H2O Precipitate Research Articles

Effect of soluble fluorine on the hydration, strength and microstructure of hemihydrate phosphogypsum

Preparing β-hemihydrate phosphogyspym (HPG) from phosphogypsum (PG) is a promising and effective method to utilize PG massively. However, the influence mechanism behind the harmful impurities on the properties of HPG is unclear. In this paper, the effect of soluble fluorine (F) on the hydration, mechanical strength, and microstructure of HPG plaster was investigated. The results show that soluble F accelerated the CaSO4·0.5 H2O dissolution and CaSO4·2 H2O precipitation. The higher soluble F content resulted in a more accelerated degree. During the hydration of HPG, CaF2 and CaSO4·2 H2O co-precipitated in the liquid phase at the beginning of the dissolution stage, which shortened the setting time of HPG. The proportion of free F was higher in the liquid phase, and F species on CaSO4·2 H2O surface contained a higher F- and CaF2 proportion resulting from the Ca2+ active site. The adsorption of free F adversely affected the 2 h and dry strength by weakening the electrostatic attraction among CaSO4·2 H2O, thereby increasing the porosity. Moreover, soluble F selectively combined with CaSO4·2 H2O on the c{111} facet, changing the morphology of hydration products from needle-like to plate-like as well as a loose structure.

Synergistic extraction of valuable metals from cyanide tailings slurry electrolyte and preparation of ferrous oxalate

A cyanide tailings slurry electrolyte (CTSE) was treated using a synergistic extraction system (P204-T) composed of di(2-ethylhexyl)phosphate (P204, (C8H17O)2PO2H, H2A2), trioctylamine (TOA, ([CH3(CH2)7]3N), R3N), and iso-alkanes, and a collaborative extraction technique involving oxalic acid stripping and photocatalysis was developed to separate and recover valuable metals and prepare ferrous oxalate. The results show that at a pH of 1.5, the P204 concentration of 15 vol%, TOA concentration of 10 vol%, O/A of 1:1, an oscillation time of 10 min, and an oscillation frequency of 190 rpm, the single-stage Fe(III) and Zn(II) extraction efficiencies of P204-T were 99.25% and 99.02%, respectively, and the saturated extraction capacities were 25.01 g•L−1 and 1.20 g•L−1, respectively. TOA destroyed the dimer structure of P204 and promoted the –OH bond and –OP group in P204 to combine with TOA through hydrogen bonding to form N–H+···O–P and N–H+···OP bonds, resulting in the R3NHA·(HA) composite structure. During the collaborative extraction process, ion exchange with FeSO4+ resulted in the formation of FeSO4A·(HA)3 and ion exchange with the intermediate product (R3NH)2SO4 and Zn(SO4)22- formed (R3NH)2Zn(SO4)2, which improves the extraction efficiency of metal ions. In addition, some amount of P204 underwent ion exchange with free Fe3+ and Zn2+ to produce FeA3(HA)3 and ZnA2(HA)2, respectively. At oxalic acid concentration of 1 mol•L−1, an O/A of 1:3, the efficiencies of Fe(III) and Zn(II) stripped from the saturated organic phase reached 99.65% and 99.41%. Zn(II) formed the ZnC2O4·2 H2O precipitate and Fe(III) separation. FeC2O4·2 H2O was successfully synthesized by ultraviolet photocatalysis using iron oxalate complex as the stock solution.

