The Effect of Pyrolysis Conditions on the Preparation of Fe2O3 Particles Using Simulated Pickling Liquor in a Venturi Reactor

Large amounts of pickling liquor are produced during the pickling of iron and steel. The Venturi reactor is used during the pyrolysis of pickling liquor to prepare high-purity Fe₂O₃ particles. Changing the pyrolysis conditions will affect the purity and concentration of Fe₂O₃ particles. In this paper, physical experiments and numerical simulations were carried out to study the effect of pyrolysis conditions on the Fe₂O₃ particles, including the pyrolysis temperature, the FeCl₂/FeCl₃ ratio in the pickling liquor, and the citric acid ratio. The results showed that a product with a higher purity, better crystallinity, and greater concentration was obtained at 923 K. Further increasing the temperature did not change the concentration of Fe₂O₃ particles. When the ratio of FeCl₂/FeCl₃ in the pickling liquor was 1:1.5, the maximum concentration of Fe₂O₃ particles at the outlet was obtained. When the added proportion of citric acid was 1:1, the particle size distribution was more uniform, and when the added proportion was 1:2.5, the maximum concentration of Fe₂O₃ particles at the outlet was achieved.

Introduction

The dense Fe₂O₃ formed on the surface of steel products during their production requires pickling, which produces 1.2–2 tons of pickling liquor per ton of steel (Zhang et al., 2014; Wang and Ma, 2017; Wang et al., 2020). HCl pickling liquor contains 1–5% HCl and 5–20% FeCl₂/FeCl₃, which causes severe environmental issues if discharged directly, as well as resource waste (Fang et al., 2010; Gao et al., 2021). In large iron and steel enterprises, the Ruthner spray pyrolysis method (Qi, 2006; Deng et al., 2007; Bian et al., 2015) is primarily used to process waste acid, in which FeCl₂ and FeCl₃ react with H₂O and O₂ to produce HCl and Fe₂O₃ particles. Nevertheless, this process has technical barriers and high investment costs. To decrease costs and increase the value of by-products, an in-depth study of the pyrolysis technique is needed. Sun et al. (2021) prepared the nanometer ferric oxide (Fe₂O₃ nanorod) by the hydrothermal method, the ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O) was the source of ferric and the ethanol absolute and ethylenediamine were used as the surfactant. Effects of the reaction temperature and the mass fraction of Fe(NO₃)₃·9H₂O on the particle size of the Fe₂O₃ nanorod were investigated. Zhang et al. (2020) prepared the α-nanometer Fe₂O₃ by microwave hydrolysis, FeCl₃ was the raw material and the triethylenetetramine (TETA) was used as the coordination agent. Taniguchi (Izumi et al., 2007) synthesized LiMn₂O₄ particles with a hollow sphere morphology from a precursor solution using spray pyrolysis from a slurry solution and prepared spherical LiMn₂O₄ nanoparticles. Liu et al. (2018) designed a spray pyrolysis reactor using SiC and prepared MgO with an average particle size of 90 μm. They also studied parameters such as the pyrolysis temperature and feed quantity. Moreover, computational fluid dynamics (CFD) was used to simulate the spray pressure and temperature distribution in a pyrolysis furnace. Lv et al. produced a mixture of gases and solids with the Venturi reactor, which improved the pyrolysis efficiency and prepared spherical CeCl₃ and Fe₂O₃ micro/nanoparticles (Lv et al., 2019a; Lv et al., 2019b; Lv et al., 2019c; Lv, 2019). Venturi reactor can enhance the mix of the gas and liquid, and solve the problem of Ruthner spray pyrolysis method that the nozzle needs to be replaced periodically. The simple structure and low investment of the Venturi reactor have resulted in its broad usage, but it is necessary to study it further. At present, there is a problem that the product particles are easy to accumulate and jam at the Venturi tube when the Venturi reactor is used for the pyrolysis reaction. Adding citric acid can well solve this problem. The carbon dioxide and water vapor are produced in the chemical reaction by citric acid. The gas-phase pressure can push the particles to flow to outlet, which prevents the jam at Venturi tube, increases the reaction efficiency and changes the morphology of the product particles.

This paper used a combination of experiments and numerical simulations to study the effects of temperature, pickling liquor composition, and citric acid ratio on the composition and morphology of Fe₂O₃ particles obtained at the outlet of a Venturi reactor. The effect on the Fe₂O₃ particle distribution in the reactor was also investigated. The results can be used to optimize the operating conditions of pyrolysis, as well as the pickling liquor, to provide a reference for the optimal design of Venturi reactors.

