Efficient Recycling and Utilization Strategy for Steel Spent Pickling Solution

Abstract

Before steel can be utilized, pickling is necessary to remove surface oxidation products. However, as the ferrous ion concentration in the pickling solution increases, the pickling rate significantly diminishes, necessitating the treatment of spent pickling solution (SPS) to mitigate its hazardous effects prior to disposal. Current industrial methods predominantly rely on neutralization and precipitation techniques, which are cost-prohibitive and generate substantial by-products, thus failing to meet environmental protection standards. In this study, a new method, which is based on the formation of FeC₂O₄·2H₂O precipitate in a strong acid solution, is proposed to treat the SPS. Initially, the SPS undergoes a two-step impurity removal process, followed by the controlled addition of oxalic acid dihydrate (H₂C₂O₄·2H₂O) to precipitate iron. The resulting precipitate is filtered, washed, and vacuum-dried, and the regenerated acid is recycled back into the pickling tank. When 1 g/10 mL of H₂C₂O₄·2H₂O is used, the iron removal rate achieves 60%, and the acidity of the regenerated acid increases by 11.3%. X-ray diffraction pattern (XRD) and thermogravimetric–differential scanning calorimetry (TG-DSC) characterization showed that the precipitate was α-FeC₂O₄·2H₂O, with an average particle size of about 3.19 μm and a purity of 95.24%. This process innovatively achieves efficient recycling of acid and iron resources, offering a potential solution to the industrial challenge of difficult SPS treatment in the steel industry and meeting the urgent need for sustainable development.

1. Introduction

During the processing, storage, and transportation of steel, rust (Fe₂O₃, FeO, Fe₃O₄) often forms on the surface [1,2,3]. Pickling, covering the reactions between acids, metals, and metal oxides, is an indispensable and important pre-treatment process in steel processing [4,5,6]. Common acids used for pickling include sulfuric acid, hydrochloric acid, nitric acid, organic acids, etc. Due to its advantages, such as small volume, easy inhibition, and no need for heating, hydrochloric acid has become the most commonly used pickling solution in the industry [7,8]. However, when the pickling solution is put into use for a period of time, the content of Fe²⁺ and Fe³⁺ inside will accumulate too high, while the concentration of H⁺ will continue to decrease due to continuous consumption, which will have an adverse effect on the pickling speed and effect. When the concentration of FeCl₂ in the acid exceeds 300 g/L, the acid will almost lose its pickling ability and cannot continue to be used. At this time, it needs to be discharged and new acid needs to be added to the pickling tank to ensure pickling efficiency [9]. The spent pickling solution, commonly referred to as steel spent pickling solution (SPS), contains a significant amount of heavy metal ions and is mildly acidic. SPS poses a significant threat to both flora and fauna, human health, and the sustainable development of the ecological environment. Hence, it must be effectively treated before discharge [10,11,12,13,14,15].

The main methods for treating SPS include neutralization precipitation [16], ion exchange resin adsorption [17], membrane technology [18] and osmosis [19,20], solvent extraction [21], and spray roasting [22,23]. In the industrial field, the traditional alkali neutralization treatment is usually adopted. This method mainly uses carbide slag or lime digestion products to react with acids and metal ions in the SPS, generating Fe(OH)₂ precipitate. However, this method requires a large amount of alkali and flocculant and generates a large amount of sludge during the treatment process. This sludge is either landfilled or stored for further treatment, leading to the waste of metal value and secondary pollution issues [24]. Currently, membrane technologies such as membrane diffusion dialysis, membrane electrodialysis, solar water purification and membrane distillation are considered simple, effective, and sustainable treatment methods as they do not require the addition of chemicals to the SPS, and the treatment equipment occupies a small area, facilitating large-scale use [25,26,27]. However, in the field of industrial SPS cleaning solution treatments, the high ion concentration to be treated leads to high total treatment costs [28,29], and current membrane technologies are still mainly in the laboratory research stage and have not been applied to actual large-scale treatment. Therefore, there is an urgent need for a low-cost, low-equipment investment and environmentally friendly SPS treatment method for the steel pickling process to address the current issues.

