Near-complete recycling of real mix electroplating sludge as valuable metals via Fe/Cr co-crystallization and stepwise extraction route.

In electroplating sludge, iron (Fe) and aluminum (Al) are common impurities that need to be separated before recycling valuable heavy metals. However, the traditional Fe/Al separation process often leads to significant losses of heavy metals. To address this issue, a new approach was developed to sequentially separate Fe/Al and recycle chromium (Cr) and nickel (Ni) from real electroplating sludge. The sludge contained 4.5% Cr, 1.2% Al, 1.1% Ni, and 14.6% Fe. Initially, the sludge was completely dissolved in a mixture of hydrochloric and nitric acids. The resulting acid solution was then heated to 160 °C for 10 h with the addition of saccharose. This hydrothermal treatment led to the hydrolysis and crystallization of 98.3% of Fe, 31.8% of Cr, 1.1% of Al, and 4.9% of Ni, forming akaganeite-bearing particles. It was observed that the excessive amount of saccharose also improved the removal of Cr, Al, and Ni, but decreased the removal of Fe. After the hydrothermal treatment, the remaining supernatant was adjusted to different pH levels (1.9, 2.9, and 4.5, respectively), and then Al, Cr, and Ni were stepwise extracted using di-(2-ethylhexyl) phosphate acid (P204). The recycling efficiencies achieved were 97.4% for Al, 61.2% for Cr, and 89.3% for Ni. This approach provides a promising method for the stepwise separation of Fe/Al and the recycling of heavy metals from electroplating sludge.

Speciation of toxic metals in metal finishing filter cake by X-ray absorption spectroscopy.

The speciation of Cr, Zn, Cu and Pb in two metal finishing filter cakes (TX and ST) was investigated by X-ray absorption spectroscopy (XAS) complemented by X-ray fluorescence (XRF) and X-ray diffraction (XRD). XRF showed that concentrations of Cr, Zn, Cu and Pb were 1.4%, 0.19%, 0.20% and 0.01%, respectively, in TX, and 12.6%, 3.3%, 1.3% and 0.21% in ST. No crystalline phases were detected in TX by XRD whereas ST was dominated by calcite. Cr and Fe K edge XAS showed Cr to be trivalent and octahedrally coordinated, co-precipitated with Fe as CrxFe1-x-(oxy)hydroxides in both filter cakes. Zn, P and Ca K edge XAS showed that 2ZnCO3∙3Zn(OH)2 and Zn3(PO4)2 were the dominant zinc-containing phases, with combined tetrahedral and octahedral coordination; Zn phases were slightly more crystalline in TX than ST. Pb L3 edge X-ray absorption near edge spectroscopy (XANES) found that Pb was likely adsorbed on amorphous SiO2. Cu, Si and S K edge XAS showed that all Cu was divalent, and the dominant copper phases were found to be Cu2Cl(OH)3, Cu(OH)2 and CuSO4·5H2O for ST, whereas Cu appeared to adsorb to amorphous SiO2 for TX, which contained much less Pb. Cr is thus immobilized in the filter cakes in a phase with low solubility at environmentally feasible pH values, whereas Zn, Cu and Pb could be released when the pH decreases below 8 or above 11. These findings are significant for the development of waste management regulations and/or metal recovery methods (e.g., hydro/pyrometallurgy).

Stepwise recycling of Fe, Cu, Zn and Ni from real electroplating sludge via coupled acidic leaching and hydrothermal and extraction routes.

