Homogeneous Recrystallization Research Articles

Redox-Driven Recrystallization of PbO2.

Lead(IV) oxide (PbO2) is one of the lead corrosion products that forms on the inner surface of lead pipes used for drinking water supply. It can maintain low dissolved Pb(II) concentrations when free chlorine is present. When free chlorine is depleted, PbO2 and soluble Pb(II) will co-occur in these systems. This study used a stable lead isotope (207Pb) as a tracer to examine the interaction between aqueous Pb(II) and solid PbO2 at conditions with no net change in dissolved Pb concentration. While the dissolved Pb(II) concentration remained unchanged, significant isotope exchange occurred that indicated that substantial amounts (24.3-35.0% based on the homogeneous recrystallization model) of the Pb atoms in the PbO2 solids had been exchanged with those in solution over 264 h. Neither α-PbO2 nor β-PbO2 displayed a change in mineralogy, particle size, or oxidation state after reaction with aqueous Pb(II). The combined isotope exchange and solid characterization results indicate that redox-driven recrystallization of PbO2 had occurred. Such redox-driven recrystallization is likely to occur in water that stagnates in lead pipes that contain PbO2, and this recrystallization may alter the reactivity of PbO2 with respect to its stability and susceptibility to reductive dissolution.

Fabrication of high-performance low silicon non-oriented electrical steels by a new method: Low-finishing-temperature hot rolling combined with batch annealing

The grain size prior to cold rolling plays an important role in the microstructural, textural evolution and magnetic properties of non-oriented silicon steel sheets. In this work, we proposed a new way to obtain large grain size before cold rolling and applied it to a 1.0 wt% Si-0.2 wt% Al non-oriented silicon steel, i.e., appropriate low-finishing-temperature hot rolling combined with batch annealing. The grain growth process during batch annealing was investigated using a quasi in-situ electron backscattered diffraction (EBSD) technique. It was demonstrated that significant grain growth took place through a strain-induced boundary migration mechanism. Then a large number of in-grain shear bands were formed in the cold rolled microstructure and could provide nucleation sites for Goss ({110} 〈001〉 ) and λ-fiber (〈001〉//ND) recrystallized grains, thus resulting in a coarse-grained and homogeneous recrystallization microstructure with weakened γ-fiber (〈111〉//ND) texture, strengthened Goss and λ-fiber texture. Consequently, superior magnetic properties (B50 = 1.763 T, P1.5/50 = 4.30 W/kg) were obtained. For comparison, the conventional processing route without batch annealing was also investigated. A small-grained and inhomogeneous recrystallization microstructure with pronounced γ-fiber texture was produced in the finally annealed sheet, which could significantly deteriorate both the magnetic induction and iron loss. This work provided a new way to enlarge the grains in hot rolled sheets by low temperature batch annealing instead of the conventional high temperature normalization. Thus a new processing route to fabricate low silicon non-oriented electrical steels with high permeability and low iron loss was established.

The Influences of Stirring on the Recrystallization of Ammonium Perrhenate

Ammonium perrhenate is widely used in alloy manufacturing, powder processing, the catalytic industry, and other fields. Recrystallization can improve the specific surface area of ammonium perrhenate, reduce its particle size, and improve its particle size distribution uniformity. Therefore, recrystallized ammonium perrhenate can obtain better application benefits in the above fields. Stirring is an important factor that affects the recrystallization of ammonium perrhenate, and this paper systematically analyzes the influence of the stirring paddle types and stirring intensities on ammonium perrhenate during the homogeneous recrystallization process, ultimately revealing the relationship between the growth rate of ammonium perrhenate and the stirring process. Particle image velocimetry physical simulation results showed that the flow field in the reactor was more evenly distributed when using the disc turbine impeller, and a relatively uniform velocity liquid flow area was formed in the whole reactor, while the low-velocity liquid flow area was smaller. Therefore, this information, combined with SEM test results, suggests that under the same recrystallization time and stirring intensity, the stirring effect of a disc turbine impeller is more suitable than a propelling propeller and an Intermig impeller for the recrystallization process of ammonium perrhenate. Moreover, the XRD patterns and SEM analysis showed that if the agglomeration in the systems was too strong or too weak, the growths of the (101) crystal plane and (112) crystal plane were restrained, which caused an attenuation in the growth rates along the crystallographic directions that were orthogonal to the crystal faces. Finally, the reduction experiments show that the recrystallization of ammonium perrhenate could improve the phase parameters of rhenium powders.