Herein, we propose a novel strategy for the recycling of SPS from steel processing. The core design of this work actually lies in the fact that oxalate can react with Fe²⁺ in an acidic solution to form ferrous oxalate, which dissolves very slowly in hydrochloric acid. Through prompt filtration and separation, the recovery of Fe²⁺ can be achieved. Additionally, since the acidity of the waste acid is not very high, there are fewer free hydrogen ions in the system, resulting in limited inhibition of the dissociation of hydrogen ions from oxalic acid. Therefore, the hydrogen ions in oxalic acid can dissociate, increasing the acidity of the regenerated acid and enhancing the acid pickling effect.

Initially, physical filtration was used to remove sludge and large particulate impurities from the SPS. Subsequently, polyacrylamide (PAM) was added as a flocculant to capture the remaining impurities, forming flocs that quickly settle to the bottom. The SPS was then filtered again to obtain purified SPS. Next, Na₂C₂O₄, (NH₄)₂C₂O₄·H₂O, and H₂C₂O₄·2H₂O were added to react with the ferrous ions in the SPS to form iron-containing precipitates. These precipitates were alternately washed with water and ethanol until neutral and finally vacuum dried at 60 °C for 24 h. When the amount of H₂C₂O₄·2H₂O added was 1 g/10 mL, the concentration of Fe²⁺ in regenerated acid decreased by 60%, and the acidity increased by 11.3%. The regenerated acid exhibited good pickling and rust removal effects on Q235 steel plates. Additionally, FeC₂O₄·2H₂O with an average particle size of 3.19 μm and a purity of 95.24% was obtained. The innovative recycling process proposed in this paper achieves rapid and efficient recovery of acid and iron resources, avoiding resource waste and the generation of polluting by-products. It offers a promising solution to the industrial challenge of treating SPS in the steel industry, with significant application prospects.

2. Materials and Methods

2.1. Materials

Steel SPS were taken from a hot-dip galvanizing plant in Jiangxi Province. Polyacrylamide (PAM, M_w = 150,000), sodium oxalate (Na₂C₂O₄, AR, ≥99.8%), ammonium oxalate monohydrate ((NH₄)₂C₂O₄·H₂O, AR, ≥99.5%), ammonium persulfate ((NH₄)₂SO₄, AR, ≥99.5%), and oxalate dihydrate (H₂C₂O₄·2H₂O, AR, ≥99.5%) were obtained from Aladdin (Shanghai, China). AR stands for Analytical Reagent. A Q235 steel plate was laser cut into dimensions of 30 mm × 20 mm × 2 mm. Potassium permanganate titration solution standard substance (KMnO₄, 0.01 mol/L), sulphuric acid (95%–98%), and phosphoric acid (≥85%, in H₂O) were purchased from Shanghai Titan Co., Ltd. (Shanghai, China).

2.2. Processing Methods

First, we conducted the first suction filtration on a 500 mL original SPS to remove sludge and large particle impurities. Then, 2.5 g of PAM was added, stirred vigorously for 10 min and then left to stand for 5 min. After small particle impurities were deposited at the bottom of the acid cleaning solution, we conducted the second suction filtration.

The filtrate of 20 mL was respectively mixed with 0.5 g, 1.0 g, 1.5 g, and 2.0 g of Na₂C₂O₄, (NH₄)₂C₂O₄·H₂O and H₂C₂O₄·2H₂O. The mixture was reacted in a constant temperature heating magnetic stirrer at 25 °C for 1 h with the rotation speed ranging from 950 to 1000 rpm. After the reaction, the mixture was filtered by suction to obtain the regenerated acid and FeC₂O₄·2H₂O. The FeC₂O₄·2H₂O was washed alternately with deionized water and anhydrous ethanol until neutrality and then placed in a vacuum oven to dry at 60 °C for 24 h.

We refer to the regenerated acid cleaning solution obtained after reacting with Na₂C₂O₄, (NH₄)₂C₂O₄·H₂O, and H₂C₂O₄·2H₂O as Na₂C₂O₄-REA, (NH₄)₂C₂O₄·H₂O-REA, and H₂C₂O₄·2H₂O-REA, respectively. The FeC₂O₄·2H₂O precipitates obtained are referred to as Na₂C₂O₄-FCO, (NH₄)₂C₂O₄·H₂O-FCO, and H₂C₂O₄·2H₂O-FCO accordingly.

