Separation and Purification Technology 94 (2012) 34–38

Contents lists available at SciVerse ScienceDirect

Separation and Purification Technology

journal homepage: www.elsevier .com/locate /seppur

Rapid and direct magnetization of goethite ore roasted by biomass fuel

Yan Wu a,⇑, Mei Fang b, Lvdeng Lan a, Ping Zhang a, K.V. Rao b, Zhengyu Bao a

a Faculty of Materials and Chemistry, China University of Geosciences, Wuhan 430074, Chinab Department of Materials Science, The Royal Institute of Technology, Stockholm, Sweden

a r t i c l e i n f o

Article history:Received 5 January 2012Received in revised form 6 April 2012Accepted 9 April 2012Available online 17 April 2012

Keywords:Biomass fuelReduction roastingGoethite oreMagnetization

1383-5866/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.seppur.2012.04.008

⇑ Corresponding author. Tel.: +86 27 67883731.E-mail address: wuyan@cug.edu.cn (Y. Wu).

a b s t r a c t

Biomass is a renewable and carbon neutral solid fuel. Utilization of biomass in iron ore roasting process asheating agent and reducing agent contributes to energy conservation and emission reduction, and canpartially replace for coal and coke. The biomass instead of coke was mixed together with iron ore powderfrom the north of Hainan province into ball roasting process to investigate the effects of mixture compo-sition, reduction temperature, reaction time, the thermal reduction and magnetic properties of the mix-ture. The results show that the reduction temperature, reaction time and dosage of the biomass arecorrelated to the quality of the reduction and the magnetism of the iron ore, within the experimental con-ditions. The mechanism of the biomass reducing the weakly magnetic goethite into stronger magneticiron oxide has been discussed. The results show that the goethite ores is dramatically reduced and mag-netized by about 20 wt.% biomass at low roasting temperature. Application of biomass energy in iron oresroasting process is prospective to the effective use of biomass and for decreasing the consumption of fos-sil fuels in the steelmaking process.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Goethite (a-FeOOH) ore is formed in terrestrial environment, asa result of oxidative weathering and acid soil formation. It is theone most common and abundant iron oxide and oxyhydroxideminerals in soils. The overwhelming part of these ores is repre-sented by low-grade varieties which cannot be used in iron andsteel industry without preliminary beneficiation. Conventionaltechnique of magnetization and roasting goethite ore is naturalgas or coal based reducing roasting method [1], which consumesmost of the energy and resources and brings about environmentalpollution. Furthermore, metallurgical coke, one of indispensableraw materials for blast furnace, is in increasing shortage and cokingprocess causes serious pollution. Therefore, it is of great signifi-cance for the interests of iron and steel enterprises and society toachieve a low-carbon, energy-saving, emission-cutting and lowcost ironmaking process.

Biomass is a renewable solid fuel, which is feasible for the sub-stitution of coke and coal in the manufacturing due to its lowercontents of harmful elements (S, P and others) and abundant rawmaterial sources. Few studies were carried out on ironmakingusing biomass [2,3]. They studied the possibility of using biomassfor ironmaking by using dehydrated goethite ore and biomass.The reducing agent is carbon monoxide (CO) from the pyrolysisof biomass. Firstly goethite ore was dehydrated at 450 �C to be

ll rights reserved.

porous (the dehydration is to create pores) while the pyrolysis ofbiomass occurred within this temperature range and the reducegas, CO generated; subsequently, the produced CO around thesurface or among the porous of the iron ore forms the reducingatmosphere for the reduction of iron oxide.

Dehydroxylation is the structural transformation leading to thedestruction of the structural OH-groups or loss of mineral crystal-line bound water [4]. This is a step which naturally occurs in thereduction process as a result of heating. This is followed by rebuild-ing of the crystalline structure. The effect of calcination in thereduction efficiency of goethite ore and its subsequent productsdo not appear to have received much attention, and the magnetismof the goethite ore during the reduction roasting process is still notwell understood. This study involves an investigation of the effectof direct reducibility and magnetization of pine tree biomass ongoethite ore.

2. Experimental

2.1. Reduction roasting and structural analysis

The tropical goethite ore used in this study originated from theNorth of Hainan Island, which was grounded into �100 lm pow-der. The typical chemical composition of the ore is given in Table 1,and the loss on ignition (LOI) is 12.28%. The main phase structuresof the raw ore are SiO2 and FeOOH.

