Features of Aluminum Bath in Rietveld Analysis for Minerals and Mining Industries

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 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 with Alcan reference data demonstrates the outstanding accuracy of the method. Furthermore, it is shown how to improve conventional Potflux analysis by the Rietveld method. Finally, the automated investigation of aluminium bath samples in AXSLAB is demonstrated.

Introduction to Aluminum Bath Analysis

Aluminum metal is produced from alumina (Al2O3) by electrolytic 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-Herault process).

Effect of Additives on the Properties of Alumina

The influence of additives on the performance of the electrolytic 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 electrical 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.

Bath Conditions for Quantitative XRD Analysis

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

Measurement Methods for Controlling Bath Composition

Two traditionally used measures for controlling the composition 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 %.

Crystalline phases appearing in the congealed aluminum electrolyte during electrolysis.

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

Determining the Bath Concentration

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.

Methods for Standardizing Samples

In order to obtain reproducible quantitative results the sample preparation needs to be standardized. Typically, the congealed 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 process. The whole procedure takes about 3 min. The pressed samples are either transported via conveyor belt to the diffractometer or collected at a sample tray.

Traditional Quantitative Bath Analysis

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.

Latest Development in Bath Analysis

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 scintillation counter is almost a factor of 200, allowing extremely fast measurements with the resolution and the peak profile virtually identical to point detector measurements.

Aluminium Bath Full Pattern Analysis with TOPAS

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 analysed by the Rietveld method using DIFFRACplus TOPAS. The process relevant parameters such as ExAlF3, BR, or total CaF2 are adjacently computed with DIFFRACplus DQUANT. Furthermore, DQUANT allows including additional information from the Ca-channel measurement.

DIFFRACplus 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 consuming 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 simultaneously 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 DIFFRACplus TOPAS software.

Electrolytic Bath Analysis Using TOPAS Rietveld Analysis

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 different compositions [5,6]. Data from a TOPAS Rietveld refinement are exemplarily given in table 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

Figure 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 of Rietveld Analysis with TOPAS

The repeatability for 90 sec measurements was investigated for the Alcan reference samples 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 deviations 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 absolute and relative single standard deviations (SD).

Reference Samples

Chemical Formula

Value / wt-%

SD

Rel. SD / %

Corundum

á-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 of Rietveld Analysis with TOPAS

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 difference 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 Cacryolite also agree extremely well with the reference values. The average 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.

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.

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.

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.

Figure 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.

Automated Preparation Systems for Aluminum Electrolysis Plants

The large amount of samples in an aluminum electrolysis plant calls for a high throughput solution with a maximum degree of automation. 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 spectrometers - 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.

AXSLAB - Automated Measurement and Analysis Process Control

The whole preparation, measurement and analysis process is controlled through AXSLAB. The calculated analysis data are stored in a SQL database and may automatically be limit checked or analyzed otherwise, according to the customer needs. Before the results are transferred to the electrolysis operator a validation procedure allows identifying outliers due to difficulties with the sample collection from the bath. The respective cells can be re-probed and AXSLAB's high-priority measurement of these samples allow fast process decisions by the electrolysis operator. AXSLAB is easy to use and designed for non-technicians. Consequently, only minimum 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.

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).

Figure 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).

Summary

The LynxEye detector makes rapid data collection of complete diffraction patterns within just a few seconds possible. The intensity gain of the detector facilitates the high sample throughput being indispensable for Rietveld quantitative 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 parameters 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. Furthermore, 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 interpretation 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 weighting 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.

This information has been sourced, reviewed and adapted from materials provided by Bruker X-Ray Analysis.

For more information on this source, please visit Bruker X-Ray Analysis.

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