2.3. Measurement of the Content of Fe³⁺ and Fe²⁺

A 1.00 mL acid sample was accurately measured and transferred to a 100 mL volumetric flask. Deionized water was diluted to the calibration mark. After completion, the pH value was first tested with pH paper to ensure that the pH value was between 1.8 and 2.5. If not, the pH was adjusted by adding the proper amount of ammonia or hydrochloric acid until it was within this range. Subsequently, 10.00 mL of the diluted pickling solution sample was measured from the volumetric flask into a 100 mL conical flask. An amount of 1.0 mL of 20% sulfosalicylic acid was added as an indicator and 0.1 mol/L EDTA standard solution was used as a titrant, gradually adding drops of the solution until the red-purple color disappeared. The volume reading of the EDTA solution was recorded as a. Then, 1.0 mL of 10% (NH₄)₂SO₄ solution was added to the Erlenmeyer flask and heated in a constant temperature water bath at 70 °C for 5 min. The solution in the flask would regain its red-purple color. At this point, the titration was continued using a 0.1 mol/L EDTA standard solution until the red-purple color disappeared. The volume reading of the EDTA standard solution was recorded as b. The titration experiment was then completed. The concentration of Fe³⁺ and Fe²⁺ can then be calculated using the following formula:

$${\mathrm{C}}_{(\mathrm{Fe}3+)}=\frac{0.1·\mathrm{a}·56}{\mathrm{V}}$$

(1)

$${\mathrm{C}}_{(\mathrm{Fe}2+)}=\frac{0.1·\mathrm{b}·56}{\mathrm{V}}$$

(2)

where C_(Fe3+) is the mass concentration of Fe³⁺ (g/L), C_(Fe2+) is the mass concentration of Fe²⁺ (g/L), a and b are the volume of EDTA standard solution, 56 is the relative atomic mass of iron, and V is the volume of SPS.

2.4. Measurement of Raw Material Utilization Rate

In practical applications, the treatment effect of SPS is undoubtedly the primary consideration, and at the same time, the cost of raw materials also plays a pivotal role. Given that there are usually tens of tons of SPS in the pickling tank, the required amount of raw materials will also be substantial. Therefore, it is necessary to calculate the utilization rate of raw materials to reduce production costs. The calculation formula is as follows:

$$\mathrm{W}=\frac{\mathrm{m}\left(\mathrm{act}\right)}{\mathrm{m}\left(\mathrm{tot}\right)}$$

(3)

where m(act) is the quality of actual participation in the reaction, and m(tot) is total mass invested.

2.5. Measurement of Acidity

Due to the high acidity of SPS, directly measuring the original sample with a pH meter would result in significant errors. Therefore, we used a pipette to accurately dispense 1.00 mL of the acid sample into a 100 mL volumetric flask, dilute it to the mark line with deionized water, and then measure its acidity using a pH meter.

2.6. Purity Determination of FeC₂O₄·2H₂O

An amount of 0.1 g of FeC₂O₄·2H₂O solid was dissolved in dilute sulfuric acid (prepared by mixing 8 mL of concentrated sulfuric acid with 92 mL of deionized water). The solution was placed in a constant temperature water bath at 60 °C and heated for 30 min. The solution was removed and allowed to cool, then titrated with a standard solution of potassium permanganate at a concentration of 0.01 mol/L until the end point was reached. The volume consumed was recorded as V (KMnO₄). The purity was calculated using the following formula:

$$\mathrm{W}=\frac{5·\mathrm{C}·\mathrm{V}·0.001·\mathrm{M}}{3·\mathrm{m}}·100\%$$

(4)

where W is the purity of FeC₂O₄·2H₂O, C is the molar concentration of KMnO₄ standard solution, M is the relative molecular weight of KMnO₄ tandard, and m is the mass of FeC₂O₄·2H₂O.

2.7. Characterization

XRD spectra of FeC₂O₄·2H₂O were characterized by X-ray diffraction (XRD) (SmartLab SE, Rigaku, Japan) with Ni-filtered Cu Kα radiation (λ = 1.54184 Å) ranging from 10 to 80° at a scanning speed of 10° min⁻¹. FTIR spectra of FeC₂O₄·2H₂O were recorded on a Spectrometer (Thermo Scientific Nicolet iS20, Waltham, MA, USA, spectral range of 4000–400 cm⁻¹, resolution of 4 cm⁻¹, 32 scans per spectrum). Particle size distribution of FeC₂O₄·2H₂O were recorded by Laser particle size analyzer (Malvern Mastersizer 2000, Malvern, UK). The SEM images of FeC₂O₄·2H₂O were taken by using a Hitachi S4800 scanning electron microscope (Tokyo, Japan). The thermal degradation pattern of the FeC₂O₄·2H₂O was also examined with a dual thermogravimetric analyzer and differential scanning calorimetry (TG-DSC, Netzsch STA 449 F5, Selb, Germany). FeC₂O₄·2H₂O was loaded onto an alumina pan for the measurements using the following conditions (ramping temperature of 10 °C/min under in air).

