Production and resource utilization of flue gas desulfurized gypsum in China - A review

A critical review on the use of flue gas desulfurization gypsum to ameliorate saline-alkali soils and its prospect for reducing carbon emissions

Flue gas desulfurization gypsum (FGDG), a solid waste produced during sulfur removal in coal-fired power plants, has applications in saline-alkali soil amelioration due to its function of calcium‑sodium ion exchange. Existing research has focused on the use of gypsum to improve saline-alkali soils in non-coastal areas. However, coastal areas are not only extensively salinized, but an important source of methane, and surprisingly, FGDG may assist to decrease methane formation mainly by the action of sulfate radical. This is the first critical review to systematically discuss the effects of FGDG on both saline-alkali soil improvement and carbon emission control in tidal flats, including application status, amendment principles, environmental risks and methane emission control. After adding FGDG, soil salinization degree was weakened via adjusting soil structure, pH, exchangeable sodium percentage and electric conductivity, introduction of nutrients also promotes crop growth. The optimal FGDG dosage in tidal flats seems to be higher (>2 %) than that in non-coastal areas (<1 %). Its environmental risks regarding heavy metals and eutrophication are evaluated safe. In tidal areas, more methane is produced in hot seasons and ebb tides. Plants and invertebrates also promote methane release. FGDG controls methane production by promoting the activity of sulfate-reducing bacteria and inhibiting methanogens. Considering methane flux levels and seawater erosion, FGDG use in low tidal beach needs more research, while that in high and middle tidal beach is recommended. This review will expand applications and appropriate use of FGDG for reducing carbon emission and improving ecological services in coastal areas.

Effect of Mineral Admixtures on Physical, Mechanical, and Microstructural Properties of Flue Gas Desulfurization Gypsum-Based Self-Leveling Mortar

The production of flue gas desulfurization gypsum poses a serious threat to the environment. Thus, utilizing gypsum-based self-leveling mortar (GSLM) stands out as a promising and effective approach to address the issue. β-hemihydrate gypsum, cement, polycarboxylate superplasticizer, hydroxypropyl methyl cellulose ether (HPMC), retarder, and defoamer were used to prepare GSLM. The impact of mineral admixtures (steel slag (SS), silica fume (SF), and fly ash (FA)) on the physical, mechanical, and microstructural properties of GSLM was examined through hydration heat, X-ray diffractometry (XRD), Raman spectroscopy, and scanning electron microscopy (SEM) analyses. The GSLM benchmark mix ratio was determined as follows: 94% of desulfurization building gypsum, 6% of cement, 0.638% each of water reducer and retarder, 0.085% each of HPMC and defoamer (calculated additive ratio relative to gypsum), and 0.54 water-to-cement ratio. Although the initial fluidity decreased in the GSLM slurry with silica fume, there was minimal change in 30 min fluidity. Notably, at an SS content of 16%, the GSLM exhibited optimal flexural strength (6.6 MPa) and compressive strength (20.4 MPa). Hydration heat, XRD, and Raman analyses revealed that a small portion of SS actively participated in the hydration reaction, while the remaining SS served as a filler.

High value-added utilization of desulfurized building gypsum as self-leveling mortar: the comprehensive effect of cement

The utilization of desulfurized building gypsum as raw material for gypsum-based self-leveling mortar (GSL) is limited by its low strength and poor water resistance. The objective of this study is to enhance comprehensive properties of GSL and prepare qualified desulfurized building gypsum-based self-leveling mortar that can be effectively applied in practical engineering projects. The influence of cement on water consumption rate of initial fluidity (W/M ratio), fluidity, setting time, mechanical strength, and water resistance of GSL were evaluated. Additionally, rheological parameter, heat of hydration, crystal morphology, and pore structure were also analyzed. Cement significantly improved the fluidity of slurry. Moreover, the compressive strength and softening coefficient of GSL reached 20.6 MPa and 0.56 at 10% cement, respectively. Furthermore, cement reduced the 30-min-fluidity loss and improved fludity by reducing the yield stress and increasing the plastic viscosity of screed. The transformation of hydration kinetics of GSL could be due to Ca2+ and OH- released by cement, thus resulting in the shortening of initial setting time and the prolongation of the interval between initial and final setting time. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) showed that CSH gel and AFt crystal would generate on the surface of CaSO4·2H2O crystal, making the structure more compact. Mercury intrusion porosimetry (MIP) indicated that cement greatly reduced the porosity through the water reduction effect in the early stage and continuous hydration in the later stage. The continuous hydration of cement also increased the shrinkage rate. This work was expected to provide reference for promoting the application of desulfurized building gypsum as the high value-added screed.

