Pin Sample Problems

[Montecristo]

I was wondering if any of you might have some ideas why this is happening and what we might be doing wrong?


All of a sudden we are having problems with our pin samples. I don't think we've changed anything from the hundreds of times we've done this before. I'll describe our process hoping it will provide some clues.

We use a large Electromelt furnace, generally melting between 2500 and 2800 grams of gold scrap at a time.

We let the furnace heat up to about 1000 degrees Celsius and then start putting in the scrap. We set the maximum temperature to about 1010 degrees Celsius. We keep adding until it is all in and melted, stirring occasionally with a carbon rod. We then add about a tablespoon of borax. Stir again and clean off any slag from the top of the crucible.

At this point we have tried two methods of taking a pin sample with a vacuume tube.

1. For a year or so the process was we would stir the gold in the crucible and insert the vacuum tube and draw out our sample all pretty much simultaneously. This started to yield bad samples a few weeks ago.

2. When the previous method started yielding bad results we changed the above. We stirred the gold and then let the gold "cook" for about 5 minutes before inserting the tube. This is also yielding a bad sample.

After withdrawing the pin sample we then roll it down to about .6 mm and then assay with an x-ray fluorescence machine.

When I say we are getting bad samples I mean specifically the pin sample is reading lower than the actual bar. This is happening about 50 % of the time, the other 50% the samples are fine.

The problem is not with the x-ray machine I regularly check the readings against know fire assayed standards that I made.

Does anyone have any ideas what is going on? I would be happy to describe any other details of our process if you need more information before giving any suggestions.

About 6 weeks ago we started adding a very small amount of soda ash to the borax at the recommendation of the guy at the store where we buy our borax and vacuume tubes. Could this be the problem?

 

[Montecristo]

Bueller? Bueller?

Did I post this in the wrong forum or is this just an inexplicable phenomenon?

 

[goldsilverpro]

Maybe no one knows how to answer your question. Pin sampling is fairly straight forward. You just stir well and then sample. I assume you're plunging the glass tube deep enough so it picks up no slag? Are the pins solid or are there gaps in them? I can't see how the soda ash would cause problems, but I see no need for it.

 

[Fournines]

Couple of questions:

1) How deep are you dipping the vacuum tube into the molten gold?

2) Are you lowering the temperature of the melt before you take the sample?

3) How do know the the sample is reading lower than the bar? Are you x-raying the bar as well? If so, x-ray several parts of both sides. As the bar cools, the denser gold settles to the bottom, so you will have a different reading from the top and bottom of the bar.

4) Why are you cleaning the slag off the melt before taking the sample and pouring the bar?

5) How much lower is the pin reading from the bar?

6) How many samples do you pull from each melt?

7) Not to question your methods or knowledge, but how you do you know that your x-ray calibration standards are correct? You said you made them, but are you positive that they are not the problem?

8) How long do you run the x-ray test? 30 seconds? 90 seconds?

I know...a lot of questions, but if you can lend some further details, perhaps we can figure out where the problem lies.

 

[Montecristo]

I know...a lot of questions, but if you can lend some further details, perhaps we can figure out where the problem lies.

Thank You for taking the time to write out all the questions. I'll do my best to answer them.

1) How deep are you dipping the vacuum tube into the molten gold?

I put the glass tube in about half way to three quarters of the way into the cylinder.

2) Are you lowering the temperature of the melt before you take the sample?

No, we take the sample melt at about 1010 Celsius

3) How do know the the sample is reading lower than the bar? Are you x-raying the bar as well? If so, x-ray several parts of both sides. As the bar cools, the denser gold settles to the bottom, so you will have a different reading from the top and bottom of the bar.

I've determined the sample is reading lower from a couple sources.
1.) I am x-raying the bar in 6 places when done, 3 on the top, 3 on the bottom. The bar is reading higher.
2.) When I determine from the pin sample what the bar should yield in ozs. of gold, the refinery is returning more. The results from the refinery more closely match the results of the bar tests.
3.) I have sent part of my pin sample in for a fire assay and compared it to the assay from the refinery and they don't match.

4) Why are you cleaning the slag off the melt before taking the sample and pouring the bar?

I thought this would give me a better pour if I remove the slag first.

5) How much lower is the pin reading from the bar?

Usually several percent. When the pin sample is correct I am usually within 1% of the refinery.

6) How many samples do you pull from each melt?

I only pull one sample.

