Assaying, Microscopy, Mineralogy & XRF/XRD

Assaying, Microscopy, Mineralogy & XRF/XRD 2017-04-04T06:57:57+00:00
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XRF to Reduce Sampling Uncertainties (20 replies)

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

Field measurements performed on soil, rock, ore, waste etc using instruments such as portable XRF may not always be as accurate as laboratory analyses, but they can be performed in large numbers with little cost. They may therefore help to select the right samples by providing a good estimate of variability.

I would be very happy to discuss your own experiences, good or bad, or your interrogations about this. 

Bob Mathias
1 year ago
Bob Mathias 1 year ago

Wouldn’t it be nice to develop courses where you can learn the best coring techniques combined with this first portable analysis of the cores and learning to adapt your coring strategy in situ?

Sturmbann
1 year ago
Sturmbann 1 year ago

Surely the sample is the variable and not the instrument - laboratory or portable?

Bill Rico
1 year ago
Bill Rico 1 year ago

If you include sampling error in your total uncertainties, then you might be able to change the paradigm. Sampling errors contribute up to 80 or 95 % of uncertainties compared to the analytical ones according to many authors in ToS. If the pXRF allow you to reduce the sampling uncertainties by a fair amount, maybe that will more than compensate for the increased uncertainties associated with the pXRF. Ramsey & Boon published an interesting paper on this topic. The instrument itself is pretty good; the problem is field heterogeneities (individual minerals) which sample pulverisation might solve.

Ramsey M.H. and Boon K.A. (2012) Can in situ geochemical measurements be more fit-for-purpose than those made ex situ? Applied Geochemistry, 27, 969-976.

David Kano
1 year ago
David Kano 1 year ago

I carried out a drill program looking at IOCG mineralization in South Australia. We used a FPXRF for our initial work. We were trying to use the FPXRF as a discriminatory tool to select samples based on Cu grade. I also had a 2 CRM's that I used (along with Niton Standards) the Cu standards of 0.23 % and 0.54% Cu returned values of approximately 90% of expected values i.e. the 2300 ppm returned values around 2100-2250. This gave me the confidence that the machine was working correctly. Blanks were reporting from the FPXRF at below DL so they worked fine. This allowed for us to relegate all the samples from the drilling to the waste pile. Very disappointing. However I think if you use a good QAQC policy with the FPXRF and also look at submitting a percentage of samples to the Lab to build up a correlation between the FPXRF and Lab results you can then extrapolate to submitting say 10% of the samples taken to a Lab with 100% having been FPXRF'd this will continue to build the correlation and also potentially save significant money in assay costs.

Dizzy Flores
1 year ago
Dizzy Flores 1 year ago

I agree with your opening statement as long as you would provide more details on:

XRF may not always be as accurate'. Provide the accepted error

Linked to the above provide the minimum requirements for operator's expertise hence maximum tolerance qualitatively speaking.

Provide a study that measures in a scientific manner that 1 and 2 could be used as methodology basis.

Bob Mathias
1 year ago
Bob Mathias 1 year ago

Another solution is hiring a containerised XRF Line Core Scanner. What price per month would make the extra, fast and in-situ information valuable in your case?

Hauptsturm
1 year ago
Hauptsturm 1 year ago

From my experience you can have good results with portable XRF with the following conditions: the portable XRF is used more as a guide than as a final assay result, it is efficient if you´re analyzing a pulp or a more desegregated sample (soil - horizon B), and you should always include QC samples (standards).

This way, I'm able to identify preferable set of samples to be analyzed at a commercial lab, instead of spending money by sending an entire soil campaign to be analyzed, for example. The comparison between the portable XRF with the lab results is an excellent orientate study, and it helps you to identify the main elements of interest that your equipment is better on assaying. The standard calibration curves are usually pretty good, but you also have the option of building your own curve for a specific material, if you have enough standard matrixes to do it properly.

Marshal Dienes
1 year ago
Marshal Dienes 1 year ago

The sample is going to be the issue here, not the instrument. Analytical error for the portable instrument, particularly accuracy (and your use of CRMs in the field) will be greater than an assay lab but the sampling error is going to be the primary error source. This will lead to large precision errors.

In some sampling situations the accuracy and precision is not as critical - we are only identifying if any mineralisation occurs at all, the "grade" not being as important. The field instrument may be good for this. In other situations (e.g. drillcore or chips) the accuracy and precision will be more important. The unprepared nature of the sample will cause precision errors and be prone to more variation from individual operators.

What can we do about this? Where field prep is performed (on site lab) we can use the instrument on the prepared sample which should much improve precision error; this will save a reasonable amount of cost but not as much as envisaged. Averaging a number of samples taken by different operators will certainly help. Ensuring CRMs are used and calibrations performed as per guidelines is important.

But let’s remember - at these early stages the results of a very small number of samples will decide the direction of the project - positively and negatively.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

About skills, experience and interpretation, it must always be kept in mind that pXRF results are highly dependent on sample condition (raw soil or rock, field crushed sample, or properly milled and homogenised sample, the same way as for the lab).

