Precipitation Of MgO Research Articles

Experimental Studies on the Removal of Chromium from Tannery Wastewater using Chemical Precipitation and Adsorption Techniques

Despite trivalent chromium’s manifold functionalities in industrial manufacturing processes, when it is improperly treated and disposed to the environment, due to external environmental factors it could get converted to hexavalent chromium that has toxic effects on humans and our environment. Hence, there is a stringent need to optimize the current industrial practices of chromium handling, management, and disposal. This paper puts forth the inferences from comparative studies conducted using chemical precipitation and biochar adsorption to ascertain if the current chromium removal practice in the chrome tanning leather industry under study is an optimal method or not. Results indicate that the current industrial practice of achieving 86~90 % removal efficiency using 2000 mg/L of MgO at pH6 can be optimized by carrying out chemical precipitation at pH8 for 96~99% removal efficiency; a combination of NaOH and Ca(OH)2 in varying ratios resulted in 80-96 % removal efficiency at pH7 and ~99% at all the ratios at pH8, but the sludge volumes were higher than twice the volumes generated by MgO precipitation; inactivated biochar in similar proportions to that of chemical precipitants did not yield encouraging results and the maximum removal efficiency was 73.2% at 48000 mg/L dosage. Therefore, if not for complete treatment of tannery wastewater with biochar, residual chromium in the supernatant should be treated with biochar as it could be effective in treating lower concentrations of chromium.

Pretreatment of Biogas Slurry by Modified Biochars to Promote High-Value Treatment of Wastewater by Microalgae

High concentrations of contaminants such as ammonia nitrogen and organic matter in full-strength wastewater severely inhibit the growth of microalgae, contributing to lower biomass accumulation and contaminant removal efficiency. To overcome this limitation, modified biochars prepared from pine sawdust and sugarcane bagasse were used in this study as an adsorbent–desorbent for the pretreatment of wastewater to promote the growth of microalgae. The results showed that the two modification methods (acid/alkaline modification and magnesium salt modification) used in the experiment could increase the abundance of oxygen-containing functional groups. Moreover, magnesium salt modification could effectively improve the pore structure of biochar surfaces and increase the specific surface areas. Compared with the pristine biochars, the adsorption performance of the modified biochar was found to be significantly higher for nutrients in wastewater. The adsorption capacity of the acid/alkaline-modified pine sawdust biochar reached 8.5 and 16.49 mg∙g−1 for ammonia nitrogen and total organic carbon in wastewater, respectively. The magnesium salt modified pine sawdust biochar achieved a more comprehensive nutrients adsorption capacity of 15.68, 14.39, and 3.68 mg∙L−1 for ammonia nitrogen, total organic carbon, and total phosphorus, respectively. The mechanism of ammonia nitrogen adsorption was mainly the complexation of surface -OH functional groups, while the adsorption mechanism for phosphate was mainly the complexation of -OH and Mg-O functional groups and the chemical precipitation of MgO or Mg(OH)2 attached to the surface.

Co2.8Mg0.2O4 as a promising thermochemical energy storage material with lower reduction onset temperature and higher energy density

Co-based oxides have been considered as one of the most promising materials for thermochemical energy storage (TCES) systems, however, the high operation temperature limits their applications. Specially, when Co3O4-based materials are used in concentrated solar power (CSP) system, a large mirror field area is required to meet the high endothermic temperature, which significantly increases the cost of a CSP system. In addition, the high cost of Co-based materials also limits its application. Improving the energy density can effectively reduce the use of the materials. Thus, in this work, to reduce the reduction onset temperature and increase the energy density of Co-based oxides, the TCES performance of Co3-xMgxO4 oxides is investigated. Co2.8Mg0.2O4 shows the best TCES performance. The reduction onset temperature of Co2.8Mg0.2O4 is ∼775.6 °C, which is 133.7 °C lower than that of the pure Co3O4 (∼909.3 °C). Mg-decoration can also improve the energy density of Co3O4 by 8.02 %. Furthermore, the in situ XRD spectra of the oxides indicate that precipitation of MgO from the Co3O4 lattice during the reduction reaction result in a metastable structure of the oxide, which is conducive to its reduction reaction. Density functional theory calculations indicate that Co2.8Mg0.2O4 possessed a higher defect formation energy and shorter chemical bonds than the pure cobalt oxide, which result in its lower reduction onset temperature and higher energy density. The cost-benefit analysis results showed that Mg-decoration could significantly reduce the area of mirror field and the use of energy storage material for a CSP system.

