International Journal of Mineral Processing 100 (2011) 172–178
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International Journal of Mineral Processing
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A further study on the interaction between one of organic active fractions of the MHAbinder and iron ore surface
Tao Jiang, Guihong Han ⁎, Yuanbo Zhang, Guanghui Li, Yanfang HuangSchool of Minerals Processing & Bioengineering, Central South University, 410083, Changsha, P.R. China
Binder is essential in the iron ore agglomeration. Fulvic acid (FA) is one of organic active fractions of the MHAbinder. A further study of the interaction between FA and iron ore surface was conducted via elementalanalysis, chemical group analysis, batch adsorption experiments, X-ray photoelectron spectroscopy (XPS)investigation, zeta potential measurements and scanning electron microscope (SEM) imaging. It is found thatthe adsorption amount of FA onto iron ore surface obviously increases with increasing the solution pH. X-rayphotoelectron spectroscopy (XPS) analysis indicates that ligand exchange (coordination) as a specialchemical adsorption occurs at the interface of iron ore and FA. Zeta potential results display that theadsorption of FA onto iron ore surface can be characterized by specific ligand exchange and electrostaticforces. Scanning electron microscope (SEM) imaging shows that flocculation sedimentation is observedduring the adsorption process. Ball-shaped FA–mineral colloidal complex with a diameter of 20–100 μm ispresented in the SEM micrograph of FA-coated mineral granules. Ligand exchange, hydrophobic interactionsand electrostatic force are contributive to the adsorption, with ligand exchange and hydrophobic interactionspredominant.
Binder is essential in the iron ore agglomeration for the improve-ment of green pellet strength (Allen, 1989; Jiang et al., 2001). Thesebinders mainly comprise inorganics or organics, or a combination ofthese fractions. For example, bentonite as a representative inorganicshas been the iron ore industry's binder standard. Peridur (tradename),a famous organic binder, has been accepted practice in America, Japanand Brazil for many years (Murr and Englund, 1989). The practiceshows that organic binder has the ability to replace inorganicbentonite to ensure good strength of green pellet in the agglomerationprocess. Furthermore, the organic binder is easily burned out duringinduration and does not contaminate the product. The effects ofbentonite and Peridur binders on the chemical composition had beenbest expounded in former literatures (Dennis and David, 1989; Haaset al., 1989). However, Peridur has not been successfully applied in theproduction of iron ore pellets due to its high cost, complexity of ironore properties, as well as special production process of pellet (grate-kiln process) in China (Jiang et al., 2007; Han et al., 2010).
The obvious strengths promote the development of domesticorganic or organic-based binders with the rapid development of ironand steel industry in China. One promising type of modified humicsubstance based binder, namely MHA binder, has been patented and
proven by laboratory studies to be suitable for iron ore agglomeration.Humic substances (HS) are the major functional fraction of the MHAbinder (Han et al., 2010; Jiang et al., 2011; Zhang et al., 2011). Thestudies showedHS performed a chemical function aswell as a physicalon the iron ore surface (Han et al., 2010). Nevertheless, there still existsa general lack of understanding of the interaction betweenHS and ironore surface.
Humic substances in the MHA binder can be further separated intotwo fractions, according to their solubility, such as fulvic acid (FA) andhumic acid (HA). FA is the fraction of humic substances that is solubleunder all pH conditions and is referred to as moderate molecularweight substances ranging from several hundreds to several thousands(Thurman and Malcolm, 1981). HA is defined as the fraction of humicsubstances that is not soluble in water under acid conditions (below pH2), but becomes soluble at greaterpH.HA is often referred toasbeing thehigh molecular weight fraction, with being estimated above tenthousand (Thurman and Malcolm, 1981). FA, compared with HA, hasmore active groups, such as carbonyl, carboxyl and alcohol hydroxyl.The former studieshad shown that the adsorptionof FAon the surfaceofmagnetite was stronger than that of HA. It can be concluded that theadsorption of FA onto iron ore surface plays a dominant role in the ironore agglomeration (Han et al., 2010). It is therefore needed for a fullunderstanding of adsorption of FA on the iron ore surface.
