Precipitation Of Impurities Research Articles

Ведение ВХР с применением реагентов ВТИАМИН на ТЭЦ с сезонным режимом работы

The main equipment of the thermal power plant (TPP) includes a E-120-7.0-500GM steam boiler produced by LLC “Belenergomash – BZEM” with a single-stage evaporation circuit. The operating mode of the thermal power plant has seasonal characteristics. During the period of February to the end of August the equipment is stopped as a reserve. Dosing of AMINAT PK-3 does not provide the required pH level in steam and feed water, so the reagent for water-chemical regime and preservation has been substituted with VTIAMIN KR-33. The complex reagent VTIAMIN KR-33 ensures both the maintenance of the required quality indicators of the coolant along the path and promotes the formation of a protective barrier film on the internal heating surfaces. To check water-chemical regime, the existing automatic chemical control system has been updated. Panels have been installed for automatic chemical control of pH value, electrical conductivity of direct and H-cationized samples. The filter material has been replaced and the mechanical filters have been connected. The first stage of the filter has been replaced, the second stage of the filter and final purification filters Н-OH have been installed. The authors have demonstrated the possibility to use boilers with seasonal operating modes and measuring the electrical conductivity of cooled water and steam samples as a chemical control of water chemistry. It is found out that dosing the VTIAMIN KR-33 reagent with periodic dosing of the VTIAMIN D-8 reagent into the boiler water prevent the precipitation of hardness salts and other impurities coming with the return condensate. Analysis of the indicators of the water chemistry regime at the thermal power plant when dosing the VTIAMIN KR-33 reagent at two points has shown that the pH value of the feed water is in the range of 8,8–9,0 and the indicators of the water chemistry regime in terms of the content of iron, silicic acid and sodium are also within the norm limits. The composition of the reagents is selected. Flexibility of water chemistry management is ensured. It has been shown that the dosage of the amine-containing reagent VTIAMIN neutralizes the negative effects of aggressive gases (carbon dioxide and oxygen) due to their binding and the formation of a protective film of the magnetite-amine type on heating surfaces. Introduction of the water chemical regime when the equipment is stopped as a reserve does not require additional preservation measures.

Modeling scalability of impurity precipitation in downstream biomanufacturing.

Precipitation during the viral inactivation, neutralization and depth filtration step of a monoclonal antibody (mAb) purification process can provide quantifiable and potentially significant impurity reduction. However, robust commercial implementation of this unit operation is limited due to the lack of a representative scale-down model to characterize the removal of impurities. The objective of this work is to compare isoelectric impurity precipitation behavior for a monoclonal antibody product across scales, from benchtop to pilot manufacturing. Scaling parameters such as agitation and vessel geometry were investigated, with the precipitate amount and particle size distribution (PSD) characterized via turbidity and flow imaging microscopy. Qualitative analysis of the data shows that maintaining a consistent energy dissipation rate (EDR) could be used for approximate scaling of vessel geometry and agitator speeds in the absence of more detailed simulation. For a more rigorous approach, however, agitation was simulated via computational fluid dynamics (CFD) and these results were applied alongside a population balance model to simulate the trajectory of the size distribution of precipitate. CFD results were analyzed within a framework of a two-compartment mixing model comprising regions of high- and low-energy agitation, with material exchange between the two. Rate terms accounting for particle formation, growth and breakage within each region were defined, accounting for dependence on turbulence. This bifurcated model was successful in capturing the variability in particle sizes over time across scales. Such an approach enhances the mechanistic understanding of impurity precipitation and provides additional tools for model-assisted prediction for process scaling.

New Processes to Extract and Purify Phosvitin by Using Aqueous Salt Solutions, Precipitation and Ultrafiltration Techniques