Experimental

Physical Experiments

A diagram of the equipment used in the jet pyrolysis experiments is shown in Figure 1. Oxygen was added by a high-speed air pump 1 into the Venturi reactor 4. The waste acid 2 mixed with citric acid, and then the mixed liquor dripped into the heating furnace 3. Pyrolysis occurred in the Venturi reactor 4. Next, the pyrolysis product Fe₂O₃ was collected by a particle collector, and tail gas was recovered by the alkali liquor in the recovery device 6. Venturi reactor is a place for evaporation, phase change and pyrolysis reaction. The reaction mechanism is not clear now, so the investigation of it should be focused on. Because of the mature separation technology of the gas and solid phase, we did not lay emphasis on it in this study. Employing the pyrolysis conditions summarized in Table 1 and O₂, CO₂, HCl(g), H₂O(g), HCl(l), H₂O(l), FeCl₂, FeCl₃, and Fe₂O₃ as the chemical species involved in the reactions. Physical parameters of the substances were obtained from the handbook of practical inorganic thermodynamic data (Ye and Hu, 2002).

FIGURE 1

FIGURE 1. Diagram of equipment. 1 oxygen bottle; 2 waste acid solution; 3 heating furnace; 4 Venturi reactor; 5 Solid product collector; 6 alkali liquor.

TABLE 1

TABLE 1. Pyrolysis conditions.

Numerical Simulations

According to the Venturi reactor dimensions, a geometric model with identical proportions was created in SolidWorks 2016, and ICEM 19.0 was used to divide the geometric model into a structured grid with a quantity of 410′000 (Figure 2).

FIGURE 2

FIGURE 2. Geometric models and mesh generation.

Eulerian multiphase model, standard k-ε turbulence model and SIMPLE algorithm were selected in Fluent 19.0, the control equations were differentially treated using the second-order upwind scheme, and all items were converged to 10^–4 except for energy (to 10^–6). The time step was set to 0.001 s, the computing time was 60 s and the simulation results were steady at the moment. The main reactions involved include:

${\text{H}}_2\text{O}\left(\text{l}\right)\to {\text{H}}_2\text{O}\left(\text{g}\right)(1)$

$\text{HCl}\left(\text{l}\right)\to \text{HCl}\left(\text{g}\right)(2)$

${2\text{FeCl}}_2\left(\text{s}\right){+2\text{H}}_2\text{O}\left(\text{g}\right)+1/2{\text{O}}_2\left(\text{g}\right)\to {\text{Fe}}_2{\text{O}}_3\left(\text{s}\right)+4\text{HCl}\left(\text{g}\right)(3)$

${2\text{FeCl}}_3\left(\text{s}\right){+3\text{H}}_2\text{O}\left(\text{g}\right)\to {2\text{Fe}}_2{\text{O}}_3\left(\text{s}\right)+6\text{HCl}\left(\text{g}\right)(4)$

${2\text{C}}_6{\text{H}}_8{\text{O}}_7{+9\text{O}}_2\left(\text{g}\right)\to {12\text{CO}}_2\left(\text{s}\right){+8\text{H}}_2\text{O}\left(\text{g}\right)(5)$

Besides the mass conservation equation, momentum conservation equation and energy conservation equation, it also involved the chemical reaction equation:

$\frac{\partial \left(\rho Y_i\right)}{\partial t}+\nabla \cdot \left(\rho \overline{\nu }{Y}_i\right)=-\nabla {\overline{J}}_i+R_i+S_i(6)$

where Y_i is the mass fraction of the ith substance, R_i is the net produce rate of chemical reaction of the ith substance, S_i is the discrete phase of the ith substance and it is also the extra produce rate caused by the user-defined source item, J_i is the diffusive flux of the ith substance produced by the concentration gradient.

As shown in Figure 2, the section on the left side in the horizontal direction of the jet reactor was set as the O₂ inlet, the section on the right was the free outlet, and the section directly below the reactor in the vertical direction was the waste acid solution inlet.

