GELDENHUIS, J.M.A., PH.ETORIUS, E.B., 'LIld D1PPENAAR, R.J, A simple experimentill technique fur the determination of MnO ;ll':livilic.~ 011

I600°C in (MnO-MgO-Si02) slags !iaturatcd with (MnO-MgO) solid sulutions. INri\CON 6. PmCl'ulillg.l· vjlJIl' I,I'III/lemolirmal Chromillm SIf'cI

and Alloys COllgre.\·.I'. Cape Towll. Volumc 2. Johanncsburg, SAIMM, 1992. pp. 105-110.

A Simple Experimental Technique for the Determinationof MnO Activities at 1600°C in (MnO-MgO-Si02) Slags

Saturated with (MnO-MgO) Solid Solutions

J.M.A. GELOENHUIS*, E.B. PRETORlUS*, and R.J. OIPPENAARt

* fscor Ltd, Pretoria, SOl/th AfricatUniversity a/Pretoria, Pretoria, SOlltll Africa

Oxide mixtures containing MilO, MgO, and Si02 were equilibrated with manganese metalat l600°C in MgO crucibles under an argon atmosphere, and the high-temperature phaseequilibria were preserved by rapid quenching of the crucibles and their contents to roomtemperature. The (MnO-MgO-Si02) slags in equilibrium with the manganese metal weresaturated with (MnQ-MgO) solid solutions at 1600°C, and the chemical composition ofthese phases were determined by electron-microprobe analysis. As theactivity--composition relationships in (MnO-MgO) solid solutions are known, the activityof MnO in the slag could be determined from the composition of the (MnO-MgO) solidsolutions in equilibrium with the slag. The MnO activities of the slags determined in thisway arc in good agreement with the MnO activities of (MnO-MgO-SiO,) slagsdetermined by previous workers using the gas-slag-metal equilibrium technique.

[21K= 1/2

'"{M'd( Po,)The change in standard free energy (.6.GO) for the reactiondepicted by equation [1.1 is -2,41795 x 10' 1/111012. Inequation [2] there are three unknowns: the activity ofmanganese in the alloy (aMn) the oxygen panial pressure(Po,), and the activity of MnO in the slag (a MaO).

Various techniques have been proposed and used todetennine the activity of MnO in slags. In one technique3-7,the molten slag is equilibrated with a manganese alloy forwhich the activity-composition relationships have beenestablished previously, for example Pt-Mn. Equilibrationis done in an atmosphere of known oxygen potential, andthe Mn content of the Pt-Mn alloy is determined afterequilibration. The activity of MnO can then be calculatedfrom equation [2J. A knowledge of the prevailing oxygenpotential in the system is usually acquired in one of twoways. The oxygen potentia] can be fixed by usc of a gasmixture of known oxygen activity or, if the oxygenpotential is not fixed, it can be measured instanwncolls!ywith an electrochemical oxygen probe. However. if theoxygen potential is fixed by use of a gas mixture.equilibrium between the gas, slag, and metal phases isachieved only after reaction periods varying between 12and 48 hours3·7.

If the electrochemical technique is used to determine (heprevailing oxygen potential within the melt, reactionperiods of only 30 to 60 minutes are required to establish

BackgroundThe governing equation for the determination of theactivity of MnO (aMno) in silicate melts is

(MIl)al/a)' +t[O,l,:=><MIlO>,'a, [I]

where < >, [ ], and ( ) denote the solid, gas, and liquidphases respectively. The equilibrium constant withreference to pure solid MnO can be expressed as

IntroductionIn steels of the SAE 200 series, nickel is partially rcplaced bymanganese. This type of stainless steel is an attractivealternative product from an economics point of view,especially when the price of nickel is high. The nickel pricehas fluctuated between 3 and 22 US dollars per kilogramduring the past five years l , causing instability in the price ofstainless steel. Such price instability can, to some extent, becounteracted by the production of SAE 200-type stainlesssteels. Manganese ore is freely available on the local marketand, with the exciting prospect of a direct route for theproduction of stainless steel, it is evidently of greatimportance that information should be available on thethermodynamic behaviour of manganese oxide in slags thatare likely to be used in the production of stainless steel. Suchinformation can be used to optimize the metallurgicalrefining process. There is little information in the literature2

on the thennodynamics of MilO in silicate-slag systems at1600°C, and there is consequently much inccntive todetermine the activity of MnO in slags of industrial interest.

