AN_380_Quantitative_XRD_phase_analysis_DOC-A88-EXS020_V2_low_res.pdf

AN 380

Quantitative XRD phase analysis in Minerals & Mining: Aluminum bath

Abstract

Rietveld analysis emerged as a routine tool in quantitative phase analysis of crystalline powder samples. The report describes the rapid analytical method from ultra-fast data collection using the LYNXEYETM detector to automatic Rietveld analysis with TOPAS. It is shown how to derive well established measures such as bath ratio and excess AlF3 from the Rietveld results. These measures widely required in aluminum production are readily determined with very high precision.The excellent agreement for Alcan reference mate-rial demonstrates the outstanding accuracy of the method. Furthermore, it is shown how to improve conventional Potflux analysis by the Rietveld method. Finally, the auto-mated investigation of aluminium bath samples in AXSLAB is demonstrated.

Introduction to Aluminum bath analysis

Aluminum metal is produced from alumina (Al2O3) by electro-lytic reduction. The melting point of alumina is above 2300 K, which renders the direct production of aluminum from alumina uneconomic. Instead, alumina is decomposed in a cryolite (Na3AlF6) electrolyte at about 1230 K (Hall-Hérault process).

The influence of additives on the performance of the elec-trolytic cell (aka bath or pot) is manifold. The admixture of CaF2 and AlF3 further decreases the melting temperature of alumina, therefore reducing the energy consumption. However, the solubility of alumina decreases concomitantly, thus decreasing the efficiency of the pot. Alumina addition also needs to be monitored since to low concentrations result in “anode effect”-failure of the bath while sludge formation is observed for to high concentrations. The addition of LiF to the bath also increases the efficiency by reducing the liquidus temperature and increasing the conductivity of the bath. The presence of sodium in the electrolyte increases the electri-cal conductivity and hence, decreasing efficiency of the pot. In summary, the operating conditions of the bath need to be found by optimising the six component system Al-Na-Mg(Li)-Ca-F-O.

The bath conditions are inferred from quantitative XRD analysis of the congealed electrolyte that may contain mineral phases such as cryolite (Na3AlF6), chiolite (Na5Al3F14), Ca-cryolite (NaCaAlF6 or Na2Ca3Al2F14), fluorite (CaF2), weberite ( Na2MgAlF7), neighborite (NaMgF3), corundum (Al2O3), spinel (MgAl2O4), villiaumite (NaF), and others.

Quantitative result

SmelterLYNXEYE detector

Solid electrolyte

TOPAS analysis

Two traditionally used measures for controlling the composi-tion of the bath are the bath ratio BR (defined as the weight ratio NaF/AlF3) and excess AlF3, ExAlF3. The ratios of some phases in the crystallized electrolyte are shown in Fig. 1. Once the chiolite (Na5Al3F14) lines disappear the bath is depleted of aluminium ions and alumina (Al2O3) has to be added. Typically, aluminum is deposited within the BR range 1.1 — 1.4. For pure cryolite (BR = 1.5) ExAlF3 is 0 % and for pure chiolite (BR = 0.833) the ExAlF3 value is 24.24 %.

Fig. 1: Crystalline phases appearing in the congealed aluminum electrolyte during electrolysis.

Experimental requirements

The determination of the bath concentration in the plant is typically repeated every two to three days. Since there are hundreds to thousands of baths, the available time for the measurement and data analysis is just several minutes.

In order to obtain reproducible quantitative results the sample preparation needs to be standardized. Typically, the con-gealed sample is roughly crushed in a small crusher and the sample is automatically screened for pure metal parts that may disturb the subsequent automated mill and press proc-ess. The whole procedure takes about 3 min. The pressed samples are either transported via conveyor belt to the dif-fractometer or collected at a sample tray.

Traditional quantitative analysis based on single diffraction peaks is fast with measurement times below 100 seconds. However, bath analysis based on single peak analysis is hampered by peak overlap of several phases, texture etc. Therefore latest developments aim at full pattern analysis. Here, the challenge is combining rapid data acquisition with the counting statistics necessary for obtaining statistically

sound Rietveld results. The LYNXEYE is a 1-dimensional detector based on compound silicon strip technology. It allows for quick data collection without compromising the data quality. The intensity gain compared to a standard scintil-lation counter is almost a factor of 200, allowing extremely fast measurements with the resolution and the peak profile virtually identical to point detector measurements.

