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Вюник Дшпропетровського унiверситету. Сер^я: Геологiя, география Visnik Dnipropetrovs'kogo universitetu. Seria Geologia, geographia Dnipropetrovsk University bulletin. Series: Geology, geography
Dnipropetr. Univ. Bull. Ser.: Geol., geogr. 2015. 23(1), 24-32. doi: 10.15421/111503
ISSN 2313-2159 print ISSN 2409-9864 online
http://geology-dnu.dp.ua
УДК 553.3+54.01+542.9+549.5
Phase transformation of goethite into magnetite by reducing with carbohydrates
N. O. Dudchenko*, V. P. Ponomar**
*M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of NASU, Kyiv **Taras Shevchenko National University of Kyiv
Phase transformations of synthetic goethite and goethite ore from Kryvyi Rih region by reducing with different carbohydrates (starch, glucose, fructose, sucrose and ascorbic acid) were investigated by thermomagnetic analysis. Thermomagnetic analysis was carried-out using laboratory device that allows automatic registration of sample magnetization with the temperature (the rate of sample heating/cooling was 65-80°/min). The reduction reaction of synthetic goethite for all carbohydrates starts at the temperature of ~250°C while reduction of goethite ore for all carbohydrates starts at the temperature of ~450°C. We could relate this increasing of reduction start temperature with shielding effect of admixtures in the ore. Reduction of synthetic goethite at this temperature range leads to formation of magnetic phase with saturation magnetisation ~70 A*m2/kg. At the same time, reduction of goethite ore leads to formation of magnetic phase with saturation magnetisation ~25 A*m2/kg. One could attribute this decreased value of saturation magnetisation to the presence of other minerals (quartz, etc.) in the ore. It was shown by X-Ray Diffraction method that goethite completely transforms into magnetite under heating with different carbohydrates up to 650°C. All carbohydrates reduce goethite to magnetite.
Key words: goethite, magnetite, phase transformations, thermomagnetic analysis.
*1нститут reoxiMii, мшералогй та рудоутворення iMeHi М. П. Семененка НАН Украши, просп. Академжа Палладша, 34, Кжв, 03680, Украша.
M.P. Semenenko Institute of Geochemistry, Mineralogy and Ore Formation of NASU, pr. Akademika Palladina, 34, Kyiv, 03680, Ukraine.
Tel.: +38-067-408-47-97. E-mail: ndudchenko@igmof.gov.ua
**Ки!вський нацюнальний ушверситет iмeнi Тараса Шевченка, вул. Володимирська, 64/13, Ки1в, 01601, Укра1на
Taras Shevchenko National University of Kyiv, Volodymyrska Street, 64/13, Kyiv, 01601, Ukraine. Tel.: +38-063-597-29-33. E-mail: vitaliyponomar.vp@gmail.com
Dnipropetr. Univ. Bull. Ser.: Geol., geogr. 2015. 23(1)
Фазов1 перетворення гетиту в магнетит за його вщновлення
вуглеводами
Н. О. Дудченко*, В. П. Пономар**
*1нститут геохгмИ] мтералоги та рудоутворення шет М. П. Семененка, Кшв **Кш'вський нацгональний унгверситет шеш Тараса Шевченка
Дослвджено фазов1 перетворення синтетичного гетиту i гетитово!" руди з Кривор1зького зал1зорудного басейну за допомогою термомагштного аналiзу при вiдновленнi рiзними вуглеводами (крохмаль, глюкоза, фруктоза, цукроза та аскорбшова кислота). Термомагнггш досл1дження проведено за допомогою лабораторно!" установки автоматично!" реестращ!" намагнiченостi залежно в1д температури (швидк1сть нагрiву/охолодження 65 - 80°/хв). З'ясовано, що реакцiя ввдновлення синтетичного гетиту для вах вуглеводiв починаеться за температури ~250°С, а ввдновлення гетитово!" руди для вс1х вуглевод!в починаеться за температури ~450°С. Наведено припущення, що пiдвищення температури початку реакцй" вiдновлення пов'язано з ефектом екранування домiшками в руд1. Ввдновлення синтетичного гетиту в цьому температурному д!апазош зумовлюе формування магштно!" фази з намагшчешстю насичення ~70 А-м2/кг, у той час як ввдновлення гетитово!" руди призводить до формування магштно!" фази з намагнiченiстю насичення ~25 А^м2/кг. Зменшення намагнiченостi насичення можна пояснити наявшстю 1нших мшерал!в (кварц та 1н.) у рудь За допомогою рентгенофазового аналiзу показано, що гетит повшстю перетворюеться на магнетит у раз! нагр!вання з р!зними вуглеводами до температури 650°С. вс1 вуглеводи ввдновлюють гетит до магнетиту.
