General class of multiparticulate dissolution models

The dissolution of multiparticulate systems under sink and nonsink conditions can be described rigorously according to a generally applicable formula on the basis of the single-particle dissolution model and the initial particle distribution. The kinetic model for log-normal systems dissolving under sink conditions is extended to nonsink conditions as a specific example. The equation presented describes a general class of multiparticulate models for various values of the dispersion parameter and the dissolution capacity coefficient.     

Toward prediction of microstructural evolution during laser surface alloying

The stability of tungsten carbide particles in iron-rich and nickel-rich liquid during the laser surface alloying (LSA) process was investigated. Kinetic calculations indicate a rapid dissolution of tungsten carbide particles in iron-rich liquid, as compared with the dissolution rate in nickel-rich liquid. Optical microscopy indicated a heterogeneous microstructure around the tungsten particles that is in agreement with concentration gradients predicted by kinetic calculations. The work demonstrates the applicability of computational thermodynamics and kinetic models for the LSA process.     

Dissolution of particles in binary alloys: part I. computer simulations

A detailed numerical study has been made of the dissolution kinetics of particles in binary alloys during isothermal annealing. In earlier models, the assumption was made that the dissolution reaction could be described by the dissolution of only one particle in an infinite matrix or the dissolution of a regular array of particles of equal size. This assumption has been relaxed and a log-normal size distribution of particles has been introduced instead. The calculations have been done numerically by applying a finite difference technique to a spherical particle in a spherical cell of finite size. The presence of a size distribution of particles was found to have a great effect on the dissolution kinetics and, therefore, must be included in a reliable model for the dissolution of particles. The results have been presented in diagrams, giving the volume fraction as a function of the dimensionless annealing time with the geometrical standard deviation as a parameter, and thus should be useful in making accurate predictions of the dissolution kinetics of binary alloys. The curves can be used for all volume fractions provided that all of the particles can be dissolved completely at the temperature considered. Also, equations have been derived that can easily be used to give an estimate of the annealing time to dissolve 90 pct of the initial volume fraction.     

微生物分解难处理金矿的动力学研究

在均匀混合的间歇式反应器中用氧化亚铁硫杆菌分解难处理金矿,研究了不同粒度矿粒分解的动力学。结果表明,细菌在矿粒表面的吸附符合兰格穆尔等温吸附式,等温吸附常数KA不受粒度大小的影响,而单位质量矿粒的最大吸收量XAm随着粒度的增大而减小。建立了用于描述细菌生长和矿石分解的动力学方程式,其与试验数据能很好的吻合,矿石的初始粒度对吸附在矿粒表面的细菌的生长以及矿石的分解都有很大的影响。     

Dissolution kinetics of NbC particles in the heat-affected zone of type 347 austenitic stainless steel

Dissolution kinetics of NbC particles in the heat-affected zone (HAZ) of Type 347 austenitic stainless steel were experimentally studied by a rapid cooling method. Coupons with 1.5-mm thickness were water quenched on a GLEEBLE thermomechanical simulator at various instances during the heating portion of a welding thermal cycle. Particle dissolution kinetics data were obtained by statistical analysis of digital images of resulting microstructure. For most of the test alloys, a good correlation exists between the dissolution kinetics and the susceptibility to cracking in the HAZ. The faster the Nb-carbide particles dissolve, the more resistant the alloy is to constitutional liquation cracking. The rate at which particles dissolve seems to be affected by their stoichiometry. If Nb:C ratios in bulk alloys are closer to the ideal stoichiometry of 7.7, NbC particles formed in the alloy are more likely to be stable and, hence, slower to dissolve.     

The response of carbo-nitride particles in hsla steels to weld thermal cycles

The behaviour of carbo-nitride particles in HSLA steels when subjected to a weld thermal cycle corresponding to heat inputs of 2, 5, 10 or 20 kJ/mm has been studied by analytical electron microscopy. Three types of response were observed:

• 1.(a) complete dissolution followed by reprecipitation on cooling (in a Nb-V containing steel),
• 2.(b) partial dissolution accompanied by precipitate coarsening (in two Ti bearing steels)
• 3.(c) dissolution followed by reprecipitation at the peak temperature of the thermal cycle (in Ti-Nb bearing steels).