3. Results and Discussion

3.1. Strategy for Recycling and Utilization of the Steel SPS

In practical industrial production, pickling baths often contain a large amount of sludge and other large-particle impurities. Therefore, it is necessary to filter the SPS, followed by adding 0.5 wt%~1.0 wt% PAM as a flocculant (Figure S1). The polar functional groups in PAM molecules, such as amino groups and amide groups, can interact with small-particle impurities in the SPS, adsorbing onto the surface of particles and forming bridging effects with them, thus aggregating them into larger particles and ultimately forming flocs, which facilitates sedimentation and removal. Subsequently, Na₂C₂O₄, (NH₄)₂C₂O₄·H₂O, and H₂C₂O₄·2H₂O are added to the clean SPS to react with Fe²⁺ in the solution, resulting in regenerated acid and FeC₂O₄·2H₂O precipitate (Figures S2 and S3). The regenerated acid is recirculated back into the pickling bath for direct use, while the FeC₂O₄·2H₂O precipitate is vacuum-dried (Figure 1). The obtained FeC₂O₄·2H₂O is a high value-added product that can be an important raw material for the preparation of lithium iron phosphate. The recovered regenerated acid can continue to be used for steel pickling. The proposed process can be conducted under normal temperature and pressure, with low environmental and equipment requirements. The use of acid also has advantages such as low cost and fast reaction speed. However, it should be noted that the acidity of the SPS to be treated should not be too high, as excessive H⁺ within it can inhibit the dissociation of H₂C₂O₄·2H₂O, making it difficult for oxalate ions to dissociate, potentially resulting in a significant reduction in iron removal efficiency. Nevertheless, Na₂C₂O₄ and (NH₄)₂C₂O₄·H₂O can still react with Fe²⁺ in SPS with higher acidity.

3.2. Effect of Addition Quality on the Concentration of Fe²⁺ and Utilization Rate

Firstly, the concentration of Fe²⁺ in the original sample of SPS was tested to be 73.47 g/L, while the concentration of Fe³⁺ was 0.81 g/L. Due to the low concentration of Fe³⁺ and its minimal impact on the pickling effect, the influence of Fe³⁺ on this experiment was subsequently ignored.

The effect of different amounts of raw materials addition on the concentration of ferrous ions was investigated, and the results are shown in Figure 2a. As the amount of addition increased from 0.5 to 2.0 g, the concentration of residual ferrous ions in the regenerated acid decreased significantly. Specifically, the concentration of ferrous ions in Na₂C₂O₄-REA decreased to 43.40 g/L, in (NH₄)₂C₂O₄·H₂O-REA to 39.98 g/L, and in H₂C₂O₄·2H₂O-REA to 29.23 g/L. The iron removal rate reached 60%, demonstrating a better iron removal effect compared to the first two. Simultaneously, we calculated the utilization rates of these three raw materials, and the results are presented in Figure 2b. When the addition amount was 0.5 g, the utilization rate of (NH₄)₂C₂O₄·H₂O was the highest, reaching 60.38%, while the utilization rate of H₂C₂O₄·2H₂O was 47.74%, and that of Na₂C₂O₄ was the lowest, only 34.20%. As the amount of addition gradually increased, the utilization rate of Na₂C₂O₄ showed a trend of first increasing and then decreasing, reaching a maximum of 56.17%. The utilization rate of (NH₄)₂C₂O₄·H₂O gradually decreased and then stabilized, ultimately hovering around 50%. On the other hand, the utilization rate of H₂C₂O₄·2H₂O gradually increased and then stabilized, ultimately reaching 70%.

