In-situ remediation of phosphogypsum in a cement-free pathway: Utilization of ground granulated blast furnace slag and NaOH pretreatment

Role of polyferric sulphate in hydration regulation of phosphogypsum-based excess-sulphate slag cement: A multiscale investigation

Current demand for waste recycling, phosphogypsum-based excess-sulphate slag cement (PESSC) as a sustainable cement prepared by solid wastes, urges enhancing its performance development based on microstructure optimisation. For the purpose of improving the performance and durability of PESSC used in normal or corrosive environments, it is deemed an efficient technique to produce iron-doped compounds with high thermodynamic stability. This paper presents a systematic study of the effect of iron modification on PESSC binders by introducing 0%-2% polyferric sulphate (PFS) from a multiscale viewpoint. XPS, 29Si and 27Al NMR, and TEM were used to characterise the nanostructure of solid particles firstly at Level I. Then, the chemical composition and phase assemblage of PESSC binders were revealed at Level II in terms of ICC, ICP, DTG-DSC, FTIR, BSE-EDS and XRD. Finally, setting time and strength development were determined at Level III. Results indicated that the soluble FeOH4- supplied by the hydrolysis of PFS promotes the generation of iron-doped ettringite with a greater length-to-diameter ratio and thermodynamic stability. Seeding effect of iron doping also promotes the production of spherical and retiform gels with a slight influence on the chemical components and polymerisation. Despite the fact that iron doping weakens the early strength of PESSC mortars, it promotes the persistent hydration rate by retarding precipitation and encapsulation of hydrates on the surface of the slag, showing excellent strength in the later stages. In view of microstructure evolution and performance development during each stage, PFS supplementation within 1.0% is considered a feasible modification of PESSC relying on the formation control of iron-doped hydrates.

Enhanced solidification/stabilization (S/S) of fluoride in smelting solid waste-based phosphogypsum cemented paste backfill utilizing biochar: Mechanisms and performance assessment

Phosphogypsum (PG) cemented paste backfill (CPB) is a primary non-hazardous method for treating PG. However, using traditional binders like cement increases global carbon emissions and mining operational costs while complicating the reduction of fluoride leaching risks. This study introduces a novel PG-based CPB treatment method using steel slag (SS) and ground granulated blast furnace slag (GGBFS) as binders, calcium oxide as an exciter, with biochar serving as a fluoride-fixing agent. We investigated the effect of biochar addition on the hydration and solidification/stabilization (S/S) of fluoride in SS and GGBFS-PG-based materials (SSPC). The results indicated that the optimal strength and performance for fluoride S/S were achieved with a biochar addition of 0.2 wt%. Compared to the control group without biochar, the strength increased by 54.3%, and F leaching decreased by 39.4% after 28 days of curing for SSPC. The addition of 0.2 wt% biochar facilitated heterogeneous nucleation and acted as a microfiller, enhancing SSPC's properties. However, excessive biochar reduced the compactness of SSPC. Additionally, the distribution of fluoride was strongly correlated with P, Ca, Fe, and Al, suggesting that fluoride S/S is linked to the formation of stable hydration products like fluorapatite, fluorite, and complexes such as [AlF6]3- and [FeF6]3-. These findings offer a promising approach for the safe treatment of PG and the beneficial reuse of solid waste from SS and GGBFS.

Co-treatment of steel slag and oil shale waste in cemented paste backfill: Evaluation of fresh properties, microstructure, and heavy metals immobilization

The environmentally sustainable treatment of steel slag (SS) and oil shale waste (OSW) is a significant concern in the field of industrial development. The mining industry also faces challenges related to the high costs and carbon emissions associated with ordinary Portland cement (OPC), leading to environmental pollution. To address these challenges, this study aimed to develop a cost-effective and environmentally friendly binder for cemented paste backfill (CPB) by utilizing SS and calcined oil shale waste (COSW) as primary precursors. Extensive investigations were conducted to evaluate the properties of the CPB sample with varying COSW content, including rheological properties, mechanical strength, and microstructure. The binder sample was comprehensively characterized using isothermal calorimetric analysis, X-ray diffraction (XRD), thermogravimetry (TG), and scanning electron microscopy (SEM). Based on systematic experimentation, an optimal blend ratio for the binder was determined, consisting of 60 wt% SS, 15 wt% COSW, 15 wt% phosphogypsum (PG), and 10 wt% OPC. The exceptional performance of the binder was attributed to the substantial formation of precipitated ettringite (AFt), resulting in a more compact structure and improved mechanical strength. Additionally, a sequential extraction test revealed that the heavy metals in the CPB sample were mainly present in the residual fraction, demonstrating the effective immobilization of heavy metals by the binder.

