Search Results (169)

Although the dissolution of sparingly soluble salts is of interest to many fields, such as material science, dentistry, and geochemistry, the simplicity of these reactions provides its own motivation for study. Three features of these reactions are examined in this paper: (i) the unusual forms of the kinetic expression that have been used to describe their rates of reaction, (ii) the observation that the rate of dissolution is correlated with the potential difference across the solid-solution interface, and (iii) the observation of non-stoichiometric dissolution. Mechanistic descriptions of the kinetics of dissolution in current use do not account for all these factors, while the surface vacancy model does. In this paper, it is shown that linear kinetics arise from a symmetry of the rates of removal and deposition of anions and cations. On the other hand, non-linear kinetics arise from an asymmetry in the rates of removal and deposition of anions and cations. Because the surface vacancy model is an electrochemical model, the influence of potential difference on the rate of reaction is inherent to the model. A transient, or non-stationary state, version of the model is used to explain how non-stoichiometric dissolution arises. Full article

This study investigates the dual mechanisms by which SiO₂ deteriorates magnesium phosphate cement (MPC) performance. MgO-SiO₂ clinkers were prepared using lightly calcined magnesia (MgO) with SiO₂ additions ranging from 1% to 9%, followed by calcination at temperatures between 1100 °C and 1500 °C. Through XRD–Rietveld refinement, workability, compressive strength, and hydration heat analyses, the damaging effects of SiO₂ were systematically evaluated. Results reveal that SiO₂ degrades MPC through two concurrent mechanisms: chemical consumption and physical deactivation of reactive MgO. Chemically, SiO₂ reacts with MgO during calcination to form inert forsterite (Mg₂SiO₄), irreversibly reducing reactive MgO content. Physically, SiO₂ and its reaction products lower the crystallinity and reactivity of remaining MgO while diluting reactive components. A calcination temperature of 1200 °C was optimal, yielding the highest compressive strength (3 d strength > 30 MPa). Increasing SiO₂ dosage monotonically reduced strength; at 1200 °C, 9% SiO₂ reduced 3 d strength by ~40% compared to 1%. Hydration heat analysis showed that both heat flow rate and cumulative heat release increased with SiO₂ content due to enhanced heterogeneous nucleation from Mg₂SiO₄. Critically, this increased heat output did not translate into higher strength, indicating that microstructural quality—not reaction extent—governs mechanical performance. Rietveld quantification confirmed that Mg₂SiO₄ formation increased linearly with SiO₂ dosage and temperature (reaching 72.24% at 1500 °C with 9% SiO₂), providing the material basis for dual damage. This work offers mechanistic insights and experimental support for utilizing low-grade magnesite and optimizing MPC performance. Full article

Ferronickel slag, as a major solid waste in the stainless-steel industry, poses a serious threat to the environment due to its large-scale production and low utilization rate. In this study, magnesium oxide in the ferronickel slag was leached out and converted into high-purity magnesium sulfate, while the leach residue was utilized for cement clinker production. During the complete utilization of ferronickel slag, the Mg leaching efficiency reached 90.75% and was significantly enhanced by reducing the particle size of the ferronickel slag with H₂SO₄ solution as the sole solvent. High-purity magnesium sulfate with a purity of 99.92% was prepared from the leachate through a multi-step process involving primary crystallization, purification, and secondary crystallization. The leach residue, accounting for 68.20% of the original mass, was primarily composed of 79.4 wt% SiO₂ and less than 6.1 wt% MgO and is used as a key raw material in the production of Portland cement. Sintering temperature significantly influenced the structure and properties of the resulting cement. Both the Portland clinker and cement were successfully produced at sintering temperatures of 1400 °C and 1450 °C when the leach residue was used as a primary raw material, with well-developed cementitious phases of calcium silicate and aluminate formed during calcination. The setting time, soundness, and compressive and flexural strengths of the hardened C1400 and C1450 mortars met the requirements specified in relevant standards. Through this integrated process, the overall utilization rate of the ferronickel slag reached 100%. Based on a preliminary estimate, full utilization of the annual ferronickel slag production in China could substitute at least 19.5 million tons of magnesite and 15.0 million tons of silica and reduce CO₂ emissions by 10.3 million tons. Full article

