By Roger Meier on Aug 29, 2017 9:48:12 PM
The role of full phase analysis in the mining industry has grown in past 10 to 15 years as a result of significant improvements in XRD instrumentation and corresponding analysis software. Today the measurements take only a few minutes and a full pattern Rietveld refinement can be performed in just a few minutes.
The added value of a full XRD analysis and the related full quantitative characterisation of the minerals present in the sample provide the analyst important information to improve the processes and to control the quality at the various process steps in an efficient way.
Taking a representative sample and applying the best possible sample preparation methodology is a must to fulfill the requirement to obtain the best information about the sample and the material at the different steps in the process.
One of the potential biggest sources of errors is the tendency that many of the minerals will have, even after size reduction by grinding, a non-ideal spherical shape of the crystallites. This causes preferred orientation of the crystals that are pressed into the sample holder. The subsequent XRD analysis will show different peak intensity ratios than used in an ideal case where the experiment has been performed under perfect conditions. This happens especially with phases that have a low symmetry, like clay minerals.
The best option to eliminate this source of errors or reduced accuracy of the quantification is to prepare the materials by using the ASP (automatic soft press) that reduces preferred orientation effect by a maximum and removes human errors from this essential step of the sample preparation. Studies have shown a huge variation between samples prepared by different operators, but also moderate variations when the same operator prepares more samples of the same homogenised material.
The introduced integrated errors by preferred orientation and the subsequent Rietveld quantification including a March-Dollase preferred orientation correction can go up to a factor of 8 in certain cases. An automated reproducible, in respect of preferred orientation optimised sample pressing does not only improve the data quality of the results, is increases as well the overall laboratory efficiency and reduces potential health risks by minimisation of the exposure of the personal to materials that could be potentially hazardous.
In many applications (semi-)automated sample preparation is categorical excluded where the sample preparation volume is below certain threshold, but in these cases the gain in higher data quality and improvement of the laboratory efficiency provides in most of the times such huge advantages that the investment for an automated soft press can be justified, even for a limited number of samples processed.