Low Impurity Concentration Research Articles

Low-Acid Leaching for Preferential Lithium Extraction and Preparation of Lithium Carbonate from Rare Earth Molten Salt Electrolytic Slag

In this work, lithium was preferentially recovered through the low-acid leaching from rare earth molten salt electrolytic slag (REMSES) with a leaching temperature of 60 °C. The influence on lithium extraction was investigated in detail in different leaching conditions. The optimal conditions were as follows: liquid-to-solid ratio (10 mL/g), sulfuric acid concentration (0.8 mol/L), leaching time (60 min) and leaching temperature (60 °C). This yielded a lithium extraction rate of 98.52% and a lithium carbonate purity of 99.5%. It was fitted using an empirical model; the kinetics showed that internal diffusion control conformed to the low-acid leaching reaction, which had an apparent activation energy of 10.81 kJ/mol for lithium. The total profit from the whole process was USD 0.2576 when dealing with 1.0 kg of REMSES. Moreover, in the sulfuric acid system, the leaching reaction mechanism was carefully investigated between 30 and 90 °C. An innovative process of recovering lithium from REMSES was achieved with environmental friendliness and good economic returns. Compared to traditional leaching using concentrated sulfuric acid, this cleaner recycling method conforms to the concept of green, low-carbon sustainable development, with high lithium selectivity, low impurity content in the filtrate and low acid consumption.

Thermodynamic Analysis and Synthesis of Metallic Molybdenum Powder via the Solution Combustion Synthesis Process

ABSTRACT This study investigates the production of metallic molybdenum powder through the solution combustion synthesis (SCS) method. Despite thermodynamic challenges posed by potential back-oxidation under equilibrium conditions, the non-equilibrium nature of SCS facilitates the formation of metallic molybdenum. Experimental evidence suggests that Mo6+ actively participates as an oxidizer in the process, leading to the reduction of Mo6+ to Mo4+. Subsequent reduction of MoO2 is primarily driven by reactions with methane, a byproduct of the SCS process. Characterization of the gel precursor reveals the formation of a molybdenum-citric acid dimer. The resulting molybdenum powder exhibits a semi-amorphous structure with a developed specific surface area and low metallic impurity content but contains a significant amount of residual organic compounds. While heat treatment does not effectively remove these impurities, future research efforts should focus on developing purification strategies to minimize organic contamination.

Ethyl biodiesel production from crude soybean oil using enzymatic degumming-transesterification associated process

Brazil is one of the largest soybean producers in the world. Biodiesel production in Brazil (specially soybean oil biodiesel) has been growing every year and demanding more effective and sustainable technologies, which is the case of enzymatic processes. Enzymatic degumming could be an alternative to provide better quality products, before biodiesel production but also, in a one reaction step as a proposal to reduce time and costs. Therefore, this work was aimed at evaluating enzymatic degumming (previously optimized) of crude soybean oil using a phospholipase cocktail associated with transesterification using lipase from Aspergillus oryzae for ethyl biodiesel production. For this, transesterification was optimized for ethanol:oil (E:O) ratio, water and lipase % through a central composite rotatable design (CCRD). Optimal conditions were used to evaluate two degumming-transesterification associated processes: i) a one-pot reaction (OPR) where degumming and transesterification were performed at the same reactor; and ii) a two-pot reaction (TPR) where oil was first degummed, followed by transesterification. The optimal transesterification condition were achieved for E:O = 4.48:1, water = 3.41 % and lipase = 2.43 %, where 97 % fatty acid ethyl esters (FAEE) were obtained. Both OPR and TPR provided biodiesels with FAEE > 94 %: TPR was the best with 97.5 % and 99.98 % before and after biodiesel purification. Mineral elements (including phosphorus) and other impurities (anions) were low, and within quality standards. Glycerol produced also presented very low content of impurities which is quite advantageous. Although lipase achieves good conversion to FAEE (95.7 ± 0.29 %) using crude oil (Control), the final biodiesel carries many impurities (P=80.07 ± 0.1 mg/kg), thus requiring subsequent biodiesel purification steps. In addition, the high impurity content generates a biodiesel that does not comply with ANP legislative standards, P<10 mg/kg. The use of enzymatic degumming in the biodiesel production process generates a biodiesel with low impurities and higher final quality, in addition to being a process that generates less effluent. Enzymatic degumming was essential for obtaining high quality biodiesel and its association with transesterification showed to be a great option for decreasing time and costs for biodiesel production.

