November 2014
Independent Assay Labs (IAL) recommend gravimetric gold assaying as the most accurate method to use for defining gold values.
The fundamental difference is that all the metal is quantified in the measuring process (and not just a snapshot of a small percentage of the sample as in instrumental methods).
The method concentrates on accuracy rather than precision, and the absolute accuracy is constant over the entire weighing range. The relative accuracy actually improves with higher values whereas with the instrumental methods it gets worse.
Perhaps not commonly known or understood by non-laboratory personnel is that modern instruments have as their fundamental function the ability to read very small, or trace, amounts of material, not large amounts. This can mean that they may not be best suited to measuring elements at or approaching ore grade. As an example consider the following facts:
Suppose a sample has a gold content of 5 g/T and that a 50g sample is taken for assay. Consider also that the prill from the fire assay is diluted to 10mL after digestion. The solution then has a concentration of:
5 µ/g in 50 grams, or 250µg in total
Therefore, the concentration is (250/10) µg/mL, or 25 µg/mL (25ppm)
In an AAS reading, a reading could take about 5 seconds.
In that time a total of about 0.06mL is introduced into the flame. (The uptake rate is 5mL per minute and only about 10% is aspirated; the rest goes into the drain)
So, of the total of 250µg in the sample, only about 1.5µg is actually quantified!!
Therefore, because the actual reading is such a small snapshot, it is critical that the reading be accurate. Any variation will translate directly into a variation in the result for the sample.
By comparison, in the gravimetric method, all 250µg are weighed. Furthermore, analytical balances are intrinsically accurate, if for no other reason than they can internally calibrate themselves (unlike AAS and ICP which must be referenced to external standards). The readings are accurate to the sixth decimal place of a gram (1 microgram), so the theoretical accuracy in the above example is 0.4%.
Although somewhat protracted, the above example is produced to give any concerned person in the industry a meaningful insight into the differences between the methods. Independent Assay Labs believes there should be a reliable alternative available to verify assays produced by rapid turnaround methods.
The above comments in no way are intended to imply that instrumental methods do not produce correct figures. Quite the contrary, it emphasizes the commendable work done by the laboratories who produce good results using those methods.
The above calculations do not touch on the errors of each method, but the AAS or ICP methods have many variables, starting with the accuracy and stability of calibration standards. Gravimetric assays are referenced to the balance masses which are stored internally and re-calibrate automatically when conditions change.
Explanatory note
We have had an occasion when a primary laboratory has reported much lower values than we did as the umpire laboratory.
Our method includes an inspection of the gold recovered in the fire assay process so makes our result virtually irrefutable.
The possibility of the differences being explained by spotty gold is unlikely in this case because of
- Uniform results from the gravimetric assays, and
- The results were on the (relatively uniform) minus fraction and would take the average above the figures reported by the primary laboratory.
Relevant technical facts
- A high silver to gold ratio is required to effect dissolution of silver from a gold/silver prill. The preferred ratio of silver to gold is 3:1 for the silver to dissolve (when done as part of the gravimetric process this is called inquarting, i.e. making the gold one quarter of the alloy). Ratios of 2:1 can also dissolve but get more unreliable in the totality of the digest.
If the silver ratio is low the silver won’t dissolve completely in nitric acid.
- High silver ratios in gold-silver alloys will inhibit the dissolution of gold by Aqua Regia.
If the gold ratio is low the gold won’t dissolve completely in Aqua Regia.
The coming together of these two facts means that there will be critical levels when neither of these two requirements are met and hence neither the silver nor gold will be dissolved.
Assumptions on the primary assays
Commercial laboratories – as far as is known – universally use an AAS or ICP finish for quantifying gold. It is not known how much silver is added by the primary laboratory but 6mg is common. To achieve this they use a two stage process whereby the silver is dissolved first, leaving the gold which is then digested in Aqua Regia (formed by adding HCl to the nitric acid).
If the gold in the sample was high, and the silver added wasn’t sufficient to dissolve in nitric acid the silver would have remained.
When the HCl was added to produce Aqua Regia the silver that was left probably inhibited the dissolution of the gold by the acid.
Summary
Curiously, the effect of this phenomenon means that very high values could report as very low, and the higher the gold content the greater the risk. For that reason caution is expressed and advice given that geology should be taken into account when getting gold samples checked, not just those that have high values.
Gravimetric (part and weigh) assays will not report low, but would report high because the total of the silver and gold will be weighed if not noticed.
Independent Assay Laboratories use a gravimetric procedure where the silver is dissolved (in a much more controlled manner) and the gold left behind. That gold is weighed on a microbalance after being inspected under a microscope. Because every piece of gold is inspected such errors are virtually impossible.
If the gold plus silver prill weighs higher than it should for the silver we have added we inquart regardless, so the ratio always is correct for the final parting
If you have problems, or don’t understand some weird events, talk to us.
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