Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning

Pyrometallurgy: Roasting, Smelting, Refining & Electrowinning 2017-03-23T09:55:18+00:00
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Fire Assay (25 replies)

Ace Levy
1 year ago
Ace Levy 1 year ago

When testing for Au concentrations using Lead Collection Fire Assay Methods; what valuable information is lost when only conducting Fire Assays?

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

Fire assay is great for assaying Au, however if you are not also assaying for other metals commonly associated with Au such as Ag or other PGMs you may not realize those values which could be significant.

Oberfuhrer
1 year ago
Oberfuhrer 1 year ago

Fire assays are so outdated because of the fact though that 95 percent are completely cooked off and the colloidal and nano. I have my own procedure to a sure all precious metals are identified x ray spectrophotometry used using Jewelry standards to compare to the Excalibur spectrophotometer manufactured in Kiev Ukraine to Russian jewelry standards after running this 5 times and overall average is made with a PDF spreadsheet this generalizes the collectors evident in the ore is also tells the type of ore being addressed, this is very important in achieving whether or not the ore high grade or low grade turn this over the geochemist.

Victor Bergman
1 year ago
Victor Bergman 1 year ago

Are you saying that fire assaying using lead collection 'misses' the colloidal and nano-sized gold? What does "95% are completely cooked off" mean and in reference to what?

Initially, for everyday assaying of gold bearing samples, I would use an experienced gold assayer. Assuming that you realized that you had 'problems' [i.e. not assaying the total gold value], then assistance from experienced gold geochemists is required.

But from a metallurgical point of view, the fact that one cannot readily 'recover' some of the gold by traditional assaying methods would sound alarm bells since it is likely that this type of gold may not be recoverable in practice.

Certainly important to know about, however this information could be misused to overestimate the potential value of an exploration property.

Maya Rothman
1 year ago
Maya Rothman 1 year ago

Over the years there has been more nonsense talked about gold, the forms it may or may not exist in and its assaying. The first thing to realize about using fire assay is that you only get what you pay for. First and foremost if you are looking for the best and most accurate result you must be prepared to pay a premium for the expertise of an assayer who has sufficient experience to treat every sample on its merits and negate many of the elements that affect the collection of precious metals in lead. Sulphur, copper, tin and tellurium just to mention a few! Secondly, as with any analytical method, the actual method employed determines the outcome and fire assay is no exception. For example, when making determinations for commercial purposes it is mandatory to produce a second prill by crushing the slag and cupel and doing a second fusion in the original crucible. The bottom line is that you cannot argue with a lump of gold, an expert fire assayer with an accurate micro-balance will always produce a better result than alternative methods until the limits of the micro-balance are approached at which time an AAS finish is usually employed. Much of the above also applies to silver which can be determined in conjunction with gold, platinoids require a different approach. One final word of caution regarding fire assay/AAS, do not be fooled into believing you are getting an accurate result because results are quoted to three decimal places, all of the above applies, you only get what you pay for and although it is possible to attain this degree of accuracy with the appropriate degree of precision, it is not within the province of routine fire assays, you only get the result that the method allows you to get.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Totally agree. Thank you for reminding people that multiple digits do not improve accuracy.
The concept of significant figures and detection limits gets 'lost' in today’s world of spreadsheet calculations. The reported value has to be consistent with the methods detection limits.

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

With respect to your education, I still must express how very wrong you are; in fact you are utterly incorrect in what you are stating.

Let’s agree not to spread incorrect information and state things as facts that have never been proven with modern science. Until such time as we can honestly say, with empirical data, what you claim it is not only not fair, but is extremely misleading to state these assumptions as fact.
Fact is that fire assay is the only truly definitive way outside complex methods, to properly assay gold. This is a fact.

I can tell you from my own experience, working with precious metals of many different alloys that there is no significant difference between fire assay, XRF or ICP when assaying for gold. Matter of fact, the XRF technologies are very limited in that they should never be used as a quantitative analysis, the reason for this is that XRF will only test the surface of whatever solution or solid you are fluorescing. If for example, a sample is melted incorrectly, and then poured into a bar to be fluoresced, the heavier metals will sink to the bottom of the bar, this is a old trick some less reputable refineries use to cheat customers. So if you XRF the top of the bar you will find significantly less of the heavier metals (Precious Metals, PGMs, etc) than on the bottom. If drilled top, and bottom, and then XRFed, you will still find a lower percentage of precious metals.

When fire assayed is properly employed, with the correct calibrated scale and cupel, etc, you can define purity of Au down to 99.999. You cannot do that with XRF no matter how much spread sheet averaging you are doing.

