Help with XRF results (99.70% Au 0.30% Ni)

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about.ag

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Jun 28, 2017
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Hi,

A friend of mine had a gold bar tested with XRF (Olympus Xpert) with the results showing 99.70% Au and 0.30% Ni (both +/- .01).

I know very little about the refining process, so the results seem odd to me. With the 99.70% Au, I would have expected to see other elements in there, like copper. Does this seem normal?
 
At that purity range, I have zero faith in XRF. However, a person that really knows what they are doing, like lou, could get more real answers. The bar, if made by a big known company, like Credit Suisse, or a country, like Canada, is probably of the marked purity of 4 Nines.
 
In my opinion 4 9's would be pure beyond the retail store/consumer grade XRF's ability to measure.

XRF only measures the very surface, just a few molecules deep. Plus any surface you measure needs to be extremely clean and shiny. Any salts from your hands, scratches from handling it, impurities from whatever the bar was laid on, slid across, pocket lint and so on will affect the readings.

Throw in calibration issues, quality of the device's manufacture, the expertise, (or lack of), on the part of the user and you might as well just use the acid/stone test to check purity.
 
If he's that concerned, he can come and we can remelt the bar and analyze it by difference with ICP-OES per ASTM B562 - 95(2012), which covers up to 99,995 purity.

An average energy dispersive bench top XRF cannot determine impurities and distinguish readily between 99.5 and 99.95 purity. If you wish to truly know with XRF, you need to have the surface machined perfectly flat, run it on wavelength dispersive, under vac or helium, and wait about 30 minutes while it raster scans the whole of the surface at a powerlevel 100X that of a usual 40W EDX system (or 1000X in the case of a handheld, which are 4 W). Even then, that instrument, very expensive as it is, is subject to certain limitations for certain elements and require different crystal/filters.
 
Lou said:
If he's that concerned, he can come and we can remelt the bar and analyze it by difference with ICP-OES per ASTM B562 - 95(2012), which covers up to 99,995 purity.

I think that's what is going to have to happen.

The issue is one specific refiner, where I have had multiple reports of concerns over purity. I've heard reports of gold marked .999 assaying well below that, bars tested with an Olympus Gold Xpert XRF consistently showing .3% to .8% copper (so around .992-.997), and a bar marked .9999 fine falling outside the expected range on a Sigma Metalytics tester. The one that would be closest to a "smoking gun" would be the assay test, but unfortunately the details leave a lot to be desired (e.g. it's not even completely clear that it was an assay rather than XRF).

So there is enough information to suggest a problem, but still room for improper testing results, inaccurate information, etc.
 
Unless I use thick copper plate behind small parts I get spurious elements popping up when using my Niton. It's a lot more money than that Olympus so I expect more from it but as Lou and Chris have pointed out all these tools have differing limitations.

I might have missed the answer but who stamped it as .9999+?
 
anachronism said:
I might have missed the answer but who stamped it as .9999+?

This may not come as a surprise (either because there actually are issues, or people are concerned there may be): Elemetal.
 
Is it a big bar?

If it's a small bar, it's not enough to even worry about if it's off spec. Might also be the XRF that is off spec.

If you want to know for sure, send me a private message and I can tell you whether or not it is pure. Alternatively, Ledoux, Inspectorate, or Alfred Knight, Alex Stewart, etc. They can all do the analysis too.
 
Ok, never mind. I found your website.


Purity concerns happen more routinely than most refiners would like to admit. Some of the big Tier 1 refineries have oxygen issues with their silver.
Usually the solution is dilution--by that I mean, big batches are much harder to throw off than small batches. Anyone who can make 6N gold on the gram scale is going to have a much harder time of it than 6N on a production scale.

In any event, I am willing to analyze the bar. It is well within our capabilities to analyze gold that is 100 X more pure than 4N.
 
Lou said:
In any event, I am willing to analyze the bar. It is well within our capabilities to analyze gold that is 100 X more pure than 4N.

And that is one of the many, many reasons I like this forum. You won't find any better advise on PM recovery and refining anywhere.
 
Often times with small bars, surface contamination with W, Ni, Co, and certain other alloying agents from the the tool steel that strikes the bar can be seen with XRF. On poured bars, that should not be the case.

While XRF is useful, it is not as useful as GD-MS (which can resolve impurities, layer by layer as it burns into the sample).

This is why some methods are appropriate for certain situations, and some are not.

For instance, if you remelted the bar in a clean graphite crucible, let it cool into a solid piece, then analyzed with XRF, chances are the Ni would go away.

Perhaps one of the easiest nondestructive possibilities would be sending the bar to a color lab to be compared against a pure gold standard. Nickel (palladium too) is a whitening and hardening agent for gold, so perhaps even 0.30% would be noticeable with the right lab.
 
From experience, if the XRF is showing up 0.3% Nickel on that bar, and isn't showing up 0.3% Nickel when used to read other things then the bar has 0.3% Nickel in it.
 
Lou said:
In any event, I am willing to analyze the bar. It is well within our capabilities to analyze gold that is 100 X more pure than 4N.

I wanted to follow up on this.

First, a big thank you to Lou for your help with this.

Although the bar in question was melted before I had a chance to try to buy it, I was able to find another suspicious bar by the same refiner. This one was marked .9999 fine, but failed a Sigma Metalytics (electrical resistivity) test for .9999 fine gold (and outside the expected limits for .999 fine gold), whereas other bars by the refiner did pass. Although I did not get XRF results for the bar, the testing results were similar to the bar that tested 99.70% with XRF: either a sub-par bar, or a case where the testing was not as accurate as desired.

The analysis showed that the bar did contain the required amount of gold. My guess is that there was something unique about the impurities that caused a higher electrical resistivity for this bar.
 
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