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A kinetic study of the strontium extraction by metallothermic reduction using submerged SrO powders injection

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A kinetic study of the strontium extraction by metallothermic reduction using submerged SrO powders injection
  A kinetic study of the strontium extraction by metallothermic reduction usingsubmerged SrO powders injection R. Muñiz  a, ⁎ , A. Flores  a  , J. Torres  a  , S. Luna  a  , N. Rodríguez  b a  CINVESTAV Unidad Saltillo, Saltillo-Monterrey highway Km. 13.5 P.O. Box 663, 25000, Saltillo, Coahuila, México  b  Instituto Tecnológico de Saltillo V. Carranza Blvd. 2500, Saltillo, Coahuila, México Received 3 May 2007; accepted 8 June 2007Available online 16 June 2007 Abstract This work reports the results of laboratory experiments conduced to follow the kinetics of strontium recovery into the Al – Mg alloy bymetallothermic reduction of SrO. The reagent was incorporated to molten alloy by the use of submerged powders injection technique. Thevariables analyzed were the injection time, the melt temperature and the initial magnesium content. Magnesium is added to the melt to increase thereactivity and reduce the surface tension of the molten aluminum. It was possible to increase the strontium content from 0 to 5 wt.% after 60 minof treatment. The results were fitted to a general kinetic equation, which allowed it to obtain the kinetic parameters, i.e. order of reaction andactivation energy of the process. As the main mechanism of the strontium recovery process is of diffusive type, the global process rate increases asthe temperature and initial amount of the magnesium increased.© 2007 Elsevier B.V. All rights reserved.  Keywords:  Strontium recovery; Submerged powder injection; Metallothermic reduction; Reaction kinetic 1. Introduction Until recently, strontium was one of the less important elements from the production volume point of view. Thissituation has been changed due the increased application for strontium in the automobile and aerospace industries resultingin growing demand for the metal. This metal is frequently usedin master alloy form, such as Al – Sr and Al – Sr  – Mg [1].Strontium is a reactive metal making it difficult and costly to produce except under carefully controlled processes. Nowadays, the most effective method for strontium production is the metallothermic reduction of its oxide. This process, named  “ Melt Leach Evaporation ”  (MLE), is under development for the extraction of valuable Group IIA metals of the Periodic Table. The MLE process consists of mixing andcontacting the value metal source material, which might be anore or concentrate, with an excess of molten metal which isacting as a reductant and lixiviant. In the process, the valuemetal extracted from the source material is dissolved in theexcess molten metallic solvent and is subsequently extracted asa vapour by vacuum distillation [2]. For the extraction of strontium from its oxide, different methods have beendeveloped, which work under vacuum conditions and hightemperatures. These conditions increase the production cost.In light of the foregoing, the process of strontium extractionfrom SrO through the use of submerged powders injection of strontium oxide is very attractive alternative from both thetechnological and the economic points of view. So in this paper,the results obtained about the feasibility to incorporate metallicstrontium to Al – Mg liquid alloy from metallothermic reductionof strontium oxide at scale laboratory are presented. 2. Materials and methods The experimental trials were carried out in a high frequencyinduction furnace 15 kg molten aluminum capacity. The powders injection equipment, whose scheme is presented inFig. 1, allowed the continuous and controlled feeding of solidmaterial through an inert carrier gas. This equipment isfrequently used in the process of Mg and Sb removal frommolten aluminum alloy [3,4]. The reactants used in theexperimentswere asfollows:Al(99% purity); Mg(99% purity);SrO ( − 70+140 mesh [5]).  Available online at Materials Letters 62 (2008) 637 –  ⁎  Corresponding author. Tel./fax: +52 844 4389600.  E-mail address: (R. Muñiz).0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2007.06.024  The selected experimental factors and their levels weretemperature (973, 1023, 1073 K), magnesium content in thealloy (1, 2, 3 wt.%) and injection time (15, 30, 45, 60 min). Tosimplifytheprocessofpowderinjection,thefollowingparameterswere kept constant: amount of liquid alloy: 10±0.1 kg; powder flowratetocarriergasflowrateratio:17gSrO/min/12LAr/min.The typical experimentaltrial was conducted as follows: First,the alloy with the initial chemical composition was melted in theinduction furnace. Then, the injection system was attached to thefurnaceandfixed.Thegraphitelancewassubmergedintothemelt at the specified depth and position (Fig. 1) [6]. Argon gas (UHP 99.99%)wasblownintothemeltfor3minbeforeintroducingthe particles of the reactive powder to avoid powder clogging. Nofluxing or degassing of the melt was performed after melting, sothe alloy was only treated for strontium increase.Finally, to obtain kinetic information and metallographic,samples were taken at 15 min intervals and analyzed by usinginductively coupled plasma spectrometry (ICP) and ScanningElectron Microscope (SEM) respectively. Dross samples weretaken at the end of each experiment and analyzed by X-raydiffraction (XRD) for the qualitative identification of thecrystalline compounds. 