Colloidal and Nano particle gold does exist, however it has only been proven that it is created when gold is weathered. But I am curious, if you truly believe a valued quantity of this type of material exists, can you explain to me how you can test for this type of gold, and what your proof might be? I know how to test for colloidal gold, but that is because I use certain processes to test for gold in processing precious metals. I also make colloidal gold, and silver, for different reasons for different customers. But can you explain a definitive method to test for gold of this type? Because if you cannot and if you cannot provide information on how it's done, then you are not basing your argument in fact, but only belief.

It's so very important, especially because of the fact that it is very difficult to obtain proprietary information about processingand assaying precious metals, that we clearly state fact and are able to back this information up by empirical data so that we do not miss-inform others who are seeking information that is so difficult to come by.

If you can find the time to do so, I would learn more about refining precious metals if I were you. It's only a suggestion, but until you have hands on experience using hydrometallurgical, pyrometallurgical and electrochemistry processes you will never have a good solid understanding of what you are talking about. And while I could probably teach you how to accurately fire assay in about 2 weeks, you would still need to increase your skill over a period of time before you became proficient enough to do so accurately to the degree I have stated is possible. Until you have done so, you will never understand how incredibly useful and accurate fire assay actually is.

Sudhirkumar
1 year ago
Sudhirkumar 1 year ago

Having worked as a Fire Assayer in Death Valley during the 80's & a fan of Bugbee, your statements resonate!

Gruppen
1 year ago
Gruppen 1 year ago
1 like by daniel777

In my experience fire assay assays for all gold in a sample. In that respect fire assay has one major draw-back and that is fire assay tells you the total gold value in a sample but cannot differentiate between free gold and refractory gold which may not be extractable (in a CIP plant for instance) and may require a significantly different processing method. This is something that people using Fire assay results must be aware of early in the establishment of resource estimation. Some have failed to see this and sometimes the consequences are not good.

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

I understand what you are stating, very clearly. Sulfides for example require being roasted prior to other processes. But please bear with me and I shall explain why what you say is correct, but also incorrect.

I can use two examples to illustrate a point if you allow me. First, chemistry will state that gold cannot dissolve in HCl alone, this is strictly true, however it's also not true because I have seen it happen and have recreated the conditions in which it does. When oxygen is present and heat is applied the oxygen acts as the oxidizer and in HCl solutions will dissolve Au. Chemistry would tell you that to make Aqua Regia you are supposed to use 3 parts HCl and one part HNO3, however that is not how the solution is applied when digesting gold into solution in this way you don't end up with excess HNO3 when all the metal has been dissolved into solution. What you are suggesting is that because a Fire assay cannot distinguish between "free" or milled gold and refractory (Gold Sulfide).

You are right in that a fire assay cannot give you the amounts of each, but what is fire assays used for specifically? Usually to give a very good idea of what PPM in any given material of Au is. It's not meant to explain everything about the gold.

But I would also disagree that it causes problems. This is why. If you took a correctly worked representative sample, this means you used the correct sample splitting method and equipment; you get a good average composition of the material you are processing. Then you fire assay, regardless of the type of gold you are still accumulating and measuring it.

If you start to process the material and are realizing a dramatic difference between the fire assay and your end results, then you know you are missing gold somewhere. Observing the results and constant QA testing insures that you are on the gold, so to speak. When something goes wrong you know it.

As well, while mining, if constantly fire assaying you will know when the material is getting weak or strong and can follow veins that might not be quartz, it could be in the different strata where you might not test with XRF shooting it at the wall of the rock face.

Because I am a small refinery, I tend to roast any material I am able to without actually incinerating the material, the reason is so that I burn off any organics, or sulfides, or other biological unknowns like the oil from your fingers that gets on material that has been touched. I am always assured of collecting all the available values just simply because I cover my bases. I can do this on a small scale but a larger scale it might simply not be possible.

So you can use material scavengers to collect anything in waste waters below a certain PPM, you can use other methods like activated carbon in an electrolytic process. You can use organic solvent to collect the Au, you can test a small sample of cons, roast it and if it smells bad you know there are sulfides and the material should be processed accordingly.

In modern operations that mostly do use Fire Assay, samples would probably be sent off to a professional lab to be interrogated as well, and this would probably be ICP. There are some many different forms of so many different minerals that this is important. Not only has that but nobody really knows what is in old mining tailings.

