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Bruker AXSSolidelectrolyteTOPAS analysisSmelterLynxEyedetectorAN 380Quantitative XRD phase analysis inMinerals & Mining: Aluminum bathQuantitativeresultAbstractRietveld analysis emerged as a routine tool in quantitativephase analysis of crystalline powder samples. The reportdescribes the rapid analytical method from ultra-fast datacollection using the LynxEye TM detector to automatic Rietveldwith TOPAS. It is shown how to derive well establishedmeasures such as bath ratio and excess AlF 3 from theRietveld results. These measures widely required in aluminumproduction are readily determined with very highprecision.The excellent agreement with Alcan referencedata demonstrates the outstanding accuracy of the method.Furthermore, it is shown how to improve conventionalPotflux analysis by the Rietveld method. Finally, the automatedinvestigation of aluminium bath samples in AXSLAB isdemonstrated.Introduction to Aluminum bath analysisAluminum metal is produced from alumina (Al 2 O 3 ) by electrolyticreduction. The melting point of alumina is above 2300 K,which renders the direct production of aluminum fromalumina uneconomic. Instead, alumina is decomposed in acryolite (Na 3 AlF 6 ) electrolyte at about 1230 K (Hall-Héraultprocess).The influence of additives on the performance of the electrolyticcell (aka bath or pot) is manifold. The admixture ofCaF 2 and AlF 3 further decreases the melting temperatureof alumina, therefore reducing the energy consumption.However, the solubility of alumina decreases concomitantly,thus decreasing the efficiency of the pot. Alumina additionalso needs to be monitored since to low concentrations resultin “anode effect”-failure of the bath while sludge formationis observed for to high concentrations. The addition of LiF tothe bath also increases the efficiency by reducing the liquidustemperature and increasing the conductivity of the bath. Thepresence of sodium in the electrolyte increases the electricalconductivity and hence, decreasing efficiency of the pot.In summary, the operating conditions of the bath need to befound by optimising the six component system Al-Na-Mg(Li)-Ca-F-O.The bath conditions are inferred from quantitative XRDanalysis of the congealed electrolyte that may contain mineralphases such as cryolite (Na 3 AlF 6 ), chiolite (Na 5 Al 3 F 14 ), Cacryolite(NaCaAlF 6 or Na 2 Ca 3 Al 2 F 14 ), fluorite (CaF 2 ), weberite(N a 2 MgAlF 7 ), neighborite (NaMgF 3 ), corundum (Al 2 O 3 ), spinel(MgAl 2 O 4 ), villiaumite (NaF), and others.

Two traditionally used measures for controlling the compositionof the bath are the bath ratio BR (defined as the weightratio NaF/AlF 3 ) and excess AlF 3 , ExAlF 3 . The ratios of somephases in the crystallized electrolyte are shown in Fig. 1.Once the chiolite (Na 5 Al 3 F 14 ) lines disappear the bath isdepleted of aluminium ions and alumina (Al 2 O 3 ) has to beadded. Typically, aluminum is deposited within the BR range1.1 — 1.4. For pure cryolite (BR = 1.5) ExAlF 3 is 0 % and forpure chiolite (BR = 0.833) the ExAlF 3 value is 24.24 %.Fig. 1:Crystalline phases appearing in the congealed aluminumelectrolyte during electrolysis.Experimental requirementsThe determination of the bath concentration in the plant istypically repeated every two to three days. Since there arehundreds to thousands of baths, the available time for themeasurement and data analysis is just several minutes.In order to obtain reproducible quantitative results the samplepreparation needs to be standardized. Typically, the congealedsample is roughly crushed