Molten Barium Hydroxide as Defect Selective Drop Etchant for Dislocation Analysis on Aluminum Nitride Layers

In this article, the suitability of the etchants NaOH/KOH and Ba(OH)2 for defect etching of metalorganic vapour‐phase epitaxy (MOVPE) grown AlN layers with a MgO‐assisted “drop method” is compared. Defect selectivity for the new etchant Ba(OH)2 is confirmed by the local TEM analysis. Temperature dependence (420–500 °C) of etch pit sizes of a‐, mixed‐, and c‐type dislocations for both etchants are investigated by statistical evaluation of scanning electron microscopy (SEM) images. Additionally, the etch pit shape is analyzed for samples etched at 460 °C by atomic force microscopy (AFM) measurements. While a‐ and mixed‐type dislocations result in pits of comparable size and shape for both etchants, c‐type dislocations are already strongly etched with NaOH/KOH at low temperatures leading to over‐etching and MgO precipitation. Ba(OH)2, in contrast, generates smaller c‐type etch pits at low and medium etching temperatures and no MgO precipitates and is therefore preferable for drop etching.

Magnesium partitioning between silicate melt and liquid iron using first-principles molecular dynamics: Implications for the early thermal history of the Earth's core

The high conductivity of the Earth's core discovered through first-principles and experimental studies requires that the core must start very hot and cool down slowly to generate the Earth's magnetic field by thermal buoyancy. The requirement is difficult to satisfy due to the fast cooling of the overlying magma ocean and consequently of the underlying core. This is in direct conflict with the early appearance of the Earth's paleomagnetic field. Recently, it was proposed that significant amount of magnesium (Mg) can be partitioned into the core through the high temperature created by the Moon-forming Giant Impact. Due to its intrinsic low solubility, subsequent cooling would cause Mg precipitation to generate compositional buoyancy to power the geodynamo in the early history of the Earth. Here we show using first-principles molecular dynamics simulations that the equilibrium constant of magnesium dissolution in molten iron depends on temperature, entirely consistent with recent experimental data. We further show that Mg partitioned into the core during giant impacts and reaching a concentration of about 2 wt% can precipitate out at around 3.5 Ga, much earlier than the onset of inner core nucleation. During the subsequent evolution of the Earth, silicon (Si) concentration of the Earth's core will remain constant while Mg and oxygen (O) concentrations decrease significantly. Consequently, the current Si concentration in the core reflects the accretion processes of the Earth while O and Mg concentrations in the core is the combined result of both accretion and the subsequent evolution of the Earth core. Forward modeling shows that for MgO precipitation to provide enough power to generate the magnetic field in the early history of the Earth, initially high silicon content of the core is preferred, which is accommodated readily in the Grand Tack accretion scenario. The geodynamo driven by MgO precipitation explains the secular decline of palaeomagnetic field intensity in the early history of the Earth.

Selection of a sustainable alternative for the reduction of chromium pollution in leather tanning wastewater

Various solutions that have been proposed to address the contamination problem of leather tanning wastewater at El Cerrito (Valle del Cauca, Colombia), have generated conflicts between stakeholders. Therefore, with the purpose of selecting a sustainable alternative for the reduction of chromium present in liquid effluents of the local tanning sector, the Social Multi-Criteria Evaluation (SMCE) method was used to analyze three proposed treatment options. These consisted of a physicochemical treatment based on MgO precipitation and acid re-dissolution, a biotechnological treatment with native yeasts coupled to Cleaner Production (CP) and a non-treatment option. During the evaluation, the social, economic, environmental and technical criteria considered, allowed for the selection of the biotechnology alternative, as the treatment option with the highest sustainability. The physicochemical alternative demonstrated a low acceptance while the non-treatment option proved unfeasible, by affecting the natural environment and social component and was favored only by the economic criterion