Effects of the three amendments on NH3 volatilization, N2O emissions, and nitrification at four salinity levels: An indoor experiment

The marked salinity and alkaline pH of coastal saline soil profoundly impact the nitrogen conversion process, leading to a significantly reduced nitrogen utilization efficiency and substantial gaseous nitrogen loss. The application of soil amendments (e.g. biochar, manure, and gypsum) was proved to be effective for the remediation of saline soils. However, the effects of the three amendments on soil nitrogen transformation in soils with various salinity levels, especially on NH3 volatilization and N2O emission, remain elusive. Here, we reported the effects of biochar, manure, and gypsum on NH3 volatilization and N2O emission under four natural salinity gradients in the Yellow River Delta. Also, high-throughput sequencing and qPCR analysis were performed to characterize the response of nitrification (amoA) and denitrification (nirS, nirK, and nosZ) functional genes to the three amendments. The results showed that the three amendments had little effect on NH3 volatilization in low- and moderate-salinity soils, while biochar stimulated NH3 volatilization in high-salinity soils and reduced NH3 volatilization in severe-salinity soils. Spearman correlation analysis demonstrated that AOA was significantly and positively correlated with the NO3--N content (r = 0.137, P < 0.05) and N2O emissions (r = 0.174, P < 0.01), which indicated that AOA dominated N2O emissions from nitrification in saline soils. Structural equation modeling indicated that biochar, manure, and gypsum affected N2O emission by influencing soil pH, conductivity, mineral nitrogen content, and functional genes (AOA-amoA and nosZ). Two-way ANOVA further showed that salinity and amendments (biochar, manure, and gypsum) had significant effects on N2O emissions. In summary, this study provides valuable insights to better understand the effects of gaseous N changes in saline soils, thereby improving the accuracy and validity of future GHG emission predictions and modeling.

Selective CO2 reduction to CH3OH over atomic dual-metal sites embedded in a metal-organic framework with high-energy radiation

The efficient use of renewable X/γ-rays or accelerated electrons for chemical transformation of CO2 and water to fuels holds promise for a carbon-neutral economy; however, such processes are challenging to implement and require the assistance of catalysts capable of sensitizing secondary electron scattering and providing active metal sites to bind intermediates. Here we show atomic Cu-Ni dual-metal sites embedded in a metal-organic framework enable efficient and selective CH3OH production (~98%) over multiple irradiated cycles. The usage of practical electron-beam irradiation (200 keV; 40 kGy min-1) with a cost-effective hydroxyl radical scavenger promotes CH3OH production rate to 0.27 mmol g-1 min-1. Moreover, time-resolved experiments with calculations reveal the direct generation of CO2•‒ radical anions via aqueous electrons attachment occurred on nanosecond timescale, and cascade hydrogenation steps. Our study highlights a radiolytic route to produce CH3OH with CO2 feedstock and introduces a desirable atomic structure to improve performance.

Investigating the synergistic effects of magnesia-coal slag based solid waste cementitious materials and its basic characteristics as a backfill material