New promising antiseptics for individual antiviral protection against seasonal infections including COVID-19

Seasonal viral infections of the upper respiratory tract are widespread throughout the world. The SARS-CoV-2 pandemic has revealed the importance of timely personalized prevention of acute viral diseases and related bacterial complications. One of the main directions in the preventing viral infections relies on the development of effective antiseptics to inactivate viruses on the hands and mucous membranes. To effectively destroy SARS-CoV-2, the WHO recommends two antiseptics: a 70% ethanol solution and sodium hypochlorite. The proposed antiseptics are outdated and have an irritating effect on the skin and mucous membranes. In connection with this, a promising research direction includes development of new antiseptics with selective toxicity, low volatility, low coefficients in the oil-water system, weak lipophilic properties, low absorption into the vacuolar skin structures and low content of toxic impurities. One of these antiseptics is anavidin. The development and selection of molecules has been carried out, the next planned stage will be assessment of the effectiveness and safety of new molecules, including preclinical and early phase clinical trials, also analyzing prevention of viral infections, e.g. COVID-19.

Enhanced reliability of Cu-Sn bonding through the microstructure evolution of nanotwinned copper

Kirkendall voids are detrimental to the Cu-Sn bonding interface, causing the failure of the high-density package. Herein, the perpendicularly aligned nanotwinned Cu (p-ntCu) and the horizontally aligned nanotwinned Cu (h-ntCu) are prepared by controlling the electrodeposition procedure. The p-ntCu shows advantages both in fast-bonding process and in void suppression through the in-situ microstructure evolution. Compared with h-ntCu, the abundant perpendicularly aligned twin boundaries in p-ntCu provide fast interdiffusion paths to build a bonding interface. In the bonding process, p-ntCu grows to ultra-large-grained Cu with an average grain size of 6.68 μm. The reduced density of normal grain boundaries in p-ntCu lowers the Cu diffusion rate and contributes to more balanced interdiffusion at the bonding interface, which is confirmed by molecular dynamics simulation and kinetic calculations of intermetallic compound (IMC) growth. In addition, the low impurity content in p-ntCu further reduces the diffusion flux imbalance and limits the nucleation of Kirkendall vacancies. Consequently, the p-ntCu/Sn interface keeps void-free during 150 °C long-term thermal aging due to the synergistic effect of reduced grain-boundary diffusion and lower impurity content, which will be beneficial for achieving high-reliability Cu-Sn bonding.