in a small crusher and thesample is automatically screened for pure metal parts thatmay disturb the subsequent automated mill and press process.The whole procedure takes about 3 min. The pressedsamples are either transported via conveyor belt to the diffractometeror collected at a sample tray.Traditional quantitative analysis based on single diffractionpeaks is fast with measurement times below 100 seconds.However, bath analysis based on single peak analysis ishampered by peak overlap of several phases, texture etc.Therefore latest developments aim at full pattern analysis.Here, the challenge is combining rapid data acquisition withthe counting statistics necessary for obtaining statisticallysound Rietveld results. The LynxEye is a 1-dimensional detectorbased on compound silicon strip technology. It allows forquick data collection without compromising the data quality.The intensity gain compared to a standard scintillation counteris almost a factor of 200, allowing extremely fast measurementswith the resolution and the peak profile virtuallyidentical to point detector measurements.Diffraction data for Alu-bath full pattern analysis with TOPASare collected with the D4 ENDEAVOR, Cu radiation and theLynxEye 1-dimensional detector plus an additional sealedproportional Ca-channel for fluorescence analysis. Thetotal scan time is about 94 sec for the angular range 11° to65° 2Theta. The fluorescence data are simultaneously collected.Therefore, the quality of the Ca-channel data is largelyimproved compared to single peak measurements. The completemeasurement time including sample transfer is about2:30 min.The diffraction data are analysed by the Rietveld methodusing DIFFRAC plus TOPAS. The process relevant parameterssuch as ExAlF 3 , BR, or total CaF 2 are adjacently computedwith DIFFRAC plus DQUANT. Furthermore, DQUANT allowsincluding additional information from the Ca-channel measurement.DIFFRAC plus TOPAS quantitative Rietveld phaseanalysisXRD is the most direct and accurate analytical method fordetermining the presence and the absolute amounts ofmineral species in a sample. There are several advantages ofRietveld phase analysis over conventional methods:• Full pattern quantitative phase analysis applying theRietveld method does generally not require time consumingcalibration.• Multi-phase samples are easily analyzed without beingconstrained by peak overlap.• The adding of new phases found in qualitative XRD isstraightforward.• Additionally, crystallinity and crystallite size that influencethe reactivity of the mineral components can simultaneouslybe derived from the peak profiles.Fast and reliable Rietveld based quantitative analysis becameroutinely possible by combining fast modern computertechnology and optimised mathematical algorithms withthe fundamental parameters approach [1] in the DIFFRAC plusTOPAS software.

How to calculate free AlF 3 , bath ratio and the total CaF 2 amount from the Rietveld resultsThe amount of free AlF 3 (ExAlF 3 ) is conventionally defined as the %-amount of AlF 3 in solution not bound as cryolite, Na 3 AlF 6 .Apart from cryolite other phases as chiolite or Ca-cryolites may be found in the congealed electrolyte.Conventionally, ExAlF 3 is calculated from the combination of the amounts of chiolite and CaF 2 determined by quantitative phaseanalysis and the total Ca content (Ca tot ) from the fluorescence channel, e.g. [2].