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1505.16.09 - Материаловедение (по отраслям) (технические науки) DOI: 10.25712/ASTU.2072-8921.2020.04.021 УДК 621.762
KINETIC PARAMETERS OF PRODUCING NICKEL METAL POWDER UNDER ISOTHERMAL CONDITIONS
T. H. Nguyen, Nguyen Van Minh, Nguyen Thai Ha, Do Thanh Lich
The kinetic parameters of producing nickel metal powder, which is obtained through hydrogen reduction from NiO micron powder under isothermal conditions were studied. The hydrogen reduction process of NiO micron powder under isothermal conditions was carried out in a tube furnace in the temperature range 275-316 °C. The study of the crystal structure and composition of the powders was performed by XRD phase analysis. The specific surface area of the samples was measured using BET method with adsorption of nitrogen in low temperature. The average particle size of powders was determined via the measured specific surface area. The size characteristics and shape of the particles were investigated by scanning electron microscope. The calculation of the kinetic parameters of synthesizing process of nickel metal powder under isothermal conditions, was carried out using the Gray-Weddington model and Arrhenius equation. It was found that the rate constant of the reduction process at 316 °C is approximately 2,6 times larger than in the case of reduction at 275 °C, while the duration of the process was reduced by more than 2 times. The activation energy of hydrogen reduction process of NiO micron powder under isothermal conditions equal to ~ 48 kJ/mol, which indicates a mixed reaction mode. In this mode, increasing the temperature as well as eliminating the diffusion layer of the reduction product by intensive mixing are two rational ways to accelerate the process.
Keywords: nickel, nickel oxide, micron powder, kinetic parameters, hydrogen reduction, isothermal conditions.
INTRODUCTION
Today, nickel powder and materials based on it have found wide applications in many fields of science, technology and medicine. For example, in the metallurgical industry, nickel is especially important for the production of heat-resistant and heat-resistant alloys, stainless steels, etc. In addition, nickel powder has shown great advantages in printing electrically conductive pastes due to its excellent properties [1, 2]. In the field of powder metallurgy, nickel-based powders are used in the manufacture of products with complex shapes and excellent characteristics, to obtain products by selective laser melting [3-5].
Metallic nickel powder (NiMP) is obtained by various mechanical and physicochemical methods, which generally have major drawbacks, such as high energy consumption, difficulties in controlling product properties, low productivity, environmental hazard, etc. [1, 6-9]. The chemical and metallurgical methodwhich includes the stage of hydrogen reduction, is a highly efficient method in terms of saving energy, enhancing the quality of the obtained powder metals (homogeneity, stability, narrow particle size distribution), environmental friendliness [1014].
Hydrogen reduction of hydroxide or oxide compounds is an important way to obtain nickel powders. The widespread introduction of reduced nickel-based powders is constrained by their high cost, which is a consequence of the
slow reduction process due to kinetic limitations when it is necessary to obtain products with specified properties in size and shape [15, 16].
Thus, the study of the kinetics of synthesizing process of Ni MP by hydrogen reduction in the search for ways to accelerate the process while guaranteeing the necessary properties of the products obtained, is an important scientific and practical problem.
In connection with the above, the aim of this work is to study the kinetic parameters of the process of obtaining Ni MP by hydrogen reduction under isothermal conditions and to study the properties of the products obtained.
MATERIALS AND METHODS
Micron powder (MP) NiO (nickel (II) oxide; MRTU: 6-09-6600-70, Ural factory of chemical reagents, Verkhnyaya Pyshma, Russia) was used as a starting material for studying the kinetic parameters of the process of obtaining nickel powder.
The NiO MP reduction process was carried out in a SNOL 0,2/1250 tube furnace (Lithuania) in a hydrogen atmosphere at various temperatures. During the process, the following reaction took place:
NiO + H2 = Ni + H2O. (1)
To select a suitable temperature for the hydrogen reduction process, the initial sample was investigated by the thermogravimetric (TG) method on an SDT Q600 analyzer (USA) in a hydrogen atmosphere with linear heating at a
rate of 5 °C/min in the temperature range 25450 °C.