Procedures

FeCl₂ and FeCl₃ solutions, each with a mass ratio of 10%, were prepared using deionized water, HCl, FeCl₂ and FeCl₃ solution (FeCl₂10%FeCl₃5%, FeCl₂10%FeCl₃7.5%, FeCl₂10%FeCl₃10%, FeCl₂10%FeCl₃12.5%, FeCl₂10%FeCl₃15%, the mass ratio of the two was 1:0.5, 1:0.75, 1:1, 1:1.25, and 1:1.5). Based on the prepared FeCl₂10%FeCl₃10% waste acid solution, a certain amount of citric acid was added to prepare a precursor solution with FeCl₂: C₆H₈O₇ mass ratios of 1:0.5, 1:1, 1:1.5, 1:2, and 1:2.5. FeCl₂, FeCl₃, HCl, and C₆H₈O₇ were provided by Tianjin Fengchuan Chemical Reagent Technologies Co., Ltd. The real-time monitoring of the pyrolysis temperature was performed by the pyrolysis furnace. When the furnace reached the set temperature, the oxygen entered the Venturi reactor from the left inlet at a speed of 21.15 m/s under the carrier gas pressure, and the waste acid solution entered the Venturi reactor from the material inlet. The furnace temperature was set to 773, 873, 923, 973, 1023, and 1123 K, and the pyrolysis products Fe₂O₃ were collected at the cyclone separating unit.

Results and Discussion

Verification of Simulation Result

The boundary conditions are shown in Table 2. The basic standard conditions of the research are 10%FeCl₂, 10%FeCl₃, 5%HCl, 0%C₆H₈O₇.

TABLE 2

TABLE 2. Boundary conditions.

As shown in Table 3, the data results of Fe₂O₃ and CO₂ at outlet of the two representative conditions: adding no citric acid and adding 10% citric acid were compared. The errors between the experimental results of the collected Fe₂O₃ and CO₂ at the outlet and the numerical simulations was within 10%, which indicated the validity of the chosen model and the boundary conditions of the simulation. As shown in Figure 3, to further study the distribution of Fe₂O₃ in the reactor, five sections in the Venturi reactor were monitored.

TABLE 3

TABLE 3. Content comparison at outlet.

FIGURE 3

FIGURE 3. Determination of monitoring surface.

Effects of Pyrolysis Temperature on the Pyrolysis Performance

The XRD phase analysis of products prepared under different pyrolysis temperatures was carried out from 10 to 80° at a scan rate of 7 °/min. As shown in Figure 4, the products prepared by jet pyrolysis under different temperatures all consisted of Fe₂O₃. Upon increasing the pyrolysis temperature, the intensity of the diffraction peak of (B-L) Fe₂O₃ first decreased and then gradually increased. The intensity of the diffraction peak of F −923 K was the smallest without an obvious peak of impurity phases, different crystal forms of Fe₂O₃ were possibly produced at 923 K.

FIGURE 4

FIGURE 4. XRD patterns of the pyrolysis products obtained at different temperatures. B-773, D-873K, F-923K, H-973K, J-1023K, L-1123K.

According to the numerical simulation results (Figure 5), the average Fe₂O₃ concentration on various monitored surfaces in the reactor increased significantly upon increasing the pyrolysis temperature. When the pyrolysis temperature reached 923 K, further increasing the pyrolysis temperature did not affect the average concentration of Fe₂O₃ particles significantly. This indicates that 923 K is the optimal pyrolysis temperature.

FIGURE 5

FIGURE 5. The average Fe₂O₃ concentration on monitored surfaces at different temperatures.

Effects of Pickling Liquor Composition on Pyrolysis

When the pyrolysis temperature reached 923 K, the XRD phase analysis of products prepared using different FeCl₂/FeCl₃ ratios in the pickling liquor was conducted, and the results are shown in Figure 6. The products prepared by jet pyrolysis under different FeCl₂/FeCl₃ ratios all consisted of Fe₂O₃. The height of the crystal face 104 and 110 changed. As the FeCl₂/FeCl₃ ratio increased, (B-J) Fe₂O₃ grew along different crystal faces. When FeCl₂:FeCl₃ = 1:1, the Fe₂O₃ with the least impurities among all the cases was obtained, with the strongest diffraction peak intensity.

FIGURE 6

FIGURE 6. XRD patterns of the pyrolysis products obtained at different FeCl₂/FeCl₃ ratios. B-1:0.5, D-1:0.75, F-1:1, H-1:1.25, J-1:1.5.