Applying solid waste resources as backfill material can reduce both the cost of backfill and the environmental problems caused by solid waste landfills. In this paper, the synergistic reaction effects of solid waste modified magnesia slag (MMS), coal gasification slag (CGS), and desulfurized gypsum (DG) as magnesium-coal slag based cementitious materials (MCC) and their preliminary feasibility as mining cementitious materials in synergy with coal gangue for the preparation of backfill materials are investigated. The results show that the order of the compressive strength of the cementitious systems is ternary system > binary system > monolithic system, which proves the existence of synergistic effect among MMS, CGS, and DG and determines the optimal dosing of each raw material in the ternary system. At early ages, the physical effect of CGS and the chemical effect of DG in the ternary system can promote the hydration reaction of MMS, but the synergistic effect between the three is weak; At later ages, a synergistic effect occurred among silica-aluminate depolymerization in CGS, dissolved sulfate from DG and hydration products from MMS, which promoted the production of more hydration products calcium-silicate(aluminum)-hydrate (C-S(A)-H) and AFt, and improved the compressive strength. In addition, the strength, fluidity and leaching of the backfill material prepared by MCC in collaboration with coal gangue can meet the preliminary feasibility for mine backfill. In the present work, the full solid waste MCC is developed to completely replace cement and use it to prepare backfill materials, which is of great importance to the comprehensive utilization of bulk solid waste, the reduction of backfill costs, and the enhancement of the economic and ecological interests of mines.

Heavy metal pollution risk of desulfurized steel slag as a soil amendment in cycling use of solid wastes

The by-product of wet flue gas desulfurization, desulfurized steel slag (DS), had chemical characteristics like natural gypsum that can be used to improve saline-sodic soil. However, contamination risk of heavy metals for cycling utilization of DS in agriculture was concerned mostly. Both pot and field experiments were conducted for evaluating the potential pollution risk of DS as the amendment of saline-sodic soil. Results showed that application of DS decreased the contents of Cd, Cu, Zn, and Pb, while significantly increasing chromium (Cr) content in DS-amended soils. The field experiment demonstrated that the migration of heavy metals (Cd, Zn, Cu, and Pb) in the soil profile was negligible. The application of DS at the dosage of 22.5-225 tons/ha significantly increased the Cr content in alfalfa (Medicago sativa L.) but lower than the national standard for feed in China (GB 13078-2017). DS altered the chemical fraction of heavy metals (Zn, Cu, and Pb), transferred exchangeable, reducible into oxidizable and residual forms in DS-amended soil. Application of DS combined with fulvic acid (FA) could effectively reduce the movement of heavy metals in soil and the accumulation of Cr in alfalfa. Based on our results, DS was a safe and feasible material for agricultural use and presented relatively little pollution risk of heavy metals. However, the results also showed that DS to a certain extent had a potential environmental risk of Cr if larger dosages of DS were used.

Application of desulfurization gypsum as activator for modified magnesium slag-fly ash cemented paste backfill material

Recycling industrial solid waste for mine backfill is one of the best ways to achieve green production in multiple industries. In this paper, the desulfurization gypsum (DG) as an activator is combined with the modified magnesium slag-fly ash cementitious paste backfill (MFPB) technology for the co-disposal of solid waste and goaf treatment, and the influence of DG on the performance of MFPB was comprehensively analyzed through rheological properties, mechanical properties, durability, microscopic analysis and environmental characteristics experiments. The results show that the fresh MFPB mortar conforms to the Herschel-Bulkley model at different maximum shear rate (γ̇_max) conditions. When the γ̇_max is 100 s^-1, the mortar exhibits shear-thickening properties. The apparent viscosity, yield stress and static yield stress of mortar decreased first and then increased with the increase of DG content, and all had the minimum value when DG was 2.5 %. The thixotropy of the mortar was significantly increased with the addition of DG, and the change in thixotropy was significantly correlated with the difference between the two yield stresses. Both the rheological and mini-slump results demonstrate that DG can improve the flowability of MFPB mortars. In addition, the UCS of D0 under steam curing and standard water curing conditions for 28 d were 4.342 MPa and 2.827 MPa, and the sample containing DG were 6.109-8.241 MPa and 6.669-9.492 MPa, respectively. The addition of DG not only improves the strength of MFPB, but also improves the durability of MFPB. Microscopic analysis (XRD, SEM, and TG-DTG) indicated that this was mainly because DG promoted the hydration reaction of the MMS-FA system and accelerated the generation of C-S(A)-H and AFt. Finally, in the results of in situ leaching based on durability and leaching based on standard HJ 557, all the indexes of MFPB meet the standard of class III groundwater in GB/T 14848-2017, and it has an effective stabilization/solidification effect on heavy metals (As, Cu, Ni, Ba, Zn and Mo, etc.). To sum up, the collaboration of DG and MFPB technology can not only efficiently clean and utilize a variety of solid wastes (MMS, FA and DG), but also greatly improve the performance of MFPB to promote its application.

