1. Introduction
Recently, steel has been the widest applied material among diverse metals due to the rapid development of technology [
1]. Nevertheless, the quality of stainless steel in appearance and corrosion resistance are more exceptional, which makes it pervasive in modern society. According to the International Stainless Steel Forum (ISSF) report, stainless steel consumption was primarily used for metal products and mechanical engineering in 2021. Furthermore, ISSF also forecast that the global demand for stainless steel will increase in the future, so the treatment of the wastes such as slag [
2] and sludge [
3,
4] is a critical issue. Stainless steel sludge was generated during the operation of large-scale machinery for the manufacturing industries. The solidified landfill is the most common waste treatment method. However, the leakage of the heavy metals not only results in environmental disruption but also harm to animals [
5,
6]. Especially for Cr(III), the conversion of Cr(III) to Cr(VI) is fatal to human beings as Cr(VI) causes health effects on the respiratory system, immune system, liver, and kidney [
7,
8]. Consequently, hydrometallurgy techniques have been developed to deal with the waste, such as solvent extraction, ion exchange, chemical precipitation [
9,
10,
11], and electrochemistry [
12], making the procedure more environmentally friendly.
In order to recover the valuable metals from stainless steel sludge, the hydrometallurgical method was applied in this study due to its high efficiency, low energy consumption, and easy implementation. The operations including acid leaching, solvent extraction, and chemical precipitation were carried out to separate the valuable metals. In hydrometallurgical procedures, employing an inorganic acid as a lixiviating agent such as HCl [
13],
[
14], or
[
15] is the most common method. According to those given in the literature, HCl,
, and
can efficiently dissolve the metals out of stainless steel. Hence, this research focused on investigating the leaching ability of these acids and choosing a suitable lixiviating agent.
Because the chemicals perform different extraction behaviors under various conditions, several extractants, resins [
16,
17], and precipitating agents were applied to dispose of the wastes in the separation procedure, such as Cyanex 272/Cyanex 301/Cyanex302 [
18,
19,
20,
21,
22], LIX984N-C [
23], LIX 54 [
24], TEA [
25], and D2EHPA [
26,
27,
28,
29]. Sole et al. [
30] used Cyanex 272, Cyanex 301, and Cyanex 302 to investigate the extraction efficiency of the metal ions under diverse pH conditions. Nonetheless, based on the literature, the pH values of the best extraction efficiency for Ni(II) and Cr(III) are beyond 2.0, which leads to the precipitation of Fe(III) and co-precipitation. Therefore, it is necessary that Fe(III) should be removed at first before separating nickel and chromium. Hu et al. [
31] used D2EHPA to extract Fe(III) from the leaching solution, and the results indicated that D2EHPA has an excellent extraction efficiency and selectivity of Fe(III) over other metal ions. In addition, according to the Pourbaix diagram [
32], Ni(II) can be separated from Cr(III) by adjusting oxidation-reduction potential (ORP) and pH value. Although it is a simple way to cope with the sludge, the generation of Cr(VI) still needs to be considered. The conventional method for Cr(VI) removal is to reduce Cr(III) at pH 2.0 and precipitation of
with lime at pH 9–10 [
33]. Dettmer et al. [
34] used sodium sulfite to reduce hexavalent chromium of the leather wastes at pH 2.0 and produced the basic sulfate chromium which had similar basicity properties compared with the commercial product.
Apart from hydrometallurgical methods, pyrometallurgy treatment was also employed to address the stainless steel sludge and dust. Liu et al. [
35] recovered iron, chromium, and nickel from stainless steel dust and reached the goals of high metal recoveries through direct reduction and self-pulverization separation. The recoveries of the three metals were respectively 92.50%, 92.02%, and 93.74%. Tang et al. [
4] utilized a coal-based smelting reduction process to deal with pickling sludge. The metals in the sludge were recovered in the form of Fe-Cr-Ni-C alloys. Although pyrometallurgy treatment was simple and straightforward, the procedure at high temperature (>1000 °C) would result in high energy consumption. Therefore, hydrometallurgy was adopted as the primary technique in this wok to attain high metal recoveries and purities, which makes the metal products have various applications.
In this study, the oil and water content of stainless steel sludge has been removed by pre-treatment procedures. Afterwards, HCl, , and were employed to leach the remaining solids, and the leaching percentages were also examined. Solvent extraction of Fe(III) was conducted through D2EHPA as an extractant. Moreover, to optimize the extraction efficiency, the influences of the pH value, extractant concentration, aqueous-organic ratio, and reaction time were also investigated. Lastly, chemical precipitation was applied to separate nickel and chromium according to the Eh-pH diagrams. To sum up, this research aimed to design an improved recovery system and combine the advantages of leaching, solvent extraction, and chemical precipitation mentioned above to achieve effective results.