Pilot-scale study of methane-assisted catalytic bitumen partial upgrading

The direct utilization of heavy and extra-heavy crude oils presents a formidable challenge due to their inherent physical and chemical properties such as high C/H ratio, extremely high viscosity and density, low APIo, super low mobility, high asphaltene and impurity (Fe, Ni, Co, S, N, etc.) contents. To tackle these problems cost-effectively, we have proposed and established a novel technique, distinct from conventional hydrotreating, for catalytic partial upgrading of extra heavy crudes with co-fed methane and a multi-functional catalyst. This technique has been further optimized using lab-scale batch reactors (100 mL, 300 mL), bench-scale and pilot-scale fixed bed reactors with their processing capacity of 250 mL/day and 20 L/day, respectively. The feasibility, stability, and profitability of this technique have been successfully verified using all these facilities and a wide variety of feedstock. Yet, further scale-up is necessary to advance this technique towards commercialization in industry. In this study, a pilot-scale prototype unit (processing capacity of 1 barrel/day) was designed and manufactured based upon the previous achievements, and a bitumen sample recovered from the Steam Assisted Gravity Drainage (SAGD) process was chosen as a typical extra heavy crude for the upgrading. A 30-day upgrading has been conducted smoothly without clogging and a liquid yield of 96.7 % was observed with remarkable enhancements in product quality. The notable decreases in density, viscosity, TAN, asphaltene content, and sulfur content were confirmed and consistent with previous results. A low olefin content implies excellent stability and compatibility of the liquid product. Additionally, a preliminary TEA (Techno-Economic Assessment) and LCA (Life-Cycle Analysis) have been conducted and the beneficial features of this novel technique have been confirmed with higher profitability, lower cost, and lower carbon footprint. This study further consolidates the advantages of this promising technique as a cost-effective and environmentally friendly alternative to hydrotreating for processing extra heavy crudes.

Experimental studies of bearing capacity of pipe steel of sewage systems

The results of experimental studies on the kinetics of crack growth, its relationship with the parameters of crack resistance and long-term strength of pipe steels of different service life and their structural and phase composition are presented. It was found that the value of the critical stress SK for all test steels increases with the increase in the service life, and the impact viscosity decreases, which indicates the structural embrittlement of the pipe steels associated with their sudden flooding. It is shown that the new 06G2BA steel, which is economically modified with a carbide-forming element (vanadium) and has a fine-grained structure and a low content of harmful impurities (sulfur, phosphorus), has the highest visco-plastic properties and resistance to brittle fracture. The microstrain of the α-Fe crystal lattice, as well as the quantitative decay of cementite and the redistribution of carbon between ferrite and pearlite, were evaluated by X-ray structural methods. 06G2BA steel is recommended for use in the construction of pipelines and, for example, bridge structures, which are constantly under cyclic loads with simultaneous contact with a corrosive-aggressive environment. The influence of the service life of water pipes on the hydrogen content and microcracks in pipe steels was determined. A diagram of the relationship between long-term and static strength depending on the hydrogen content in steels is recommended, which can be used by designers to rationally choose the type of steels with high crack resistance in aggressive technological environments. In order to extend the service life of pipe structures, it is necessary to use economically modified steels.

The superior adsorption capacity of biogenic α-Mn2O3 in comparison to chemogenic α-Mn2O3 for five divalent heavy metal cations and the adsorption mechanism of biogenic α-Mn2O3: Experimental and DFT investigations

Manganese oxides play a crucial role in the adsorption of heavy metals in the environment. However, there is currently a lack of information regarding the comparative adsorption efficiency of heavy metals between biogenic and chemogenic manganese oxides. In this work, we compared the adsorption performance of biogenic and chemogenic α-Mn2O3 towards Pb(II), Cd(II), Cu(II), Zn(II) and Ni(II). Mineral characterization analyses revealed that biogenic α-Mn2O3 exhibited a polycrystalline nature, whereas chemogenic α-Mn2O3 displayed a monocrystalline nature with a lower impurity content compared to biogenic α-Mn2O3. Under acidic conditions, biogenic α-Mn2O3 demonstrated a significantly higher adsorption capacity, ranging from 3 to 6 times greater than its chemogenic counterpart, within an initial heavy metal concentration range of 0.2–1.0 mM. This superiority can be attributed to the presence of more abundant functional groups and a greater content of hydroxyl O species on the surface of biogenic α-Mn2O3 in comparison to chemogenic α-Mn2O3. During the adsorption process of biogenic α-Mn2O3, there were observed ionic exchanges between Mn(II) and these cations. More importantly, biogenic α-Mn2O3 bound with these cations probably through the formation of -O/OH/S complexes. The adsorption selectivity of biogenic α-Mn2O3 towards the five heavy metals was experimentally determined to follow the order of Ni(II) > Pb(II) > Cu(II) > Zn(II) > Cd(II). The electronic interaction between biogenic α-Mn2O3 and these cations indicated that Ni(II), Pb(II) and Cu(II) were more likely to share electrons with O atoms on the surface of biogenic α-Mn2O3 compared to Zn(II) and Cd(II).

