[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

A 40 mm pellet is produced by pressing at 25 tons fig

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A 40-mm pellet is produced by pressing at 25 tons Fig. 4. Preparation of calibration standard with wax: 6 g of the powder is mixed with 2 g of FLUX- ANA wax. A 40-mm pellet is produced by pressing at 25 tons Pressed Pellets The high error of loose powder analysis can be reduced by milling the powder and reducing the particle size to < 60 µ m. This can be achieved with a disc mill, ball mill or “swing” mill (ring and puck mill) using a milling container which is hard enough to grind the material. At this point contaminants in the container material have to be taken into account. Usual materials are agate, aluminium oxide, zirconium oxide or tungsten carbide. (For more info: www.fluxana.com ) To present a good surface and a consistent density to the X-rays, a pressed pellet has to be prepared although. Not all materials will form a stable pellet under pressure. Therefore the use of a binder or grinding additive is recom- mended. Properties of Grinding Additives/Binders for X-ray Fluorescence: good grinding performance, binding property under pressure with a high variety of materials, good stability under X-ray radiation, long term stability necessary for standard preparation, non-hygroscopic, easy to handle. Depending on the original grain size the sample is milled at 1400 s 1 for 15–60 s. Then a pressed pellet will be produced. There are two ways of adding the binder: (1) a grinding additive like Multimix is added before milling (see Fig. 3). (2) The milling is done with the pure sample and then in a second step a binder like wax is mixed with the sample homogeneously (see Fig. 4).
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4 Dr. Rainer Schramm and Hubert Dzierzawa Ca in cement 800 820 840 860 880 900 920 940 960 980 1000 0 5 10 15 20 25 30 tons Intensity STD = 0.4% Fig. 5. Preparation of 32 mm pressed pellets from cement using different pressure Fig. 6. Fully automated fusion machine VULCAN Figure 5 shows that the influence of the pressure becomes negligible if the particle size is below 60 µ m. Fused Beads The minimum error achieved with pressed powder technique is not below 0.5%. For more precise analysis the fusion technique is required. In fusion the sample will be dissolved totally in borate glass. Particle size effects disappear and based on the dilution, matrix effects will be reduced. Also the surface of the bead represents an ideal sample. Principle For a successful fusion the sample must be presented in an oxidic form. This is mostly the case for materials like cement, quartz, clays, ores and limestone. These materials can be fused directly after drying. A typical recipe would be 8 g of lithium tetraborate (FLUXANA FX-X100) and 1 g of sample.
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Sample Preparationfor X-ray Fluorescence Analysis 5 Fig. 7. Principle of preparation of a fused bead Cement CaO % 50 55 60 65 70 75 50 55 60 65 70 75 Given Analyzed X0207 SD = 0,10 X0206 SD = 0,10 BCS 372 SD = 0,17 BCS 354 SD = 0,14 X0204 SD = 0,06 Fig. 8. Certified reference materials from BAS, GB (BCS) and Dillinger, D (X) 5 times prepared as fused bead with HD Elektronik VULCAN, SD = Standard Deviation The mixture is heated in a fully automated fusion machine (HD Elektronik VULCAN, see Fig. 6) to 1050
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