A material known as clinker is obtained when cement raw materials are calcined in the kiln. In order to ensure cement quality, free lime in clinkers have to be closely monitored. Excess free lime results in undesirable effects such as increased setting time, volume expansion or reduced strength.
Furthermore, by monitoring free lime constantly, the operator can determine and maintain the optimum operating point of the kiln in order to obtain maximum reactivity and to reduce thermal consumption. With high reactivity, raw meal grinding can also be reduced resulting in further economies of energy.
The X-ray fluorescence technique (XRF) is used for performing chemical elemental analysis on cement making materials. Concentration for the major oxides is derived from this analysis. As mineralogical information is not available from XRF spectra, wet chemical methods such as titration or separate X-ray diffractometry (XRD) equipment is required to determine the phase content in clinker or cement.
The Thermo Scientific ARL 9900 series with IntelliPower is an innovative instrument that can handle both techniques. A diffraction system can be integrated which can perform qualitative scans and qualitative analysis. The Thermo moiré fringe positioning mechanism ensures the highest repeatability of angular positioning. There has been an improvement in the performance of this diffraction system by careful optimization of the crystal discriminator and the detector.
Free Lime Analysis Results
Six clinker standards were analyzed for free lime. Grinding of the powders was done for 10s with two 200mg grinding pills. They were pressed into pellets at 15t for 40s and measured on the ARL 9900. The diffractogram recorded with the diffraction system on three different clinker pellets is shown in Figure 1. It is observed that the two peaks assigned to C3S and CaO phases, are resolved and not interfered by any fluorescence from the sample. It can be observed from the peak representing 0.5% of free lime that the increased sensitivity of the diffraction system is more than enough to monitor the free lime content even at low levels (0.1% to 0.5%).
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Figure 1. Diffraction pattern obtained with the ARL 9900 Series in the free lime region for clinker samples with varying CaO concentrations.
The intensities of the characteristic diffraction peak for free lime (d=2.41Å) on the six clinker standards is recorded with the calibration program. The results are shown in Figure 2 along with the relevant parameters in Table 1. The innovative diffraction system produces a sensitivity of 600 cps % for free lime analysis. This is about 10 times more than a conventional X-ray diffractometer. The average of the differences between chemical and found concentrations is the standard error of estimate. The value obtained (0.08 %) is well within the capability of wet chemistry.
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Figure 2. Calibration curve of free lime in clinker using the ARL 9900 Series.
Peak Intensity [Kcps]
A clinker sample was measured 11 times for 40s on free lime with the integrated XRD system and 40s on the fixed XRF channels. The short term stability was tested by measuring a clinker sample 11 times for 40s on free lime with the integrated XRD system and 40s on the fixed XRF channels. Table 2 summarizes the results where XRF results on major oxide are also included for comparison. The fine repeatability of the system is shown approximately 1% relative standard deviation. This is approximately 10 times better than what is achievable by wet chemistry. Lastly a long-term stability test was done for 50 hours. Excellent results were obtained with sigma values well within statistics (1.8% free lime ±0.02%).
Table 1. Regression results on free lime.
| |
-CONCENTRATION |
| SAMPLE DIFFERENCE ABSOL. % |
NAME |
Kcps |
NOM. % |
CAL’D% |
| Clinker 1 |
1.54 |
0.80 |
0.93 |
0.13 |
| Clinker 2 |
1.30 |
0.53 |
0.53 |
0.00 |
| Clinker 3 |
2.98 |
3.30 |
3.32 |
0.02 |
| Clinker 4 |
1.62 |
1.10 |
1.07 |
-0.03 |
| Clinker 5 |
2.09 |
1.90 |
7.84 |
-0.06 |
| Clinker 6 |
1.41 |
0.77 |
0.71 |
-0.06 |
| Standard error of estimate |
0.08% |
|
| BEC |
1.636% |
|
| Q (sensitivity) |
600 cps% |
|
| Limit of Detection |
156ppm |
|
Table 2. Typical reproducibility (10 runs) of free lime analysis in clinker
| RUN |
XRD FREE LIME |
CaO |
SiO2 |
Al2O3 |
Fe2O3 |
MgO |
K2O |
Na2O |
SO3 |
| 1 |
0.67 |
65.54 |
21.42 |
5.59 |
3.97 |
1.55 |
0.68 |
0.58 |
1.01 |
| 2 |
0.68 |
65.52 |
21.41 |
5.59 |
3.96 |
1.55 |
0.68 |
0.58 |
1.00 |
| 3 |
0.67 |
65.52 |
21.41 |
5.59 |
3.96 |
1.55 |
0.68 |
0.58 |
1.00 |
| 4 |
0.66 |
65.50 |
21.40 |
5.58 |
3.96 |
1.55 |
0.68 |
0.58 |
1.00 |
| 5 |
0.67 |
65.50 |
21.39 |
5.58 |
3.96 |
1.55 |
0.68 |
0.58 |
1.00 |
| 6 |
0.66 |
65.52 |
21.41 |
5.58 |
3.96 |
1.55 |
0.68 |
0.58 |
1.01 |
| 7 |
0.66 |
65.52 |
21.41 |
5.59 |
3.96 |
1.54 |
0.68 |
0.58 |
1.00 |
| 8 |
0.67 |
65.51 |
21.41 |
5.58 |
3.96 |
1.55 |
0.68 |
0.58 |
1.00 |
| 9 |
0.67 |
65.51 |
21.41 |
5.59 |
3.96 |
1.55 |
0.68 |
0.58 |
1.00 |
| 10 |
0.65 |
65.50 |
21.39 |
5.59 |
3.96 |
1.55 |
0.68 |
0.58 |
1.00 |
| Average |
0.67 |
65.51 |
21.41 |
5.59 |
3.96 |
1.55 |
0.68 |
0.579 |
1.00 |
| St Dev |
0.007 |
0.014 |
0.011 |
0.009 |
0.004 |
0.003 |
0.001 |
0.002 |
0.001 |
| RSD |
1.04 |
0.02 |
0.05 |
0.16 |
0.10 |
0.18 |
0.09 |
0.27 |
0.08 |
Conclusions
Free lime in the clinker can be quantified with high reliability, sensitivity and excellent stability by using the diffraction system integrated into the ARL 9900. It is also possible to monitor limestone additions to cement using this integrated XRF-XRD instrument, as well as other clinker phases like C3S, C2S, C3A or C4AF.
The benefits of this combined system are:
- It is possible to perform XRF and XRD on the same sample and under identical conditions thus cutting down all extra costs for additional hardware and ensuring stable and reliable total analysis
- By adding this diffraction system, XRF performance is not affected
- Software and data treatment methods are common to XRF and XRD measurements.
Integrating XRF and XRD in the same instrument can offer complete quality control of clinker and cement. Separate instruments or methods are no longer required resulting in significant savings from increased operator efficiency and lower maintenance and running costs.
This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.
For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers.