Determination of phosphorus in raw materials for ceramics: comparison between X-ray fluorescence spectrometry and inductively coupled plasma-atomic emission spectrometry

Analytica Chimica Acta 432 (2001) 157–163

Determination of phosphorus in raw materials for ceramics: comparison between X-ray fluorescence spectrometry and inductively coupled plasma-atomic emission spectrometry M.A. Marina a , M.C. Blanco López b,∗ a

b

Fundación ITMA, Cayés, Oviedo, Spain Departamento de Qu´ımica F´ısica y Anal´ıtica, Facultad de Qu´ımicas, Universidad de Oviedo, Oviedo, Spain Received 5 June 2000; received in revised form 4 October 2000; accepted 22 November 2000

Abstract This paper compares methods for phosphorus determination in refractory silicoaluminous materials based on inductively coupled plasma-atomic emission spectrometry (ICP-AES) and X-ray fluorescence (XRF) spectrometry. Chemical interferences were studied by using several calibration standards and standard additions. The limits of detection obtained by XRF spectrometry (0.03 and 0.06 mg P2 O5 per gram of sample) are clearly advantageous as compared with those obtained by ICP-AES (0.2 mg g−1 ). Phosphorus can successfully be determined by XRF spectrometry with either pressed sample pellets or diluted sample beads. Matrix effects were minimised by using a certified bauxite as a standard, but several less-similar materials could be used, such as siderurgic slag, chamotte and even dolomite for low P2 O5 content samples. The linear calibration range is larger (up to 100 mg g−1 ) when analysis is carried out by ICP-AES, using the 213.628 nm line. Repeatability is similar with both methods, but XRF spectrometry is preferred on the grounds of the better sensitivity achieved, and the simpler sample preparation requirements. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Phosphorus; X-ray fluorescence spectrometry; Inductively coupled plasma-atomic emission spectrometry; Ceramics

1. Introduction Phosphorus is present in many ceramics and glasses, as a minor component or trace element, at concentration levels very often <0.1% [1]. Its origin may be due to its initial presence in raw materials, or the addition of phosphorus-based polyelectrolytes (phosphoric esters, Calgon, etc.) as deflocculants or processing additives. On the other hand, many glasses and derived products include P2 O5 in their formulation [2–3]. P2 O5 plays an important role during ∗ Corresponding author. E-mail address: [email protected] (M.C. Blanco L´opez).

ceramic microstructure development, since it reacts with the surrounding oxides to form very stable bonds at relatively low temperatures (1000◦ C) [4]. However, the refractory properties of the ceramic (high thermal shock resistance and high chemical resistance) can be seriously degraded by the presence of phosphorus: at high temperatures (∼1800◦ C), P2 O5 decomposes with corresponding porosity development and consequent detrimental effect on microstructure. It is, therefore, necessary to include a precise and reliable determination of phosphorus in routine quality control analysis of raw materials for ceramics. In this work, two analytical techniques have been investigated and compared with the purpose of determining P2 O5 in refractory materials. NIST-certified reference

0003-2670/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 0 0 ) 0 1 3 3 7 - 4

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bauxites were chosen for this study. Due to its high alumina content, this aluminosilicate is used for high refractoriness applications, and a more strict control of impurities is usually demanded. Literature related to spectrophotometric procedures for the determination of phosphorus in silicates is extensive. The phosphorus content can be determined directly from the absorbance of the yellow molybdovanadophosphoric acid [5,6]. The absorbance occurs maximum at 315 nm, but the preferred wavelength for rock analysis is within the 420–470 nm range, where interference from iron is decreased or eliminated [6]. Several modifications have been introduced in this method, and the most used is the reduction of the ternary heteropolyacid to the blue molybdophosphoric acid [7–16]. Silicon is the main interferent in this method, forming an analogous yellow vanadomolybdate which could be eliminated by either HF attack [5,7–9,14], addition of tartaric acid [10], pH control [13], on pH control and addition of Fe in excess [15]. It is also recommended to remove fluoride ions from solution [5]. The most common reducing agents are ammonium iron(II) sulphate [13–14], ascorbic acid [7–12] and tin(II) oxalate [15]. With the aim of reducing the analysis time of this troublesome method, a flow injection modification has been developed for silicates (with molybdenum blue spectrophotometric detection) [16]. Although detection limits are improved by a factor of 20 with the introduction to reduction to molybdenum blue and determination at the absorbance maximum at 830 nm, lower values could be reached with simple matrices (such as those of environmental aqueous samples), by using an ion exchange resin with the double function of preconcentration and separation of silicate [17]. These methods are very laborious and their application to silicoalumia samples should be carried out very carefully in order to avoid volatilisation of phosphorus during the successive evaporation steps, or positive errors if silicon is not fully eliminated [5,6]. Nevertheless, they have been the basis of ISO and ASTM standard procedures for the determination of phosphorus in ceramics [18] and glasses [19], and are still the basis of more recent modified molecular spectroscopic methods, such as the photoacoustic spectrometric determination of phosphorus as molybdenum blue adsorbed on an anion-exchange resin [20].