Mitigating soil salinity stress with titanium gypsum and biochar composite materials: Improvement effects and mechanism

Titanium gypsum and biochar are considered effective amendments for mitigating soil salinity stress. However, the knowledge is inadequate regarding their efficiency and application as an improvement. In this study, TG-B composite was prepared by using industrial by-products titanium gypsum and biochar as raw materials and then modified by ball milling method, to characterize its microscopic characteristics and explore the improvement effect on saline-alkali soil and plant growth. Besides, we explored the mechanism of TG-B in improving saline-alkali soil and the dynamic balance of the solution reaction process. Our results showed that the CaSO₄·2H₂O particles in TG-B were finer, dispersed evenly, and contacted fully with soil gelatinous particles, which was more conducive to the improvement of saline-alkali soil. The results of TG-B with different ball milling ratios and different materials dosages indicated that the application rate of TG-B was 5%, and the optimum ratio of TG-B was TG: B (mass ratio) = 10:1, with the best soil improvement effect. The pot experiment proved that the indicators of indicating soil salinity such as pH, EC, SAR, and soluble Na⁺ decreased by 20.74%, 77.24%, 68.77%, and 44.70%, respectively, thus playing a good role in improving saline-alkali soil. In addition, pot experiments demonstrated that compared with the control group, the soil porosity and soil moisture content in the TG-B group increased by 15.95% and 38.71%, respectively, and further improve the structure and diversity of soil bacterial community when compared with titanium gypsum and biochar alone. Finally, the application of TG-B promoted the germination and growth of rice significantly through the synergistic effects of composite material components. These results all suggested that the application of TG-B was an effective strategy to improve soil salinity and promote plant growth. Therefore, it might provide new insights into the utilization of solid waste resources to improve saline-alkali lands.

In Situ Reconstruction of Active Catalysis Sites Triggered by Chromium Immobilization for Sulfite Oxidation

Hexavalent chromium (Cr(VI)) is a highly toxic substance in wastewater, triggering grievous detriment to aquatic life and human health. Magnesium sulfite is spawned along with the desulfurization process in coal-fired power plants, which is usually disposed of as solid waste. Here, a "waste control by waste" method was proposed upon the redox of Cr(VI)-sulfite, in which highly toxic Cr(VI) is detoxicated and sequent enriched on a novel biochar-induced cobalt-based silica composite (BISC) due to the forced electron transfer from chromium to surface hydroxyl. The immobilized Cr on BISC gave rise to the reconstruction of catalytic active sites "Cr-O-Co", which further enhance its performance in sulfite oxidation by elevating O₂ adsorption. As a result, the sulfite oxidation rate increased by 10 times compared with the non-catalysis benchmark together with the maximum chromium adsorption capacity being 120.3 mg/g. Therefore, this study provides a promising strategy to simultaneously control highly toxic Cr(VI) and sulfite, achieving high-grade sulfur resource recovery for wet magnesia desulfurization.

Basic characteristics of magnesium-coal slag solid waste backfill material: Part I. preliminary study on flow, mechanics, hydration and leaching characteristics

