The effect of particle size and water content on XRF measurements of phosphate slurry

doi:10.21203/rs.3.rs-1532974/v2

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The effect of particle size and water content on XRF measurements of phosphate slurry

https://doi.org/10.21203/rs.3.rs-1532974/v2

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Phosphate slurries obtained from Moroccan mines are studied using the XRF technique and the effect of the particle sizes and the water content parameters are analyzed and reported for the first time. Samples of the phosphate slurry with different particle sizes (425 µm, 300 µm, 250 µm, 200 µm, 160 µm and 106 µm) and different water content (30%, 40%, 50%, 60%) were analyzed with an energy dispersive x-ray spectrometer (EDXRF). The results show that the relative error of measurement varies with the particle size of the analyzed sample, the water content and the element measured. The relative error increases with the increase of the particle size for the compounds P₂O₅, Al₂O₃, K₂O, Cr₂O₃, Fe₂O₃ and Sr. The ratio between the relative errors obtained from the maximum and the minimum grain size was 1.50 for P₂O₅, 4.01 for Al₂O₃, 15.58 for K₂O, 1.22 for Cr₂O₃, 1.51 for Fe₂O₃ and 1.11 for Sr. On the other hand, an opposite evolution has been observed in the case of compounds CaO and SiO₂. The relative error increases with increasing water content for all compounds existing in the slurry. Depending on the measured compound, the relative error increases by a factor that varies between 1.39 and 2.39. The results obtained for P₂O₅ do not show a clear correlation between the measurement error and the water content. We will evaluate the impact of particle size and water content on XRF measurements in the case of phosphate slurry, aiming to develop an online XRF analyzer system for phosphate slurry.

phosphate mineral slurry

X-ray fluorescence

particle size effect

water content effect

EDXRF

Phosphate mineral slurry is a mixture of dry phosphate and water. It is the raw material to be exploited to produce fertilizers and phosphoric acid. The phosphate slurry is processed to produce phosphorus (P), which is one of the three main nutrients most used in fertilizers (the other two are nitrogen and potassium), and the most important macronutrients essential for the growth and development of a plant. It is a building block of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) in plant cells and is responsible for energy storage and transfer. Plants acquire all their P from fertilizers in the soil (Holford 1997).

The quality of the phosphate slurry is checked regularly in the laboratory during all the treatment processes using conventional analytical methods (Gaft et al. 2007). These methods are expensive, time-consuming and generate hazardous waste. An online X-ray fluorescence technique (XRF) analysis system could overcome these drawbacks and allow a rapid chemical analysis of the phosphate slurry. A few studies have demonstrated the potential of the XRF technique to analyze dry phosphate rock (Issahary and Pelly 1982; Addas et al. 1995; Safi et al. 2006; Hasikova et al. 2014). However, there is a lack of information on the analysis of phosphate slurry using the XRF technique.

This article will explore some challenges of analyzing phosphate slurry using the XRF technique. These challenges are related to the physical parameters of the slurry, namely particle size and water content. These parameters affect the accuracy of XRF measurements and more specifically in the case of slurry. Indeed, analyzing the slurry is a challenging process compared to solids analysis, one of these challenges is the absorption of X-rays by the water.

XRF is a rapid technique for quality control of mineral samples (Rose et al. 1986). The time required to analyze a sample can be reduced from hours (in the case of conventional analysis methods) to minutes (in the case of XRF) (Perring and Andrey 2003). Like many other techniques, XRF has some limitations regarding the analysis of light elements (Al-Eshaikh et al. 2016). In the case of phosphate slurry, the particle size of the sample and the water content may affect the accuracy of the XRF measurements. (Melquiades et al. 2011; Kalnicky and Singhvi 2001; Laperche 2005; Clark et al. 1999; Ge et al. 2005; Kido et al. 2006; Tjallingii et al. 2007; Parsons et al. 2013; Weindorf et al. 2014). When analyzing samples with different particle size distribution and light elements or having analytes with long-wave characteristic lines (e.g., Si, Al), peak intensities can be attenuated by 30% (Mzyk et al. 2002). F. Demir et al. (2008) studied the effect of particle size distribution on XRF measurements of cement samples with different grain sizes. The results show a difference between the maximum intensities of the \({K}_{\alpha }\) peaks which can reach 17%, depending on the particle size of the sample analyzed. In another study, samples of different grain sizes extracted from the Nile River in Egypt were analyzed with the XRF technique. The results show that the XRF intensities of \({K}_{\alpha }\) radiation can increase or decrease when decreasing grain size and depending on the atomic number of the analyte. For small particle size samples, the characteristic radiation penetration depth increases, and therefore the probability of the particle size effect on the characteristic radiation decreases (Shaltout et al. 2011). Presence of water in a slurry and its capability to absorb x-ray could affect the accuracy of the XRF measurements as well, and can also causes diffusion of primary radiation from excitation sources, resulting in a decrease of the intensity of the characteristic X-rays and an increase of the intensity of the X-rays scattered in the fluorescence spectrum (Ge et al. 2005). Three certified reference materials with different water contents were analyzed with the XRF technique. The measurements show that the elements identified by the \({L}_{\alpha }\) spectral line with the highest Z atomic number were more affected by the water content than the elements identified by the \({K}_{\alpha }\) line with the lowest Z. Ti, Cr and Fe were not significantly influenced by water content, while the Sr was the most affected (Santana et al. 2019). In some cases, the influence of water can be corrected by calculating the attenuation coefficients for each measured element (Schneider et al. 2016).