Fe/S-bearing erdite flocculant has been proven to be effective in the precipitation of heavy metals from real electroplating wastewater, with the only drawback being the huge production of sludge. This sludge was rich in Fe/S/Zn/Cu/Ni and refractory to be recycled due to the extractant pollution by free Fe and the dissolution of sulphide. Herein, a multistep separation method was developed to dissolve sulphide and separate Fe prior to Zn/Cu/Ni. Results showed that more than 92% sludge was dissolved as Fe/Zn/Cu/Ni-rich leachate after the sludge was leached by nitric acid, with the rest of the remaining undissolved elemental sulphurs. When the leachate was directly extracted by using commercially extractant Acorga M5640 and Di-(2-ethylhexyl) phosphoric acid (P204), Fe was complexed by the phosphate group of the extractant. The Fe was effectively removed prior to Zn/Cu/Ni to avoid the extractant pollution. The Fe removal efficiency was only 38.34% without sucrose, but it rose to 99.94% with the addition of 0.5 g sucrose. The added sucrose reacted with nitrate to consume H+, which showed a similar rate to the H+ release from Fe hydrolysis. Thereafter, the Fe hydrolysis was continued to remove, the Fe at a high level. The removed Fe was in the form of high-purified hematite nanorod with a diameter and length of 300-600 nm and 0.5-2.5 µm, respectively. After Fe removal, Cu/Zn/Ni was extracted by using Acorga M5640 and P204 to form three halite, including a mixture of copper sulphate hydrate and bonattite (96.8% CuSO4·H2O/CuSO4·3H2O), gunningite (97.5% ZnSO4·H2O) and dwornikite (97.9% NiSO4·H2O). The rest of the solution was neutralised by lime water to remove sulphate as gypsum (95.9% CaSO4) to meet the discharge standard of the electroplating industry. In summary, the recycling efficiency of Fe/Cu/Zn/Ni from the sludge reached 94.4%, 92.6%, 94.7% and 95.3%, which provided an alternative strategy to resource utilise Fe/S-bearing solid waste.

Recovery of Ni matte from Ni-bearing electroplating sludge.

In this study, a novel process for the recovery of Ni from Ni-bearing electroplating sludge (ES) is proposed, which involves the carbothermic reduction stage and smelting stage. In the reduction stage, the CaSO4, Fe2O3, and NiO in the ES were reduced by carbon at 1000 °C, and the Ni3S2 and Fe4Ni5S8(Ni-rich phases) were generated. After that, the reduced ES was mixed with SiO2 and smelted at 1500 °C. During the smelting stage, Ni3S2 and Fe4Ni5S8 were melted to form liquid Ni-Fe-S matte and separated from the molten slag by gravity. Finally, 58.5%Ni-13.8%Fe-27.7%S (in weight) matte and vitrified slag were obtained. The recovery ratio of Ni (97.2%) was much higher than that of Fe (14.7%). Besides, the Ni/Fe mass ratio of the ES was 0.7, while the ratio of the prepared matte was about 4.2. Therefore, the selective recovery of Ni was achieved. The obtained Ni matte can be used as the raw material for pure Ni or Ni-bearing chemicals.

Pyrometallurgy treatment of electroplating sludge, emulsion mud and coal ash: ZnAlFeO4 spinel separation and stabilization in calcium metasilicate glass.

Electroplating sludge was a hazardous waste comprised of heavy metals and other Fe/Al/Ca/Si impurities, and produced massively in surface treatment industry. In the past, it was commonly purified via hydrometallurgy, chlorination and reduction calcination routes, but also blended as additive in rotary kiln, to stabilize the heavy metals in geopolymer. Herein, an alternative strategy was developed to treat a real electroplating sludge for recycling magnetic Zn-rich spinel and stabilizing Zn in calcium metasilicate glass via a facile pyrometallurgy route with the blending of emulsion mud and coal ash. The sludge contained 35.6% Zn and 0.54% Cr and then was blended with 50% emulsion mud. After calcination at 1200 °C, the product was highly dispersed, whilst octahedral ZnAlFeO4 spinel with Zn content of 40.0% were formed and separated by using magnet, in accordance with the recycling efficiency of 51.2% Zn from the electroplating sludge. But after calcination at 1400 °C, the gypsum in emulsion mud was decomposed as CaO and accelerated the dissolution of Si-bearing substance as calcium metasilicate glass for covering ZnAlFeO4 spinel, resulting in the Zn leaching of 1568 mg/L. By adding 50% Si-rich coal ash in the calcination system, more calcium metasilicate glass were generated, and then the Zn concentration in the toxic leaching test was only 12.09 mg/L. During the calcination, Cr showed similar performance to Al/Fe and involved in the spinel formation. This provided a new route to recycle Zn from Zn-rich electroplating sludge and to solidify heavy metals via calcium metasilicate glass route.