The environmental damage caused by surface subsidence and coal-based solid waste (CBSW) is a common problem in the process of coal mining. Backfill mining can control the mining-induced subsidence and solve the problem of bulk solid waste storage. In the present work, a magnesium-coal slag solid waste backfill material (MCB) with modified magnesium slag (MS) as binder and CBSW (fly ash (FA), flue gas desulfurization gypsum (FDG) and coal gasification slag (CGS)) as supplementary cementitious material/aggregate was proposed to meet the needs of coal mining in Northern Shaanxi, China, to realize the comprehensive treatment of goaf and CBSW. The results show that: (1) The rheological curve of the fresh MCB slurry is highly consistent with the Herschel-Bulkley (H-B) model, and its fluidity meets the basic requirements of mine backfill pumping. With the addition of FDG and MS, the yield stress, apparent viscosity and thixotropy of MCB slurry increase, while the pseudoplastic index and slump decrease. (2) The strength of MCB develops slowly in the early stage (0∼14 days) and increases rapidly in the later stage (14∼90 days). Except for the ratio of M20F1 and FDG = 0%, the strength of samples at other ratios (at 28 days) is between 6.06∼11.68 MPa, which meets the strength requirement of 6 MPa for coal mine backfill. The addition of MS and appropriate amount of FDG is beneficial to the development of strength. In contrast, MS exhibits a significant improvement in early strength, and FDG has a significant improvement in late-age strength. (3) Corresponding to the compressive strength, the hydration products C-S(A)-H and AFt of MCB are less in the early stage and greatly increased in the later stage. The active substance in FA/CGS will undergo pozzolanic reaction with the MS hydration product CH. The addition of FDG and MS can promote the reaction and increase the amount of hydration product, but in contrast, the promotion effect of FDG is more significant. (4) The amount of heavy metal leaching of MCB meets the requirements of national standards. The hardened MCB has a solidification/stabilization effect on heavy metal elements, which can significantly reduce the amount of heavy metal leaching. The results imply that MCB is a safe, reliable, and eco-friendly solid waste backfill material, and its application is conducive to the coordinated development of coal resource mining and environmental protection.

Study on the desulfurization performance of calcium-based desulfurizer and NaHCO3 desulfurizer

The commonly used calcium desulfurizers have low desulfurization efficiency. NaHCO₃ desulfurizers can meet the requirements of desulfurization efficiency, but the high price and the difficulty in handling desulfurization products make dry flue desulfurization technology quite difficult to realize the large-scale application. Preliminary research found a new calcium desulfurizer, to understand its performance, comparing investigation into the desulfurization performance of different calcium desulfurizer and NaHCO₃ desulfurizer. The results showed that with the high-performance calcium desulfurizer, conventional NaHCO₃ desulfurizer, and ultrafine NaHCO₃ desulfurizer, the operating time with 100% desulfurization efficiency is 25,200, 21,600, and 6000 s, when the flue temperature of 373.15-573.15 K, the "break-through" temperature is 533.15, 473.15, and 373.15 K, expand the use range of desulfurizer flue gas temperature. Regarding the desulfurizer per unit mass, the production costs of ultrafine NaHCO₃ desulfurizer are 5.36 times higher than calcium desulfurizer. Compared with NaHCO₃ desulfurizer, high-performance calcium desulfurizer is characterized by several advantages, including high desulfurization efficiency, wider applicable temperatures, and low preparation cost, allowing for significant development potential in flue gas desulfurization.

Review on treatment and utilization of barium slag in China

Barium slag (BS) is generated as a by-product waste during the production of barium salts from barite. A large amount of BS is discharged annually threating the ecological environment and restricting the development of the barium salts industry. In China, BS is classified as hazardous waste due to its corrosivity, and more importantly because of its extraction toxicity of barium. Soluble barium is toxic and can result in barium poisoning for environment and human beings. The current review presents a detailed summary on general characteristics, discharge and disposal status, harmless treatment pathways and comprehensive utilization of BS in China. BaO, SiO₂, CaO, and SO₃ occur as main chemical compositions in BS, especially BaO accounting approximately for 35-40%. The mineral compositions include unreacted barite, quartz, clay minerals, newly-formed phases from the side reactions such as BaCO₃, BaSiO₃ and BaSO₃, and residual carbon. A special attention is given to the assessment of the harmless treatment methods for BS from hazardous waste to general waste, which will decrease its management costs. Precipitation and solidification of soluble barium is the common pathway for harmless treatment of BS, and the using of other industrial waste can realize cost-saving. Methods for comprehensive utilization of BS include recovery of barium and carbon, application in building materials, and using as adsorbents for wastewater treatment. In particular, we analyzed and discussed the advantages and disadvantages of these existing process routes, intending to promote potentials for comprehensive utilization of BS in the future.

