Iron removal from extremely fine quartz and its kinetics
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Iron removal from extremely fine quartz and its kineticsHouquan Huang,Jingsheng Li ⇑,Xiaoxia Li,Zhizhen ZhangSchool of Chemistry and Chemical Engineering,Jiangsu University,Zhenjiang 212013,PR Chinaa r t i c l e i n f o Article history:Received 18August 2012Received in revised form 25January 2013Accepted 28January 2013Available online 6February 2013Keywords:Purification KineticsExtremely fine silica Oxalic acid Mechanisma b s t r a c tThe iron impurity removal from extremely fine quartz and its kinetics have been explored.The investi-gation reveals that the product layer diffusion in heterogeneous reaction system is the rate-control step and the activation energy was found to be 45.37kJ/mol.But the mechanism seems to have a trend towards homogeneous reaction system with the second order rate model from the heterogeneous sys-tem.Because the homogeneous reaction system’s average linear correlation coefficient is up to 0.978,which is very close to 0.995of its counterpart in heterogeneous system.This result was confirmed through examination of the homogeneous system models with the first and the second order rates,as well as the heterogeneous system models with interface film,product layer diffusion and chemical reac-tion models.Additionally,other interesting results revealed that the leaching efficiency for such extre-mely fine silica sand decreased with increasing stirring speed –this observation was opposite to that for coarse ually,leaching efficiency of coarse particles increases with increasing stirring speed or higher ultrasonic power.Ó2013Elsevier B.V.All rights reserved.1.IntroductionHighly pure silica is usually used as a high-tech raw material such as optical fibers [1,2],electro-magnetic materials [3,4],and catalysis materials [5,6]as well as separation and purification materials [7–9].However,the prerequisite of silica to be used as a high-tech raw material is that it should be highly purified.Unfor-tunately,the industrial pure silica usually contains over 10impu-rities.Among the impurities,iron oxides cause the highest damage by both their color and properties.Therefore,removing iron impurities as much as possible from industrial pure quartz sand by an effective,environment-friendlier,affordable method and understanding its mechanism are particularly critical.Up to now,considerable efforts have been devoted to these targets [10–20],and some agreements have been obtained [10–14].For example,leaching percentages of impurity iron from quartz sand by oxalic acid increase with higher leaching temperature [10,12–18,20],the smaller particle sizes [11,15],and the higher acid con-centrations [14–17]usually;however,some investigations still ob-tained discrepant conclusions because of the differences in systems,particle sizes,leaching temperatures and concentration of oxalic acid.For example,Taxiarchou et al.thought concentration of oxalic acid had almost ‘‘no effect’’on leaching efficiency of impu-rity iron removal [10];again,Du et al.’s experiments showed that leaching percentages increased with higher stirring speeds and ultrasound power [18],but Raman and Abbas indicated that the particle size of a similar hard material was independent of the in-put ultrasound power [19].There also exist different conclusions on the rate-controlling step of silica sand leaching processes.For example,Martinez-Luev-anos et al.[15]and Lee et al.[17]leached impurity iron by oxalic acid at 25–60°C from low grade kaolin (60l m on average,the same below)or silica sand (123l m),they thought that the product layer diffusion was the rate-controlling step in the shrinking core model.Mgaidi et al.and his colleagues [20]dissolved silica by NaOH at 150–220°C,and concluded that the mechanism of the dissolving silica sand (154l m)was a combination model,i.e.chemical reaction was the rate-controlling step at the initial stage,then transferred to the rate-controlling step of product layer diffusion.The aim of this work is to examine the kinetics of iron impurity removal from extremely fine quartz (45l m)by oxalic acid solu-tions,and to find the discrepancies between the fine and coarse sil-ica (80l m)in a leaching process with parameters such as reaction temperatures,particle sizes,acid concentrations and stirring speed.The kinetics of iron impurity removal from the extremely fine silica sand will be examined very carefully by the first and the second order reaction rate models in homogeneous system (be-cause of the very fine samples used)and by film diffusion,chemical reaction and product layer diffusion control models in heteroge-neous system.Oxalic acid was used in this project as a leaching reagent be-cause of two reasons:one is oxalic acid to be generally thought as environment-friendlier and effective solvent in leaching iron impurities from industrial pure quartz;and the other is the acid1383-5866/$-see front