7) Not to question your methods or knowledge, but how you do you know that your x-ray calibration standards are correct? You said you made them, but are you positive that they are not the problem?

Please do question me. I have no illusions that I know what I'm doing. I'm grateful for any information people are willing to share no matter how trivial they may think it is.

I believe my sample are accurate because I made the samples then cut them into 3 parts. One part I kept, the second part went for an independent fire assay and the 3rd part went to the refiner for assay with their x-ray fluorescence machine (which is the same as the one I use). I then calibrated the machine using the results from the assayer and the refiner.

8) How long do you run the x-ray test? 30 seconds? 90 seconds?

After we roll out the pin sample I take 7 tests, of 30 seconds each and then use the average for my expected results.

 

[Oz]

While I have not performed the procedure being discussed I noticed that you are rolling out your pin samples, then testing. Is there the possibility you are contaminating the samples from exposure to your rollers or is this a dedicated machine for only fine gold? This would only be surface contamination of course, but you are testing the surface with x-ray fluorescence.

 

[Montecristo]

Maybe no one knows how to answer your question. Pin sampling is fairly straight forward. You just stir well and then sample. I assume you're plunging the glass tube deep enough so it picks up no slag? Are the pins solid or are there gaps in them? I can't see how the soda ash would cause problems, but I see no need for it.

I agree the process should be pretty straight forward, and I really hesitated to make my first comment on here such an elementary question, but it is driving us crazy that we are not getting consistent results. So I decided to bite the bullet, and no matter how ridiculous it makes me seem, I had to ask the question in hopes of figuring this out.

 

[Montecristo]

While I have not performed the procedure being discussed I noticed that you are rolling out your pin samples, then testing. Is there the possibility you are contaminating the samples from exposure to your rollers or is this a dedicated machine for only fine gold? This would only be surface contamination of course, but you are testing the surface with x-ray fluorescence.

Hmmm..... the roller is not a dedicated roller, we use it daily for various tasks in our shop. But, the pin samples from the melt aren't pure gold, they vary with whatever the scrap is that we melt down, Usually between 10 and 16 karat.

The reason that we roll out the pin sample is that I was told that rolling it out to approximately 3/4 mm would yield a more consistent assay from the x-ray machine. I have never tested to see if this is true.

 

[Montecristo]

Maybe no one knows how to answer your question. Pin sampling is fairly straight forward. You just stir well and then sample. I assume you're plunging the glass tube deep enough so it picks up no slag? Are the pins solid or are there gaps in them? I can't see how the soda ash would cause problems, but I see no need for it.

We will try the next melt with just plain borax.

I was told by the someone, who just so happened to be a salesman from where we buy our borax that soda ash would "give us a better pour by lowering the melting temperature". I didn't really understand what he was talking about, but I decided to try it anyhow.


The pin samples are coming out nice and solid all the way up the tube.

 

[Palladium]

I believe my sample are accurate because I made the samples then cut them into 3 parts. One part I kept, the second part went for an independent fire assay and the 3rd part went to the refiner for assay with their x-ray fluorescence machine (which is the same as the one I use). I then calibrated the machine using the results from the assayer and the refiner.

To convert processed spectral data into quantitative
determinations requires mathematical models that relate
sample X-ray countrates to thickness and composition.
These models are called calibrations. An XRF tool can be
calibrated using a variety of empirical and fundamental
parameters techniques.

Empirical Calibration

By measuring the X-ray count rates from a coating/base
combination of known thickness and/or composition (standard),
we obtain the reference data needed to calculate the
thickness and/or composition of unknown quantities of the
same coating and base combinations. In practice, a series
of standards with different values are measured to obtain the
necessary reference countrates. Statistical methods are used
to correlate the standard values to the measured XRF count
rates.

Accurate calibration of the XRF tool is dependent on the
availability of high-quality standards for the specific application
under analysis.

Fundamental Parameters Calibration

The major practical drawbacks to empirical calibration
methods are:

• The time required to measure a series of calibration
standards
• The difficulty of fabricating standard sets for intricate
applications.

Highly accurate mathematical techniques based on physical
properties have been developed for a wide range of applications,
enabling the XRF tool to be calibrated without (or
with a reduced number of) standards.

In a fundamental parameters (FP) calibration, an extensive
range of instrument parameters are first quantified. This
information is incorporated into a mathematical model of
the fundamental physics of X-ray interaction with the specific
materials under analysis.