Phosvitin is the most phosphorylated naturally occurring protein and it is concentrated in the granular fraction of egg yolk. It has interesting antioxidant and cation chelating properties that can be used to preserve food and cosmetics, but its industrial application is limited as its separation relies on the use of organic solvents and chromatographic techniques, which are expensive and difficult to assimilate in a continuous procedure. In this study, we propose a new phosvitin separation process using substances legally accepted for use in the food industry (NaCl and HCl), employing egg yolk granules as raw material. In this case, the NaCl concentration and the pH of the solution of granules were screened in order to obtain a phosvitin-rich supernatant after centrifugation. Additionally, two new processes were proposed to purify this phosvitin-rich solution. The first was the precipitation of impurities during the desalting stage at optimized pH values. The second was ultrafiltration under selected pH value conditions. A low nitrogen/phosphorous (N/P) atomic ratio is considered a quality parameter, with 3.6 ± 0.2 being the value of the phosvitin-rich supernatant. The two purification processes provided highly purified phosvitin with a similar N/P value of 2.5 ± 0.1. The high level of purification of the phosvitin was confirmed using electrophoresis and ion-exchange chromatography. In particular, the purified phosvitin obtained via ultrafiltration is already desalted and membrane technology is more easily scalable than that based on chromatography, thus facilitating the industrial separation and commercialization of the phosvitin.

Selective leaching process for efficient and rapid recycling of spent lithium iron phosphate batteries.

With the continuous development of new energy vehicles, the number of decommissioned lithium iron phosphate (LiFePO4) batteries has been constantly increasing. Therefore, it is necessary to recover metal from spent LiFePO4 batteries due to the high potential for environmental protection and high resource value. In this study, sodium persulfate (Na2S2O8) was selected as the oxidant to regulate and control the oxidation state and proton activity of the leaching solution through its high oxidizing ability. Selective recovery of lithium from LiFePO4 batteries was achieved by oxidizing LiFePO4 to iron phosphate (FePO4) during the leaching process. This paper reports an extensive investigation of the effects of various factors, including the acid concentration, initial volume fraction of the oxidant, reaction temperature, solid-liquid ratio, and reaction time, on lithium leaching. Li+ reached a high leaching rate of 93.3% within 5 minutes even at a low concentration of sulphuric acid (H2SO4), and high-purity lithium carbonate (Li2CO3) was obtained through impurity removal and precipitation reactions. In addition, the leaching mechanism was analysed by both X-ray diffraction and X-ray photoelectron spectroscopy characterization. The results show that the obtained high lithium-ion (Li+) leaching efficiency and fast Li+ leaching time can be ascribed to the superior oxidizing properties of Na2S2O8 and the stability of the crystal structure of LiFePO4 during the oxidative leaching process. The adopted method has significant advantages in terms of safety, efficiency and environmental protection, which are conducive to the sustainable development of lithium batteries.

Paraffinic oil residues after heating as a basis for fuel

The possibility of reuse of heavy residues (oily sludge) of low-sulfur, highly paraffinic Mangyshlak oils as a basis for fuel with improved environmental characteristics for engines to marine, river and railway transport, gas turbine and boiler plants, steam boilers and industrial furnaces was shown. A variant of the joint application of the methods of nuclear magnetic resonance, laser confocal microscopy and rheology for study of micro- and mesoscopic structure and some physicochemical properties of crude oil products has been developed. It has been shown that preliminary heating to 90 ° C leads to disruption of the structure and hydrogen bonds between oil associates and complexes, increasing sample homogeneity due to precipitation of solid impurities, partial removal of water content, air bubbles and dissolved gases. The addition of medium distillate diesel fractions to preliminary heated oil residues will make it possible to obtain a higher quality version of the fuel with a reduced resinous and asphaltene substances, lower pour point, good energy content and low sulfur content. Thus, valuable energy-containing raw materials can be reused. It should also be noted that the reuse of accumulated oil waste will reduce the harmful effects of landfills, will preserve the natural landscape, and will cut the cost of extracting natural resources.

Processing of serpentenites of the Arvaten deposit of Uzbekistan with the use of ammonium sulphate

The article presents the results of a study of the processes of obtaining magnesium oxide by fusing serpentinites of the Arvaten deposit in the presence of ammonium sulfate, followed by grinding and treatment with water to separate silica from the suspension, neutralizing the filtrate with ammonia to pH 8.5 for precipitation and separation of impurity metal ions, precipitation and separation of ions magnesium from solution using ammonia and ammonium carbonate, thermal decomposition of magnesium corbanate to obtain magnesium oxide. It has been established that the use of ammonium sulfate for fusion of serpentite at a ratio of serpentenite: (NH4)2SO4 = 100: 201.77 and further processing of the resulting solutions allows you to obtain magnesium oxide with a composition (wt.%): MgO - 97.55; Fe2O3 - 0.86; A12O3 - 0.12; CaO - 0.18; SO3 - 0.48.