Oxide-Inclusion Evolution in the Steelmaking Process of 304L Stainless Steel for Nuclear Power

The inclusions formed in 304L stainless steel for nuclear power produced by the electric arc furnace (EAF)-argon oxygen decarburization furnace (AOD)-ladle furnace (LF)-continuous casting (CC) process were investigated by thermodynamics calculations and experimental results. The results showed that the inclusions after AOD and LF refining were almost the same as the slag composition. The types of inclusions (sizes larger than 5 µm) were mainly CaSiO3 with high SiO2 content at the end of AOD, and Ca2SiO4 with high CaO content at the end of LF. The Al2O3 and MgO content of inclusions increased from AOD to LF. There were two types of inclusions in the tundish: CaO-SiO2-Al2O3-MgO and CaO-SiO2-Al2O3-MgO-MnO inclusions with MgO·Al2O3 spinel precipitation. The content of Al2O3 in the inclusions increased rapidly with the decrease in temperature from the end of LF refining to continuous casting, as calculated using FactSage6.3 software. The rapid increase of Al2O3 in the CaO-SiO2-Al2O3-MgO-(MnO) inclusions promoted the precipitation of MgO·Al2O3 spinel in continuous casting tundish, suggesting mechanisms for the formation of inclusions in the 304L stainless steel.

Non-ammonia enrichment of rare earth by magnesium oxide from rare earth leaching liquor in magnesium salt system

In order to solve the problem of ammonia-nitrogen pollution in the enrichment process of the ion-adsorption type rare earth ore, the technology of non-ammonia precipitation with magnesium oxide precipitant was carried out. It is determined that the rare earth precipitation efficiency is 99.6% and the purity of rare earth concentrates is only 85.89 wt% under the optimum precipitation conditions. And the contents of MgO, SO3 and Al2O3 in the rare earth concentrates are 5.12 wt%, 6.77 wt% and 1.78 wt%, respectively. Furthermore, the thermo-decomposition process of precipitates was investigated by TG-DSC, XRD and FI-IR. The thermal decomposition process consists of two stages: the dehydration of rare earth hydroxide and alkaline rare earth sulfate within 900 °C and the thermal decomposition of RE2O2SO4 at 900–1300 °C. Therefore, a high-temperature calcinations method for removing SO3 from precipitates is proposed. When the precipitates are calcined at 1300 °C for 2 h, the rare earth concentrates with a purity of 92.03 wt% can be acquired. Moreover, the content of SO3 in the concentrate is only 0.46 wt%. In the MgO precipitation and high-temperature calcinations process, the raw material cost is low and the quality of rare earth concentrates is acceptable. It could have great significance for non-ammonia enrichment of rare earth from the rare earth leaching liquor, and finally solve the problem of ammonia nitrogen in the extraction process of the ion-adsorption type rare earth ore within magnesium salt system.

Preparation of Fine-Grained Y2.5Nd0.5Al5O12 + MgO composite ceramics for Inert Matrix Fuels by Spark Plasma Sintering

We have studied the feasibility of preparing high-density (98.6–99.5%) Y2.5Nd0.5Al5O12 (YAG)–xMgO (x = 5, 10, 20 vol %) composite ceramics by spark plasma sintering. YAG–MgO powder materials have been prepared via MgO precipitation from an aqueous solution of magnesium nitrate, Mg(NO3)2, on the surface of garnet particles. The sintering rate of the YAG–MgO composites has been shown to be controlled by volume diffusion at low temperatures and by grain-boundary diffusion at elevated temperatures.

