Alumina Agglomerates Research Articles

Modelling the formation and dissolution behavior of alumina agglomerate in the cryolite

The presence of alumina agglomerates seriously affects the current efficiency of the aluminum electrolysis process. Clarify the dynamic dissolution process of agglomerates is essential to improve the current efficiency of aluminum electrolysis. A mathematical model is proposed to describe the different phenomena from the formation until complete dissolution of agglomerates. Considering permeation and solidification processes of cryolite, a semi-analytical mathematical model is developed to formulate the formation, melting and dissolution processes of agglomerates, and the time duration for each stage is deduced. Porosity and heat mass transfer of agglomerates are explored based on the packing theory and mechanism of heat mass transfer in wet porous media. Dimensionless approach is applied to investigate the main factors affecting the dissolution stages and porosity of agglomerates. The results show that the superheat has a great influence on the formation and melting stage, the diameter of agglomerates can reach 14.93 mm for 200 particles agglomerated. The density decreases with the increase of agglomerated particle number, which varies in the range of 2.27–2.28 g/cm3. The average dissolution rate of agglomerates is about 1.83×10−5−2.95×10−5 kg/s within the range of alumina concentration in this study.

Experimental investigations of AA6061/Al2O3/WC/SiC hybrid metal matrix composite fabricated by two step stir casting

ABSTRACT Hybrid metal matrix composites have emerged as an epicentre of material research due to their vast applications and a wide array of reinforcement combinations. In this experiment, two-step stir casting method had been used to fabricate hybrid aluminium 6061 metal matrix composites. The micro-structural and mechanical evaluation (density measurement, impact, tensile, hardness, and flexural) of the fabricated aluminium (AA6061) hybrid metal matrix composite had been performed. The reinforcements used were Al2O3 (3 wt.%), RHA (2 wt.%), WC (2 wt.%), and SiC (5 wt.%, 10 wt.%, 15 wt.%, 20 wt.%). The mechanical properties had been found to improve with increasing wt.% of SiC. The maximum experimental density (2999.2 Kg/m3), impact energy (6 J), tensile strength (130.15 N/mm2), hardness (35 HRA), and flexural load (6720 N) had been obtained with 20 wt.% SiC reinforcement. The presence of reinforcements (Al2O3, RHA, WC, SiC) and their uniform distribution in the Al6061 matrix was confirmed by micro-structural analysis. Other surface morphological information like agglomeration of less soluble alumina and presence of dendrite structures at higher SiC concentration were also revealed. Further, the observations from this investigation have been compared with already existing literature. In conclusion, prospects for future research have also been explored.

Improved CFD modeling of full dissolution of alumina particles in aluminum electrolysis cells considering agglomerate formation

The full alumina dissolution process in aluminum electrolysis cells was investigated using an improved computational fluid dynamics (CFD) model based on the previous researches by consideration of agglomerate formation. The results show that the total mass of alumina agglomerate and its maximum size are mainly dependent on the feeding amount and increase with increasing it. Higher superheat can effectively inhibit the agglomerate formation and thus promote the full alumina dissolution behavior. The full alumina dissolution process mainly includes a fast stage and a slow stage, with an average dissolution rate of 17.24 kg/min and 1.53 kg/min, respectively. About 50% (mass percentage) of the total alumina particles, almost all of which are the well-dispersed alumina fine grains, dissolve within the fast dissolution stage of about 10 s. The maximum values of the average dissolution rate and final percentage of the cumulative dissolved alumina mass are obtained with a feeding amount of 1.8 kg for a superheat of 12 °C. The formation of the alumina agglomerates and slow dissolution characteristics play a dominant role in the full dissolution of alumina particles.

Diversified Aggregated Patterns of Alumina Inclusions in High-Al Iron Melt

High-concentration Al deoxidation resulting in diversified aggregated patterns of alumina inclusions in high-Al iron melt were observed, especially, a new alumina agglomerate with a ring shape was reported and discussed in the current work. A large number of small alumina particle originated from the explosive nucleation of alumina in Fe-Al-O reaction zone with high-concentration Al in a short time is the critical condition for forming the alumina-agglomerated ring. To understand the stabilities of the ring, the relationships among the liquid capillary force, the equivalent inner radius of ring, the filling angle, and the separation distance h were quantitatively investigated based on a mathematical model and experimental results. The new phenomenon and formation mechanism of alumina-agglomerated ring could be important to high-Al steel metallurgy.