A Modified Method of Total RNA Isolation for Mango Leaf Tissues

Complex metabolic components in mango leaves lead to high difficulty in RNA extraction. In order to improve the quality of total RNA extraction, on the basis of the existing methods reported in the literature, an RNA extraction method combining acetone washing liquid nitrogen abrasive material with 0.3 mol/L lithium chloride (LiCl) solution and 2 times the volume of anhydrous ethanol as the second precipitator was developed. The experimental results showed that the improved method significantly reduced the amount of impurity precipitation in the RNA extraction process. The electrophoretic bands of total RNA extracted were complete. The absorbance ratio of OD 260/280  was about 1.9, and the average was 329.8 g/g FW±11.2 g/g FW. Further validation experiments of RNA reverse transcription and the polymorphic amplification of double-stranded cDNA related sequences (RAP-PCR) obtained clear polymorphic bands, showing that the total RNA from mango leaves had high quality and was suitable for molecular biology experiments.

Thermal system for a new flexible composite tank

A flexible thermal insulation system is proposed in this study, which considers the application and promotion of flexible composite materials in the field of marine engineering. This system is designed to provide a cost-effective and convenient thermal insulation solution for avoiding impurity precipitation and formation of wax plugs caused by extremely low temperature during mining, transportation, and storage of oil. The thermal insulation system includes two main parts: the insulation layer and the heating source. In accordance with the laws of thermodynamics and the primary characteristics of the flexible structure, two different insulation systems were designed and model tests were performed to compare performance in heating efficiency, thermal insulation, and temperature field distribution under identical environmental conditions. Compared with the plan 1 thermal insulation system, the plan 2 system has lower requirements for heating power, more uniform temperature field distribution inside the flexible structure, and a difference of only 7.18°C between the highest and lowest temperatures.

A review of thermodynamic data for lanthanum, iron, and thorium applied to rare earth extraction

A consolidated and revised database for thermodynamic simulation of lanthanum (representing the rare earth elements), iron, and thorium in water, sulfate, and phosphate systems was built. Thermodynamic parameters (∆Gf°, ∆Hf°, and Sf°) collected from the original sources were compared with those reported in thermochemical software databases. The inconsistencies found in the latter were mainly ascribed to typographical errors, incorrect unit conversions, and errors generated by combining data from different sources. The Eh-pH diagrams illustrate the significant effects of data discrepancies on the stability regions of aqueous and solid species. The results obtained with the revised database helped the identification of operating windows for the selective precipitation of common impurities (e.g., Fe and Th) in rare earth extraction. In the presence of sulfate, the complex LaSO4+(aq) is the only stable species up to pH = 8 and then La(OH)3(s) is formed. In phosphate solutions, lanthanum salts precipitate at lower pH. The precipitation pH varies with the nature and crystal features of the different lanthanum phosphate phases. Under oxidizing conditions, wide operational windows favor the selective precipitation of Fe2O3(s) or FePO4.2H2O(s) at pH 1–5. Thorium hydroxide is formed at pH 4. The inclusion of phosphate complexes in the simulation expands the thorium solubility region from pH 4 to 6.5 (approximately), depending on phosphate concentration.

Separation of rare earth elements from a South American ionic clay lixivium by sequential precipitation

Ionic clays, formed by natural weathering of rare earth element (REE)-containing minerals and surface adsorption of liberated ions, are a critical REE resource. Here, the sequential precipitation of impurities and REEs from the desorption of a South American ionic clay is investigated. Selective sequential precipitation profiles are constructed by stepwise addition of 1.3 M ammonium bicarbonate to the lixivium. During sequential precipitation, ammonium bicarbonate first behaves as a pH adjustor to remove co-extracted impurities as hydroxides, and then behaves as both a carbonate ion source and a pH adjustor to precipitate the REEs as carbonates. Impurity precipitation is completed at pH 6.0 and REE precipitation is completed at pH 6–7.5; however, total carbonate concentration during the impurity precipitation step must be kept low to prevent REE loss. To gain a better understanding of the REE precipitation step, thermodynamic calculations using the Davis model are performed. Dysprosium is studied as a model REE and its speciation diagram as a function of pH and total carbonate and sulfate ion concentrations is constructed. It is found that the primary factors controlling REE precipitation are pH, carbonate, and sulfate concentrations. The knowledge gained in this work provides a deeper understanding of the sequential impurity and REE precipitation processes from the desorption lixivium of a unique ionic clay from South America. The results would help guide future work on REE separation from this distinct ionic clay.