So if you are using your best practices, specific types of assaying are used for specific applications, no one is the end all in other words. If you are not properly collecting your samples then you might as well not even assay.

So while you are right, and I can appreciate the intellectual discourse, you are also at the same time wrong in assuming that you would lose any values. It all really depends on the refiner in the end, how well they process the material, how well their process fits the material, how well they are able to fire assay.

Maya Rothman
1 year ago
Maya Rothman 1 year ago

Just to put it a little more simply, fire assay tells you how much gold is present in a sample, not necessarily what processes may be required to recover it. That is the job of the metallurgist not the fire assayer.

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

I tend to beat a concept with words until I feel like I have done a good job attempting to explain what I mean, but yes, that was a much better way to say what I was attempting to explain.

Truth is told I utilize XRF a lot; it's quick and easy and gives a fairly good representation of the composition of most materials. However, if you are only relying on the software to translate the values, instead of learning how to interpret the actual information, you can get a lot of incorrect readings, or reading that are right on the border between one element or another. I am a fan of ICP, and send off samples that I am skeptical about to an assayer who uses ICP, I have been very happy with the results as it's able to help me create the correct process for difficult materials.

But I use fire assay a lot, especially with certain types of materials. I am fortunate in that much of the material I process, I know its manufactured values so I know more or less what to expect with very few if any surprises. Regardless, I think the point in all this is that there are many ways to look at any one problem, and there is no one way to assay or process that can cover all options. I will tell you this however, if I were representing gold and taking it to a refiner to process, I would insist on a fire assay, this is really its intended purpose.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Interesting stuff!
As a metallurgist, who wants to know the different mineral associations, I do use different assay methods as a guide to imply mineralogy. So Fire assay for total, then the different dissolution methods to solubilise the metal prior to AA or ICP (or Fire assay on solution).

This can be taken to a method called Diagnostic Leaching, where a sample is attacked by progressively stronger 'acids' with a cyanide leach after each attack. Essentially a series of liberation by 'acid' dissolution, followed by cyanide recovery of liberated metal.

There does seem to be a trend to use Multi-acid dissolution with ICP, in place of Fire assay. My sense is this is driven by assay cost and detection limit. We are all looking for lower and lower grade deposits.

Standartenfurer
1 year ago
Standartenfurer 1 year ago

A couple of points that I will make! It is unlikely that gold will be present in any form other the elemental. Do not confuse particle size with chemistry. To my knowledge the only time gold may ever appear in solution (other than process) is if it is a telluride or amalgam with Hg. You only need to explore this possibility is if your ICP analysis identifies Te or you know Hg is present. In that case you will have to identify and confirm with electron microscopy. Just because Hg is present it does not mean it is an amalgam with Au.

Fire assaying is the only sure way of correctly collecting and quantifying Au, and some of the Pt group metals. ICP, AA and XRF are great tools but have their specific strengths. ICP & AA for base and trace metals and XRF for refractory (SiO, Cr, Mg, Ma, etc) Hydride for Hg and possibly As, Bi, Sb, etc if ppb quantities are required.

Silver is best done by 4 acid digest and ICP or AA finish if below 500 gmt. Corrected fire assay above this value. XRF and ICP are great tools for looking at impurities in Dore but the question here was stated as concentrate.

Rahil Khan
1 year ago
Rahil Khan 1 year ago

I concur with the comments about the value of Fire Assaying techniques for Au analysis. And if it means Au transactions, not a single microgram is traded without a fire assay analysis. It is the only technique of transaction. The instrumental techniques have "low" precision levels in this regard. When one applies instrumental precision at +- 1% and even better based on replications, it does not satisfy the dollar value in that precision. Instrumental techniques have their value, but it must be assessed in the proper context, in the proper matrix and in sample origin. This was a valuable discussion. And it surprises me that after so many years it is still debated. Take heart in the fact that on the early voyages of exploration, a Fire Assayer was on board the Long Ships.

Zander Barcalow
1 year ago
Zander Barcalow 1 year ago

Fire Assay is the Industry Standard for determination of total gold. For exploration purposes, it would be difficult to use anything else and have people believe the results. There is a whole plethora of information that is often determined by other analytical methods done in addition to Fire Assay. It includes geochemistry, metallurgy, permitting (ABA accounting, pH neutral metal leaching to name a few). It all depends on what type of deposit you have, and what stage it is at in development.