Atomic spectrometric methods for the determination of phosphorus in ceramics and glass are mainly based on atomic emission techniques such as direct current plasma (DCP) or inductively coupled plasma atomic emission spectrometry (ICP-AES), and XRF spectrometry. Direct atomic absorption spectrometry (AAS) is not sensitive [21,22], but some indirect AAS methods have been developed based on selective extraction of the heteropoly acid formed with bismuth in the presence of molybdophosphate [21] or of molybdophosphate itself [23], followed by the determination of the bismuth or molybdenum released into aqueous solution. This has been applied to the determination of phosphorus in steels [21] and rocks [23]. For simpler matrix samples, such as organic soils, a method to evaluate phosphate in solution by using graphite furnace AAS with La as matrix modifier has been reported [24]. AES is widely used [25–30], although major drawback sites are the few emission lines that this element provides [31], and their poor sensitivity [32], and, therefore, the impossibility to achieve low detection limits. Emission lines below 190 nm (used with a vacuum monochromator to prevent adsorption of radiation by air) are free from spectral interferences and in many cases allow lower detection limits to be attained [33]. However, increased background effects might restrict their use with refractory samples. Better detectability is usually achieved by using ICP mass-spectrometry (MS), as for the determination of phosphorus in an organic matrix [34]. However, with a complex matrix such as coal or coal ash, ICP-MS was not found to be advantageous for the determination of phosphorus due to strong spectral interferences [35]. Inspite of the development of instrumental techniques, the main difficulties of the analysis of refractory materials remain sample decomposition [36] (as a consequence of their high resistance to chemicals) and the complexity of the matrix, which restricts the application of potential new methods for phosphorus determination. For instance, a determination procedure for phosphorus involving atomisation in a furnace and molecular non-thermal spectrometry of PO and HPO formed encountered difficulties when applied to inorganic samples [37,38]. Sometimes an extraction step for complete isolation of the analyte from the matrix is required, such as, for the determination of phosphorus in zircon (ZrSiO4 ) [30]. A novel

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indirect method has been suggested for phosphorus determination in ilmenite rocks (high titanium content minerals), by using a Pb-ion selective electrode for the potentiometric titration with Pb(ClO4 )2 of the solution resulting after K2 CO3 fusion [39]. Sample dissolution is critical for the analysis of ceramic and refractory materials [40]. Acid digestion and fusion with a flux (LiBO2 + Li2 B4 O7 or Na2 CO3 ) have traditionally been used as solubilisation procedures [41,42]. When combined with microwave oven digestion it is possible to reduce sample preparation time [43–45]. It was reported that 4 min is enough to dissolve a rock sample [45], although the strong negative errors accompanying phosphorus determination indicate the high risk of losing volatile compounds. Another possibility is the introduction of the sample into the atomiser as a slurry [46–48] but problems were still encountered, possibly because transport efficiency and slurry sampling for a good analysis depend very closely on particle morphology and critical particle size distribution [46]. Study of the suspension stability before analysis is also recommended if low relative standard deviation values are required [48], at the cost of extending the analysis time. Solid sampling techniques would be advantageous, and are currently under research. Laser ablation has been applied to ICP-AES and ICP-MS with the aim of determining phosphorus in coals and coal ashes, but poor accuracy is the main feature reported for the determination of this element [35]. Electrothermal vaporisation has been tested, too, but reproducibility problems were found [49]. Radiofrequency glow discharge is emerging as a powerful technique for elemental analysis of solids, and could be particularly useful when applied to these non-conductive and high chemical resistant solids [50–52]. Sample preparation is simple for phosphorus determination by XRF spectrometry. Its determination in refractory materials, however, is complicated by their matrix, since the net-intensity of an X-ray emission line strongly depends on both the concentration and absorption coefficients of the analyte and those of other elements present in the sample. The choice of calibration standard is critical in order to minimise matrix absorption and enhancing effects. A new calibration procedure has been proposed for non-destructive analysis of small and irregular ceramics fragments, which is based on modifications of

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some procedures and software [53] although its suitability for phosphorus determination was not tested. ICP-AES (with both flux and acid digestion solubilisation) and XRF spectrometry were the two techniques chosen for this comparative study. Matrix effects were tested by ICP-AES by comparing the calibration graphs for aqueous standards and standard additions. Minimisation of matrix effects with XRF was attempted by the use of four different external standards, and by dilution of the sample with a flux (beads formation). Analytical characteristics of the methods used are compared, and their reliability tested against several NIST bauxite standards. The results shown are expected to highlight the advantages and disadvantages of these methods and their scope of application.