The latter behaviour was shown to be dependent upon the reheat temperature used during manufacture and was only found in a steel reheated to 950°C; this treatment promoted the formation of Nb-rich particles having a narrow size range in the base metal. The rates of dissolution observed in the experiments were supported by numerical calculations, which gave dissolution times of the order of 1 s or less at the peak temperature (1350°C) of the weld thermal cycle. The changes in the particle size distributions in the Ti and Ti-Nb bearing steels and the composition changes observed for (Ti, Nb) carbo-nitride particles were explained by modelling the precipitation behaviour in the austenite phase field.     

Weld thermal cycles and precipitation effects in Ti-V-containing HSLA steels

Particle dissolution, precipitation, and grain-coarsening effects were examined in the heat-affected-zone (HAZ) of submerged are welds deposited at 3 and 6 kJ/mm heat inputs on three Ti-V-containing steels. The three steels showed different responses to the thermal cycle of the weld. The Ti-bearing steel (0.014 pct Ti) showed little change in the TiN particle distribution on welding, in agreement with calculations which demonstrated very limited TiN solution during the weld thermal cycle. For the high-V, low-Ti steel (0.093 pct V,0.002 pct Ti), the V-rich particles in the baseplate dissolved, and Ti-rich precipitates formed during the heating leg of the cycle, while V-rich particles precipitated again on cooling. The third steel (0.014 pct Ti, 0.08 pct V) contained a wide distribution of particle compositions. On welding, the Ti-rich cuboidal precipitates did not dissolve, while the V-rich spheroidal precipitates did dissolve. No reprecipitation was observed on cooling in this steel. The different behaviors of the three steels were related to the dissolution of the precipitates in the baseplate and the available thermodynamic driving force for reprecipitation to occur on cooling.     

Studies on dissolution kinetics of dolime in electrothermal magnesium slag

The electrothermal process of magnesium metal production is a promising route, where large sized internally heated reactor is used for magnesium production resulting in less energy and labour intensive and high space-time yield process. However, the dissolution behavior of dolime in the electrothermal slag has been found critical for the process optimization. In this paper, the dissolution kinetics of the dolime in the slag was discussed. Quaternary slag (CaO-Al₂O₃-SiO₂-MgO) was prepared having basicity CaO/SiO₂ ≥ 1.8 and Al₂O₃/SiO₂ ≥ 0.26 for dolime dissolution studies by static hot dip method. Prior to the experiments, FactSage calculations were carried out varying temperatures and slag compositions. In the kinetic studies, dolime particles 10–15 mm size was added in slag melted at 1450, 1500 and 1550°C and samples were taken at various time intervals. The chemical analysis of slag sample was carried out to investigate the dissolution kinetics to establish the rate expression. The activation energy for the process was calculated for different models used in study and was found to be in the range of 130–270 kJ/mol. SEM analysis was done for surface analysis of reacted particles. This study would be helpful in optimizing the dolime charging rate during pilot scale trials for electrothermal magnesium production at CSIR-NML, Jamshedpur.     

Dissolution of particles in binary alloys: part II. experimental investigation on an Al-Si alloy

A detailed experimental study of the dissolution kinetics of Si particles in an Al-Si alloy has been carried out in order to test the validity of the two models presented in the accompanying article.[1] In these models, the dissolution kinetics are dependent on the particle size distribution of the alloy. An alloy with composition C_o = 0.77 at. pct Si was heat-treated in order to obtain rather coarse spherical particles (1 to 10 μn). The size distribution of the particles was found to be close to the log-normal distribution. At high temperatures, when the solvus concentration was well above C_o, the experimental values were very close to the values of the model which predicted the highest dissolution rates. At lower temperatures, when the solvus concentration was closer toC _o, the experimental values lay in between the values predicted by the two models. The results clearly demonstrate that a size distribution of particles must be included in the model if an accurate prediction of the dissolution kinetics is to be achieved.     