Prospects for an ancient dynamo and modern crustal remanent magnetism on Venus

Venus lacks an internally generated magnetic field today. Whether one existed in the past is unknown, but critical to atmospheric evolution and potential habitability. Canonical models assume the core of Venus is cooling too slowly for convection and thus a magnetic dynamo to occur today. Core/mantle heat flow is suppressed in these models after a putative transition in mantle dynamics associated with widespread, volcanic resurfacing. However, recent studies of impact craters and other surface features support more steady heat loss over geologic time. Precipitation of MgO and/or SiO2 from the core can also drive compositional convection even with slow cooling. Here we reevaluate the likelihood that Venus has an “Earth-like” (at least partially liquid and chemically homogeneous) core using numerical simulations of the coupled atmosphere–surface–mantle–core evolution. An Earth-like core is only compatible with the modern lack of a dynamo if the thermal conductivity of core material is towards the higher end of modern estimates (i.e., >100 W m−1 K−1). If lower estimates like ∼40–50 W m−1 K−1 are actually correct, then we favor recent proposals that the core has completely solidified or preserved primordial stratification. Any simulation initialized with a homogeneous, liquid core predicts a global magnetic field with Earth-like surface strength for >2–3 billion years after accretion—consistent with all available observations—and also sporadic activity within the surface age while temperatures remain below the Curie point of magnetite. Therefore, future spacecraft missions should prioritize the first-ever magnetometer measurements below the ionosphere to search for crustal remanent magnetism.

Precipitation Behaviors of TiN Inclusion in GCr15 Bearing Steel Billet

There are many types of non-metallic TiN-based inclusions observed in GCr15 bearing steel, including single-particle TiN, multi-particle polymerized TiN, and complex inclusions like TiN-MnS, TiN-MgO-MgAl2O4 (TiN-MgO-MA), and TiN-MgAl2O4-MnS (TiN-MA-MnS). Thermodynamic calculations suggest that single-particle TiN precipitates dominate the mushy zone of GCr15 bearing steel. Kinetic calculations regarding TiN growth suggest that the final size of the single-particle TiN ranges between 1 and 6 μm in the initial concentration range of [pct Ti] = 0.0060 to 0.0079 and [pct N] = 0.0049 to 0.0070, at 1620 to 1640 K and a local cooling rate of 0.5 to 10 K/s. The multi-particle polymerized TiN are formed by single TiN particles in three stages: single-particle TiN inclusions approach each other drawn by the cavity bridge force (CBF), local active angles consolidate, and neck region sintering occurs. Based on the thermodynamic calculations of TiN, MnS, and MgO precipitation, the formation behaviors of complex inclusions of TiN-MnS, TiN-MgO-MA, and TiN-MA-MnS were investigated.

High temperature selective sensing of hydrogen with MgO-modified SrMoO4 micro-fibers

Micro-fibers of Mg-doped SrMoO4 (SrMoO4/MgO) were synthesized using a two-step hydrothermal technique. The SrMoO4/MgO micro-fibers were used as the sensing material within a solid-state, resistive-type sensor architecture. The material showed selective detection of hydrogen (H2) up to 1000°C with high sensor response and stability for the given concentrations. The maximum relative resistance change values (Rmax) for SrMoO4/MgO for 4000ppm H2 in a N2 atmosphere (1% O2) were −31, −85, and −87.5 for 600, 800 and 1000°C, respectively. At 1000°C, and the same background atmosphere, the Rmax was only −2.5 and −12.5 to 4000ppm CO and 2000ppm SO2, respectively. Chemical characterization showed that the Mg was dissolved into the SrMoO4 structure, in addition to residual MgO precipitate found within the final SrMoO4 powder. The surface MgO was shown to affect the H2 adsorption and dissociation processes in addition to high selectivity for H2 in comparison to SO2 and CO, while the dissolved Mg contributed to the alteration of the electronic properties, where the Schottky barrier height, band gap, and work function were all lowered. These combined effects resulted in the favorable sensitivity and high selectivity to H2, and also contributed to the increased stability at the elevated sensing temperatures.

Polymorph transformation kinetics of Ca3SiO5 with MgO

Tricalcium silicate (Ca3SiO5 (C3S)) is the primary cement phase in Portland cement. An understanding of its transformation kinetics is important to make use of its polymorphism. In this study, X-ray diffractometry and Rietveld refinement were used to qualitatively and quantitatively analyze the phase compositions of C3S with 2.0% MgO. The results show that C3S with 2.0% MgO is a monoclinic polymorph, M3. Heat treatment changes this monoclinic polymorph to a triclinic polymorph via the precipitation of MgO. The transformation kinetics follows the first-order exponential decay function .

