Hi Lubo,
the difference between 6 and 7 % MgO is bigger than 15 % relatively,
thus the error by wrong re-scaling (e.g. if 10 % "amorphous" are
missing) is smaller than the error discussed. And, if the "true" content
of periclase should be about 5 %, than a measured value of 7 % is worth
to be discussed to be a real systematic error. This is an
over-estimation by 40 %.
In the very past (30 years and more ago) people who applied single line
methods carefully have been able to determine "simple" phases like free
lime, quartz, anatase in low concentrations very accurate (of course
after optimizing their technique), and I believe that such methods are
still in use in routine product control. So I feel it is worth to think
about why a Rietveld refinement should perform worse.
I agree with you that the cement business is full of dubious
advertisement for machines and methods for accurate QPA. But this is not
a reason to finish thinking about how to improve QPA methods.
Best
Reinhard
Lubomir Smrcok schrieb:
Hi,
I really wonder why do you bother about 1% difference when the error
of the method (XRD, quantitative phase analysis) could reach 10%
(absolute) ...
People in the discussions appearing here seem to forget about two things:
i) "quantitative" phase analysis done by rietveld method is always
re-scaled to 100%, i.e. the accuracy of % attributed to the individual
phase directly depend on %'s attributed to the another phases present
in the mixture (i.e. when you forget or ignore one phase having 10%
share, those %'s are redistributed among the other phases);
ii) this method completely ignores any amorphous parts (i.e. those not
providing Bragg peaks, to be more exact) - ignores, because it cannot
do anything else about it. Obviously, when you compare XRF and XRD you
can find yourself in troubles as XRF dos not bother about the state of
the compounds.
Finally, my personal advice. If someone tells you you can analyze
clinkers or even cements by XRD accurately, do not trust him. He, most
probably, thinks of selling you a machine you can hardly use for this
purpose :-)
Bad luck if you have already bought it ...
Best,
Lubo
On Thu, 29 May 2008, [iso-8859-1] José Carlos Cordeiro wrote:
My samples are cements (fine samples with 30 microns) and don't need
micronize its. The problem in my refinements is that periclase
quantified with fixed slit is bigger than MgO by Fluorescence, and
its impossible. Ex:
Periclase with fixed slit: 7,14%
MgO by XRF = 6,84%
We know that part of MgO (1,5-2,0%) go to the structure of C3S an C2S
phases, so periclase expected in this sample is 4,8% - 5,3%. If we
consider this, the errors of refinement is not very slim.
Sorry for my bad English!!!
regards
===========================================
José Carlos Cordeiro
Diretoria Técnica - Laboratório Central
Centro Técnico - Curitiba
Rodovia PR-092, 1303, Abranches, 82130-570 Curitiba-PR
Votorantim Cimentos
[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
www.votorantimcimentos.com.br
Fone: + 55 041 3355-1380
Fax: + 55 041 3355-1358
===========================================
-----Mensagem original-----
De: Whitfield, Pamela [mailto:[EMAIL PROTECTED]
Enviada em: quinta-feira, 29 de maio de 2008 17:40
Para: José Carlos Cordeiro; rietveld_l@ill.fr
Cc: Mitchell, Lyndon
Assunto: RE: ADS
Hi Jose
Errors in the region of 1wt% aren't unheard of in complex mixtures
(and the liklihood of the second decimal place being meaningful is
very slim). However there are other possible explanations and one in
particular comes to mind.
Did you micronize your sample? If yes then feel free to ignore the
following! If not then it might be worth thinking about.
We did a systematic study on the effect of particle statistics on the
repeatability of quantitative analysis on one of the NIST cements
last year (presented at the ICCC in Montreal - should really go
somewhere more accessible). We compared different divergence slits,
micronized and unmicronized samples, spun and static, and
remounted/repeated the experiments 3 times each for real statistics
as opposed to esds.
Funnily enough the phase that showed the biggest anomaly in one of
the unground sample was the periclase - obviously a big (by XRD
standards) lump fell into the sample from the kiln that made up the
NIST batch and made it through the mill at the plant. Without
grinding it we got 0.1, 0.1 and 2wt% one series of 3, versus 0.6, 0.6
and 0.6 wt% (this is from memory so don't don't quote me!) for the
micronized sample.
This was the result that we should have got theoretically but it was
nice to see it work in practice, and the periclase did such a nice
job for us by sticking out like a sore thumb even without a Rietveld
analysis! BTW there was no statistically significant difference (95%
limits) in the results of the C3S and C2S phases between ground and
unground samples - mostly because the errors in the unground samples
were so large! The minor phases were another matter. In the
micronized samples the choice of divergence slit size made little
difference, but with the unground samples they made a significant
difference.
To cut a long story short it all comes down to particle statistics -
with an ideal powder the choice of fixed versus variable slits (or
even different fixed slits) should make no difference (assuming that
the Rietveld software can handle the difference in profiles). Moral
of the story - sample prep, sample prep and more sample prep!
Pam
-----Original Message-----
From: José Carlos Cordeiro [mailto:[EMAIL PROTECTED]
Sent: May 29, 2008 3:53 PM
To: rietveld_l@ill.fr
Subject: ADS
I have a Programmable Divergence Slit (incident beam) in my
difratctometer, that can work in automatic mode (irradiated length)
or fixed mode (0.5 degree or 1.0 degree etc). What is the best
collect mode for Rietveld Refinament??
I did refinement two difractograms collected in booth
configuration (fixed 0.5 degree and automatic mode), the results in
the fase "Periclase" (cement fase) changed, see below:
Periclase with 0.5 degree = 6.08%
Periclase wich automatic mode = 7.14%
Note.: The refinement of the difractograms collected with
automatic mode went converted to fixed mode by software.
What do you think about this?
regards
===========================================
José Carlos Cordeiro
Diretoria Técnica - Laboratório Central
Centro Técnico - Curitiba
Rodovia PR-092, 1303, Abranches, 82130-570 Curitiba-PR
Votorantim Cimentos
[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>
www.votorantimcimentos.com.br
Fone: + 55 041 3355-1380
Fax: + 55 041 3355-1358
===========================================
