Assaying, Microscopy, Mineralogy & XRF/XRD

Assaying, Microscopy, Mineralogy & XRF/XRD 2017-03-23T09:37:54+00:00
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Effective reading area/volume read by hand held XRF (21 replies)

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Can somebody tell me the effective area/volume that is analyses by a hand held XRF? I'm experimenting with a Niton hand held GOLDD+ device reading some fine grained samples and want to get an idea of what proportion of the lot is being analysed with 5 readings

Obersturmbann
1 year ago
Obersturmbann 1 year ago

I'm using the Innovex unit.
On soil mode about 5c coin. Mining modes less than half that.

You need to be happy that the sample is homogenized, that is most important. 5 readings is a bit excessive, I normally go with 1 reading of 1 to 1.5 mins.

Zapping through the paper bag/plastic bag has a negligible effect overall, as long as the bags are the same make/manufacture.

You should realize that the readings you get are not absolute. But they should be close enough to give you a good idea of what is in there.

For example, on pulps, I found Ni = 100% of wet chem values, Cu about 60% and Zn 40%. Au, forget it.
Hope it helps.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

The reason behind the 5 readings is I am concerned about homogeneity as this HM concentrate (sands) sample. I'm hoping five readings are enough to offset the obvious heterogeneity. The question about area and volume is related to this as I can then make an estimate as to how many grains are being analyzed in each reading. I have some fusion XRF and AutoSEM results for these samples so will see how well the device shapes up. If 5 readings don't seem too good I may go back for 10.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

I haven't had experience with handheld / area but as an alternative to the handheld I have seen the "suitcase" version working very well for assessing grades compared to lab analysis of the same sample (only small 100g samples). We found that the correlation between lab and the portable XRF was very good for the situation (within 0.5% absolute grade per sample).

I would recommend doing a trial with paired lab samples and a T-test? (Scary how statistics comes to mind at times). The original trial conducted included 100 samples with grades from 2 to 10%.

Obersturmbann
1 year ago
Obersturmbann 1 year ago

Ah mine sands.
Have you sachet logged your HM concentrates? Viewed under a binocular microscope? That will give you both a grain count for field of view plus an idea on heterogeneity. I reckon your HM will be mixed quite well, so maintain that 1 decent long scan is enough for your purposes.

What you can do is draw a little circle on the HM sachet which is the same size as the field of view of the XRF. Then view this circle under microscope for grain count, geol desc. Then Zap with XRF (recommend at least 1 to 1.5minutes). Rinse and repeat for other areas of the sachet to satisfy your curiosity on the distribution/hetergeneity question.

Just remember that the longer the beam time, the better the repeatability (and accuracy) of the unit.

Ace Levy
1 year ago
Ace Levy 1 year ago

We have experimented with our Niton with a lot of success on some "detailed" field readings of coarse AC cuttings through 150µ bags; with readings evenly spread using a field template. The aggregate of 8-16 readings per bag correlated very well with wet chemistry values, much better than with a single reading. The material read was typically quite high in organic matter and characterized by high grade variability (supergene).

John Koenig
1 year ago
John Koenig 1 year ago

The type of bag actually does matter. And beam time affects precision, not so much accuracy. Accuracy is your moisture, grain size, bag type, etc.

Marshal Dienes
1 year ago
Marshal Dienes 1 year ago

If sample homogeneity may be a problem, it is important to control carefully the way the lab sample was prepared - and if possible have an idea on how it was sub-sampled in the lab.

The small volume analysed by the pXRF instrument may be a problem if shooting only once. It may be your best asset for checking sample homogeneity, because multi-shooting is cheap and will give you a direct measurement of primary sample homogeneity. On routine fine-grained samples such as sediments, I start shooting 3 times on various parts of a sample until I am comfortable with measurements repeatability.

As mentioned earlier, the quality of preparation (and similarly, moisture or bag type) will affect absolute concentrations but will not affect sample ranking as long as all samples are prepared similarly. We published in GEEA an assessment of moisture effect and reduction last year.