2. Materials and Methods
2.1. Materials, Reagents, and Instruments
In this research, the sludge which had been the accumulation of stainless steel scraps with lubricant was collected from the local recycling center for the use of the experiment. In the pre-treatment procedure, oil and water were removed by a muffle furnace (LE 6/11, Naberthem, Li-lienthal, Germany), and the residue would be ground by a ball mill at 260 rpm for 24 h and sieved with a 100-mesh screen to increase the leaching percentages of iron, nickel, and chromium. The sulfuric acid (, 98%, Sigma-Aldrich, St. Louis, MI, USA), hydrochloric acid (HCl, 36.5%, Sigma-Aldrich, USA), and nitric acid (, 65%, Sigma-Aldrich, USA) were used as leaching agents and diluted in deionized water. Sodium hydroxide (NaOH, 97%, SHOWA, Gyoda, Japan) and hydrochloric acid were employed to adjust the pH value. Bis(2-ethlhexyl) phosphate (D2EHPA, 95%, Alfa Aesar, Haverhill, MA, USA) diluted into kerosene was used as an extractant to separate Fe(III) from the leach liquor. Hydrogen peroxide (, 36.5%, Sigma-Aldrich, USA) and sodium hydroxide were applied to adjust the oxidation-reduction potential (ORP) and the pH value in chemical precipitation according to the Pourbaix diagram. Sodium sulfite (, 100%, Honeywell, Charlotte, NC, USA) was utilized as a reductant of chromate ions. Chemical reagents used in the experiment were all analytical grades.
2.2. Pre-Treatment
The stainless steel sludge is mainly composed of oil, water, and ash. The proportions analysis by weight of the components were observed and determined by Differential Thermal Analysis/Thermogravimetry Analysis (DTA/TG, NETZSCH-409PC, Netzsh, Selb, Germany). After the sludge was calcined by a muffle furnace, the remaining solids were ground into powder and sieved with a 100-mesh screen. The crystal structure analysis was analyzed by X-ray Diffraction Meter (XRD, Dandong DX-2700, Dandong Kemait Ndt Co., Ltd., Dandong, China). The metal concentrations for separation and leaching efficiencies were analyzed by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Varian, Vista-MPX, Palo Alto, CA, USA).
2.3. Acid Leaching
Leaching procedures were conducted applying standard laboratory leaching equipment. The powder of the residue was dissolved in 4 mol/L HCl,
, and
to investigate the leaching efficiency and select the lixiviating agent. To achieve a better leaching percentage, the effect of calcination temperature was also studied and tested at 300 °C, 600 °C and 900 °C for 8 h. The leaching efficiency was calculated according to Equation (1):
where
is the leaching efficiency,
is the measured quantity of metal leached, and
is the quantity of metal in remaining solids.
2.4. Solvent Extraction
In this study, D2EHPA was used to efficiently extract Fe(III) from the leach liquor. The extractant was first diluted into kerosene and was then thoroughly mixed with the leaching solution for extraction. The extraction mechanism of D2EHPA can be written as Equation (2) [
26]:
The distribution ratio, D, is the concentration ratio of the metal in the organic phase to that in the aqueous phase at equilibrium. Hence, the distribution ratio can be written as Equation (3):
where
and
are the metal concentrations in the organic and aqueous phases.
is the initial concentration of metal ions in the aqueous phase and
is the equilibrium concentration of metal ions in the aqueous phase.
From the distribution ratio, D, the extraction percentage, %E can be calculated by Equation (4):
where D is the distribution ratio.
and
are the volumes of the aqueous phase and organic phase.
The separation factor, β, defines the selectivity for target metal (
) over another metal (
). From the distribution ratios of two metals, the separation factor, β can be calculated by Equation (5):
where
and
are the distribution ratio of target metal and the others.
and
are the metal concentrations in the organic phase after solvent extraction.
and
are the metal concentrations in the aqueous phase after solvent extraction.
2.5. Chemical Precipitation
According to the Pourbaix diagram of nickel and chromium, NaOH and
were applied as the reagent to separate these two metals in the chemical precipitation process. The following chemical equations illustrate the separation and precipitation of Ni(II) and Cr(III) [
34]:
The precipitation percentage is calculated by Equation (10):
where P is the precipitation percentage,
is the metal concentration of the leach liquor, and
is the metal concentration of the leach liquor after precipitation.
and V were the volumes of the solution before and after chemical precipitation, respectively. The whole procedure of the experiment is shown in
Figure 1.
4. Conclusions
This research investigated the purpose of recycling iron, nickel, and chromium from stainless steel sludge through hydrometallurgical methods, namely acid leaching, solvent extraction, and chemical precipitation. The leaching efficiencies of iron, nickel, and chromium were respectively 97.6%, 98.1%, and 95.7% by applying 4 mol/L HCl to lixiviate the sludge calcined at 300 °C. Furthermore, this study was dedicated to maximizing the efficiency of recovery and separation through solvent extraction. The results indicated that 0.1 mol/L D2EHPA could efficiently extract 80% of Fe(III) at pH 1.5 with an A/O ratio of 1 and contacting time of 10 min. The separation factors for Fe/Ni and Fe/Cr were 1616.22 and 906.06, respectively. To reach a higher extraction efficiency, this study carried out a two-stage extraction to achieve over 99% of the extraction percentage. Finally, the separation of nickel and chromium was conducted according to the Pourbaix diagram. The recovery rates were 99.5% and 75%, respectively. In addition, sodium sulfite was used to reduce , and then the Cr(III) precipitated as at pH 10. In this way, the metal products recovered from the stainless sludge can be reused in the industries to decrease the waste and reach the goal of resource recycling. However, there are still some improvements needed in this recovery system. For instance, the oil in the sludge could also be recycled to reduce the emission of carbon dioxide during calcination, and the parameters of leaching and stripping should be investigated further to increase the application potential.