Indirect bioleaching recovery of valuable metals from electroplating sludge and optimization of various parameters using response surface methodology (RSM).

Electroplating sludge contains amounts of valuable/toxic metals as a typical hazardous solid waste, but existing technology is hard to simultaneously gain the high recovery of valuable metals and its convert into general solid waste. In this study, indirect bioleaching process was optimized by using RSM for high recovery of four valuable metals (Ni, Cu, Zn and Cr) from electroplating sludge and its shift into general waste. The results showed that the maximum leaching rate respectively was 100% for Ni, 96.5% for Cu, 100% for Zn and 76.1% for Cr at the optimal conditions. In particular, bioleaching saw a much better performance than H2SO4 leaching in removal of highly toxic Cr (76.1% vs. 30.2%). The extraction efficiency of Cr by H2SO4 leaching sharply rose to 72.6% in the presence of 9.0 g/L Fe3+, suggesting that Fe3+ played an important role in the bioleaching of Cr. Based on bioleaching dynamics analysis, it was speculated that Fe3+ passes through the solid shell and enter inside the sludge to attack Cr assisting by extracellular polymeric substances (EPS), leading to high extraction and low residue of Cr. Meanwhile, due to high-efficient release and removal of valuable/toxic metals by bioleaching, the bioleached residues successfully degraded into general based on TCLP test and can be reused as construction material safely.

Stepwise extraction of Fe, Al, Ca, and Zn: A green route to recycle raw electroplating sludge.

Electroplating sludge is a hazardous waste produced in large quantities in the electroplating industry during production. It is rich in heavy metal resources and can be recovered as value-added heavy metal products. To recover Zn in electroplating sludge, Fe/Al/Ca impurities were effectively removed as hematite, boehmite, and calcium sulfate, respectively, via a facile hydrothermal method with reduction of nitric acid by addition of glucose. After the sludge was dissolved in nitric acid, the generated solution contained 6.1 g/L of Zn, 2.2 g/L of Fe, 2.5 g/L of Al, and 2.9 g/L of Ca. First, approximately 100% Fe was extracted as hematite nanoparticles containing 94.6 wt% Fe2O3 after the solution was treated at 190 °C for 6 h. Second, when the temperature was elevated to 270 °C, nearly 99% Al was isolated as boehmite particles containing 95.2 wt% Al2O3. Third, more than 98% Ca was removed as anhydrite, which contained 95.9 wt% CaSO4, by adding sulfuric acid. During the steps, the total loss of Zn was less than 3%, and 5.75 g/L of residual Zn was recovered as zincite containing 92.2 wt% ZnO by adjusting the pH to 8. The dissolved Fe, Al, and Ca impurities were successfully removed as purified hematite, boehmite, and anhydrite, respectively, through the stepwise separation method by adjusting reaction temperatures and pH. The high content of Zn in the electroplating sludge was finally purified as zincite.

Recycling electroplating sludge as a monolithic catalyst for effective catalytic purification of volatile organic compounds.

Electroplating sludge had a high content of heavy metals and usually lacked high-value-added utilization. In this work, Cu-containing sludge was used to synthesize a spinel catalyst, which was applied in catalytic oxidization of toluene. As a result, the sludge-derived spinel removed 50% of toluene (1000 ppm, 9600 h-1) at 280 °C. In comparison, a reagent-synthesized spinel with a similar component removed 50% of pollutant at 294 °C. The sludge-derived spinel also showed a stable performance for over 50 h at 370 °C. Even when the initial concentration was increased to 5000 ppm, or the gas hourly space velocity was increased to 40,000 h-1, the temperature for 50% removal was only increased to 303 °C. According to characterizations, surface oxygens of the sludge-derived spinel were more active than those in the reagent-synthesized one. Besides, the former had more active surface oxygens (207.9 µmol/g) than the latter (183.1 µmol/g). Furthermore, the sludge-derived spinel was coated on a monolithic honeycomb, which were also effective in catalytic oxidization of toluene. The main results of this work were in favor of high-value-added utilization of hazardous solid waste and promoting its real industry application.