3. Results and discussion 3.1. Chemical analysis From the chemistry analysis results (Fig. 2a) is observed that strontium oxide is being reduced by metallothermic mechanism, because the present strontium content in the liquid bath is increasedthrough injection time. Strontium content in the Al – Mg alloy wasincreased to 5 wt.% by using the following conditions: temperature,1073 K; magnesium content, 3 wt.%; particle size, − 70+140 mesh; the powder flow rate to carrier gas flow rate ratio, 17 g SrO/min/12 L Ar/ min; and pressure, 1 atm (out vacuum).The effect of temperature and initial concentration of magnesium onthe strontium recovery rate could be observed in Fig.2a.As can be seenfor all cases, increasing temperature, the reaction rate between SrO particle and melt alloy is increased, obtaining higher final strontiumlevels in the treated alloy. This behavior may be explained due to thethermally activated nature of the mechanisms governing the kinetics of the process, diffusion and chemical reaction [7].Theratesofstrontiumrecoverytothemeltwithmagnesiumadditionsincreasedsteadilywithtimeforallexperiments,suchasshowninFig.2a.A previous investigation on strontium recovery from SrCO 3 , establishedthat magnesium improved the rate and extent of extraction by excessmolten aluminum. These experiments were performed in a mechanicallystirred molten aluminum bath with different levels of magnesium at 1273 K. It concluded that the beneficial effect was due to magnesiumdecreased the surface tension favoring the wettability and, therefore,improvingthesolid – liquidcontact.Thiswayenhancingthereactionsthat takeplaceintheinterphase,aswellasincreasingtheamountofstrontiumdissolved into the excess molten reactive [8].The analysis of  Fig. 2 b shows the magnesium loss from the molten bath, asa functionof time, for different strontiumcontentsobtained attheend of treatment. The magnesium content decreases respect to theincrement of strontium into the molten bath. Preliminary tests wereconduced todetermine the magnesium lossdue tooxidation. The Al – Mgmolten alloy was kept at 1073 K during 2 h. The decrease of magnesiumcontent into the Al – Mg alloy was not significant. Therefore, the overallmagnesium loss is due to the occurring reaction between strontium oxideand liquid magnesium. Consequently, increasing the initial amount of  Fig. 1. Schematic diagram of the powder injection system.Fig. 2. Results of progresses of elements from the molten bath as a function of time. a) Strontium recovery for the temperature and initial concentration of magnesium indicated. b) Magnesium loss for the temperature, initialconcentration of magnesium and final concentration of strontium indicated.638  R. Muñiz et al. / Materials Letters 62 (2008) 637   –  640  reactive (Mg) in the molten bath, the reactivity between Mg and SrO isenhanced, which contributes to the final strontium recovery. 3.2. X-ray diffraction and scanning electron microscope The compounds present in the dross sample included: MgO (79-0612),MgAl 2 O 4 (21-1152),SrF 2 (88-2294),Al(4-0787),KCl(72-1540),SrO (72-0057). The presence of strontium oxide in the dross sample isduetothepartiallyreactedSrOparticle,whileSrFandKClcompoundareformed duetothe addition offlux at the endofexperiment. The presenceof MgO and MgAl 2 O 4  as reaction products, establishes that aluminumandmagnesiumarereducingtostrontiumoxidethroughametallothermicmechanism. This may indicate that the principal reaction that takes placeduring the injection of SrO is the following:SrO  þ  Mg  ¼  MgO  þ  Sr   ð 1 Þ The strontium content in the Al – Mg alloy was increased to 5 wt.%after 60 min of treatment. The high strontium content benefits the precipitation of primary Al 4 Sr phase. It can be seen in Fig. 3.The primary Al 4 Sr phase exhibits two-dimensional planar growthslablike form (faceted growth mode). The primary Al 4 Sr phase in this process is slablike, growing fast in two planar directions and slowly inthe normal direction of the slabs. Previous investigation [9] about theformation of microstructure of an Al-10 wt.