matter Ó2013Elsevier B.V.All rights reserved./10.1016/j.seppur.2013.01.046Corresponding author.E-mail address:jli5154@ (J.Li).we have used for many times and understood its properties well. The possible chemical reactions could be referenced to papers [10,12,14].2.Materials and methods2.1.Silica sand sampleThe extremelyfine silica in this investigation was purchased from Ninglu Corp.in Hebei Province,China.Before use,the silica sand samples were ground and sieved,two fractions of mesh size (À55+35l m andÀ100+60l m)were used in this project(the lat-ter was used just for comparison of the effect of sizes).The size of the former(45l m on average,equivalent to325mesh)is much smaller than the size150l m on average(equivalent to100mesh by Tyler mesh)usually used by industry.The complete chemical analysis of the silica sand is reported in Table1.In order to know the mineralogy of the samples,X-ray diffrac-tometer(XRD,Bruker D8Advance diffractometer)with Cu K a radi-ation was used,the analyzed result is as shown in Fig.1.It can be seen from Fig.1that the samples showed very high percentages of quartz and very small percentages of montmorillonite,implying the purity of these samples are very high–this is in agreement with our analyzed results of the chemical components.2.2.Experimental procedureThe leaching experiments were carried out in a250mL round-bottomflask at atmospheric pressure.A mechanical stirrer was used;a thermostat wasfixed to keep the reaction medium at con-stant temperature.In a leaching process,100mL of oxalic acid solution was put into theflask.After the required temperature was reached,10.0g of the silica sand was added into the solution and the stirring started.After a certain period of time,the solution wasfiltered as soon as the reaction ended.Then5mlfiltrate and corresponding residue were taken out respectively for determina-tion of the iron content and for consistency of component percent-ages.Deionized water was used throughout all the experiments. The iron content in these investigations was determined by ICP-OES(Vista-MPX,Varian,and USA).3.Results and discussionBefore searching for the appropriate kinetics of iron removal from extremelyfine silica sand,we had to examine effects of some important parameters such as reaction temperature,acid concen-tration,particle size,etc.on leaching behavior of the extremelyfine silica sand.There is a great need for data to set up the kinetics model and comparison of leaching behavior discrepancies between the coarse and veryfine silica particles.3.1.Effect of reaction temperaturesThe effects of temperature on iron impurity extraction were investigated in the temperature range of45–90°C(further higher temperatures would cause technical problems)and are shown in Fig.2.The leaching percentages of iron from the quartz sand al-ways increased with increasing temperature,implying that higher temperature has a strong influence on the leaching efficiency. Especially,at the highest temperature90°C,the leaching percent-ages remarkably increased compared with those at lower temper-ature range of45–75°C.Although usual explanations for higher yields at the higher temperature are due to increasing reaction and diffusion rates of reactants and products,the dramatic incre-ment of the leaching percentages from20.69%to37%at the tem-peratures from75°C to90°C still remains a surprise as compared to20.69%from13.52%at temperatures from45°C to 75°C.3.2.Effect of acid concentrationsThe influence of oxalic acid concentrations on the percentage of iron removal was examined with solutions of concentrations2,4, 6,and8g/l of oxalic acid at90°C by a stirring speed of500rpm and using the extremelyfine silica sand sample(45l m).The re-sults are as shown in Fig.3.Two points can be observed immedi-ately from Fig.3:one is that the iron leaching percentages increased remarkably from23.5%to37.5%as the concentrations of oxalic acid increased from2g/l to6g/l,but the increase of leach-ing percentages was slowed as the concentrations further in-Table1Chemical analyses of silica sand samples with particle sizeÀ55+35l m.Comps.SiO2Al2O3CaO Fe2O3K2O TiO2Na2O Others a Sum (wt.%)99.3180.2170.2300.0890.0750.0170.0130.01999.978 a Oxides such as ZrO2,MnO2and SO2.46H.Huang et al./Separation and Purification Technology108(2013)45–50creased from6g/l to8g/l.Additionally,it can be observed that the longer the time,higher the percentages of iron removal.These observations from extremelyfine silica samples are similar to that from coarse silica samples,and indicate that higher acid concentra-tions and longer leaching time are always favorable to obtain high-er leaching percentages.The reason for the higher removal percentages obtained from higher concentrations and longer time may be due to increasing acid concentration,which is equivalent to have increased concentration of the reactant and successively raised reaction driving force.Hereafter,6g/l was chosen as an opti-mal concentration with