(1)ExAlF 0.2429w CaFactor Ca wtot3 chiolite CaF2The CaFactor is the ratio Ex j AlF 3 to the mass fraction m j CaF 2 per formula unit of the j-th CaF 2 bearing phase. The CaFactor is0.717 for pure NaCaAlF 6 and 0.487 for pure Na 2 Ca 3 Al 2 F 14 . Traditional quantitative analysis that is based on single peak evaluationcan hardly determine the amount of NaCaAlF 6 and Na 2 Ca 3 Al 2 F 14 due to strong peak overlap of the two phases. That0.2744w10.2303w2 w1w2totprevents a precise ExAlF 3 determination. 0.2429wchiolite Therefore, the result is always biased by guessing Cathe wCaFreal 2 phase content, which isw1w2 0.3826w10.4818w2essentially dependent on the cooling rate of the electrolyte. Typically, an intermediate CaFactor of 0.6 is assumed [2].0.2744w10.2303w2tot totEx 0.2429 AlF3 w0.2429 chiolitewchiolite CaFactor CaCa wwCaF CaF20.3826w210.4818w2Quantitative phase analysis by the Rietveld method gives the phase content for all crystalline phases in the sample. Consequently,the CaFactor can exactly be calculated by weighting Ex j AlF 3 and the m j CaF 2 with the weight fractions w j of NaCaAlF 6and Na 2 Ca 3 Al 2 F 14 , respectively:(2) tot0.2744w 0.2303w w wExAlF AlF 0.2429w ExAlFCa wj1 2 1 2tot33 jchiolite 3CaF2jw1w2 0.3826w10.4818w20.2744w 0.2303w1 2 tot0.2429wchiolite Ca 0.3826w10.4818w2w2CaFtotExAlF The Ex j 30.2429wchiolite CaFactor Ca wCaF2AlF 3 (Tab. 1) are total derived NaFBR from the factors A and C in the decomposition of the respective phases into cryolite, AlF 3 andCaF 2 according to the stoichiometry total AlF3given by the general formulatotjEx 0.6 AlF wCryolite 3 w 0.454jExwChiolite AlF 0.172 wCaCryo1 0.2wCaCryo23j0.4w A phase ↔ B cryolite + C AlF 3 + D CaF 2 ,Cryolite0.546wChiolite 0.2744 0.345 wCaCryo1 10.2303 0.411w wCaCryo22w1w2totExAlF 30.2429 chiolite totCawCaFExAlF 230.2429wchiolite CaFactor w1Ca w w2 CaF 20.3826w10.4818w2the mass per formula unit m j of phase j, and the mass of AlF 3 (83.98). tot0.2744 ExAlF w130.23030.2429 wwchiolite CaFactor Ca wtotCaF 220.2429wStraightforward, total Ex chiolite CawCaF2total AlF NaF 3 can0.3826directly w1be 0.4818calculated w2from the Rietveld results without the need for a separate Ca-channel,BR tottotaljust by summation Ex over AlF CaFtotal the 32 0.2429 AlFwtheoreticallyCaF w0.4817 chiolitedetermined CaFactor w CaEx j 3AlF 2CaCryo2 0.3826wCaCryo1CaF 3 of 2 the j phases weighted by the amount of the phases (w j ) in themixture, obtained through 0.6w the Rietveld quantitative0.2744w1phase0.2303w analysis:2w1w2totExAlF Cryolite0.454wChiolite 0.172wCaCryo1 0.2w30.2429wchiolite CaCryo2CawCaF2w1w2 0.2744 0.3826 w1w0.230310.4818w2 w2w1w2tot0.4wCryolite 0.546wExAlF Chiolite0.345 30.2429 w wCaCryo1chiolite 0.411 wCawCaCryo2CaF 2totj(3)Ex AlF3 w0.2744 jExAlF w130.2303ww2 tot1w2 0.3826w10.4818w20.2429wchiolite CawjCaF 20.3826w0.2744 10.4818 w w10.2303 20.2744 ww2 10.2303w21wtot2totExAlF 30.2429w 0.2429 chiolite wchiolite CawCaF2Table 1: Selected parameters of bath constituents.CawCaF2w1w0.3826w2 10.4818 0.3826w210.4818w2Phase name Formula m j0.2744wA B C D m j CaF 2 Ex j 10.2303w2totAlFtotal 0.2429 CaFw32chiolitew CaF0.4817w 2CaCryo2 0.3826 CawwCaCryo1CaF2Chiolite Na 5 Al 3 F 14total NaF461.87 0.3826w3 10.4818 5 w4 2 0 0.0 0.2429totjEx BR AlF 3Ca-cryolite 1 NaCaAlFtotal 6AlF 204.04w jExAlF331 2 3 0.3826 0.2745j 3totjEx AlF3 Ca-cryolite 2 Na 2 Ca0.6 3 Alw 2 F 14 486.15 3 2 w jExAlF34 9 0.4818 0.2303Cryolite0.454wChiolite 0.172 j wCaCryo1 0.2wCaCryo2tot 0.4wCryolite 0.546wChiolite j 0.345wCaCryo1 0.411wCaCryo2Furthermore, bath Ex ratio AlF and 3the total wjExamount AlF3of CaF 2 can directly be calculated from the quantitative Rietveld results. The bathjratio BR is defined as the total weight NaF ratio NaF/AlF 3 in the electrolyte. As a result of the Rietveld refinement the amount of