Phenomena in late period of seeded precipitation of sodium aluminate solution

1 Introduction The seeded precipitation from supersaturated sodium aluminate solution is a crucial step in the Bayer process for the production of alumina, which is a complicated crystallization operation involving a variety of physico-chemistry sub-proc…     

Effect of 20kHz ultrasound on alumina hydrate precipitation from seeded sodium aluminate solution

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Kinetics of crystal growth on seeded precipitation of sodium aluminate solutions with new device

A new device was designed, which can effectively avoid the undesired nucleation and agglomeration of fine particles on the experimental results during the seaded precipitation of sodium aluminate solution, and moreover, the experimental conditions are nearly kept constant during the experiment. With the new device, it is proven that a good result can be obtained on the kinetics study of the crystal growth in seeded precipitation of sodium aluminate solution. Experiments were carried out with the concentration of Na2O (Nk)170 g/L, the mole ratio of Na2O to Al2O3(ak) all between 1.52 to 2.01, at 65, 70, 75℃, respectively. And the kinetics equation of crystal growth of gibbsite was deduced.     

Preparation of Al（OH）3 by ion membrane electrolysis and precipitation of sodium aluminate solution with seeds

The preparation of AI（OH）3 by the ion exchange membrane electrolysis followed by the precipitation of sodium aluminate solution with seeds was made. During the process of ion membrane electrolysis, the sodium aluminate solution is rapidly acidified and the caustic ratio （αK） is decreased due to oxygen evolution in the anodic region. And the causticity of solution is increased due to hydrogen evolution in the cathode region, producing the high concentration of caustic soda solution. Regulating the acidity of the anodic solution by controlling the electric quantity in the electrolysis and subsequent decomposing the solution, AI（OH）3 could yield with very large rate and high efficiency. The experiments also indicate that the quality of aluminum hydroxide product is greatly affected by the impurity silicon.     

Influence of supersaturation on structure of sodium aluminate solutions with medium concentration:a solution X-ray diffraction study

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Adsorption effect mechanism of sodium oleate on feldspar and quartz

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Combined effect of melt thermal treatment and solution heat treatment on eutectic Si particles in cast Al–Si alloys

The present results showed that in Al&hyphen ;Si alloys, Sr modified (SrM), superheat (SH) and SrM melt thermal treatment (SrMTT) processed castings provide fine eutectic Si particles, with the SrMTT process giving the best modification results. Both size and morphology of the eutectic silicon particles are affected by the modification process used. The SrM, SH and SrMTT castings show well modified fibrous Si particles, whereas the melt thermal treatment (MTT) casting exhibits Si particles that, although refined to a certain extent, still retain their acicular morphology. Cooling rate affects the eutectic Si particle size in that a higher cooling rate produces finer Si particles. However, within the range of cooling rates provided by the end chill mould used in this work, the cooling rate does not affect the morphology of the Si particles. During solution heat treatment at 540°C, the eutectic Si particles undergo fragmentation, spheroidisation and coarsening, affecting the Si particle morphology. The spheroidisation process is determined by the size and morphology of the Si particles in the as cast condition. The SrM, SH and SrMTT processed castings with their refined Si particles require much less solution treatment time for the spheroidisation process to take place than do the non-modified and MTT castings.     

The effect of yttrium on densification and grain growth in α-alumina

The grain growth and densification have been investigated in very high-purity α-alumina doped with varying amounts of yttrium (0 to 3000 wt ppm of yttria) and sintered in air at 1450, 1550 and 1650 °C. Yttrium doping inhibited densification and coarsening at 1450 °C, but had very little effect at 1550 °C and no effect at 1650 °C. The change in densification behaviour is suggested to be related to the transition with increasing temperature from grain boundary diffusion to lattice diffusion controlled densification. The coarsening rate increases faster with temperature than the densification rate. This was correlated with a higher measured activation energy for grain growth than for the diffusion processes, which control the densification.     