Mechanical Properties and Coagulation Characteristics of Flue Gas Desulfurization Gypsum-Based Polymer Materials

To resolve problems caused by the accumulation of flue gas desulfurization gypsum (FGDG) in the environment, a polymer material was prepared using FGDG, granulated blast furnace slag (GBFS), fly ash (FA), and solid sodium silicate (SSS). The compressive strength of these polymer specimens cured for 3, 28, and 60 d was regularly measured, and their condensation behavior was analyzed. Both the formation behavior of mineral crystals and microstructure characteristics were analyzed further using X-ray diffraction and scanning electron microscopy. The compressive strength of pure FGDG polymer specimen (whose strength is generated by particle condensation crystallization) is insufficient and the condensation is slow. The addition of appropriate amounts of GBFS, FA, and SSS can continuously and considerably improve the compressive strength and shorten the setting time. The optimal proportions of FGDG, GBFS, and FA are 50%, 20%, and 30%, respectively, with the SSS addition amount of 20 g. The incorporation of GBFS, FA, and SSS can promote the polymerization of calcium, silicon, and aluminum in FGDG to form silicate and aluminosilicate minerals. Their formation is the main reason for the increased compressive strength and accelerated coagulation.

Geopolymer: A Systematic Review of Methodologies

The geopolymer concept has gained wide international attention during the last two decades and is now seen as a potential alternative to ordinary Portland cement; however, before full implementation in the national and international standards, the geopolymer concept requires clarity on the commonly used definitions and mix design methodologies. The lack of a common definition and methodology has led to inconsistency and confusion across disciplines. This review aims to clarify the most existing geopolymer definitions and the diverse procedures on geopolymer methodologies to attain a good understanding of both the unary and binary geopolymer systems. This review puts into perspective the most crucial facets to facilitate the sustainable development and adoption of geopolymer design standards. A systematic review protocol was developed based on the Preferred Reporting of Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist and applied to the Scopus database to retrieve articles. Geopolymer is a product of a polycondensation reaction that yields a three-dimensional tecto-aluminosilicate matrix. Compared to unary geopolymer systems, binary geopolymer systems contain complex hydrated gel structures and polymerized networks that influence workability, strength, and durability. The optimum utilization of high calcium industrial by-products such as ground granulated blast furnace slag, Class-C fly ash, and phosphogypsum in unary or binary geopolymer systems give C-S-H or C-A-S-H gels with dense polymerized networks that enhance strength gains and setting times. As there is no geopolymer mix design standard, most geopolymer mix designs apply the trial-and-error approach, and a few apply the Taguchi approach, particle packing fraction method, and response surface methodology. The adopted mix designs require the optimization of certain mixture variables whilst keeping constant other nominal material factors. The production of NaOH gives less CO₂ emission compared to Na₂SiO_3, which requires higher calcination temperatures for Na₂CO₃ and SiO_2. However, their usage is considered unsustainable due to their caustic nature, high energy demand, and cost. Besides the blending of fly ash with other industrial by-products, phosphogypsum also has the potential for use as an ingredient in blended geopolymer systems. The parameters identified in this review can help foster the robust adoption of geopolymer as a potential "go-to" alternative to ordinary Portland cement for construction. Furthermore, the proposed future research areas will help address the various innovation gaps observed in current literature with a view of the environment and society.

A Simple and Efficient Method for Preparing High-Purity α-CaSO4·0.5H2O Whiskers with Phosphogypsum