theBR mineral phases (w j ) in the total congealed AlF electrolyte is obtained. Applying the known stoichiometry of the mineral phases BR is3total NaFBR determined by: 0.6wCryolite 0.454w totalChiolite0.172 AlFw30.2wCaCryo2total CaF2 wCaF 0.4817w 2CaCryo2 0.3826wCaCryo10.4wtotal NaFCryolite0.546w 0.6Chiolitew0.345 Cryolitew0.454 CaCryo1w0.411Chiolitew0.172 wCaCryo2CaCryo10.2wCaCryo2(4)BR total AlF30.4wCryolite 0.546wChiolite 0.345wCaCryo1 0.411wCaCryo2The total amount of CaF 0.6w 2Cryolite in the electrolyte 0.454wChiolite is 0.172wCaCryo1 0.2wCaCryo20.4wCryolite 0.546wChiolite 0.345wCaCryo1 0.411wCaCryo2(5)total CaF2 wCaF 0.4817w 2CaCryo2 0.3826wCaCryo1total CaF2 wCaF 0.4817w 2CaCryo2 0.3826wCaCryo1that can be compared to total-Ca from the fluorescence measurement.total CaF w 0.4817w 0.3826w2 CaF2CaCryo2 CaCryo1

Bruker AXS Alu-bath solution: electrolytic bath analysis using TOPAS RietveldFigure 2 shows a typical powder diffraction pattern from Alu-bath analysis together with the results from the TOPAS Rietveldquantitative analysis. The congealed electrolyte contains fluorite, corundum, cryolite [3], chiolite [4] and Ca-cryolite of two differentcompositions [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 analysisemploying TOPAS V4. The measurement time is 90 sec, the agreement parameters of the modelcalculation and the experiment data are Rwp = 7.14 and GoF = 1.7.PrecisionThe repeatability for 90 sec measurements was investigatedfor the Alcan reference samples BA-01 to BA-11. For eachof the runs the sample was unloaded and reloaded to thediffractometer. Table 2 contains typical results averaged fromthe analysis of 12 scans. The wt%-quantity of the mineralphases, derived values such as the bath ratio BR, Ex tot AlF 3 ,total CaF 2 , and the respective standard deviations are given.The reproducibility is very good with absolute standard deviationsof the concentrations below 0.2%. The relative standarddeviations of minor phases (amount of phase below 1%)seem large. However, this simply implies that the method isclose to its detection limits.Table 2: Average values of 12 measurements and their absoluteand relative single standard deviations (SD).Value /wt-%SD Rel. SD /%Corundum a-Al 2 O 3 0.24 0.03 15.00Fluorite CaF 2 0.08 0.03 40.00Cryolite Na 3 AlF 6 59.01 0.15 0.25Chiolite Na 5 Al 3 F 14 31.08 0.19 0.62Ca-cryolite NaCaAlF 6 0.78 0.11 14.00Na 2 Ca 3 Al 2 F 14 8.80 0.09 1.09Total CaF 2 TOPAS 4.62 0.05 1.21Ex tot AlF 3 TOPAS 9.76 0.04 0.44BR TOPAS 1.156 0.001 0.10

Accuracy1.51152TOPAS1.41.31.2Bruker AXS / %105y = 0.9377xR 2 = 0.99761.111 1.1 1.2 1.3 1.4 1.5Alcan standard00 5 10 15Alcan standard / %TOPAS / %1284030 4 8 12Alcan standard / %Fig. 3:Accuracy plot for (1) bath ratio BR,(2) Ex tot AlF 3 , and (3) total CaF 2 and a-Al 2 O 3 .All standard deviations are smaller than thesymbols. The symbols in (1) represent theTOPAS results (Eq. 4) plotted versus theAlcan reference data, the line representsthe fit of a linear trend line. Filled circles inpanel (2) represent values derived by theoptimized Ca-channel method (Eq. 2) whileopen squares stand for the Rietveld TOPASderived data (Eq. 3), the trend line for theRietveld TOPAS data is also given. In panel(3), total CaF 2 (Eq. 5) is given by filled squaresand circles represent a-Al 2 O 3 . The line is therespective linear trend.The Rietveld analysis with TOPAS was checked using eleven Alcanreference samples [7]. The excellent agreement between the TOPASresults and the reference values directly follows from figure 3. Thescatter of the data points around the trend lines is very small. Thereis