Effect of alkaline leaching of phosphogypsum on sulfate reduction activity and bacterial community composition using different sources of anaerobic microbial inoculum

Phosphogypsum (PG), a by-product of the phosphate industry, is high in sulfate, (SO42-), which makes it an excellent substrate for sulfate-reducing bacteria (SRB) to produce hydrogen sulfide. This work aimed to optimize SO42- leaching from PG to achieve a high biological reduction of SO42- and generate high sulfide concentrations for subsequent use in the biological recovery of elemental sulfur. Five SRB consortia were isolated and enriched from: IS (Industrial sludges), MS (Marine sediments), WC (Winogradsky column), SNV (petroleum industry sediments) and PG (stored Phosphogypsum). The five consortia showed reduction activity when using PG leachate (with water) as source of SO42- and lactate, acetate, or glucose as the electron donor. The highest reduction rate (81.5 %) was registered using lactate and the IS consortium (81.5 %) followed by MS (79 %) and PG (71 %). To enhance the concentration of leached SO42- from PG for future utilization with the isolated consortia, PG was treated with NaOH solutions (2 % and 5 %). SO42- release of 97 % was achieved with a 5 % concentration and the resulting leachate was further diluted to target a SO42- concentration of 12.4 g·L-1 for utilization with the isolated consortia. Compared to water leachate, a significantly higher reduction rate was registered (2 g·L-1 of SO42) using the IS consortium, demonstrating limited inhibition effect of sulfide- concentration on SRB functionalities. Moreover, metagenomic analysis of the consortia revealed that using PG as a source of SO42- increased the abundance of Deltaproteobacteria, including known SRB like Desulfovibrio, Desulfomicrobium, and Desulfosporosinus, as well as novel SRB genera (Cupidesulfovibrio, Desulfocurvus, Desulfococcus) that showed, for the first time, significant potential as novel sulfate-reducers using PG as a SO42- source.

Fluorides immobilization through calcium aluminate cement-based backfill: Accessing the detailed leaching characterization under torrential rainfall

Urbanization and economic development have increased the demand for fertilizers to sustain food crop yields. Huge amounts of by-products, especially phosphogypsum (PG), are generated during the wet processing of rock phosphate to produce fertilizers. Chronic exposure to fluoride in phosphogypsum in groundwater as a result of the weathering of fluoride-containing waste poses a significant health risk to millions of people. We propose a method for using calcium aluminate cement (CAC) to remediate high fluoride contents in solid waste. Column leaching tests under harsh rainfall conditions confirmed the efficient fluoride immobilization capacity of a CAC binder. Although the fluoride concentrations in leachates during the first 1-2 days (1.25 mg/L) slightly exceeded the threshold of 1.00 mg/L, the concentrations over 3-28 days (ranging from 0.98 to 0.83 mg/L) consistently remained well within the acceptable range. Furthermore, our characterization and geochemical modeling revealed the fluoride retention mechanisms of CAC-stabilized PG under laboratory-simulated conditions of torrential rainfall. During leaching, physical encapsulation prevents fluoride from contacting leachate. However, an unfavorable pH value can cause the release of fluoride from the cement matrix, which is subsequently captured by aluminate hydrate through adsorption or co-precipitation. We quantified the carbon footprint of CAC for immobilizing 1 mg of fluoride in PG, obtaining a remarkably low value of 4.4 kg of CO2, in contrast to the emissions associated with the use of ordinary Portland cement (OPC). The findings suggest a unique opportunity for extensive PG remediation. This opportunity extends the horizons of achieving zero-waste emissions in the phosphorus industry and has practical significance in the context of reducing carbon emissions.

Control of Fluoride Pollution in Cemented Phosphogypsum Backfill by Citric Acid Pretreatment

Using phosphogypsum (PG) as the aggregate of cemented backfill is an economical and effective method of PG utilization. However, the stability and performance of cemented backfill are challenged by the rich fluoride content in PG. In this study, the effects of citric acid pretreatment on PG defluorination, backfill performance and environmental behavior were investigated by washing PG with different concentrations of citric acid and washing times. The results showed that the citric acid pretreatment could significantly reduce the fluoride content in PG and promote the hydration reaction with the binder, thus greatly reducing the usage and cost of the binder in actual production. Considering the efficiency of defluorination, the optimal citric acid concentration and washing times were determined to be 4% and 7-8 times, respectively. In addition, after citric acid pretreatment, the viscosity and setting time of the backfill slurry and the porosity of the backfill reduced, and the strength of the backfill improved, which was conducive to slurry pipeline transportation and underground mine stability. Finally, a further analysis of environmental behavior was conducted and it was found that the citric acid washing greatly reduced the content of fluoride in the bleeding water of slurry and the backfill leachate, which met the integrated wastewater discharge standard in China. The results of this study can provide important guidance for the large-scale recycling and environmental management of PG.