On oxoacidic properties and solubilities of beryllium and magnesium oxides in molten (CsCs + KCl + NaCl) eutectic at T = 783 K

Reactions of Be2+ and Mg2+ with O2– in molten eutectic mixture (CsCs+KCl+NaCl) (0.455:0.245:0.30) at T=783K were studied by a potentiometric method using Pt(O2)|ZrO2(Y2O3) indicator electrode. Addition of O2– ions to the melt containing Mg2+ results in precipitation of MgO (pKs,MgO=11.89±0.3, molality) whereas interaction of Be2+ with O2– is accompanied with sequential formation of Be2O2+ (pK=15.68±0.5, molality) and precipitation of BeO (pKs,BeO=9.62±0.3, molality). On the basis of the obtained and known data pKs,MgO–T−1 dependence in molten (CsCs+KCl+NaCl) eutectic is constructed. The slope of the said dependence in T/K=(from 583 to 1073) range is in good agreement with the value predicted by the Shreder equation, that extends the range of use of the Shreder equation for predictions of metal oxide solubilities in molten halides.

A study of Lux-Flood acid-base reactions in KBr melts at 800°C

The dissociation of CO32− (pK = 2.4 ± 0.2) and precipitation of MgO (pLMgO = 10.66 ± 0.1) in a KBr melt at 800°C were studied potentiometrically with the use of a Pt(O2)|ZrO2|(Y2O3) membrane oxygen electrode. The direct calibration of the electrochemical circuit allowed only the equilibrium concentration of O2− (of strong bases) to be determined in the melt. The total concentration of oxygen-containing impurities, including CO32− and CO42− weak bases, can be found by the potentiometric titration of a sample of KBr by adding MgCl2 (Mg2+), a strong Lux-Flood acid, which causes the decomposition of these oxygen-containing anions. This reaction can also be used to remove oxo anions from alkali metal halide melts.

Percolated perpendicular medium of FePt‐MgO/MgO bilayer films for ultra‐high magnetic recording

AbstractPercolated perpendicular (FePt)100–x‐(MgO)x thin films with x = 0–10.07 vol.% were fabricated on glass substrate with MgO(200)underlayer. When the substrate temperature was 580 oC, a significant enhancement in coercivity was observed at 1.23 vol.% of MgO, which was due to the precipitation of MgO in FePt film causing the hindrance in domain wall motion. As the amount of MgO was increased above 1.23 vol.%, the percolated perpendicular structure began to transform into granular perpendicular structure. Meanwhile, the MgO was found to segregate along grain boundaries of FePt grains, leading to the reduction of FePt grain size. As a result the coercivity and ordering parameter became smaller. In this study the percolated perpendicular medium using FePt film with doping adequate amount of MgO has shown possibility to be an efficient method to increase recording density if the film processing conditions are optimized further. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of divalent dopants on defect structure and electrical properties of Bi 2WO 6

High-temperature tetragonal Bi 4V 2O 11 has been reported as a good oxygen ionic conductor. The structure can be stabilized to ambient temperature by doping Cu, Ti or Nb to substitute V. In this work, Bi 2WO 6, which is almost isostructural with Bi 4V 2O 11 but has no oxygen vacancy, was doped with Cu and Mg to induce extrinsic oxygen vacancies to enhance its ionic conductivity. Both solid solutions were synthesized by solid-state reaction method using oxides as the raw materials. Such solid solution may be represented by the formula of Bi 2W 1− x M x O 6−2 x , where M=Cu and Mg. The sintered samples were examined by XRD and SEM. The conductivity was measured at 300, 400, 500, 600, and 700 °C by the two-probe technique, using HP-34970 multi-meter. Both Bi 2W 0.9Cu 0.1O 5.8 and Bi 2W 0.9Mg 0.1O 5.8 exhibited a mixture of tetragonal and orthorhombic lattices from XRD patterns. However, the SEM micrograph of Bi 2W 0.9Cu 0.1O 5.8 showed CuO precipitation while no MgO precipitation in Bi 2W 0.9Mg 0.1O 5.8. Although Mg ions may incorporate into the structure of Bi 2WO 6, a new Mg-rich tetragonal phase may still appear when Mg is no longer to be surrounded by six oxygen atoms. The Mg-rich tetragonal phase is highly conductive because of its high oxygen vacancy concentration. The conductivity of Bi 2W 0.8Mg 0.2O 5.6 is 1.12×10 −1 Ω −1 cm −1 at 700 °C and that is higher than 7.97×10 −2 Ω −1 cm −1 of Bi 2W 0.8Cu 0.2O 5.6. Thus, the dissolution of Cu into the W cation-sublattice is limited.