The pine sawdust was used as biomass; before use, it was pul-verized into �630 lm fine powder. The elemental compositions of

Table 1Main composition of raw ore (mass fraction, %).

Composition TFe FeO SiO2 Al2O3 MgO CaO Mn TiO2

Wt.% 40.01 0.26 16.76 8.32 0.079 0.005 0.70 2.51

As V Mn Cu Co Ni P S LOI

Wt.% 0.0016 0.027 0.49 0.0034 0.007 0.0056 0.53 0.064 12.28

Y. Wu et al. / Separation and Purification Technology 94 (2012) 34–38 35

the biomass used were C/H/O = 51.00:6.00:43.00 (mass%). The bio-mass was homogeneously mixed with goethite ore and water toroll into small balls with the diameter about 10 mm for the reduc-tion roasting in the preheated muffle furnace. The mass ratio ofbiomass was varying from 10% to 30%, the heating temperatureranging from 500 to 700 �C, and the roasting time from 10 to60 min.

The solid phase structure was analyzed by X-ray Diffractionme-ter, X’ Pert PRO MPD. Thermal decomposed properties were char-acterized by STA 409 PC simultaneous ThermogravimetricAnalyzer and Differential Scanning Calorimeter (TGA–DSC), withthe heating rate of 10 �C/min in air.

2.2. Magnetism analysis

The magnetization (M) of a substance, in general, depends onthe magnetic field (H) acting on it. Its magnetic properties are char-acterized not only by the magnitude and sign of M and also by theway in which M varies with H. The ratio of these two quantities iscalled the susceptibility j, i.e. j = M/H. Generally the mass suscep-tibility v, the ratio of susceptibility and the density, is used.

The measured magnetic susceptibility, in a relatively low ap-plied magnetic field, can be represented by the Owen–Hondamethod [5,6]:

x ¼ xp þ xd þMS

H¼ x1 þ

MS

Hð1Þ

Where xp and xd are paramagnetic and diamagnetic specific sus-ceptibilities, respectively, x1 is the specific magnetic susceptibilityextrapolated to the infinite magnetic field, and MS is the saturationmagnetization of the ferromagnetic component of the mixture. x1is characteristic of a mineral, and is nearly constant for samples ofthe same mineral from different localities. On the other hand, sat-uration magnetization MS, which indicates the degree of ferromag-netism, varies widely between samples from differing localitiesand between size fractions of the same sample [5,8]. If a magneti-zation versus field curve shows evidence of both paramagnetic andferromagnetic components, then it is possible to construct aHonda–Owen plot and separate the two aspects of the magnetiza-tion. This involves plotting the magnetic susceptibility of the sam-ples against the reciprocal of the field in the dM=dH ¼ a region.This plot would show a linear relationship, with the y-interceptbeing the magnetic susceptibility, x, of the paramagnetic compo-nent. As x for a paramagnetic (or diamagnetic) material is constant,the paramagnetic fraction of the total sample magnetization can becalculated, and this can be subtracted from the total magnetizationof the sample to obtain the ferromagnetic magnetization. The roomtemperature magnetic property was characterized by vibratingsample magnetometer (VSM, Model 155 EG&G Princeton AppliedResearch).

Fig. 1. Magnetic hysteresis loop of raw iron ore. The inset is the enlarged loop atlow field.

3. Results and discussion

The oxide of goethite ore after dehydration is a-Fe2O3, and thereduction processes are carried out step by step: FeOOH ?a-Fe2O3 ? Fe3O4 ? c-Fe2O3. The gradual deoxidizing mechanism

(H2 and CO as reduction agents) is described by the following reac-tions shown in Eqs. (2)–(4) [1,7]. When the cooling procedure isprocessed in air at lower temperature, the oxidation of magnetiteoccurs: Fe3O4 ? c-Fe2O3, shown in Eq. (5) [8].

