Study on kinetics of thermal decomposition of low LOI goethetic hematite iron ore

Study on kinetics of thermal decomposition of low LOI goethetic hematite iron oreBeuria P.C.a,⇑,Biswal S.K.a ,Mishra B.K.a ,Roy G.G.ba Mineral Processing Department,CSIR –Institute of Minerals and Materials Technology,Bhubaneswar 751013,India bIndian Institute of Technology,Kharagpur 721302,Indiaa r t i c l e i n f o Article history:Received 29October 2016Received in revised form 26December 2016Accepted 26January 2017Available online 18July 2017Keywords:Loss on ignition Goethite Kaolinite Gibbsite Roasting Pellet feedKinetic analysisa b s t r a c tIn the present study,the kinetics of thermal decomposition of hydrated minerals associated in natural hematite iron ores has been investigated in a fixed bed system using isothermal methods of kinetic anal-ysis.Hydrated minerals in these hematite iron ores are kaolinite,gibbsite and goethite,which contribute to the loss on ignition (LOI)during thermal decomposition.Experiments in fixed bed have been carried out at variable bed depth (16,32,48and 64mm),temperature (400–1200°C)and residence time (30,45,60and 75min)for iron ore samples.It is observed that beyond a certain critical bed depth (16mm),100%removal of LOI is not found possible even at higher temperature and higher residence time.Most of the solid-state reactions of isothermal kinetic analysis have been used to analyze the reac-tion mechanism.The raw data are modified to yield fraction reacted ‘‘a ”versus time and used for devel-oping various forms of ‘‘a ”functions.f (a )is the inverse of first derivative of g (a )with respect to a .The study demonstrates that decomposition of hydrated mineral in hematite follows the chemical kinetics.The estimated activation energy values in all the experimental situations are found to high,of the order of 60kJ/mol,reinstating that the reactions are indeed controlled by moving phase boundary and random nucleation.Ó2017Published by Elsevier B.V.on behalf of China University of Mining &Technology.This is an openaccess article under the CC BY-NC-ND license (/licenses/by-nc-nd/4.0/).1.IntroductionIndian iron ore resources are mostly hematite.High grade hematite iron ore is depleting very fast.The low grade resource has become the major resources to produce iron and steel in India.The majority of low grade hematite iron ore resource at present is fragile in nature because of its association with good amount of goethite,kaolinite,and gibbsite.As per iron ore formation,goethite generates from hematite due to weathering.The goethite associ-ated with hematite is called vitreous goethite which is hard and crystalline.The goethite associated with clayey materials i.e.,kaolinite and gibbsite is called ochreous goethite [1].The liberation of iron phase minerals in this type of ore is at finer sizes.During the beneficiation of these low grade hematite iron ores,vitreous goethite comes with hematite and partly ocherous goethite along with kaolinite and gibbsite which contribute to the loss on ignition (LOI)in iron ore concentrate.The generation of fines is more during size reduction because of its fragile nature that leads to high Blaine number of the concentrate [2].The utilization of low grade iron ore is need of the hour in respect to meet the high quantity of production of steel in India as per the steel policy of government of India.In order to meet the future demand of steel,low grade iron ore fines need to be ben-eficiated to provide raw materials for steel plants in form of pellet.The presence of goethite,gibbsite,and kaolinite leads to high LOI and high Blaine number of the iron ore concentrate after benefici-ation.The presence of high LOI in the final concentrate deteriorates the quality of pellets [3].Due to the presence of chemically bound water in the matrix of goethite,kaolinite and gibbsite,high pres-sure steam is released at high temperature during induration pro-cess resulting in cracks inside the pellets thereby reducing the strength.The production of pellets and its use in blast furnace has increased largely over the past decade and hence there is con-siderable amount of research interest to remove LOI from the iron ore sample before making the pellet.Gibbsite and goethite release their water molecules within the temperature of 300–400°C whereas kaolinite releases its water molecule after the tempera-ture of 850°C.The kinetics of many solid-state reactions can be represented by Eq.