76 X-RAY FLUORESCENCE SPECTROMETRY: DEVELOPMENT OF EFFECTIVE FUSION PROCEDURES FOR LIME PRODUCTS Jean-Philippe Gagnon Corporation Scientifique Claisse, 35 rue Franquet, Québec, QC, G1P 4P3, CANADA ABSTRACT The industrial lime production requires a strict control of material composition to ensure a constant production. XRF combined with borate fusion is commonly used in production laboratories because it provides fast and accurate analysis. Sample preparation problems arise with samples containing large amounts of carbonates. They are reported to foam and run over the crucible during the fusion. The sample also sticks badly on the side of crucible. New robust fusion procedures for lime products such as calcium carbonate, calcium oxide (dolomitic and high calcium limes), calcium hydroxide and calcium supplements are reported. A precise control over mixing and heat transfer allows to get specimens free of bubbles. The precision of the procedures is evaluated by calculating the standard deviation of net intensities of silica (SiO 2 ), alumina (Al 2 O 3 ), iron oxide (Fe 2 O 3 ), calcium oxide (CaO) and magnesium oxide (MgO). Special attention is paid to minor elements. The use of pre-fused flux with an integrated non-wetting agent is a factor to achieve a good precision on these elements. INTRODUCTION The quality for chemical analysis of lime is imposed mainly by construction and steel industries. In the literature there are fusion procedures for limestone but in fact, few of them produce a perfect specimen, free of bubbles and undissolved particles [1]. The purpose of this work is to find out sturdy fusion procedures for lime products. The word lime refers to natural minerals, as well as compounds such as calcium carbonate (CaCO 3 ), calcium oxide (CaO) and calcium hydroxide (Ca(OH) 2 ). Calcium carbonate is a sedimentary rock, commonly called limestone. For the majority of industrial applications, calcium carbonate comes from quarries and it contains impurities such as clay, sand, iron oxide and organic compounds. On a different note, the pharmaceutical industry requires very pure calcium carbonate to produce antacids, calcium supplements and base material for other drugs [2]. The calcination of calcium carbonate (Figure 1) is carried out at about 9 o C and this produces calcium oxide, called quicklime [3, 4]. Calcium oxide is a very corrosive product. The hydration of calcium oxide is very exothermic and produces calcium hydroxide, or slaked lime. The calcium hydroxide is very useful for many industrial applications. For example, it is an active ingredient in mortars, it is used in water treatment and it also neutralizes ground acid in the agricultural world. Calcium hydroxide reacts slowly with carbon dioxide (CO 2 ) contained in the air or generated by combustion, and produces calcium carbonate. After processing, products derived from limestone have the unique property to return to their initial chemical state.
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77 Setting Carbonation H 2 O Limestone Calcium Carbonate CaCO 3 CO 2 Burning Calcining CO 2 Heat H 2 O Figure1. Lime cycle. Ca(OH) 2 Hydrated lime Calcium Hydroxide Heat Slaking Hydration CaO Quicklime Calcium Oxide An essential condition for an efficient fusion is to get reproducible elemental intensities. When this requirement is fulfilled, an XRF method can be developed. Ideally, the net intensities must be known without error, but this is impossible. It must be assumed that a slight fluctuation on the intensity has a small effect on the concentration precision. FUSION OF CALCIUM CARBONATE All fusion procedures presented in this study are designed with a sample flux ratio of 1/7 on a calcined basis. During the fusion, the calcium carbonate is decomposed in calcium oxide and carbon dioxide. CaCO 3 CaO + CO 2 The sample preparation procedure is the following: 1. Weigh 3.2g of 67/33/LiBr flux in a platinum crucible (Bis!). 2. Add 1.3g of sample. 3. Add 4.g of 67/33/LiBr flux on the top. 4. Place the crucible and 32mm mould (11 o ) on the M4 fluxer. 5. Select the calcium carbonate program and start the fusion. Adding the remaining flux on the top helps to reduce sputtering during the decarbonation process. In regards to casting, a new type of mould was used with the wall at 11 degree angle (Figure 2). Even though the melt is not sticky, this angle reduces the cracking probability. In this connection, the fusion program (Figure 3) shows a decarbonation period followed by a sequence of stop and go motions; it is perfect to remove trapped bubbles. On the far left graph, the apparent power is simply the integration of the gas curve which represents the energy put in the system. Figure 2. Wide angle mould.