Modified thick thermal barrier coatings: Thermophysical characterization

Thermophysical properties of modified zirconia based thick thermal barrier coatings (8Y 2O 3–ZrO 2, 25CeO 2–2.5Y 2O 3–ZrO 2 and 22MgO–ZrO 2) were characterized at temperature range of RT-1300 °C. Coatings were studied in laser-glazed and phosphate sealed state, and their properties were compared to as-sprayed coatings. Laser glazing affected only slightly on thermal conductivity of the studied coatings. If the segmentation cracks, induced by laser glazing, were oriented vertically like in the case of laser-glazed 8Y 2O 3–ZrO 2 coating, thermal conductivity was increased in some degree. But if the orientation of the segmentation cracks was deviated from the vertical direction or if the cracks were branched, thermal conductivity was decreased. This was the case with the laser-glazed 25CeO 2–2.5Y 2O 3–ZrO 2 and 22MgO–ZrO 2 coatings. Phosphate based sealing treatments were found to increase the thermal conductivity of all coatings. Aluminium phosphate sealing also lowered the high temperature stability of the 8Y 2O 3–ZrO 2 coating down to 1000 °C. In 8Y 2O 3–ZrO 2 and 25CeO 2–2.5Y 2O 3–ZrO 2 based coatings thermal conductivity was increased in consecutive measurement cycles, caused mainly by the sintering based phenomena in which the contact between overlapping lamellae was improved. Thermal conductivity of the 22MgO–ZrO 2 based coatings was increased significantly in the first measurement cycle because of the unstabilization of zirconia-caused by precipitation of MgO.

Precipitation from a reactive silicate on MgO

For this paper, high-temperature interactions between a reactive silicate liquid and the (001) surface of a MgO single crystal were studied. The paper discusses the influence of the morphology of the MgO surface on both the dewetting of a silicate liquid and the mechanism of precipitation of excess MgO out of this silicate liquid. Alternative pathways were considered for MgO precipitation; it may occur by nucleation and growth of plateaus on the MgO surface or by MgO absorption at surface steps. On flat MgO(001) surfaces, precipitation of MgO from the continuous layer of silicate liquid led to the formation of plateaulike precipitates. Precipitation from disconnected silicate droplets onto stepped MgO(001) surfaces resulted in the growth of ridges at steps on the surface.

Atomic-scale studies of silver segregation at [formula omitted] heterophase interfaces

Atom-probe field-ion microscopy (APFIM) is used to measure quantitatively the absolute values of the Gibbsian interfacial excess of Ag, Γ Ag, at semi-coherent {222} MgO Cu(Ag) heterophase interfaces. Atomically clean metal oxide/metal heterophase interfaces were obtained by internal oxidation of a pure ternary Cu(Mg,Ag) single-phase alloy to produce octahedral-shaped MgO precipitates, 10 to 30 nm in diameter, in a single-phase Cu(Ag) matrix. Precipitates are faceted on {222} planes. Random-area APFIM analysis was performed in the 〈111〉 direction of the Cu(Ag) matrix, perpendicular to one pair of {222} facets of every MgO precipitate. For a single precipitate dissected on an atomic scale, segregation levels at both “front” and “rear” interfaces are determined. The average measured value of Γ Ag, at 500°C, is (2 ± 0.6) × 10 15 atoms cm −2. This corresponds to about one monolayer coverage.