In general, three mechanisms are reported in the adsorption of HSonto minerals, including (i) specific adsorption with ligand exchange(coordination) (Murphy et al., 1990; Yang et al., 2009), (ii) entropy-drivenhydrophobic humic–humic interactions (Meier et al., 1999) and
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(iii) electrostatic force caused by attraction between the negatively(positively) charged mineral surface and positively (negatively)charged HS fractions (Thurman and Malcolm, 1981). For instance,coordinative and hydrophobic interactions occur between FA andsynthetic goethite (Ochs et al., 1994; Fu and Xie, 2006). Hydrophobicinteraction is one of the leading adsorption mechanisms of FA ontokaolinite (Li et al., 2008). HA adsorption by nano-oxides is mainly dueto electrostatic attraction and ligand exchange (Yang et al., 2009).
In this investigation, elemental analysis, chemical group analysis,batch adsorption experiments, X-ray photoelectron spectroscopy (XPS)investigation, zeta potential measurements and scanning electronmicroscope (SEM) imaging were used to analyze the FA sample, theadsorption of FA onto iron ore and the surface properties of FA–mineralcolloidal complex.
2. Materials and methods
2.1. Materials
The FA sample was isolated from a typical kind of brown coal inHunan province of China. The experimental procedures for separationof FA from the brown coal were shown in Fig. 1. The supernatant waspurified with a strongly acidic styrene type cation exchange resin toremove bivalent and trivalent cations, and subsequently freeze-dried.
The FA sample was characterized by elemental analysis, chemicalgroup analysis, and visible spectrum (E4/E6) as described elsewhere(Han et al., 2010). Themain characteristic of FAwas listed in Table 1. Asshown in Table 1, O/H and H/C weight ratio was respectively 0.74 and0.12, indicating that FA was polar substance (Namjesnik et al., 2000).
The natural iron ore (magnetite)was obtained from Shanxi Provinceof China. Firstly, the magnetite samples were ground and passedthrough a 10 μm vibration sieve. Then undersize magnetite granuleswere used for test. Specific surface area of undersized granules was3575 cm2/g,whichwasmeasured bya cementblaine tester. The particlesize distribution ofmagnetite granuleswas determined by laser particlesize analyzer (CILAS 1064) and the results were given in Table 2.
Main chemical compositions of magnetite granules were testedand presented in Table 3. It can be seen from Table 3 that themagnetite samples were composed of about 94% of ferroferric oxide,and the main gangue mineral is quartz.
2.2. Batch adsorption experiments
The magnetite granule suspension was first prepared by puttingsomemagnetite into a known volume of distilled water. Test solutions
Fig. 1. Flowchart outlining procedures for separation of FA from brown coal.
at the dry mass ratio of FA to magnetite were gotten by diluting someFA powders in a proper amount of magnetite granules suspension. Thesolutionswere quickly adjusted to needed pH (that is, within±0.1 pHunits) by small additions of 0.1 MNaOH buffer andwere stirredwith amagnetic stirrer at 25 °C. When it came to the fixed adsorption time,the suspensions were filtered. The filtered supernatants were dilutedwith the 0.1 MNaOHbuffer, and then the remaining FA concentrationswere measured at wavelength 465 nm with a model 722N visiblespectrophotometer immediately as well as 0.1 M NaOH buffer asreference solution. All batch adsorption experiments were conductedtwice.
The adsorbed amount (Qm) of FA onto mineral was estimated asthe difference in FA before and after adsorption according to Eq. (1)(Zhou et al., 2001):
Qm =C0−Ctð ÞMmag•V
ð1Þ
where, Qm (mg g−1) is the amount of FA adsorbed by unit weight ofmineral at different adsorption time; C0 (mg/L) and Ct (mg/L) are thefirst and final concentrations of FA in the solution, V (ml) andMmag (g)represent the volume of the liquid phase and the mass of magnetiteadded into liquid phase, respectively.