3.3. Effect of Regenerated Acid on Acidity and Actual Pickling

In addition to the concentration of ferrous ions, acidity is also an important factor that affects the pickling rate and effectiveness of the SPS. The impact of different raw material addition amounts on the acidity of regenerated acid was investigated, and the results are shown in Figure 3a. As the addition amount gradually increased from 0 to 2.0 g, the acidity of Na₂C₂O₄-REA and (NH₄)₂C₂O₄·H₂O-REA barely changed, whereas the acidity of H₂C₂O₄·2H₂O-REA showed a significant increase, rising from an initial value of 3.19% to 14.49%. To validate the actual pickling effect of regenerated acid, we respectively placed 25 mL of Na₂C₂O₄-REA, (NH₄)₂C₂O₄·H₂O-REA, and H₂C₂O₄·2H₂O-REA into a constant temperature water bath at 25 °C. We then immersed Q235 steel plates with similar surface corrosion degrees, prepared in the earlier stage, into the recycled acids. After the same intervals, we observed and recorded the residual conditions of the rust on the surfaces to evaluate the rust removal capabilities of the recycled acids, as shown in Figure 3b. After 2 min of immersion, the rust on the surface of the SPS slightly decreased, but there was no significant change after 4 min, indicating a very slow pickling rate. When using Na₂C₂O₄-REA for pickling, part of the steel plate’s substrate emerged after 2 min, proving that its pickling effect was slightly better than the SPS. With (NH₄)₂C₂O₄·H₂O-REA, the outer layer of rust could be removed within 2 min, and a significant reduction in rust thickness was observed. However, after 4 min, the rust layer on the surface still remained. The rust removal effect of H₂C₂O₄·2H₂O-REA was significantly better than the first three. After 2 min of pickling, the steel plate’s substrate had already emerged, and nearly 80% of the rust on the steel plate’s surface had been removed after 4 min.

As H₂C₂O₄·2H₂O dissociates into hydrogen ions (H⁺) and oxalate ions(C₂O₄²⁻) in the SPS, the gradual consumption of C₂O₄²⁻ due to their participation in the reaction leads to an increasing degree of dissociation, resulting in a significant increase in the acidity of the recycled acid. Therefore, H₂C₂O₄·2H₂O-REA exhibits excellent rust removal capabilities through pickling, promising to meet the needs of practical industrial production.

3.4. Characterization of Na₂C₂O₄-FCO, (NH₄)₂C₂O₄·H₂O-FCO, and H₂C₂O₄·2H₂O-FCO

To investigate the properties of solid products, their X-ray diffraction patterns are first presented in Figure 4a. The diffraction peaks of Na₂C₂O₄-FCO, (NH₄)₂C₂O₄·H₂O-FCO, and H₂C₂O₄·2H₂O-FCO all match the characteristic spectral lines of the standard card PDF#22-0293 for α-FeC₂O₄·2H₂O, indicating that the product is orthorhombic α-FeC₂O₄·2H₂O. For Na₂C₂O₄-FCO and (NH₄)₂C₂O₄·H₂O-FCO, the diffraction peaks at 2θ = 18.4°, 22.7°, 29.3°, 34.3°, and 42.7° are consistent with those of α-FeC₂O₄·2H₂O. In addition, the diffraction peaks of Na₂C₂O₄-FCO at 2θ = 24.7° and 33.6° also align well with those of α-FeC₂O₄·2H₂O. By referencing the standard PDF card for α-FeC₂O₄·2H₂O, the cell parameters are found to be a = 9.921 Å, b = 5.556 Å, c = 9.707 Å; α = 90°, β = 104.5°, γ = 90°. Since the three samples we prepared showed good agreement with the standard card, we used H₂C₂O₄·2H₂O-FCO as an example to calculate its lattice constants. The crystallite size and strain from XRD data were calculated using Scheller’s formula and the Williamson–Hall (W-H) plot method, and the detailed calculation process can be found in the supporting materials. By calculation, the lattice constants of H₂C₂O₄·2H₂O-FCO are a = 9.911 Å, b = 5.8 Å, and c = 9.812 Å, crystallite size (D) is 28.71 nm, and lattice strain (ε) is 1.58 × 10⁻³.

The infrared spectrum, as shown in Figure 4b, exhibits two characteristic absorption peaks of hydrated oxalate compounds: the C=O asymmetric stretching vibration peak at 1630 cm⁻¹ and the absorption peak of crystal water at 3360 cm⁻¹. Additionally, the stretching vibration absorption peak of O-C-O at 1320 cm⁻¹ and the bending vibration absorption peak of O-C-O located at the band of 820 cm⁻¹ both confirm that the precipitate is hydrated oxalate. The peaks at 494 cm⁻¹ and 821 cm⁻¹ may correspond to the vibrational absorption of Fe-O.