Cyclic thermal shock resistance for MgAlON–MgO composites obtained with additions of spent MgO–C brick: Microstructure characteristics, thermal shock parameter and thermal shock mechanism

Thermal shock parameters (R, R''', R'''' and Rst) of MgAlON–MgO composites obtained with additions of spent MgO–C brick were calculated using measured mechanical properties and thermal expansion coefficient, determining their resistance to fracture initiation and crack propagation. The cyclic thermal shock experiments of MgAlON–MgO composites performed from 1398 K to ambient temperature indicate that as number of thermal shock cycle increases, retained strength ratio of MgAlON and MgAlON–4.2 wt%MgO sharply decrease and then keep constant, while that of MgAlON–10.5 wt%MgO and MgAlON–15.7 wt%MgO slowly decrease. The reason for the difference is that MgAlON and MgAlON–4.2 wt%MgO show low value of R''' and R'''', and high value of R and Rst. Moreover, precipitation of impurity containing Fe may play a positive role in improvement of thermal shock resistance of MgAlON–MgO composites. MgAlON‒4.2 wt%MgO has the maximum retained strength (55 MPa) even after 5 thermal shock cycles, which is expected to be used in the metallurgical industry.

High-Temperature Precipitation of Impurities in Metals

High-Temperature Precipitation of Impurities in Metals

Physicochemical foundations of the pioneering technology for the carbonaceous uranium ore processing

The article presents the results of the first known industrial trials for the processing of uranium-containing slag obtained during blast-furnace smelting of carbonaceous iron-uranium ore (Zhovtorichenske deposit, Ukraine). It is shown that the fractional precipitation of iron and aluminum impurities allows reducing the losses of uranium and increasing the degree of utilization of nitrate nitrogen and ammonia nitrogen, additionally producing the ammonium nitrate as fertilizer. The scheme for chemical concentrate processing ensured a significant increase in the degree of uranium deposition into the finished product – up to 60%. Nitric acid sludge processing technology made it possible to maintain the water balance and completely prevented the discharge of off-balance effluents into the river Dnipro.

Employing Constant Rate Filtration To Assess Active Pharmaceutical Ingredient Washing Efficiency.

Washing is a key step in pharmaceutical isolation to remove unwanted crystallization solvents and dissolved impurities (mother liquor) from the active pharmaceutical ingredient (API) filter cake to ensure the purity of the product whilst maximizing yield. It is therefore essential to avoid both product dissolution and impurity precipitation during washing, especially precipitation of impurities caused by the wash solvent acting as an antisolvent, affecting purity and causing agglomerate formation. This work investigates the wash solvent flow through a saturated filter cake to optimize washing by displacement, taking account of diffusional mechanisms and manipulating the wash contact time. Constant rate filtration/washing is employed in this study using readily available laboratory equipment. One advantage of using constant rate filtration in this work is that it allows for the collection of separate aliquots during all stages of filtration, washing, and deliquoring of the API cake. This enables a wash profile to be obtained, as well as providing an overall picture on the mass of API lost during isolation and so can assist in optimizing the washing strategy. Particle size analysis of damp cake obtained straight after washing is also performed using laser diffraction. This allowed for agglomerate formation caused during washing to be distinguished from agglomeration that would be caused by subsequent drying of the wet filter cake. This work aims at improving pharmaceutical product quality, increasing sustainability, and reducing manufacturing cost.

Aluminum particle production on alumina rod surface by continuous-wave laser ablation

Continuous-wave laser ablation has been applied to alumina reduction with the eventual goal of lunar resource utilization. Aluminum particles were detected on the alumina rod surface after laser ablation. Impurity precipitation has been proposed as the aluminum particle production mechanism. For this study, distributions of aluminum particles and temperatures after laser ablation on an alumina rod surface were measured using SEM and two-color method. Laser power affected the aluminum particle distribution. The mean aluminum particle radius varied: 0.11–1.7 μm. The minimum value was consistent with the critical radius for aluminum nucleation in heterogeneous nucleation. The time history of surface temperatures was found to be independent of laser power. Therefore, aluminum particle production is expected to change with oxygen deficiency, and to increase with laser power. The obtained condition for aluminum particle production is applicable for an aluminum collection system in the alumina reduction method using laser ablation: the plate temperature of T = 933–2300 K and the cooling rate of 600–800 K/s.