The phase of the powder samples was determined by X-ray phase analysis on a Difrey-401 diffractometer (Russia) (CrKa radiation) at room temperature.
The specific surface area (S) of the samples was measured by the BET method with adsorption of nitrogen in low temperature, on a NOVA 1200e analyzer (USA). The measurement accuracy is ± 5%. The average particle size of the powders was calculated from the measurement data of the S value using the formula:
ried out by the formula:
D = -h
p s
(2)
where p - the pycnometric density of the material, kg/m3; D - average particle size, m.
The dimensional characteristics and morphology of the powders were investigated by electron microscopy using a HITACHI TM 1000 scanning electron microscope (SEM) (Japan).
The calculation of the degree of reduction a(i.e.) during the reduction of samples was car-
mt
a = — ,
mo
where mo - the initial weight of the NiO sample, g; mt - the reacting mass of NiO after time t, g.
The kinetics of producingnickel metal powder by hydrogen reduction of NiO was studied using the Gray-Weddington equation [17]. Using this model allows a calculation of the rate constant (k) of the process by the formula:
k t =1 - (1 - a)13 , (4)
where k - the rate constant, s-1; t - the reaction time, s.
The calculation of the activation energy Ea (J/mol) was carried out according to the experimental data obtained under isothermal conditions using the integral form of the Arrhenius equation:
1Пк = - R
+ InA ,
(5)
where A - preexponential factor, s-1; T - temperature, K; R - gas constant, J/(mol K).
1
T
Figurel - XRD pattern (a) and SEM image (b) of the initial sample of NiO MP
RESULTS AND DISCUSSION
The XRD pattern and SEM image of the initial sample NiO MP are shown in Figure 1.
The result of XRD phase analysis (Fig. 1, a) shows that the sample under study contains a purely crystalline oxide phase NiO with a trigonal lattice, no other phases were found. The SEM image (Fig. 1, b) reveals that the NiO MP sample mainly consists of polydisperse particles less than 20 ^m in sizewith a tendency to aggregate.
According to the BET data, the specific surface area S of the sample of the initial NiO powder was 6,0 m2/g. Accordingly, the average particle size which results from the specific surface
area is 0,15 ^m.
Analysis of the TG curves (Fig. 2) obtained through the hydrogen reduction of the NiO MP sample according to the reaction (1) under the condition of linear heating shows that the reduction proceeds in two stages. At the first stage, under the temperature range 275-330 °C, the reduction of the smallest particles of NiO MP occurs. The maximum specific rate of the process was achieved at a temperature of 316 °C and its value was 9,710-8 kg/s. At the second stage, the bifurcation of the peak is a consequence of the fact that large oxide particles take longer to recover than small ones.
a - mass change; b - the rate of mass change
Figure 2 - TG curves of the hydrogen reduction Of NiO MP
In order to determine the rate constants and activation energy of the reduction process under isothermal conditions, the dependences of the degree of the reduction (a) on time (t) were obtained at different temperatures. Based on the TG data (Fig. 2), the temperatures of the reduction process were selected: 275, 285, 295, 305, 316 °C. These temperatures are in the range of intensive reduction processes. In Figure 3 the graph illustrates the dependence of the degree of conversion on time a (t) at different temperatures. The figure shows that the recovery process at a temperature of 275 °C is slowest with an incubation period from the begin to the fortieth minute. At a temperature of the maximum rate of reduction (316 °C), the complete reduction time was 72 min, that is approximately more than 2 times compared with the case of recovery at 275 °C (at this temperature, the complete reduction time was 136 min). The reduction process of NiO MP at temperatures above 300 °C proceeds practically without an incubation period.
Figure 3 - Dependence a (t) of the reduction process of NiO MP at different temperatures
The rate constant k of the reduction process at different temperatures was determined using the Gray-Weddington model (4). The values of the rate constant (depending on temperature) are presented in Table 1.