2.3. Zeta potential measurements
Zeta potentials of samples were measured with a Coulter Delsamodel 440sx by taking the average of four measurements. Magnetitegranule suspension of 10 g/L was prepared by putting 10 g magnetiteinto 1 L distill water. Test solutions at a wished FA to mineralconcentration were gotten by diluting some FA powders with a fittingamount of magnetite granule suspension. Mineral solutions with andwithout the presence of FA were checked by an acidometer PHSJ-4A.Small amounts of 0.1 MHCl or NaOHwere added to achieve pH valuesbetween 2 and 12. Ionic strength of solutions was set equally at thesame time. The temperature of the tested solution was controlled at25 °C. Each measurement was repeated four times and an averagevalue was calculated.
Samples were from the flocculation sedimentation of batchadsorption experiments. X-ray photoelectron spectroscope (XPS)Model PHI-5400 (America PE Corporation) was used to test thesurface composition of magnetite granules coated with FA fractions.The XPS measurements were made on a VG ESCALAB MkII spectrom-eter with an AlKα X-ray source (1486 eV photons). The X-ray source
Table 2Particle size distribution of magnetite granules.
Note: R2 is the coefficient of determination, used in curve fitting to show the goodness-of-fit.
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was produced by a reduced power of 150 W (15 kV and 10 mA). Thepressure in the analysis chamber was kept at 7.5×10−9 Torr or lowerduring the measurement. In peak syntheses, the linewidth (full widthat half maximum or FWHM) of the gaussian peaks was upheldconstant for all parts in a particular spectrum. Surface elementalstoichiometries were measured from peak–area ratios, after correctedwith the experimentally determined sensitivity index, and werereliable to ±5%.
2.5. Scanning electron microscope (SEM) imaging
First, samples from the flocculation sedimentation of batchadsorption experiments were pipetted onto a glass slide surface andplaced into a desiccator, where samples are dried at room temper-ature. The testing samples were sprayed with gold powder and pasteda conducting resin to improve the conductivity. Finally, the sampleswere analyzed by a SEM, Model Quanta 200 (FEI Corporation, Holand;EDAX, US).
3. Results and discussions
3.1. Adsorption kinetics and effects of the solution pH
Short-term batch experiments on the adsorption kinetics of FAonto magnetite granule surfaces at different solution pH values werefirstly studied. The adsorption amount and the fitting parameters foradsorption speed of FA ontomagnetite against the adsorption time areplotted in Fig. 2 and listed in Table 4, respectively.
The results in Fig. 2 and Table 4 indicate that adsorption amount ofFA ontomagnetite increases with prolonging the adsorption time. Thefitting equations of adsorption speed show good fits to the practicaladsorption. As adsorption time is increased to 110–120 min, theadsorption amounts at acid and basic pH conditions almost keepunchanged, which shows adsorption of FA onto magnetite quicklyreaches the equilibrium.
0 20 40 60 80 100 120 1400
2
4
6
8
10
12
14
Ads
orpt
ion
amou
nt (
mg.
g-1)
Adsorption time (min)
pH 4
pH 7
pH 10
Fitting curves
Fig. 2. Adsorption amount curves of FA adsorbed onto magnetite at a given solutionchemistry (FA solution concentration of 500 mg/L, and FA to mineral weight ratio of1.0%).
Under conditions of the same adsorption time, adsorption amountincreases obviously with increasing the solution pH value. In the viewof amuch faster adsorption equilibriumandhigher adsorption amountat higher pH value in this study, the interactions of FA fractions withmagnetite must be stronger than electrostatic repulsion. It can beconcluded that electrostatic interaction is not a chief interactionbetween the magnetite and FA.