The work presented in this article is part of a project to develop an online XRF analyzer to control the quality of phosphate slurry directly on the production line. Particle size and water content are parameters that can adversely affect XRF measurements. Several studies have addressed the effect of these two parameters in the case of XRF analysis of other materials. However, the case of XRF analysis of phosphate slurry has not been reported in the literature, mainly because of the following parameters such as the specific chemical composition, mineralogical structure and other characteristics of the sample being analyzed. In this study, we investigate the effect of these two parameters for the analysis of phosphate slurry with XRF. We analyzed 07 samples of phosphate slurry with different particle sizes (from 106 µm to 425 µm) and 16 samples with different water content ranging from 30–60%. Based on the obtained data, we will propose new formulas to correct the concentration considering the particle size distribution and water content. Also, solutions are envisaged in the future to obtain these data in real-time with a view to implementing an XRF online analyzer system for phosphate slurry.

Preparation of samples to investigate the effect of particle size distribution

The phosphate slurry samples were prepared in the laboratory based on dry phosphate ore extracted from phosphate mines located in Morocco. Six samples were prepared from a same batch of the ore weighing 2 kg. The sieving operation yielded to six samples of different particle sizes of 425 µm, 300 µm, 250 µm, 200 µm, 160 µm and 106 µm. These samples were dried in an oven for 12 hours at 60°C. The preparation of the slurry samples was done by adding 50% of water content to each sample. for all the samples. Table 1 summarizes the characteristics of the six samples.

Table 1

The six samples prepared for studying the effect of particle size
Sample	Brut_106	Brut_160	Brut_200	Brut_250	Brut_300	Brut_425
Particle size (µm)	106	160	200	250	300	425
Water content (%)	50	50	50	50	50	50

Preparation of samples to investigate the effect of water content

Sixteen samples were used to study the effect of water content. They were prepared from four reference samples S1, S2, S3 and S4 extracted from different phosphate mines located in Morocco. The four samples were ground to a fineness of 160 µm and dried for 12 hours at a temperature of 60°C. Based on each sample, we prepared four new slurry samples of different water content (30%, 40%, 50%, 60%). The preparation of the slurry samples was done by adding a well calculated amounts of water to the reference samples. The solutions were then stirred for 5 min to obtain slurry samples. Table 2summarizes the 16 samples with water content.

Table 2

The 16 samples prepared for studying the effect of water content
Basic samples ID	Prepared samples ID	Water content (%)	Particle size (µm)
Sample 1 (S1)	S1_60	60	160
	S1_50	50	160
	S1_40	40	160
	S1_30	30	160
Sample 2 (S2)	S2_60	60	160
	S2_50	50	160
	S2_40	40	160
	S2_30	30	160
Sample 3 (S3)	S3_60	60	160
	S3_50	50	160
	S3_40	40	160
	S3_30	30	160
Sample 4 (S4)	S4_60	60	160
	S4_50	50	160
	S4_40	40	160
	S4_30	30	160

Xrf Setup

Each sample was divided into two sets for analysis by two different methods. The first set of samples was sent to external laboratories for analysis with conventional methods to determine the exact chemical composition of the samples and the amount of chemical compounds. The second set of samples was analyzed using XRF Epsilon 1 spectrometer (Malvern Panalytical, United Kingdom). The results of XRF measurements will be compared with those obtained by external laboratories. The XRF Epsilon 1 equipment can analyze both solid and liquid samples. To analyze the slurry samples, we used plastic cups and covered the bottom with a thin film of Mylar polymer with a thickness of 3.6 µm. Then, the slurry sample was poured into the cup and placed inside the XRF analyzer. Figure 1 shows the main components that make up the Epsilon 1 XRF analyzer. The X-ray tube has a power of 15 W with a 50 kV generator. The spot size on the sample is typically 10 x 14 mm. The detector has a high resolution of 135 eV. The matrix of phosphate slurry contains heavy and light elements in low and high concentrations. Thus, each sample was analyzed using four measurement parameter configurations, as shown in Table 3.

