Article

Efficient Recycling and Utilization Strategy for Steel Spent

Pickling Solution
Qi Liu 1,2, Yuqing Cao 3,4, Meng Zhou 2,4, Zehao Miao 1, Jinkun Yang 1, Zhaokai Du 5, Baoyang Lu 1,2, Guiqun Liu 6,*,
Jianhong Li 7,* and Shuai Chen 1,4,*

1 Jiangxi Province Key Laboratory of Flexible Electronics, Nanchang 330013, China;

liuqi1903794147@163.com (Q.L.); mzh15867450093@163.com (Z.M.); jky746111238@163.com (J.Y.);
luby@jxstnu.edu.cn (B.L.)
2 Flexible Electronics Innovation Institute, School of Pharmacy, Jiangxi Science and Technology Normal

University, Nanchang 330013, China; zhoumeng1512022@163.com

3 School of Chemistry and Chemical Engineering, Jiangxi Science and Technology Normal University,

Nanchang 330013, China; caoyuqing0917@163.com

4 Jiangxi Provincial Engineering Research Center for Waterborne Coatings, Nanchang 330013, China

5 School of Business, Jianghan University, Wuhan 430060, China; zhaokai7272@gmail.com

6 School of Materials Science and Technology, North Minzu University, Yinchuan 750021, China

7 The Institute of Metaverse, Jiangxi Science and Technology Normal University, Nanchang 330013, China

* Correspondence: gqliu10b@alum.imr.ac.cn (G.L.); masterlijh@jxstnu.edu.cn (J.L.); shuaichen@jxstnu.edu.cn

(S.C.)

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 signif-
icantly diminishes, necessitating the treatment of spent pickling solution (SPS) to mitigate its haz-
ardous 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 addi-
tion of oxalic acid dihydrate (H2C2O4·2H2O) to precipitate iron. The resulting precipitate is filtered,
Citation: Liu, Q.; Cao, Y.; Zhou, M.; washed, and vacuum-dried, and the regenerated acid is recycled back into the pickling tank. When
Miao, Z.; Yang, J.; Du, Z.; Lu, B.; Liu, 1 g/10 mL of H2C2O4·2H2O is used, the iron removal rate achieves 60%, and the acidity of the regen-
G.; Li, J.; Chen, S. Efficient Recycling erated acid increases by 11.3%. X-ray diffraction pattern (XRD) and thermogravimetric–differential
and Utilization Strategy for Steel scanning calorimetry (TG-DSC) characterization showed that the precipitate was α-FeC2O4·2H2O,
Spent Pickling Solution. Coatings with an average particle size of about 3.19 µm and a purity of 95.24%. This process innovatively
2024, 14, 784. https://doi.org/ achieves efficient recycling of acid and iron resources, offering a potential solution to the industrial
10.3390/coatings14070784
challenge of difficult SPS treatment in the steel industry and meeting the urgent need for sustainable
Academic Editor: Joaquim Carneiro development.

Received: 1 May 2024

Keywords: steel anticorrosion; spent pickling solution; regenerated acid; ferrous oxalate dihydrate
Revised: 11 June 2024
Accepted: 20 June 2024
Published: 22 June 2024

1. Introduction
Copyright: © 2024 by the authors. During the processing, storage, and transportation of steel, rust (Fe2O3, FeO, Fe3O4)
Licensee MDPI, Basel, Switzerland. often forms on the surface [1–3]. Pickling, covering the reactions between acids, metals,
This article is an open access article and metal oxides, is an indispensable and important pre-treatment process in steel pro-
distributed under the terms and cessing [4–6]. Common acids used for pickling include sulfuric acid, hydrochloric acid,
conditions of the Creative Commons nitric acid, organic acids, etc. Due to its advantages, such as small volume, easy inhibition,
Attribution (CC BY) license and no need for heating, hydrochloric acid has become the most commonly used pickling
(https://creativecommons.org/license solution in the industry [7,8]. However, when the pickling solution is put into use for a
s/by/4.0/). period of time, the content of Fe2+ and Fe3+ inside will accumulate too high, while the