Enhanced boron removal by CrMnFeNi-based high-entropy alloys during purification of silicon

• CrMnFeNi-based HEAs was employed as a novel impurity getter for MG-Si purification. • Refined Si crystals can be better agglomerated and separated. • The maximum distribution ratio of B between alloy and Si-enriched region was 19.9. • The segregation coefficient of B in Si-HEA melts is reduced to 0.036. A novel approach is proposed to prepare high-purity Si by enhancing B removal from metallurgical-grade Si with high-entropy alloys as solvents. CrMnFeNiM (M = Al, Co, Ti, and Cu) alloy is melted with metallurgical-grade Si at 1773 K. After a period of holding time, the recrystallization of high-purity Si is realized by electromagnetic directional solidification. High-purity Si enrich in the upper region of the sample, and impurities precipitate in the bottom Si-based alloy. The enrichment percentage of Si is more than 92.9%, up to 97.4%. The maximum distribution ratio and minimum segregation coefficient of B are obtained in Si-CrMnFeNiAl solvent, which are 19.9 and 0.036, respectively. The primary cause why Si-20 at. %CrMnFeNiAl solvent stands out is that the activity coefficients of metals and B in the melt are large, which makes the formation of boride easier. This method provides a potential possibility for improving the B removal effect and reducing the cost to prepare high-purity Si.

Separation of cobalt, nickel, and manganese in leach solutions of waste lithium-ion batteries using Dowex M4195 ion exchange resin

Current methods to separate and recover critical metals from waste lithium-ion battery (LIB) scrap require multiple unit operations that involve selective leaching and a combination of separation methods. These methods include impurity removal, solvent extraction, ion exchange, and precipitation, to achieve high purity Co and Ni products. In this paper an efficient extraction strategy was developed to individually separate Ni and Co from a LIB leachate that was generated from an electrochemical leaching process. By using Dowex M4195 resin, as the only method of separation, additional impurity removal operations were not needed. After loading and eluting, three different fractions were obtained: 99.0% Ni concentrate, 98.5% Co concentrate, and a Li/Mn rich concentrate allowing the separation of Ni, Co and Mn. The developed process is then presented as an alternative for metal recovery from leachates from scrap LIBs with minimum chemical addition and pH adjustment.

High performance of Er-doped Sb2Te material used in phase change memory

To date, operation speed and data retention are still bottlenecks for phase-change memory. Doping impurity is a practical strategy to improve these performances of phase-change memory. Here, to avoid the precipitation of impurity, we choose erbium (Er) dopant due to the most matched structure with parental Sb2Te (EST), which is manifested by both experimental and simulation results that Er locates at lattice position. The device tests show the excellent comprehensive performances: operation speed as fast as 6 ns and 10-year data retention as high as 121 °C. This excellent amorphous thermal stability stems from the strong ErTe bonds, and these stable local patterns stabilize glassy states and result in high stability. Our study not only demonstrates that the performance of designed EST meets the requirement of phase-change memory, but also provides general doping rules to obtain better comprehensive performance.

(Invited) Use of Secondary Materials in NMC Co-Precipitation: Effect of Impurities on the Cell Performance