Obersturmbann
1 year ago
Obersturmbann 1 year ago

There are inappropriate applications of fire assay which lead to low bias or total loss of contained precious metals. Commercial fire assayers, for cost reasons, typically apply mass production protocols to fire assay which can generate inappropriate values in specialty matrixes. Perhaps, at high concentrations and common alloy matrixes, it may serve all his needs. If he is only risking his own money, this is good enough. However, for the general public, “experts” that declare fire assay as outdated and then recommend “my own procedure”, is often the first warning of large future financial losses. Listen to the points made earlier.

I hope there is no confusion about the point that fire assay is for quantifying total precious metals, not speciation. There are identified weaknesses in the typical mass-production application of fire assay; these weaknesses can be addressed via appropriate adaption of well-established fire assay theory. As other contributors indicated, there are a host of additional methods applied for determining appropriate cost-effective engineering for precious metal recovery. A metallurgist or process engineer may also choose from a host of non-fire methods because one may be more cost-appropriate for managing a specific precious metals operation or process. However, appropriate application of fire assay, inter-laboratory checks (“round robins”) and other assay quality review methods remain critical to assuring that money is not spent on recovery of “fantasy” precious metal.

As noted by other contributors cost considerations may lead to using non-fire methods:

I use XRF routinely to obtain information that is not affordable by other methods, including fire assay. XRF is cheap and can be quick. However, users often don’t recognize sampling limitations, such as those mentioned. In addition, XRF is not one-size-fits-all. It can and does deliver false precious metal outputs. Vendors may not be much help UNLESS you have the background to demonstrate that instrumental error has occurred.

Multi-assayed dissolution, followed by ICP-OES, if applied correctly with appropriate dissolution techniques, also delivers much more data for the dollar. However, like any other method, be aware of the potential for false values in specific circumstances.

Be wary of promises of low cost “improved” detection by other methods, such as ICP-MS. I have tested commercial labs that buy instruments, use them as a mass volume “magic box” and then deliver brutally precise wrong values.

Failures to apply science, appropriately skeptical cross-method quality assurance and leaping on alternative method promises have cost many people large fortunes. Fire assay is in general the most reliable method for total precious metals, but like any method, specialized adoptions and skill is necessary for best results.

Be an informed client. Poor sampling and sample prep generates more poor outcomes than any specific matrix factor. Know how your mineralogy impacts an assay – a normal-skilled gold fire assayer may not have the appropriate skill set or protocols in place for rare metals. Precious metals in tellurides and other less common minerals may, indeed, associate with “lost” value by routine fire assay protocols. Silver assays in many matrixes may do better by dissolution–instrumental methods than by fire assay. Avoid facilities that use “special” methods that find the precious metal in ways no-one else can reproduce. I’ve enjoyed the times I was assigned to test and reproduce some of these methods, but, in those cases where small investors never tested until the money was gone, it is tragic to see good people discover that their entire savings will never be recovered because they believed someone who touted a “special” method that recovered precious metal that fire assay “missed”. 

Helena Russell
1 year ago
Helena Russell 1 year ago

I agree with and will echo with you. Fire assay is the industry standard for total contained gold. Whether that gold is recoverable is another story. Additional methodology, such as ICP, AA, and XRF can shed light on some of the rest of the story. I also very much appreciate your statement: “However, for the general public, “experts” that declare fire assay as outdated and then recommend “my own procedure”, is often the first warning of large future financial losses.” Agreed! Any time anyone doubts the results of fire assay results from a reputable laboratory and says “l have my own procedure for assaying gold,” that should raise a big red flag.

Jean Rasczak
1 year ago
Jean Rasczak 1 year ago

I use the fire assay always the end result of the gold content, and XRF is an approximation this tool is only a guide to buying or selling gold, but the end result is taken as you say cupellation. I must say that modern techniques can lead us to know that the presence of other metals can be found in my bullion in order to establish a more precise when refining process, is not the same refining ingot with high concentration of iron. Then it is very important to know that other metals have associated with my gold, but this will happen if other processes to gold. If it is only to sell it is not very significant to know about the other metals. Always fire assay is the method of universal testing and more reliable, to know the law really gold.