The mathematical model is often enhanced through the
use of physical standards (the Standard-FP method). The
improvement in test process throughput enabled by reducing
or eliminating the need for physical standards can be substantial.
FP techniques offer versatility and often enhanced capability.
For certain applications, however, empirical calibration
offers a more accurate, simple, flexible, and reliable solution.
An XRF tool should offer both calibration strategies.

:arrow:

 

[Platdigger]

You might try polishing or buffing somehow, one flat side of the pin after you roll it out.
I know when they test for different stainless alloys at the scrap yard, they grind a spot first to expose some fresh metal, before using the XRF.

 

[4metals]

I assume that the melter you are using, the electromelt, is one of those units which is self contained, you pick it up and pour the molten metal out? I personally have never had any luck with those melters, sure they melt the metal but they never lasted for long with any of my clients. If anything is suspect here it is the temperature of the melt throughout the melter.

As GSP pointed out, dip samples are straight forward, dip it in and either a pin is pulled into the tube or not. I think your problem is the melter, you're not getting hot enough for the sample to be mixed properly and the results are inconsistency.

I would try this: do a melt in your electromelt, take a dip sample, pour a bar. Now read the dip sample, some drillings from the top of the bar, some drillings from the bottom of the bar. If the melt was hot enough for good mixing they should all be very close.

If there is a difference, take the bar to your refiner and re-melt it, take a dip and drillings and see if the results are any different.

My opinion, upgrade the melter, you're not getting hot enough.

 

[Montecristo]

Palladium,

Thank You for the info on x-ray fluorescence. I printed it out and will read it this weekend.

 

[Montecristo]

I assume that the melter you are using, the electromelt, is one of those units which is self contained, you pick it up and pour the molten metal out? I personally have never had any luck with those melters, sure they melt the metal but they never lasted for long with any of my clients. If anything is suspect here it is the temperature of the melt throughout the melter.

As GSP pointed out, dip samples are straight forward, dip it in and either a pin is pulled into the tube or not. I think your problem is the melter, you're not getting hot enough for the sample to be mixed properly and the results are inconsistency.

I would try this: do a melt in your electromelt, take a dip sample, pour a bar. Now read the dip sample, some drillings from the top of the bar, some drillings from the bottom of the bar. If the melt was hot enough for good mixing they should all be very close.

If there is a difference, take the bar to your refiner and re-melt it, take a dip and drillings and see if the results are any different.

My opinion, upgrade the melter, you're not getting hot enough.

Thanks for the suggestion!

This is a definite possibility. It hadn't occurred to me until you mentioned it, but there has been some inconsistency with the melting times lately. Sometimes it will heat up and do a melt as it normally would, while other times it has been noticeably slower. It may be having some issues with the thermostat. We have used this excessively over the past year it may be ready to die on us.

 

[goldsilverpro]

If your volume warrants it, I would certainly look in to a small induction furnace.

 

[Montecristo]

If your volume warrants it, I would certainly look in to a small induction furnace.

I have thought about it. I do feel it would be something very useful and worth the cash outlay, but we are really pressed for space around here and I'm hesitant to move into a larger space based on the gold business alone.

 

[qst42know]

I don't know what you had in mind as a space issue. Fournines has a photo of an induction furnace as an avitar.

Fournines what does your furnace measure?

 

[Palladium]

How much are you trying to melt at once ? They make a small counter top model.

 

[4metals]

An induction melter like in fournines avitar is not for everyone. It is a fixed crucible furnace. That means when the crucible needs replacement, the crucible has to be cut out of the box, repacked with a new "spider" ground fault detector and a new crucible and topped with the refractory cement before it can be used again. Oh, and you have to let the refractory cap dry too. This process takes time and labor, often time when you are not melting with that unit. Fixed crucible units are usually used by refiners with multiple melt capacities so the down time is not an issue.

A small furnace with a # 6 crucible should suffice. That size should handle up to 300 ounces of karat jewelery and while it's bigger and more costly than an electromelt, it can melt quickly and quietly. Plus changing crucibles takes no time at all. It will also be relatively efficient with a # 2 or #4 crucible.

Another option is a small gas melter, they are cheap, but loud. One of the good things about an electromelt is it works on a table top next to an open window! No hoods or exhaust.

 

[Fournines]

True. The induction melter pictured in my avatar can melt about 1000 oz at a time. It's one of several that we use. Cost is also a big issue. The power transformer and water cooling setups for those bad boys cost over $50k brand new I believe. Not quite practical if you only need to do a hundred ounces at a time.

I would go with whatever 4metals recommends.