DETERMINATION OF MnO ACTIVITIES AT 1600°C IN (MnO-MgO-SiO,l SLAGS 105

·equilibrium between the slag and the metalS-I'.A third method can be employed for the deten-nination of

the activity of MnO in silicate slags. The slag phase can besaturated with, and brought into equilibrium with, an MnO­containing solid solution of which the acl.ivity-compositionrelationship is known, and the activity of MnO in the slagcan then be calculated from the relationship

GMoo(slag) = GM"o(solid soL), [3]

where GMoo(slag) and GMoo(solid soL) are respectively theactivities of MnO in the slag and solid-solution phases withreference to pure solid MnO, If this partially moltenslag-solid solution system is equilibrated with an Mn­containing alloy of which the activity-compositionrelationships are not known but the oxygen partial pressureis known, then the aMn in the alloy can readily be calculatedfrom equation [2]. Alternatively, if the aMn in the metal isknown, the oxygen partial pressure of the system can becalculated,

The aim of the present study was to determine theactivity of MnO in partially liquid (MnO-MgO-Si02)

slags, These slags were saturated with (MnO-MgO) solidsolutions, and were brought into equilibrium withmanganese metal at 1600°C. The activity-compositionrelationships in (MnO-MgO) solid solutions, recentlydetermined by TsaP, were then used to calculate GMnD inthe slag from equation [3] and the oxygen potential 01' thesystem from equation [2]. The activities of MnOdetermined by this experimental technique, where the slagwas in equilibrium with Mn metal as well as with(MnO-MgO) solid solutions, were compared with theexperimental findings of previous researchers6.7 who usedthe gas-slag-metal equilibrium technique in which theoxygen partial pressure was fixed and the (lMn in the alloyknown.

The experimental technique used in this study could beutilized very elegantly to determine the feasibility andaccuracy of the electrochemical measuring technique. Theparlial pressure of oxygen determined by the use ofelectrochemical oxygen probes can be compared with thatcalculated in this study, since the melts are in equiLibriumwith Mn and the activity of MnO in the melts is known.Should it be possible to accurately verify the ability ofelectrochemical probes to determine the oxygen potential inthese Mn-MnO-MgO silicate-slag systems, theelectrochemical technique could easily be expanded tomelts where the activity of MnO is not known.

Experimental ProcedureReagent-grade oxides supplied by Merck (Pty) Ltd andSarchem (Pty) Ltd were used as starting materials, and thevarious slags were prepared by carefully weighing the exactamount of each reagent. The oxide mixtures were groundunder acetone to a particle size of less than 300 11m in anagate mortar. Then, I g of each of the oxide mixtures andmanganese metal powder were mixed thoroughly beforeequilibration, and were placed in a magnesia crucible(outside diameter 16 mm, inside diameter 10 mm, andheight 20 mm). The initial oxide mixtures were chosen sothat the crystalline phases of the slags would be at least 25per cent when equilibrium between the slag, the solidsolution, and the metal was attained 13 . This choice ensuredthat some of the (MnO-MgO) solid solution in equilibrium

106

with the slag precipitated onto the walls of the MgOcrucible to form a protective layer between the slag and thecrucible, thereby minimizing interaction with the crucible.

The porosity of commercially available MgO crucibles isapproximately 25 per cent, and preliminary tests showedthat the slag and metal penetrated these crucibles anddestroyed their integrity. It was therefore necessary tomanufacture MgO crucibles with a lower porosity in thelaboratory, This was accomplished in the followingmanner.

'Seawater' MgO, which contained approximately 3 percent A1 20 3, supplied by Vereeniging Refractories (Pty) Ltdas ball-mill fines, was ground to a particle size of less than100 11m. A slip, containing 0,2 ml of ethanol per gram ofMgO, was then slipcast into a plaster mould, The wallthickness of the eventual MgO crucible casting wasdetermined by the contact time of the slip with the mouldbefore decantation, and this contact period varied betweenI and 5 seconds to yield a wall thickness of approximatelyI to 2 mm, The plaster mould, which consisted of twosymmetrical parts, was then carefully removed from thecrucible casting, The casting was allowed to dry for 24hours at room temperature before it was heated at a rate ofIOO"C per hour to l700"C, sintered for 2 hours, and thencooled slowly. The porosity of the crucibles manufacturedin this way was approximately 15 per cent, and evaluationtests indicated that the crucible contained the slag andmetal satisfactorily and that little interaction between theslag-metal and the crucible occurred at 1600"C.