To develop and design new alloy materials with lightweight and superior comprehensive performance. In this study, a MgAlTiVFeCo lightweight high-entropy alloy (LW-HEA) was fabricated via mechanical alloying and spark plasma sintering (SPS) to investigate the effects of sintering temperature on its phase structure, microstructure, densification, microhardness, high-temperature oxidation resistance, and corrosion resistance. The results indicate that the ball-milled MgAlTiVFeCo LW-HEA formed a simple solid solution phase with a BCC structure. After spark plasma sintering, the phase structure of the alloy changed, maintaining the BCC phase as the primary phase while accompanying the precipitation of secondary phases. When the sintering temperature reached 1000 °C, the alloy achieved a densification of 96.7% and a microhardness of 1235.5 HV. Its hardness value is comparable to the typical range of cemented carbides, demonstrating outstanding mechanical properties. The oxidation kinetics of MgAlTiVFeCo high-entropy alloys sintered at different temperatures at 900 °C follow a parabolic law, which is diffusion-controlled and can be divided into two stages: rapid growth and slow stabilization. At a sintering temperature of 1000 °C, the fitted oxidation rate constants, k_p1 (0–25 h) and k_p2 (25–60 h), are 3.76 × 10⁻² mg²·cm⁻⁴·s⁻¹ and 1.10 × 10⁻¹ mg²·cm⁻⁴·s⁻¹, respectively, outperforming those of alloys sintered at other temperatures. In a 3.5 wt% NaCl solution, the corrosion resistance of the alloy improves with increasing sintering temperature. Compared to alloys sintered at medium-to-low temperatures (850–950 °C), the alloy sintered at a high temperature (1000 °C) exhibits a more positive corrosion potential (−0.438 V) and a lower corrosion current density (1.07 × 10⁻⁶ A·cm⁻²), indicating excellent corrosion resistance. It is evident that 1000 °C is the optimal sintering temperature, and the MgAlTiVFeCo LW-HEA demonstrates superior comprehensive properties. Full article

Sulfur-based autotrophic denitrification (SAD) is limited by low efficiency and poor stability in carbon-deficient photovoltaic (PV) wastewater treatment. This study developed four sulfur-based composite fillers (S⁰-CFs) comprising 75% elemental sulfur and mineral additives (boron mud, magnesite, and/or siderite) fabricated via melt mixing–jet granulation. Lab-scale operation showed that at a hydraulic retention time (HRT) of 1 h, all S⁰-CFs achieved high TN removal (89.1–93.8%) with effluent NO₃⁻-N below 1.5 mg/L (>93% nitrate removal efficiency) and stable pH. Although effluent COD increased with a short HRT (1 h) due to biofilm detachment, no leaching of organic or inorganic pollutants from the fillers was observed, and TP was consistently removed. 16S rRNA sequencing confirmed enrichment of autotrophic denitrifiers Thiobacillus and Sulfurimonas, verifying SAD as the dominant pathway. In a 270-day pilot-scale operation, nitrate removal varied with temperature (7.3–27.2 °C) and HRT, reaching 88.2% on average (range: 86.6–90.0%) at 1 h HRT during warm periods (25.8–27.2 °C), dropping to 13.5–38.1% under cold conditions (7.3–16.0 °C) at 0.5 h HRT, and then stabilizing at 64.1% by adjusting HRT to 1 h. Fluoride was removed at 0.51–1.49 mg/L. Additionally, operational cost was 34.5% lower than heterotrophic denitrification. These results demonstrated that S⁰-CF enabled efficient, stable, and cost-effective nitrogen removal, making SAD more suitable for low-carbon industrial wastewater treatment. Full article

