron sized gold or what the metallurgist consider i.e. gold that fails to be revealed by assaying and yet turns up after conventional gold processing.

Surely if it is fine gold that is recognised (by presumably mineralogical studies) or released by finer grinding, then it is surely not "invisible gold".

Please correct me if I have misunderstood the basis of this discussion.


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