corresponding percentage iron removal of 37.5%.3.3.Effect of particle sizesThe effects of particle sizes on iron removal percentages were examined at a stirring speed of500rpm,an acid concentration of 6g/l,and a solution temperature of90°C.The results are shown in Fig.4.As is seen in Fig.4,the leaching percentages of the both coarse(80l m)and extremelyfine particles(45l m)increased as the leaching time was extended,and higher leaching rate was eas-ier to obtain by thefine particle sizes than by the coarse particles in the leaching process because the interfaces of the liquid–solid reactants for thefine particles are much larger than that for the coarse particles.This may have resulted in enlarged reaction sur-face for thefine samples.Concretely,the extracted iron amount was22.78%for the coarse particle and37.50%for the extremely fine particle in120min respectively.3.4.Effect of stirring speedsThe effects of stirring speed on percentages of extremelyfine silica sand were examined at stirring speeds of300,500,700and 900rpm and the interesting results are given in Fig.5.The exper-imental results showed an opposite trend:higher leaching effi-ciency of iron impurities was obtained by decreasing stirring speed although the influence was not very significant.This observation still puzzles us,because usual observations are that higher stirring speed results in higher leaching percentages [18,21].Because,usually higher stirring speed reduces resistance of interfacefilm and makes the solution more turbulent on the one hand,on the other hand,higher stirring speed will break the large solid particles into smaller ones,this is equivalent to enlarg-ing the reaction surfaces and to speed up chemical reactions.How-ever,leaching percentages of the iron impurities now decreased with increasing stirring speed for extremelyfine silica sand,the real reason behind is still unclear,but a possible explanation for such an extremelyfine quartz might be the too short contact time between oxalate ions and the iron oxides on the silica sand surface for such hard and compact material like silica at the high stirring speeds,therefore,the reactions was slowed down and resulted in fewer leaching yields.3.5.Kinetic testsFor such an extremelyfine quartz sample,homogeneous reac-tion models arefirstly considered to describe the leaching pro-cesses.In these investigations,homogeneous reaction models with thefirst and the second order rate equations can be written as follows[22,23]:Àlnð1ÀXÞ¼k hf tð1Þð1ÀXÞÀ1À1¼k hs tð2ÞHere k hf and k hs are coefficients of thefirst and the second order rate equations in homogeneous systems,X is conversion fraction,and tH.Huang et al./Separation and Purification Technology108(2013)45–5047is instantaneous time.The plots of conversion fraction to time are given in Figs.6a and6b.Clearly,the experimental data are slightly scattered,especially at the high temperature for120min where the reaction speed is slower so that the experimental points fell below the straight lines in Figs.6a and6b.Correspondingly,linear correlation coefficients for thefirst and the second order rate equations are in ranges of 0.9619–0.9800and0.9716–0.9835respectively.Therefore,homo-geneous system models with either thefirst or the second order rate equations are not desirable to describe the leaching process of our even extremelyfine quartz samples very well.However,be-cause the correlation coefficient(0.9835)of second order rate model at the high temperature in Fig.6b is very close to the straight line index0.9976of product layer diffusion model in het-erogeneous systems with the deviations from the straight lines only0.0141at the longest time(i.e.at120min)(Fig.7a).This indi-cates that the rate model in homogeneous system seems to be close to applicable to description of this particular leaching process before100min.When the time is long enough,or in other words, the product layer is thicker enough,the homogeneous rate equa-tion would be replaced by the rate equation model in heteroge-neous system.Then we considered the heterogeneous reaction models care-fully,the reactions in these models are thought to take place at the outer skin of the unreacted particle.With increasing conver-sion rates,the reaction zone then moves into further interior of the solid particles,and the unreacted core of the particle shrinks gradually.Thoughfive steps are proposed successively between fluid–solid reactions[22],however,because thefive steps proceed simultaneously,the important resistance is following three steps [23]:(1)diffusion of thefluid reactant through the interfacefilm surrounding the particle over the surface of a solid particle with to-tal coefficient k tf;(2)penetration and diffusion of thefluid reactant through the blanket of ash to the surface of the unreacted core with total effective diffusion coefficient k ted;(3)fluid–solid chemical reaction at the reaction surface with total coefficient k tr.By this way,thefilm diffusion control model is given by[22,23]: X¼k tf tð3ÞThe chemical reaction control model by:1Àð1ÀXÞ1=3¼k tr tð4ÞThe product layer diffusion control model by:1À3ð1ÀXÞ2=3þ2ð1ÀXÞ¼k ted tð5ÞThe experimental data were regressed using Eqs.