Diffraction data for Alu-bath full pattern analysis with TOPAS are collected with the D4 ENDEAVOR, Cu radiation and the LYNXEYE 1-dimensional detector plus an additional sealed proportional Ca-channel for fluorescence analysis. The total scan time is about 94 sec for the angular range 11° to 65° 2Theta. The fluorescence data are simultaneously collected. Therefore, the quality of the Ca-channel data is largely improved compared to single peak measurements. The complete measurement time including sample transfer is about 2:30 min.

The diffraction data are analyzed by the Rietveld method using DIFFRAC.TOPAS. The process relevant parameters such as ExAlF3, BR, or total CaF2 are adjacently computed with DQUANT. Furthermore, DQUANT allows including addi-tional information from the Ca-channel measurement.

TOPAS quantitative Rietveld phase analysis

XRD is the most direct and accurate analytical method for determining the presence and the absolute amounts of mineral species in a sample. There are several advantages of Rietveld phase analysis over conventional methods:

� Full pattern quantitative phase analysis applying the Rietveld method does generally not require time consum-ing calibration.

� Multi-phase samples are easily analyzed without being constrained by peak overlap.

� The adding of new phases found in qualitative XRD is straightforward.

� Additionally, crystallinity and crystallite size that influence the reactivity of the mineral components can simultane-ously be derived from the peak profiles.

Fast and reliable Rietveld based quantitative analysis became routinely possible by combining fast modern computer technology and optimised mathematical algorithms with the fundamental parameters approach [1] in the TOPAS software.

How to calculate free AlF3, bath ratio and the total CaF2 amount from the Rietveld results [9]

The amount of free AlF3 (ExAlF3) is conventionally defined as the %-amount of AlF3 in solution not bound as cryolite, Na3AlF6. Apart from cryolite other phases as chiolite or Ca-cryolites may be found in the congealed electrolyte.

Conventionally, ExAlF3 is calculated from the combination of the amounts of chiolite and CaF2 determined by quantitative phase analysis and the total Ca content (Catot) from the fluorescence channel, e.g. [2].

(1)

The CaFactor is the ratio Ex

jAlF3 to the mass fraction m

jCaF2 per formula unit of the j-th CaF2 bearing phase. The CaFactor is 0.717 for pure NaCaAlF6 and 0.487 for pure Na2Ca3Al2F14. Traditional quantitative analysis that is based on single peak evalu-ation can hardly determine the amount of NaCaAlF6 and Na2Ca3Al2F14 due to strong peak overlap of the two phases. That prevents a precise ExAlF3 determination. Therefore, the result is always biased by guessing the real phase content, which is essentially dependent on the cooling rate of the electrolyte. Typically, an intermediate CaFactor of 0.6 is assumed [2].

Quantitative phase analysis by the Rietveld method gives the phase content for all crystalline phases in the sample. Conse-quently, the CaFactor can exactly be calculated by weighting Ex

jAlF3 and the m

jCaF2 with the weight fractions wj of NaCaAlF6 and Na2Ca3Al2F14, respectively:

(2)

The Ex

jAlF3 (Tab. 1) are derived from the factors A and C in the decomposition of the respective phases into cryolite, AlF3 and CaF2 according to the stoichiometry given by the general formula

A phase ↔ B cryolite + C AlF3 + D CaF2,

the mass per formula unit mj of phase j, and the mass of AlF3 (83.98).

Straightforward, total ExtotAlF3 can directly be calculated from the Rietveld results without the need for a separate Ca-channel, just by summation over the theoretically determined Ex

jAlF3 of the j phases weighted by the amount of the phases (wj) in the mixture, obtained through the Rietveld quantitative phase analysis:

(3)

Table 1: Selected parameters of bath constituents.

Phase name Formula mj A B C D m jCaF2 Ex

jAlF3

Chiolite Na5Al3F14 461.87 3 5 4 0 0.0 0.2429

Ca-cryolite 1 NaCaAlF6 204.04 3 1 2 3 0.3826 0.2745

Ca-cryolite 2 Na2Ca3Al2F14 486.15 3 2 4 9 0.4818 0.2303

Furthermore, bath ratio and the total amount of CaF2 can directly be calculated from the quantitative Rietveld results. The bath ratio BR is defined as the weight ratio NaF/AlF3 in the electrolyte. As a result of the Rietveld refinement the amount of the mineral phases (wj) in the congealed electrolyte is obtained. Applying the known stoichiometry of the mineral phases BR is determined by:

(4)

The total amount of CaF2 in the electrolyte is

(5)

that can be compared to total-Ca from the fluorescence measurement.