78 1 75 Apparent power: 28 13 Cooling 5 25 5 1 15 2 Figure 3. Fusion program for CaCO 3 samples. In this study, the replicability of elemental intensities is evaluated following the analysis of 1 specimens prepared by fused beads. Table 1 presents the net intensities of several analytes contained in samples from two different lime plants. Generally, the values of residual standard deviations for major and minor elements are lower than 1.32% which is acceptable. The sulfur is an exception but it can be explained by RSD magnification due to weak sulfur intensities. Table 1. Replicability of intensities from CaCO 3 samples (n=1). Analyte Plant no.1 Plant no.2 x RSD x RSD (kcps) (kcps) Si 2.511 ±.36 11.9229 ±.32 Al.345 ±.74.6134 ±.65 Fe.3476 ± 1.23.72 ±.9 Ca 366.12546 ±.6 176.5587 ±.6 Mg 1.582 ±.35 12.2795 ±.1 S.636 ± 4.5.1184 ± 7.64 K.5714 ± 1.32.72 ±.42 Na.2766 ± 1.11.317 ±.78 FUSION OF CALCIUM OXIDE The fusion of calcium oxide is the same as calcium carbonate except the decarbonation period is reduced. Remember that calcium oxide can fix the CO 2 contained in the atmosphere and the addition must be considered for the weighing process. The sample preparation procedure is the following:
79 1. Weigh 3.2g of 67/33/LiBr flux in a platinum crucible (Bis!). 2. Add.72g of sample. 3. Add 4.g of 67/33/LiBr flux on the top. 4. Place the crucible and 32mm mould (11 o ) on the Claisse M4. 5. Select the calcium oxide program and start the fusion. 1 75 Apparent Power: 24 8 5 25 Cooling 5 1 15 2 Figure 4. M4 fusion program for CaO samples. Like calcium carbonate samples, the values of residual standard deviations (table 2) for major and minor elements are lower than 1.28% which is acceptable except for sulfur. Table 2. Replicability of intensities from CaO samples (n=1). Analyte Plant no.3 Plant no.4 x RSD x RSD (kcps) (kcps) Si 2.3789 ±.23 2.4462 ±.52 Al.3193 ±.61.339 ± 1.8 Fe.635 ± 1.23.3158 ± 1.28 Ca 216.723822 ±.15 362.869 ±.8 Mg 15.518 ±.13.8266 ±.48 S.1412 ± 4.56.115 ± 6.18 K.3688 ± 1.1.4566 ±.62 Na.317 ±.47.2737 ±.56
8 FUSION OF CALCIUM HYDROXIDE Ca(OH) 2 CaO + H 2 O The fusion program is the same as the calcium carbonate program but instead of producing CO 2, water is produced. The sample preparation procedure is the following: 1. Weigh 3.2g of 67/33/LiBr flux in a platinum crucible (Bis!). 2. Add.95g of sample. 3. Add 4.g of 67/33/LiBr flux on the top. 4. Place de crucible and 32mm mould (11 o ) on the Claisse M4. 5. Select the calcium oxide program and start the fusion. 1 75 Cooling Apparent power: 28 13 5 25 5 1 15 2 Figure 5. M4 program for Ca(OH) 2 samples. Residual standard deviations (Table 3) for major and minor element are lower than 1.25% which is acceptable. Again the sulfur value is higher but the signal is very low. Table 3. Replicability of intensities from a Ca(OH) 2 sample (n=1). Analyte Plant no.5 x RSD (kcps) Si 2.573 ±.3 Al.3122 ± 1.25 Fe.284 ±.87 Ca 364.81421 ±.6 Mg.6732 ±.68 S.793 ± 2.51 K.411 ±.83 Na.2763 ±.71
81 FUSION OF CALCIUM SUPPLEMENTS In this case, the calcium carbonate is included in an organic matrix. Each tablet contains 8 mg of potassium tartrate hemihydrate and 15 mg of calcium carbonate. This drug is used for the prevention of hypokalaemia and the treatment of potassium depletion. The two step fusion program consists first of all in burning the organic material by alternatively switching on and off the flame and, secondly, fusing the ashes. The sample preparation procedure is the following: 1. Grind 1 tablets of calcium supplement with a ring & puck grinder. 2. Weigh 8.2g of 67/33/LiBr flux and 2.g of sample in a platinum crucible (Bis!). 3. Thoroughly mix the sample and the flux with the Claisse mini-vortexer. 4. Place the crucible and 32mm mould (11 o ) on the Claisse M4. 5. Select the calcination program and start it. 6. Select the fusion program and start it. 1 8 6 4 1 8 6 4 Cooling Apparent power: 78 5 2 2 2 4 6 8 1 Figure 6. M4 calcination program. 5 1 15 2 25 3 Figure 7. M4 fusion program for ashes. Table 4. Replicability of intensities from a calcium supplement sample (n=1) Analyte Calcium supplement x RSD (kcps) Si 2.6753 ±.23 Al.2246 ± 2.15 Fe.25 ± 1.12 Ca 46.14551 ±.36 Mg.5169 ±.46 S.328 ± 4.93 K 274.174 ±.11 Na.3651 ±.68
82 The intensity replicabilities (Table 4) are acceptable. However, the idea of having a large standard deviation seems probable since the fusion process implies a calcination step. Actually, this is not the case. For example, Figure 8 shows that the standard deviation of calcium and sodium are comparable to other products. Calcium Sodium Figure 8. Probability density functions. CONCLUSION This study shows that all fusion procedures suggested for lime products are replicable for Si, Al, Fe, Ca, Mg, S, K and Na. The remaining glass beads are homogenous and bubble free. Moreover, the calcination of calcium supplements does not have any negative effects on elemental replicabilities. ACKNOWLEDGMENT I thank Mrs. Melanie Houghton from Carmeuse Technology Center for her help to obtain lime samples. REFERENCES [1] Bennett, O.; Oliver, G., XRF Analysis of Ceramics, Minerals and Allied Materials, J. Wiley & Sons, New York, 1992. [2] Lime, Wikipedia, The Free Encyclopedia. March 26 [3] Guide to lime Lime Cycle, Limebase Products website, March 26 [4] Lafuma, H.; Freyssinet, E., Liants Hydrauliques, Dunod, Paris, 1965