Assaying, Microscopy, Mineralogy & XRF/XRD

Assaying, Microscopy, Mineralogy & XRF/XRD 2017-04-04T06:57:57+00:00
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Pressed Pellets in XRF (17 replies and 1 comment)

John Koenig
2 years ago
John Koenig 2 years ago

I am interested in hearing people's views on pressed pellets. What is the maximum pressures you use? What is the most common pressure you use? What is the minimum pressure you use?

Maya Rothman
2 years ago
Maya Rothman 2 years ago

I think the issue of pressed pellets for XRF goes well beyond just considering the pressure used to make your pellets. The answer is it depends……

Some factors to consider include:

  • type- silicate, organic rich this determines how much binder you might need
  • size- finer particles bind better than coarse ones
  • type- wax, cellulose – mixture- Different binders often require different pressures
  • to binder ratio- more binder usually means less pressure required
  • of sample- How much do I have and what are my XRF goals
  • goals- how much material do I need to make the pellet infinitely thick for the elements I want to measure
  • I using automation- automation usually means you are pressing into steel rings and adding binder/ grinding aide during the milling. This uniformly mixes the binder with the sample and usually means a more homogenous pellet that only needs to be pressed at 20T.

You might be interested in an application note on the sample preparation of raw meal for the cement industry using pressed pellets. It gives a good look at what reproducibility can be achieved for even light element oxides such as Na2O.

Maya Rothman
2 years ago

I would add that the homogeneity of the mixture is a key factor, too.

Bill Fraser
2 years ago
Bill Fraser 2 years ago

The most difficult part is how to get standards for a pressed pellet (pp) calibration. If you buy CRMs they mostly do not fit to the local materials. Therefore we analyse the real materials (the unknowns) in our lab first and then use the concentrations to establish so called secondary standards from the material. These are then used to performa app calibration. Mostly this fits much better than a calibration with external CRMs.

Tony Verdeschi
2 years ago
Tony Verdeschi 2 years ago

Have people tried running an XRF on a sample prepared using XRD sample preparation techniques (raw sample in sample holder) ? Significant benefit is that the sample doesn't get mixed up with a binder or even fused. What are your views?

David Kano
2 years ago
David Kano 2 years ago

First of all you would need an XRF measuring from the top or if not you need a film to avoid the sample falling down. For many elements XRF is only a surface measurement and then it will be difficult to prepare such a sample reproducible. That is why you would press at a certain pressure to have at least a reproducible density and surface. As higher the atomic number of the element of interest as better it would work what you suggest. Remains the problem of a reproducible thickness of the sample.

Helena Russell
2 years ago
Helena Russell 2 years ago

We are using 10 - 20 drops of 5% mowiol solution in water as binder. This minimizes binder component in pellet and we measure pellet corresponding practically to the 100% of the sample. In pure clays with particle size around 1 micron we get accurate results even for light elements. In samples with larger grain size softer material embraces harder grains (e. g. clay embraces quartz) and we get lower values for components of hard minerals (e. g. SiO2) masked by softer material.

John Koenig
2 years ago
John Koenig 2 years ago

I appreciate binders and good mixing is vital. However, one of my questions was what is the maximum tonnage you would press at? Any answers would be appreciated.

Bob Mathias
2 years ago
Bob Mathias 2 years ago

To answer your original question. the force/tonnage that you press at depends on the size of the die. For a 40mm die, I typically press at 20 tonnes. I usually do not use less than 12 tonnes otherwise I find that I do not get pellets that hold together sufficiently. I have pressed up to a maximum of 40 tonnes which is the limit of my hydraulic press. I usually find no advantage of pressing over 20 tonnes for typical mineral and geological samples. For smaller dies such as 32 mm you can press at lower force/tonnage to produce the same pressure inside the pellet which binds it together.

Helena Russell
2 years ago
Helena Russell 2 years ago

Do any of you see advantages in NOT destroying the samples (fusion or binding) for XRF?

