Precious metal catalyst sampling: Not a trivial pursuit (ERTC 2023)
Precious metals are likely among the most recycled substances on earth as their value always seems to make it worth someone’s time and effort to retrieve, regardless of the form they may take. For those whose day-to-day activity does not involve precious metals or the catalyst, let us start with a brief introduction.
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Many petroleum and petrochemical processes use catalysts containing precious metals (platinum, palladium, ruthenium, and rhodium, commonly referred to as platinum group metals, or PGMs) or another valuable precious metal, rhenium. Whatever their precious metals composition, all PGM and rhenium-bearing catalysts must eventually be replaced with fresh catalysts (or ‘changed out’) to restore process efficacy or speed up process reactions; typically, this happens every three to five years. The spent catalysts are sent to a precious metals refiner to determine the value contained and return the precious metals (or the monetary equivalent) to the catalyst owner.
That Which Truly Matters
When it comes to determining the value inside your precious metal spent catalyst asset, what truly matters? In this context, it is not the processing fees or the shipping charges; it is obtaining the correct value for your precious metals. That is quite literally where the money is. At the precious metals reclaimer, this breaks down into six fundamental service qualities and, like the game, you must have all wedges to ‘win’ (Figure 1):
The no-compromise essentials:
υ Having the technical knowledge and experience to accomplish the job
ϖ Following all proper regulatory, environmental, and safety rules
ω Adhering to strict ethical standards
The scope of work itself:
ξ The correct weight
ψ A representative sample
ζ An accurate and precise assay.
The PGM value determination is made by processing the material, sampling it, and then assaying that sample for the appropriate metals. Weight times assay of the sample equals the value. Without correct values for all three of these data points, there is no chance of determining the PGM value.
• Weight accuracy is verified on the spot: the customer or their representative witnesses the certified test weights being utilised and checks every scale used for a proper zero at the start
• Great attention has been give n to the precision and accuracy of the precious metals assay, and rightly so, but. . .
• The best lab in the world is of no use if presented with a poor sample. Let us, therefore, talk about sampling.
To underscore the importance of sample accuracy, please note that eventually, at the analytical stage, one gram or less will become the aliquot. The aliquot is the sample portion being used for analysis. It is not unusual for 8 or 10 MT (metric tonnes) of catalyst, worth approximately €500,000 at today’s platinum prices, to be valued based on an aliquot of one gram or less.
Uniformity of the Material to Be Sampled
It is in the best interest of all parties to process the material in question to the highest reasonable level of uniformity before proceeding to sampling. A useful illustration here is a swimming pool full of fruit. Let us picture hundreds of kilos of apples, watermelons, cherries, bananas, and so on filling an Olympic-sized swimming pool, and we need to take an accurate sample. One could mix well, take a long section of PVC pipe, and shove it into this fruit salad to get a core sample, but is this really a representative sample?
This is where we should introduce one of the fundamentals of proper sampling: particle size reduction. Using a melon baller, we could reduce everything to the size of the cherries and the grapes. Now, when we mix and take our core sample, we can see that we have a better chance of accuracy. Best of all, of course, would be to juice the entire thing and separate the seeds, stems, and pits, as we would then be that much closer to molecular uniformity, but that would be very expensive. The point here is that thought must be given to the overall value and difficulty in processing the precious metal-bearing catalyst in order to determine the most appropriate processing method(s).
In some cases, organic contamination has become excessive, and thermal treatment can volatilise the carbon, moisture, and benzene. Screening can remove spacer balls or other support media and trash. The goal is to have a clean parcel of dry and free-flowing catalyst that is both constitutionally uniform (the similarity of each particle) and distributionally uniform (how well-mixed the particles are overall).
Blending has been found to be of dubious value when sampling precious metal-bearing catalysts. Much greater success is achieved using a sampling system that utilises a multiple aliquot philosophy to achieve statistical precision. Some of the crucial aspects of a top-notch sampling system:
υ The consistent and constant flow of material
ϖ The entirety of the lot must go through the system
ω Draw many small samples in equal amounts at regular intervals
ξ The smaller the device, the smaller the particle size.
ψ The entire sampling system must be thoroughly cleaned before and afterwards.
Accuracy AND Precision
The difference between accuracy and precision deserves a quick mention here. Many people use these words interchangeably, but they have different meanings in the sampling and lab context.
High accuracy is getting close to the centre of the bull’s eye every time (see Figure 2). In our lab, this would translate to repeatedly getting the platinum assay close to correct. High precision is getting a very consistent ‘shot group’ on your archery target or a set of assay results that are very close to one another. The high precision/low accuracy laboratory is, in effect, precisely wrong. In both sampling and assay, we clearly need both accuracy and precision.
When creating a sampling system, we can prove accuracy and precision by running the same material (with known content, such as a manufactured standard) through the sampling system three or four dozen times. The 40 or so samples can then go to the lab and see if the results are a) accurate – all very close together, and b) precise – close to the known metal content of the standard material. Achieve both accuracy and precision, and you have a great sampling system.
In the analytical lab, every sample that arrives is run multiple times through multiple assay methods alongside quality control standards and blanks. Consistent, accurate, and precise results must be confirmed before any final numbers are reported commercially.
The critical nature of the sample is clear: once it has been drawn from the customer lot, any error or mis-step is compounded proportionally. Each time the sample is reduced in size, the mistake is multiplied again, which means a larger and larger amount of money is moving in the wrong direction.
To find out more about this topic and others in the world of precious metals, please visit us at www.sabinmetal.com.
This short article originally appeared in the 2023 ERTC Newspaper, which you can VIEW HERE
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