The slag and the metal were equilibrated within thesemagnesia crucibles in a vertical molybdenum-woundresistance furnace containing an alumina reaction tube andequipped with a PID temperature controller. High-purityargon gas was flushed through the alumina reaction tubeand the MgO crucible containing the metal and the slag,which was suspended in a molybdenum wire basket asshown schematically in Figure 1. The molybdenum basketwas suspended by a thin molybdenum wire (0,3 mm

J ..",.""f-----Galvanastat

~----Ma-wlre (0,9 mm)

f----- Rubber stopper

Watercooled brass ca

AI 20, sheath

1--- AI 20 3 reaction tube

FIGURE 1. Schematic illUslmtion of the crucible assembly for thequenching cxpcrimcnL<;

INCSAC 1

diameter), which, in turn, was fixed at the ends of twomolybdenum extension wires of 0,9 mm diameter as shownin the diagram. These wires were protected by an aluminasheath, and the whole assembly was lowered slowly intothe hot zone of the furnace at I600°C. The tempcrature wasregulated within ±3°C. In order to quench the crucible. anelectric current was pulsed by means of a galvanostutthrough the molybdenum-wire circuit, causing the thinmolybdenum wire to melt at the position where it was fixedto the basket. The cruciblc feU under gravitational forceinto a water bath at room temperature, the water balh beinglocated in a glass container and positioned at the bottom ofthe reaction tube.

The quenched crucible with its contents was cut in halfwith a diamond saw, and was then prepared for opticalmicroscopy and electron-microprobe analysis. Thechemical compositions of the individual phases weredetennined by usc of a JEOL 733 electron microprobe at anacccicrating potcntial of 25 kV and a beam current of 2,5nA. The counting time varicd between 60 and 100 seconds,and oxide and silicate standards werc used to quantify theanalyses of MnO, MgO, and SiO,. On average, threedifferent grains of each phase were analysed in detail.

Results and DiscussionBefore the equilibrium phase relationships could beobtained, it was necessary 10 determine the reaction timerequired to establish equilibrium between the(MnO-MgO-SiO,) slag, thc (MnO-MgO) solid solution,and the manganese metal. To achieve this. slags of twodifferent compositions containing at least 50 per cent MilOwere equilibrated with manganese metal for differentperiods (15, 30, 60, and 120 minutes) and qucnched aspreviously described. Metallographically prepared sectionsof the quenched slags were then analysed on themicroprobe. The composition of the liquid slag phase in thcquenched sample was determined and was found to be thesame in all the samples equilibrated for 30 minutes orlonger. The chemical composition of various grains of thesolid solution in each sample was also determined, and itwas also found to be essentially the same.

Accordingly, there is good reason to believe that a periodof 30 minutes is sufficient 10 altain equilibrium between theslag, the solid solution, and the metal. Furthcrmore theho":,ogeneity of the (MnO-MgO) solid-solution phaseeqUilibrated for 30 mmutes was checked in a number ofsequential analyses with an eiectron·microprobe beam alongstraight lines in several directions across the exposedsurface of several grains of the solid solution. No significantdeviations in the composition of the solid solutions wereobserved. confirming that equilibrium between the slag andthe metal is attained within 30 minutes. However, a reactiontime of 60 minutes was used in all subsequent experimentsto ensure that equilibrium was attained.