(1):f ða Þ¼ktð1Þ/10.1016/j.ijmst.2017.06.0182095-2686/Ó2017Published by Elsevier B.V.on behalf of China University of Mining &Technology.This is an open access article under the CC BY-NC-ND license (/licenses/by-nc-nd/4.0/).⇑Corresponding author.E-mail address:pcbeuria@immt.res.in (P.C.Beuria).International Journal of Mining Science and Technology 27(2017)1031–1036Contents lists available at ScienceDirectInternational Journal of Mining Science and Technologyjournal homepage:www.else v i e r.c o m /l o c a t e /i j m stwhere a is the conversion factor(fraction of LOI removed)in time t, and the function f(a)depends on the thermal decomposition mech-anism of the iron ore with respect to temperature and pressure and the physical properties of the particle,i.e.,grain size,shape,and crystallinity.The activation energy during the transformation of goethite to hematite has been well documented by many researchers.Lima-de-Faria found that the activation energy for a crushed single crys-tal goethite is82.8kJ/mol[4].Pollack et al.quoted121±13kJ/mol activation energy of two natural limonite(amorphous goethite) [5].Thrierr-Sorel et al.reported activation energy of88kJ/mol for afibrous goethite[6].Keller also gave activation energies in the range of96–167kJ/mol for different variety of goethite types[7]. Goss reported the activation energy of154±15kJ/mol for the sed-imentary goethite[8].Prasad et al.examined the in-situ FTIR study on dehydration of natural graphite and reported that activation energy for pure goethite is85kJ/mol[9].In this paper,the most commonly methods for isothermal kinetic analysis are used in order to determine the mechanism of decomposition of hydrated minerals associated in hematite iron ore using heating cycle process in muffle furnace and thermo-gravimetric analyzer and evaluate the activation energy.The ther-mal decomposition of goethite,gibbsite and kaolinite has given in Eqs.(2)–(4)respectively:2FeOOH¼Fe2O3þH2Oð2Þ2AlðOHÞ3¼Al2O3þ3H2Oð3ÞAl2Si2O5ðOHÞ4¼Al2O3þ2SiO2þ2H2Oð4Þ2.Materials and methodsThe iron ore sample collected from Barbil region of Odisha, India is selected for the study,which contain LOI of5.46%.Miner-alogical characteristic are analyzed through quantitative X-ray diffraction(XRD).The XRD study was carried out by X-ray diffrac-tometer(PANalytical,X’pert)and quantitative analysis was done through high score plus software.The mineralogical characteristic of the sample is given in Table1.The chemical analysis of the sam-ple was carried out through wet chemical and XRF analysis tech-niques.The detailed chemical analysis of the sample is given in Table2.The particle size distribution is given in Table3.Initially,the iron ore samples are subjected to thermo gravimet-ric analysis to determine the rate of weight loss with increase in temperature by TGA(Thermo gravimetric Analysis)/SDTA(Simula-tion of Differential Thermal Analysis)supplied by Mettler,USA.In order to calculate the cumulative weight loss for high mass(250–1000g)samples infixed bed with temperature,the sample with different bed depths are taken after removal of surface moisture and kept inside the furnace for afixed time of30min at a particu-lar temperature from200°C to1200°C with an incremental increase of50°C to compare the TGA data.Further,the iron ore samples are dried at about120°C for 2hours for complete removal of surface moisture.Then the samples are put in four different crucibles of identical size having 96mm internal diameter and91mm height.All the experiments are made in batch mode in afixed bed system.The crucibles are made of high alumina refractory to withstand high temperature. The different weights i.e.250,500,750and1000g of samples are taken for the experiments.The depth of the ore bed varies from 16mm to64mm depending on the weight of the sample.The top of the crucibles is kept open.The samples are decomposed in the muffle furnace isothermally at different temperatures(ranging from400°C