Ключовi слова: гетит, магнетит, фазовi перетворення, термомагнiтний ан^з.
Introduction. Goethite (a-FeOOH) is an iron-containing mineral named by Johann Wolfgang von Goethe. It is widely spread in ores, sediments and soils and is one of the most thermodynamically stable iron oxide at ambient temperature [4]. Goethite powder has yellow colour. Goethite is often formed as sedimentary rock, and its formation requires the presence of water. That's why it is often found within spring water wetlands, and at lake and creek bottoms. It is always present in ore deposit oxidation zones. At the beginning of the 21st century, Mars Exploration Rover «Spirit» discovered goethite on the Red Planet's surface.
Goethite is antiferromagnetic [4] with Neel temperature (TN) of 120°C. At high temperatures goethite can easily loose water and transforms into hematite. Transformation temperature depends on crystallinity of goethite.
Goethite now is mainly used for production of pigments and magnetic iron oxides [8], as an adsorbent for different toxic anions [6] and cations [3].
Nowadays, the usage of weakly magnetic iron ores for iron production is of very importance, because the deposits of magnetite iron ores are becoming exhausted. The main problem, that interferes the usage of weakly magnetic goethite ores for iron production is complexity of their beneficiation, i.e. separation of goethite from other admixtures in the ore. Therefore, number of investigations concerning transformation of goethite into magnetic iron oxides (mainly, magnetite, that is suitable for magnetic separation) by reduction with different reducing agents (coal [9], hydrogen and carbon monoxide [7], biomass etc. [10; 11] at increased temperatures have been carried-out in order to develop new technologies of goethite ore beneficiation.
Dnipropetr. Univ. Bull. Ser.: Geol., geogr. 2015. 23(1)
The aim of this work was to investigate the phase transformations of synthetic goethite and goethite ore from Kryvyi Rih region into magnetic phase by reducing with different carbohydrates, i.e. starch, glucose, fructose, sucrose and ascorbic acid.
Materials and methods. Samples of synthetic goethite were synthesised by procedure [2]. Samples of goethite ore from Kryvyi Rih region were grinded up to 0,07^0 mm. Starch, glucose, fructose, sucrose and ascorbic acid («Chimlaborreactive», Ukraine) were used as reducing agents.
The changes of magnetization with the temperature were determined with laboratory build facility. This facility enables to measure the force that effects on the sample in nonuniform magnetic field. This force is proportional to magnetization and the gradient of magnetic field.
Initial samples were mixed with 4% (m/m) carbohydrates (starch, glucose, fructose, sucrose and ascorbic acid) and this mixture (0,2 - 0,5 g) was used further for thermomagnetic analysis. Reduction of goethite and goethite ore with different carbohydrates was performed in quartz mini-reactor, isolated from atmospheric oxygen (Vreactor=4 cm3), under heating up to 650°C. The rate of sample heating/cooling was 65-80°/min.
The initial and obtained samples were investigated by the methods of X-Ray Diffraction (XRD) and magnetometry. XRD measurements were performed with a diffractometer DRON-UM1 in filtered emission CuKa with recording geometry by Bragg-Brentano. Measurements of saturation magnetisation were performed using a magnetometer with Hall sensor (Ukraine).
The XRD phase diagnostics was performed using [1] by detected d-spacing.