An example of a back-scattered electron image of aquenched sample is shown in Figure 2. Three diffcrent phasesare clearly discernible. The white phase consists of Mn metaldroplets, while the darker-grey, spherical phase represents thc(MnO-MgO) solid solution. The solid solution and themanganese metal are embedded in the slag. the colour ofwhich is slightly darker than that of the solid solution.If a proper interpretation is to be given to the

FIGURE 2. Back-scallered electron image of a quenched slag sample.showing the phases in equilibrium at 1600°C:

A = Quenched liquid slagB = (Mno-MgO) solid SOIUliol1

C = Quenched liquid manganese metal

metallographic features in these samples. it is essential thatthe structure observed at room temperature is indeedrepresentative of the phase equilibria that existed at1600°C. This prerequisite deserves some comment withregard to the experimental procedure followed in this study.It is of the utmost importance that the mass and volume ofthe crucible, and of its contents, should be minimized toensure that the phases that are in equilibrium at 1600°C areretained upon quenching to room temperature. Should thesamples not be quenchcd rapidly enough from 1600°c, theliqUId slag WIll crystallize partially and the composition ofthe slag phase Will not be homogeneous. However, extremecare wa~ taken in this study to ensure a sufficiently rapidquenching rate, and the homogeneity of the phasesachieved upon quenching, as detcnnined by the microprobeanalyses, indicated that the quenching rate was indeed rapidenough to fulfil this requirement.

!he chemical compositions of the different oxidemIxtures used as feed materials are summarized in Table Iand are superimposed in Figure 3 on an isothermal sectionat 1600°c of the MnO-MgO-SiO, system proposed byGlasser and Osborn 13. The mixtures 3M I, 3M2, and 3M3were selected to fall within the phase field where the(MnO-MgO) solid solution and the liquid are stable and inequilibrium at 1600°c, while mixture 3M4 was selected tofall within lhe phase field where the olivine the(MnO-MgO) solid solution, and the liquid are st~ble.Mixture 3M6 was chosen to contain the olivine and theliquid at equilibrium.

TABt.EICIIEMICAL COMPOSITIONS OF STARTING OXIDE MIXTURES AND THE

EXPECTED PI lASE CONSTITUTIONS AT IC.OO·C

3MI 3M2 3M3 3M4 3M6MgO.~ 7 18 25 J{) J{)

MnO.% T1 61 II 42 3'5;0,." 16 18 1JJ 28 38

Mctaladdcd. % 5/) 5/) 5/) 5/) 5/)

ExlXClCd phases at (Mfl~."'gO~+ (MnQ.~li,'O~+ (~InO-~lj,'Ol,..+ (MnQ.MsO~+ OIj\"i~+

equilibrium lJq11id liquid liquid liquid liquid

DETERMINATION OF MnO ACTIVITIES AT I600°C IN (MnO-MgO-SiO,) SLAGS 107

FIGURE 3. Equilibrium phase cOlllpusitiom ill the syslemMnO-MgO-Si02 in contact with rnl'tallil.: manganese at 1600°C as

determined in this study compared with thuse ohtained by Ghl.~scr andOsburn

'.'.MgSIO] ...

(O~>.... . ..,. '.

l 3.. ' .....A '..-....:::-:-._ ......

'\ (OJ)u..... -3~ ..,~~h&, A

\ ~ /' ; 'MJI \'"(MnO-MgO)s;" j A, J,Al ,

(o~" 1/: 1\(MnQ.MgO)ss "/:' (",;,no-~goj*!

Liquidus (Glasser and Osburn)Liquidus (This sludy)Conjugation lines (lliis study)Conjugation line (Glasser :lin! Osburn)Starting mixtures

M92 SiO. 'P"__

1.0

1-16000 C!0.8

0.6

0c::f

a 0.4

0.2

TABLE IIINORMALIZED COMPOSITION OFTIIE LIQUID I'!-IASE IN EQUILIBRIUM

WITII TIlE SOLID SOLUTION I\ND MANG,\NESE METAL (MOLARr:ni\C':TIONS), THE MnO CONTENT OF TilE SOUD SOLUTION. THE

ACTIVITY OF MnO. AND THE CALClJl.t\TED Po".VALlJES OF THESESLAGS ~

oMgO 0.2 0.4 0.6 0.8 MnO

FIGURE 4. AClivily-eOlllpositioll rdl1lilJ1lships of MnO in (MnO-MgO)sulid solutions at IGOnoe (llner TsaP)