Efficient synthesis of Co2+–Co3+ hydrotalcite-like compounds via NaOH precipitation in methanol–water under mild alkaline conditions

Divalent/trivalent cobalt hydrotalcite-like compounds (Co2+–Co3+ HTlc) were synthesized via precipitation under mild alkaline conditions by using NaOH as precipitant and methanol–water as solvent. The influences of synthetic parameters including CH3OH:H2O volume ratio, precipitation pH, and hydrothermal treatment on the HTlc formation were investigated. The crystalline phase, morphology, and chemical composition of the synthesized samples were characterized by XRD, FTIR, SEM, TG–MS, and iodometric titration. The formation of Co2+–Co3+ HTlc was strongly affected by the methanol content and a single Co2+–Co3+ HTlc phase was obtained by raising the CH3OH:H2O ratio to 4:1–8:1. It was suggested that the presence of methanol increased the concentration of dissolved oxygen and promoted the oxidation of Co2+ into Co3+, leading to complete transformation of α-Co(OH)2 into Co2+–Co3+ HTlc. Moreover, single-phased Co2+–Co3+ HTlc was available at pH between 10 and 12, and they were hydrothermally stable by treatment at 60 °C for 24 h. Under the synthetic conditions, complete precipitation of cobalt ions was achieved. This highlights that the developed synthesis process is an efficiency way for quantitative synthesis of cobalt HTlc.

Cover Feature: Shedding Light on the Enigmatic TcO2 ⋅ xH2O Structure with Density Functional Theory and EXAFS Spectroscopy (Chem. Eur. J. 59/2022)

Radioactive Tc in deep nuclear waste repositories can be immobilized by reduction of the highly soluble TcO4− anion by agents like magnetite (Fe3O4) and H2, which create an amorphous TcO2⋅x H2O precipitate. EXAFS measurements combined with DFT calculations reveal that the precipitate is an inorganic polymer formed of zigzag TcO2⋅2 H2O chains and give evidence of an increasing polymerization over time. More information can be found in the Research Article by A. F. Oliveira, A. C. Scheinost, and co-workers (DOI: 10.1002/chem.202202235).

The Influence of Precursor on the Preparation of CeO2 Catalysts for the Total Oxidation of the Volatile Organic Compound Propane

CeO2 catalysts were prepared by a precipitation method using either (NH4)2Ce(NO3)6 or Ce(NO3)3, as CeIV or CeIII precursors respectively. The influence of the different precursors on catalytic activity was evaluated for the total oxidation of propane with water present in the feed. The catalyst prepared using the CeIV precursor was more active for propane total oxidation. The choice of precursor influenced catalyst properties such as surface area, reducibility, morphology, and active oxygen species. The predominant factor associated with the catalytic activity was related to the formation of either CeO2.nH2O or Ce2(OH)2(CO3)2.H2O precipitate species, formed prior to calcination. The formation of CeO2.nH2O resulted in enhanced surface area which was an important factor for controlling catalyst activity.

Facile synthesis of WOx/ZrO2 catalysts using WO3·H2O precipitate as synthetic precursor of active tungsten species

Zirconia-supported tungsten oxide (WOx/ZrO2) catalysts were successfully synthesized using a suspension containing amorphous hydrous zirconia precipitates [ZrOx(OH)4-2x·yH2O]n and tungstate monohydrate (WO3·H2O) precipitates. The procedure involved the dissolution of the WO3·H2O precipitate during the aging process with the release of oxyanion [WO4]2- species, interaction of this species with the surface of the [ZrOx(OH)4-2x·yH2O]n precipitate and, formation of active WOx species after thermal treatment. Non-bridging hydroxyl (OH−) groups present in the [ZrOx(OH)4-2x·yH2O]n precipitate act as an active agent for the WO3·H2O dissolution. N2 physisorption, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), temperature-programmed reduction using hydrogen (H2-TPR), temperature-programmed desorption of ammonia (NH3-TPD), Fourier-transform infrared (FTIR) spectroscopy of adsorbed pyridine, and Raman spectroscopy were used to elucidate the catalyst structure–performance relationship. The catalytic activity was evaluated for the oxidative desulfurization (ODS) of a model fuel containing dibenzothiophene (DBT). For a fixed WO3·H2O content, longer aging times improved the catalyst activity, reaching a maximum when WO3·H2O was completely dissolved. The increase in surface area and formation of more active Zr-WOx clusters and polytungstates are observed for the highest active catalysts. A synergetic effect between local Lewis and Brønsted acid sites seems to have contributed to the observed superior activity. The proposed strategy provides an efficient approach to produce active WOx/ZrO2 catalysts and may be applicable for designing other heterogeneous catalytic systems.