Raje Singh
1 year ago
Raje Singh 1 year ago

This is in reference to the assay of gold in the samples by fire assay. So many thoughts have been emerged during discussions. However, I would like to share my view in this regard on the following points:
The fire assay is an art and requires high skilled persons to carry out the analysis of gold along with other metals especially lead, silver, copper.
A proper recipe is required for the complete collection of gold and silver in the lead matrix. The slag requires further processing for complete gold recovery from slag. I think two or more treatment of slag is required for further collection of gold in the lead button.
Cupellation poses another problem. A proper correction is needed for the correctness of results.
Sampling of the heterogeneous material for preparing a representative sample for the analysis of precious metals is another problem which may affect the accuracy of gold and other precious metals. This area has to be studied and well established procedure is to be adopted.
A standard addition of gold equivalent to the amount of gold present in the sample is added and processes the analysis if you wish to continue the method by Fire Assay. The recovery may be calculated from the replicate analysis and see the repeatability and reliability of the method performed at your laboratory.
It is suggested to determine gold and other precious metals by ICP-OES after dissolution of precious metals completely and carrying out simultaneously with a known addition of gold and determine the recovery of gold by the method. You have to ensure that the method is followed as that of the sample. Results obtained by ICP-OES with Fire Assay may be compared.
Another approach is to treat the solution with suitable Ion -exchange resin or suitable IBC MRT ligands for the extraction of gold from the solution and complete the analysis by FAAS or ICP-OES and compare the results with Fire Assay. Evaluate your studies and adopt the method if it suits you.

Oberfuhrer
1 year ago
Oberfuhrer 1 year ago

You can all believe what you want to believe but I as a Chemist believe it to be only a qualitative TOOL for identification purposes only. I as a researcher work with that you cannot so less than a human hair and it is this particle state that gives all assays so much confusion when trying to achieve accuracy there your fire assay methods are out of date when it comes to working with the gold you cannot dee which has a great deal of interference with the extraction of METALLIC GOLD all over the world. I keep my research up and will in fact prove you wrong with the book I am about to publish Buy one you may just like the book.

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

You are a chemist, your background is in chemistry, not metallurgy, not smelting, not processing metals.

I had a chemist argue with me recently on two points. First that Aqua Regis is made by exact measurements of Hydrochloric and Nitric acids. If you are strictly going by the chemist view, this would be true. However when dissolving metals in Aqua Regis you only add as much Nitric acid as is required to dissolve the metals, this is done by small additions so that there is little or no leftover Nitric acid to remove after the metals have been dissolved.

Next was if hydrochloric acid alone could dissolve gold, I would like you to answer this simple question, as a chemist. It is a very simple answer, and if you are worth your salt as a chemist you should be able to answer no problem. You are wrong in how you express your argument. Until you accept the fact you are wrong, you will never accept facts, or the experience of others who are actually doing the work.

It's a wonderful thing that you have your degree in chemistry, but be careful not to allow your knowledge and degree to mislead you into believing things that simply are not strictly true.

Raje Singh
1 year ago
Raje Singh 1 year ago

I want to know what information is lost while carrying out the analysis of gold by Fire Assay. From the affiliation he seems to be a geologist and practising the analysis of gold by Fire assay. Lot of corrections is necessary while carrying out the analysis by Fire Assay in the determination of gold and other precious metals. I think you will agree with me that the fire assay is an art and not a science. It requires more practical knowledge in smelting, and other metallurgical processing. Starting from sample preparation to obtain a representative sample for analysis, characteristics of the sample to last stage of slag treatment to obtain the occluded precious metals in the slag require systematic approach is needed before coming to the conclusion.

Carl Jenkins
1 year ago
Carl Jenkins 1 year ago

I agree with you that fire assay is an art. Matter of fact, pyrometallurgy is an art unto itself, a different, more ancient form of chemistry to be sure. A pure inorganic chemist who has not learned how to properly fire assay really cannot comment on the accuracy of assaying in this way. However, I know more than a few chemists who also recover and refine precious metals who swear by it.

I don't usually call how well a person is able to do the work of a refiner as art, but rather craft work. It sometimes comes down to things such as lab glass and keeping it clean, or being able to understand the chemistry to such a degree.

Sachin Prakash
1 year ago
Sachin Prakash 1 year ago

If you are looking for gold content, fire assay is the best. We have been doing it since more than 20 years on different grades of gold and have been fairly close to the most accurate analysis. This is reflected in results of national / international PTP program in which we have participated so far.

The problem with fire assay is its limitation to detect insoluble PGMs like Ir and Ru in the representative lot as if presence in small quantity. Our input in refinery is either jewelry scrap or Dore. So much knowledge from so many experts in one place, it is really fascinating and overwhelming to learn so much in place.

Hauptsturm
1 year ago
Hauptsturm 1 year ago

Concentration and temperature are two fundamental kinetic drivers in any chemical reaction. Could it be that as you increase cyanide concentration you overcome the activation energy required for chemical reactions in preference to gold? Assay your solutions for other metals you suspect may be dissolving, look at your kinetic profile to get more idea.

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