Results and discussion. It was shown by the XRD-data that the initial sample of synthetic goethite was pure goethite (fig. 1, a) and the initial sample of goethite ore was composed by quartz, kaolinite, goethite and hematite (fig. 1, b). Saturation magnetisation of synthetic goethite was 0,14 Am2/kg and of initial goethite ore was 0,7 Am2/kg.
Fig. 1. XRD patterns of the initial samples of synthetic goethite (a) and goethite ore (b). The numbers correspond to the phases:
1 - goethite y-FeOOH; 2 - quartz; 3 - hematite Fe2O3; 4 - caolinite
Dnipropetr. Univ. Bull. Ser.: Geol., geogr. 2015. 23(1)
The thermomagnetic curves for synthetic goethite with starch are shown in the fig. 2. Fig. 2, a presents first cycle of heating/cooling and fig. 2, b presents second cycle of heating/cooling.
Temperature [deg °C]
a
0 100 200 300 400 500 600 Temperature [deg °C] b
Fig. 2. Thermomagnetic curves for synthetic goethite with starch:
a - first cycle; b - second cycle
Thermomagnetic analysis of synthetic goethite shows that the reaction of iron reducing with starch starts at the temperature of 260°C with the maximum at 465°C. Magnetisation of the sample after cooling increases considerably.
Dnipropetr. Univ. Bull. Ser.: Geol., geogr. 2015. 23(1)
The disappearance of magnetisation of the sample above Curie temperature and the cycle of heating and cooling provide us additional information about the present phases. Curie temperature, determined by cooling curve is ~560°C that is close to Curie temperature of magnetite (580°C). Therefore one could conclude that the phase of magnetite is formed in the reaction.
Saturation magnetisation, determined for obtained sample was ~70 Am2/kg (magnetisation curve is shown at fig. 3), that is close to saturation magnetisation of pure magnetite (92 A-m2/kg). The result of XRD-measurements (fig. 4) shows that obtained
sample consists of magnetite.
80-i
2 so- ............
■
< 40- ■
I
D I
20-
J
),6 -0,4 1 ' - 1 -0,2 0 -201 -4Q- I _V6o - -80- 0 0,2 0,4 B, T 0,6
Fig. 3. Magnetisation curve of obtained sample
Fig. 4. XRD pattern of obtained sample. The number corresponds to the phase:
1 - magnetite.
Thermomagnetic curves for synthetic goethite with other carbohydrates (data not shown) are similar to the thermomagnetic curve for synthetic goethite with starch. Characte-
Dnipropetr. Univ. Bull. Ser.: Geol., geogr. 2015. 23(1)
ristics of these samples (temperature of reaction start, temperature of reaction maximum at heating curve, Curie temperature, phase composition of obtained sample, saturation magnetisation of obtained sample) are shown in the tab. 1.
Table 1
Temperature of reaction start, temperature of reaction maximum at heating curve, Curie temperature (Tc),
phase composition of obtained sample, saturation magnetisation (Ms) of goethite, reduced ___with different carbohydrates___
Sample name Reductant Ms, Am2/kg Tc, °c Temperature of reaction start, °C Temperature of reaction maximum, °C Phase composition
Goethite 4 % starch 71 564 260 465 Magnetite
Goethite 4 % glucose 69 548 250 480 Magnetite
Goethite 4 % sucrose 69 548 250 470 Magnetite
Goethite 4 % fructose 67 547 260 475 Magnetite
Goethite 4 % ascorbic acid 69 560 280 493 Magnetite
So, we could conclude that reduction of synthetic goethite by different carbohydrates leads to formation of magnetite with rather high saturation magnetisation. For all types of carbohydrates, the reduction reaction starts at ~260°C with the maximum at ~470°C.
The thermomagnetic behaviour of iron oxides in the presence of glucose was analysed in [5]. It was shown, that in the presence of 5-10% of glucose, synthetic goethite starts transform into highly magnetic mineral at 420°C with Curie temperature of 580°C. Authors proposed the formation of highly magnetic maghemite, which then turns into haematite, which is slightly magnetic. In our case, we have obtained highly magnetic magnetite after thermal treatment of synthetic goethite with different carbohydrates, including glucose, which does not loose its magnetisation during further thermal treatment (fig. 2, b).