Facile one-pot fabrication of ZnO2 particles for the efficient Fenton-like degradation of tetracycline

Abstract This work presented a facile one-pot fabrication of zinc peroxide (ZnO2) particles for Fenton-like degradation of tetracycline (TC). The effects of precipitants and their addition sequences on the synthesis of ZnO2 were first investigated in detail. Phase-pure ZnO2 could be obtained when H2O2 was added before adding precipitant (NH3·H2O/NaOH) or after NH3·H2O precipitation. The Fenton-like degradation performance of ZnO2 was evaluated using 100 mL of TC solution (50 mg/L). It was found that 10 mg of ZnO2 could degrade about 90% of TC within 1 h in the presence of 3 mg of Fe2(SO4)3·xH2O at ambient temperature. Besides, increasing ZnO2 dosage, Fe2(SO4)3·xH2O dosage and temperature were all beneficial to TC degradation, and the degradation process could be well described by the pseudo-second order kinetic model. The catalytic degradation process could be efficiently conducted in a wide pH range of 4–8, while the coexisted PO43− and HCO3− were unfavorable for TC degradation. Furthermore, radical quenching experiments and electron spin resonance tests indicated that hydroxyl radical was the dominant reactive oxygen species generated in the degradation system.

Ammonium vanadate/ammonia precipitation for vanadium production from a high vanadate to sodium ratio solution obtained via membrane electrolysis method

Vanadium is an important strategic metal and has been widely applied in a multiple of industries. However, the current process for the production of vanadium compounds suffers from the generation of a significant amount of high salinity ammonium containing wastewater due to the mixing of sodium salts, ammonium salts, and acid during vanadium precipitation operation. In order to avoid the generation of wastewater at source, a clean and recyclable V2O5 production process was proposed and investigated in this study. Using NH4VO3 and NH3·H2O as precipitants, (NH3)2·3V2O5·H2O and (NH3)2·V2O5·H2O were obtained by precipitation of vanadate (VxOyn−) from a high vanadate to sodium (V/Na) molar ratio acidic solution obtained via membrane electrolysis method, avoiding the generation of wastewater or solid waste. After calcination, high purity V2O5 product could be obtained. The vanadium precipitation process was investigated using ICP, XPS, SEM, XRD techniques, and the precipitation rate was investigated using solution conductivity method. Critical precipitation parameters such as excessive coefficient, solution temperature, solution V/Na molar ratio and solution pH were examined in detail. The results showed that increase the excessive coefficient, solution temperature and solution V/Na molar ratio, as well as reduce the solution pH would be beneficial for obtaining high purity vanadium products (≥99.9% V2O5). Optimum parameters were obtained at the V/Na molar ratio of 2.58, the excessive coefficient of 2, and the solution temperature of 363 K. The vanadium products obtained using both precipitants have a similar purity (more than 99.9%), however, due to a higher degree of hydrolysis reaction, the NH4VO3 precipitation method showed a higher VxOyn− precipitation rate (66.36%) in comparison with the NH3·H2O precipitation method (38.76%), indicating that the first method was superior. Based on this, a complete process with zero waste emission for V2O5 production was designed. The methods proposed and the results obtained from this study could provide fundamental information for the design of cleaner vanadium production process.

