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 Fe 2 O 3 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% FeCl 2 /FeCl 3 , 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 FeCl 2 and FeCl 3 react with H 2 O and O 2 to produce HCl and Fe 2 O 3 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 (Fe 2 O 3 nanorod) by the hydrothermal method, the ferric nitrate nonahydrate (Fe(NO 3 ) 3 ·9H 2 O) 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(NO 3 ) 3 ·9H 2 O on the particle size of the Fe 2 O 3 nanorod were investigated. Zhang et al. (2020) prepared the α-nanometer Fe 2 O 3 by microwave hydrolysis, FeCl 3 was the raw material and the triethylenetetramine (TETA) was used as the coordination agent. Taniguchi (Izumi et al., 2007) synthesized LiMn 2 O 4 particles with a hollow sphere morphology from a precursor solution using spray pyrolysis from a slurry solution and prepared spherical LiMn 2 O 4 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 CeCl 3 and Fe 2 O 3 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 Fe 2 O 3 particles obtained at the outlet of a Venturi reactor. The effect on the Fe 2 O 3 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 Fe 2 O 3 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 O 2 , CO 2 , HCl(g), H 2 O(g), HCl(l), H 2 O(l), FeCl 2 , FeCl 3 , and Fe 2 O 3 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).

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).
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: Besides the mass conservation equation, momentum conservation equation and energy conservation equation, it also involved the chemical reaction equation: 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 O 2 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.

Verification of Simulation Result
The boundary conditions are shown in Table 2. The basic standard conditions of the research are 10%FeCl 2 , 10%FeCl 3 , 5%HCl, 0%C 6 H 8 O 7 . As shown in Table 3, the data results of Fe 2 O 3 and CO 2 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 Fe 2 O 3 and CO 2 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 Fe 2 O 3 in the reactor, five sections in the Venturi reactor were monitored.

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 Fe 2 O 3 . Upon increasing the pyrolysis temperature, the intensity of the diffraction peak of (B-L) Fe 2 O 3 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 Fe 2 O 3 were possibly produced at 923 K.   According to the numerical simulation results (Figure 5), the average Fe 2 O 3 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 Fe 2 O 3 particles significantly. This indicates that 923 K is the optimal pyrolysis temperature.

Effects of Pickling Liquor Composition on Pyrolysis
When the pyrolysis temperature reached 923 K, the XRD phase analysis of products prepared using different FeCl 2 / FeCl 3 ratios in the pickling liquor was conducted, and the results are shown in Figure 6. The products prepared by jet pyrolysis under different FeCl 2 /FeCl 3 ratios all consisted of Fe 2 O 3 . The height of the crystal face 104 and 110 changed. As the FeCl 2 /FeCl 3 ratio increased, (B-J) Fe 2 O 3 grew along different crystal faces. When FeCl 2 :FeCl 3 1:1, the Fe 2 O 3 with the least impurities among all the cases was obtained, with the strongest diffraction peak intensity.
As shown in Figure 7, the Fe 2 O 3 concentration in the reactor increased upon increasing the concentration of FeCl 3 in the pickling liquor. When FeCl 2 :FeCl 3 1:1.5, the maximum average Fe 2 O 3 particle concentration on various monitored surfaces in the reactor was obtained. This was because upon increasing the FeCl 3 concentration, the Fe ion content at the material inlet increased; thus, according to the

Effects of the Citric Acid Ratio During Pyrolysis
When the pyrolysis temperature reached 923 K, FeCl 2 :FeCl 3 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 Fe 2 O 3 . Upon changing the pickling liquor ratio, there was no obvious change in the intensity of the diffraction peak of (B-J) Fe 2 O 3 , and the addition of more citric acid had no obvious effect on the product purity. Figure 9 shows the SEM images of pyrolysis products using solutions with different citric acid ratios. As the citric acid ratio increased, the Fe 2 O 3 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).
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 Fe 2 O 3 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 Fe 2 O 3 particle distribution in the reactor-at a higher citric acid ratio, less Fe 2 O 3 appeared on the first half of the monitored surface, and a higher Fe 2 O 3 content appeared on the second half. This was because a large amount of gas (CO 2 and H 2 O) 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.

CONCLUSION
As the pyrolysis temperature increased, the concentration of Fe 2 O 3 particles in the reactor increased. At 923 K, the purity and concentration of Fe 2 O 3 were maximized, and further increasing the pyrolysis temperature had negligible effects on the concentration of Fe 2 O 3 particles at the outlet. When FeCl2:FeCl3 1:1, the Fe2O3 with the least impurities among all the cases was obtained. When FeCl 2 / FeCl 3 1:1.5, the concentration of Fe 2 O 3 particles at the outlet was maximized. The ratio of citric acid possibly changed size distribution of Fe 2 O 3 particles and their distribution in the reactor. When the ratio of FeCl 2 /C 6 H 8 O 7 was1:1, the size distribution of Fe 2 O 3 particles was homogeneous, and when the ratio of FeCl 2 /C 6 H 8 O 7 was1: 2.5, the concentration of Fe 2 O 3 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.