Correction Factor for Unbiased Estimation of Weibull Modulus by the Linear Least Squares Method

In material science, linear least squares is the most popular method to estimate Weibull parameters for stress data. However, the estimation $$ {\hat{m}} $$ of the Weibull modulus m is usually biased due to the data uncertainty and shorcoming of estimation methods. Many researchers have developed techniques to produce unbiased estimation of m. In this study, a correction factor is considered. First, the distribution of $$ {\hat{m}} $$ is derived mathematically and proved through a Monte Carlo simulation numerically again. Second, based on the derived distribution, the correction factor that depends only on the probability estimator of cumulative failure and stress data size is presented. Then, simple procedures are proposed to compute it. Further, the correction factors for four common probability estimators and typical sizes are displayed. The coefficient of variation and mode are also discussed to determine the optimal probability estimator. Finally, the proposed correction factor is applied to four groups of stress data for the unbiased estimation of m correspondingly concerning the alumina agglomerate, ball stud, coated conductor and steel, respectively.

Synthesis optimization and fluoride uptake properties of high capacity composite adsorbent for defluoridation of drinking water

A novel low cost composite adsorbent has been successfully synthesized from waste eggshell (ES), alum, and chitin for removal of excess fluoride from drinking water. Synthesis of chitin based adsorbent (CBA) was optimized by varying different synthesis parameters such as alumina loading, ES:chitin ratio, calcination temperature, calcination time, shaking time, drying temperature, and washing time. CBA is a composite type material having heterogeneous phases of Ca(OH)2, Al(OH)3, Al2O3 encapsulated with carbon, Al(OH)3 and Al2O3 deposited on carbon and Al2O3. Presence of multiple phases with prominent presence of crystalline as well as amorphous phases of alumina has been substantiated by using XRD and SEM analysis. The elemental composition of material and EDX also suggests composite nature of material with different phases. The SEM image of CBA reveals the presence of both coarse and fine particles with irregularly shaped particles of alumina agglomerates adhering to calcite particles of ES. Defluoridation performance was systematically studied for CBA, which has outperformed several reported adsorbents with its higher adsorption capacity of 30.3 mg g−1. No significant influence on the percent fluoride removal was observed in presence of Cl−, and ions, whereas the fluoride removal decreases significantly by 36, 24.84 and 24% in presence of , and ions, respectively. CBA follows pseudo second order kinetic and fluoride adsorption mechanism was rather complex one, wherein both intraparticle diffusion and surface diffusion are involved in the rate‐limiting step. Negligible leaching of Al+3 ions was observed. © 2015 American Institute of Chemical Engineers Environ Prog, 34: 1576–1585, 2015

Efeito do processamento em misturas de alumina/ligantes orgânicos usadas na moldagem por injeção em baixa pressão

A moldagem por injeção em baixa pressão (MIBP) é uma técnica que já vem sendo empregada na produção de peças cerâmicas com formas e geometrias complexas. A homogeneidade da mistura de ligantes orgânicos e pós cerâmicos é um fator determinante que deve ser controlado para minimizar a formação de imperfeições no processamento de feedstocks para MIBP. Defeitos típicos de processamento por MIBP, como bolhas de ar e aglomerados, geram gradientes de densidade nas misturas que, após conformação, possuem poucas possibilidades de remoção. Essas imperfeições comprometem o desempenho dos produtos obtidos por essa técnica. Este trabalho está focado na avaliação dessas heterogeneidades e como elas podem ser correlacionadas com a variação da densidade aparente e com o comportamento reológico dessas misturas. Para tanto, aluminas submicrométricas, como recebida e desaglomerada, foram adicionadas a uma mistura fundida de ligantes a base de parafinas, ceras e aditivos e processada em dois tipos diferentes de misturadores, com e sem o auxílio de vácuo. Foi observada a presença de aglomerados existentes na alumina como recebida, possivelmente gerados durante a etapa de calcinação. Também foi observado que o tipo de misturador e a aplicação ou não de vácuo durante a etapa final do processamento têm grande influência no tempo de mistura necessário para reduzir a viscosidade do feedstock para a injeção.