Bioleaching of Copper-Containing Electroplating Sludge.

The recovery of precious metals from solid waste through bioleaching has become a research hotspot in recent years. Thus, in this study, different strategies, such as chemical sulfuric acid leaching and mixed consortium bioleaching, were adopted to extract copper from Copper-Containing Electroplating Sludge. The results showed that, compared to chemical leaching, bioleaching showed a much better performance. Indeed, copper bioleaching efficiency reached 94.3% on day 7 (21.1% higher than that of chemical leaching). The results also indicated that the process of bioleaching involved more mechanisms and reactions than that of chemical leaching. The SEM and EDX tests showed that the surface morphology of the sludge changed significantly after bioleaching, and that an insignificant amount of copper remained in the leached residues. Furthermore, the leached residues passed the characteristic leaching toxic test and thus can be considered as non-hazardous raw materials for the construction industry. Hence, adopting a mixed consortium leaching process to extract copper from Copper-Containing Electroplating Sludge will not only significantly reduce environmental pollution, but will also use metal resources more efficiently.

Synthesis of a perovskite-type catalyst from Cr electroplating sludge for effective catalytic oxidization of VOC.

Chromium-containing electroplating sludge usually lacked proper disposal and recycling. High-temperature melting was a technology aiming to form glass-phase slag for the stabilizations of heavy metals. This work investigated the possibility of forming perovskite-like phase by chromium-containing sludge using high-temperature melting. The formed material was applied in catalytic oxidization of volatile organic compound. As a result, Ca2+-doped LaCrO3 was formed according to XRD and HRTEM. When Ca2+ doping reached 33%, i.e., La0.67Ca0.33CrO3, surface oxygen species of the obtained catalyst was increased to 65.7%, which was detected by XPS, resulting in a toluene removal of 50% at 302 °C. Besides, the activity was stable for over 50 h. In addition, the doping amount was as high as 40 mol% of Cr in the catalyst. Based on these results, a high-value-added catalyst was produced by the hazardous waste, which was in favor of hazardous-waste recycling as well as high-temperature-melting development.

Upcycling of Electroplating Sludge into Ultrafine Sn@C Nanorods with Highly Stable Lithium Storage Performance.

Sn-based anode materials have become potential substitutes for commercial graphite anode due to their high specific capacity and good safety. In this paper, ultrafine Sn nanoparticles embedded in nitrogen and phosphorus codoped porous carbon nanorods (Sn@C) are obtained by carbonizing bacteria that adsorb the Sn electroplating sludge extracting solution. The as-prepared Sn@C rod-shaped composite exhibits superior electrochemical Li-storage performances, such as a reversible capacity of approximate 560 mAh/g at 1 A/g and an ultralong cycle life exceeding 1500 cycles, with approximately no capacity decay. The ultrastable structure of the Sn@C was revealed using in situ transmission electron microscope at the nanoscale and indicated that the Sn@C composite could restrict the volume expansion of Sn nanoparticles during the lithiation/delithiation cycles. This work provides a new insight into addressing the electroplating sludge and designing novel lithium ion battery anodes.

Heavy metal leaching and distribution in glass products from the co-melting treatment of electroplating sludge and MSWI fly ash.

Melting is a common solidification treatment that concentrates and encapsulates heavy metals into a glass matrix for waste containing heavy metals (Chae et al., 2016). To control the risk of heavy metal leaching into the glass product, a reduction in the amount of heavy metal was achieved in a pilot-scale furnace by co-melting electroplating sludge (EPS) and municipal solid waste incineration fly ash (MSWI FA). Through the melting process, the chloride from MSWI FA led to heavy metals volatilization in the form of chlorine salts. The fly ash additionally increased heavy metals volatilization by 4%-91%. The highest volatilization ratios of Zn, Pb, Cu, Cd, Cr and Ni were 33%, 96%, 33%, 79%, 81% and 31%, respectively. The concentrations of Pb and Zn in the secondary fly ash were close to the Pb and Zn concentrations in lead-zinc ore that are required in smelting industry. Moreover, glass sand was produced after the melting treatment. With an increase in the fly ash mixing ratio, the leaching concentration of Zn and Cu decreased to 3.8 mg/L and 2.1 mg/L. The leaching concentrations of other heavy metals stayed below 1 mg/L in all cases. When the ratio of MSWI FA reached 10%, the glass sand contained the least amount of impurities and a large amount of phosphate and silicate, which were probably responsible for the stability of the heavy metals. Therefore, our results provided a promising approach to the stability of the waste by the recovery of heavy metals in the co-treatment of heavy metal-bearing wastes.