Alan Carter
2 years ago
Alan Carter 2 years ago

the advantages I would find for not fusing samples for XRF is when they are temperature sensitive and that I would lose analytes of interest when fusing them. Typical analytes that do not behave during fusion are Hg, Se, Tl, As and all the halides with the exception of F. It would also be useful for elements that do not fuse easily - that contain PGMs such as for autocatalyst samples that typically contain Pt, Rh and Pd. The disadvantages in the analysis would remain the mineralogical effects that cannot be overcome without destroying the matrix, as a result one would need empirical calibrations and all their associated problems.

Victor Bergman
2 years ago
Victor Bergman 2 years ago

XRF samples are pressed generally at 20 ton Pressure. PP technique generally used in Cement Industries because of its some advantages over fusion bead i.e easy, speedy and inexpensive sample preparation. Although PP technique has some serious problems e.g particle size effect, mineralogical effect , low homogeneity and so on but due to narrow calibration range of element of interest it is being used successfully.

JohnnyD
2 years ago
JohnnyD 2 years ago

I'm using Insmart system for pressed pellet. The maximum operating pressure is 200 bar and Max. load is 40 tons.

David Kano
2 years ago
David Kano 2 years ago

Thirteen tonnes. Other details: 32 mm pellet, 90 seconds dwell with slow pressure release, Licowax binder. Why 13 tonnes? That's a mystery lost in time, but I have always used 13 tonnes for CRMs and unknowns and it works fine.

Bob Mathias
2 years ago
Bob Mathias 2 years ago

In my old application lab the sample types varied a lot depending on what someone sent you in the mail. 20-30T usually worked well with a cellulose wax mixture for binder. There are lot of different binders available from pure cellulose to PVC and I have even seen asprin used. I am curious on the groups experience on the positives and negatives of different binders ?

Bill Fraser
2 years ago
Bill Fraser 2 years ago

In my experience with catalyst and cement applications both with and without binder the maximum used was 25T. As part of the method and application development we would run curves of pressure vs. intensity for each element and insure that the pressure we used would intensities that were well along the flat part of the curve.

Tony Verdeschi
2 years ago
Tony Verdeschi 2 years ago

An explanation of 13 tonnes you use is not a mystery. Very many people use 20 tonnes for 40 mm diameter pellets. If you want to make 32 mm diam. pellet with an equal recipe, then you need to apply 12.8 tonnes. In this case the same pressure per sq. area unit will be applied.

David Kano
2 years ago
David Kano 2 years ago

irrespective of sample/binder recipe it is good practice to validate pressed powder methods for all elements analysed by generating grinding and pressure curves.

Mean particle size contributes significantly to cohesion. Especially with wax, underground samples will result in the face of the sample adhering to the die and breaking away from the pellet leaving a less than ideal analytical surface.

For most silicates a good starting point is 1:4 C-Wax:Sample ratio and 20 tonnes for a 40mm dia. pellet

I use 22 tonnes to ensure I am on the plateau. Grind 80% passing 75um in a ring mill - this gives a d50 sufficient to form a good pellet.

John Koenig
2 years ago
John Koenig 2 years ago

In Summary:

Following are the Key Parameters responsible for preparing a Pressed Pellet to obtain an accurate and reproducible XRF results;

  1. Selection of binder compatible to the sample matrix. To determine the compatibility of binder, pellets are prepared using the different binders such as waxes, Celluloses; etc with same proportion and Std. deviation is calculated for each binder. The binder is considered best one which has the low value of Std. deviation.
  2. The proportion of binder can be determined in the same way by calculating the low sigma value. Generally 20 % proportion of binder is used.
  3. An important parameter is optimal grinding time which is the time period after which the fineness does not change any more resulting stable output XRF intensities. It can be determined by preparing the pellet at different grinding time i.e. 30, 60, 90, 120,150,180,210 seconds. Now curves are plotted keeping grinding time at x-axis and XRF intensities at y-axis for the elements of interest to obtain a plateau. The midpoint of plateau is a time suitable for grinding.
  4. Applied pressure and pressing time (generally 20 ton pressure is applied for 20 seconds which gives the required value of density).
  5. Use of matrix matched CRM with unknown samples to be analyzed for developing application.
  6. For inter element correction empirical coefficients might be used.
  7. Keep narrow range of Calibration curves.

 https://www.911metallurgist.com/equipment/laboratory/assaying/pellet-press/

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