Numerical simulation of dendrite white spot formation during vacuum arc remelting of INCONEL718

White spot is the term for a particulate dispersion lean in niobium found in vacuum arc remelted (VAR) ingots of niobium containing nickel-based superalloys, such as INCONEL718, that can be detrimental to the mechanical properties. While spot can result from exogenous fragments that fall into the VAR melt pool and remain incompletely melted. In this study, white spot formed when dendrite clusters fall-in from the shrinkage pipe of vacuum induction melted (VIM) electrodes is considered by simulations. The motion and dissolution of the dendrite cluster particles were simulated in the framework of a macroscopic heat and fluid flow model of the VAR process. Two scales of heat and mass transfer are considered within the cluster: interdendritic solute diffusion within particles and the thermal interaction between the particle and the bulk convective melt. The dissolution behavior of dendrite cluster fall-in was investigated for a range of initial particle conditions including solid fraction, Nb content, drop height, and initial temperature. The operational window where the exogenous particles completely dissolve was determined as a function of cluster size, density, and location. It was found that particles smaller than 3 mm are completely dissolved under all conditions simulated in this study. All factors studied demonstrated significant influence on particle dissolution. Particles with a solid fraction less than 0.5, a Nb content greater than 4 pct, or an initial temperature greater than 1400 K are likely to be dissolved immediately after entering the melt pool. Drop height and initial density had the greatest effect on particle dissolution.     

The Dissolution of Sphalerite in Ferric Chloride Solutions

The kinetics of dissolution of both sintered sphalerite disks and untreated sphalerite particles in ferric chloride-hydrochloric acid solutions have been investigated. Over the temperature interval 25 to 100°C, the dissolution occurred according to a linear rate law and with an associated apparent activation energy of about 10 kcal/mole. Most of the oxidized sulfide ion reported as elemental sulfur in the leach residues. The leaching rate was independent of the disk rotation speed and this fact, together with various hydrodynamic calculations, indicated that the reaction was chemically controlled. The dissolution rate increased as the 0.36 power of the ferric chloride concentration and it also increased substantially in the presence of dissolved CuCl₂. The accumulation of the ferrous chloride reaction product severely retarded the leaching reaction, but the presence of dissolved zinc chloride only slightly impeded it. The leaching rate was relatively insensitive to low levels of HC1 (>1 M), but increased dramatically at higher acid concentrations because of direct acid attack of the ZnS.     

Physical Simulation and Experimental Examination of ε-Cu Particles Dissolution Evolution During Welding of Copper Precipitation Strengthening Steel

The kinetics of ε-Cu particles dissolution in the matrix during welding of a copper-precipitation strengthening steel was determined by a combination of GleebleTM physical simulation, TEM examination and hardness measurement. The ε-Cu particles underwent a coarsening and part dissolution and then complete dissolution reaction as the peak temperature increased from 750 to 1 000 ℃, which resulted ir the decrease in the number density of ε-Cu particles and hardness in the heat-affected zone (HAZ). The results can be used to understand the evolution of this transformation and a softening behavior of the HAZ during welding of this type of steel.     

Dissolution of iron intermetallics in Al-Si Alloys through nonequilibrium heat treatment

Conventional heat treatment techniques in Al-Si alloys to achieve optimum mechanical properties are limited to precipitation strengthening processes due to the presence of second-phase particles and spheroidization of silicon particles. The iron intermetallic compounds present in the microstructure of these alloys are reported to be stable, and they do not dissolve during conventional (equilibrium) heat treatments. The dissolution behavior of iron intermetallics on nonequilibrium heat treatment has been investigated by means of microstructure and mechanical property studies. The dissolution of iron intermetallics improves with increasing solution temperature. The addition of manganese to the alloy hinders the dissolution of iron intermetallics. Nonequilibrium heat treatment increases the strength properties of high iron alloys until a critical solution temperature is exceeded. Above this temperature, a large amount of liquid phase is formed as a result of interdendritic and grain boundary melting. The optimum solution treatment temperature for Al-6Si-3.5Cu-0.3Mg-lFe alloys is found to be between 515 °C and 520 °C.     

颗粒在液相中溶解-析出过程的蒙特卡罗模拟

利用蒙特卡罗方法,通过建立合理的模拟规则,对单个圆形颗粒在液相中被溶解并形成溶质、溶质扩散、以及溶质析出等过程进行模拟。模拟结果表明:颗粒溶解度随模拟时间延长而逐渐增加,液相中的溶质浓度相应趋于饱和;升高模拟温度会加快颗粒的溶解速率,使溶质在液相中达到饱和所需要的时间缩短,饱和浓度值相应增加;尺寸越小的颗粒表现出越高的溶解活性,通过对不同初始尺寸的颗粒在液相中达到溶解平衡过程的模拟,所得平衡尺寸及饱和浓度之间的关联性与Gibbs-Thomson关系较为吻合。上述模拟结果均与实际溶解情况较一致。     

