Electrolytic manganese residue disposal based on basic burning raw material: Heavy metals solidification/stabilization and long-term stability

Characterization, pollution, and beneficial utilization assessment of dredged sediments from coastal ports in China

China, the world's largest dredging nation with over 2 billion cubic meters dredged annually, urgently needs tailored strategies for reusing those dredged sediments based on their physicochemical traits and pollution levels. The properties (grain size, pH, density, and conductivity), nutrient contents (organic matter, N, P, K), and heavy metal risks (Cu, Pb, Zn, Cr, Cd, Ni, Hg, As) of dredged sediments were investigated across 14 ports in China. Notably, 80 % of dredged sediments in China coastal ports are composed of silt. Significant variations in organic matter, nutrients, and heavy metals concentrations were observed. Environmental indices (m-ERM-Q < 0.5, Igeo < 1 and PERI <150) indicate sediment-associated nutrients & heavy metals pose low environmental risks, except for Cd and Hg, which are potential pollution concerns. This research offers insights into tailored strategies for managing dredged sediments and proposes resource utilization approaches, including wetland restoration, for practical application in China.

Anti-seepage reinforcement property and pollution control effect of bio-cemented fracture zone in electrolytic manganese residue dump

The heavy metal (HM) pollutants in electrolytic manganese residue (EMR) can easily diffuse through the seepage channel of the dump under the leaching action of rainfall. Particularly, the fracture zone, as one of the widely distributed seepage channels in the manganese residue dump (MRD), poses a greater threat to the ecological environment due to its weak mechanical properties, strong ductility, and numerous fractures. In this study, the microbially induced calcium carbonate precipitation (MICP) method was used for anti-seepage reinforcement of the fracture zone in an MRD to be constructed. The optimal conditions for the anti-seepage reinforcement of the fracture zone using the MICP method were proposed, and the control effect of the bio-cemented fracture zone on the major HM pollutant in the leachate of EMR was illustrated. Results showed that after grouting at a rate of 3 mL/min for 12 cycles with a grouting slurry of high urease activity (9 mM urea/min) and high cementation solution concentration (1.5 mol/L), the permeability coefficient of the fracture zone decreased from 10-4 to 10-5, and the unconfined compressive strength was approximately 5.58 times that of uncemented. Furthermore, the cubic calcite crystal clusters with high purity, good stability, dense arrangement, and high cementation properties were generated using the optimal conditions, and the pores were transformed from long columnar to spherical pores. Additionally, the Mn2+ concentration in the effluent from the fracture zone at the stabilization stage was only 663 mg/L, significantly lower than that of the leachate of EMR, and the risk of leaching changed from very high risk to low risk. The research outcomes can provide guidance and reference for laboratory model testing and in-situ testing of bio-cemented fracture zone, and it is expected to provide the theoretical and technological support for green and economic anti-seepage reinforcement of the fracture zone.

Highly efficient CO2 mineralization: Mechanisms and feasibility of utilizing electrolytic manganese residue as a feedstock and implementing ammonia recycling

Electrolytic manganese residue (EMR) and CO2 emissions from the electrolytic manganese metal (EMM) production process present significant challenges to achieving cleaner production within the industry. Given the high capacity for CO2 sequestration and the stability of the sequestered forms, CO2 mineralization methods utilizing minerals or industrial residues have garnered considerable research interest. The efficacy of such methods is fundamentally dependent on the properties of the materials employed. EMR, due to its calcium sulfate dihydrate (CaSO4·2H2O) content, possesses an intrinsic potential for CO2 solidification. In this study, we propose a novel method for CO2 mineralization utilizing EMR, coupled with NH3·H2O recycling. Experimental results indicated that under conditions of a reaction temperature of 55 °C and a pH of approximately 8, each ton of EMR can sequester 0.16 t of CO2, with equilibrium achieved within 10 min. The mineralization mechanism was elucidated using SEM, TG curves, and XRD analyses, which revealed that Ca2+ ions are initially leached from CaSO4·2H2O in the EMR, subsequently precipitating with CO32- ions to form CaCO3. This CaCO3 layer effectively covers the surface of CaSO4·2H2O, inhibiting further Ca2+ release and stabilizing the reaction equilibrium. Furthermore, the ammonia in the solution is regenerated into NH3·H2O, facilitating its reuse and preventing secondary pollution. The utilization of EMR for CO2 mineralization not only mitigates carbon emissions in the EMM production process but also promotes environmentally sustainable practices in the industry. This study highlights a promising pathway towards achieving carbon neutrality and cleaner production in electrolytic manganese production.

