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).

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: H 2 O ( l ) → H 2 O ( g ) (1) HCl ( l ) → HCl ( g ) (2) 2 FeCl 2 ( s )   +   2 H 2 O ( g ) + 1 / 2 O 2 ( g ) → Fe 2 O 3 ( s ) + 4 HCl ( g ) (3) 2 FeCl 3 ( s )   +   3 H 2 O ( g ) → 2 Fe 2 O 3 ( s ) + 6 HCl ( g ) (4) 2 C 6 H 8 O 7   +   9 O 2 ( g ) → 12 CO 2 ( s )   +   8 H 2 O ( g ) (5) Besides the mass conservation equation, momentum conservation equation and energy conservation equation, it also involved the chemical reaction equation: ∂ ( ρ Y i ) ∂ t + ∇ ⋅ ( ρ ν ¯ Y i ) = − ∇ 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.

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.

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

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.

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.

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.

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.

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.

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 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).

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.