Coatings 2024, 14, 784. https://doi.org/10.3390/coatings14070784 www.mdpi.com/journal/coatings

Coatings 2024, 14, 784 2 of 13

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 con-
tinue 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, com-
monly 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 [10–15].
The main methods for treating SPS include neutralization precipitation [16], ion ex-
change resin adsorption [17], membrane technology [18] and osmosis [19,20], solvent ex-
traction [21], and spray roasting [22,23]. In the industrial field, the traditional alkali neu-
tralization 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 pre-
cipitate. However, this method requires a large amount of alkali and flocculant and gen-
erates a large amount of sludge during the treatment process. This sludge is either land-
filled or stored for further treatment, leading to the waste of metal value and secondary
pollution issues [24]. Currently, membrane technologies such as membrane diffusion di-
alysis, 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 [25–27]. However, in the field of industrial SPS cleaning solu-
tion 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 ur-
gent 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. Addition-
ally, 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 acid-
ity 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.
Coatings 2024, 14, 784 3 of 13

2. Materials and Methods

2.1. Materials
Steel SPSs were taken from a hot-dip galvanizing plant in Jiangxi Province. Poly-
acrylamide (PAM, Mw = 150,000), sodium oxalate (Na2C2O4 AR, ≥99.8%), ammonium oxa-
late 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 tem-
perature 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 regener-
ated acid and FeC2O4·2H2O. The FeC2O4·2H2O was washed alternately with deionized wa-
ter 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 volu-
metric 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 stand-
ard 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 com-
pleted. The concentration of Fe3+ and Fe2+ can then be calculated using the following for-
mula:
0.1·a·56
C(Fe3+) = (1)
V

0.1·b·56
C(Fe2+) = (2)
V

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.
Coatings 2024, 14, 784 4 of 13

2.4. Measurement of Raw Material Utilization Rate

In practical applications, the treatment effect of SPS is undoubtedly the primary con-
sideration, 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:
m(act)
W= (3)
m(tot)

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 dis-
pense 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 (pre-
pared 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:
· · · . ·
W= · 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 rec-
orded 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 distribu-
tion 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 (ramp-
ing 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, fol-
lowed 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
Coatings 2024, 14, 784 5 of 13

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 dis-
sociation of H2C2O4·2H2O, making it difficult for oxalate ions to dissociate, potentially re-
sulting in a significant reduction in iron removal efficiency. Nevertheless, Na2C2O4 and
(NH4)2C2O4·H2O can still react with Fe2+ in SPS with higher acidity.

Figure 1. Schematic illustration for the recycling and utilization process of steel SPS.

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 sub-
sequently ignored.
The effect of different amounts of raw materials addition on the concentration of fer-
rous ions was investigated, and the results are shown in Figure 2a. As the amount of ad-
dition increased from 0.5 to 2.0 g, the concentration of residual ferrous ions in the regen-
erated 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 bet-
ter iron removal effect compared to the first two. Simultaneously, we calculated the utili-
zation 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, reach-
ing 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%.
Coatings 2024, 14, 784 6 of 13

Figure 2. The effect of addition quality of Na2C2O4, (NH4)2C2O4·H2O, and H2C2O4·2H2O on (a) the
concentration of Fe2+ in SPS and (b) the raw material utilization rate.

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 ma-
terial addition amounts on the acidity of regenerated acid was investigated, and the re-
sults 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 acid-
ity 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 con-
stant 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 sur-
faces 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.
Coatings 2024, 14, 784 7 of 13

Figure 3. (a) The acidity of the regenerated acid varying with the change in the mass of added
Na2C2O4, (NH4)2C2O4·H2O, and H2C2O4·2H2O. (b) Photographs of the surface of Q235 steel plate after
rust removal through pickling with regenerated acid (The changes on the surface of the steel plate
are labeled by red dashed lines).