to1200°C)for a particular residence time.The heating rate of the furnace is maintained at15°C per minute.The temper-ature of the furnace is maintained precisely by PID controller.After removal from the furnace,final weight of each sample is measured to calculate the percentage of weight loss.The experiments are car-ried out for different residence time from30min to75min.For dif-ferent experiments,fresh samples are used.In overall,the heating cycle is conducted in the muffle furnace at different temperatures (400–1200°C)at incremental increase of50°C each,residence time(30–75min)and bed heights(16–64mm).Changes in the iron ore phases with increase in temperature were supplemented through XRD analysis.3.Results and discussion3.1.Non-isothermal conditionInitially a few experiments are carried out non-isothermally to understand the effect of increasing temperature on the LOI removal.Firstly,some low mass SDTA analysis is done tofind out the temperature when the chemical water starts releasing the hydrated hematite ore.The change in LOI of iron ore sample is observed through the loss of weight at different temperatures using SDTA/TGA as shown in Fig.1.Fig.1shows that the removal of LOI starts at a temperature of 200°C and loses around77%and86%at350°C and500°C,respec-tively.Following this some non-isothermal experiments are car-ried out for high mass system infixed bed.Here,temperature is increased in step wise fashion and the sample is soaked for 30min at each temperature.Fig.2shows the cumulative loss with increase in temperature taken with different bed depths.In this case the sample is contin-uously roasted at temperatures ranging from200°C to1200°C with incremental increase of50°C at different bed depths(16–64mm)for a residence time of30min at each temperature.In these experiments the sample is loaded once in the beginning and not changed after each observed temperature to know the cumulative removal of LOI.Similar experiments are carried out for different bed depths.It is observed that LOI removal follows S-shaped curve.Initially up to300°C,the LOI removal is insignifi-cant and thereafter there is a steep rise in LOI removal to around 90%at temperature of700°C for all depths of bed.Subsequently, the LOI removal almost comes to standstill with increasing tem-perature in irrespective of depth.In respect to bed depth,initial percentage of removal of LOI shifts towards higher temperature with increase of bed depth in the range of400°C to700°C.This is in contrast to observe in low mass TGA experiments where LOI is removed at low temperature between250°C and350°C.The shift in temperature range between low(around36mg in TGA) and high(1000g in experiment)mass experiments may be attrib-uted to decrease in specific reaction surface area for large mass sample.In fact,the reaction surface area(cross section of the bed)remains the same irrespective of the mass in the bed.There-fore,higher temperature is required to enhance the surface reac-tion rate to compensate for the reduction in reaction rate due to lower specific surface area for larger mass infixed bed.While theTable1Mineralogical constituents of thesample.Details Sample(wt.%)Hematite44.96Goethite47.41Kaolinite 6.5Gibbsite0.9Quartzite0.11032P.C.Beuria et al./International Journal of Mining Science and Technology27(2017)1031–1036TGA experiments indicate the actual range of temperatures for LOI removal for low mass,the fixed bed experiments indicate the actual furnace temperature required for larger mass experiments in fixed bed.Moreover,LOI of the iron ore sample is 5.46%and as the sample contains around 6.5%kaolinite,total LOI cannot be removed at lower temperature,because kaolinite needs more time to release its water molecule from its matrix after 850°C.The LOI released by this process at lower temperature is mostly from goethite and gibbsite minerals.3.2.Isothermal conditionIn these experiments fresh samples are taken for each temper-ature and roasted isothermally from 400°C to 1200°C in 50°C interval with a variation of bed depth from 16mm to 64mm and residence time from 30min to 75min.In this condition the removal of LOI with respect bed depth and residence time were studied and analyzed.3.2.1.Mineralogy