( )2

tot3 chiolite CaFAlF 0.2429 CaEx w CaFactor w= + −

( )

( )

2

2

tot1 2 1 23 chiolite CaF

1 2 1 2

tot1 2chiolite CaF

1 2

0.2744 0.2303AlF 0.2429 Ca 0.3826 0.4818

0.2744 0.23030.2429 Ca0.3826 0.4818

w w w wEx w ww w w w

w ww ww w

+ += + −

+ ++

= + −+

( )tot3 3AlF w AlFj

jj

Ex Ex= ⋅∑

3

Cryolite Chiolite CaCryo1 CaCryo2


total NaFtotal AlF

0.6 0.454 0.172 0.2

0.4 0.546 0.345 0.411

BR

w w w w

w w w w

=

+ + +=

+ + +

22 CaF CaCryo2 CaCryo1total CaF 0.4817 0.3826w w w= + +

( )2


( )

( )

2

2


1 2 1 2

tot1 2chiolite CaF

1 2

0.2744 0.2303AlF 0.2429 Ca 0.3826 0.4818

0.2744 0.23030.2429 Ca0.3826 0.4818


w ww ww w

+ += + −

+ ++

= + −+

( )tot3 3AlF w AlFj

jj

Ex Ex= ⋅∑

3



total NaFtotal AlF

0.6 0.454 0.172 0.2

0.4 0.546 0.345 0.411

BR

w w w w

w w w w

=

+ + +=

+ + +


( )2


( )

( )

2

2


1 2 1 2

tot1 2chiolite CaF

1 2

0.2744 0.2303AlF 0.2429 Ca 0.3826 0.4818

0.2744 0.23030.2429 Ca0.3826 0.4818


w ww ww w

+ += + −

+ ++

= + −+

( )tot3 3AlF w AlFj

jj

Ex Ex= ⋅∑

3



total NaFtotal AlF

0.6 0.454 0.172 0.2

0.4 0.546 0.345 0.411

BR

w w w w

w w w w

=

+ + +=

+ + +


( )2


( )

( )

2

2


1 2 1 2

tot1 2chiolite CaF

1 2

0.2744 0.2303AlF 0.2429 Ca 0.3826 0.4818

0.2744 0.23030.2429 Ca0.3826 0.4818


w ww ww w

+ += + −

+ ++

= + −+

( )tot3 3AlF w AlFj

jj

Ex Ex= ⋅∑

3



total NaFtotal AlF

0.6 0.454 0.172 0.2

0.4 0.546 0.345 0.411

BR

w w w w

w w w w

=

+ + +=

+ + +


( )2


( )

( )

2

2


1 2 1 2

tot1 2chiolite CaF

1 2

0.2744 0.2303AlF 0.2429 Ca 0.3826 0.4818

0.2744 0.23030.2429 Ca0.3826 0.4818


w ww ww w

+ += + −

+ ++

= + −+

( )tot3 3AlF w AlFj

jj

Ex Ex= ⋅∑

3



total NaFtotal AlF

0.6 0.454 0.172 0.2

0.4 0.546 0.345 0.411

BR

w w w w

w w w w

=

+ + +=

+ + +


( )2


( )

( )

2

2


1 2 1 2

tot1 2chiolite CaF

1 2

0.2744 0.2303AlF 0.2429 Ca 0.3826 0.4818

0.2744 0.23030.2429 Ca0.3826 0.4818


w ww ww w

+ += + −

+ ++

= + −+

( )tot3 3AlF w AlFj

jj

Ex Ex= ⋅∑

3



total NaFtotal AlF

0.6 0.454 0.172 0.2

0.4 0.546 0.345 0.411

BR

w w w w

w w w w

=

+ + +=

+ + +


Bruker Alu-bath solution: electrolytic bath analysis using TOPAS Rietveld

Figure 2 shows a typical powder diffraction pattern from Alu-bath analysis together with the results from the TOPAS Rietveld quantitative analysis. The congealed electrolyte contains fluorite, corundum, cryolite [3], chiolite [4] and Ca-cryolite of two dif-ferent compositions [5,6]. Data from a TOPAS Rietveld refinement are exemplarily given in table 2.

Fig. 2: Typical powder XRD pattern of an Alu-bath sample together with the results from Rietveld analysis employing TOPAS V4. The measurement time is 90 sec, the agreement parameters of the model calculation and the experiment data are Rwp = 7.14 and GoF = 1.7.

Precision

The repeatability for 90 sec measurements was investigated for the Alcan reference materials BA-01 to BA-11. For each of the runs the sample was unloaded and reloaded to the diffractometer. Table 2 contains typical results averaged from the analysis of 12 scans. The wt%-quantity of the mineral phases, derived values such as the bath ratio BR, ExtotAlF3, total CaF2, and the respective standard deviations are given.