..Mixture NMnO NSi02 N MgO N(MnO) lIMnO PO,no·

solid sol. [atm.1

3MI 052& 0.]00 0.172 0.76 0.78 1.983xlO·IJ

3M2 OA17 0.307 0,2(,6 0.56 Q61 1.253xllTlJ

3M3 O.J88 0195 OJI7 0.+1 0.5-1 9.5Ol'l:1O-15

3M4 llJ61 0185 OJ54 (t'-l 0,46 6.895xllT I5

The compositions of the liquid, olivine. and solidsolutions of the different quenched samples arcsummarized in Table II and are also shown in Figure 3.A1,0, levels of up to 5 per cent by mass were detected insome of the liquid slags, the compositions of these slagphases being indicated by triangles in Figure 3. (Thealumina content of the liquid slag probably originated fromlhe MgO crucible. because the MgO powder used in themanufacture of the crucibles contained approximately 4 percent alumina by mass.) The slag compositions indicated bythe solid circles within lhe centre of the triangles were usedin the conslruclion of a liquidus curve. The liquidus curvesdetermined by Glasser and Osborn 13 are also shown inFigure 3, and there is reasonably good agreement with theresults of the present study, indicating thai the limitedprescnce of A 1,03 in the slags of this study did not result ina serious discrepancy. Conjugation lines determined in thisstudy, which link the compositions of the equilibriulll slagand solid solution at 1600°C arc also shown. It is evidemIhal Ihe compositions of bOlh the final slag and the solid­solution phases of mixtures 3M I, 3M2, and 3M3 wereenriched in MgO compared with the starting mixtures.This does not apply to mixtures 3M4 and 3M6, which werechosen to canlain olivine at equilibrium. The higher MgOcontent in the final slag and the solid-solution phases ofmixtures 3MI, 3M2, and 3M3 is probably due to the initialreaction of the MgO crucible with the liquid slag. However,a protective layer 01" (MnO-MgO) solid solutionprecipitated on the cruciblc wall as thc reaction proceedcdulltil equilibriulll was :.tllaincd. A conjugation line betweenthe solid solution and the liquid, obtained by cxtrapolntionor Glasser and Osborn's data l3 to 1600°C, is also shown inFigure 3, and this conjugation line is likewise in goodagreement with the results orthe present study.

The activity of MnO in solid solutions of MnO and MgOat 1600'C has been determined by Tsai" and his results arcpresented in Figure 4. Tsai's data were used to determinethe activity of MnO in the slag from equation [3J, and theseactivities are summarized in Table m.

TABLE IICOMPOSITIONS OFTI-lE EQUILlllRJUM PHASES OFTI-lE SM"\I'LES THAT

WERE L~ EQUILIBRIUM WITI-l MANGANESE METAL AT 1600'C

rigurcs in bmckcl:\ IIldlCalc slandanJ dcvla!lon

3Mt 3M2 3M3Liquid Solid Liquid Solid Liquid Solid

soln soln soln

I MgO,% 93 15.2 I~' ]1.1 "'-9 42j

(01' (OJ) (0,1) til') (Ill (lj)

MoO." 58JI ... 10J "'9 .1(", sutOJI (OJ, (o.9) (0.7) Col) llj)

SiOl-':f l8.5 - ?J.9 - JAJJJ -(0.9) (IAI fl.O

Alp).';; U - 3.7 - !J -(o.S) (1.0) (0.9)

3M4 3M6Liquid Snlid Olivine Liquid Olivine

suln

M,oO." 2.\•.1 53.11 .jJ.l "", -I2J)

\1.2\ tU.!, 1U.1) to,li1 m.MMnO. Wi "'.6 "" 20.1 {l,1 IIJ.I

t111 (1l.!1 (011 1131 to.7,

Si01'~ 2'11 - "'-' JI.I .~,l)

(O,/\) (0,1) (2.1) (n,ll)

AI~O,l''.l> I.' - - 5,2 -10.') (UJ)

108 LNCSAC I

70

The saturation limit of MnO in the liquid component ofsome (MnO-MgO-Si02) slags determined in the presentstudy is shown in Figure 7 as a function of the MgO-to­MnO ratio at a constant Si02 content of approximately 30per cent by mass. As expected. the solubility of MnO in theslag decreases with an increase in the MgO-to-MnO ratio.The solubihty of MnO will have to be limited in slags thatarc to be lIsed in the production of steels of high manganesecontent. The results of the present study, as shown inFigure 7, are consequently experimental confirmation thatthis can be achieved through all increase in the MgO-to­MnO ratio in the slag.