It must also be kept in mind that pXRF is subject to matrix effects and that results depend upon possible interferences. I used purposely "results" instead of "analyses". Maybe this looks the same on the instrument's screen, but their accuracy and significance depends on what was measured, and how.

Several levels of accuracy can be obtained on the same samples, depending upon the time spent on preparing them and the measurement parameters. These levels range from qualitative geochemical information (errors up to relative 100%) to true analyses, comparable to lab results (errors below 10% in mid-range).

Error values quoted here may seem high (when compared with pure analytical errors) but one must remember that sampling errors are always much larger. It requires experience and training, from the operator, to know the level of accuracy he can expect from his measurements, and the level of confidence he can use them for operating decisions. This experience is much easier to obtain for skilled geochemists or geologists than for lab chemists or field technicians, as the mineralogy issues are more easily understood.

One of the best documents you may read on pXRF accuracy is the CAMIRO review http://is.gd/3DCXkm You may also read with interest the two volumes recently issued by GEEA on pXRF, for instance http://is.gd/SweVbX

Methodologies to be used for assessing pXRF accuracy can be developed by specialised geochemists on a given site, in collaboration with local geologists and engineers. Thank you all for commenting so far. In order to facilitate further discussion, I will post separate replies. One is about applications of field analysis for drilling projects.

Another one is about skills, experience and interpretation QA/QC and sampling error issues. There are more possible topics in this group, such as data set density vs. sparse analysis accuracy, which applies as well to mapping contaminated sites as to mineral exploration.

Further replies, as promised above.

Core scanners.

These are fantastic instruments, providing much more information to geologists than any other instrument on a core drilling site, or in a core storage facility. This is however not light field equipment and the early stages of exploration will rather need handy instruments like pXRFs.

If considering purchasing a core scanner, I would advise strongly a model with the XRF option, besides image recording or gamma measurement, as it enhances widely its capabilities. XRF data will need the same precautions as those from portable instruments.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

The portable XRF (or any on-line analytical device) error makes up a small portion of the total error.

We should also consider:

The presentation of sample to pXRF (are we achieving repeated geometry to the sample)
Sample error (method of taking the sample and the bias of such method),
Sample handling error (further separations of the primary lot down to aliquots),
Human error, and
Laboratory analyser error (how was its calibration determined?).

There are other errors to consider, but I consider the above to be the most relevant.

Having previously serviced and calibrated on-line analytical instruments and in-stream sampling machines, the biggest analyser errors were generated from the sampling. Poor selection of mechanical sampling machine for the accuracy expected was generally most commonly seen.

From a geo point of view, which I had some limited exposure to, most of the methods of "sampling" that I am aware of, would generate far greater errors than the error of the pXRF. Thus, this would have an influence on how the pXRF results are interpreted, as we are led to believe that the lab results are more accurate.

pXRF is for spot checks on drill cores ,etc. as a quick method to determine if drilling to determine an ore body is going to be successful.

All analytical machines (portable, in-stream and lab) have limitations. It is important to remember that we need to be able to accept a particular level of error/bias in the methods we choose to use when analysing our ore ore bodies (raw or processed).

Thanks for generating this discussion topic; I certainly find it very interesting.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

Back to an early and interesting comment, in which you mentioned the need of training.

Manufacturers do usually a good job training their customers in the operation and maintenance of their instruments. They also can help users on specific calibrations.

However, the key skills are:

Understanding the representativity of measurements vs. bulk analysis, which implies experience with the analysed material, minerals and heterogeneity? This can be achieved by a geologist, mineralogist or geochemist on rocks and ores during mineral exploration operations or by a mining geologist or process engineer during mining operations. On other media (soil, waste), an environmental geochemist will do best. What comes on the instrument screen can be really different from the lab result on a bulk sample, without any mistake. It is just a matter of analysed field and volume, and of elements distribution;

Understanding thoroughly sample heterogeneity issues, and sample preparation procedures. This is certainly familiar with this group's members, but I am surprised how far many professionals are unaware of this. In this regard, improperly sampled or prepared material will come back from an excellent lab with meaningless results, sampling errors in excess of 100% in many cases.

Training sessions for field instruments are thus highly needed, but they must contain a large section on sampling methods. This is particularly important for drill-hole analysis projects, which have high QA/QC requirements, and in which the results have direct impacts on big financial decisions.

Sturmbann
1 year ago
Sturmbann 1 year ago

The instrument can only be as good as the sample (or specimen) it receives.

Sachin Prakash
1 year ago
Sachin Prakash 1 year ago

Interesting topic, right in line with the work i am partly involved in. I pretty much agree with all previous comments. But you hit a point of a particular importance when speaking about sample presentation for p-XRF analysis.