At different FA solution concentrations, results show no greatdifferences with the results presented in Fig. 2 and are therefore notshown. However, effects of the solution pH on the interaction betweenFA and magnetite granules in a higher FA solution concentration areremarked during the experiments (Fig. 3). Fig. 3 displays that,flocculation sedimentation of magnetite granules in suspensionsolution occurs both at acid and basic pH conditions. In previousstudy of adsorption, flocculation has also been viewed. The paperreiterated adsorption and flocculation must occur simultaneously towork and should not be thought of as separate mechanisms (Hogg,2000). With prolonging the adsorption time, interestingly, thesedimentation degree increases noticeably at pH 10. It is revealedthat effects of the solution pH on the adsorption between FA andmagnetite granules are intense, which are also reflected in theadsorption kinetics. Themost likely explanation for the pH-dependentadsorptionof FA ontomagnetite is the high ability ofmagnetite to formstrong complexeswith FA. Since complex formation of magnetite withFA involves a ligand exchange with protons, complex formationincreases with increasing of pH; that is, the higher the pH, the morecomplex is formed.
3.2. XPS investigation
TheXPS spectrumprovides information on the chemical changes ofa given atom or ionic forms in themineral (Wagner and Ringgs, 1979).XPS, therefore, was used to find out whether chemical adsorption orreaction occurs on magnetite surfaces in the presence of FA. The XPSspectrums of magnetite granules and magnetite granules adsorbedwith FA is plotted in Fig. 4. Meanwhile, carbon-containing chemicalgroups of FA fractions adsorbed onmagnetite are calculated as listed inTable 5.
As seen from Fig. 4(A) and (B), the binding energy of Fe2p3/2 in themagnetite granules coatedwith FA is 712.33 eV, lower than that in themagnetite granules alone (713.45 eV). Besides, the distance betweenFe2p1/2 and Fe2p3/2 in magnetite granules coatedwith FA is also lowerthan that in the magnetite granules. Those results suggest there isspecial chemical reaction on the magnetite surface in the presence ofFA fractions (Wagner and Ringgs, 1979; Teermann and Jekel, 1999).Among the interactions between HS and minerals, ligand exchange isconsidered to be the most important and strongest interaction whichis able to cause chemical reaction on the minerals surface (Greenland,1971). Ligand exchanges as special chemical adsorption, therefore, arebelieved to occur at the link ofmagnetite and FA. Just because of strongligand exchange, FA fractions adsorb onto the magnetite quickly evenunder the condition of some electrostatic repulsion existing.
Fig. 4(C) and Table 5 show that the content of functional groups ofFA is increased on the surface of magnetite. According to the calcu-lation, content of polar groups containing carbon of FA is about
Fig. 3. Photograph of effects of the solution pH on the interactions between FA and magnetite granules in suspension (FA solution concentration of 1000 mg/L, and FA to mineralweight ratio of 2.0%) (A) at pH 4; (B) at pH 10.
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42.84%, with nonpolar groups of 57.16%. It shows that FA fractionshave been adsorbed onto magnetite surfaces. The result is in agree-ment with the existing works which show that adsorption of naturalorganic matter (NOM) onto mineral surfaces is assigned to their polarcarboxylic and phenolic groups (Mcknight et al., 1992).
3.3. Zeta potential study
Although XPS provides qualitative analysis, it cannot explainclearly adsorption of FA onto mineral surface. To complement XPS,zeta potentialmeasuringwas used to analyze the interactions betweenFA and magnetite at acid and basic pH conditions.
Zeta potential, often, can be used to estimate the likely effect ofvarious adsorbate on the properties of minerals (Hunter, 1981). The
Fig. 4. XPS spectrums of magnetite granules and magnetite granules coated with FA. (A) Bingranules coated with FA at pH 10, FA solution concentration of 1000 mg/L, and FA to miner
zeta potentials versus pH curvesmatching to the suspendedmagnetitegranules in the absence and presence of FA are shown in Fig. 5.
From Fig. 5, it can be seen that the zeta potential of magnetiteparticles with and without FA is affected with the changing of thesolution pH. The pH value where the zeta potential changes sign orisoelectric point (IEP) characterizes the electrical properties of amineral (Hunter, 1981). The IEP of the magnetite is at pH 6.1 in purewater solution, which is close to the reported IEP of iron oxides at pH6.5 (Wang, 1994). As seen from Fig. 5, FA adsorption is observed largerthan the magnetite IEP value, indicating that electrostatic effects arelikely not the main controls on adsorption (Mahir et al., 2005). Thisconclusion is in line with the results of adsorption kinetics of FA ontomagnetite and XPS results.