80 .-----,--~-----,--_--__,

,,/

/,• Glasseralal. (1600

0 qO.H

The activity of MnO in the slag is shown in Figure 5 as afunction of the molar fraction of MnO. The activity ofMnO in slags in the MnO-MgO-SiO, system at lGOO°Chad not been determined previously and, for the sake ofcomparison, the results obtained by Mehta and Richardson6

at t650°C are included in Figure 5. The. activitiesdetermined in this study are in good agreement with thosedetermined by Mehta and Richardson for slags with similarchernical compositions. The composition of the liquid inthc MnO-Si02 binary system, whcre MnO saturation atI600°C is obtained, as dctcrmined by Glasscr 14, is alsoshown in Figure 5. This valuc is also in good agreementwith thc value determined in the present study byextrapolation of the experimental results.

1.0 '-'''''''','",'',"'""',""''''''',.,-----,---...,0..-----,• Molha and Richardson (1650°C)• Glasser (1600'Cj

• I " ~ ..v " J

60

0.6

0.4

0.2

LiquidiL'f limit

.11650'C J

• II

II,,

/

"

•

NSi01

=O.29

For melL'I saturated with

(MnO-MgO) solid solutions

50

40

T=1600 °c

30

0.0 "'-'----'------'------'----'-----'nn n? fld fl~ fiR 1"

FIGURE 5. Relationships between the activity or MnO and the MnOcuntent or (MnO-MgO-SiO l >slags s:lIurated with (MnO-MgO) solid

soluLions at 1600"C

?II '----'-__~ L___~__-..J

FIGURE 7. Rela1ionship between the MnO content and the MgO-to-MnO" c

ratio or the liquid phase ill the Mno-MgO-SiO~ system at 1600"C asdetermined in this study

0.0 '-~--'---~--'---~~-'---~--'---~-n" n~ nA ftC no

FIGURE 6. Relationship between the lltlivity or MnO :md the MnOwnlent of different MnO-eontaining sl,lgs at 160(Y'C

The activity of MnO as a function of the molar fractionof MnO in the binary MnO-Si02 system determined atl650°C by Abraham et al.' is shown in Figure G. The effectof CaO additions to the MnO-SiO, systems on the activityof MnO is highlighted, and shows that CaO increases theactivity of MnO. The resu.lts of the present study arc alsoshown in this diagram and indicate that additions of MgOalso increase the activity of MnO.

1.0 r-~--'-~~-'-~--'-~-r-~-

In conclusion. it was shown that Ihe activity of MnO in(MnO-MgO-SiO,J slags that are saturated with(MnO-MgOJ solid SOIUliol1s and in contact with puremolten manganese metal can be determined from adetermination of the chemical compositions of theconstituent phases in equilibrium at high temperature. Thisfinding is in good agreement with the results of previousworkers, who employed different experimental techniques.The results obtained in this study can also be used todetermine tbe oxygen potential of a slag-metal systemunder investigation. Accordingly. the experimentaltechniques used in this study can be utilized very elegantlyto evaluate the accuracy of the electrochemical techniquc tomeasure the oxygen activity in liquid slags of similarcomposition. The partial pressure of oxygen determinedwith the aid of electrochemical oxygen probes can becompared with the values determined in this study, since themolten slag is in equilibrium with liquid Mn metal and theaClivity or the MnO in the melts has been established. Theuse of the electrochemical technique could then easily beexpanded to melts where the activity of MnO is not known.

AcknowledgmentsThe authors express their sinccre appreciation to Iscor Ltd,to Mintek. and to the University of Pretoria for financialsupport and for the provision of laboratory facilit.ies. Theassistance of Dr J. Smuts with the SEM analyses and that ofMr S.D. McCollough with the quenching experiments aregratefully acknowledged.

o MnO+Si02+CaOG10/Si02=6 : 4

(Abraham el al.)

• MnO+SiO~+CaO

CaolSiO,_ 3:7

(Abraham et at)

6. MnO+SiO~

(Abraharn et al.)

o MnO+Si01+MgO (This ~tlJl1y)

0.6

0.8

0.2 -

DETERMINATION OF MnO ACTIVITIES AT IGOO°C TN (MnO-MgO-SiO,J SLAGS 109

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110

chemical sensing techniques for the determination ofactivity-<:omposition relations in liquid alloys and slagsat 1873 K. Ph.D. thesis, University of Pretoria, SouthAfrica.

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lNCSAC I