The effect of atmospheric precipitation on the corrosion of ferrous metals buried in soils

Data from the US NBS study for corrosion of buried objects are employed to show that average annual atmospheric precipitation directly influences corrosion of ferrous objects buried in various soils. This is attributed to the softness of precipitation waters and their delivery of oxygen. Also, as surrogate for time of wetness at the buried object within the soil, atmospheric precipitation has a clearer effect on corrosion loss and localised corrosion than does ground surface time of wetness. The relationship and the amount of corrosion also depend on soil type, a matter not previously considered. The results allow an explanation for the generally higher rate of corrosion of cast iron water pipes in cities such as Sydney with mainly clay-loam soil backfill and high precipitation compared with many locations in the UK with much lower annual average precipitation.     

FIB-SEM investigation on corrosion propagation of aluminium–lithium alloy in sodium chloride solution

Abstract

Microstructural characterisation of 2A97-T4 aluminium–lithium alloy was carried out using electron probe microanalysis and transmission electron microscopy (TEM). Scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy facilities has been employed to examine localised corrosion sites after immersion in sodium chloride solution. A dual beam microscope, which integrates a focused ion beam and an electron beam in one powerful instrument, has also been employed to investigate the development of intergranular corrosion from both surface and cross-section. It was found that localised corrosion is generally initiated at θ phase particles, which represents only 8.4% of the intermetallic (IM) particles in 2A97-T4 aluminium–lithium alloy. θ phase particles exhibit preferential dissolution of aluminium during corrosion testing, with trench formed at their periphery as well. Initiation of intergranular corrosion is relatively late with respect to the attack of IM particles. Owing to the presence of θ phase particles at intergranular corrosion sites and non-uniform distribution of T1 (Al₂CuLi) grain boundary precipitates, it is supposed that dealloyed θ phase particles and grain boundary precipitates cooperate to provide the driving force for grain boundary attack.     

Investigation of adsorption and corrosion inhibition effect of 1,1’-thiocarbonyldiimidazole on mild steel in hydrochloric acid solution

The adsorption and inhibition effect of 1,1′-thiocarbonyldiimidazole (TCDI) on the corrosion of mild steel (MS) in 0.5 M HCl solution was studied in both short and long immersion time (120 h) with the help of electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR) techniques. For long-time tests, the hydrogen gas evolution (VH₂-t) and the change of open circuit potential with immersion time (E _ocp-t) were also utilized in addition to the former two techniques. The surface morphology of MS after its exposure to 0.5 M HCl solution with and without 1.0 × 10⁻² M TCDI was examined by scanning electron microscopy (SEM). It was demonstrated that the inhibition efficiency of studied inhibitor is concentration depended and increased with TCDI concentration. The higher value of inhibition efficiency was obtained after longer immersion time merely on the basis of strong increase of corrosion rate of mild steel in the blank solution. The high inhibition efficiency was discussed in terms of adsorption of inhibitor molecules and protective film formation on the mild steel surface which was substantiated by SEM micrographs. The adsorption of TCDI on MS was found to obey Langmuir adsorption isotherm.     

Influence of AlF3 and hydrothermal conditions on morphologies of α-Al2O3

Homogeneous α-Al2O3 platelets were synthesized by introducing AlF3 to alumina precursor.The effects of AlF3 additive on the phase transformation and morphology of the prepared α-Al2O3 platelets were investigated.The results show that a single phase of α-Al2O3 with an average particle size of 8μm can be obtained at 900℃ with 2% AlF3 additive.The transformation temperature decreasing IS attributed to introduction of Al3 vacancy and to the formation of intermediate compound of AlOF,which is considered to accelerate the mass transportation from transitional Al2O3 to α-Al2O3.AlF3 concentration and hydrothermal temperature can also affect the morphology of α-Al2O3.When hydrothermal temperature is 120℃,the morphology of α-Al2O3 transforms from irregular to flat hexangular platelet with increasing AlF3 concentration.As hydrothermal temperature increases,the morphology of α-Al2O3 with 2% AlF3 additive changes from polyhedron to hexangular platelet and then to vermicular.     

Passivity breakdown and stress corrosion cracking of α-brass in sodium nitrate solutions

The anodic behaviour and SCC susceptibility of pure copper and four α-brasses of different zinc alloy concentration in a 1 M NaNO₃ solution was studied by means of potentiodynamic polarisation curves and constant potential slow strain rate experiments. SCC was exclusively observed when the potential was equal to or higher than a certain critical value (E_c) at which pitting initiated under slow dynamic straining. It is concluded that the same SCC mechanism should be operating during SCC of copper and α-brasses in sodium nitrate, sodium nitrite and copper (II) nitrate solutions.     