TECXY simulations of the power exhaust in the multi-impurity plasma of DTT reactor

Reduction of the heat load to plasma-facing components is a crucial problem for future fusion reactors like Divertor Test Tokamak (DTT). Mitigation of the power load via increased plasma radiation with the use of puffed ions of impurities would be one way to mitigate power in the scrape-off layer. This paper presents a numerical investigation of the impact of seeded impurities on the radiation pattern and the power load to the divertor plates of the high-field DTT reactor in the single null (SN) configuration. The simulations have been done with the use of the TECXY code, which solves multi-species plasma transport equations for multiple impurity species simultaneously and all associated ionization stages in a two-dimensional poloidal geometry. TECXY represents the model of plasma transport in the scrape-off layer region by a classical set of transport equations of multi-species plasma derived by Braginskii. The paper aims to compare the mitigation capabilities of neon and argon impurities seeded in the plasma of the DTT device and to obtain a significant energy flux reduction to the target plate at the smallest possible impurity concentration. Performed investigations showed the effects of neon and argon impurities seeding separately for constant electron density at the separatrix. It has been found that the decrease in electron temperature on the divertor plates up to 3 eV at the outer and the inner divertor plates and the peak power load below 15 MW m−2 at the outer divertor plate can be achieved with argon seeding and much lower impurity concentration than that in the case of neon impurity seeding. Studies have shown the complexity of the effect of neon and argon impurities on the boundary plasma. It was found that the reduction of temperature and the power on both divertor plates was the most effective for the high upstream plasma densities. The results show also that diffusive perpendicular transport strongly affects impurity radiation and thus plasma condition at divertor plates.

Large-scale production of cefazolin in a microreactor with a low impurity content and high yield

Cefazolin is crucial in the field of healthcare because of its efficacy against bacterial diseases, but trace amounts of polymer impurities produced during synthesis may cause severe allergic reactions. In this work, an efficient system based on a membrane dispersion microreactor was designed for scalable cefazolin production with a high yield and low impurity content. At a production scale of 200 kg, the yield could reach 90 %, and the polymer impurity content could reach <0.008 %. Density functional theory calculations were used to investigate the formation mechanisms of the polymer impurities. The results revealed that the activation energy of cefazolin ring-opening and substitution reactions decreased by 22.4 kJ/mol and 6.4 kJ/mol respectively under alkaline conditions, resulting in increased polymer content. Computational fluid dynamics simulation indicates that the organic base in the substrate can be rapidly dispersed in the membrane dispersion microreactor, thus avoiding excessive local base concentration. In conclusion, the development of a membrane dispersion microreactor system offers a scalable and efficient method for cefazolin production with significantly lower polymer impurity content, thereby addressing a critical challenge in antibiotic synthesis and ensuring safer and more reliable healthcare applications.

Strength comparison for fully dense zirconium diboride ceramics tested by different methods

AbstractThe strength of zirconium diboride ceramics was tested by three different methods, 3‐point flexure, 4‐point flexure, and compression. Nearly full‐density ceramics were obtained by hot pressing commercial ZrB2 powder with the addition of .5 wt.% carbon as a sintering aid. The thermal properties and hardness were studied for ZrB2 milled with ZrB2 and WC media. Based on phase purity and higher thermal conductivity, ZrB2 ceramics prepared from powders milled with ZrB2 media were selected for mechanical property studies. The strength in 3‐point flexure was 546 ± 55 MPa. The flexure strength was 476 ± 41 MPa in 4‐point bending, which was ∼20% higher than the previously reported value of 398 MPa for ZrB2 with similar grain sizes due to higher relative density and lower impurity contents. Compression testing was performed at room temperature, and the strength was 1110 ± 358 MPa. Finally, the fracture toughness of pure ZrB2 ceramics was determined by the chevron‐notched beam method to be 3.6 ± .7 MPa m1/2. The strength and fracture toughness values are higher than those previously published for ZrB2 ceramics and can be attributed to higher density and lower grain size. The strength‐limiting flaw sizes were comparable to the grain size, suggesting that porosity and impurity phases did not play a significant role in the strength of these ceramics.