Iron separation from iron-rich manganese ore leachate: Comprehensive optimization of operating parameters and economic viability

In the current electrolytic manganese industry, iron separation and reuse from iron-rich manganese ore leachate (IRMOL) has become one of the most pressing challenges. This study aimed to investigate the optimal conditions for iron separation from IRMOL and to assess the economic and practical advantages of iron separation or removal in industrial manufacturing. To identify more cost-effective and technologically advanced production circumstances, we examined five key elements that weaken Fe(OH)3 colloidal production conditions in enterprises: reaction temperature, pH, crystal species, aging and reaction time. The screening results showed that when the conditions were optimized, the efficiency of reducing manganese loss decreased from 6.15% to 4.69%. Additionally, the generation of iron-rich electrolytic manganese residue (IREMR) was decreased by 44.32%, and the filtration velocity of IREMR increased from 0.0030 to 0.0220 mL/(s·cm2) compared to the production conditions before optimization at the enterprises. Through multiphase equilibria modeling with Visual MINTEQ, we have determined that raising the temperature and pH levels increases the expenses associated with chemicals and energy usage and results in an elevation of Fe(OH)2+ concentration. This can lead to the creation of Fe(OH)3 colloidal, causing a high water content in IREMR, inefficient filtration, and significant loss of manganese. This strategy is highly significant for the production of electrolytic manganese and the reduction of electrolytic manganese residue.

Environmental performance of unfired bricks produced from co-disposal of mine tailings and municipal solid waste incineration fly ash based on comprehensive leaching tests

The potential leaching of heavy metals is a crucial concern for construction materials produced from solidification/stabilization (S/S) treatment of wastes. This study comprehensively evaluated the leaching characteristics of heavy metals from the unfired bricks produced from co-disposal of Pb-Zn mine tailings and municipal solid waste incineration fly ash using batch, sequential, and semi-dynamic leaching tests. The results show that S/S treatment drastically reduced the leachability of heavy metals from the unfired bricks through lowering their distribution in the acid-soluble fraction. The effective diffusion coefficients of heavy metals within unfired bricks were all below 1.55 × 10-13 cm2/s, which is indicative of low mobility in the environment. The release of heavy metals from the unfired bricks was primarily governed by diffusion and dissolution. Slaking treatment of fly ash significantly reduced the leaching of heavy metals from the unfired bricks due to their improved structural integrity and compactness, which minimizes the surface area in the solid matrix accessible by the leaching medium. The leachability indices of heavy metals within the unfired bricks ranged from 13.12 to 18.10, suggesting that they are suitable for "controlled utilization" in specific scenarios. Compared to untreated mine tailings, converting them into unfired bricks could reduce the releases of heavy metals by several to hundreds of folds. These findings demonstrate that S/S can be an effective and sustainable strategy for co-disposal of mining tailings and incineration fly ash to produce construction materials with sound long-term environmental performance.