Nitrogen release of hydrothermal treatment of antibiotic fermentation residue and preparation of struvite from hydrolysate

Hydrothermal treatment (HT) is an appropriate treatment method for organic hazardous wastes such as antibiotic fermentation residue (AR). However, there is no effective way to recycle hydrolysate with high nitrogen content. In this study, penicillin fermentation residue (PR, a type of AR) was used as raw material to study the release and redistribution of N during hydrothermal process. And the influences of pH, ion ratio and reaction time on the preparation of struvite were analyzed. The results showed that the nitrogen in PR consists of Inorganic-N and Amino-N. Most of N (~70%) that entered hydrolysate was converted into org-N, NH4+-N and NO3–-N. At 260 °C, the NH4+-N concentration was 2842.78 mg/L, accounting for 45.2% of total nitrogen. The remaining amino-N in the hydrochar was gradually converted to pyridine-N, pyrrole-N and quaternary-N with the increasing of temperature. At pH = 9.5, Mg2+: NH4+: PO43- = 1.3: 1: 1.15, struvite was prepared by hydrolysate. And over 95% removal rate of NH4+-N could be achieved. XRD analysis showed that the main component of the product was struvite, which was further confirmed by SEM-EDX and FT-IR. It was found that there was trace amount of MgKPO4·H2O precipitation in the product. In addition, Mg3(PO4)2 precipitation might also be formed at pH = 10.

Recovery of Rare Earth Oxide from Waste NiMH Batteries by Simple Wet Chemical Valorization Process

Nickel metal hydride (NiMH) batteries contain a significant amount of rare earth metals (REMs) such as Ce, La, and Nd, which are critical to the supply chain. Recovery of these metals from waste NiMH batteries can be a potential secondary resource for REMs. In our current REM recovery process, REM oxide from waste NiMH batteries was recovered by a simple wet chemical valorization process. The process followed the chemical metallurgy process to recover REM oxides and included the following stages: (1) H2SO4 leaching; (2) selective separation of REM as sulfate salt from Ni/Co sulfate solution; (3) metathesis purification reaction process for the conversion REM sulfate to REM carbonate; and (4) recovery of REM oxide from REM carbonate by heat treatment. Through H2SO4 leaching optimization, almost all the metal from the electrode active material of waste NiMH batteries was leached out. From the filtered leach liquor managing pH (at pH 1.8) with 10 M NaOH, REM was precipitated as hydrated NaREE(SO4)2·H2O, which was then further valorized through the metathesis reaction process. From NaREE(SO4)2·H2O through carbocation, REM was purified as hydrated (REM)2CO3·H2O precipitate. From (REM)2CO3·H2O through calcination at 800 °C, (REM)2O3 could be recovered.

Stability of Neptunium(V) in Concentrated Acetic Acid Solutions

Np(V) acetate in glacial acetic acid and aqueous acetic acid solutions is resistant to disproportiona-tion and occurs in equilibrium with the NpO2(Ac)·H2O precipitate. Within the time of dehydration with excess acetic anhydride, Np(V) disproportionates. The inverse reproportionation process occurs after adding excess water to anhydrous acetic acid containing acetic anhydride, Np(IV), and Np(VI). Addition of HClO4 causes rapid disproportionation of Np(V) in anhydrous acetic acid and in concentrated acetic acid solutions. The spectra of suspensions containing Np(V) and potassium acetate in concentrated acetic acid contain absorption bands with maxima at 990 and 1020 nm, assigned to NpO2(Ac)32- anions and K2NpO2(Ac)3 particles, respectively.

Advanced chemical stability diagrams to predict the formation of complex zinc compounds in a chloride environment.

A chemical stability map is advanced by incorporating ion complexation, solubility, and chemical trajectories to predict ZnO, Zn(OH)2, ZnCO3, ZnCl2, Zn5(CO3)2(OH)6, and Zn5(OH)8Cl2·H2O precipitation as a function of the total Zn content and pH of an NaCl solution. These calculations demonstrate equilibrium stability of solid Zn products often not considered while tracking the consumed and produced aqueous Zn ion species concentrations through chemical trajectories. The effect of Cl-based ligand formation is incorporated into these stability predictions, enabling enhanced appreciation for the local corrosion conditions experienced at the Zn surface in chloride-containing environments. Additionally, the complexation of Cl− with Zn2+ is demonstrated to compete with the formation of solid phases, making precipitation more difficult. The present work also extends the chemical stability diagram derivations by incorporating a Gibbs–Thompson curvature relation to predict the effect of nanoscale precipitate phase formation on species solubility. These thermodynamic predictions correlate well with experimental results for Zn corrosion in full and alternate NaCl immersion, and have far-reaching utility in a variety of fields requiring nanoscale, semiconductor, and/or structural materials.

