Assaying, Microscopy, Mineralogy & XRF/XRD

Assaying, Microscopy, Mineralogy & XRF/XRD 2017-03-23T09:37:54+00:00
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Fluorine XRF Analysis (7 replies)

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

We are a mining company producing fluorspar (F2Ca). We have been so far analyzing our mineral by the chemical method. We just bought an XRF spectrometer for complete analysis of: -F,Ca,Si,Al, Fe, Mg, Pb, S.
The method is useful for us only if we can do pressed pellets. The question is, is there an optimum method to prepare the samples? How can we determine the optimum grain size for analysis?

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Pressed pellets is definitely the correct approach for this type of material. grain size will be important for the light elements. I would suggest that you grind to 50um or less. I would go for making pellets 40mm in diameter if your XRF has a sample mask wide enough to take advantage of the larger diameter( eg. 35-37mm). This will provide the best signal for the lightest elements like F.

* of sample and 3g of a wax-cellulose binder pressed at 20-30T will make a nice robust pellet. getting a fine and consistent grind for the powder is likely to be the hardest part. Not all mills can deliver that level of consistency for grinding. You may also want to consider an automated mill and press for the best and most consistent analytical results. This application Note will give you an idea of what type of reproducibility is possible.

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

I checked the document you linked, and understand that it demonstrates the reproducibility of your equipment for a determined sample preparation procedure. But what I need to know is the optimum sample preparation procedure itself, as we have measured significant differences in F analysis while milling to different sizes.
Please note that there´s part of your text missing where the *

Bill Fraser
1 year ago
Bill Fraser 1 year ago

Fluorine analysis is significantly effected by particle size ( shadow effects) effects because the sampling depth is so small. The grain size of the powder you use to make the pellets needs to be less than <50um. Then you can make a robust pellet 40mm diameter pellet using 12g of your sample and 3g of binder. These should be mixed well, preferably in a mill or shaker and then pressed at 25-30T for 2-3 minutes and with a pressure release over abut 1 minute. You can scale down the sample and binder proportionally if you are using 32mm pellets. I would use a cellulose-wax binder, but would measure the binder as a blank as some waxes contain Fluorine. There are many ways to make pressed pellets but this one should work for you. Good luck

David Kano
1 year ago
David Kano 1 year ago

<40uM particle size, 20+tons of pressure, minimum binder at 5-20%, thickness critical to escape depth ie F is only a few uM depth.

JohnnyD
1 year ago
JohnnyD 1 year ago

We are doing Fluorspar analysis by fusion and made good experience also with F analysis, definitely an alternative to pp.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

When referring to is the X-ray sampling depth for Fluorine. While the primary X-ray beam can penetrate deeply in to the sample and generate secondary X-rays of all the elements of the sample, the X-rays can be reabsorbed by the sample before they can travel back out of the sample to the detector. The lower the atomic number of the element, the longer the wavelength of the X-ray and the easier it is re-absorbed by the sample. The sample depth at which an X-ray for a given element will make it out of the sample is called the critical escape depth. It is different for different elements. The escape depth for low atomic number elements such as Fluorine is only a few 10's of microns. This is why it is so critical that the sample be extremely homogeneous when measuring these elements. Having a sample with a consistently small particle size, that is well compacted and has a very flat surface is really important.

John Koenig
1 year ago
John Koenig 1 year ago

Yes, the 'nugget effect' can be critical in geological samples. The sample for XRF needs to be both representative and of a particle size smaller than the critical escape depth of the F X-rays. Taking lots of samples won't help much as there will be a self-absorption bias on the results. F is quite a challenge for XRF.

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