Beware of direct comparisons between pXRF and wet chemical methods. When the element of interest happens to be linked with more refractory phases, pXRF may actually be more accurate than digestion-dependent methods such as ICP, provided that the same care was given to sample preparation. The real reference for lab comparisons should be a physical technique like pressed pellet or glass bead lab XRF, or even INAA.

Oberfuhrer
1 year ago
Oberfuhrer 1 year ago

Thank you for sharing your great information and experiences.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

All well the first past testing was interesting with reasonable results for iron and titanium but a systemic bias on zirconium (quite high) and manganese (less so).

A college suggested to me that the there will be biases occurring due to the fact that some minerals in the sands are dull and opaque (more X-ray absorbent and returning less X-rays to the sensor) and others (like zircon) will be more reflective and also refractive and giving more returned energy (perhaps this explains the Zr bias ?).

Also regarding visual sachet logging - my information suggests that this is has not that reliable in terms of quantifying mineralogy proportions - have you any information that suggest otherwise?
The next bit of fun is to try for light elements (Al, Si, Mg)

Sachin Prakash
1 year ago
Sachin Prakash 1 year ago

I am currently experimenting with the Niton XL3t XRF Analyzer on underground platinum mine. We cut underground samples and analyse them in the Lab and then along the same channel where the samples were cut we take five 1minute readings on each sample. The results from the lab are then compared with the Niton analyser results. The shortcoming at the moment is that all the PGEs (Platinum Group Elements) are below the detection limit of these analysers and only Cu, Ni, Cr report consistent results.

Obersturmbann
1 year ago
Obersturmbann 1 year ago

Regarding Grain Counts-

You need a binocular microscope with grids in the field of view. The grid allows a more accurate grain count. You can choose how many of the grid squares you want to use.

I found the visual grain count method to be more reliable than using XRF actually, and the sachets were very homogenous. The advantage to going visual was you can "see through" grain coatings (e.g. clay, rock, FeOx) where they don't completely cover the grains. pXRF will be seeing the coatings, so the info would be not as accurate.
I highly recommend visual counts rather than pXRF for HM sachets, so long as you can recognise the minerals.

Marshal Dienes
1 year ago
Marshal Dienes 1 year ago

Zircon is one of the most refractory minerals to digestion. Was your comparison made with wet analyses? (ICP, AAS)
Other tricky elements include Th, Nb, Ta, Sn and W.

PGE is particularly prone to nugget effects, so it is easy to miss them even with five readings. Did you try shooting the pulps with the pXRF, or the cut samples?

Sachin Prakash
1 year ago
Sachin Prakash 1 year ago

We did shoot the cut samples not the pulps.

Dizzy Flores
1 year ago
Dizzy Flores 1 year ago

I do not know if you are aware of the following publication:

CANADIAN MINING INDUSTRY RESEARCH ORGANIZATION (CAMIRO) EXPLORATION DIVISION CAMIRO PROJECT 10E01, Phase I

Quality Control Assessment of Portable XRF Analysers: Development of Standard Operating Procedures, Performance on Variable Media and Recommended Uses

Gwendy Hall
Angelina Buchar
Graeme Bonham-Carter

In addition the Journal of the Association of Applied Geochemists has recently published an entire issue dedicated to Hand Held XRF instruments.

A believe that given the limitations of the instrument with regard specific elements this is a major advance that we have to embrace with enthusiasm.

Consider aspects such as the calibration mode selection. It is however not to be used by the ignorant and then try to base far reaching financial decisions on the results.

"Garbage in garbage out".

Sampling theory still apply. Think carefully how representative your sample is.
Pulverize and produce a pressed surface that can be analysed in a desktop environment under standardized conditions to get repeatable results.
Maintain a proper quality Assurance/ Quality control protocol, as should be done for any analytical method.

Remember as with standard XRF this is not a method to be used for elements such as Au and the PRE's where pre-concentration may be required.

The same matrix problems that affect normal XRF will apply here and in order to overcome that one may have to select in many instances a laboratory environment.

I will only sign off on analytical results for resource estimation if I am satisfied that best practice procedures have been adhered to irrespective of the chemical analytical method used.