Enhancement of Signal‐to‐Background Ratio in Molecular Vibrational Signal Extraction by Stimulated Emission Depletion Mechanism

Herein, a novel approach is presented to mitigate the fluorescence interference during the detection of vibrational signal via the stimulated emission depletion (STED). STED is the mechanism commonly employed in optical imaging; however, its application should not be confined solely to this field. To explore additional possibilities, a novel application of STED in vibrational spectroscopy detection is introduced. Vibrational spectroscopy is a widely used technique for the material detection and identification, but its sensitivity is influenced by impurity signals, especially the fluorescence. The proposed method is capable of suppressing fluorescence without influencing vibrational signal. At the low concentration of fluorescent impurities, the signal‐to‐background ratio of vibrational spectroscopy is 2.6 times as high as that without this method. The introduction of depletion light can enhance the detection of vibrational signals, resulting in more optimal signal detection. A promising new application of STED other than super‐resolution imaging is investigated.

Preparation and properties of especially pure Ge-Sb-As-S glasses for IR optics

The novel multi-stage method for preparation of especially pure GexAsySbzS100-x-y-z (x = 10–13; y = 20–23; z = 4–6) glasses is presented. The method includes the stages of deep purification of initial elements using different chemical getters, transport reactions and vacuum distillation. The glass samples with compositions close to stoichiometric ones and a low content of gas-forming impurities are synthesized; their structure, optical, thermal and magneto-optical properties are investigated. The studied characteristics of the samples meet the requirements for their use in the IR optics and are relevant for multifunctional applications.

Complex-shaped titanium components fabricated via metal injection molding using hydride-dehydride titanium powder

Metal injection molding (MIM) is a net-shape manufacturing method that lessens the costs of production and increases the affordability of titanium goods. A range of CP-Ti specimens was prepared to confirm the primary manufacturing parameters for titanium metal injection molding utilizing affordable hydride-dehydride (HDH) powders. Mixture of powder and polyformaldehyde-based binder was injection molded, debonded, and then sintered. The optimum powder loading is 55 vol%. The ideal conditions of catalytic debinding were attained by placing the specimens in the catalytic debinding system at 120 °C temperature. Samples with minimal deformation and low impurity content were obtained when thermal debinding was carried out at a modest heating rate of 1.0 K/min and a holding time of 1.5 h. All the sintered samples have a similar microstructure, that is, lamellar α + β structure. The sample exhibits a strength of 739 MPa and an elongation of 6.1 % at the optimum sintering conditions (sintering at 1250 °C for 2 h). The samples prepared by MIM have excellent wear resistance. This process provided a straightforward and cost-effective means of producing complex-shaped titanium components from HDH Ti powder.

Ultrafine-grained refractory high-entropy alloy with oxygen control and high mechanical performance