a strong linear correlation for the here determined concentrations,ExAlF 3 and BR with the independently determined reference values.For the group of reference samples investigated here, the average differencebetween the certified free CaF 2 and the Rietveld determinedvalues is below 0.5% which is just excellent.The Ex tot AlF 3 values calculated according to Eq. 3 from the Rietvelddetermined quantities of chiolite and the two different types of Cacryolitealso agree extremely well with the reference values. The averagedeviation from the linear trend line is about 0.25 %. Conventionalvalues derived from the Ca-channel but weighted with the properconcentrations for the Ca-cryolite closely resemble the pure TOPASdata. This proves the high reliability of the results derived from theRietveld method. Finally, the bath ratio BR shows a brilliant agreementwith reference data having a mean deviation as low as 0.02.The Bruker AXS Alu-bathsolution• Automated push-button AXSLAB• High-speed XRD data collectionusing LynxEye detector• Full profile calculation (TOPASRietveld) as a new primarymethod in bath analysisDerivation of industry standard•control parameters: Bath ratio,total CaF 2 , Excess AlF 3

Fig. 4:Flow of an Alu-bath analysis in AXSLAB. The schematic arrangement of the cells (A) is mapped within theLabControlCenter, 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) anda job list is created that is automatically executed (C). The resulting XRD patterns are analysed with TOPASand results, stored in a database, are statistically evaluated (D), before the validated data are send to theelectrolysis operator (E).

Automation with AXSLAB andD I F F R AC plus TOPAS BBQThe large amount of samples in an aluminumelectrolysis plant calls for a high throughputsolution with a maximum degree of automation.AXSLAB provides the respectivepush-button solutions that may range fromoperating a standalone diffractometer upto the integration of automated preparationsystems, sample transport, control of severalXRD diffractometers and/or XRF spectrometers— including data analysis — into alaboratory information system.The AXSLAB interface (Fig. 4) graphicallymaps the number of production halls, linesand cells at the customer site and therefore,allows to unambiguously assign the probedbath to a particular sample. The probing plansare stored as batches and can be repeated atany time.The whole preparation, measurementand analysis process is controlled throughAXSLAB. The calculated analysis data arestored in a SQL database and may automaticallybe limit checked or analyzed otherwise,according to the customer needs. Beforethe results are transferred to the electrolysisoperator a validation procedure allowsidentifying outliers due to difficulties with thesample collection from the bath. The respectivecells can be re-probed and AXSLAB’shigh-priority measurement of these samplesallow fast process decisions by the electrolysisoperator.AXSLAB is easy to use and designed fornon-technicians. Consequently, only minimumoperator training is needed. The highthroughput reduces the costs per analysis.The automated sample preparation ensuresconstant quality of the samples, which is theprerequisite for the quality of the results.ConclusionsThe LynxEye detector makes rapid data collection of completediffraction patterns within just a few seconds possible.The intensity gain of the detector facilitates the high samplethroughput being indispensable for Rietveld quantitativeanalysis in aluminum industries. While the setup of theRietveld analysis needs expert knowledge, the integrationof