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 param-
eters 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 stretch-
ing 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.
Coatings 2024, 14, 784 8 of 13

Figure 4. (a) X-ray diffraction patterns, (b) FTIR spectra, (c) particle size distribution and (d) particle
size distribution histogram of Na2C2O4-FCO, (NH4)2C2O4·H2O-FCO, and H2C2O4·2H2O-FCO.

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 corre-
sponding 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 diam-
eter 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 diam-
eters 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, respec-
tively. 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, respec-
tively. 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):
[ ] [ ]
U= (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.
Coatings 2024, 14, 784 9 of 13

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 par-
ticles (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.

Figure 5. SEM images of (a,b) Na2C2O4-FCO, (c,d) (NH4)2C2O4·H2O-FCO and (e,f) H2C2O4·2H2O-
FCO.

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 re-
action 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
Coatings 2024, 14, 784 10 of 13

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 phenome-
non 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%).

Figure 6. TG-DSC curves of (a) Na2C2O4-FCO, (b) (NH4)2C2O4·H2O-FCO and (c) H2C2O4·2H2O-FCO.
(d) Purity of Na2C2O4-FCO, (NH4)2C2O4·H2O-FCO and H2C2O4·2H2O-FCO obtained from the spent
pickling solution.

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%, correspond-
ing 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 sig-
nificant impact on the conductivity, impedance, electrochemical capacity, and other prop-
erties of lithium iron phosphate. High purity FeC2O4·2H2O can be used to prepare lithium
Coatings 2024, 14, 784 11 of 13

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 commer-
cially 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 meth-
ods 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 environ-
mental 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 in-
creasing 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 appli-
cations.
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 pro-
cess, 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 regener-
ated acid with high acidity. In addition, the raw materials used in our work are inexpen-
sive, and the overall process is relatively simple and easy to implement. This work is ex-
pected to achieve practical application and has comprehensive advantages.

Supplementary Materials: The following supporting information can be downloaded at:

https://www.mdpi.com/article/10.3390/coatings14070784/s1, Figure S1: Photos of the reaction after
adding (a) 0 g, (b) 0.5 g, (c) 1 g, (d) 1.5 g, and (e) 2 g H2C2O4·2H2O; Figure S2: Treatment process of
FeC2O4·2H2O. (a) washing, (b) stewing, (c) filtering, (d) drying, (e) the pH of the washing solution
ranges from 3 to 7; Figure S3: Photos of (a) Na2C2O4-FCO, (b) (NH4)2C2O4·H2O-FCO, and (c)
H2C2O4·2H2O-FCO; Figure S4: (a,b) SEM images and (c,d) EDS analysis of Na2C2O4-FCO; Figure S5:
(a,b) SEM images and (c,d) EDS analysis of (NH4)2C2O4·H2O-FCO; Figure S6: (a,b) SEM images and
(c,d) EDS analysis of H2C2O4·2H2O-FCO; Table S1: Comparison of the iron removal rate and products
between this work and previously reported waste acid treatment methods.
Author Contributions: Q.L.: conceptualization, software, and writing—original draft; Y.C. and
M.Z.: revision and review, formal analysis; S.C. and B.L.: conceptualization, writing—revision and
editing, supervision; Z.M. and J.Y.: formal analysis. G.L., Z.D. and J.L.: software. All authors have
read and agreed to the published version of the manuscript.
Funding: We are grateful to the Jiangxi Educational Committee for a Postgraduate Innovation Pro-
gram grant (YC2022-s789), and the Academic Development Project of TongXin Funds (grant num-
ber 2024161806) for their financial support of this work.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available from the corresponding
author upon request.
Conflicts of Interest: The authors declare no conflicts of interest.
Coatings 2024, 14, 784 12 of 13