at different temperaturesQuantitative mineralogy by XRD analysis have been carried out on the bulk sample and after thermal decomposition at 400°C and700°C.The results are shown in Fig.3a–c.As per the mineralogical analysis,major minerals in the bulk sample are hematite,goethite,gibbisite,kaolinite and quartzite.It is also observed that hydrated minerals (gibbsite,goethite and kaolinite)peak gradually vanishes by increasing the temperature.Most of the LOI removal takes place within 700°C.Goethite converts to hematite [10],which is indi-cated by increase in hematite peaks at the expense of goethite peaks with increase in temperature,gibbsite changes over to alu-mina and kaolinite matrix breaks into silica and alumina after releasing the hydrated molecule,gibbsite and kaolinite goes to amorphous phase and they do not appear in the XRD analysis after conversion [11].3.2.2.Effect of bed depthFig.4shows the effect of bed depth on the removal of LOI with for 45min residence time.Rate of LOI removal depends on the bed depth in fixed bed system.Bed depth reduces the kinetic of LOI removal because of lower specific reaction surface area for deeper bed.For the lowest bed depth (16mm)removal of almost all LOI is found to be possible at 1050°C;whereas for highest bed depth (64mm)it is only 81%with a minimum residence time of 30min.At temperature of 900°C,80%of LOI could be removed;whereas at 1200°C,more than 90%LOI can be removed even at higher bed depth of 64mm.Finally it is observed that beyond a certain critical bed depth (16mm),100%removal of LOI is not found possible even at higher temperature and higher residence time.3.2.3.Effect of residence timeThe pattern of variations of LOI reduction with temperature at different residence times is shown in Fig.5.Four residence times viz.30,45,60and 75min are considered in fixed bed system in a bed depth of 16mm over the temperature range of 400–1200°C.It is observed that the percentage of removal of LOI is more with higher residence time at particular bed depth and tem-perature.For example,at 400°C,while the removal of LOI is less than 20%for residence time of 30min,it is around 70%for 75min residence.However,the effect of residence time diminishes at higher temperature.For example,at 900°C,variation of resi-dence time between 30min and 75min,only enhance the LOI removal from 91%to 96%.The study further indicates that since maximum LOI in the sample is contributed by goethite,which releases its water molecule from its matrix from 300°C onwards,maximum LOI gets removed prior to 850°C,beyond which kaolin-ite start releasing its water content.3.3.Kinetic analysisThe rate determining step in any solid-phase reaction can be evaluated either diffusion or chemical reaction process [4,12].In these two fundamental processes,surface diffusion rapidly coats the surface of the reacting particle with a continuous product layer.Another approach is also to consider that there is nucleation of products active sites [13,14].According to the above statements,kinetic functions f (a )have been classified into three groups:the diffusion,the chemical reaction,and the nucleation model.Several mechanisms of the solid state transformation of goethite to hema-tite have been reported.It has been reported that thermal decom-position of goethite directly converts to hematite without any intermediate phase during transformation [15,8].Wolska hasTable 2Chemical analysis of the sample (%).Details Fe Al 2O 3SiO 2CaO K 2O MgO MnO 2P 2O 5TiO 2Na 2O LOI Sample61.272.953.130.0540.0230.0710.0630.2130.1140.0845.46Table 3Size analysis of the sample.Size (l m)Sample (wt.