Fire assay Why some ores fail. (Parkes process)

[spoke27]

Considering that lead is used as the collector in most of the assays and it is possible to talk about the presence of zinc in almost every ore.. In order to improve the fire analysis, taking the ore into the HCL bath first and getting rid of the zinc first may change the results.

Parkes process - Wikipedia​

Thank you for each reply.​

 

[Rick & Carrie]

Doing so is a pretreatment to an ore before assay, and such steps are taught when taking a course to become an assayer, which I am.

There are several pretreatment options to remove unwanted problematic elements from an ore before a fire assay is done.

 

[shrewdly]

My memory is a bit foggy on this, perhaps Rick & Carrie can help e on this. I seem to remember that in the refining process of lead, the cathodic lead is melted and poured into ingots. As the lead is melted the Zinc sublimates and is drawn off in the air flow and collected in a bag house. If so I would think that in the fire assaying process as the lead is evaporated away any Zinc would sublimate and be drawn off as well.

 

[Rick & Carrie]

My memory is a bit foggy on this, perhaps Rick & Carrie can help e on this. I seem to remember that in the refining process of lead, the cathodic lead is melted and poured into ingots. As the lead is melted the Zinc sublimates and is drawn off in the air flow and collected in a bag house. If so I would think that in the fire assaying process as the lead is evaporated away any Zinc would sublimate and be drawn off as well.

You are correct to an extent.

Yes zinc sublimates, but extra zinc is rabbaled into the molten lead, then skimmed off, thus extracting the silver from the lead, and is the primary reason that is done.

When cupeling a lead/zinc prill, the precious metals will remain in the cupel, but zinc is no fun to cupel.

It does not turn into an oxide that the cupel can absorb, thus you must wait for it to vaporize away, and there is the chance of pm loss to some degree as a result.

Therefore, a pretreatment to remove the zinc content is advisable.

I would.

I was hating life with a high zinc assay as part of my course to become an Assayer.

 

[PeterM]

Doing so is a pretreatment to an ore before assay, and such steps are taught when taking a course to become an assayer, which I am.

There are several pretreatment options to remove unwanted problematic elements from an ore before a fire assay is done.

In many ores that don't contain free gold but are complexed by interfering elements like Chromium, Iron, Silicates, Tellurium and a few other elements. a pre-treatment is almost academic in order to get a good assay. There are many ores that are bound so tight that it may require a chemical treatment, then an acidic roast, followed by an alkaline roast. However, it's a lot of work but upon the right ores most rewarding.

 

[kurtak]

Yes - pyro chemistry (like hydro chemistry) is a VERY DEEP subject

Oxidation - reduction - re-dox --- all things that need to be considered/understood in HIGH temp (pyro) work/processes

Just like hydro processes - research is your friend

Kurt

 

[Rick & Carrie]

Well said.

In many ores that don't contain free gold but are complexed by interfering elements like Chromium, Iron, Silicates, Tellurium and a few other elements. a pre-treatment is almost academic in order to get a good assay. There are many ores that are bound so tight that it may require a chemical treatment, then an acidic roast, followed by an alkaline roast. However, it's a lot of work but upon the right ores most rewarding.

 

[shrewdly]

Thank you for your explanation on Zinc and sublimation. Who says an old dog can't learn new tricks.

 

[Rick & Carrie]

Yes - pyro chemistry (like hydro chemistry) is a VERY DEEP subject

Oxidation - reduction - re-dox --- all things that need to be considered/understood in HIGH temp (pyro) work/processes

Just like hydro processes - research is your friend

Kurt

Also well said.

I studied Fire Refining long before I found this forum and then bought C.W. Amen's book on "Recovery and Refining of Precious Metals" and started learning wet chemistry.

 

[emanuel.dutra]

In many ores that don't contain free gold but are complexed by interfering elements like Chromium, Iron, Silicates, Tellurium and a few other elements. a pre-treatment is almost academic in order to get a good assay. There are many ores that are bound so tight that it may require a chemical treatment, then an acidic roast, followed by an alkaline roast. However, it's a lot of work but upon the right ores most rewarding.

I am working with a tricky ore that has a high iron content (15-30% by weight). When analyzed by laboratories via ICP-OES and ICP-MS, every sample comes back with a Gold content reading between 5 and 20 ppm. When sent to SGS for their standard fire assay analysis, however, we get negligible results for Au. Would you have any suggestions of pre-treatments, or could you point me out to a reference?