The growing demand for raw materials in lithium-ion batteries increasingly requires the increased use of secondary raw materials in the co-precipitation of NMC battery chemicals. Traditionally, pure raw materials as metal solutions are required for co-precipitation, which is a challenge for battery producers. Removal of impurities is also expensive and challenging step. Aim of this research is to study the impact of impurities present in metal sulfate solutions on the battery cell performance. These metal sulfate solutions were obtained from metal industry’s side-stream and from recycled spent batteries.Metal sulfate solutions with known amount of impurities were used in (Ni0.8Co0.1Mn0.1)OH2 (NMC811) precursor co-precipitation. Commercial pure manganese and nickel sulfates were used as reference samples. Key impurities in the solutions were zinc, iron, calcium, potassium, lithium, and magnesium. Composition of samples was determined using ICP-OES. We also demonstrate actions to control precipitation of impurities during the NCM precursor co-precipitation. Solubilities of impurities were calculated using MEDUSA and HYDRA.Dried hydroxide precursor (sieved <25µm) was mixed with LiOH in molar ratio of Li:(Ni0.8Co0.1Mn0.1:Al) 1.04:0.996. LiOH excess was used to compensate lithium loss during high temperature calcination and ensure homogenous lithiation. The mixture calcined with 2.5°C/min heating ramp and 6 hours holding time at 800 °C under O2 flow. Material was milled and sieved <40µm in the dry room conditions. Residual lithium washed from surface with certain amount of de-ionized water and dried in vacuum oven.Cathode slurry was mixed with Thinky ARE 250 mixer. Slurry composition was 4% of PVDF, 4% carbon and 92% active material and NMP as solvent. Cathode slurry was spread on aluminum foil with 100 µm applicator, then dried at 50 °C for few hours and finally at vacuum oven at 120 °C overnight. Cathode foil was calendered three times before coin cell assembly. The active material loading on the foil was about 12 mg/cm2. Theoretical capacity used to calculate the C-rate was 200 mAh/g. Electrochemical performance was carried out using one electrode pair pouch cell (50 mAh) was prepared with graphite anode, and electrolyte was 1.15M LIPF6 in EC:DMC:EMC (2:4:4) and 1 % of vinylane carbonate. All foils and coin cells were prepared in the dry room. Cells tested between 4.2V-2.8V. 1100 cycles at 1C/1C currents including capacity check every 100 cycles at 0.2C/0.2C currents. Before the capacity check, cell was discharged at 0.2C.The microstructure shown by field emission scanning electron microscope (FESEM) images was obtained using a Zeiss Sigma FESEM operating at 5kV at the Centre for Material Analysis of the University of Oulu. X-ray diffraction (XRD) analysis was done for the calcined NCM811 cathode materials by using Rigaku SmartLab 9 kW with Co-Kα radiation at 40 kV and 135 mA. Diffractograms were collected in the 2θ range (5–120° at 0.01° intervals). Diffraction peaks and crystallite parameters were identified using International Centre for Diffraction Data (PDF-4+ 2020). The specific surface areas, pore sizes and pore volumes of samples were determined from nitrogen adsorption desorption isotherms at the temperature of liquid nitrogen (−196 ◦C) using a Micromeritics ASAP 2020.The tapped density of powders was measured based on the ISO EN 787/11 standard. Residual lithium was measured with an automatic titrator. Particle size distribution (PSD) during the co-precipitation was measured with a Malvern Mastersizer 3000.Based on the results, co-precipitation of impurities to the NMC particles followed the thermodynamic calculations. As expected, iron, zinc, magnesium and calcium co-precipitated in NMC811 to a large extent, but potassium and lithium remain in the mother liquor. The presence of impurities was also confirmed by several characterizations. Impurities did not affect the particle morphology or tap density of NMC811. Surprisingly, the effect of impurities was also minor to the cell performance. During the cycling, these cells provided good cyclability and remain high retention (>80%) after 1100 cycles compared to reference samples.

Temperature-dependent near-surface interstitial segregation in niobium

Niobium’s superconducting properties are affected by the presence and precipitation of impurities in the near-surface region. A systematic wide-temperature range x-ray diffraction study is presented addressing the effect of low temperatures (108 K–130 K) and annealing treatments (523 K in nitrogen atmosphere, 400 K in UHV) on the near-surface region of a hydrogen-loaded Nb(100) single-crystal. Under these conditions, the response of the natural surface oxides (Nb2O5, NbO2, and NbO) and the changes in the subsurface concentration of interstitial species in Nb are explored, thereby including the cryogenic temperature regime relevant for device operation. The formation and suppression of niobium hydrides in such conditions are also investigated. These treatments are shown to result in: (i) an increase in the concentration of interstitial species (oxygen and nitrogen) occupying the octahedral sites of the Nb bcc lattice at room temperature, both in the near-surface region and in the bulk. (ii) A decrease in the concentration of interstitials within the first 10 nm from the surface at 130 K. (iii) Hydride formation suppression at temperatures as low as 130 K. These results show that mild annealing in nitrogen atmosphere can suppress the formation of superconducting-detrimental niobium hydrides, while subsurface interstitial atoms tend to segregate towards the surface at 130 K, therefore altering the local concentration of impurities within the RF penetration depth of Nb. These processes are discussed in the context of the improvement of niobium superconducting radio-frequency cavities for next-generation particle accelerators.