Fabrication and characterization of Ag3PO4/TiO2 heterostructure with improved visible-light photocatalytic activity for the degradation of methyl orange and sterilization of E.coli

ABSTRACTAg3PO4/TiO2 heterostructure photocatalysts were prepared by the hydrolysis of TiCl4 and NH3· H2O, microwave calcination, and chemical precipitation method. A series of techniques, such as XRD, SEM, TEM, DRS, FT-IR, PL and XPS, were used to characterize the Ag3PO4/TiO2 heterostructure. The as-prepared Ag3PO4/TiO2 heterostructure was employed to degrade methyl orange and to inactivate E.coli under visible light irradiation, which exhibited high photocatalytic activity, could degrade 95.5 % of MO and kill almost all the E.coli. Recycling experiments confirmed that the Ag3PO4/TiO2 heterostructure had superior cycle performance and structural stability. The photocatalytic activity enhancement of Ag3PO4/TiO2 heterostructure could be mainly attributed to the improved light absorption and the efficient separation of photo-induced electron–hole pairs. This study provided a new way to design and prepare high efficiency and stable photocatalysts for photocatalytic degradation of organic pollutants and photocatalytic sterilization of E.coli.

Adsorption of uranium from sulfate leach liquor using rice straw impregnated with chlorophenyl hydrazinyl pyrazoloquinazolinon

ABSTRACTThe impregnation of rice straw with 2-amino-3-(2-(4-chlorophenyl)hydrazinyl)-8,8-dimethyl-8,9-dihydropyrazolo[1,5-a]quinazolin-6(7H)-one (CPHPQ) has been used for the recovery of uranium from sulfate leach liquor. The uranium adsorption and elution of solvent impregnated mercerized rice straw (SIMRS) were carried out using a batch technique. The uranium adsorption controlling factors include pH, initial uranium concentration, contact time, S/L ratio, and temperature. Thermodynamic characteristics showed that the adsorption process is exothermic with enthalpy change ΔH = −152.1 kJ/mol. The kinetics data fit well with a pseudo-second-order model. The equilibrium data fit well with the Langmuir isotherm. Uranium cake was obtained from the eluate solution using hydrogen peroxide as UO4.2H2O precipitate.

Experimental study on SO2 recovery using a sodium–zinc sorbent based flue gas desulfurization technology

A sodium–zinc sorbent based flue gas desulfurization technology (Na–Zn-FGD) was proposed based on the experiments and analyses of the thermal decomposition characteristics of CaSO3 and ZnSO3·2.5H2O, the waste products of calcium-based semi-dry and zinc-based flue gas desulfurization (Ca–SD-FGD and Zn–SD-FGD) technologies, respectively. It was found that ZnSO3·2.5H2O first lost crystal H2O at 100°C and then decomposed into SO2 and solid ZnO at 260°C in the air, while CaSO3 is oxidized at 450°C before it decomposed in the air. The experimental results confirm that Zn–SD-FGD technology is good for SO2 removal and recycling, but with problem in clogging and high operational cost. The proposed Na–Zn-FGD is clogging proof, and more cost-effective. In the new process, Na2CO3 is used to generate Na2SO3 for SO2 absorption, and the intermediate product NaHSO3 reacts with ZnO powders, producing ZnSO3·2.5H2O precipitate and Na2SO3 solution. The Na2SO3 solution is clogging proof, which is re-used for SO2 absorption. By thermal decomposition of ZnSO3·2.5H2O, ZnO is re-generated and SO2 with high purity is co-produced as well. The cycle consumes some amount of raw material Na2CO3 and a small amount of ZnO only. The newly proposed FGD technology could be a substitute of the traditional semi-dry FGD technologies.