Barite recrystallization in the presence of 226Ra and 133Ba

Despite the long history of studies on (Ba,Ra)SO4, various recent investigations aimed at improving our understanding of its formation processes and thermodynamics. Accumulation of natural Ra isotopes (mainly 226Ra and 228Ra) in (Ba,Ra)SO4 plays an important role in many geotechnical applications and water desalination facilities. In the near field of a nuclear waste repository, barite formation may be expected e.g. as a consequence of contact of spent nuclear fuel or vitrified high level waste with sulfate containing ground water, and may control the potential release of Ra from waste forms upon leakage.Here, we present results of long term batch-type barite recrystallization experiments conducted in the simultaneous presence of 226Ra and 133Ba as a function of initial Ra2+ concentration and pH with the same type and charge of barite powder as used in previous studies (Curti et al., 2010; Klinkenberg et al., 2014; Brandt et al., 2015). Due to the simultaneous measurement of 133Ba and 226Ra our data allow for a direct relation of 226Ra uptake with barite recrystallization, which leads to more accurate partition coefficients compared to previous studies. During a reaction period of five years, barite is continuously recrystallizing. Within the investigated radium concentration range (Ba(1−X)RaXSO4 with X < 0.0006), we measure a partition coefficient of D = 2.1 ± 0.5. The partition coefficient is constant within uncertainty during almost five years (1793 days) of experimental duration. This value is in line with a description of (Ba,Ra)SO4 as an ideal solid solution based on the solubility products (KSP) of the endmembers barite (log10(KSP(barite)) = −9.97) and radium sulfate (log10(KSP(RaSO4)) = −10.26; dimensionless Guggenheim parameter, a0 = 0.0 ± 0.3). Apparent discrepancies to previous theoretical results (a0 = 1.0 ± 0.4) may be resolved when the uncertainties related to the solubility of RaSO4 are considered.Compared to results of previous publications, recrystallization is extremely slow in the experiments presented here. While previous authors suggested complete equilibration of bulk microcrystalline barite within less than three years, a recrystallization of less than 7% of the barite mass is observed within five years. We describe the progress of recrystallization with a new modified homogeneous recrystallization model. Observed recrystallization rates are in the range 0.11–1.5 nmol/(m2 s) and increase with decreasing pH. According to this modified homogeneous recrystallization model, complete bulk barite equilibration is expected in about 1400–16,900 years.The strongly decreased recrystallization kinetics in our experiments is likely related to a strongly prolonged pre-equilibration time (0.8 years), which according to XRD investigations, leads to a higher crystallinity (higher crystal domain size and lower Debye-Waller parameters) of the barite powder.

Effects of hot rolled microstructure after twin-roll casting on microstructure, texture and magnetic properties of low silicon non-oriented electrical steel

In this work, a 0.71wt%Si+0.44wt%Al as-cast strip was produced by novel twin-roll casting. Some as-cast samples were respectively reheated and hot rolled at different temperatures in order to obtain different microstructure prior to cold rolling and annealing. The effects of the hot rolled microstructure on microstructure, texture evolution and magnetic properties were investigated in detail. A coarse deformed microstructure with λ-fiber texture was formed after hot rolling at 850–1050°C, finally leading to an inhomogeneous recrystallization microstructure with strong λ-fiber, Goss and extremely weak γ-fiber texture. By contrast, a fine transformed microstructure was formed after hot rolling at 1150–1250°C, finally leading to a fine and homogeneous recrystallization microstructure with stronger α-fiber, γ-fiber and much weaker λ-fiber texture. It should be noted that both the magnetic induction and core loss non-monotonically decreased or increased according to the hot rolling temperature. The unfavorable α-fiber and γ-fiber textures in the annealed sheets were much weaker than those of the conventional products regardless of the hot rolling temperature, thus contributing to a much higher magnetic induction. However, the average grain size in the annealed sheets was much lower than those of the conventional products regardless of the hot rolling temperature, thus leading to a higher core loss except the case of 1050°C. Hence, it is underscored that better integrated magnetic properties than those of the conventional products can be obtained by optimizing the hot rolled microstructure to produce final desirable recrystallization microstructure and texture.