The flotation efficiency of magnesite in the slurry system is critically influenced by its surface wettability. In this work, molecular dynamics (MD) and density functional theory (DFT) calculations were employed to investigate the interactions between water molecules and the magnesite (104) surface. To elucidate the underlying mechanisms, systematic evaluations were conducted, encompassing frontier orbital energies, water molecule adsorption behavior, and the water wetting process. Results indicate that electrons readily transfer from the highest occupied molecular orbital (HOMO) of water to the lowest unoccupied molecular orbital (LUMO) of magnesite. Specifically, the chemisorption of a single water molecule onto the magnesite surface was observed, with a calculated adsorption energy of −91.6 kJ/mol. This process involves an interaction between the oxygen atom of water and a surface magnesium atom, leading to the formation of an Mg–O_W bond. This bond primarily arises from hybridization between the Mg 2p, Mg 2s, and O_W 2p orbitals. Furthermore, water molecules within the first adsorbed monolayer exhibited an average adsorption energy of −66.3 kJ/mol, which further confirms the occurrence of chemisorption. Notably, minimal changes were observed in the orbital interactions between water molecules and surface Mg atoms, a trend consistent with the single-molecule adsorption case. The average adsorption energies for the second and third water layers were calculated to be −63.2 kJ/mol and −45.6 kJ/mol, respectively. The stabilization of the hydration layer structure is attributed to the hydrogen-bonding network formed among water molecules in the outer layers. As the number of water layers increases, the structural disorder of water molecules on the magnesite surface progressively intensifies. This decrease in adsorption energy with increasing layer number is attributed to the progressively enhanced contribution of hydrogen-bonding interactions between water molecules across different layers. Consequently, the magnesite surface exhibits a low contact angle, indicating high intrinsic hydrophilicity. Collectively, these findings provide molecular-level insights into the wettability of the magnesite surface, thereby contributing to a more fundamental understanding of magnesite flotation mechanisms. Full article

In view of the depletion of high-grade magnesite resources in China, this study presents a comparative analysis of two industrial fused magnesia products produced via a flotation–fusion route. A low-grade magnesite (DSQLM-3, MgO 41.48 wt.%) was upgraded by reverse flotation to a concentrate (FDSQLM-3, MgO 47.55 wt.%) with >97% SiO₂ removal. Two fused magnesia samples (FM-1 from natural high-grade ore DSQLM-1; FFM-3 from concentrate FDSQLM-3) were produced under identical arc-furnace melting (2800 °C, 4 h), followed by natural cooling. Although FFM-3 showed higher MgO (97.61 vs. 97.25 wt.%), its bulk density was comparable to FM-1 (3.45 vs. 3.46 g/cm³). XRD/Rietveld refinement and SEM-EDS indicated that CMS dominated the Ca–silicate assemblage in FM-1, whereas β/γ-C₂S was observed in FFM-3, coinciding with a higher CaO/SiO₂ (C/S) ratio (2.85 vs. 0.68). Image analysis further showed higher grain boundary microcrack metrics in FFM-3. These observations are consistent with reports in the literature stating that the β → γ transformation of C₂S during cooling involves ~12% volume expansion that can contribute to cracking; however, cooling history and composition were not independently controlled in this industrial comparison, so the relationships are interpreted as data-supported associations rather than isolated causality. The results suggest that beneficiation strategies may benefit from managing residual oxide balance (especially C/S ratio) in addition to reducing total impurities. Mechanical and thermomechanical properties were not measured and should be evaluated in future work. Full article

Magnesite tailings are by-products of magnesite mining, yet their utilization rate remains extremely low. Although previous studies have explored their basic physical properties and potential use in cementitious or geotechnical materials, research on cement-stabilized magnesite tailings-particularly regarding their mechanical behavior, engineering applicability, and microstructural evolution-remains limited. Key scientific gaps include the lack of systematic evaluation of their compaction characteristics, strength development, stiffness evolution, and bearing capacity, as well as insufficient understanding of the stabilization mechanisms governing their performance. Addressing these gaps is essential for assessing their feasibility as road construction materials. In this study, magnesite tailings were selected as the primary raw material and mixed with ordinary Portland cement to prepare mixtures for evaluating their suitability as highway subgrade fillers. The compaction characteristics, unconfined compressive strength (UCS), ultrasonic pulse velocity (UPV), and California Bearing Ratio (CBR) of the mixtures were systematically examined. Furthermore, the evolution of composition and stabilization mechanisms of the mixtures was analyzed using X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis. The results show that cement incorporation effectively improves the poor particle gradation of magnesite tailings, leading to a denser and more homogeneous structure. Adding 7% cement increases the maximum dry density and optimum moisture content by 3.7% and 5.1%, respectively. The unconfined compressive strength rises by 100.9–126.3% within 3–28 days, and the maximum uniaxial stress is 119.6% higher than that of the 1% cement mixture. These improvements demonstrate the potential of cement-stabilized magnesite tailings as a sustainable subgrade material and provide insight into their microstructural and mechanical behavior. Full article