2. Experimental 2.1. Materials Several NIST bauxite reference standards were used. Their certified composition is shown in Table 1. 2.2. Sample preparation 2.2.1. ICP-AES analysis Two solubilisation procedures were followed: 1. Acid digestion. Concentrated HClO4 (2.0 ml), concentrated HNO3 (2.0 ml) and HF (20 ml) were Table 1 Materials used (mg P2 O5 /g)

Al2 O3 Fe2 O3 SiO2 TiO2 ZrO2 P2 O5 V2 O5 Cr2 O3 CaO MgO MnO ZnO K2 O SO3

NIST 600

NIST 696

NIST 697

NIST 69b

NIST 698

400 170 203 13.1 0.60 0.39 6.0 0.24 2.2 0.5 0.13 0.03 2.3 1.55

545 87.0 37.9 26.4 1.4 0.50 0.72 0.47 0.18 0.12 0.04 0.014 0.09 1.50

458 200 68.1 25.2 0.65 9.7 0.63 1.00 7.1 1.8 4.1 0.37 0.62 0.770

488 71.4 134.3 19.0 2.9 1.18 0.28 0.11 1.3 0.85 1.10 0.035 0.68 5.51

482 196 6.9 23.8 0.61 3.7 0.64 0.80 6.2 0.58 3.8 0.29 0.10 1.43

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added to 0.5000 g of dried sample (slightly humidified with few drops of water) in a PTFE vessel. This procedure was repeated three times. The residue was extracted with 20 ml of (1 + 1) HCl, filtered through filter paper (narrow pore), and thoroughly rinsed. The solution was brought up to 100 ml with distilled, demineralised water. 2. Fusion. Dried sample (1.000 g) was mixed with 2.000 g of flux (NaKCO3 :Na2 B4 O7 , 25:10) in a platinum crucible which was covered with a lid and placed in a muffle furnace at 1100◦ C for 2 h. The crucible was taken out and let cool. Both crucible and lid were subsequently placed in a glass containing 100 ml of (1 + 1) HCl and warmed on a hot plate until full dissolution of the melt. Crucible and lid were thoroughly rinsed, and dissolution and washing liquid were transferred to a 200 ml flask, and brought up to volume with distilled, demineralised water. 2.2.2. XRF analysis 1. Pellets (pressed discs). Sample (6.0000 g) was weighed and thoroughly mixed with 0.3 g of stearic acid in a vibrating mill. The resulting mixture was placed on an aluminium die previously covered with ground sucrose. The sample was pressed to 40 Tm. 2. Beads. Dry sample (0.7500 g) was transferred to a platinum crucible. Li2 B4 O7 (7.5 g) were added as flux and 0.05 g of LiBr as releasing agent. These compounds were thoroughly mixed with the sample in the crucible. Beads were formed by keeping the sample for 14 min over a flame. Once the sample was melted, the content of the crucible was cast in a platinum mould and allowed to cool. 2.3. Apparatus and reagents Analytical grade reagents (Merck) were used all throughout the study. A Perkin-Elmer Plasma 1000 apectrometer was used. Outer gas flow was set at 15 l min−1 , auxiliary gas flow at 1.0 l min−1 and the nebulizer uptake rate at 1.0 ml min−1 . The radiofrequency power was 1022 W. Phosphorus concentration was measured using the 213.618 nm line. Aqueous PO4 3− standards (1000 ppm solution, Merck) were used for calibration.

XRF analysis was carried out with a Siemens SRS 303 spectrometer using the P K␣ line, under vacuum. Tube voltage was 37 kV and tube current was 80 mA. The crystal used was pentaerythritol (PET). Pellets for the XRF analysis were prepared by pressing sample powder and additives with an Instron Press (40 Tm). Fused beads were prepared with a LECO FX-200 apparatus. Calibration for the XRF analysis was carried out using several certified reference materials, in order to minimise matrix effects. Dolomite and chamotte standards were used for calibration with diluted standards (beads) and slag and bauxite for undiluted samples (prepared as pellets).