% Sr alloy prepared byelectrolysis and mixing, established that the primary Al 4 Sr phase isdendritic in the electrolysis-prepared alloy and slablike in the mixing- prepared alloy. These forms of growth are product of the undercoolingdegreethatoccurs inthesolidification process.WhentheprimaryAl 4 Sr  phase grows from the melt prepared by mixing method (undercooling278.7 K  [9]), the preferred direction is [110] and it exhibits a planar growthmode,andfinallygrowsintoslabs,asseeninFig.3.Forthemelt from electrolysis, however, the relatively higher undercooling (289.4 K [9]) enhances the instability of the solid – liquid interface and theoccurrence of a dendritic growth mode, and facilitates the branches of dendrites. The undercooling for this investigation was 285 K.Also in Fig. 3 is observed a photomicrograph (SEM) of a partiallyreacted SrO particle. According to the results of the energy dispersive X-Rayanalysis,thecenteroftheparticleisunreactedSrO,asonlystrontiumand oxygen were determined. The layer surrounding the center of the particle contains Sr, Mg, Al, and O, indicating the presence of MgO as areaction product. Thismay indicatethatthe occurringreaction during theinjection of SrO is given by Eq. (1). This assumption is supported by theX-Ray diffraction analysis of the slag. Finally, intermetallic compoundsurrounding the partially reacted SrOparticle isconstituted by Sr and Al,indicating the presence of the primary phase Al 4 Sr. 3.3. Kinetic study According to the classical kinetic theory (differential method), therate of strontium recovery during the injection of SrO could bedetermined by the following equation: V  dC  Sr  dt   ¼  K C  Sr  − C  Sr eq ð Þ   r  S  m  ð 2 Þ Where  V  represents the volume (m 3 ),  t   is the time (s),  K   is the masstransfer coefficient (m (wt.%) 1 − r  s − 1 ),  r   is the reaction order,  S  m  is theinitial concentration of magnesium (wt.%) in the molten bath and  C  Sr.eq is the volumetric concentration of the Sr at equilibrium (wt.%).Eq. (2) can be simplified considering that the value of   C  Sr(eq)  isclose to zero at the investigated reaction temperature range. The valueof the constant   K  m  considers the effects of the initial concentration of magnesium and the volume of the melt. With the considerations made previously, Eq. (2) can be written as follows: dC  Sr  dt   ¼  K  m  C  Sr  ð Þ r  ð 3 Þ It is possible to measure the kinetic values of Eq. (1) as a function of temperature. Thus, the logarithm format of Eq. (3) can be written as:ln  dC  Sr  dt     ¼ ln  K  m  þ  r   ln C  Sr   ð 4 Þ Fig. 4. Linear plotof ln(d C  Sr  /d t  ) vs.ln C  Sr  ,forthe determinationofreactionorder.Fig. 3. SEM image showing a partially reacted particle and EDS characteristic patterns of the indicated zone.639  R. Muñiz et al. / Materials Letters 62 (2008) 637   –  640  Fig. 4 shows the linear plot of ln(d C  Sr  /d t  ) against ln C  Sr   for the sameexperimental condition as that shown in Fig. 2a. The slope andinterception values of each line, in Fig. 4, indicate the reaction order ( r  )and the value of the ln  K  m , respectively. Using the least squares method,the reaction order was found to be about 1, indicating that the reactioncorresponds to a first order kinetics. The value of   K  m  depends ontemperature according to the Arrhenius equation:  K  m  ¼  f T  ð Þ ¼  k  m  exp  −  E  RT     ð 5 Þ Where  E   is the activation energy for the reaction (J mol − 1 ) and  R  isthe universal constant of gases (8.314 J mol − 1 K  − 1 ).Fig. 5 is a linear plot of ln  K  m  versus 1/  T  , from which the activationenergy of the process was obtained to be 102 kJ mol − 1 . This value isclose to the one reported for typical diffusion processes [10]. 4. Conclusions The viability of strontium extraction from strontium oxide byusing submerged powders injection has been proved at alaboratory scale. The strontium content in the Al – Mg alloy wasincreased to 5 wt.% after 60 min of treatment.The rate of reaction enhanced with increasing the initialamount of magnesium.The most important reaction during the injection process isthat occur between SrO and the Mg.The experimental data of study kinetic were fitted to a generalkinetic formula, having determined that the reaction between Mgin the molten metal and SrO is approximately first order.The studies were conducted in the range of temperatures between 973 and 1073 K, at a magnesium concentration of 3 wt.% Mg. It was obtained a value of the activation energy for theoverall process equal to 102,004 J mol − 1 K  − 1 . This valueindicates that strontium recovery is controlled by diffusion of Mg from the bulk to the powder interface. References [1] J.E. Gruzleski, B.M. Closset, AFS USA (1990) 1 – 102.[2] Z. Wang,  “ Master Thesis ” , McGill University, Montreal Canada, 1990.[3] J. Castrejón, D. Cortes, A. Flores, Light Metals (2000) 705 – 710.[4] C.R.MuñizValdez, “ MasterThesis, ” Cinvestav-UnidadSaltillo,México2005[5] ASTM E11, Annu. book ASTM stand. 04.01 (1989) 490 – 492.[6] S. Ohguchi, D. Robertson, Ironmak. Steelmak. 11 (5) (1984) 262 – 273.[7] O. Levenspiel, Ingeniería de las Reacciones Químicas, Editorial Reverté,2a. Edición Barcelona, España, 2002, pp. 406 – 415.[8] J. Langlais, R. Harris, Can. Metall. Q. 31 (2) (1992) 127 – 131.[9] Z. Zhang, X. Bian, Y. Wang, Mater. Lett. 57 (2003) 1261 – 1265.[10] J. Szekely, N.J. Themelis, Rate Phenomena in Process Metallurgy, JohnWiley and Sons, Inc., 1971, pp. 369 – 371.Fig. 5. Linear plot of ln  K  m  vs. 1/  T   for determination of the activation energy of strontium recovery.640  R. Muñiz et al. / Materials Letters 62 (2008) 637   –  640
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