Sample presentation with p-XRF should come together with the below further similar considerations:

Depth of penetration of the X-rays within the sample layer presented Surface area acquisition and x-ray spot size (these 3 are generally supplied by manufacturers; and are of the order of um and mm respectively depending on the nature of the sample)

Both the above bullet points are closely related to the density of the sample. If samples are presented as loose powder then the error is more important than with samples presented as pressed pellets or fusion beads (which have all their own error and specific issues)

Having said that, field XRF are very good at screening the samples to go at the assay lab but as said above rigorous Qa/Qc should be applied as well as the use of CRM or Matrix-Match-CRM for calibration and routine blind insertion into sample strings.

And when a field preparation facility is available, some more considerations apply:

Properly chosen equipment: drying oven with temperature selection and air recirculation, jaw crushers with adjustable nominal output size, splitting equipment of know precision (a motor driven spinning riffler would be best with its error being assessed on a regular basis), milling apparatus with timer to ensure repeatability.

Qa/Qc on sample processing would also apply: sieve sizing tests on crushed and pulp material, use of blank material to evaluate cross contamination level, duplicate samples for assay precision and splitting equipment accuracy, written SOPs, etc.

All these may be discussed in depth as they depend upon the commodities analyzed and the uncertainty these bring in. But a pluridisciplinary and team work approach always lead to maximized benefit.

A last but not least comment is to remind that portable XRF technology can't easily be used to quantify light elements (Z below to that of phosphorus). Sulfur also can show some issues as well as PGE.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

Thanks for providing input for these important points.

It is important to pay attention to sample preparation (most pXRF failures result from neglecting it) but not beyond what is needed. Glass beads are by no way required for pXRF. This preparation scheme is only needed for 100% total (+/- 1.5%) oxide analysis of rocks. It comes from lab petrographic analysis. For pXRF, powder or pressed pellet samples are enough.

In most cases, pressed pellets improve analysis reproducibility, but not beyond marginal effects <5%, unless the material is really heterogenous. Instrument manufacturers provide capsules with thin films which do a nice job too. My favourite approach, recommended by US-EPA too, is to shoot various places on a thin PE sample bag, properly homogenised first.

Under the pre-requisite of proper calibration, pXRF may go beyond screening. Calibration is particularly essential for light elements such as P or S. 

Oberstorm
1 year ago
Oberstorm 1 year ago

I agree with your comment comparing field XRF to laboratory analysis but would go a bit further suggesting that XRF is rarely, if ever, as accurate as proper laboratory analysis. For results of a test of a fixed XRF measuring 50 mm x 0 coal on a moving belt see paper ID JAI 12072 available at http://is.gd/G3lsGD.

John Koenig
1 year ago
John Koenig 1 year ago

The instrument can only be as good as the sample. The only time the instrument would be suspect is if it is not calibrated properly.

The biggest contributor would thus be the sample itself. Particle size and homogeneity (not forgetting whether the sample is representative or not) of the sample in obtaining precise and accurate results using pXRF on any other instrument for that matter.

Sachin Prakash
1 year ago
Sachin Prakash 1 year ago

Thanks for emphasizing that each sample presentation (be it pressed pellets or fusion beads) has specific issues and commodities they fit well to. Though, instances where handheld XRF are used with fusion beads do exist as well.

Dizzy Flores
1 year ago
Dizzy Flores 1 year ago

I found your comments interesting, however too academicals in my opinion for an industry best practice approach not only for exploration grade hand held devices, but also for mining grade ones. I also looked at the presentation linked, and I found it quite sadly, focused on XRF ego. What about Raman Spectroscopy? As a geophysical engineer, and former student of material science, I feel compelled to bring forth the humble reminder that the known interaction of the EM radiation with matter translated into applied measurement methodologies, and filtered (by market related constraints) into industry best practices deserves broader options for consideration.

I do not deny the importance of strict Work Operation Procedures, in PXRF industrial use, as a matter of fact I am the first to insist upon that, however, I question the probability of instances where resources are being calculated relying solely on 'field' XRF measurements pressed pellets or fused balls or not.

Usually in order to reduce uncertainties, in more advanced stages of a project companies go to great lengths to create field laboratories, successful ones even certified. Other companies use dedicated personnel specially trained. However due to the wide scope of applicability of the PXRF the reliability can only be achieved by appropriately setting boundaries of relevance, in favour of balanced field and laboratory testing methodology involving elemental analysis among other processes. It is very important to create clear and transparent guidelines and perhaps have a joint academic-technology manufacturer approach instead of competition? After all PXRF is not a smart phone.

Nevertheless, I want to thank you for the great comments.

Carmen Ibanz
1 year ago
Carmen Ibanz 1 year ago

The idea of using regular XRF's fused beads with a pXRF is good. It may help to improve measurements results and get them closer to real analyses. However, fused beads equipment on field is uncommon.

Your idea may fit a specific need: investigate quickly existing bead collections for more elements than analysed at the time they were prepared (typically: major oxides only). I am sure that can be helpful for earlier prospect's revaluation, when a new geochemist explores previous archives.

Yes we should not limit ourselves to pXRF, and portable Raman is a very promising technology, along with FTIR and pXRD. Should we open a new thread for this? 

Moneer_Noor
1 month ago
Moneer_Noor 1 month ago

is it work for Magnesite Ore?

 

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