Table 3

The measurement parameters of the four configurations
	Excitation energy (kV)	Current (µA)	Filter	Measurement time (S)
Config 1	50	100	Ag	100
Config 2	50	100	Cu	300
Config 3	12	416	Al	120
Config 4	10	316	None	300

Generally, the choice of the film is based on factors such as X-ray transmission, impurities, chemical resistance, and cost (Margui and Van Grieken 2013). The analysis of some certified reference materials using the XRF technique showed that the energy absorption varies according to the material constituting the thin films (Hall et al. 2011). The study confirmed that the transmittance of some light elements (Al, Mg, Si, K, S and Ca) has a high sensitivity to the thin film material. In general, greater transmittance (minimal absorption) is observed for polypropylene (PP) than for Mylar. This difference is mainly explained by differences in density between the materials, Mylar being denser than PP. However, the film used in our study allows transmittance of 65% K-line intensity of element Al, a transmittance of 75% K-line intensity of Si, and a transmittance greater than 80% of K-line intensity of other elements analyzed in this study, as shown in Fig. 2. Also, all light elements analyzed in this study are major elements and not trace elements. Thus, Mylar 3.6 µm film is suitable for this study.

The validation of the XRF equipment used in this study

To validate the XRF measurements obtained using Epsilon 1 spectrometer, we prepared and analyzed three certified Moroccan phosphate samples. Water was added to the three powder samples to prepare slurry samples with three water content 51%, 58% and 66%. The average error obtained for the measurements is 0.035% for K₂O, 0.083% for SO₃, 0.21% for SiO₂, 0.34% for Fe₂O₃, 0.70% for Al₂O₃, 0.82% for MgO and 1.19% for P₂O₅. The results obtained are comparable to the certified laboratory analysis. Based on this first stage of measurements, we performed our experimental analysis using this equipment for different samples.

In this work, we studied the effect of particle size and water content on XRF measurements of phosphate slurry. The results showed that the relative error of XRF measurements increases as the particle size of the sample increases. This is for most compounds except SiO₂ and CaO. We quantified the evolution of the error as a function of the relative grain size for the elements contained in the phosphate slurry. The variation in the relative error as a function of the particle size could be explained by the mineralogy of the samples measured and the roughness of the measured surface. Also, this experimental study showed that the relative error of XRF measurements increases as the water content of the measured sample increases. This variation was expected, as the X-rays are absorbed by the water contained in the slurry. However, this assumption does not justify the results obtained for the P₂O₅ measurements. Indeed, the minimum relative error was obtained for the water content of 50%. These results can be expressed by the mineralogy of the phosphate slurry, the size of the tricalcium phosphate Ca₃(PO₄)₂ particles and the diameter of x-ray beams derived from the x-ray tube. For a water content between 30% and 60%, we quantified the evolution of the relative error by mathematical equations. Thus, we proposed new formulas to correct the concentration considering the water content.

Competing Interests

Ismail Ben amar has received fellowship funding from the University UM6P in Morocco.
Mourad Roudjane receives a salary from Prof. Younes Messaddeq where he is a Senior Research Development Scientist. Hafid Griguer receives a salary from UM6P University where he is a director of the Innovation Lab For Operation. Amine Miled is the co-supervisor of the Ph.D. student Ismail Ben Amar and he has no competing interests to declare that are relevant to the content of this article. Younes Messaddeq is the main supervisor of the PhD student Ismail Ben Amar and he received research support from the Sentinel North program, the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), and The OCP Group.

Authors contributions

Ismail Ben Amar, Mourad Roudjane and Prof. Younes Messaddeq wrote the main manuscript text. preparation of reference samples and processing of interlaboratory test results was done by Hafid Griguer. Prof. Amine Miled made a critical analysis of the work, he helped to perform a deep interpretation of the data and to redo some tests. Preparation of slurry samples, planning of this work, data collection and analysis were performed by Ismail Ben Amar, Mourad Roudjane and Prof. Younes Messaddeq. All authors reviewed the manuscript. Prof. Younes Messaddeq and Prof. Amine Miled are the main supervisor and the co-supervisors of the Ph.D. Student Ismail Ben Amar.

Acknowledgments

This research was supported by the Sentinel North program, the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), and The OCP Group.

Availability of Data and Materials

The datasets generated and/or analysed during the current study are available in the OneDrive Cloud repository, you can access to all the data through this link.

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The effect of particle size and water content on XRF measurements of phosphate slurry

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Version 2

Abstract

Figures

Introduction

Materials And Methods

Preparation of samples to investigate the effect of particle size distribution

Preparation of samples to investigate the effect of water content

Xrf Setup

Results And Discussion

The validation of the XRF equipment used in this study

Conclusion

Declarations

References

Competing Interests

Supplementary Files

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