Effect of the primary particle morphology on the micromechanical properties of nanostructured alumina agglomerates

Depending on the application of nanoparticles, certain characteristics of the product quality such as size, morphology, abrasion resistance, specific surface, dispersibility and tendency to agglomeration are important. These characteristics are a function of the physicochemical properties, i.e. the micromechanical properties of the nanostructured material. The micromechanical properties of these nanostructured agglomerates such as the maximum indentation force, the plastic and elastic deformation energy and the strength give information on the product properties, e.g. the efficiency of a dispersion process of the agglomerates, and can be measured by nanoindentation. In this study a Berkovich indenter tip was used for the characterisation of model aggregates out of sol–gel produced silica and precipitated alumina agglomerates with different primary particle morphologies (dimension of 15–40 nm). In general, the effect of the primary particle morphology and the presence or absence of solid bonds can be characterised by the measurement of the micromechanical properties via nanoindentation. The micromechanical behaviour of aggregates containing solid bonds is strongly affected by the elastic–plastic deformation behaviour of the solid bonds and the breakage of solid bonds. Moreover, varying the primary particle morphology for similar particle material and approximately isotropic agglomerate behaviour the particle–particle interactions within the agglomerates can be described by the elementar breaking stress according to the formula of Rumpf.

Effect of Mechanical Milling on the Thermal Behavior of Polyethylene Reinforced with Nano-sized Alumina

Abstract In the current study the dependency of morphology and thermal behavior of medium density polyethylene (MDPE) powder reinforced with 0, 5, 10 and 15 wt.% nano-sized Al2O3 particles on mechanical milling were studied. The effects of milling time on the morphology and the thermal behavior were investigated by Transition Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Differential Scanning Calorimetery (DSC) and Thermo Gravimetric Analyzer (TGA). The TEM micrographs obtained from the nanocomposites proved that mechanical milling could be an adequate method to disperse nano-sized hard particles throughout the polymer. The SEM micrographs of pure MDPE showed that regular shape powder converted to the flake shapes as milling time increased. Also the results illustrated that an increase in milling time caused to decrease the agglomeration of alumina. The DSC profiles of samples elucidated that ball milling had little effect on crystalline temperature and melting point of all materials including MDPE and its nanocomposites. The nanocomposite exhibited more thermal stability than pure polyethylene proved by TGA tests.

Instrument for ceramic particle dispersion

Abstract Agglomerated sub-micrometer-sized alumina particles were pulverized using a novel method. A high-density polyethylene (HDPE) cup equipped with stainless steel mesh turned at a 500–2000 rpm revolution rate and a 200–800 rpm rotation rate. A suspension passes through the small mesh openings, and turbulent flow and high shear stress was generated. The sample's particle size distribution before mesh treatment exhibited a broad peak at 2–70 μm, reflecting the particles' agglomeration. This suspension's viscosity was 19.8 mPa s. After mesh treatment at 2000 rpm revolution rate for 10 min, the distribution showed two peaks at 0.2 and 2–70 μm. The alumina agglomerates shrank concomitantly with increased treatment time and dispersed completely after 30 min mesh treatment. The resultant suspension's viscosity was 9.2 mPa s, which is half that of the pre-treatment suspension.

