A discrimination among microbial iron oxidation mechanisms

The oxidation mechanism of ferrous ion (Fe¹¹) by whole cells of Thiobacillus ferrooxidans was studied. Product inhibition by ferric ion (Fe¹¹¹) was observed experimentally at the pH = 2.0 and initial inhibitory iron concentration of 2.58 mM ≤ Fe¹¹¹ ≤ 15.0 mM. Experimental results indicated that the hyperbolic expressions (i.e. Monod type mechanisms) were valid to describe the microbial oxidation. A Bayesian statistical method for discrimination was utilized to identify the inhibition mechanism. In order to use the data efficiently the integrals of the inhibition models were taken (concentration versus time). Such analysis indicated that a non-competitive mechanism with two inhibition parameters was superior to others describing the microbial oxidation. Not only was this the best model, it was also a good one in the sense that the errors of predictions made from it were consistent with experimental error. Estimates for the parameters of the non-competitive inhibition model were obtained.     

Scale effect in high-rate (spall) failure

Experimental results on the spall strength of copper in which the scale of the system was changed by a factor of ten show that the scale effect for high-rate one-dimensional strain depends on energy. The spall energy per unit surface area for failure increases with time. Arzamas-16. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 6, pp. 88–93, November–December, 1993.     

Microstructural investigation of long-term degradation mechanisms in GFRP dowel bars for jointed concrete pavement

Steel dowel bar is used to transfer loads in concrete pavement slab. However, once the steel dowel bar corrodes, it may cause faults, such as joint freezing in concrete pavement, level differences resulting from spalling or decreased efficiency of load transfer, etc., which are the same problems experienced by typical reinforcing steel. This study evaluated the applicability of glass fiber-reinforced polymer (GFRP) dowel bar as a substitute for steel dowel bar. A microstructural analysis was conducted to examine the decrease in durability of GFRP dowel bar exposed to deterioration environments. To analyze the deterioration mechanism of GFRP dowel bar, scanning electron microscopy was employed and the porosity was measured by the gas absorption method. It was concluded that the longer the GFRP dowel bar was exposed to deterioration environments, the more the interlaminar shear stress decreased. This result was validated by the microstructural analysis. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Microstructural evolution under load and high temperature deformation mechanisms of a mullite/alumina fibre

A two-phase mullite alumina fibre, the 3M Nextel 720 fibre, has been studied in tension and creep. The fibre shows the highest creep resistance of all current commercial fine oxide fibres up to 1500 °C. The creep mechanisms involve progressive dissolution of mullite and simultaneous reprecipitation of alumina into elongated oriented grains and grain boundary sliding by a thin alumino-silicate liquid phase. The rate of grain growth in creep at a given temperature is dependant on the applied stress. The combination of sub-micron size mullite crystallites and alumina grains gives rise to a high sensitivity to alkaline contamination. Stress enhanced diffusion of the contaminants from the fibre surface results in crack nucleation, dissolution of mullite, formation of a liquid phase and slow crack growth. From 1200 °C, this process is coupled with a fast α-alumina grain growth at the fibre surface.     

Quantification of the mechanisms governing the packing of iron ore fines

An investigation of the effect of moisture content on the bulk density of iron ore fines has been carried out. Particle size distribution, density of agglomerates and angle of repose in both wet and frozen states were measured as a function of moisture content. Based on the experimental results, the mechanisms governing the relationship between bulk density and moisture content, i.e. the density of agglomerates, the inter-particle friction, tightened size distribution and agglomerate deformation, were quantitatively analysed. The results suggest that when moisture content is small, less than 6.5% for the ore fines considered the mechanism of agglomerate deformation can be ignored, and the mechanisms of inter-particle friction and tightened size distribution result in a decrease in bulk density. When moisture content is large, the mechanism of agglomerate deformation becomes effective too, which, together with the mechanism of inter-particle friction, results in an increase in bulk density after it reaches a minimum value. The density of agglomerates also affects the relationship between bulk density and moisture content, resulting in a decrease in bulk density as moisture contents increases, like the mechanism of tightened size distribution but with a smaller effect. The findings should be useful to the development of strategies for controlling the bulk density of ore fines in practice.     