2FeOOH ¼ a� Fe2O3 þH2O ð2Þ

3a� Fe2O3 þ CO ¼ 2Fe3O4 þ CO2 ð3Þ

3a� Fe2O3 þH2 ¼ 2Fe3O4 þH2O ð4Þ

4Fe3O4 þ O2 ¼ 6c� Fe2O3 ð5Þ

3.1. Magnetic properties of raw goethite ore

The room temperature magnetization (M) of raw iron ore asfunction of the applied field (H) is presented in Fig. 1. The insetis the enlarged loop at smaller magnetic field. It apparently revealsparamagnetic behavior within the complete cycle of magnetic fieldfrom 20 to �20 kOe, and at smaller field, it presents weak ferro-magnetic characteristic with the coercivity of about 150 Oe.Goethite is an antiferromagnet with a weak ferromagnetism belowthe Néel temperature TN = 120 �C [9].This hydroxide has a very sta-ble magnetization at room temperature. The specific magnetic sus-ceptibility is between 26 � 10�8 and 280 � 10�8 m3 kg�1 [10,11].Goethite belongs to the group of ‘‘hard magnetic materials’’, whichneed high magnetic fields in order to reach magnetic saturation.Although both minerals have similar remanent coercivities, thesaturation remanent magnetization of goethite is obtained withhigher magnetic fields. Some samples containing goethite couldnot reach their saturation at fields of 10 T [12].

Fig. 3. Effect of reduction temperature on saturation magnetization (MS) of rawgoethite ore and mixture (goethite mixed with 20 wt.% biomass).

Fig. 4. Effect of reduction time on saturation magnetization (MS) of mixture(goethite mixed with 20 wt.% biomass).

36 Y. Wu et al. / Separation and Purification Technology 94 (2012) 34–38

3.2. Magnetic properties of goethite ore reduced by biomass

Above result of raw goethite ore points out that it is not feasibleto directly separate iron oxide minerals from the gangues. Improv-ing the magnetism of iron oxide minerals through reducing agentscould solve this issue. We investigate the magnetization of the goe-thite ores roasted with the ratio of biomass of 20 wt.% (mixture) atdifferent roasting temperature (from 550 to 700 �C) and the roast-ing time of 30 min. Fig. 2 presents the magnetic hysteresis loops formixture roasted at different temperatures. It is obvious that thebiomass shows huge enhancement of the magnetization of themixture, compared with the negligible magnetization of raw goe-thite ore. The relationship of the saturation magnetization andthe reduction temperature is shown in Fig. 3. Adding with 20% bio-mass, the magnetic loops of all samples completely become ferro-magnetic characteristics, and the saturation value of themagnetization (MS) goes up to 25.9 emu/g at 550 �C, when the ap-plied magnetic field up to about 1 kOe (�80 kA/m). Furtherincreasing the roasting temperature, the MS value slightly in-creases to 28.9 emu/g, and then almost keeps it constant. There-fore, we could conclude that the biomass was decomposed toreduced gases at large when the temperature is around 500–550 �C, which efficiently reduce the goethite ore into strongermagnetic oxides [13].

Figs. 4 and 5 present the relationships of reduction time, andbiomass percentage on saturation magnetization of goethite oremixed with biomass, respectively. For the goethite ore mixed with20 wt.% biomass, as shown in Fig. 4, at all reduction temperaturesthe saturation magnetization (MS) increases to the maximum valuewith the roasting time up to 30 min, then it turns to decrease whenthe time prolongs further. Thereby, we delude that the reductiontime of 30 min is the optimum time for reduction. Shorter timeleads the reduced reaction incomplete and longer roasting timemake the reduced product oxides again in the air, decreasing itsmagnetic property. Fig. 5 demonstrates the saturation magnetiza-tion (MS) of the product (roasting was conducted at 550 �C for30 min) varies with different percentages of biomass in the mix-ture. The saturation magnetization MS value is dramaticallyincreasing with the percentage of biomass up to about 20%. Whileafter the content of the biomass is more than 20%, the MS valuestarts to slightly decrease down. In general, more biomass pro-duces higher concentration of reducing gases, which could have abetter reduction on the iron ore, leading to the high content ofFe2+. We deduce that there could have a critical concentration forthe reducing gases, higher concentration of the reduction gasescould over-reduce the iron oxide, leading to the lower magneticproperty of the product [14].

Fig. 2. Magnetic hysteresis loops for mixture (goethite ores with 20 wt.% biomass)roasted at different temperatures.

Fig. 5. Effect of percentage of biomass with goethite on saturation magnetization(MS) of mixture roasted at 550 �C.