3. Results and discussion The choice of the emission line and their low sensitivity are among the difficulties reported for the determination of phosphorus in rock and refractories by atomic emission spectroscopy. Suitable lines reported (free from spectral interferences) are 214.914 nm for direct current plasma (DCP) [25], 178.287 [26], 213.618 [27,30], 214.914 nm [28,30], 253.565 [30] for the ICP. The 213.618 nm was used here, on the basis of its better detectability (signal to background ratio) [54] and because it was free from major spectral interferences (Cr, Ti) [48]). For the samples under study, it offered better sensitivity than the VUV lines, because of background elevation caused by other elements present in the matrix (Al, Cr, Fe, V) was lower. Only Fe could cause partial wing overlap [54] and this interference could be minimised by narrowing the window. Other strong spectral interferents reported such as Zr, Hf [29] and Mo [30] were not expected in the samples analysed. Matrix effects could be evaluated by comparing the slopes of the calibration graphs obtained with aqueous P2 O5 standards and the standard additions plot (Table 2) for both procedures (acid digestion and flux). The slope was greater for aqueous standards, decreased slightly when flux reagents were added, and was slightly lower when several phosphate aliquots were added to either acid digested or fluxed samples. This indicates that both flux components and sample matrix elements (mainly) caused a decrease of the phosphorus detectability by ICP-AES.

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Table 2 Calibration results for the ICP-AES methods used (mg P2 O5 /g)a

Aqueous Aqueous Standard Standard a

standards standards + flux additions on fluxed samples additions on acid digested samples

Slope

sb

Intercept

sa

r2

0.4550 0.4025 0.3645 0.3595

0.0111 0.0102 0.0014 0.0095

1.8095 1.8391 4.3849 2.6293

0.0535 0.0438 0.0058 0.0084

0.9970 0.9968 0.9999 0.9988

sb : standard deviation of the slope; sa : standard deviation of the intercept (n = 6).

A better power of detection was achieved by XRF spectrometers (Table 3), especially with undiluted standards (prepared as pressed pellets). Matrix effects during the XRF analysis were studied by using four different external standards: bauxite (the closest matrix match), chamotte (calcined clay material, with silicoalumina basis but lower content of Al2 O3 and higher SiO2 than bauxite), dolomite (Ca and Mg carbonate) and siderurgic slag (∼50% CaO, with SiO2 , Fe, MnO2 and P2 O5 as major constituents). When the sample was not diluted (pellets), the calibration lines with standards of different composition, such as bauxite or slag, were very similar. Lower sensitivity is achieved with diluted standards of dolomite or chamotte standards (prepared as diluted beads). The slope value obtained for the dolomite standards is half that obtained for the calibration with chamotte standards, indicating strong chemical interferences from the dolomite matrix. Theoretical estimations and a standard addition method have been proposed in order to minimise this effect when phosphorus is determined in biological materials by XRF spectrometry [55], although its application to bauxite or other silicoalumia samples with a complex matrix might be difficult. The greater slopes obtained by XRF resulted in lower limits of detection (Table 4). Up to 0.06 mg g−1 can be determined by XRF spectrometry using diluted standards (beads), and 0.03 mg g−1 using undiluted

standards and samples prepared as pellets, whereas the limit of detection obtained by ICP-AES is 0.2 mg g−1 . This is mainly a consequence of the poor sensitivity of the emission line used for ICP-AES. The values reported here are similar to those reported for phosphorus determination in rocks by ICP-AES [33] or by other techniques [39]. The authors have not come across any reported limit of detection for the determination of phosphorus in rock samples by XRF spectrometry. Table 4 shows too that the calibration graph by ICP-AES was linear over a wide range of concentrations (0.1–100 mg g−1 ). This calibration range is larger than that of the well-established molybdovanadophosphate method, which was linear up to 10 mg g−1 [5], and also than the linear range observed for XRF (0.01–10 mg g−1 ). Repeatability (calculated using the relative standard deviation of five successive measurements, R.S.D.) was similar for the methods followed in this study (Table 4). The best values were obtained by XRF with undiluted pellets (1.5%) and by ICP-AES with fluxed samples (1.6%), whereas 2.0% was found for ICP-AES with acid digestion solubilisation and 2.1% for XRF with samples prepared as beads. A comparison of the reliability of the methods used is shown in Table 5. It can be observed that, for the low P2 O5 concentration NIST standards (696 and 600), the

Table 3 Calibration results for the XRF methods (mg P2 O5 /g)a Sample preparation

Calibration standards

Slope

sb

Intercept

sa

r2

Pellets

Bauxite Slag

0.2936 0.2862

0.0090 0.0183

−0.0977 0.0415

0.0518 0.0975

0.9912 0.9985

Fused beads

Dolomite Chamotte

0.0202 0.0432

0.1519 0.4824

0.0139 0.0118

0.1069 0.4055

0.9012 0.9908

a

sb : standard deviation of the slope; sa : standard deviation of the intercept (n = 6).

