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25HOLLOW HEMATITE PARTICLES SYNTHESIZED BY SPRAY PYROLYSIS OF THE SPENT PICKLING SOLUTION
Kargin J.
PhD
L. N. Gumilyov Eurasian National University, Almaty, Kazakhstan
Mukhambetov D. Doctor of Physical and Mathematical Sciences Almaty Academy of Economics and Statistics, Almaty, Kazakhstan
Kozlovskiy A. PhD
Institute of Nuclear Physics, Almaty, Kazakhstan
Biseken A.
PhD
Almaty University of Power Engineering &Telecommunication
ABSTRACT
It is shown that the powder synthesized from the spent pickling solution by spray pyrolysis consists of hollow microparticles, the shell of which is formed by nanoparticles of iron oxide. A scheme for the formation of hollow spheres in the spray roaster process is proposed.
Keywords: hematite, hollow microspheres, spent pickling solution, spray roaster process.
Introduction. Powdered iron oxides is widely used in various fields of science and technology. In particular hollow spheres of these oxides are promising in terms of their applicability. Nanoscale hollow particles are intensely discussed recently with regard to their functionality (large specific surface, low specific weight, container-type morphology) and technical application (catalysis, gas storage, low-weight building materials, lithium-ion batteries, drug delivery) [1]. Therefore, a study of methods for obtaining such materials and its properties are relevant at the present time.
The most common techniques to produce hollow spheres are based on the use of core hard templates such as silica and carbon spheres, polymer latex colloids, spheres and block copolymers [2] or soft templates, such as emulsion droplets, surfactants vesicles [3]. The hard template technique is effective for controlling the morphology of the final product. But this technique requires the tedious procedures of synthesis. Some other drawbacks of the hard templates methods include limited sphere size, significant cost of production, and low temperature capability of the obtained particles [1].
Recently different free-template approaches have been developed to produce hollow spheres. These methods are based on simultaneous blowing and melting hidrogels, Kirkendall Effect, among others. However the average size of these are as a rule larger than 10 ^m [4].
One of the most economically feasible methods of producing iron oxides is the synthesis of hematite from spent pickling solution at steel rolling production [5, 6].
The purpose of this work is to study the powders of iron oxide obtained from the spent pickling solution of steel production.
Material, apparatus and methods of investigation.
The investigated material is a secondary product of metallurgical production, which is formed as follows.
The oxide layers arise on steel surface during the hot rolling processes. They should be removed for subsequent processing of steel strip. For this the steel strip passes through a bath with aqueous solution of hydrochloric acid HCl. Iron oxides dissolve in an acid solution to form ferric chloride FeCl2. Sludge and other impurities settle on the bottom of the bath and are removed from the liquor.
The spent pickling solution is fed through nozzles into a furnace at a temperature of 600 °C. In the furnace spray pyrolysis takes place in which the ferric chloride decomposes into dispersed hematite and hydrochloric acid vapor in accordance with the reactions:
12FeCl2 + 3O2 = 8FeCls + 2Fe2Os| (1) 2FeCls + 3H2O = 6HC1T + Fe2Os| (2)
Hydrochloric acid vapors are extracted from the top of the furnace and used for re-etching. Particles of iron oxide a-Fe2O3 are deposited on the bottom of the furnace and pneumatically transported to the hopper [6]. Dispersed hematite powder has a high purity due to what it is a valuable material for manufacturing industries.
Research of structural characteristics and element composition of the powder was carried out using the Hitachi TM3030 scanning electron microscope (SEM) with the Bruker XFlash MIN SVE microanalysis system at the accelerating voltage of 15 kV before and after the thermal annealing.
X-ray diffractometry studies were performed by the D8 ADVANCE ECO diffractometer using X-ray tube radiation with a Cu anode and a graphite mono-chromator on a diffracted beam. The tube operation mode is 40 kV, 25 mA. XRD patterns were recorded in the angular range of 10 - 90° 20 with the step of 0.02° 20.
The average particle size (coherent scattering region) D was calculated from the Scherrer formula:
D =
0,94 -Â p- cos S
(3)
Results and discussion
Fig. 1 shows the SEM image of the oxide powder being studied.
where p is the true physical broadening of the line of the sample under study, calculated by the approximation method and by Fourier analysis, in radians.
Fig. 1. SEM image of the oxide powder
Analysis of the morphological data showed that the oxide powder is hollow spheres, the average size of which is 2-7 microns. In a detailed study of the obtained particles it was found that the shells of hollow particles are aggregations of nanoparticles whose average size is about 50 nm.
On the obtained X-ray diffractogram (Fig. 2), peaks characteristic for X-ray diffraction on polycrys-talline objects are observed. Analysis of the peaks in the diffractogram showed the presence of a phase characteristic of iron oxide a-Fe2O3 (PDF, 130534).
The crystal lattice parameter was determined using the Nelson-Taylor extrapolation function:
a = f
f
cos20 cos0
Y
sin 0
0
(4)
The value and error in determining the parameter a are determined by linear extrapolation of this function to the zero value of the argument (0 = 90 °). The value of the lattice parameter a is equal to 5.413 A. The average crystallite size (CSR) calculated using the Scherrer equation was 45.3 nm.
Fig. 2. X-ray diffraction pattern of the oxide powder
Let us consider the mechanisms of formation and growth of spherical particles.
In general, the template technique involves four stages: preparation of the templates, functionalization of their surface to achieve suitable surface properties, coating the templates with desirable materials and removal of the templates in corresponding solvents or calcination of samples to obtain the hollow particles.
The gas-bubble template method is free from these shortcomings to a certain degree . It involves the production of gas microbubbles during the chemical preparation of nanoparticles by using selected ligands. It is considered that the nanoparticles cover the surface and form the shell of the hollow spheres after calcinations at high temperatures [7]. However the exact mechanism for the bubble nucleation and grow is unclear.
The process of formation of hollow spheres in our case can be represented as follows. The stages of formation of a hollow spherical particle of a-Fe2O3 as a result of pyrolysis are shown in Fig.3.
a b c d
Fig. 3. The stages offormation of the hollow sphere of a-Fe2O3 during pyrolysis: a is the initial drop of an aqueous solution offerric chloride; b is the formation of an oxide shell according to reaction (1) on the surface of a drop; c is the formation of an oxide layer on the inner surface of the shell by reaction; d is the hollow spherical
particle a-Fe2O3.
The droplet of an aqueous solution of ferric chloride, which has arisen upon injection into the furnace, enters the atmosphere of heated air, being in a suspended state (Fig. 3a). Molecules of iron chloride on the surface of the drop interact with oxygen of the air by reaction (1) to form a-Fe2O3 molecules, which precipitate and form the layers of the shell of the hollow sphere on the surface of the drop (Fig. 3b). The resulting molecules of FeCl3 interact with water molecules according to the formula (2) and form a-Fe2O3 particles on the inner surface of the sphere shell (Fig. 3c). The result is a hollow sphere the surface whose is a set of conglomerates of iron oxide nanoparticles (Fig. 3d).
Conclusions.
The results of X-ray diffraction analysis and electron microscopic studies showed that the powder synthesized from the spent pickling solution by spraying pyrolysis consists of hollow microparticles, the shell of which is formed by hematite nanoparticles.
The formation of hematite hollow spheres in the spray roaster process occurs as a result of two chemical reactions of the interaction of FeCl2 with oxygen and FeCl3 with water vapor during pyrolysis.
Acknowledgements. This work was supported by a grant from the Ministry of Education and Science of Kazakhstan on Science Development program (Agreement No. 45 of February 12, 2015).
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