As shown in Figure 7, the Fe₂O₃ concentration in the reactor increased upon increasing the concentration of FeCl₃ in the pickling liquor. When FeCl₂:FeCl₃ = 1:1.5, the maximum average Fe₂O₃ particle concentration on various monitored surfaces in the reactor was obtained. This was because upon increasing the FeCl₃ concentration, the Fe ion content at the material inlet increased; thus, according to the law of conservation, the concentration of Fe₂O₃ particles at the outlet also increased.

FIGURE 7

FIGURE 7. Fe₂O₃ concentration on the monitored surfaces at different FeCl₂/FeCl₃ ratios.

Effects of the Citric Acid Ratio During Pyrolysis

When the pyrolysis temperature reached 923 K, FeCl₂:FeCl₃ = 1:1, the XRD phase analysis of products prepared with different proportions of citric acid was carried out, and the results are shown in Figure 8. Adding citric acid contributed to a sufficient pyrolysis reaction and a disappearing of impurity peak. The products prepared by jet pyrolysis using different citric acid ratios all consisted of Fe₂O₃. Upon changing the pickling liquor ratio, there was no obvious change in the intensity of the diffraction peak of (B-J) Fe₂O₃, and the addition of more citric acid had no obvious effect on the product purity.

FIGURE 8

FIGURE 8. XRD patterns of the pyrolysis products obtained using different citric acid ratios. B-1:0.5, D-1:1, F-1:1.5, H-1:2, J-1:2.5.

Figure 9 shows the SEM images of pyrolysis products using solutions with different citric acid ratios. As the citric acid ratio increased, the Fe₂O₃ particles aggregated and finally evolved into irregular sheets. This was because upon increasing the citric acid ratio, a great amount of gas was produced during the pyrolysis of citric acid under a high temperature, and the aggregated products exploded into fragments due to the gas pressure (Yang et al., 2017).

FIGURE 9

FIGURE 9. SEM images of the pyrolysis products (A) 1:0.5 (B) 1:1 (C) 1:1.5 (D) 1:2 (E) 1:2.5.

The integral area of each curve in Figure 10 is shown in Table 4. When the concentration of the citric acid changed, increasing the citric acid ratio did not obviously affect the total amount of Fe₂O₃ particles in the reactor because the addition of citric acid did not change the total amount of Fe ions at the material inlet. The added citric acid changed the Fe₂O₃ particle distribution in the reactor—at a higher citric acid ratio, less Fe₂O₃ appeared on the first half of the monitored surface, and a higher Fe₂O₃ content appeared on the second half. This was because a large amount of gas (CO₂ and H₂O) was produced during the pyrolysis of citric acid. The flow of this gas changed the distribution of particle products in the Venturi reactor, which caused the particle product content to decrease in the first half of the reactor, while the particle product content increased in the second half.

FIGURE 10

FIGURE 10. Average Fe₂O₃ concentrations on monitored surfaces in the reactor at different citric acid ratios.

TABLE 4

TABLE 4. Integral areas of different curves.

Conclusion

As the pyrolysis temperature increased, the concentration of Fe₂O₃ particles in the reactor increased. At 923 K, the purity and concentration of Fe₂O₃ were maximized, and further increasing the pyrolysis temperature had negligible effects on the concentration of Fe₂O₃ particles at the outlet. When FeCl2:FeCl3 = 1:1, the Fe2O3 with the least impurities among all the cases was obtained. When FeCl₂/FeCl₃ = 1:1.5, the concentration of Fe₂O₃ particles at the outlet was maximized. The ratio of citric acid possibly changed size distribution of Fe₂O₃ particles and their distribution in the reactor. When the ratio of FeCl₂/C₆H₈O₇ was1:1, the size distribution of Fe₂O₃ particles was homogeneous, and when the ratio of FeCl₂/C₆H₈O₇ was1:2.5, the concentration of Fe₂O₃ particles at the outlet was maximized.

Data Availability Statement

The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.

Author Contributions

CL: Conceptualization, Funding acquisition, Project administration; M-HS: Data curation, Writing—original, draft; H-XY: Software.

Funding

This research was supported by the National Natural Science Foundation of China (51904069), the open fund of the Key Laboratory of iron and steel metallurgy and resource utilization of Wuhan University of science and technology (FMRU19-1), the Scientific Research Fund project of Northeastern University at Qinhuangdao (XNY201808), the Natural Science Foundation of Hebei Province of China (E2019501085).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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