This study evaluated the ability of microalgae to produce carbonate minerals through CO₂ uptake, in comparison with abiotic, direct chemical synthesis through CO₂ absorption. A freshwater microalga (Synechococcus elongatus) isolated from garden soil in East Anglia, UK, was cultivated under laboratory conditions with CO₂ injection to generate a bicarbonate-rich aqueous solution, in which Fe²⁺, Mg²⁺, and Ca²⁺ ions were added to facilitate carbonate formation. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analyses revealed distinct morphologies and mineral types. The algae-based process precipitated calcite, siderite, magnesite, and dolomite, whereas the abiotic process yielded, respectively, calcite, siderite, high-Mg calcite and nesquehonite. Biogenic minerals were finer and more morphologically diverse than their abiotically formed counterparts. The results indicated that microalgae produced 0.21 mol/L of calcium carbonate, compared to 0.51 mol/L obtained through abiotic CO₂ sequestration, and a comparable yield of about 0.25 mol/L reported for Sporosarcina pasteurii-induced precipitation. Acid resistance tests showed that algae-induced minerals had similar or improved resistance to acidic conditions compared to minerals formed through abiotic CO₂ consumption. Overall, despite slower kinetics, algae-induced carbonate precipitation offers advantages for soil stabilization by biocementation in the context of environmental sustainability, climate change mitigation and circular economy. Full article

The Global Cement and Concrete Association (GCCA) estimated that by 2050, 36% industry-wide sustainable value will be created, which includes sequestering CO₂ into the cement and concrete industry to produce commercially feasible high-value products. Direct utilization of CO₂ in the cement and concrete industry, which utilizes natural and sustainable materials, is gaining momentum. Naturally occurring mixtures of hydro magnesite and huntite are important industrial minerals which, upon endothermic decomposition over a specific temperature range, will release water and CO₂. This unique chemistry has led to such mixtures being successfully utilized as fire retardants, replacing aluminum hydroxide or Alumina Tri-Hydrate (ATH). Despite the developed marketplace for magnesium-based fire-retardant products, there is little mention of CO₂ mineral carbonation methods, which attempt to recover and convert magnesium from natural seawater or industrial waste into oxides or carbonates as part of the carbon sequestration initiative. The hypothesis to be proven in this work states that if the process of seawater mineral carbonation is prematurely quenched, Mg²⁺ ionic species in seawater adsorbed on the calcite lattice formation will be trapped and therefore recovered in various oxidized forms, such as magnesium oxides, magnesium hydro magnesite, and magnesium carbonate precipitates. A novel method to recover magnesium Mg²⁺ ions from seawater was successfully explored and documented; as such, from an initial concentration of 1250 ppm Mg²⁺ in raw seawater, the average concentration of spent Mg²⁺ ions after the reaction was as low as 20 ppm. A very efficient near-total recovery of Mg²⁺ from the seawater into the solid precipitates was recorded. Subsequently, the process for continuous seawater mineral carbonation for the production of magnesium/brucite/huntite products was successfully proven and optimized to operate with a 30 s reaction time, a dynamic feedstock concentration, [CaO] at 1 gpl in seawater and a room temperature reaction temperature (30 °C), where the average yield of the fire-retardant magnesium-based compounds was 26% of the synthesized precipitates. Approximately 5000 g of the hydro magnesite materials was molded into a fire-retardant brick or concrete wall, which was subjected to an accredited fire performance and durability testing procedure BS476-22:1987. There were encouraging results from the fire resistance testing, where the fire-retardant material passed BS476-22:1987, with performance criteria such as physical integrity failure, the maximum allowable face temperature, and a minimum duration before failure, which was up to 104 min, evaluated. Full article