Toxicity assessment of untreated/treated electroplating sludge using human and plant bioassay.

The purpose of this work was to assess the risk to the environment arising from the electroplating sludge from both chemical and toxicological point of view. Both approaches were used for the assessment of the treatment efficiency which consisted of CaO based solidification followed by thermal treatment at 400°C. The elemental composition was determined in the bulk samples and the leachates of untreated sludge. The toxicity of the leachate was determined using two human colorectal adenocarcinoma cell lines (Caco-2 and SW 480) and Hordeum vulgare L. based plant bioassay. The same toxicity tests were employed to the leachate of the treated sludge. Untreated sludge showed extremely high cytotoxic effect to both human and plant bio-system in dose-dependent manner. The percentages higher than 0.5% and 0.05% of the leachate caused significant cytotoxic effect on Caco-2 and SW 480 cells, respectively. The percentages of the leachate higher than 0.05% also showed significant toxic effect to H. vulgare L. bio-system with complete arrest of seed germination following the treatment with 100% to 5% of the leachate. The leachate of the treated sludge showed no toxicity to any of the test systems confirming the efficiency and justification of the employed procedures for the detoxification of electroplating sludge.

Electroplating sludge derived zinc-ferrite catalyst for the efficient photo-Fenton degradation of dye.

A zinc-dominant ferrite catalyst for efficient degradation of organic dye was prepared by the calcination of electroplating sludge (ES). Characterizations indicated that zinc ferrite (ZnFe2O4) coexisted with Fe2O3 structure was the predominant phase in the calcined electroplating sludge (CES). CES displayed a high decolorization ratio (88.3%) of methylene blue (MB) in the presence of H2O2 combined with UV irradiation. The high efficiency could be ascribed to the photocatalytic process induced by ZnFe2O4 and the photo-Fenton dye degradation by ferrous content, and a small amount of Al and Mg in the sludge might also contribute to the catalysis. Moreover, the degradation capability of dye by CES was supported by the synthetic ZnFe2O4 with different Zn to Fe molar ratio (n(Zn): n(Fe)), as 84.81%-86.83% of dye was removed with n(Zn): n(Fe) ranged from 1:0.5 to 1:3. All synthetic ferrite samples in the simulation achieved adjacent equilibrium decolorization ratio, the flexible proportioning of divalent metal ions (M2+) to trivalent metal ions (M3+) applied in the synthesis indicated that the catalyst has a high availability. Therefore, an efficacious catalyst for the degradation of dye can potentially be derived from heavy metal-containing ES, it's a novel approach for the reutilization of ES.

Effect of inorganic anions on hydrothermal removal of impurities Fe/Al from Cu-bearing polymetallic leachate.

