DOI 10.1007/s10717-017-9892-5 Glass and Ceramics, Vol. 73, Nos. 9 – 10, January, 2017 (Russian Original, Nos. 9 – 10, September – October, 2016)

AT ENTERPRISES AND INSTITUTES UDC 666.76:541.128.13:541.18.02:543.422

CALCIUM FERRITE STRUCTURE FORMATION DURING MECHANOCHEMICAL INTERACTION IN THE SYSTEM FeC2O4 × 2H2O–Ca(OH)2 R. N. Rumyantsev,1, 2 A. A. Il’in,1, 3 K. O. Denisova,1 A. P. Il’in,1, 3 and A. V. Volkova1 Translated from Steklo i Keramika, No. 10, pp. 24 – 28, October, 2016.

The mechanochemical synthesis of calcium ferrite from iron oxalate FeC2O4 × 2H2O and calcium hydroxide Ca(OH)2 is studied by means of x-ray phase, x-ray diffraction, and simultaneous thermal analysis and scanning electron microscopy. The optimal conditions for obtaining calcium ferrite are determined. It is shown that the use of mechanochemical synthesis can significantly reduce the heat treatment temperature and duration. Key words: calcium ferrite, mechanical activation, production, automatic thermal reduction.

Due to its unique properties calcium ferrite is widely used in various industries. It is used as a ceramic pigment, semiconductor, refractory, and adsorbent [1 – 3]. One promising application of ferrites is catalysis. This compound exhibits activity in medium-temperature conversion of carbon monoxide by steam and the purification of exhaust gases by removal of nitrogen oxide (I) and carbon monoxide [3 – 6]. Analysis of the published data shows that different methods are used to obtain ferrites: ceramic, mechanochemical, precipitation, sol-gel, microemulsion, sintering during microwave treatment, and others. However, all these methods have a number of drawbacks, such as high product formation temperatures and the presence of waste waters and gas emissions. These drawbacks can be avoided by using mechanochemical synthesis (MCS), as dry powders can be used as precursors in this case. For example, the authors of [7] suggest obtaining calcium ferrite by joint mechanical activation (MA) of a mixture of FeOOH and Ca(OH)2. In this case anion-modified calcium ferrite forms at 400°C. In [8] it is shown CaFe2O4 can be obtained from a-Fe2O3, and CaCO3

by processing the feedstock in a ball mill for 10 h and then heat-treating at 1100°C. In [9, 10] it is proposed that calcium ferrite be obtained by joint mechanical activation (MA) of a mixture of calcium oxide or calcium carbonate with iron oxide. Recently, new trends have emerged in the development of MCS which are directed toward more efficient use of expensive mechanical energy to accelerate chemical reactions: using chemical reactions with low activation energy [11]. Such reactions include the reactions of solid acids and bases, hydrated compounds, and salts and other substances, which occur during relatively weak mechanical actions; a method based on these reactions is called ‘soft MCS’ [11, 12]. The advantages of this method are higher reaction rates compared to anhydrous mixtures, which saves energy and permits obtaining products that are less contaminated by the milling-body material [11 – 13]. The present article investigates the synthesis of calcium ferrite Ca2Fe2O5 in the mechanochemical interaction of stoichiometric quantities of Ca(OH)2 and FeC2O4 × 2H2O. EXPERIMENTAL PART

1

2 3

Scientific Research Institute of Thermodynamics and Kinetics of Chemical Processes, Ivanovo State Chemical Technology University, Ivanovo, Russia. E-mail: [email protected]. E-mail: [email protected].

Iron oxalate FeC2O4 × 2H2O and calcium hydroxide Ca(OH)2 were used for solid-phase synthesis calcium ferrite Ca2Fe2O5. The mechanoactivation of a mixture of oxides with durations 5, 15, 30, 45, and 60 min was conducted in a 374 0361-7610/17/0910-0374 © 2017 Springer Science+Business Media New York