Grain boundary engineering plays a significant role in the improvement of strength and plasticity of alloys. However, in refractory high-entropy alloys, the susceptibility of grain boundaries to oxygen presents a bottleneck in achieving high mechanical performance. Creating a large number of clean grain boundaries in refractory high-entropy alloys is a challenge. In this study, an ultrafine-grained (UFG) NbMoTaW alloy with high grain-boundary cohesion was prepared by powder metallurgy, taking advantages of rapid hot-pressing sintering and full-process inert atmosphere protection from powder synthesis to sintering. By oxygen control and an increase in the proportion of grain boundaries, the segregation of oxygen and formation of oxides at grain boundaries were strongly mitigated, thus the intrinsic high cohesion of the interfaces was preserved. Compared to the coarse-grained alloys prepared by arc-melting and those sintered by traditional powder metallurgy methods, the UFG NbMoTaW alloy demonstrated simultaneously increased strength and plasticity at ambient temperature. The highly cohesive grain boundaries not only reduce brittle fractures effectively but also promote intragranular deformation. Consequently, the UFG NbMoTaW alloy achieved a high yield strength even at elevated temperatures, with a remarkable performance of 1117 MPa at 1200 °C. This work provides a feasible solution for producing refractory high-entropy alloys with low impurity content, refined microstructure, and excellent mechanical performance.

Sulfuric acid leaching of ferronickel and preparation of precursor materials for power batteries: Strategy of enhanced leaching through thermal activation and its mechanism

With the development of the new energy industry, the demand for iron phosphate and nickel–cobalt hydroxide precursor materials in the power battery industry has increased sharply. Ferronickel with low content of other impurities, derived from electric furnace smelting of laterite nickel ore, contains iron, nickel, and cobalt components used for the production of two kinds of power batteries. In this study, the sulfuric acid leaching behaviors of the water quenched ferronickel particles under atmospheric pressure was investigated. Practically, the direct leaching performance of these ferronickel particles in sulfuric acid is unsatisfactory. The leaching of Ni, Co and Fe was only 4.52 %, 3.58 % and 3.83 %, respectively, at the initial acid concentration of 3 mol/L, liquid–solid ratio of 12 mL/g, leaching temperature of 95 °C and time of 4 h. However, through thermal activation, the face-centered cubic (FCC) Fe-Ni phase was transformed into the body-centered cubic (BCC) structure, which significantly improved its acidic leaching activity. Under the thermal activation at 800 °C for 2 h, the leaching of Ni, Co and Fe reached 91.27 %, 90.12 % and 89.19 %, respectively, at the same leaching conditions. After oxidizing the Fe(II) in lixivium to Fe(III) using hydrogen peroxide, 89.3 % of Fe was selectively precipitated into battery-grade iron phosphate by adding phosphoric acid and coordinating solution pH. Nickel and cobalt in residual solution was then recovered to nickel–cobalt hydroxide by precipitation after removing impurity ions in advance. This study provides a new pathway for the value-added utilization of ferronickel in the power battery industry, not only in the production of stainless steel.

Isolation, screening, and characterization of polyhydroxyalkanoate (PHA)‐producing Cellulosimicrobium cellulans: A promising approach for eco‐friendly biopolymer production

AbstractThe replacement of synthetic plastics with eco‐friendly biopolymers is a notable approach in the modern world. Polyhydroxyalkanoates (PHAs) have gained a lot of attention among researchers in recent decades. This study aimed to isolate and screen microorganisms capable of producing PHA from composite samples. Several composting unit samples were collected and used to isolate 15 distinct colonies which were then screened for PHA synthesis. Three isolates‐SS2, SSNA 1a, and SF2‐showed promising PHA production. Among these isolates, SS2 had the highest PHA content of 70.1%. The isolate SS2 was determined as Cellulosimicrobium cellulans DSM 43879 by gene sequencing. The polymer produced by SS2 was further characterized. The presence of carbonyl groups and ester linkages in the PHA polymer was established by Fourier transform infrared analysis. The polymer's crystalline nature was validated by X‐ray powder diffraction measurement. Scanning electron microscope study revealed thread‐like forms with a rough surface while energy dispersive X‐ray analysis revealed a low impurity content that was primarily composed of carbon and oxygen with traces of other elements. Overall, the findings indicate C. cellulans potential as an ideal candidate for efficient PHA synthesis supporting its commercial potential in diverse applications.