The reproducibility is very good with absolute standard devia-tions of the concentrations below 0.2%. The relative standard deviations of minor phases (amount of phase below 1%) seem large. However, this simply implies that the method is close to its detection limits.

Table 2: Average values of 12 measurements and their abso-lute and relative single standard deviations (SD).

Value / wt-%

SD Rel. SD / %

Corundum a-Al2O3 0.24 0.03 15.00

Fluorite CaF2 0.08 0.03 40.00

Cryolite Na3AlF6 59.01 0.15 0.25

Chiolite Na5Al3F14 31.08 0.19 0.62

Ca-cryolite NaCaAlF6 0.78 0.11 14.00

Na2Ca3Al2F14 8.80 0.09 1.09

Total CaF2 TOPAS 4.62 0.05 1.21

ExtotAlF3 TOPAS 9.76 0.04 0.44

BR TOPAS 1.156 0.001 0.10

Accuracy

1

1.1

1.2

1.3

1.4

1.5

1 1.1 1.2 1.3 1.4 1.5

Alcan standard

TOPA

S

Fig. 3: Accuracy plot for (1) bath ratio BR, (2) ExtotAlF3, and (3) total CaF2 and a-Al2O3. All standard deviations are smaller than the symbols. The symbols in (1) represent the TOPAS results (Eq. 4) plotted versus the Alcan reference data, the line represents the fit of a linear trend line. Filled circles in panel (2) represent values derived by the optimized Ca-channel method (Eq. 2) while open squares stand for the Rietveld TOPAS derived data (Eq. 3), the trend line for the Rietveld TOPAS data is also given. In panel (3), total CaF2 (Eq. 5) is given by filled squares and circles represent a-Al2O3. The line is the respective linear trend.

The Rietveld analysis with TOPAS was checked using eleven Alcan reference samples [7]. The excellent agreement between the TOPAS results and the reference values directly follows from figure 3. The scatter of the data points around the trend lines is very small. There is a strong linear correlation for the here determined concentrations, ExAlF3 and BR with the independently determined reference values. For the group of reference samples investigated here, the average dif-ference between the certified free CaF2 and the Rietveld determined values is below 0.5% which is just excellent.

The ExtotAlF3 values calculated according to Eq. 3 from the Rietveld determined quantities of chiolite and the two different types of Ca-cryolite also agree extremely well with the reference values. The aver-age deviation from the linear trend line is about 0.25 %. Conventional values derived from the Ca-channel but weighted with the proper concentrations for the Ca-cryolite closely resemble the pure TOPAS data. This proves the high reliability of the results derived from the Rietveld method. Finally, the bath ratio BR shows a brilliant agreement with reference data having a mean deviation as low as 0.02.

1 2

0

4

8

12

0 4 8 12Alcan standard / %

TOPA

S / %

3

The Bruker Alu-bath solution

� Automated push-button AXSLAB

� High-speed XRD data collection with the D4 ENDEAVOR process diffractometer using LYNXEYE detector and optionally a Ca fluo-rescence channel

� Full profile calculation (TOPAS Rietveld) as a new primary method in bath analysis

� Derivation of industry standard control parameters: Bath ratio, total CaF2, Excess AlF3

Fig. 4: Flow of an Alu-bath analysis in AXSLAB. The schematic arrangement of the cells (A) is mapped within the LabControlCenter, where unambiguous sample names are defined from the hall, raw and cell numbers, shift label and the day of the year. The measurement method is taken from a simple drop-down list (B) and a job list is created that is automatically executed (C). The resulting XRD patterns are analysed with TOPAS and results, stored in a database, are statistically evaluated (D), before the validated data are send to the electrolysis operator (E).

Conclusions

The LYNXEYE detector makes rapid data collection of complete diffraction patterns within just a few seconds pos-sible. The intensity gain of the detector facilitates the high sample throughput being indispensable for Rietveld quantita-tive analysis in aluminum industries. While the setup of the Rietveld analysis needs expert knowledge, the integration of preparation, measurement and data analysis in AXSLAB allows routine operation of the whole analysis process by non-specialists in the plant.

Standardless TOPAS analysis appears as the new primary standard method for the determination of electrolytic bath compositions. This breakthrough in Alu-bath analysis became possible by means of the fast and robust TOPAS algorithms. Consistent and most reliable results are obtained, irrespective of tube ageing, probing and preparation issues. Bath param-eters such as ExAlF3, BR or total CaF2 are calculated from the concentrations of the different crystalline phases in the electrolyte with outstanding accuracy and precision. Further-more, TOPAS analysis facilitates the quantification of calcium mixed-crystal phases in the electrolyte and sample properties such as preferred orientation or crystallite size are entirely considered.