Calcium Ferrite Structure Formation During Mechanochemical Interaction

VM-4 vibratory ring-roller mill with vibration frequency 930 min – 1 and energy input 0.146 kJ/(g × sec); the energy input was calculated by the method described in [14]. The milling bodies were rollers and rings made of chemically resistant steel SHKH15, the total mass of the bodies was 1194 g, and the amount of activated material was 100 g. X-ray phase analysis (XPA) was conducted in a DRON-3M diffractometer using CuKa-radiation (l = 0.15406 nm, Ni-filter). The crystallographic data bases MINCRYST and PDF-4 were used to identify the x-ray diffraction data. The broadening the x-ray diffraction profiles makes it possible to determine how the coherent scattering region (CSR) changes and to find the value of the rms microstrain. The substructure parameters of the samples were determined using software that implements the GAFRL method. Thermogravimetric analysis of the reaction products was conducted using an STA 449 F3 Jupiter simultaneous thermal analyzer and heating rate 5 K/min. The specific surface was determined by the method of BET nitrogen adsorption from a nitrogen-helium mixture in an automatic mode using the Sorbi-MS apparatus. Microscopic images of the sample surfaces were obtained in a VEGA 3 TESCAN scanning electron microscope (SEM). An Analysette 22 Laser-PartikelSizer was used for laser analysis of the particle size. Pulse reduction of calcium ferrite was performed in a flow-type setup. Samples of the catalysts (0.15 – 0.25 mm fraction) were loaded into the reactor and reduced by H2. The reduction temperature was increased automatically in the range of 150 – 400°C at rate 5 K/min for 50 min. The amount of hydrogen absorbed in the reduction process was measured every minute.

375

c b

a

2q, deg

Fig. 1. Change of the diffraction patterns during MA: a) initial mixture FeC2O4 × 2H2O and Ca(OH)2; b, c) after 15 and 60 min MA, respectively; 1 ) FeC2O4 × 2H2O; 2 ) Ca(OH)2; 3 ) CaC2O4.

I

100 mm

2 mm

100 mm

2 mm

100 mm

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RESULTS AND DISCUSSION Figure 1 shows the x-ray diffraction patterns of the initial and mechanoactivated mixtures of iron oxalate and calcium hydroxide. Their analysis shows that the reflections characteristic for the phases FeC2O4 × 2H2O and Ca(OH)2 vanish even during the initial period of MA (0 – 15 min). During the activation process intense interaction occurs between the components of the mixture and calcium oxalate a-CaC2O4 forms as an intermediate product; in addition, as the processing time increases to 15 – 60 min the reflections characteristic for this compound become more intense. It follows from this that during MA the starting components undergo amorphization and the following exchange reaction occurs:

III

a

FeC2O4 × 2H2O + Ca(OH)2 ® CaC2O4 + Fe(OH)2 + 2H2O.

b

(1)

Fig. 2. SEM image of the initial (I) and mechanoactivated mixture for 15 (II) and 60 (III) min.

Along with this reaction the partial decomposition of iron and calcium oxalates as well as the oxidation by atmospheric oxygen Fe(OH)2 to FeOOH can occur in parallel during MA.

It is evident in the SEM photographs of the original sample that it is composed of both large and small aggregates (Fig. 2, I a). The photomicrographs show imbedding of fine

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R. N. Rumyantsev et al. DSC, mV/mg

Mass losses, %

b

a – T, °C

2q, deg

Fig. 3. The results of simultaneous thermal analysis of MA products.

Fig. 4. X-ray diffraction pattern of the products of MA, calcinated at 380°C (a) and 620°C (b ): 1 ) CaCO3; 2 ) Fe2O3; 3 ) Ca2Fe2O5.

particles (presumably Ca(OH)2; 0.1 – 1.0 mm) in a looseopen structure comprised of larger aggregates (presumably FeC2O4 × 2H2O; > 20 mm) (Fig. 2, I b ). The microscopic images taken of the sample after 15 min of MA show an increase in the number of particles smaller than 10 mm (Fig. 2, II a). Many particles have a quite distinct spherical shape and enter into agglomerates of size 5 – 20 mm (Fig. 2, II b ). In the images of a sample subjected to activation for 60 min (Fig. 2, III a and b ) one can distinguish particles of spherical and irregular shapes 0.1 – 0.5 mm in size, which randomly coalesced into larger aggregates. Laser analysis was used to determine the particle size more accurately. It was found that comminution and aggregation occur during MA. For example, the particle size of the initial sample does not exceed 100 mm. Mechanoactivation for 15 min leads to the formation of reaction products with 50 – 70 mm particles. At longer processing time 45 min aggregates with particle sizes up to 120 mm form. The aggregates are destroyed when the MA time increases to 60 min. These changes occur because when mechanical action is used for dispersion the minimum particle size is reached quite quickly, the comminution process practically stops, and secondary aggregation starts to prevail, which occurs because the system strives to reduce the excess free energy [12, 15]. Thus, the microstructure of the product obtained from processing samples in the grinding machine is the result of two competing processes: size reduction of individual particles by their fracture and formation of quite large aggregates from these particles [12, 15]. Calcination is accompanied by mass losses and four heat effects (Fig. 3). The first one is endothermal and occurs in the temperature range 120 – 220°C; it is due to removal of carbon dioxide adsorbed from air and adsorbed and crystallization water. The second one is exothermal and occurs in the temperature range 330 – 460°C; it is due to several processes: the oxidation of carbon monoxide, formed as a result