A simple and efficient approach for the high-purity CaSO₄·2H₂O (DH) whiskers and α-CaSO₄·0.5H₂O (α-HH) whiskers derived from such phosphogypsum (PG) was proposed. The impact of different experimental parameters on supersaturated dissolution–recrystallization and preparation processes of α-CaSO₄·0.5H₂O was elaborated. At 3.5 mol/L HCl concentration, the dissolution temperature and time were 90 °C and 20 min, respectively. After eight cycles and 5–8 times cycles, total crystallization amount of CaSO₄·2H₂O was 21.75 and 9.97 g/100 mL, respectively, from supersaturated HCl solution. The number of cycles affected the shape and amount of the crystal. Higher HCl concentration facilitated CaSO₄·2H₂O dissolution and created a much higher supersaturation, which acted as a larger driving force for phase transformation of CaSO₄·2H₂O to α-CaSO₄·0.5H₂O. The HCl solution system’s optimum experimental conditions for HH whiskers preparation involved acid leaching of CaSO₄·2H₂O sample, with HCl concentration 6.0 mol/L, reaction temperature 80 °C, and reaction time 30 min–60 min. Under the third cycle conditions, α-CaSO₄·0.5H₂O whiskers were uniform in size, clear, and distinct in edges and angles. The length range of α-CaSO₄·0.5H₂O whiskers was from 106 μm to 231 μm and diameter range from 0.43 μm to 1.35 μm, while the longest diameter ratio was 231. Purity of α-CaSO₄·0.5H₂O whiskers was approximately 100%, where whiteness reached 98.6%. The reuse of the solution enables the process to discharge no waste liquid. It provides a new reference direction for green production technology of phosphogypsum.

Selenium migration mechanism in wet FGD slurry: Experimental and DFT analysis

Selenium (Se) is one of the hazardous trace elements emitted from coal-fired power plants. The Se migration behavior in wet flue gas desulfurization (FGD) slurry is still unclear, and the species of Se in FGD gypsum remains controversial. In this research, the bubbling experiments using simulated slurry with/without gypsum crystallization process were conducted. The experimental results indicated that pure gypsum has poor capability to capture Se components, and only selenite could be trapped in gypsum during its crystal growth stage. Furthermore, the DFT calculation was conducted to provide the microscopic information of Se adsorption and substitution characteristics during gypsum crystallization process. The research findings of this study could help understand the mechanism of Se migration process in FGD slurry, and facilitate the development of effective Se emission control technologies in the future.

Pilot-scale study on seeded precipitation assisted nanofiltration for flue-gas desulfurization wastewater softening

In order to reduce the cost of chemical softening, the seeded precipitation assisted nanofiltration (NF) process was introduced into zero liquid discharge (ZLD) of flue-gas desulfurization (FGD) wastewater treatment. A pilot-scale system was developed and run for 168 h in a coal-fired power plant. The system mainly consists of lime softening, ambient temperature crystallizer (ATC) and NF, in which the raw water treatment capacity was 1 m³/h. The results indicated that the system operated stably, the softening cost was 13.30 RMB/m³, and the electricity cost was 3.39 RMB/m³ for the FGD wastewater in this pilot system. High quality gypsum was got from the ATC unit, of which the purity was 95.8%. Through this system, the hardness removal rate was higher than 98.9% and the water recovery rate reached 96%. In addition, the pressure and permeate flux kept stable in the ultrafiltration (UF) unit and NF unit, indicating no scaling occurred in the two units during 168 h test. Thus, a feasible and cost-effective process was provided by using the seeded precipitation assisted NF to deal with the FGD wastewater.

Effect of Electrolytic Manganese Residue in Fly Ash-Based Cementitious Material: Hydration Behavior and Microstructure

Electrolytic manganese residue (EMR) is a solid waste with a main mineralogical composition of gypsum. It is generated in the production of metal manganese by the electrolysis process. In this research, EMR, fly ash, and clinker were blended to make fly ash-based cementitious material (FAC) to investigate the effect of EMR on strength properties, hydration behavior, microstructure, and environmental performance of FAC. XRD, TG, and SEM studied the hydration behavior of FAC. The pore structure and [SiO₄] polymerization degree were characterized by MIP and ²⁹Si NMR, respectively. The experimental results indicate that FAC shows excellent mechanical properties when the EMR dosage is 10%. Moderate content of sulfate provided by EMR can promote hydration reaction of FAC, and it shows a denser pore structure and higher [SiO₄] polymerization degree in this case. Heavy metal ions derived from EMR can be adsorbed in the hydration products of FAC to obtain better environmental properties. This paper presents an AFt covering model for the case of excessive EMR in FAC, and it importantly provides theoretical support for the recycling of EMR in cementitious materials.