239 posts
Diffrence Between XRF & XRD or chemical analysis
Dear Members What is the diffrence in XRF & XRD Do we get a change in chemical composition of clinker/Raw Meal/Free Lime etc. for the same sample in XRD & in XRF? If yes , why? People say that XRD is better than XRF , Why? Some say that XRD gives real phases of clinker while XRF phases are calculated. Also calculated phases are without free lime ? Please answer my query. Thanks Mahendra
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537 posts
re Diffrence Between XRF & XRD or chemical analysis
Hello Mahendra,
XRF and XRD measure different things, each giving different information about the same sample.
XRF, or X-Ray Fluorescence analysis, measures the intensity of x-rays flouresced by individual elements in a sample, irrespective of the different compounds present that may contain those elements. eg. in cement the XRF analysed Ca percentage is the total Ca contributed by all calcium compounds in the cement.
XRD, or X-Ray Diffraction analysis, measures the intensity of crystal diffraction peaks due to the individual chemical compounds in the sample. ie CaCO3, CaO, CaSO4 etc. The result is estimated percentages for each compound of interest.
One method is not necessarily better than the other, they are simply complementary techniques which, when combined, give the total picture.
XRD is becoming more popular because it can estimate the quantity of clinker minerals more accurately than the traditional Bogue equations. which use the XRF chemical results. And XRD can also quickly analyse important compounds such as Free lime which are time consuming to analyse by any other method. With advances in computer power and programming sofware the complex calculations required to estimate clinker phases have become faster and more accurate, enabling this method to be used in on-line analysers for both clinker and cement.
Lastly, the formulae for calculating clinker phases can allow for free lime by simply subtracting the free lime from the XRF CaO value and using that value in the equation.
Regards,
Ted.
77 posts
re Diffrence Between XRF & XRD or chemical analysis
Dear Ted / every one
As you mention in XRF we mesured the % of different elemental analysis and in XRD we analyse the different phase analysis .
can you please share some more information regarding XRF and XRD , like
1.)some details about both the technique
2.) In XRF if we detect the different element why we convert it in to it's oxide form for bogue calculation. why we are not measuring only Ca, Al,Fe etc.
3.) Are we measure the sulphure in ( So3 form) clinker or hot meal accuretly in XRF ?.
4.) If XRD give the different phase information presence in clinker or any raw material so we can also measure the different phases like C2S,C3S etc in clinker is it correct? if yes are this mesuerment give the apparox same value as microscopy.
5.) if we want to measure total carbonate in lime stone it is possible in XRD only, please correct me if I am wrong?
6.) Calcualtion done by XRF ( Bogue) and Microscopy, Always we get difference in clinker phases% like C3S,C2S ets. what is the main reasons for this? ( but mostly we get higher value in microscopy )
7.) % analyze by XRF or XRD is the volumetric or wt%?
best regards
raju verma
i
537 posts
re Diffrence Between XRF & XRD or chemical analysis
Hello Raju,
To answer your questions;-
1.) An XRF analyser measures the intensity of fluorescent radiation given off by the electrons in the various elements as they fall back into a more stable state after being temporarily excited to a higher energy state by the primary X-ray beam. The intensity of the fluorescent radiation is measured by moving an x-ray detector (or fixing it, in the case of monochromators) to a specific angle with respect to an analysing crystal. This is similar to viewing the individual colours which refract at different angles when white light passes through a glass prism.
An XRD analyser measures the intensity of x-rays which are diffracted into wave patterns by the atoms in the crystal compounds contained in a sample. The angles and intensities of the diffracted beams indicate a three-dimensional density of electrons within the crystal which are unique to that compound. X-rays work best for this because their wavelength is of a similar magnitude to the spacing between the atomic planes within crystals.
2.) Many industries (eg. metals & alloy manufacture) use software calibration curves that report the results of the XRF analysis in % of each element directly. ie. Fe, Sn, Al, Cu etc
However, in the cement industry we are not normally dealing with individual elements. The elements we are interested in are almost always present in minerals, combined with oxygen. Therefore in the cement industry, XRF calibration software still uses the raw intensity of the element being analysed, but since the calibration data is input as % oxides, the instrument converts the elemental intensity to an oxide percentage. The sum of the oxides plus L.O.I. also adds up to close to 100%, which it would not do if using elemental concentrations.
Oxides must be used in the Bogue equations because the elements in the minerals C3S, C2S, C3A and C4AF are all present as oxides. eg. C3S = Tricalcium Silicate = Ca3SiO5 = 3CaO.SiO2
3.) Some lighter elements like sulphur are harder to measure accurately by XRF. However, if sample preparation is carefully controlled and consistent, SO3 can be analysed quite accurately down to levels of +/- 0.01% with modern XRF equipment. This is particularly important when using fused glass beads are being used. SO3 is volatile in at high temperature in a reducing atmosphere, so to minimise the loss of SO3 during the fusion process, the flame should be maintained in a state of strongly oxidising conditions by careful control of the O2 / Gas ratio, and by minimising the fusion temperature. There is also the dilution factor of the sample:flux ratio in fused bead analysis however if the element being analysed is present in too low a concentration to accruately measure by fused bead, it can be analysed by pressed pellet.
4.) Yes, XRD can give direct information about the different compounds such as C2S, C3S etc present in a sample. However, due to the complex overlapping nature of the peaks produced by the clinker phases, differentiating them to obtain an accurate percentage of each phase present is a very difficult and time consuming job. Special software (eg Rietveld) is required which is usually quite expensive.
Simple peaks such as CaO on the other hand are much more straight-forward and do not require any special spectral analysis software and can be calibrated into most XRF/XRD analysers. If the XRD analyser is properly calibrated and configured, the results from XRD are comparable with microscopy.
5.) It is possible to use XRD to analyse the CaCO3 content of limestone, but with faster and cheaper alternatives, it is normally not necessary.
6.) The differences between Bogue and XRD or microscopy are the result of the assumptions made in the derivation of the Bogue equations. With the Bogue calculations, the clinker phases are assumed to be pure minerals of the form, C3S, C2S, C3A and C4AF. In practice, all of the minerals contain various quantities of other oxides such as SO3, Na2O, K2O, Mn2O3 and P2O5 etc.
7.) Results of XRF and XRD analysis are usually expressed in wt%.
I hope this answers your questions.
Regards,
Ted.