Gruppen
1 year ago
Gruppen 1 year ago

Would say with these sorts of specific questions it is best to contact the supplier directly. They will have undertaken systematic trials with the instrument and are usually very helpful.

Marshal Dienes
1 year ago
Marshal Dienes 1 year ago

The advice and support of manufacturers is useful indeed, but an expert view may be more useful to discover some possible assets and pitfalls.
You should view Nigel Brand's conference at http://is.gd/yVf7az

Unterstarm
1 year ago
Unterstarm 1 year ago

Some papers of the 7th World Conference on Sampling and Blending (WCSB7) will be devoted to PXRF and the associated sampling issue/challenge. It is the opportunity to share experience with the "sampling community".

Have a look on the web site http://is.gd/erqxVB for more information and have a look on the provisional program.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Thanks for your comment about sachet logging.
The data I have suggests that sachet logging and grain counting is not very reliable. Specifically, much like the logger's visual estimate of the percentage of heavy minerals, the sachet logging results tend to have poor precision and a fixed bias when compared by some machine method (AutogeoSEM or QEMSCAN) that analyses several thousand for grains. My opinion is that we humans are not good at visual estimation (individually at least) and also a visual estimates really a 2D area estimate, which is tricky in material that readily segregate and in the end the estimate not corrected for density (to mass percent). As such, I would be interested to know on what basis you think sachet logging gives good results. Have you seen any mining reconciliation data that suggests the method is in any way reliable?

This also leads to the deeper question of estimating the value of a mineral sands deposit given the reasonable precision and accuracy that can be set for heavy mineral percent but the much poorer precision and accuracy that is related to different (and different value) valuable heavy minerals.

Obersturmbann
1 year ago
Obersturmbann 1 year ago

It’s been a while since working at Iluka, but they did get a positive improvement in grade and product after implementing sachet logging. This was prior to the pXRF, so have not seen comparison studies, but I expect pXRF to be biased by the grain size and "air space" between rounded grains.

Visual estimation is just that, an estimate. The more you do, the better you get. Importantly, we would use visual estimates to then "bulk" like mineral zones and that would pass through the process of grade and recovery metallurgical studies.

Visual also gives an estimate of the mineral makeup - valuable and trash. pXRF results would require some sort of algorithm to determine the mineral makeup.

Visual gives grain size and degree of sorting/rounding. pXRF does not. This is useful in determining where you are in the Strand.
Hope it helps.

Dizzy Flores
1 year ago
Dizzy Flores 1 year ago

I will be most interested in the results of a well designed QA/QC exercise (blanks, CRM's and duplicates) on a batch of analyses by PXRF. It will then also be very informative if one can do this following on different sample prep methods and different configurations that the sample is presented for analysis.

I am all in favour of sample prep. After many years of analysing and having samples analysed in routine XRF applications, but for practical reasons I suggest at this stage to leave fluxing and the preparation of glass beats, for a laboratory environment.

However, crushing and milling can be achieved in the field (maintain good practice in getting a representative sample). I have seen some very robust equipment designed for application in the field that are commercially available. Even a method of collecting powder from a cut into the surface of solid rock.

The desktop application where an aliquot the milled sample (in some container) is placed on top of the PXRF gun seems best as it will facilitate a standardized distance between the PXRF's slit and the sample material and therefore standardize the effective reading area.
Feature that should be compared is the usage of plastic bags or specially designed containers.

From first principals we know that a powder method is not going to work well for elements present in major element quantities. So the tool should be more suitable for analyzing elements present in trace element quantities. (I would think above 50 ppm) however the tool has an application in the analyses of major elements at ore grade or in alloys but in such cases one use a different application selection (basically a pre-designed different calibration selection).

So perhaps we can arrange to compare and participate in such a sharing of information effort. I am quite prepared to assist any one in designing an appropriate QA/QC protocol that may form the basis of comparison.
I will respond in a separate comment on the visual estimate aspect.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Well the final outcome of this experiment is that I came to conclusion is that PXRF is not going to work on mineral sands due to the significant matrix effects. I may revisit this another time when I can pulverize some concentrates but unfortunately I need to preserve these test samples for other purposes. Stay tunes for a future update.

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