Table 1 - The rate constant of the reduction process of NiO MP
Sample / process NiO MP / hydrogen reduction
T, °C 275 285 295 305 316
k103, s-1 1,159 1,398 1,579 2,118 2,333
According to the data in Table 1, it can be seen that the rate constant of the reduction of NiO MP at 316 °C is approximately 2 times higher than the value obtained in the case of reduction at 275 °C, therefore, the duration of the process decreased by more than 1.89 times.
To determine the value of the activation energy Ea of the reduction process of NiO MP under isothermal conditions, a graph of the dependence lnk(1/T) was plotted according to equation (5).
It has been shown that the value Ea of producing Ni MP under isothermal conditions, is calculated from the kinetic data, equal to ~ 48 kJ/mol.
Figure 4 - Calculation of the activation energy in the coordinates of the Arrhenius equation
Comparing the obtained Ea value with the literature data [18], it can be confirmed that the reduction proceeds in a mixed reaction mode (in this case, the rate of chemical transformation and diffusion rate are approximately equal, and both stages simultaneously limit the reduction process as a whole). In this mode, increasing the temperature and eliminating the diffusion layer of the reduction product by intensive mixing are two rational ways to accelerate the process.
It should be noted that, although in this case an increase in temperature is a solution to accelerate the reduction of NiO MP, carrying out the reduction at high temperatures can lead to an accelerated aggregation and sintering of the obtained metal particles.
Figure 5 - XRD pattern (a) and SEM image (b) of the product of the hydrogen reduction
Of NiO MP at 316 °C
The following are the results of studying the properties of the product obtained by hydrogen reduction of NiO MP with the maximum rate of reduction, at 316 °C.
Figure 5 shows the XRD pattern and SEM image of the hydrogen reduction product of NiO MP at 316 °C.
The result of XRD phase analysis of the product (Fig. 5, a) shows that the obtained sample contains only the (fcc) nickel phase. Analysis of the SEM image in Figure 5, b reveals that the powder of metallic nickel is a highly porous material, and Ni particles are collected in large porous aggregates up to 10 ^m in size.
The result of measuring the specific surface area of the obtained nickel powder by the BET method is in good agreement with the result of microscopic analysis. It was found that, in the course of reduction, the processes of sintering and aggregation of the formed metal particles lead to a significant decrease in the value of S in ПОЛЗУНОВСКИЙ ВЕСТНИК № 4 2020
comparison with the initial material (from 6,0 for NiO MP to 1,8 m2/g for Ni MP).
CONCLUSION
The kinetic parameters of producing Ni MP by hydrogen reduction are investigated. It was found that the rate constant of reduction at 316 °C is approximately 2 times higher than in the case of reduction at 275 °C, accordingly, the duration of the process was reduced by more than 1,89 times.
The value of the activation energy of the hydrogen reduction of NiO MP equal to ~ 48 kJ/mol, which indicates a mixed reaction mode. In this mode, to accelerate the process, increasing the temperature and eliminating the diffusion layer of the reduction product by intensive mixing are recommended.
It was established that the Ni MP, obtained by hydrogen reduction of nickel oxide at 316 °C, is a
highly porous material, while Ni particles are collected in large porous aggregates up to 10 ^m in size.
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Tien Hiep Nguyen, lecturer, Le Quy Don Technical University (Hanoi, 100000, Vietnam). E-mail: htnru7@yandex.ru.
Nguyen Van Minh, PhD. Sci. (Eng.), Institute of Technology (Hanoi, 100000, Vietnam). Email: chinhnhan88@gmail.com.
Nguyen Thai Ha, research assistant, Institute of Technology (Hanoi, 100000, Vietnam). E-mail: thaihabwde@gmail.com.
Do Thanh Lich, postgraduate, Dalat Vocational Training College, (Lamdong, 670000, Vietnam). E-mail: lichdalat@gmail.com.