Influence of cluster mobility on Cu precipitation in α-Fe: A cluster dynamics modeling

A cluster dynamics model has been parametrized to quantitatively reproduce results obtained by atomistic kinetic Monte Carlo (AKMC) modeling on the precipitation of Cu in α-Fe under thermal aging. The cluster mobility, highlighted by AKMC, is shown to have a significant effect on the precipitation kinetics and can reconcile the experimentally observed fast kinetics with the relatively low diffusivity of Cu monomers.     

The effect of pre-existing defects on the strength and deformation behavior of α-Fe nanopillars

The effects of two types of pre-existing defects, dislocations and clusters, on the strength and deformation behavior of body-centered cubic Fe nanopillars with a diameter of ～150 nm were investigated using in situ nanocompression in a transmission electron microscope. The plastic deformation of nanopillars containing high initial dislocation densities was observed to be relatively continuous, proceeding via a series of small- and intermediate-scale strain bursts that were associated with the movement/escape of dislocations and the formation of slip bands. Mechanical annealing was observed in nanopillars with high dislocation densities. When the dislocation density was reduced by in situ heating, the nanopillars were much stronger and the plastic deformation behavior transformed to a more abrupt and explosive mode. The introduction of a dispersion of solute atom clusters into nanopillars caused further strengthening as a higher stress level is required for dislocations to pass the clusters. The strengthening effect of cluster dispersion in nanopillars is comparable to that observed in the bulk steel. These phenomena are universal for Fe nanopillars with different crystallographic orientations.     

Some observations on the films formed on α-brasses tarnished in an ammoniacal solution of copper sulphate

Tarnish films on copper and its alloys with zinc forming in a stress corrosion environment have been studied with an automatically following ellipsometer. The porosity of the films has been examined and measurements of electrical conductivity have been made. The data are discussed in terms of the incorporation of Zn in the Cu₂O film substance and its dependence on alloy composition. An explanation is thereby provided for the high tarnishing rates in terms of high electronic and ionic mobility induced by the defective nature of the tarnish.     

Effect of pH on corrosion and monotonic loading behaviour of 90Cu–10Ni in 3·5% sodium chloride solution

Abstract

Polarisation studies were performed on 90Cu–10Ni (alloy UNS C70600, hot rolled and annealed) in 3·5% NaCl solutions over a range of initial pH levels. It was found that film formation was dependent on the applied current and the relative concentrations of the ionic species H⁺, (OH)^?, and Cl^? in solution which ultimately influenced the formation of the corrosion products. Constant extension rate tests at different applied currents and pH levels suggested that a higher pH level increased the time to failure although no evidence of stress corrosion cracking was found. These observations are rationalised through surface film formation mechanisms.     

Effect of large cold deformation after solution treatment on precipitation characteristic and deformation strengthening of 2024 and 7A04 aluminum alloys

1Introduction Studies on thermomechanical treatment of aluminum alloy can date from the1960s[1,2].A great deal of achievements concerning this study has been obtained after near40years of development[3?7].Among them,the achievement of intermediate thermom…     

Influence of different solution methods on microstructure,precipitation behavior and mechanical properties of Al–Mg–Si alloy

The effects of different solution methods on microstructure, mechanical properties and precipitation behavior of Al–Mg–Si alloy were investigated by scanning electron microscope, transmission electron microscope, tensile test, and differential scanning calorimetry. The results revealed that the recrystallized grains of the alloy after the solution treatment with hot air became smaller and more uniform, compared with solution treatment with electrical resistance. The texture of the alloy after two solution treatment methods was different. More rotated cube components were formed through solution treatment with electrical resistance, which was better for improving the drawability of the alloy. The strength of the alloy under the solution treatment with hot air was higher before stamping, because of the small uniform grains and many clusters in the matrix. The alloy solution treated with hot air also possessed good bake hardenability, because the transformation occurred on more clusters in the matrix.