Effects of initial microstructure and texture on microstructure, texture evolution and magnetic properties of non-oriented electrical steel

An equiaxed grained as-cast strip and a columnar grained as-cast strip was produced by using twin-roll strip casting, respectively. Both as-cast strips mainly containing 0.71wt%Si and 0.44wt%Al were cold rolled and annealed with or without the hot rolling prior to cold rolling. Microstructure, texture evolution along the whole processing routes and the magnetic properties were investigated in detail. It was found that the equiaxed grained strip was characterized by almost random texture while the columnar grained strip was dominated by strong λ-fiber (<001>‖ND) texture. After cold rolling and annealing, all the final sheets of both the as-cast strips showed extremely weak γ-fiber (<111>‖ND) recrystallization texture. In addition, the finally annealed sheets of the equiaxed grained strip were dominated by relatively weak λ-fiber and strong Goss ({110}<001>) recrystallization texture while those of the columnar grained strip were dominated by much stronger λ-fiber and much weaker Goss recrystallization texture regardless of whether the hot rolling was adopted before cold rolling, thus the former showed much lower magnetic induction than the latter. On the other hand, even though the finally annealed sheets of the equiaxed grained strip showed a little more homogeneous recrystallization microstructure with a little bigger grain size than those of the columnar grained strip in the case of no hot rolling, a much higher iron loss was displayed. By contrast, in the case of hot rolling, the former exhibited a little lower iron loss than the latter as a result of the more significant increase in grain size and λ-fiber recrystallization texture. The introduction of the hot rolling could increase the grain size, strengthen λ-fiber texture and weaken Goss texture of the finally annealed sheets of both the as-cast strips, leading to a much improvement in both the magnetic induction and iron loss.

Recrystallization of 223Ra with barium sulfate

In this work, the kinetics of barium sulfate recrystallization has been studied in acidic 0.01 mol dm -3 sodium sulfate solution using Ra-223 and Ba-133 tracers at very low total radium concentration, i. e. less than 10(-13) mol dm(-3). It was found that the system follows the homogeneous recrystallization model and that recrystallization rates, inferred by the decrease of Ra-223 and Ba-133 in the aqueous solution, are fast. Therefore, even at very low concentrations, below the solubility limit, radium will be retained by barium sulfate-a mineral present in the deep underground repository.

Carbon Isotope Exchange During Calcite Interaction With Brine: Implications for 14C Dating of Hypersaline Groundwater

Due to its possible role in solid/water carbon isotope exchange, the effect of salinity on radiocarbon dating of groundwater was examined by batch interaction of alluvial sediment and calcite powder with freshwater (Cl– = 100 mg L–1) and Dead Sea (DS) brine (Cl– = 225 g L–1). These 2 water types were spiked with H13CO3– tracer and kept under constant agitation for about 1 yr. Several bottles were respiked twice with the tracer. The uptake of the 13C by calcite was monitored through repeated isotopic measurements of the aqueous solutions, and the effect on 14C groundwater dating was evaluated using a simple transport reaction model. The results indicate that the kinetics of water/calcite isotope exchange start with a very fast initial step followed by a slower one, which was used here to simulate the long-term water/solid exchange in “real” aquifers. The exchange model that best fits the data was homogeneous recrystallization that formed just a very thin layer of newly formed calcite. The estimated recrystallization rates for calcite powder/solution interaction were much smaller for the DS brine than for freshwater: 3 × 10–5 to 7 × 10–6 and 9 × 10–4 to 7 × 10–5 mol m2 yr–1, respectively. The 13C experimental data imply a very small effect of the brine/calcite isotope exchange on the 14C age estimate for the brines within the DS coastal aquifer. However, when calcite recrystallization reaches ∼1% of the solid, the 14C groundwater dating estimates will show aging by ∼10%.