The inefficient disposal of corn stover (CS) and the accumulation of magnesite tailings (MMTs) pose dual environmental threats. Although biomass gasification can utilize CS, its inherent drawbacks result in syngas with low heating value and high tar content. Torrefaction pretreatment can effectively improve biomass properties, and the use of steam as a reaction medium can further optimize the product’s pore structure. This study proposes a steam-assisted torrefaction pretreatment to address the inefficient utilization of CS and the disposal challenges of MMTs. The experimental results demonstrated that torrefaction at 300 °C with 30% water content for 60 min significantly improved the raw material’s properties. The optimized CSBC exhibited a well-developed pore structure and achieved a phenol removal rate of 63.4%. The addition of MMTs further enhanced the pretreatment effect, increasing the removal rate to 75.5% and confirming the superiority of the CSBC–magnesite composite system. The steam atmosphere improved phenol adsorption by regulating pore structures and surface functional groups, offering a feasible approach for utilizing solid waste resources and developing a new in situ tar control strategy. Full article

Mortars taken from the 16th century Venetian Fortress of Bergamo (Italy) were characterized (binder-concentrated fractions and aggregate fractions as well as bulk samples) with a multi-analytical approach using X-ray diffraction (XRD), inductively coupled plasma optical emission spectrophotometry (ICP-OES), optical microscopy (OM), differential scanning calorimetry (DSC) and thermogravimetric analysis (TG). The results showed the presence of calcite, hydrocalumite and hydrotalcite-type compounds, brucite, aragonite, plombierite and a large fraction of amorphous phases (ranging between 14 and 27 wt%) in the binder. Quartz and carbonate-rich sands were used as aggregates. The mortar is a Mg-rich material containing 4–5 wt% brucite. No evidence of magnesite or hydromagnesite was found in any sample, although these phases are frequently detected in the binder of buildings from the Renaissance period that are located in Northern Italy. The large average amount (12–13 wt%) of reactive silicate, such as Mg-containing phyllosilicates that can react with lime, and the presence of carbonate-containing hydrocalumite and hydrotalcite indicate hydraulic interactions between lime and reactive silicate aggregates. The CO₂/H₂O_bound ratio, evaluated from the weight loss referred to the finer fraction (<63 μm), ranges from 1.99 to 2.55, which suggests that the walls of Bergamo were constructed using lime-based mortar with hydraulic properties. Full article

Mining activities generate huge volumes of mine tailings (MTs), which pose huge environmental management challenges. Reuse as cemented paste backfill (CPB), a mixture of tailings with water and a binder—often cementitious or alkaline—is amongst the best methods to reduce surface disposal, and it is used to backfill underground mine voids. Although the most widely used binder in CPB production remains Ordinary Portland Cement (OPC), it is associated with a high carbon footprint and a high economic cost. In this study, both the economic feasibility and the environmental performance of three alkaline activators—sodium hydroxide (NaOH), sodium silicate (Na₂SiO₃), and a high MgCO₃ and MgO content calcined magnesite residue—are evaluated as OPC replacements in CPB products. A gate-to-grave life cycle assessment (LCA) was performed at a CPB plant located in southwestern Spain with the use of tailings from a massive sulfide deposit. The results from the uniaxial compressive strength test and LCA demonstrate that paste formulations using the magnesite residue achieve comparable mechanical performance while significantly reducing both the environmental footprint and total cost relative to OPC-based mixtures. These results support the use of alkaline binders as viable substitutes that enable more sustainable and cost-effective tailings management practices in the mining sector. Full article

To achieve efficient desilication and improve the grade of magnesite, an environmentally friendly surfactant, cocamidopropyl dimethylamine (PKO-H), was employed as a collector for the flotation separation of magnesite and quartz. The flotation performance and adsorption mechanism of PKO-H was systematically investigated through flotation experiments, Fourier-transform infrared spectroscopy (FTIR), contact angle measurements, zeta potential analysis, and molecular simulations. The flotation results demonstrated that PKO-H exhibited excellent selectivity, achieving a MgO recovery rate of 98.8% and a concentrate grade of 45.7% in artificially mixed mineral samples. Contact angle measurements, FTIR spectra, zeta potential analysis, and molecular simulations revealed that the adsorption of PKO-H on quartz is primarily driven by electrostatic attraction. In contrast, due to electrostatic repulsion, the interaction between PKO-H and magnesite is weak, preventing stable adsorption. This study establishes PKO-H as a sustainable and efficient collector for magnesite beneficiation and provides new insights into interfacial mechanisms for the design of eco-friendly flotation reagents. Full article