Study of microscopic properties and heavy metal solidification mechanism of electrolytic manganese residue-based cementitious materials

Electrolytic manganese residue (EMR) is a solid waste that contains a significant amount of soluble manganese and ammonia nitrogen, which can pose risks to human health if improperly disposed of. This study aimed to prepare cementitious materials containing abundant ettringite crystals by mixing EMR with various proportions of granulated blast furnace slag (GBFS) and alkaline activators (CaO, Ca(OH)2, clinker, NaOH). The resulting cementitious material not only utilized a substantial amount of EMR but also exhibited comparable strength to ordinary Portland cement. The optimal ratios were determined through mechanical testing. Additionally, the leaching toxicity of cementitious materials was assessed using ICP-MS (inductively coupled plasma mass spectrometer) tests. The microscopic properties, hydration, and mechanism of heavy metal solidification in the cementitious materials were evaluated using XRD (X-ray diffraction), SEM (scanning electron microscope), EDS (energy-dispersive spectrometer), FTIR (Fourier transform infrared spectroscopy), and TG (thermogravimetric) techniques. The results showed that the optimal ratio for the cementitious materials was 60% EMR, 36% GBFS, and 4% Ca(OH)2. The hardened mortar exhibited compressive strengths of 34.43 MPa, 41.3 MPa, and 50.89 MPa at 3 days, 7 days, and 28 days, respectively, with an EMR utilization rate of 60%. The hydration products of EMR-based cementitious materials were C-(A)-S-H, AFt, and ferromanganese compounds, which contribute to the mechanical strength. The Mn2+ and NH4+-N contents of raw EMR were 1220 and 149 mg/L, respectively. Nonetheless, the leaching of Mn2+ and NH4+-N in the alkali-EMR-GBFS system was significantly below the limits set by the Chinese emission standard GB8978-1996. Within this system, C-(A)-S-H and AFt could physically adsorb and displace heavy metals, Ca6Mn2(SO4)2(SO3)2(OH)12·24H2O could replace Al ions with Mn ions, and ferromanganese compounds Fe2Mn(PO4)2(OH)2·(H2O)8 and MnFe2O4 could chemically precipitate Mn2+.

Solidification/stabilisation behaviours of Zn2+ in magnesium potassium phosphate cement: Experiments and density functional theory study

The solidification/stabilisation behaviours of Zn2+ in magnesium potassium phosphate cement (MKPC) have not been thoroughly investigated. Herein, a series of experiments and a detailed density functional theory (DFT) study were conducted to investigate the solidification/stabilisation behaviours of Zn2+ in MKPC. The results showed that the compressive strength of MKPC reduced with the addition of Zn2+ because the formation of MgKPO4·6H2O (the main hydration product in MKPC) was delayed with the addition of Zn2+, as discovered by the crystal characteristics, and because Zn2+ exhibited a lower binding energy in MgKPO4·6H2O compared to Mg2+, as revealed by DFT results. Additonally, Zn2+ had little influence on the structure of MgKPO4·6H2O, and Zn2+ existed in MKPC as the formation of Zn2(OH)PO4, which was decomposed in the range of around 190-350 °C. Moreover, there were a lot of well-crystallised tabular hydration products before the addition of Zn2+, but the matrix was comprised of irregular prism crystals after adding Zn2+. Furthermore, the leaching toxicity of Zn2+ of MKPC was much smaller than the requirements of Chinese and European standards.

Study on phase evolution and promoting the pozzolanic activity of electrolytic manganese residue during calcination

Electrolytic manganese residue (EMR) is a harmful by-product in the electrolytic manganese industry. Calcination is an efficient method for disposing EMR. In this study, thermogravimetric-mass spectrometry (TG-MS) combined with X-ray diffraction (XRD) was used for analysing the thermal reactions and phase transitions during calcination. The pozzolanic activity of calcined EMR was determined by the potential hydraulicity test and strength activity index (SAI) test. The leaching characteristics of Mn were determined by TCLP test and BCR SE method. The results showed that MnSO₄ was converted into stable MnO₂ during calcination. Meanwhile, Mn-rich bustamite (Ca_0.228Mn_0.772SiO₃) was converted into Ca(Mn, Ca)Si₂O₆. The gypsum was transformed into anhydrite and then decomposed into CaO and SO₂. Additionally, the organic pollutants and ammonia were completely removed following calcination at 700 °C. The leaching concentration of Mn decreased from 819.9 mg L^-1 to 339.6 mg L^-1 following calcination at 1100 °C. The chemical forms of Mn were transformed from acid-soluble fraction to residual fraction. The pozzolanic activity tests indicated that EMR1100-Gy maintained a complete shape. The compressive strength of EMR1100-PO reached 33.83 MPa. Finally, the leaching concentrations of heavy metals met the standard limits. This study provides a better understanding for the treatment and utilization of EMR.