Structure, synthesis, and thermal properties of trans-[Ru(NO)(NH3)4(SO4)]NO3·H2O

In treatment of trans-[Ru(NO)(NH3)4(OH)]Cl2 with concentrated sulfuric acid on heating trans-[Ru(NO)(NH3)4(SO4)](HSO4)·H2O (I) is obtained with a yield close to quantitative. In the interaction of the saturated solution of I with a saturated NaNO3 solution a trans-[Ru(NO)(NH3)4(SO4)]NO3·H2O (II) precipitate forms whose structure is determined by single crystal XRD: space group P212121, a = 6.8406(3) A, b = 12.6581(5) A, c = 13.3291(5) A. A monodentately coordinated sulfate ion is in the trans-position to the nitroso group. Compound II is characterized by IR spectroscopy, powder XRD, and diffuse reflectance spectroscopy. The process of its thermolysis is studied; by differential scanning calorimetry the thermal effect of the dehydration reaction occurring on heating to 120°C (ΔH = 58.9 ± 1.5 kJ/mol) is estimated. The final product of the thermolysis of II is a mixture of Ru and RuO2.

Synthesis and electrochemical performance of nanostructured LiMnPO4/C composites as lithium-ion battery cathode by a precipitation technique

A fast precipitation method is adopted for synthesis of nano-MnPO4·H2O, with MnSO4·H2O, H3PO4, NH4NO3 and NaOH as raw materials. MnPO4·H2O precipitate is characterized by XRD (X-ray diffraction) and SEM (scanning electron microscope). Fine-sized, well-crystallized, carbon-coated LiMnPO4/C nano-composites are obtained by using mechanochemical activation assisted carbothermal reduction route from as-prepared MnPO4·H2O, Li2CO3 and PVA (polyvinyl alcohol). The effect of calcination temperature on the structure and properties of obtained materials is investigated by XRD, SEM, TEM (transmission electron microscopy) and electrochemical measurements. The in situ 6.8wt% carbon coated LiMnPO4/C displays discharge specific capacity of 124mAhg−1 at 0.05C rate and 108mAhg−1 at 1C rate. The capacity retention is nearly 100% after 20 cycles at 1C rate. This mechanochemical activation assisted precipitation technique is a facile approach for the fabrication of LiMnPO4 cathode materials.

Effect of precursor morphology on the hydrothermal synthesis of nanostructured potassium tungsten oxide

Graphical abstractDisplay Omitted Highlights? Nanowires/nanorods can be simply prepared by selecting suitable precursor. ? Uniform nanowires were synthesized from very fine, amorphous-like precursor. ? Non-uniform nanorods were synthesized from agglomerated, well-crystalline precursor. ? Different crystallization is ascribed to different dissolution of the two precursors. K2W4O13 with nanorod and nanowire structures were selectively-prepared by the alteration of starting precursors under hydrothermal treatment at 180?C for 6h in the presence of K2SO4. The uniform K2W4O13 nanowires with a diameter of ~10nm and a length in a range of hundreds nanometers were obtained when the amorphous-like WO3?0.33H2O precipitate was employed as the precursor, while non-uniform K2W4O13 nanorods with diameter in a range of ~10nm and diverse length ranging from tens to few hundreds nm were obtained when the well-crystalline, platelet WO3?H2O precursor was employed. The experimental finding is discussed based on solubility of the two precursors, which led to different crystallization behavior during the hydrothermal treatment.