The particle size distribution of the precipitates was measured using a laser particle size analyzer, as depicted in Figure 4c,d. D₁₀, D₅₀ and D₉₀ refer to the particle sizes corresponding to the cumulative particle size distribution of the sample reaching 10%, 50% and 90%, respectively. The physical meaning is that particles with particle sizes smaller than them account for 10%, 50% and 90%, respectively. D₅₀ is also known as the median diameter or median particle size, commonly used to represent the average particle size of a powder. D[3,2] refers to the surface area mean diameter, which physically means the total volume of the particle group divided by the total surface area of the particle group, i.e., the reciprocal of the surface area per unit volume. D[4,3] (volume average diameter) refers to the average diameter with the same volume and number of particles.

The average particle size of Na₂C₂O₄-FCO is 2.75 μm, with the average particle diameters for surface area D[3,2] and volume D[4,3] being 2.31 μm and 15.70 μm, respectively. For (NH₄)₂C₂O₄·H₂O-FCO, the average particle size is 2.40 μm, with the average particle diameters for surface area D[3,2] and volume D[4,3] being 1.73 μm and 25.49 μm, respectively. The average particle size of H₂C₂O₄·2H₂O-FCO is 3.19 μm, with the average particle diameters for surface area D[3,2] and volume D[4,3] being 2.40 μm and 3.87 μm, respectively. It can be observed that the D[4,3] value for H₂C₂O₄·2H₂O-FCO is significantly lower. Based on the values of D₁₀, D₅₀, and D₉₀, we can calculate the distribution uniformity U using Equation (5):

$$\mathrm{U}=\frac{\mathrm{D}\left[90\right]-\mathrm{D}\left[10\right]}{\mathrm{D}\left[50\right]}$$

(5)

The U values for Na₂C₂O₄-FCO, (NH₄)₂C₂O₄·H₂O-FCO, and H₂C₂O₄·2H₂O-FCO are 8.58, 52.76, and 1.99, respectively. A lower U value indicates a narrower particle size distribution and better uniformity. This is in good agreement with the analysis results of the SEM images.

Figure 5 shows SEM images of the Na₂C₂O₄-FCO (Figure S4), (NH₄)₂C₂O₄·H₂O-FCO (Figure S5) and H₂C₂O₄·2H₂O-FCO (Figure S6). Na₂C₂O₄-FCO-FCO appears in the form of granules with sizes ranging from 1 to 20 μm. Its surface is relatively rough, and the mutual attraction between particles increases, leading to easy agglomeration of some particles (Figure 5a,b). (NH₄)₂C₂O₄·H₂O-FCO also manifests as granules, exhibiting a wider range of particle sizes but with smoother surfaces (Figure 5c,d). H₂C₂O₄·2H₂O-FCO, on the other hand, assumes a relatively regular cubic shape with small and uniformly distributed particle sizes approximately between 1 and 10 μm (Figure 5e,f). Moreover, its surface is extremely smooth, making it highly valuable for utilization. The particle sizes observed in the SEM images are consistent with the results obtained from particle size distribution.

In fact, the surface morphology of FeC₂O₄·2H₂O particles can be affected by various factors, such as reaction temperature, feeding method, initial concentration of reactants, and stirring speed. In this work, since the above conditions are the same, the main factor that affects the particle size and surface morphology of FeC₂O₄·2H₂O dihydrate particles is pH. With the continuous addition of H₂C₂O₄·2H₂O, the C₂O₄²⁻ reacts with excess Fe²⁺ in the waste acid to form complexes, promoting the forward progress of the dissociation reaction of H₂C₂O₄·2H₂O. Therefore, the content of free H⁺ increases, leading to an increase in acidity and a decrease in pH, thus enhancing the pickling effect of waste acid. On the other hand, the rough surface of FeC₂O₄·2H₂O particles is more prone to agglomeration, reducing the overall contact area with the waste acid, which is not conducive to the reaction, resulting in a low iron removal rate and raw material utilization rate (such as Na₂C₂O₄-FCO), thus hindering the efficient recycling of waste acid.