%)+1002.97À100+759.80À75+4522.97À4564.26Fig.1.SDTA analysis ofsample.Fig.2.Cumulative removal of LOI with temperature at different bed height.P.C.Beuria et al./International Journal of Mining Science and Technology 27(2017)1031–10361033reported that goethite transforms into protohematite,then into hydrohematite and finally into hematite with increasing of tem-perature during thermal decomposition [16].Even Ozdemir and Dunlop found that small amounts of magnetite were formed in intermediate phase during phase transform from goethite and hematite [17].Diamandescu et al.established that the solid state reaction of goethite to hematite as a first order reaction controlled by nucleation process in isothermal process [18].The thermal transformation mechanism was studied by Fan et al.,to evaluate maximum decomposition degree under non-isothermal conditions and supported by microstructural evidence [19].The fraction decomposed (a )versus time for different tempera-ture values are presented in Fig.6for 16mm bed depth.The curves show the characteristics of sigmoidal curve as generalized a -t plot,but since the data are collected after 5min interval,initial reaction and induction period covers within this time,especially at higher temperature;whereas acceleratory period,point of inflection,decelerator period and completion of the reaction are visible in Fig.6.The maximum decomposition of 67%(a =0.67)is possible at a temperature of 400°C for residence time of 75min;whereas at higher temperature of 800°C,thermal decomposition are 83%(a =0.83)and 89%(a =0.89)for residence time of 30min and 75min,respectively.Further,the rate of decomposition for different temperature as a function of time is calculated.A typical graphical representation of d a /d t versus time at different temperatures for 16mm bed depth is shown in Fig.7.It can be assessed that rate of decomposi-tion is more at high temperature and reduces with time.In case of low temperature,the rate decomposition remains almost constant throughout the residence time because of deficiency of required enthalpy during the decomposition.The above data are further kinetically treated and analyzed to determine the controlling mechanism and underlying equations.To study the kinetics on the rate of removal of LOI from hydrated iron ore,most of the isothermal kinetics for solid state reactions is analysed.The raw data are modified to give fractional decomposition of LOI (a ),and subsequently to obtain various func-tional forms of a ,and their plots against reduced time,t /t 0.5to determine the reaction kinetics.Various isothermal kinetics for solid state reduction considered in the present study are presented in Table 4.In Table 4,D 1(a )to D 4(a )represent the cases where rate is controlled by diffusion,while F 1(a ),R 2(a )and R 3(a )represent the situation where rate mechanism follows chemical kinetics.The A 2(a ),A 3(a )represents cases where fractional reaction follows a sigmoidal variation with time,which is controlled by nucleation and growth that is mixed controlled.Table 5makes a summary of kinetic equations that best fit the experimental data with corre-lation coefficients for various temperatures and depth of thebed.Fig.3.XRD analysis of sample with 16mm beddepth.Fig.4.LOI removal with temperature at different bed depth at 30min.residencetime.Fig. 5.LOI removal of with temperature with 16mm bed depth at different residencetimes.Fig.6.Variation of thermal decomposition with time and temperature for 16mm beddepth.Fig.7.Rate of thermal decomposition with time for 16mm bed height.1034P.C.Beuria et al./International Journal of Mining Science and Technology 27(2017)1031–1036Fig.8depicts a typical representation of the various bestfit lines through the data points when various functions of a(controlling process)is plotted against reduced time for depth of32mm and 500°C.Similarly,the data generated for other conditions of varying bed depths and temperatures are given in Table5.From Table5,it is observed that decomposition and removal of LOI mostly follows F1(a),R2(a)and R3(a)equations and in some cases at higher tem-peratures,A2(a)indicating decomposition of LOI is mostly gov-erned by chemical kinetics with random nucleation.Halikia et al.have studied thermal decomposition of various other minerals like magnesium hydroxide and calcium carbonate, and predicted the bestfit model for solid-state reaction utilizing thermo gravimetric data,are chemical kinetics,viz.F1(a)and R2(a)and R3(a)[12,13].Goss investigated the kinetics and reaction mechanism for conversion through TG,TEM and XRD analysis and estimated the range of activation energy of goethite and found that the reaction is controlled by R2(a),i.e.,chemical kinetics[8].Dia-mandescu et al.reported F1,i.e.,first order decay law bestfit