投加不同形态的铁对厌氧消化的影响和作用机理

厌氧消化(AD)因具有有机负荷高、能耗低、污泥产量少、可回收再生能源等优点被广泛用于有机固废、工业废水及复杂物料处理。Fe是厌氧微生物所必需的生长因子,参与多种酶的激活反应,甚至可直接参与种群间的直接电子传递。然而,过量投加Fe会对微生物产生毒害和抑制作用。研究表明,螯合剂[次氮基三乙酸(NTA)、乙二胺四乙酸(EDTA)等]可与Fe形成螯合物,有助于提高Fe的生物利用度,进而降低Fe的投加量。重点总结Fe在AD中的作用机理,以及投加不同形态的Fe对厌氧产酸产甲烷的影响,同时阐述螯合剂提高Fe的生物利用度的效能。     

Specific features of the transformation of spall cracks to localized shear bands

Investigations of specific features of the microstructure of the region where a spall crack transforms to an adiabatic shear band are based on a spall model of strain localization, which implies that adiabatic shear bands are induced by interference of unloading waves, and the value of the negative stress in the expansion region of these waves does not exceed the dynamic strength of the material. It is shown that the transformation region contains a tremendous number of dislocation ensembles, which is much greater than the number of dislocation ensembles generated by a shock wave. Detection of micrometer-sized fracture sites in the region of interference of unloading waves implies that small fracture sites are formed in a polycrystalline material on dislocations arising in the course of dynamic tension.     

Electrical relaxation and conduction mechanisms in iron doped barium strontium titanate

Polycrystalline Ba_0.7Sr_0.3Ti_(1?x)Fe_xO₃ (x = 0.1) (BSTF) ceramics, synthesized via solid-state reaction route were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and complex impedance spectroscopy (CIS). The Rietveld refinement of the XRD data confirmed the presence of tetragonal and cubic phases in the prepared sample. The SEM image revealed that the sample has well distributed grains along with some degree of agglomerations. The electrical behaviour of the BSTF ceramic has been studied by complex impedance spectroscopy (CIS) as a function of frequency (1 Hz to 1 MHz) at different temperatures (RT to 700 K). Two semicircular arcs observed in the Cole-Cole plot confirm the contribution from the grain and grain boundary in overall impedance. Both the electrical as well as ac conduction phenomena take place via correlated barrier hopping (CBH) authenticated by detailed complex modulus analysis and ac fitted conductivity respectively. The values of activation energies calculated from electrical impedance, modulus, and conductivity data clearly reveal that the relaxation and conduction processes in BSTF ceramic are induced by doubly ionized oxygen vacancies.     

Changes in the CEC of a soil smectite–kaolinite clay fraction as induced by structural iron reduction and iron coatings dissolution

Studies of the effects of changes in iron (Fe) redox status on cation exchange capacity (CEC) and other physical and chemical properties of clays have typically focused on purified clays or clay fractions, but little attention has been given to systems with mixed mineralogy, which is more typical of natural soils. The objective of this study was to measure and establish any correlation between changes in CEC and Fe mineralogy that occurs in a mixed-mineral clay system undergoing chemical reduction. The clay fraction (SE1089) of a soil containing a mixture of smectite, kaolinite, and Fe oxide was investigated using variable-temperature Mössbauer spectroscopy, chemical analyses, and reductive dissolution in CBD media. This study revealed that in the unreduced clay fraction half of the total Fe was structural iron (Fe_Str) in the smectite and half was in goethite. The goethite particles were estimated to be 9 nm in mean crystal diameter (MCD) and to contain 9% Al substituted isomorphously for Fe.An evolution of the different Fe pools and changes in CEC of the mixture were observed. The CEC of SE1089 smectite fraction sharply increased upon reduction, similar to the SWa-1 reference clay, even though the Fe_Str content of SE1089 is much less. Results revealed a correlation of the increase in CEC with Fe_Str reduction and oxide dissolution, giving a direct connection between the total coating content and the rate of dissolution. For this reason, dissolution kinetics appears to be a key factor in understanding the control of CEC by iron coated material. In particular, in natural media governed by bacterial activity, different dissolution kinetics are expected.     

Dependence of spall strength of metals on the amplitude of a shock-wave load

Arzamas. Translated from Fizika Goreniya i Vzryva, Vol. 28, No. 1, pp. 94–98, January–February, 1992.     