preparation, measurement and data analysis in AXSLABallows routine operation of the whole analysis process bynon-specialists in the plant.Standardless TOPAS analysis appears as the new primarystandard method for the determination of electrolytic bathcompositions. This breakthrough in Alu-bath analysis becamepossible by means of the fast and robust TOPAS algorithms.Consistent and most reliable results are obtained, irrespectiveof tube ageing, probing and preparation issues. Bath parameterssuch as ExAlF 3 , BR or total CaF 2 are calculated fromthe concentrations of the different crystalline phases in theelectrolyte with outstanding accuracy and precision. Furthermore,TOPAS analysis facilitates the quantification of calciummixed-crystal phases in the electrolyte and sample propertiessuch as preferred orientation or crystallite size are entirelyconsidered.The importance of the here presented bath analysis for themanufacturers was recently summarized by Frank R. Ferret ofAlcan Int. Ltd., one of the world leading aluminum producers:“The new methodology is seen as applicable to alltypes of bath; it is the most accurate and able toconsistently produce the same result independentlyof the operator’s skill and the sample history.… fast X-ray diffraction coupled with Rietveld interpretationwill most likely constitute the future in bathanalysis […].” [8]In addition, TOPAS quantitative phase analysis improves thetraditional determination of ExAlF 3 from Ca-channel data.The data of the ExAlF 3 analysis are redundant if the weightingscheme proposed in Eq. 2 is applied. In principal, theTOPAS method supersedes the use of the Ca-channel andfinally, overcomes uncertainties introduced by the operatorat sample taking by proper determination of the Ca-cryoliteconcentrations.

KeywordsXRD / quantitative phase analysis / TOPAS / AluminumbathanalysisAuthorsKarsten Knorr, Elke Schwöbel, Heinz-Günter Granacher;Bruker AXSReferences[1] Cheary, R. W.; Coelho, A. A. & Cline, J. P.: Fundamentalparameters line profile fitting in laboratory diffractometersJournal of Research of the National Institute of Standardsand Technology, 2004, 109, 1-25[2] Lossius, L. P.; Hoie, H.; Pedersen, H. H. & Foosnaes, T.:Analysis of excess AlF 3 - Harmonization in Hydro Aluminium.Peterson, R. D. (ed.)TMS 2000: Annual Meeting and Exhibition, TMS: TheMinerals, Metals and Materials Society, 2000, 265-270[3] Hawthorne, F. C. & Ferguson, R. B.: Refinement of thecrystal structure of cryolite. Canadian Mineralogist, 1975,13, 377-382[4] Brosset, C.: Die Kristallstruktur des Chioliths. Zeitschriftfür Anorganische und Allgemeine Chemie, 1938, 238,201-208[5] Courbion, G. & Ferey, G.: Na 2 Ca 3 Al 2 F 14 : A New Exampleof a Structure with "Independent F – " A New Method ofComparison between Fluorides and Oxides of DifferentFormula. Journal of Solid State Chemistry, 1988, 76,426-431[6] LeBail, A.; Hemon-Ribaud, A. & Courbion, G.: Structureof a-NaCaAlF 6 determined ab initio from conventionalpowder diffraction data. European Journal of Solid Stateand 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 BathUsing Rietveld-XRD Method.TMS2008: Annual Meeting & Exhibition of the Minerals,Metals & Materials Society, 2008All configurations and specifications are subject tochange without notice. Order No. DOC-A88-EXS020.© 2008 Bruker AXS. Printed in Germany.Bruker AXS GmbHKarlsruhe, GermanyPhone +49 (7 21) 5 95-28 88Fax +49 (7 21) 5 95-45 87info@bruker-axs.dewww.bruker-axs.deBruker AXS Inc.Madison, WI, USAPhone +1 (800) 234-XRAYPhone +1 (608) 276-3000Fax +1 (608) 276-3006info@bruker-axs.comwww.bruker-axs.com