References
1. Santa, A.C.; Montoya, D.A.; Tamayo, J.A.; Gómez, M.A.; Castaño, J.G.; Baena, L.M. Atmospheric corrosion of carbon steel:
Results of one-year exposure in an andean tropical atmosphere in Colombia. Heliyon 2024, 10, e29391.
2. Wu, F.J.; Hu, Z.H.; Liu, X.J.; Su, C.W.; Hao, L. Understanding in compositional phases of carbon steel rust layer with a long-
term atmospheric exposure. Mater. Lett. 2022, 315, 131968.
3. SArreola-Villa, S.A.; Vergara-Hernández, H.J.; Solorio-Diáz, G.; Pérez-Alvarado, A.; Vázquez-Gómez, O.; Chávez-Campos,
G.M. Kinetic study of oxide growth at high temperature in low carbon steel. Metals 2022, 12, 147.
4. Liu, Q.; Cao, Y.; Chen, S.; Xu, X.; Yao, M.; Fang, J.; Liu, G. Hot-dip galvanizing process and the influence of metallic elements
on composite coatings. J. Compos. Sci. 2024, 8, 160.
5. Wu, M.T.; Li, Y.L.; Guo, Q.; Shao, D.W.; He, M.M.; Qi, T. Harmless treatment and resource utilization of stainless steel pickling
sludge via direct reduction and magnetic separation. J. Clean. Prod. 2019, 240, 118187.
6. El Kacimi, Y.; Touir, R.; Galai, M.; Alaoui, K.; Dkhireche, N.; Touhami, M.E. Relationship between silicon, phosphorus content
and grain number in mild steels and its corrosion resistance in pickling hydrochloric acid. Int. J. Ind. Chem. 2020, 11, 111–122.
7. Manna, M.; Dutta, M. Effect of prior electro or electroless Ni plating layer in galvanizing and galvannealing behavior of high
strength steel sheet. Surf. Coat. Technol. 2017, 316, 48–58.
8. Miranda-Alcantará, B.; Castañeda-Záldivar, F.; Ortíz-Frade, L.; Antaño, R.; Rivera, F.F. Electrochemical study of iron deposit in
acid media for its recovery from spent pickling baths regeneration. J. Electroanal. Chem. 2021, 901, 115805.
9. Regel-Rosocka, M. A review on methods of regeneration of spent pickling solutions from steel processing. J. Hazard. Mater. 2010,
177, 57–69.
10. Covaliu-Mierlă, C.I.; Păunescu, O.; Iovu, H. Recent advances in membranes used for nanofiltration to remove heavy metals
from wastewater: A review. Membranes 2023, 13, 643.
11. Tang, B.; Yuan, L.; Shi, T.; Yu, L.; Zhu, Y. Preparation of nano-sized magnetic particles from spent pickling liquors by ultrasonic-
assisted chemical co-precipitation. J. Hazard. Mater. 2009, 163, 1173–1178.
12. Tang, J.; Pei, Y.; Hu, Q.; Pei, D.; Xu, J. The recycling of ferric salt in steel pickling liquors: Preparation of nano-sized iron oxide.
Procedia Environ. Sci. 2016, 31, 778–784.
13. Shi, C.H.; Zhang, Y.Q.; Zhou, S.; Jiang, J.C.; Huang, X.Y.; Hua, J. Status of research on the resource utilization of stainless steel
pickling sludge in China: A review. Environ. Sci. Pollut. Res. 2023, 30, 90223–90242.
14. Yi, Y.; Tu, G.; Zhao, D.; Tsang, P.E.; Fang, Z. Biomass waste components significantly influence the removal of Cr(VI) using
magnetic biochar derived from four types of feedstocks and steel pickling waste liquor. Chem. Eng. J. 2019, 360, 212–220.
15. Sherif, E.S.M. Corrosion inhibition in 2.0 M sulfuric acid solutions of high strength maraging steel by aminophenyl tetrazole as