Hydrometallurgy recycling of heavy metals from electroplating sludge is of hot spot in recent decades. Such recycling was tedious in the separation of impure Fe/Al prior to heavy metals from acid leachate after sludge dissolution. Herein, a facile hydrothermal route was developed to separate Fe/Al from Cu-bearing leachate. The results showed that when the leachate was directly hydrothermally treated at 160 °C in the presence of nitrate and ethanol, Al/Cu were stable in the leachate, but nearly 100% Fe was removed as hematite nanoparticles. With the addition of chloridion, the removal efficiencies of Fe/Al/Cu did not change apparently, but the corresponding precipitate was akageneite, not hematite. By replacing chloridion with sulfate, nearly 100% Fe and 98.6% Al were separated as natrojarosite/natroalunite block, while the Cu loss was only 1.7%. However, with the supplementary of phosphate, the Fe/Al removal achieved nearly 100%, but the Cu removal also achieved by 92.6%. The thermodynamic analysis showed that Cu was precipitated rapidly via the phosphate/Cu oxyhydroxide route by adding phosphate but removed slightly via the coordination route on the Fe/Al precipitates with the addition of nitrate, chloridion, and sulfate. In summary, Fe was effectively separated as hematite, akageneite, natrojarosite, and phosphate halite, in the presence of nitrate, chloridion, sulfate, and phosphate, separately. But the removal of Al as natroalunite and AlPO4 only started by adding sulfate and phosphate, respectively. Such results enabled a short hydrometallurgy process to effectively recycle heavy metals from electroplating sludge.

A metallurgical approach for separation and recovery of Cu, Cr, and Ni from electroplating sludge.

Electroplating sludge is extensively produced in chemical precipitation-based treatment of electroplating wastewater. It poses a huge threat to environmental safety if not properly disposed, ascribed to its high contents of heavy metals. An innovative metallurgical approach was proposed a to recycle Cu, Cr, and Ni from it. Ammonia leaching was firstly performed to selectively leach Cu from Cr, in which the Cu oxide and sulfide were leached into the leachate while the Cr oxide and Ni carbide (NiCx) retained in the residue. (NH4)2SO4 increased the Cu leaching rate via increasing the dissolved oxygen amount in the ammonia leachate and converting CuS to Cu2+. Under the optimal conditions, the leaching efficiency of Cu achieved 96.5 % while that of Cr was only 0.1 %. In the followed aluminothermic reduction, C in the leaching residue could be effectively removed via a thermal oxidation, which in turn decreased the formation of a C-containing compound of high melting point and benefited the Cr and Ni recovery. Cr and Ni from the residue were reduced and recovered in a Cr-Ni alloy, and the reductant of Al first changed to a refractory Al2O3 and then transformed to a low melting point 12CaO·7Al2O3 with the additive of CaO. This transformation increased the molten slag fluidity and promoted the separation of Cr-Ni alloy from slag. Moreover, the excessive Al increased the Cr and Ni yields and concentrated all of them to be together. Partial Al was used as reductant, and the other Al transferred into Cr-Ni alloy to decrease its melting point. Cr and Ni contents in the smelting slag could be decreased to 0.11 wt% and 0.12 wt% respectively, showing an efficient recovery. This work provided a high efficiency method to recover Cu, Cr, and Ni from waste electroplating sludge.

Catalytic reduction of nitrogen monoxide using iron-nickel oxygen carriers derived from electroplating sludge: Novel method for the collaborative emission decrease of polluting gases.

The valorization of electroplating sludge (ES) for high added value presents greater economic and environmental benefits than conventional treatment methods such as thermal processing, solidification, and landfill. Inspired by the mechanism of chemical looping combustion (CLC), this study developed a novel cost-effective method for denitrification by preparing FeNi-OCs from ES to achieve the synergistic reduction of CO and NO emissions. The phase structure, micromorphology, and valence state changes of the FeNi-OC catalyst during the CO-catalyzed reduction of NO and the pathway for catalytic denitrification using FeNi-OCs were analyzed. Results showed that CO could reduce FeNi-OCs to FeNi, and the reduced FeNi was subsequently oxidized back to FeNi-OCs by NO, a process analogous to CLC. During experiments, the simultaneous consumption of CO and NO gases was observed at 350 °C. This phenomenon was highly pronounced at 600 °C, where the CO and NO concentrations decreased from initial values of 8550 and 470 ppm, respectively, to 6719 and 0 ppm, respectively, with conversion rates of 21.41 % and 100 %, respectively. Hence, synergistic emission reduction was achieved. Further experiments also indicated that the addition of 1.5 % ES during iron ore sintering could substantially reduce the CO and NO concentrations in the sintering flue gas from 1268.32 and 244.81 ppm, respectively, to 974.51 and 161.11 ppm, respectively.