Reaction mechanisms of tridymite iron phosphate with lithium ions in the low-voltage range

The electrochemical reaction mechanisms of tridymite FePO₄ with Li ions were studied in the voltage range of 2.4-0 V. The lithiation/delithiation of FePO₄ was found to be similar to that of binary 3d-metal oxides, which involve the formation and decomposition of Li₂O along with the reduction and oxidation of nanoscale 3d-metal nanoparticles. In the first discharge, FePO₄ was reduced to metallic Fe nanoparticles dispersed in a lithium compound matrix consisting of Li₃PO₄ and Li₂O. In the subsequent charge, these metallic Fe nanoparticles were partially oxidized to FeO by the decomposition of Li₂O. The Li₃PO₄ matrix was inactive during the change of potential from 0 to 2.4 V. It is notable that the re-oxidation of metallic Fe occurred only in the case of small nanoparticles, and the large Fe particles remained electrochemically inert during the reversible reaction.     

Binding mechanisms in wet iron ore green pellets with a bentonite binder

Fundamental research during the past decade has been focussed on understanding the role of viscous forces on agglomerate deformability and strength. Much of this work has been done on glass spheres using Newtonian liquids as a binder. In this work, we show the variations in plasticity and strength of magnetite iron ore green pellets with varying liquid saturations and binder dosages (viscosities). For this purpose, a new measuring instrument was built to analyze the green pellet wet compression strength, plastic deformation and breakage pattern.Industrial iron ore green pellets are over-saturated and a supporting “network” of viscous liquid is formed on the green pellet surface. At least half, probably more, of the total binding force appeared to be due to the cohesive force of the network. The other half (or less) of the total compression strength can be explained by the capillary force. Due to irregularities on green pellet surfaces, both fully developed concave pore openings and saturated areas are expected to be found at the same time.Wet green pellets started showing plastic behaviour as they became over-saturated. In over-saturated green pellets, an explosive increase in plasticity with increasing moisture content was seen, due to the contemporary increase in porosity. Plasticity is an important green pellet property in balling and should gain the status of a standard method in green pellet characterization. It is suggested that the control strategy for the balling circuits be based on plastic deformation and compression strength of green pellets instead of the rather inaccurate drop number. The results also point out the importance of knowing whether the balling process should be controlled by adjusting the moisture content (plasticity) or by adjusting the bentonite dosage (viscosity). These two operations are not interchangeable—even if they would compensate in growth rate, the green pellet properties would differ.A new green pellet growth mechanism is suggested, based on the measured over-saturation. Firstly, green pellet plasticity needs to exceed a minimum level to enable growth. This limiting plasticity defines the material-specific moisture content needed in balling. Secondly, it is suggested that the growth rate be controlled by the viscosity of the superficial water layer rather than by the mobility of the pore water.     

Modeling and simulation of iron ore pellet drying and induration process with accurate bed void fraction calculation

As key parameters of modeling iron ore pellet drying and induration process, the properties of a packing bed are calculated by our proposed method. Derived from realistic spheres-dropping phenomena, the dropping and rolling rules make the moving sphere finally reach its minimum gravitational potential energy. Furthermore, the plastic deformation of green pellets and mechanic vibration are also considered in the model. The model is based on the laws of mass and heat transfer, physical process, and chemical reaction. The partial differential equations of the process kinetic model involve derivatives in time and bed depth. Comparing the results with measured data proves that the model can be used for optimizing the iron ore pellet induration process and as an effective tool for other, similar thermodynamic process simulations.     

氧化物-非氧化物复合材料的显微结构特征

利用SEM、TEM及HREM研究了氧化物-非氧化物复合材料的显微结构特征。结果表明(1)在氧化物基体中引入非氧化物,非氧化物颗粒将穿插于氧化物颗粒构成的骨架中;在非氧化物基体中引入氧化物,氧化物颗粒将弥散于非氧化物颗粒构成的编织状结构的空隙处。(2)氧化物与非氧化物之间的结合方式可分为(a)直接结合(在没有助烧剂、直接烧结的情况下);(b)通过晶界非晶质薄膜相结合(在有助烧剂奉与的直接烧结和反应烧结的情况下)。(3)所研究的试祥中普遍存在有微裂纹,这些微裂纹将有助于材料抗热震性的改善。