Meantime, it is shown in Fig. 5 that the zeta potentials ofmagnetiteadsorbed with FA are further increased and become positive at either
ding energy of Fe in magnetite granules at pH 10; (B) binding energy of Fe in magnetiteal weight ratio of 1.0%; (C) XPS C1s spectrum of sample (B).
Table 5Carbon-containing chemical groups of FA frinteractions on magnetite surface (wt. %).
Polar groups Nonpolar groups
COO C_O C\O C\H C_C
8.38 8.96 25.98 43.33 13.73
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acidic or alkaline pH values. As the pH is increased, the zeta potentialvalue grows to reach maximum value around the IEP. While pH isincreased from the IEP to pH 12, the magnitude of the zeta potentialdecreases slightly. The trends are in good agreement with the findingsof other researchers (Wang, 1994). It has been recognized thatadsorption of FA onto magnetite particles can be characterized byspecific chemical reaction (ligand exchange) from the results of XPSstudy. Literatures show that an acidic organic ligand which iscoordinated to the metal ion on the surface of the oxide, replaces ahydroxyl group, to increase the density of acidic protonation (Tipping,1981). FA fractions are concentrated on mineral surfaces, leading tolocally increased charge densities. This would result in an accumu-lation of acidic protonation on the surface of mineral. When density ofacidic protonation comes to certain extent, it can make the charge ofmineral surface reverse even though the absorbed FA fractions carrynegative charges. Positive zeta potentials of magnetite at acidic andalkaline pH values suggest that specific adsorption (ligand exchange)of FA is so strong that electrostatic force does not have enough powerto overcome it.
The increase in zeta potential below the IEP and the decrease inzeta potential above the IEP with the pH value increasing can beexplained by considering the ligand exchange and electrostaticinteractions between the acidic moieties of FA and the magnetitesurface. Under the condition of pH values smaller than the IEP ofmagnetite, magnetite is positive. With increasing of solution pH andionization, fulvic acid carries more and more negative charges. Theincrease of zeta potential due to that ligand exchange is strengthenedby electrostatic attraction occurring at the boundary of magnetite andFA. Under the condition of pH values larger than the IEP, magnetite isnegative. The decrease of zeta potential lie in that ligand exchange isweakened by electrostatic repulsion occurring at the boundary.
From the results mentioned above, it can be concluded thatadsorption of FA onto magnetite particles can be characterized byspecific ligand exchange and electrostatic interactions.
2 4 6 8 10 12-6
-4
-2
0
2
4
6
8
10 Magnetite Magnetite coated with FA
Zet
a po
tent
ial (
mV
)
pH
IEP=6.1
Potentials increase
Fig. 5. Zeta potentials versus pH curves matching to the suspended magnetite granulesin the absence and presence of FA.
3.4. SEM imaging
Flocculation has been observed in the adsorption. SEM imaging,therefore, was conducted to provide insights into the nature ofinteractions between FA and magnetite granules and a true picture ofmagnetite–FA colloidal complex. The surface topographies of themagnetite before and after FA adsorption are shown in Fig. 6.
As shown in Fig. 6(B), compared with magnetite without FAfractions in Fig. 6(A), flocculation indicates that adsorption of FA ontomagnetite granules takes place. Magnetite particles are linkedwith FAmolecule. And FAmolecule presents on the surface of magnetite in theshape of an araneose histioid network. Adsorption of FA fractions ontomagnetite is mainly related to both their carboxyl groups and theirmolecular weight (Gu et al., 1995; Chi and Amy, 2004).
As listed in Table 2, the size of magnetite particles is smaller than10 μm, with an average particle size of 3.64 μm. However, markeddifferences are found to exist in the shape and size of particles. Ballshaped magnetite–FA colloidal complex with a diameter of 20–100 μmpresents in the SEM images of FA-coatedmagnetite samples in Fig. 6(C)and (D), while no “ball” aggregates can be seen from the blank samplesin Fig. 6(A). Appearance of large ball-shaped substance indicatescompressed conformation in the magnetite–FA colloidal complex.However, it is clear that these images cannot be directly used to inferthe inner structure of the magnetite–FA colloidal complex.