Based on the TG-DSC curve, it can be observed that there are two processes of mass loss during the thermal decomposition of Na₂C₂O₄-FCO, (NH₄)₂C₂O₄·H₂O-FCO, and H₂C₂O₄·2H₂O-FCO (Figure 6a–c). In the first stage, the remaining mass of Na₂C₂O₄-FCO at 205.20 °C is 80.29%, corresponding to a distinct endothermic peak in the DCS curve. For (NH₄)₂C₂O₄·H₂O-FCO, the remaining mass at 205.13 °C is 80.19%, and the DSC curve shows endothermic peaks at 169 °C and 275.58 °C. The possible reason for this phenomenon is the decomposition of ammonium ions due to heating. As for H₂C₂O₄·2H₂O-FCO, the remaining mass at 212.6 °C is 80.46%, and the DSC curve exhibits an endothermic peak at 202.57 °C. The proportion of mass loss corresponds exactly to the mass ratio of the two water molecules in ferrous H₂C₂O₄·2H₂O (20%).

In the second stage, when the final heating temperature reaches 600 °C, the remaining masses of Na₂C₂O₄-FCO, (NH₄)₂C₂O₄·H₂O-FCO, and H₂C₂O₄·2H₂O-FCO are 43.29%, 43.52%, and 43.99%, respectively. The total mass loss is approximately 57%, corresponding to the final thermal decomposition product of Fe₂O₃. Based on the test results and previous studies, the thermal decomposition equation of FeC₂O₄·2H₂O in air is:

4FeC₂O₄·2H₂O + O₂ → 2Fe₂O₃ + 4CO + 4CO₂ +8H₂O

(6)

It is evident from the DSC curve that their endothermic peaks occur at 291.81 °C, 351.11 °C, and 271.85 °C, respectively, which is also related to their purity.

FeC₂O₄·2H₂O is an important raw material for synthesizing a new type of lithium-ion battery cathode material, lithium iron phosphate. Its purity and particle size have a significant impact on the conductivity, impedance, electrochemical capacity, and other properties of lithium iron phosphate. High purity FeC₂O₄·2H₂O can be used to prepare lithium iron phosphate with better electrochemical performance. Currently, the purity of FeC₂O₄·2H₂O sold on the market is around 98.50%. Figure 6d shows the purity of Na₂C₂O₄-FCO, (NH₄)₂C₂O₄·H₂O-FCO and H₂C₂O₄·2H₂O-FCO. Among them, the purity of Na₂C₂O₄-FCO was the highest, reaching 96.63%, but it was still lower than the purity of commercially available products. Therefore, it is necessary to optimize the filtration and impurity removal steps to improve the purity of ferrous oxalate products.

4. Conclusions

Due to the high ferrous ion content and acidity of steel SPS, current treatment methods such as acid–base neutralization precipitation have issues of high cost and numerous by-products, making it difficult to meet the demands of energy conservation and environmental protection. To address these industrial issues, this study developed a simple and efficient process for recycling and utilizing SPS. A comparative analysis is conducted on the precipitation effect of oxalate-containing substances, such as Na₂C₂O₄, (NH₄)₂C₂O₄·H₂O, and H₂C₂O₄·2H₂O, on Fe²⁺ in SPS, as well as the acidity of regenerated acid and its rust removal effect. Among them, H₂C₂O₄·2H₂O demonstrates the best iron removal effect, achieving a 60% iron removal rate with an addition of 1 g/10 mL and increasing the acidity of SPS by 11.3%. The regenerated acid can remove most of the rust on the surface of Q235 steel within 4 min. Furthermore, through XRD and FTIR analysis, we confirm that the three precipitated components are α-FeC₂O₄·2H₂O particles, with H₂C₂O₄·2H₂O-FCO having a purity of 95.24%, an average particle size of 3.19 μm, and the most uniform particle size distribution, making it the most promising for potential applications.

To achieve practical industrial application, improvements can be made in areas such as enhancing the purity of FeC₂O₄·2H₂O by improving the filtration and separation process, as well as controlling the reaction conditions to improve the utilization rate of raw materials and the iron removal rate. Compared to some similar research work (Table S1), this method can not only efficiently remove iron from waste acid, but also obtain regenerated acid with high acidity. In addition, the raw materials used in our work are inexpensive, and the overall process is relatively simple and easy to implement. This work is expected to achieve practical application and has comprehensive advantages.