Elaboration of geopolymer package derived from uncalcined phosphate sludge and its solidification performance on nuclear grade resins loaded with 134Cs

Nuclear-grade Spent Organic Resin (SOR) contains high concentrations of radioactive nuclides and metal contaminants, while phosphate sludge contains high amount of fine clayey particles and CO₃^2-, both posing a major threat to the biosphere. In this study, a novel geopolymer package (GP) was proposed to directly solidify SOR loaded with ¹³⁴Cs by incorporating uncalcined phosphate sludge (UPS) as feedstocks, activated by NaOH/KOH. The results showed that alkali-mixed reagents-activated GP is more advantageous in terms of chemical stability and mechanical properties than NaOH-activated GP, recording compressive strength values greater than the waste acceptance criteria and OPC. The 28-day compressive strength of solidified packages can exceed 31 MPa at the highest amount of 42 wt% UPS. The addition of NaF powder into the solidified packages generates more hybrid type gels, which are more conducive to partial dissolution and bonding UPS particles, thereby producing stable and stronger GP. Leaching results of solidified GP in presence of up to 13 wt% SORs showed that only 0.15 % of total ¹³⁴Cs was leached, even under aggressive solutions. Solidification mechanism revealed that activation of UPS-MK blend forms N,K-A-S-H, (N,K,C)-A-S-H/C-S-H gels coexisting with unreacted particles, thereby solidify/stabilize metal contaminants and Cs⁺ by a synergetic immobilization action of hydration products via substitution and encapsulation. This study provides a promising paradigm for effective solidification of nuclear-grade resins and synergetic harmless treatment of industrial/phosphate mine solid wastes.

Long-Term Stabilization/Solidification of Arsenic-Contaminated Sludge by a Blast Furnace Slag-Based Cementitious Material: Functions of CaO and NaCl

Arsenic is a kind of element widely distributed in the environment that may pose a threat to the ecological environment and human health, while effective remediation and sustainable utilization of arsenic-containing sludge is a challenge. Based on stabilization/solidification blast furnace slag-based cementitious materials (BCMs), this study innovatively proposes to improve the arsenic (As) solidification efficiency and long-term stability by using the activation mode of CaO and NaCl. The effects of different factors on the properties of the BCM were measured by unconfined compressive strength (UCS) tests, X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. The long-term stability and safety of the BCM were verified by leaching toxicity and improved three stage continuous extraction method (BCR) tests. Experimental results show that the addition of CaO provides conditions for the formation of ettringite (AFt), thus promoting the crystal growth of AFt. The addition of NaCl can promote the formation of Cl-AFt and play a good long-term stabilizing role. When the content of the alkali activator is 10% and the modulus is 1.0, the contents of CaO and NaCl are 10 and 1%, respectively. The BCM has the best efficiency in terms of UCS and As solidification. The UCS at 28 days was 5.4 MPa, and the leaching concentration of As was 0.309 mg/L, and the As solidification efficiency was up to 99.9%. In the improved BCR test, the proportions of residual and oxidizable states of arsenic increased by 19.6 and 13.5%, respectively, and the stability of heavy metals improved. These findings show that the BCM has good long-term stability and safety. Overall, this study shows that CaO and NaCl significantly increase the output of AFt and achieve the purpose of efficient and stable solidification of As by the BCM.