Combustion characteristics of cotton stalk in FBC

Abstract The present work reports studies on the mixing and combustion characteristics of cotton stalk with 10–100 mm in length in FBC. Experiments on a cold model show that cotton stalk cannot fluidize, and adding bed material can improve the fluidization condition. Cotton stalk can mix well with 0.6–1 mm alumina at fluidization number N = 3–7. However, when the fluidization number is higher more than 7, the mixing bed will exist a little segregation comparing with N = 3–7. Thermogravimetric experiments show that cotton stalk can be ignited easily at a lower temperature, and its devolatilization and combustion are quick. Fluidized-bed combustion of cotton stalk was tested in a 0.2 MWth test facility. According to the temperature distribution along the bed height, when the primary and secondary air is adapted cotton stalk can be burned stably in the fluidized bed. During pure cotton stalk combustion tests, silica sand and alumina are used as bed material to compare their agglomeration characteristics. SEM/EDX analysis on agglomerate samples after combustion about 38 h suggests that the high alkali metals content causes the formation of the coating around silica sand particles. The coating consists of compounds with low-melting temperature results in agglomeration of silica sand particles. By contrast, alumina is difficult to react with alkali metals from biomass ash, and the agglomeration of alumina was not found at 910 °C. It is found that alumina is more favorable than silica sand particle for use in a fluidized bed in cotton stalk combustion.

Effect of the content of the α-phase and granulometric composition on the dissolution rate of alumina in cryolite-alumina melts

The rate and mechanism of dissolution alumina in a cryolite-alumina melt were determined starting from the results of a measurement of the dissolution rate, visual observations, and investigations of alumina agglomerates using an electron microscope. In the experiments, alumina samples delivered by different suppliers and differing in phase and granulometric compositions were dissolved. It is established that the dissolution rate of alumina decreases with the increase in the contents of α-Al2O3 and −45-μm fraction, as well as of the starting concentration of Al2O3 in the electrolyte. The particles >150 μm in size and alumina, in which their amount reaches 31%, dissolved slower than alumina containing the −45-μm fraction. Photographs of agglomerates forming after charging alumina and images of their microstructure are presented. The data on the dissolution rate are interpreted based on the behavior of the alumina charge in electrolyte, which is associated with recrystallization γ-Al2O3 → α-Al2O3 and with the formation of the network consisting of interlacing α-Al2O3 plates.

Modeling Copper CMP Material Removal Rates Using Copper Surface Nanohardness

Copper nanohardness and etch rate measurements in various CMP solutions, in addition to alumina agglomerate size distribution measurements, were used in a model to predict material removal rates (MRR). Model predictions were compared to experimental copper CMP data. In general, the model predictions improved using measured nanohardness compared to predictions using a constant nanohardness of metallic copper. The model reasonably predicts MRR, indicating this model is applicable to the copper CMP process. However, when the solution pH was low (<4) the model overpredicted MRR, possibly due to a thin, hard passivation layer. Also, the model was very sensitive to the surface hardness in solutions of glycine and H2O2.

Modeling Material Removal Rates for Copper CMP Using Copper Nanohardness and Etch Rates

Measurements of copper nanohardness and etch rate were used with alumina agglomerate size distributions in a model to predict material removal rates (MRRs), which were then compared to experimental copper chemical mechanical planarization (CMP) data. Generally, model predictions improved using measured nanohardness compared to predictions using a constant nanohardness of Cu metal. When the slurry pH was acidic the model overpredicted the MRR. An increase in the slurry pH increased the nanohardness, and MRR predictions agreed with experimental results. For slurries with small etch rates , the nanohardness had little effect on the MRR predictions, and the model agreed with experiment. The model was very sensitive to the nanohardness for slurries with large etch rates , and was unable to predict the MRR.

Size effect on strength of alumina agglomerates

The size effect on strength of alumina agglomerates was examined, and the applicability of the often used size effect laws was discussed. It is concluded that there is a significant energetic size effect, which shows that the Weibull statistical size effect does not apply. However, the size effect on the tensile strength of the agglomerates examined can be addressed well by the energetic–statistical size effect law.

Effects of CMP Slurry Chemistry on Agglomeration of Alumina and Copper Surface Hardness

The influence of common slurry additives on the colloidal behavior of alumina suspensions and the surface characteristics of copper were investigated. The effects of the addition of glycine and H2O2 to aqueous suspensions of alpha-alumina at various pH values were studied by zeta potential and agglomerate size distribution measurements. Depending on the state of copper in solution, alumina agglomerate size was found to either increase or decrease. Nanohardness of 1 µm copper films deposited onto 30 nm Ta on a silicon wafer was measured after exposure to the same slurry solutions and was found to be significantly affected by pH and addition of chemical additives. The agglomeration behavior and hardness measurements suggest that the states of the copper in solution or on the surface are consistent with potential-pH diagrams.