 

[orvi]

I am working with a tricky ore that has a high iron content (15-30% by weight). When analyzed by laboratories via ICP-OES and ICP-MS, every sample comes back with a Gold content reading between 5 and 20 ppm. When sent to SGS for their standard fire assay analysis, however, we get negligible results for Au. Would you have any suggestions of pre-treatments, or could you point me out to a reference?

ICP pretreatment most often include acid digestion of the sample, fire assay just usually pulverize and smelt. That tells a lot about nature of the ore - that classical con smelting would probably deliver not very satisfactory results.
Entrapment of gold in matrix of complicated iron oxides ? or other refractory behaviour...

Or from the other end - somehow messed up ICP ?

 

[fishaholic5]

Some ores can contain gold as oxide or hydroxide, sometimes as gold (1) chloride.
In these cases pre leaches with HCl can remove part of the gold content in the pretreatment process.

Cheers Wal

 

[emanuel.dutra]

ICP pretreatment most often include acid digestion of the sample, fire assay just usually pulverize and smelt. That tells a lot about nature of the ore - that classical con smelting would probably deliver not very satisfactory results.
Entrapment of gold in matrix of complicated iron oxides ? or other refractory behaviour...

Or from the other end - somehow messed up ICP ?

I believe ICP results are correct, as three different laboratories yield very similar readings. Fe is present in different types of oxide minerals, hematite and magnetite being the two most important. The issue here is that I need standard fire assays (as carried out by SGS and ALS) to show the presence of Au, as they are the industry standard for the sake of reserve estimation. So I need to figure out a way of pre-treating samples and making the Au content "visible" in standard fire assays. I have done digestions in citric acid which were partially successful, Au finally showed up in SGS FA, but only about 5-10% of the value I get when the same sample is analyzed by ICP in other laboratories.

 

[orvi]

I believe ICP results are correct, as three different laboratories yield very similar readings. Fe is present in different types of oxide minerals, hematite and magnetite being the two most important. The issue here is that I need standard fire assays (as carried out by SGS and ALS) to show the presence of Au, as they are the industry standard for the sake of reserve estimation. So I need to figure out a way of pre-treating samples and making the Au content "visible" in standard fire assays. I have done digestions in citric acid which were partially successful, Au finally showed up in SGS FA, but only about 5-10% of the value I get when the same sample is analyzed by ICP in other laboratories.

This would be difficult, as iron rich ores are notorious for difficult assays. But it is mystery quite a bit... Because when the assayer see iron rich material, he should use the procedures for such stuff using proper fluxing and assuring the full decomposition and liquification of the oxides to the melt. Otherwise it is very probable that using regular fire assay procedure, grains of oxides form just a suspension in a melt and does not liberate gold.
I would investigate how the fire assay was done. If they just do it as one regular procedure, or they change the procedure in accordance with the feed material.

Because - if there´s gold, and you fully decompose the matrix and stuff with proper flux, you will obtain it in the end.

If there is no chance, I would then try to grind the material finest as possible, fluffy flying powder fine. This could maybe help to open up some clusters of ore and enable lead to scavenge the gold

 

[emanuel.dutra]

Thanks Orvi, you hit the nail on the head, the assayer does not change their methodology, unfortunately. We have submitted some finely ground samples (1000 mesh) and are waiting for the results to come back.

 

[omzimm]

I believe ICP results are correct, as three different laboratories yield very similar readings. Fe is present in different types of oxide minerals, hematite and magnetite being the two most important. The issue here is that I need standard fire assays (as carried out by SGS and ALS) to show the presence of Au, as they are the industry standard for the sake of reserve estimation. So I need to figure out a way of pre-treating samples and making the Au content "visible" in standard fire assays. I have done digestions in citric acid which were partially successful, Au finally showed up in SGS FA, but only about 5-10% of the value I get when the same sample is analyzed by ICP in other laboratories.

Some ores will not fire assay without some type of pretreatment, Check out Dr. D.A. Pretorius who was the head of economic geology in South Africa report
The BLM could not assay some ores from Idaho because of the mercury contained in he ore with the gold. As for a solution assay, depends how the metals are alloyed with other metals, pt and group metals, selenides, telluride arsenide's rare earth phosphates' and many more If you dissolve and ore that contains ER, GD, YTB, ND and several more of the rare earth meals plus arsenic and selenium. The ICP will give you a false reading because most of the rare earth have a double charge that will mask the true result Some of these complex ores will respond to both solution and fire assay after a roasting with Potassium nitrate and potassium hydroxide. C.W Ammen has the recipe for this roast. Use extreme caution when roosting and dissolving samples. There are several ores that will not dissolve in aqua regia.

 

[omzimm]

Forgive some of the misspelled words as my keyboard is not working very well on my CP.