(3)–(5)and the results are given in Table2.It is seen in Table2that all the regression coefficients R2in the fourth column from the left forfilm diffusion model,k tf,were in the range of0.9405–0.9761,and that in the sixth column for thefirst order reaction model,k tr,were also in the range of0.9405–0.9760.Obviously,the linear correlation coefficients for the models of chemical reaction control and for thefilm diffusion control are smaller than that for product layer diffusion control model,which gave values in the8th column rang-ing from0.9914to0.9987.The poorer mathematicalfitting for k tf, k tr and better mathematicalfitting indicate that though the extre-melyfine sizes of quartz sample were used,the product layer dif-fusion model seems to still control all the reaction process in the time considered.The plots of product layer diffusion model versus different temperatures and different acid concentrations are given in Figs.7a and7b.From the twofigures,very well straight lines were obtained with different temperatures and acid concentra-tions at all times,these have verified our above judgment.48H.Huang et al./Separation and Purification Technology108(2013)45–503.6.Arrhenius plotThe plot of natural logarithm ln k of rate constants in Fig.7a, versus1000/T,gives a straight line graph with the slope as the acti-vation energy of the leaching process.The activation energy was calculated to be45.37kJ/mol.This activation energy confirmed that iron removal from silica sand was controlled by diffusion step through the product layer in the conditions considered.We noted that the higher activation energy could cause some discrepancy.However,we insist that it is right.There are three reasons:thefirst,the measuring point at the far left(corresponding to the highest temperature90°C)on Fig.8scatters from the other three points,it is this scattered datum that raised the straight line –thus resulting in increased activation energy.Secondly,the large datum came from quick leaching speed;we carefully checked the iron leaching yield in Fig.2and found it had a jump from75to 90°C.Therefore,this result is reasonable.The k ted values of the sec-ond column in Table2confirm this again,where the diffusion coef-ficient of product layer k ted increased500%from temperatures75–90°C,but the corresponding coefficients were only increased less than30%from45°C to75°C.Thirdly,we compared ours with other related experiments,for example,Lee studied‘‘kinetics of iron oxide leaching by oxalic acid’’and observed the sharp increase of leaching yield at the high temperature and discussed the straight line by two parts:the lower part obtained12.2kJ/mol of activation energy;and the higher part obtained50.7kJ/mol,and simple average was31.5kJ/mol.Lee’s conclusion is‘‘product layer diffusion control model’’to be rate limiting step[17].Martínez-Luévanos investigated the‘‘kinetics of iron from low grade kaolin by oxalic acid solutions’’and obtained46.32kJ/mol as the activa-tion energy,and thought that the rate control step was‘‘product layer diffusion’’[15].Sokic´et al.[24]and Ho et al.[25]also looked into the‘‘kinetics’’,Sokic´looked into‘‘chalcopyrite leaching by so-dium nitrate in sulphuric acid’’obtained the activation energy of 83kJ/mol,and concluded that both the surface reaction and prod-uct layer diffusion governed the rate control step.These indicate that activation energy alone cannot be used to determine the rate control step.4.ConclusionsThe kinetics of iron removal from extremelyfine silica sand by oxalic acid has been investigated.The results showed that the leaching process was controlled by product layer diffusion step in the heterogeneous systems with activation energy of45.37kJ/ mol.Furthermore,the average linear correlation coefficient of 0.978for the second order reaction rate model in homogeneous systems was very close to its counterpart0.995of the product layer diffusion control model in heterogeneous systems.This indi-cates that the heterogeneous reaction model is nearly to be re-placed by homogeneous reaction model.The leaching percentages of silica sand,like the coarse ones,increased with increasing temperature,acid concentrations,and decreasing sam-ple sizes;but unlike the coarse ones,increased with decreasing stirring speeds.AcknowledgementThis research is supported by Grand from NSFC(50474037).Table2Coefficients k tf,k tr,k ted of heterogeneous models.aAcid concentration(g/l)Temperature(°C)k tf(minÀ1)R2k t r(minÀ1)R2k ted(minÀ1)R22900.00150.9670 4.8651b0.96700.02010.9987 4900.00190.9527 6.3069b0.95260.03500.9970 6900.00210.9405 6.9989b0.94050.04360.9954 8900.00220.97617.1924b0.95260.05010.9976 6457.8024b0.9761 2.6013b0.97600.00510.9958 6609.0697b0.9591 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