3.3. Magnetic susceptibility analysis

Figs. 6–9 show Honda–Owen plots for raw goethite ore (sampleA), mixture with 20% biomass roasted at 550 �C (sample B), 600 �C(sample C), and 650 �C (sample D). The magnetic susceptibilityof paramagnetic component is determined from each graph.

Fig. 6. Honda–Owen plot for sample A (raw goethite ore).

Fig. 7. Honda–Owen plot for sample B (mixture roasted at 550 �C).

Fig. 8. Honda–Owen plot for sample C (mixture roasted at 600 �C).

Fig. 9. Honda–Owen plot for sample D (mixture roasted at 650 �C).

Table 2Magnetic susceptibility of paramagnetic and ferromagnetic percentages at the fieldH = 100 kA/m in different samples.

Sample Roastingtemp.(�C)

Total magnetic susceptibility(v, m3 kg�1)

Para.(%)

Ferro.(%)

A – 1.344E�6 31.20 68.80B 550 3.775E�5 3.50 96.50C 600 4.282E�5 2.02 97.98D 650 4.881E�5 1.95 98.05

Y. Wu et al. / Separation and Purification Technology 94 (2012) 34–38 37

The y-intercept is the magnetic susceptibility of the paramagneticfraction of the sample, from which the magnetization of the para-magnetic component can be plotted. Subtracting the paramagneticmagnetization from the total magnetization gives the ferromag-netic component of the magnetization.

Honda–Owen plots show that there are ferromagnetic and para-magnetic parts both in raw goethite ore and all samples treated by

biomass. Table 2 presents paramagnetic, ferromagnetic percent-ages, and the total magnetic susceptibility values of the samples. Itcan be seen clearly that after treated by biomass at 550 �C, the para-magnetic percentage considerably drops from 31.2% to less than 5%,corresponding to the ferromagnetic percentage dramaticallyincreasing from 68.80% to above 96%. The total magnetic suscepti-bility of the samples treated by biomass increases more than 30times than the raw goethite ore at the magnetic field of 100 kA/m.While the roasting temperature further increasing, the total mag-netic susceptibility and the ferromagnetic percentage of the mixturejust slightly increase.

Fig. 10 demonstrates the X-ray diffraction patterns of raw ore,and mixture (the ratio of biomass: 20%) roasted at 550 and650 �C, respectively. It is clear to see that the raw goethite oremainly contains FeOOH and SiO2. After mixing with biomass trea-ted at high roasting temperature, FeOOH in the raw ore loses thecombined water and converts into maghemite (c-Fe2O3). As weknown, at reducing atmosphere the iron oxide is firstly reducedinto magnetite [15]. Sequentially cooled down in air, the magnetitewas mainly oxidized into maghemite (c-Fe2O3), which is the finalstructure of the product roasted from the iron ore mixed with bio-mass. According to the chemical analysis of divalent iron, the max-imum percentage of Fe2+ is about 0.8%. Generally, maghemite(c-Fe2O3) is formed by weathering or low-temperature oxidation.Although it is difficult to distinguish magnetite from maghemiteby XRD [16,17], from our valence analysis of iron, divalent iron isjust around 5% of the total iron. From this consideration, it couldbe concluded that the product in the present experiment wasmaghemite (c-Fe2O3). This is consistent with the maghemite(c-Fe2O3) product of mixture, with strong magnetic property asmagnetite [18].

The susceptibility of the raw goethite ore is about1.34 � 10�6 cm3/g. However, when mixed with biomass androasted at 550 �C, it is increasing to 37.7 � 10�6 cm3/g at the fieldof 100 kA/m, 30 times larger than the raw goethite ore. The change

Fig. 10. The X-ray diffraction pattern of raw ore, and mixture roasted at differenttemperatures.

38 Y. Wu et al. / Separation and Purification Technology 94 (2012) 34–38

of the susceptibility of the iron ore is not linear when the strongmagnetic iron ore existing [19], we could not get the content ofthe c-Fe2O3. But the magnetic separation is efficient for the furtherimproving the quality of the iron ore.

4. Conclusions

The paper presents a novel direct reduction technology usingbiomass as raw material in goethite ore roasting process. Thereduction of goethite ore was conducted in a lab-scale reactor.The results show that the magnetization of goethite ore mixedwith biomass roasted at low roasting temperature could be highlyreduced and magnetized, reaching the saturation value of magne-tization about 25.9 emu/g at the magnetic filed of about 1 kOe.When the goethite ore roasted with biomass, the magnetic suscep-tibility could be enhanced 30 times than the raw goethite ore atlow roasting temperature and the ferromagnetic percentage ismore than 96%, which could significantly reduce the iron benefici-ation cost. Application of biomass energy in iron ore roasting pro-cess is prospective to the effective use of biomass and for

decreasing the consumption of fossil fuels in the steelmakingprocess.