Coarsening of SiO2 particles in copper and MnS inclusions in steel

A model to simulate the diffusion-controlled coarsening and dissolution kinetics of particles within a metallic matrix is formulated. With an arbitrary size distribution of particles, the model can be used to calculate the change in the size distribution of particles during coarsening or dissolution. Other system parameters, such as average radius of particles, volume fraction, average distance between particles, surface area, and matrix composition are also calculated. An important result is that kinetics do not generally obey the often-applied Lifshiftz-Slyozov-Wagner theory for diffusion controlled coarsening based upon concentration profiles around isolated spheres. In such a formulation, the direct effect of the surrounding particles is neglected. In our model, which is a modification of the coarsening kinetics described by Weins and Cahn, the effect of surrounding particles is incorporated because the system is taken to be a system of point potentials, each with a potential according to its radius of curvature. Calculations are on silica particles in a copper matrix and on manganese sulfide inclusions in iron, with emphasis on the latter, in order to predict their behavior during homogenization or soaking treatments. The effect of the composition of manganese, from 0.1 to 1.2 wt pct, on the coarsening of sulfides in a “high” sulfur (0.017 wt pct) steel and a “low” sulfur (0.003 wt pct) steel was investigated. As expected, the model predicts that manganese strongly reduces the rate of coarsening, particularly for times of ten hours or less in the temperature range of 1100 to 1400 °C. Calculated results also indicate that the rate of dissolution is very low at temperatures greater than the solvus for manganese sulfide inclusions in austenite.     

钢中夹杂物的分形维数及其与形貌特征的关系

 利用Matlab图像处理功能计算了钢中夹杂物SEM图像的边缘、二维和三维分形维数,并讨论了边缘分形维数与夹杂物颗粒微观凝聚机制和三维分形维数与其融入顶渣中动力学之间的关系。结果表明：边缘分形维数可定量地表征夹杂物边界轮廓线的曲折复杂程度;对非润湿夹杂物颗粒来说,其边缘分形维数越大,表明气泡易于在其表面生成,从而更加有力于夹杂物的碰撞凝聚长大;夹杂物颗粒的三维分形维数越小,更有利于夹杂物融入渣中而被去除;分形维数是一个影响不规则夹杂物颗粒碰撞、凝聚和去除的重要参数。     

Computer simulation of diffusion in multiphase systems

A general model to treat multicomponent diffusion in multiphase dispersions is presented. The model is based on multicomponent diffusion data and basic thermodynamic data and contains no adjustable parameters. No restriction is placed on the number of components or phases that take part in the calculations, as long as the necessary thermodynamic and kinetic data are available. The new model is implemented into the DICTRA software, which makes use of THERMO-CALC to handle the thermodynamics. The model is applied to carburization of Ni alloys and heat treatment of welded joints between dissimilar materials. In both cases, the diffusion is accompanied by carbide formation or dissolution. A good agreement between experiments and calculations is found, despite the fact that no adjustable parameters are needed.     

An investigation into the suitability of amidated pectin hydrogel beads as a delivery matrix for chloroquine

The aim of the present study was to delay the release of chloroquine to distal parts of the gastrointestinal tract by using a multiparticulate hydrogel formulation. Amidated pectin chloroquine beads (PC) with varying pectin-to-chloroquine ratios (PC) w/w loadings of 4:1, 2:1, and 1:1 in the dried beads were prepared by the gelation of drug-loaded pectin solutions in the presence of calcium. In vitro release studies of chloroquine from pectin-chloroquine hydrogel beads and chloroquine diphosphate powder were carried out in simulated gastric and intestinal fluids. The total release of the entrapped chloroquine from the hydrogel beads was achieved between 4 and 7 h in simulated intestinal fluid, but total release was not achieved in simulated gastric fluid. However, total release from chloroquine diphosphate powder was achieved by 1.5 and 2 h in gastric and intestinal fluids, respectively. The plasma pharmacokinetics of chloroquine from pectin hydrogel beads and chloroquine diphosphate solution following single or repeated dosing were compared in male Sprague-Dawley rats over a period of 60 h. Oral administration of the hyrogel beads to rats produced maximum plasma concentrations by 7 h, but highest plasma concentrations following chloroquine solution administration were observed by 2 h. The dissolution data and appearance of significant plasma concentrations of chloroquine 2 to 4 h after oral administration suggests release in duodenum, jejunum, or ileum.