the transformation of goethite to hematite[18].All the above experi-ments point to the fact that decomposition reactions follow the chemical kinetics,which is also observed in the present study.It may also be noted that all the investigations were made taking low mass and without varying the bed height in afixed bed for practical application in industries.The present experiments have been carried out for higher bed depth.However,up to64mm bed depth and even at high temperature of850°C,the reaction kinetics for LOI decomposition remains primarily controlled by chemical kinetics.Activation energy values are also estimated to confirm the influ-ence of chemical kinetics.These values are estimated using Arrhe-nius plots(Fig.9).Estimated activation energy values for different bed depths and are presented in Table6.The activation energy values estimated for all the bed depths are found to be around60kJ/mol.These higher activation energy values reinstates that the chemical kinetics indeed controls the dehydration reaction up to64mm bed depth and900°C.The acti-vation energy during the transformation of goethite to hematite has been well documented by many researchers.Lima-de-Faria found that the activation energy for a crushed single crystal goethite is82.8kJ/mol[20].Pollack et al.quoted121±13kJ/mol activation energies of two natural limonite(amorphous goethite) [5].Thrierr-Sorel et al.,reported an activation energy of88kJ/mol for afibrous goethite[6].Keller also gave activation energies in the range of96–167kJ/mol for different variety of goethite types[7].Table4a-functions for most commonly used solid-state reaction processes(a represents fractional reaction,and k,trepresent rate constant and time,respectively).Rate controlling process Kinetic equationDiffusion,one dimensional D1(a):a2=ktDiffusion,two dimensional D2(a):(1Àa)ln(1Àa)+a=ktDiffusion,three dimensional(spherical symmetry)D3(a):[1À(1Àa)1/3]2=ktDiffusion,three dimensional(contracting sphere model)D4(a):1À(2/3)a(1Àa)2/3=ktMoving phase boundary,two dimensional R2(a):1À(1Àa)1/2=ktMoving phase boundary,three dimensional R3(a):1À(1Àa)1/3=ktRandom nucleation,two dimensional A2(a):[Àln(1Àa)]1/2=ktRandom nucleation,three dimensional A3(a):[Àln(1Àa)]1/3=ktRandom nucleation,first order decay law F1(a):Àln(1Àa)=ktTable5Various a-functions thatfit the present experimental data for different bed depths and temperature.Temperature and bed depth16mm32mm48mm64mm400°C F1(a):(R2=0.9987)F1(a):(R2=0.9964)F1(a):(R2=0.9967)F1(a):(R2=0.9957)R2(a):(R2=0.9983)R2(a):(R2=0.9959)R2(a):(R2=0.9962)R2(a):(R2=0.995)R3(a):(R2=0.9985)R3(a):(R2=0.9962)R3(a):(R2=0.9964)R3(a):(R2=0.9953) 450°C F1(a):(R2=0.9988)F1(a):(R2=0.9987)F1(a):(R2=0.9987)F1(a):(R2=0.9978)R2(a):(R2=0.9992)R2(a):(R2=0.9994)R2(a):(R2=0.9983)R2(a):(R2=0.9978)R3(a):(R2=0.9993)R3(a):(R2=0.9993)R3(a):(R2=0.9985)R3(a):(R2=0.9978) 500°C F1(a):(R2=0.992)F1(a):(R2=0.9982)F1(a):(R2=0.995)F1(a):(R2=0.9873)R2(a):(R2=0.9984)R2(a):(R2=0.998)R2(a):(R2=0.9905)R2(a):(R2=0.9893)R3(a):(R2=0.9968)R3(a):(R2=0.9959)R3(a):(R2=0.9927)R3(a):(R2=0.99) 550°C F1(a):(R2=0.9925)F1(a):(R2=0.9882)F1(a):(R2=0.9946)F1(a):(R2=0.9762)R2(a):(R2=0.9991)R2(a):(R2=0.998)R2(a):(R2=0.9973)R2(a):(R2=0.9826)R3(a):(R2=0.9976)R3(a):(R2=0.9959)R3(a):(R2=0.9971)R3(a):(R2=0.9806) 600°C F1(a):(R2=0.9984)F1(a):(R2=0.9858)F1(a):(R2=0.989)F1(a):(R2=0.9926)R2(a):(R2=0.9976)R2(a):(R2=0.9813)R2(a):(R2=0.9886)R2(a):(R2=0.9983)R3(a):(R2=0.995)R3(a):(R2=0.9837)R3(a):(R2=0.9894)R3(a):(R2=0.9968)A2(a):(R2=0.9784)650°C F1(a):(R2=0.9786)F1(a):(R2=0.9829)F1(a):(R2=0.989)F1(a):(R2=0.9949)R2(a):(R2=0.9964)R2(a):(R2=0.9974)R2(a):(R2=0.9986)R2(a):(R2=0.9926)R3(a):(R2=0.992)R3(a):(R2=0.9939)R3(a):(R2=0.9964)R3(a):(R2=0.9944)A2(a):(R2=0.9602)700°C F1(a):(R2=0.974)F1(a):(R2=0.9785)F1(a):(R2=0.9825)F1(a):(R2=0.979)R2(a):(R2=0.9958)R2(a):(R2=0.9953)R2(a):(R2=0.9966)R2(a):(R2=0.9751)R3(a):(R2=0.9906)R3(a):(R2=0.9912)R3(a):(R2=0.9931)R3(a):(R2=0.9776)A2(a):(R2=0.9737)A2(a):(R2=0.9696)750°C F1(a):(R2=0.9649)F1(a):(R2=0.9606)F1(a):(R2=0.9772)F1(a):(R2=0.9663)R2(a):(R2=0.994)R2(a):(R2=0.9676)R2(a):(R2=0.995)R2(a):(R2=0.9869)R3(a):(R2=0.987)R3(a):(R2=0.9574)R3(a):(R2=0.9906)R3(a):(R2=0.9814)A2(a):(R2=0.9753)A2(a):(R2=0.9603) 800°C