Electroplating sludge derived CuFe2O4/MgFe2O4 metal oxide composites for highly efficient removal of Congo red.

The utilization of electroplating sludge (ES) to derive metal oxide functional materials is a key strategy, as it enables the recycling of valuable elements, mitigates environmental risks, and aligns with green, low-carbon development strategies. Nevertheless, the development of metal oxide composite functional materials with distinctive structures and properties derived from ES continues to present several challenges. Herein, we synthesized CuFe2O4/MgFe2O4 metal oxide composites from ES by one-step hydrothermal method. As-obtained CuFe2O4/MgFe2O4 metal oxide composites (MMOs) have a unique layered structure, richer mesoporous and microporous structures, activity sites. When evaluated as an adsorbent for Congo red (CR), as-synthesized CuFe2O4/MgFe2O4 with layered structure composite exhibited excellent adsorption capacity (1039.1 mg/g) and reusability (85.55% after five cycles), which was superior to most similar adsorbents reported till date. Such improvement is explored to mainly originate from two respects: the physical adsorption facilitated by the abundant pores formed through the stacking and growth of CuFe2O4 and MgFe2O4, and the chemisorption resulting from surface complexation and hydrogen bonding between the MMOs and CR. This strategy to directly transform ES into functional materials shows great promise both in waste management and preparation of robust adsorbents for wastewater treatment.

A novel method for effective solidifying chromium and preparing crude stainless steel from multi-metallic electroplating sludge.

Electroplating sludge (ES) is a globally prevalent hazardous waste that primarily contains Cr, Cu, Ni, and Fe. However, the residual Cr phases within the slag potentially poses an environmental risk in current vitrification. A novel method for effective recovering and solidifying Cr in ES is proposed in this work. ES was desulfurized and subsequently co-treated with ferrosilicon (Fe-Si) and spent carbon anode (SCA) for enhancing the recovery of Cr, Cu, Ni, and Fe to prepare crude stainless steel. Under optimal conditions, the recovery ratios of Cr, Cu, Ni, and Fe reached 96.96%, 99.45%, 99.92%, and 99.20%, respectively, signifying improvements of 21.4%, 0.2%, 1.5%, and 2.8%, respectively, compared with existing research. Meanwhile, the fluoride in SCA yielded CaF2, further progressing to the Si-Ca-F-Na-Al-O phase, with a solidification ratio of 97.87%. The Cr leaching content of the residual Cr-Cu-S phase in the slag remained below 5 mg/L across a pH range of 2-4, demonstrating enhanced stability compared to prior alloy, oxide, and chemically dissolved phases. An innovative approach for solidify Cr by forming matte holds implications for the treatment of Cr-containing solid wastes such as chromium slag, tannery sludge and stainless steel slag.

Recovery of valuable metals from electroplating sludge with reducing additives via vitrification.

In this study, vitrification was applied to treat Ni-Cu electroplating sludge. The sludge was mixed with additives (limestone:cullet = 4:6) and then heated to 1450 °C. The cooled product could be separated into slag and ingot. An atomic absorption spectrometer was used to determine the metal levels of specimens and toxicity characteristic leaching procedure (TCLP) tests, whereas the crystalline and surface characteristics were examined using quantitative X-ray diffraction (XRD) analysis and scanning electron microscopy, respectively. With a glassy structure, the slag was mainly composed of Ca, Si, and Mg. The TCLP results of slags met the Taiwan regulated standards, suggesting that slag can be used for recycling purposes. With the aid of additives, the crystalline phase of slag was transformed form CaMgSiO4 into CsSiO3. The ingots were mainly composed of Ni (563,000-693,800 mg/kg), Cu (79,900-87,400 mg/kg), and Fe (35,000-43,600 mg/kg) (target metals) due the gravity separation during vitrification. At appropriate additives/sludge ratios (>0.2), >95% of target metals gathered in the ingot as a recoverable form (Ni-Fe alloy). The high Ni level of slag suggests that the ingot can be used as the raw materials for smelters or the additives for steel making. Therefore, the vitrification approach of this study is a promising technology to recover valuable metals from Ni-Cu electroplating sludge.