Modeling of Copper CMP Using the Colloidal Behavior of an Alumina Slurry with Copper Nanoparticles

The measured agglomerate size distributions of alumina abrasives in various slurry chemistries and at different pH values were used in a model to predict material removal rates (MRRs). The alumina agglomerate size and distribution were measured both with and without the presence of copper nanoparticles in the solution for each slurry chemistry studied. Although the agglomerate sizes were measured under quiescent conditions, it is determined that the agglomerated abrasive particles remain intact during chemical mechanical planarization (CMP), hence the measurements can be used in the CMP model. The model predictions using these measurements both with and without copper in solution were compared to experimental copper CMP data. The model was unable to predict the MRR when the slurry did not have any chemical additives because the dispersion was unstable and small fluctuations in the agglomerate size and distribution caused large changes in the predicted MRR. The model predictions were in excellent agreement with experimental MRR for a slurry with glycine in alkaline conditions. The model results from the size distribution measurements with copper in solution agreed slightly more with experiment than those without copper.

Novel route to nanoparticle dispersion using supercritical carbon dioxide

An experimental study was carried out to determine the effects of supercritical carbon dioxide (scCO 2) on the break up of untreated and treated alumina agglomerates in polystyrene. Alumina nanoparticles were treated with fluoroalkanes to achieve a better affinity with scCO 2. All agglomerates were found inside pores (cells) that were created by scCO 2 expansion during depressurization, and the observed cell size increased with increasing alumina agglomerate size. When untreated alumina was used, the agglomerates were not affected by scCO 2 expansion. On the other hand, large agglomerates ( L ≥ 15 μm) of the treated alumina showed signs of catastrophic fragmentation, whereas small agglomerates of the treated alumina were intact upon scCO 2 expansion. This was attributed to the amount of scCO 2 saturation around and within small agglomerates, which does not provide enough energy to break up the agglomerate. This conclusion was also supported by the fact that cell sizes were proportional to agglomerate size. Large agglomerates can absorb more scCO 2 than smaller ones, and therefore, have an adequately larger bursting pressure to overcome interactions between individual alumina nanoparticles that make up the agglomerate.

Dispersion and Fluidity of Aqueous Aluminum Oxide Suspension with Titanate Aqueous Solution

Titanate aqueous solution (TIP-Lac) prepared by direct reaction of titanium tetraisopropoxide (TIP) with lactic acid (Lac) in water shows characteristics of a polyanion and acts as a dispersant for preparing aluminum oxide suspension. When small amounts of TIP-Lac were added to aqueous dispersions of alumina, which had a positive surface charge at pH 4, agglomeration of alumina occurred, and alumina was redispersed by further addition of TIP-Lac. The fluidity and dispersion of alumina were studied from the zeta potential, sedimentation velocity, and rheological measurements.

The Effects of Copper CMP Slurry Chemistry on the Colloidal Behavior of Alumina Abrasives

The effects of common slurry additives on the colloidal behavior of alumina suspensions used for copper chemical mechanical planarization (CMP) were investigated. The alumina suspensions were characterized by zeta potential and agglomerate size distribution measurements with various chemical additives. To simulate the slurry during copper CMP, the effect of the addition of diameter copper particles was studied. The presence of copper caused a decrease in agglomeration for pH values less than 6.5 and an increase in agglomeration ranging from for pH values greater than 7 in aqueous solutions. Addition of glycine caused the formation of a soluble Cu-glycine complex that decreased agglomeration at pH values less than 4. The addition of did not affect the effective alumina agglomerate size without copper, but with copper in the solution the majority of the alumina agglomerated to for all pH values. However, increasing concentration to decreased the agglomerate size by . The pH of the slurry had the largest effect on the zeta potential and agglomerate size distributions.