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Table 4 Analytical characteristics of the XRF and ICP-AES methods used Method

Calibration standards

Linear range (mg g−1 )

LODa (mg g−1 )

Repeatabilityb (%)

XRF

Bauxite (pellets) Slag (pellets) Dolomite (beads) Chamotte (beads)

1–10 1–10 0.1–1 0.1–1

0.03 0.03 0.06 0.06

1.5 (1.2)c

Aqueous standards Aqueous standards + flux

0.1–100 0.1–100

0.20 0.22

2.0 (4)c 1.6 (2)c

ICP-AES

2.1 (1.2)c

a

Limit of detection, the analyte concentration producing a signal to noise ratio of 3. Relative standard deviation of five successive determinations measured with the standard of concentration indicated in parentheses. c Units are expressed in mg g−1 . b

Table 5 Results obtained with the XRF and ICP-AES methods used (mg P2 O5 /g) (sample preparation procedure and calibration standard used for XRF are specified)a Standards

Certified

XRF

ICP-AES

Beads

NIST NIST NIST NIST NIST a

696 697 600 69b 698

0.50 ± 0.06 9.7 ± 0.6 0.39 ± 0.07 1.18 ± 0.04 3.7 ± 0.1

Pellets

Dolomite

Chamotte

Bauxite

Slag

0.5 ± 8.3 ± 0.3 ± 1.1 ± n.d.

0.6 ± 8.7 ± 0.5 ± 1.3 ± n.d.

0.4 ± 9.7 ± 0.4 ± 1.1 ± n.d.

0.5 ± 8.5 ± 0.5 ± 1.1 ± n.d.

0.01 0.2 0.01 0.02

0.01 0.2 0.01 0.03

0.01 0.1 0.01 0.02

0.01 0.1 0.01 0.02

Acid digestion

Flux

1.5 ± n.d. 1.3 ± 1.1 ± 4.0 ±

0.8 ± n.d. 0.6 ± 1.3 ± 3.3 ±

0.03 0.03 0.02 0.08

0.01 0.01 0.02 0.05

n.d.: not determined; mean ± S.D. (n = 3).

flux led to slightly more accurate results than the acid digestion procedure, when samples were analysed by ICP-AES. With both procedures, positive errors were found for the lowest concentration samples analysed, which can be due to the low sensitivity of the analyses. Since the calibration range by XRF was linear up to 10 mg g−1 P2 O5 , samples did not need to be diluted. With this technique, the best results for all samples analysed are obtained with undiluted samples (pellets) and when bauxite is used as external standard. When slag is used, negative errors occurred for the high concentration sample. The same type of error was observed with the high P2 O5 content samples when using dolomite bead standards. The reason for these observations could be the absence of strongly incident beam absorbing elements (such as Si and Al) in important concentrations in those compounds, and consequently this effect not being compensated when bauxite samples are analysed if slag or dolomite is used as standards. However, as it can be observed in

Table 5, for low P2 O5 content samples, the results are still close to those of the certified values. Taking into account the low detection limits achieved, the fact that no sample solubilisation is required, the rapidity of the analysis (14 min is enough), XRF spectrometry was found to be the most suitable technique for phosphorus determination in bauxite and silicoalumina materials.

4. Conclusions Development of new procedures for phosphorus determination in silicoaluminous refractory materials is restricted by the high chemical resistance to solubilisation and the complex matrix effects. Calibration is linear up to 100 mg g−1 by ICP-AES, and up to 10 mg g−1 by XRF spectrometry. Both acid digestion or flux sample solubilisation procedures lead to a similar detection limit of 0.2 mg g−1 by ICP-AES.

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The lowest detection value (0.03 mg g−1 ) is attained by XRF using pressed pellets of either bauxite or siderurgic slags as external standards. Negative errors are observed with dolomite and slag standards at the higher concentrations determined. The lowest R.S.D. values (1.5 and 1.6%) are obtained with pressed pellets by XRF and flux solubilisation by ICP-AES.

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