Synthesis and Cation Exchange of LTA Zeolites Synthesized from Different Silicon Sources Applied in CO2 Adsorption

Zeolites have a well-ordered crystalline network with pores controlled in the synthesis process. Their composition comprises silicon and aluminum, so industrial residues with this composition can be used for the synthesis of zeolites. The use of zeolites for CO2 adsorption is feasible due to the characteristics that these materials have; in particular, zeolites with a low Si/Al ratio have greater gas adsorption capacities. In this work, the synthesis of LTA (Linde Type A) zeolites from silica fumes obtained from the industrial LIASA process and light coal ash is presented. We explore three different synthesis routes, where the synthesized materials undergo cation exchange and are applied in CO2 adsorption processes. Studying the synthesis processes, it is observed that all materials present characteristic diffractions for the LTA zeolite, as well as presenting specific areas between 6 and 19 m2/g and average pore distributions of 0.50 nm; however, the silica fume yielded better synthesis results, due to its lower impurity content compared to the light coal ash (which contains impurities such as quartz present in the zeolite). When applied for CO2 adsorption, the standard materials after cation exchange showed greater adsorption capacities, followed by the zeolites synthesized from silica fume and, finally, the zeolites synthesized from coal ash. By analyzing the selectivity of the materials for CO2/N2, it is observed that the materials in sodium form present greater selectivity when compared to the calcium-based materials.

An optimisation study for leaching synthetic scheelite in H2SO4 and H2O2 solution

ABSTRACT Tungsten production primarily relies on scheelite, a secondary resource due to its complex ore composition and lower grade compared to high-grade wolframite. Synthetic scheelite gains significance for its low impurity content and accessibility in ongoing laboratory-based investigations. Recent advancements offer a feasible environment-friendly leaching method using a mixed solution of H2SO4 and H2O2 under normal pressure and moderate temperatures. However, comprehensive research on this novel method is lacking, emphasising the need for collaborative exploration and operational optimisation. The present work is to provide insights and improvements in reagent usage to promote economic and environmental sustainability. A detailed investigation into the thermal decomposition process was also conducted without compromising leaching efficiency. The findings suggest the potential for eco-friendly lixivium recycling with reduced levels of chemicals, decreasing operational costs. Notably, optimising the thermal decomposition duration to 6 hours at an L/S (mL/g) ratio of 10 enhances H2WO4 crystallization. Furthermore, experiments without H2SO4 supplementation highlight the system's optimisation potential. Finally, the leaching process was optimised by decreasing H2SO4 concentration to 1 mol/L from 3 mol/L, increasing the temperature to 60°C, and extending the leaching duration to 120 minutes. This leads to a cost-effective synthetic scheelite leaching process with environmental benefits.

Effect of impurity elements on the creep rupture strength of Gr. 91 steel welded joints at 650 °C

The upper limits for Sb, As, and Sn in Gr. 91 steel are set by the recent standardized Gr. 91 Type 2 specification. To clarify how the three impurities impact the microstructures and creep rupture strength of Gr. 91 welded joints, two types of steel with varying impurity concentrations were prepared. The first contained high impurity concentrations (Steel 1), while the other had low impurity concentrations (Steel 2). The results of the creep tests on the welded joints showed that Steel 1 had lower creep rupture strength compared to Steel 2, indicating premature failure caused by the impurities. Comparing the microstructures in the as-welded joints, it was observed that Steel 1 had more M23C6 particles in the fine-grained heat affected zone (FGHAZ) than Steel 2. This finding indicates that M23C6 dissolution during the welding process was hindered in Steel 1. It is believed that M23C6 dissolution was reduced due to the segregation of impurities at the grain boundaries or the interface between the matrix and M23C6 particles. Consequently, the microstructure in FGHAZ of Steel 1 showed lower creep resistance because the pinning force by M23C6 was decreased due to the reduction of re-precipitation on the grain boundaries during post weld heat treatment. The degradation in the creep rupture strength due to the impurities was attributed to the earlier progression of recovery process in FGHAZ during creep, leading to premature creep void formation.