The thermomagnetic curves for goethite ore with starch are shown in the fig. 5. Fig 5, a presents first cycle of heating/cooling and fig. 5, b presents second cycle of heating/cooling.
Thermomagnetic analysis of goethite ore shows that the reaction of iron reducing with starch starts at the temperature of 420°C with the maximum at 510°C and Curie temperature
~560°C. Magnetisation of the sample after cooling increases considerably.
0 100 200 300 400 500 600 0 100 200 300 400 500 600
Temperature [deg °C] Temperature [deg °C]
a b
Fig. 5. Thermomagnetic curves for goethite ore with starch:
a - first cycle; b - second cycle
Dnipropetr. Univ. Bull. Ser.: Geol., geogr. 2015. 23(1)
Saturation magnetisation determined for obtained sample was ~30 A m2/kg (magnetisation curve is shown at the fig. 6).
Fig. 6. Magnetisation curve of obtained sample
The result of XRD-measurements (fig. 7) shows that obtained sample consists of magnetite, quartz and hematite.
Fig. 7. XRD pattern of the obtained sample. The numbers correspond to the phases:
1 - magnetite Fe3O4; 2 - quartz; 3 - hematite Fe2O3
Dnipropetr. Univ. Bull. Ser. Geol., geogr. 2015. 23(1)
Thermomagnetic curves for goethite ore with other carbohydrates (data not shown) the same dependencies as for goethite ore with starch. Their characteristics (temperature of reaction start, temperature of reaction maximum at heating curve, Curie temperature, phase composition of obtained sample, saturation magnetisation of obtained sample) are shown in the tab. 2.
Table 2
Temperature of reaction start, temperature of reaction maximum at heating curve, Curie temperature (Tc), phase composition of obtained sample, saturation magnetisation (Ms) of goethite ore, reduced
with different carbohydrates
Sample name Reductant Ms, Am2/kg Tc, °c Temperature of reaction start, °C Temperature of reaction maximum, °C Phase composition
Goethite ore 4 % starch 30 559 420 510 Quartz, magnetite, hematite (traces)
Goethite ore 4 % glucose 22 567 445 535 Quartz, magnetite, hematite (traces)
Goethite ore 4 % sucrose 31 564 425 515 Quartz, magnetite, hematite (traces)
Goethite ore 4 % fructose 22 569 450 550 Quartz, magnetite, hematite (traces)
Goethite ore 4 % ascorbic acid 23 571 450 520 Quartz, magnetite, hematite (traces)
So, we could conclude that reduction of goethite ore by different carbohydrates leads to formation of magnetite. For all types of carbohydrates the reduction reaction starts at ~450°C with the maximum at ~520°C.
Conclusion. It was slight difference of different carbohydrates (starch, glucose, fructose, sucrose and ascorbic acid) influence on the reduction rate of goethite or goethite ore. Synthetic goethite reduction with carbohydrates (starch, glucose, fructose, sucrose and ascorbic acid) under heating up to 650°C leads to formation of magnetite with saturation magnetisation ~70 A m2/kg. Goethite ore reduction with carbohydrates (starch, glucose, fructose, sucrose and ascorbic acid) under heating up to 650°C leads to formation of magnetite. Saturation magnetisation ob obtained magnetic samples is ~25 Am2/kg. The reduction of synthetic goethite with carbohydrates starts at rather low temperatures, i.e. ~260°C, while the reduction of goethite ore with carbohydrates starts at ~450°C. We could attribute this increase of reduction start temperature with shielding effect of admixtures (quartz, etc.) in the ore. Obtained results are promising for development of new low-energy technologies of goethite ore beneficiation.
Dnipropetr. Univ. Bull. Ser.: Geol., geogr. 2015. 23(1)
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Hadiüwm do редкоnегií 27.02.2015
Dnipropetr. Univ. Bull. Ser.: Geol., geogr. 2015. 23(1)