Acknowledgment

The authors wish to acknowledge the financial support receivedfrom the Fundamental Research Founds for National University,China University of Geosciences (Wuhan).

References

[1] G.S. Liu, V. Strezov, J.A. Lucas, L.J. Wibberley, Thermal investigations of directiron ore reduction with coal, Thermochimica Acta 410 (2004) 133–140.

[2] S. Kudo, K. Sugiyama, K. Norinaga, C.Z. Li, T. Akiyama, J.I. Hayashi, Coproductionof clean syngas and iron from woody biomass and natural goethite ore, Fuel, inpress.

[3] Y. Hata, H. Purwanto, S. Hosokai, J.I. Hayashi, T. Akiyama, Biotar ironmakingusing wooden biomass and nanoporous iron ore, Energy & Fuels 23 (2009)1128–1131.

[4] V. Strezov, T.J. Evans, V. Zymla, L. Strezov, Structural deterioration of iron oreparticles during thermal processing, International Journal of MineralProcessing 100 (2011) 27–32.

[5] K. Honda (Ed.), Magnetic Propflies of Matter, Syokwabo and Company, Tokyo,Japan, 1928.

[6] R.E. Treece, J.A. Conklin, R.B. Kaner, Metathetical synthesis of binary andternary antiferromagnetic gadolinium pnictides (P, As, and Sb), InorganicChemistry 33 (1994) 5701–5707.

[7] S. Luo, C. Yi, Y. Zhou, Direct reduction of mixed biomass-Fe2O3 briquettes usingbiomass-generated syngas, Renewable Energy 36 (2011) 3332–3336.

[8] K. Przepiera, A. Przepiera, Kinetics of thermal transformations of precipitatedmagnetite and goethite, Journal of Thermal Analysis and Calorimetry 65 (2001)497–503.

[9] Ö. Özdemir, D.J. Dunlop, Thermoremanence and Néel temperature of goethite,Geophysical Research Letters 23 (1996) 921–924.

[10] J. Walden, F. Oldfield, E.J.P. Smith (Eds.), Environmental Magnetism: A PracticalGuide. Technical Guide, No. 6, Quaternary Research Association, London, 1999.

[11] C.P. Hunt, B.M. Moskowitz, S.K. Banerjee, Magnetic properties of rocks andminerals, in: T.J. Ahrens (Ed.), Rock Physics and Phase Relations: A Handbookof Physical Constants, 3, American Geophysical Union, Washington, DC, 1995,pp. 189–204.

[12] D.E. France, F. Oldfield, Identifying goethite and hematite from rock magneticmeasurements of soils and sediments, Journal of Geophysical Research 105(2000) 2781–2795.

[13] K. Higuchi, R.H. Heerema, Influence of sintering conditions on the reductionbehaviour of pure hematite compacts, Minerals Engineering 16 (2003) 463–477.

[14] Y.B. Wang, G.C. Zhu, R.A. Chi, Y.N. Zhao, Z. Cheng, An investigation on reductionand magnetization of limonite using biomass, The Chinese Journal of ProcessEngineering 9 (2009) 508–513.

[15] X.G. Huang, Steel Metallurgy Principle, third ed., Metallury Industry Press,Beijing, 2007.

[16] Y. Kashiwaya, T. Akiyama, Nanocrack formation in hematite through thedehydration of goethite and the carbon infiltration from biotar, Journal ofNanomaterials 2010 (2010) 1–12.

[17] K. Hanedaand, A.H. Morrish, Magnetite to maghemite transformation inultrafine particles, Journal de Physique 38 (1977) 321–323.

[18] F. Adam, B. Dupre, C. Gleitzer, Cracking of hematite crystals during their low-temperature reduction into magnetite, Solid State Ionics 32–33 (1989) 330–333.

[19] X. Chen, W.G. Zhang, L.Z. Yu, The dependence of magnetic parameters on themixing proportion of hematite and magnetite, Process in Geophysics 24 (2009)82–88.