Formation of magnetite–FA colloidal complex can be mainlyexplained as two reasons. One is that ligand exchange takes place inthe link of FA andmagnetite particles. Ligand exchange increases withincreasing of pH value, which makes the adsorption amount increaseas pH increases, and flocculation are obvious at higher pH values. Also,the adsorbed FA can have influences on the chemical properties of theunderlying mineral (for example, electrical property or chargedensity), which plays an important role in changing the interfacialbehavior (Davis, 1982). Just as described in Fig. 5, zeta potentials ofthe magnetite coated with FA are changed by ligand exchange, beingpositive at a wide range of pH values. Adsorption of the polar moietiesof FA and the magnetite onto the FA-coated magnetite are improvedby electrostatic attractions. Therefore, FA fractions, fine magnetiteparticles, and the FA-coatedmagnetite are aggregatedmore obviouslyat higher pH values.
The other reason is that organic macromolecules formed byhydrophobic FA–FA interactionsmightpromote smallmagnetite particlesor small magnetite–FA colloidal complex to form larger magnetite–FAcolloidal complex (Fig. 6(C) and (D)). A highermagnification image of thecomplex surface is shown in Fig. 6(E) and (F), which show that FAaggregates are observable on the surface of magnetite–FA colloidalcomplex. FA aggregates arebelieved togrowbecauseof polymer chains toadhere to one another on contact (Hogg, 2000). As well as the size of theaggregates, clearly, the multilayer aggregates noted in Fig. 6(F) arecomposed of manymuch smaller macromolecules as viewed in Fig. 6(E).It shows that the hydrophobic FA–FA interactions lead to multiplemolecule adsorption of FA to complex. Meanwhile, hydrophobic in-teractions on the magnetite–FA complex may provide new hydrophobicadsorption sites condensed aromatic or long chain aliphatic moieties(Brunauer et al., 1938; Murphy et al., 1990).
In brief, flocculation sedimentation is accompanied with the FAadsorption ontomagnetite. Finemagnetite particles are aggregated bythe adsorbed FA. When magnetite–FA colloidal complex grows to thesuitable size, sedimentation forms as presented in Fig. 3. Ligandexchange, hydrophobic interactions and electrostatic attractioncontribute to the adsorption, but ligand exchange and hydrophobicinteractions dominate the adsorption mechanisms.
4. Conclusions
This investigation is carried out to understand the interactionbetween FA and iron ore surface. Adsorption of FA onto magnetite
Fig. 6. SEM images of (A) magnetite granules; (B) magnetite granules coated with FA network; (C) small magnetite–FA colloidal complex; (D) hugemagnetite–FA colloidal complex;(E) small FA fraction aggregates on the magnetite–FA colloidal complex surface; and (F) huge FA fraction aggregates on the magnetite–FA colloidal complex surface.
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surface is dependent on pH value and adsorption amount of FA isapparently increased with the solution pH value increasing. Ligandexchange makes charge of magnetite surface reverse even though theadsorbed FA carries negative charges. Flocculation sedimentation isaccompanied with the FA adsorption onto magnetite. And theflocculation sedimentation may be influenced by adsorbed FAfractions. The adsorption shows that ligand exchange, hydrophobicinteractions and electrostatic attraction are contributive to theadsorption, while ligand exchange and hydrophobic interactionsdominate the mechanisms.
Acknowledgments
This work is supported by the National Science Fund forDistinguished Young Scholars (no. 50725416), the Key Program inNational Science and Technology Pillar Program during the 11th Five-year Plan Period of China (2008BAB32B06), the National NaturalScience Foundation of China (no. 50804059) and Graduate Degree
Thesis Innovation Foundation of Hunan Province and Central SouthUniversity (CX2010B063).
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