Carbon Isotope Exchange During Calcite Interaction With Brine: Implications for 14C Dating of Hypersaline Groundwater

Due to its possible role in solid/water carbon isotope exchange, the effect of salinity on radiocarbon dating of groundwater was examined by batch interaction of alluvial sediment and calcite powder with freshwater (Cl–= 100 mg L–1) and Dead Sea (DS) brine (Cl–= 225 g L–1). These 2 water types were spiked with H13CO3–tracer and kept under constant agitation for about 1 yr. Several bottles were respiked twice with the tracer. The uptake of the13C by calcite was monitored through repeated isotopic measurements of the aqueous solutions, and the effect on14C groundwater dating was evaluated using a simple transport reaction model. The results indicate that the kinetics of water/calcite isotope exchange start with a very fast initial step followed by a slower one, which was used here to simulate the long-term water/solid exchange in “real” aquifers. The exchange model that best fits the data was homogeneous recrystallization that formed just a very thin layer of newly formed calcite. The estimated recrystallization rates for calcite powder/solution interaction were much smaller for the DS brine than for freshwater: 3 x 10–5to 7 x 10–6and 9 x 10–4to 7 x 10–5mol m2yr–1, respectively. The13C experimental data imply a very small effect of the brine/calcite isotope exchange on the14C age estimate for the brines within the DS coastal aquifer. However, when calcite recrystallization reaches ∼1% of the solid, the14C groundwater dating estimates will show aging by ∼10%.

Temperature and Deformation Effects on the Recrystallization Microstructure and Texture of Wire Draw Steel

In this work, the effect of the deformation, caused by cold wire drawing, on the microstructure and the texture of low carbon steel wire (0.06 wt % C) is examined. The combined influence of the deformation level and the recrystallization temperature on the development of new grains is studied for all wires. Isothermal tests of annealing allow the determination of the critical temperature of recrystallization estimated above 450°C. The temperature effect is studied below the eutectoid level, at 500°C, 600°C and 680°C. The appearance of a homogeneous recrystallization is noted over the section of the wire. The recrystallized grains keep the same orientation as the deformed grains. The expansion of time of annealing lead to recrystallization in the ferritic grains accompanied by a spheroidization of the lamellar pearlite. The kinetics of recrystallization and spheroidization are accelerated by increasing of annealing temperature and the deformation level. The experimental techniques used in this study are: the Scanning Electron Microscope (SEM), the Electron Back Scattered Diffraction (EBSD), the X-ray diffraction and Vickers microhardness.

Attempts of whole-rock K/Ar dating of mesozoic volcanic and hypabissal igneous rocks from the Central Subbetic (Southern Spain): A case of differential Argon loss related to very low-grade metamorphism

12 samples of basic intrusives within Triassic rocks «ophites») and 11 samples of volcanic and associated intrusives within Jurassic to Early Cretaceous sequences of the Subbetic Zone were subjected to whole-rock K/Ar dating in combination with chemical/petrological analysis. Satisfactory results were obtained only from a number of samples of volcanic rocks, however, analytical ages commonly agree, within about 10 relative percent, with those deduced from stratigraphic location. «Ophite» samples, on the other hand, may reveal considerably lower analytic ages than the volcanics and show much stronger scattering, even among samples collected within a small area. It is argued that the inferred loss of Ar results from very-low-grade alpine metamorphic alteration, which affected the «ophites» more intensely than the higher volcanic rocks. Other post-emplacement chemical changes, such as the degree of secondary oxidation of Fe, are also distintive among the two groups of samples, and are to some extent consistent with the above view in that the alteration environment of the ophites should have produced conditions for more penetrative fluid-rock interactions and homogeneous recrystallization. Overall, the magmatic activity from which the ophitic rocks originated might have started in the Late Triassic and continued in the Lower Jurassic. 80th, the «ophites» and the volcanics are though to be the result of magmatic events Collowing tensional to transtensive crustal movements affecting the external basins of the Betic Cordilleras Crom Late Triassic to Early Cretaceous times.