The importance of the here presented bath analysis for the manufacturers was recently summarized by Frank R. Ferret of Alcan Int. Ltd., one of the world leading aluminum producers:

“The new methodology is seen as applicable to all types of bath; it is the most accurate and able to consistently produce the same result independently of the operator’s skill and the sample history.

… fast X-ray diffraction coupled with Rietveld inter-pretation will most likely constitute the future in bath analysis […].” [8]

In addition, TOPAS quantitative phase analysis improves the traditional determination of ExAlF3 from Ca-channel data. The data of the ExAlF3 analysis are redundant if the weight-ing scheme proposed in Eq. 2 is applied. In principal, the TOPAS method supersedes the use of the Ca-channel and finally, overcomes uncertainties introduced by the operator at sample taking by proper determination of the Ca-cryolite concentrations.

Automation with AXSLAB and TOPAS BBQ

The large amount of samples in an aluminum electrolysis plant calls for a high throughput solution with a maximum degree of auto-mation. AXSLAB provides the respective push-button solutions that may range from operating a standalone diffractometer up to the integration of automated preparation systems, sample transport, control of several XRD diffractometers and/or XRF spectrom-eters — including data analysis — into a laboratory information system.

The AXSLAB interface (Fig. 4) graphically maps the number of production halls, lines and cells at the customer site and therefore, allows to unambiguously assign the probed bath to a particular sample. The probing plans are stored as batches and can be repeated at any time.

The whole preparation, measurement and analysis process is controlled through AXSLAB. The calculated analysis data are stored in a SQL database and may automati-cally be limit checked or analyzed otherwise, according to the customer needs. Before the results are transferred to the electroly-sis operator a validation procedure allows identifying outliers due to difficulties with the sample collection from the bath. The respec-tive cells can be re-probed and AXSLAB’s high-priority measurement of these samples allow fast process decisions by the electroly-sis operator.

AXSLAB is easy to use and designed for non-technicians. Consequently, only mini-mum operator training is needed. The high throughput reduces the costs per analysis. The automated sample preparation ensures constant quality of the samples, which is the prerequisite for the quality of the results.

Keywords

XRD / quantitative phase analysis / TOPAS / Aluminum-bath analysis

Authors

Karsten Knorr, Elke Schwöbel, Heinz-Günter Granacher; Bruker AXS

References

[1] Cheary, R. W.; Coelho, A. A. & Cline, J. P.: Fundamental param-eters line profile fitting in laboratory diffractometers Journal of Research of the National Institute of Standards and Technology, 2004, 109, 1-25

[2] Lossius, L. P.; Hoie, H.; Pedersen, H. H. & Foosnaes, T.: Analysis of excess AlF3 - Harmonization in Hydro Aluminium. Peterson, R. D. (ed.) TMS 2000: Annual Meeting and Exhibition, TMS: The Minerals, Metals and Materials Society, 2000, 265-270

[3] Hawthorne, F. C. & Ferguson, R. B.: Refinement of the crystal structure of cryolite. Canadian Mineralogist, 1975, 13, 377-382

[4] Brosset, C.: Die Kristallstruktur des Chioliths. Zeitschrift für Anor-ganische und Allgemeine Chemie, 1938, 238, 201-208

[5] Courbion, G. & Ferey, G.: Na2Ca3Al2F14: A New Example of a Structure with "Independent F–" A New Method of Comparison between Fluorides and Oxides of Different Formula. Journal of Solid State Chemistry, 1988, 76, 426-431

[6] LeBail, A.; Hemon-Ribaud, A. & Courbion, G.: Structure of a-NaCaAlF6 determined ab initio from conventional powder diffraction data. European Journal of Solid State and Inorganic Chemistry, 1998, 35, 265-272

[7] Electrolytic Bath Standards, Alcan International Ltd., Quebec, Canada (2005)

[8] Ferret, F. R.: Breakthrough in Analysis of Electrolytic Bath Using Rietveld-XRD Method. TMS2008: Annual Meeting & Exhibition of the Minerals, Metals & Materials Society, 2008

[9] Knorr, K. & Kelaart, C.: Automated analysis of aluminum bath electrolytes by the Rietveld method. Minerals Engineering, 2009,22,434-439

Bruker AXS GmbH

Karlsruhe · GermanyPhone +49 721 5 09 97-0Fax +49 721 5 09 97-56 [email protected]

Bruker AXS Inc.

Madison, WI · USAPhone +1 800 234-XRAY Phone +1 608 276-3000 Fax +1 608 276-3006 [email protected]

www.bruker.com/x-ray

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