R2(a):(R2=0.995)R2(a):(R2=0.9932)F1(a):(R2=0.9741)F1(a):(R2=0.9776)R3(a):(R2=0.998)R3(a):(R2=0.9857)R2(a):(R2=0.9956)R2(a):(R2=0.9958)A2(a):(R2=0.978)A2(a):(R2=0.9761)R3(a):(R2=0.9904)R3(a):(R2=0.9908)A2(a):(R2=0.9695)A2(a):(R2=0.9709) P.C.Beuria et al./International Journal of Mining Science and Technology27(2017)1031–10361035Goss reported the activation energy of 154±15kJ/mol for the sedimentary goethite [8].Prasad et al.examined the in-situ FTIR study on dehydration of natural graphite and reported that activa-tion energies for natural goethite samples from Karnataka are 103kJ/mol and 85kJ/mol [9].Therefore,the present estimated activation energy values are found to be in tune with the literature data.Slightly lower value in the present study may be attributed naturally occurring ore having 47%of goethite is being considered in the present study in contrast to synthetic goethetic mostly reported in literature.Besides,the mass of the bed is also higher in the present study.4.Conclusions(1)The reduction of LOI from a fixed bed is prominent in thetemperature range of 300–700°C due to the release of water molecule by goethite and gibbsite from their matrix.(2)Since the kaolinite releases its matrix moisture at 850°C,more temperature and residence time is required for sample containing kaolinite along with goethite and gibbsite for maximum removal of LOI.(3)The extent of LOI removal decreases with increase in beddepth and decrease in residence time.Effect of residence time is found to be prominent at comparatively lower tem-perature.Beyond a certain critical bed depth (16mm),100%removal of LOI is not found to be possible even at higher temperature and higher residence time.(4)The rate of thermal decomposition of hydrated iron ore ismore for initial stages and reduces thereafter for tempera-tures more than 500°C,whereas it remains more or less con-stant for temperature below 500°C.(5)Kinetics of thermal decomposition of hydrated minerals inhematite has been studied through various a functions against reduced time plot.Thermal decomposition at all bed depths up to 64mm and temperatures up to 900°C,fol-lowed a -functions for chemical kinetics.The activation energy values calculated for all cases for various bed depths and temperatures in the present study are found to be more than 60kJ/mol,reinstating the role of chemical kinetics.(6)It is found that temperature required to remove LOIincreases with increase in mass in the fixed bed,which is attributed to the lower specific surface area for high mass fixed bed samples,where surface reaction controls the over-all reaction.AcknowledgmentsThe authors are very much thankful to Ministry of Steel –India,New Delhi for sponsoring the program to carry out the research work.References[1]Das SK,Das B,Saktivel R,Mishra BK.Mineralogy,microstructure,and chemicalcompositions of goethites in some iron ore deposits of Orissa,India.Miner Process Extr Metall Rev 2010;31(2):7–110.[2]Mishra BK,Das B,Prakash S,Das SK,Biswal SK,Reddy PSR.Issues relatingcharacterization and beneficiation of low grade iron ore.Steel World 2007:34–7.[3]Biswal SK.Utilization of low grade iron ore fines,slimes and tailings byphysical beneficiation to minimize the waste generation.J Sustain Planet 2010:46–58.[4]Bamford CH,Tipper CFH,prehensive chemical kinetics.TheNetherlands:Elsevier Scientific Publishing Corporation;1980.[5]Pollack JB,Pitman D,Khare N,Sagan C.Goethite on Mars:a laboratory study ofphysically and chemically bound water in 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?Fe 2O 3.Mater Chem Phys 1997;48(2):170–3.[19]Fan H,Song B,Li Q.Thermal behaviour of goethite during transformation tohematite.Mater Chem Phys 2006;98:148–53.[20]Lima-de-Faria J.Dehydration of goethite and diaspore.Zeitschrift furKristallographie1963;119:176–203.Fig.8.Best fit lines for various a -functions at 500°C and 32mm beddepth.Fig.9.Arrhenius plot for 16mm bed depth.Table 6Activation energy values estimated for various bed depths using Arrhenius plot.Bed depth (mm)Slope =ÀEa /R Ea =Activation energy (kJ/mol)16À7.907565.7432À7.790264.7748À7.343661.0564À7.065058.741036P.C.Beuria et al./International Journal of Mining Science and Technology 27(2017)1031–1036。