of the decomposition of iron and calcium oxalates, to carbon dioxide and polymorphic transformation of maghemite into hematite. The third one is endothermal and occurs in the temperature range 575 – 650°C, indicating decomposition of calcium carbonate. The presence of calcium carbonate is further confirmed by XPA (Fig. 4). Reflections characteristic for CaCO3 and g-Fe2O3 are observed in the x-ray diffraction patterns of the sample calcined at 380°C (Fig. 4a ). The fourth one is exothermal (see Fig. 3) and occurs in the temperature range 650 – 950°C; it is strongly extended and is due to the crystallization of calcium ferrite Ca2Fe2O5. Three distinct steps are observed in the mass change curve (see Fig. 3). They are due to the removal of water and carbon dioxide and the decomposition of iron and calcium oxalates as well as calcium carbonate. Calcination of the sample at 620°C for 3 h results in the formation of a single-phase crystalline compound which x-ray diffraction analysis indicates to be uniquely attributable to calcium ferrite Ca2Fe2O5 (see Fig. 4b ). The compound Ca2Fe2O5 was produced in the course of these investigations and the optimal conditions for obtaining were determined. Characteristics and optimal conditions for obtaining Ca2Fe2O5 Formula . . . . . . . . . . . . . . . . . . . . . . Ca2Fe2O5 Calcination temperature, °C . . . . . . . . . . . . . . . 620 Calcination time, h . . . . . . . . . . . . . . . . . . . . . 3 Specific surface, m2/g . . . . . . . . . . . . . . . . . . 18 CSR size, Å . . . . . . . . . . . . . . . . . . . . . . . 182 Microstrain, % . . . . . . . . . . . . . . . . . . . . . 0.12

Calcium ferrite is widely used in catalysis. In a number of cases, the active component of the catalyst is formed during preliminary reduction; the optimum ion ratio of Fe2+ and Fe3+ secures not only high activity but also selectivity. For this reason the preliminary reduction of calcium ferrite by hydrogen was investigated in the present work.

Calcium Ferrite Structure Formation During Mechanochemical Interaction T, °C

2.5

This work was supported by a President of the Russian Federation Scholarship for young scientists and graduate students engaged in promising scientific research and development work in the priority directions of modernization of the Russian economy (2016 – 2018) No. SP-3477.2016.1.

2.0

REFERENCES

3.0

Reduction rate, ml/(g × sec)

377

1.5

1.0 0

0.04 0.08 0.12 0.16 Degree of reducibility of Ca2Fe2O5, d.u.

0.20

Fig. 5. Kinetics of reduction of Ca2Fe2O5.

Analysis of the x-ray diffraction patterns shows that no phases of individual oxides are present in the reduced sample. Therefore, only stepped splitting of oxygen with a new oxygen-depleted structure being formed occurs in the course of the reduction process: Ca2Fe2O5 + xH2 = Ca2Fe2O5–x + xH2O. The curve of automatic thermal reduction is complicated. Three basic regions of hydrogen absorption can be identified in it (Fig. 5). 1. The absorption of hydrogen in the temperature range 150 – 200°C corresponds quantitatively to the removal of not more than one monolayer of oxygen. Since there is no phase reduction in this temperature range, hydrogen absorption can characterize the most weakly bound surface species of oxygen, including those adsorbed on extended defects at the sites where they emerge at the surface. 2. Hydrogen absorption in the temperature range 200 – 275°C. The amount of oxygen removed (from one to several tens of monolayers) indicates near-surface reduction of ferrite. 3. The absorption in the temperature range 275 – 400°C corresponds quantitatively to a deeper reduction and therefore can be attributed to the volume reduction of the samples, for example, the reduction of Fe3+ to Fe2+. CONCLUSIONS It was shown that the formation of the brownmillerite phase does not occur at the stage of mechanoactivation of iron oxalate and calcium hydroxide. The formation of calcium oxalate and iron hydroxide is observed during MA. It was found that crystalline calcium ferrite is formed during heat treatment of an activated mixture at 620°C for 3 h. It was found that the Ca2Fe2O5 reduction process occurs in three steps in the temperature ranges 150 – 200, 200 – 275, and 280 – 400°C.

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