a corrosion inhibitor. Appl. Surf. Sci. 2014, 292, 190–196.
16. Xu, L.; Zheng, Y.; Zhao, Y.; Chen, W. Recovery of arsenic oxide, harmless gypsum residue and clean water by lime neutralization
and precipitation. Hydrometallurgy 2023, 215, 105996.
17. Aboul-Magd, A.A.S.; Al-Husain, S.A.R.; Al-Zahrani, S.A. Batch adsorptive removal of Fe (III), Cu (II) and Zn (II) ions in aqueous
and aqueous organic–HCl media by Dowex HYRW2-Na Polisher resin as adsorbents. Arab. J. Chem. 2016, 9, S1–S8.
18. Lorenz, M.; Seitfudem, G.; Randazzo, S.; Gueccia, R.; Gehring, F.; Prenzel, T.M. Combining membrane and zero brine
technologies in waste acid treatment for a circular economy in the hot-dip galvanizing industry: A life cycle perspective. J.
Sustain. Metall. 2023, 9, 537–549.
19. Leonzio, G. Recovery of metal sulphates and hydrochloric acid from spent pickling liquors. J. Clean. Prod. 2016, 129, 417–426.
20. Ma, Y.; Wang, X.; Wang, M.; Jiang, C.; Xiang, X.; Zhang, X. Separation of V(IV) and Fe(III) from the acid leach solution of stone
coal by D2EHPA/TBP. Hydrometallurgy 2015, 153, 38–45.
21. Arguillarena, A.; Margallo, M.; Irabien, Á.; Urtiaga, A. Life cycle assessment of zinc and iron recovery from spent pickling acids
by membrane-based solvent extraction and electrowinning. J. Environ. Manag. 2022, 318, 115567.
22. Gao, Y.; Yue, T.; Sun, W.; He, D.; Lu, C.; Fu, X. Acid recovering and iron recycling from pickling waste acid by extraction and
spray pyrolysis techniques. J. Clean. Prod. 2021, 312, 127747.
23. Schiemann, M.; Wirtz, S.; Scherer, V.; Bärhold, F. Spray roasting of iron chloride FeCl2: Numerical modelling of industrial scale
reactors. Powder Technol. 2013, 245, 70–79.
24. Wang, L.; Wang, Y.; Ma, F.; Tankpa, V.; Bai, S.; Guo, X.; Wang, X. Mechanisms and reutilization of modified biochar used for
removal of heavy metals from wastewater: A review. Sci. Total Environ. 2019, 668, 1298–1309.
25. Wang, L.; Zhang, F.; Li, Z.; Liao, J.; Huang, Y.; Lei, Y.; Li, N. Mixed-charge poly(2,6-dimethyl-phenylene oxide)anion exchange
membrane for diffusion dialysis in acid recovery. J. Membr. Sci. 2018, 549, 543–549.
26. Yue, X.; Wu, W.; Chen, G.; Yang, C.; Liao, S.; Li, X. Influence of 2,2′,6,6′-tetramethyl biphenol-based anion-exchange membranes
on the diffusion dialysis of hydrochloride acid. J. Appl. Polym. Sci. 2017, 134, 45333.
27. Chavan, V.; Agarwal, C.; Adya, V.C.; Pandey, A.K. Hybrid organic-inorganic anion-exchange pore-filled membranes for the
recovery of nitric acid from highly acidic aqueous waste streams. Water Res. 2018, 133, 87–98.
28. Carrillo-Abad, J.; García-Gabaldón, M.; Pérez-Herranz, V. Treatment of spent pickling baths coming from hot dip galvanizing
by means of an electrochemical membrane reactor. Desalination 2014, 343, 38–47.
29. Azizitorghabeh, A.; Rashchi, F.; Babakhani, A.; Noori, M. Synergistic extraction and separation of Fe(III) and Zn(II) using TBP
and D2EHPA. Sep. Sci. Technol. 2016, 52, 476–486.
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