Modeling ammonia–ammonium aqueous chemistries in the Solar System’s icy bodies

The properties of ammonia and ammonium compounds in cold, subsurface brines are important for understanding the behavior of outer planet icy moons. The FREZCHEM model of aqueous chemistry was primarily designed for cold temperatures and high pressures, but does not contain ammonia and ammonium compounds. We added ammonia and ammonium compounds to FREZCHEM, and explored the role of these chemistries on Enceladus and Titan, mindful of their astrobiological implications. For the new FREZCHEM version, Pitzer parameters, volumetric parameters, and equilibrium constants for the Na–K–NH4–Mg–Ca–Fe(II)–Fe(III)–Al–H–Cl–ClO4–Br–SO4–NO3–OH–HCO3–CO3–CO2–O2–CH4–NH3–Si–H2O system were developed for ammonia and ammonium compounds that cover the temperature range of 173–298K and the pressure range of 1–1000bars. Ammonia solubility was extended to 173K, where NH3·2H2O and NH3·H2O precipitate, which is the lowest temperature in existing FREZCHEM versions. A subsurface “ocean” on Enceladus was simulated at 253K with gas pressures, 1–10bars, and with Na+,Cl-,HCO3-,CO2(g),CH4(g), and NH3(aq) that led to precipitation of ice, NaHCO3, and gas hydrates (CO2·6H2O and CH4·6H2O). The pH on Enceladus (Fig. 10) ranged from 5.74 to 6.76, and water activity, aw, ranged from 0.80 to 0.82, which are relatively favorable for life. A subsurface “ocean” on Titan was simulated with NH4+,Cl-,SO42-,CH4(g), and NH3(aq) over the temperature range of 173–273K that led to precipitation of (NH4)2SO4, ice, CH4·6H2O, and NH4Cl. The CH4 clathrate should float above the brine and is buoyant with respect to H2O ice, so has the potential to be a source of CH4 to replenish what has been photochemically destroyed in Titan’s atmosphere over time. The pH in the Titan simulation (Fig. 11) ranged from 11.24 to 18.03 (latter may not be accurate), and aw ranged from 0.28 to 0.72, which are relatively unfavorable for life as we know it. The Titan simulations, with total pressures of 10, 250, and 1000bars, led to similar depositions, except for ice that failed to form under 1000bars of pressure. In the past, there have been arguments for why Titan, given an early environment similar to Earth, could be a highly favorable body in our Solar System for life. But if Titan oceans are strongly alkaline with high pH whereas Enceladus’ oceans have moderate pH, as simulated, the latter would seem a better environment for life as we know it. But bear in mind, caution must be exercised in quantifying the ammonia/ammonium cases because of the complexities and limitations of these chemistries in the FREZCHEM model.

Direct Hydrothermal Precipitation of Pyrochlore-Type Tungsten Trioxide Hemihydrate from Alkaline Sodium Tungstate Solution

Pyrochlore-type tungsten trioxide hemihydrate (WO3·0.5H2O) powder with the average particle size of 0.5 μm was prepared successfully from the weak alkaline sodium tungstate solution by using organic substances of sucrose or cisbutenedioic acid as the acidification agent. The influences of solution pH and acidification agents on the precipitation process were investigated. The results showed that organic acidification agents such as sucrose and cisbutenedioic acid could improve the precipitation of pyrochlore WO3·0.5H2O greatly from sodium tungstate solution compared with the traditional acidification agent of hydrochloric acid. In addition, the pH value of the hydrothermal system played a critical role in the precipitation process of WO3·0.5H2O, and WO3·0.5H2O precipitation mainly occured in the pH range of 7.0 to 8.5. The precipitation rate of tungsten species in the sodium tungstate solution could reach up to 98 pct under the optimized hydrothermal conditions. This article proposed also the hydrothermal precipitation mechanism of WO3·0.5H2O from the weak alkaline sodium tungstate solution. The novel method reported in this study has a great potential to improve the efficiency of advanced tungsten trioxide-based functional material preparation, as well as for the pollution-reducing and energy-saving tungsten extractive metallurgy.