Cracking behaviour and mechanism at grain boundary of gibbsite during calcination

Limiting the cracking of gibbsites during calcination can reduce the content of ultrafine particles in the smelter grade alumina product and increase the quality of products. However, the investigation on the cracking behaviour and mechanism of gibbsites during calcination remains a challenge. In this study, we focus on the cracking behaviour and mechanism of gibbsite grain boundary during calcination. We first observe the cracking behaviour of the gibbsite grain boundary during calcination and propose a cracking mechanism. Based on the observed crack evolution at the gibbsite grain boundary, we found that the cracking of gibbsite grain boundary is initiated on the surface of particles, and then the cracks grow into the inner side of particles with the calcination. We also investigate the impact of the crystallite size and the c-axis angle between crystallites on gibbsite grain boundary. As a result, the increased crystallite size and the c-axis angle between crystallites can result in increased crack size at the gibbsite grain boundary.     

Numerical modelling of the breakage of loose agglomerates of fine particles

This paper presents a numerical study of the breakage of loose agglomerates based on the discrete element method. Agglomerates of fine mannitol particles were impacted with a target wall at different velocities and angles. It was observed that the agglomerates on impact experienced large plastic deformation before disintegrating into small fragments. The velocity field of the agglomerates showed a clear shear zone during the impacts. The final breakage pattern was characterised by the damage ratio of agglomerates and the size distribution of fragments. While increasing impact velocity improves agglomerate breakage, a 45-degree impact angle provides the maximum breakage for a given velocity. The analysis of impact energy exerted from the wall indicated that impact energy in both normal and tangential directions should be considered to characterise the effects of impact velocity and angle.     

A macroscopic agglomeration kernel model for gibbsite precipitation in turbulent and laminar flows

A macroscopic agglomeration kernel model has been developed that is capable of describing gibbsite agglomeration over a broad range of process conditions, including both the laminar and turbulent flow regimes. The agglomeration kernel model was derived using chemical reaction engineering principles and data from an extensive experimental program covering a wide range of temperatures, supersaturations, seed sizes, shear rates and mixing regimes. The experimental precipitation data in the laminar flow regime were generated using a novel Taylor-Couette precipitator. Data in the turbulent regime were generated in a stirred reactor. The developed agglomeration kernel model incorporates terms for the collision, capture, rupture and cementation rates affecting the formation of agglomerates. The model is shown to be able to predict the data from independent experiments. The proposed model also captures the complex non-linear shear rate and size-dependency observed experimentally, e.g. (1) for small particles the agglomeration kernel exhibits a maximum with respect to the shear rate, increasing at low shear rates in the laminar flow, but decreasing in the unstable Taylor-Couette flows and turbulent regimes; (2) for large particles the agglomeration kernel decreases monotonically with increasing shear.     

Dehydration reactions and kinetic parameters of gibbsite

Dehydration reactions and the corresponding kinetics of gibbsite [γ-Al(OH)₃] were analyzed using non-isothermal thermoanalysis and the Kissinger equation. It is concluded that the starting temperature of the dehydration reaction of γ-Al(OH)₃ rises with increasing heating rate of the system. At a heating rate of 10 °C min⁻¹, γ-Al(OH)₃ has lost part of its crystalline water, and was completely transformed into boehmite (γ-AlOOH)) at about 317 °C. However, γ-AlOOH did not lose the residual crystalline water entirely, and did not change into amorphous Al₂O₃ until the system was above 700 °C. The kinetic energy needed to convert γ-Al(OH)₃ to γ-AlOOH, and γ-AlOOH to amorphous Al₂O₃, was calculated by differential scanning calorimetry (DSC) with activation energies of 108.50 and 217.24 kJ mol⁻¹, pre-exponential factors of 2.93 × 10⁹ and 8.30 × 10¹³, and reaction orders of 0.96 and 1.06, respectively. The kinetic parameters of dehydration reactions for γ-Al(OH)₃ obtained using the derivative thermogravimetric method (DTG) are very similar to that of obtained by DSC.     

Particle distribution model of gibbsite precipitation process in alumina production

The precipitation is a process involving complicated crystallization and mass transfer. As a key step in the Bayer process in alumina production, it has great influences on both outputs and quality of alumina products. Thus laboratory experiments, industrial tests and numerical simulations have been carried out to study the precipitation. Based on the dynamics law and particle population balance, a combined model is developed with MATLAB/Simulink to study the process of crystal nucleation, growth and agglomeration, the model also take the variation characteristics of the particle size of the MSMPR (mixed-suspension, mixed product removal) gibbsite precipitation process into account. The particle size distribution is predicted through the method of moments at different conditions. The deviation between the model prediction and the test data is less than 9%, which is within the acceptable accuracy range for the process control.     

Parametric dependence of particle breakage mechanisms

It has been observed that the pattern of particle impact breakage in two-dimensional systems is a result of two mechanisms. “Mechanism I” accounts for the breakage induced by the stresses that appear in unbroken particles. “Mechanism II” breakage is due to the buckling of the Mechanism I fragments. The purpose of this paper is to try and understand the parameters that govern the magnitude of the breakage induced by the various mechanisms—in particular, to understand the specific effects of impact velocity on the size distribution. To that end, a dimensionless parameter that governs the magnitude of the breakage induced by Mechanism I in both two and three dimensions is developed. The two-dimensional analysis demonstrates a velocity dependence for the Mechanism I breakage that accounts for the observed velocity effect on the generated size distributions. However, the three-dimensional analysis demonstrates no such velocity effect. Both findings are supported by simulation results.     

Simultaneous calcination and spheroidization of gibbsite powders in an RF thermal plasma

Thermal treatment of gibbsite (Al(OH)₃) powders of different particle size was studied in inductively coupled thermal plasma operating at normal pressure. Calcination and spheroidization took place simultaneously although spheroidization did not occur with coarse feed particles. Microsized or nanosized Al₂O₃ particles of different phase composition were produced depending on the size of injected grains. The microsized powders were mainly formed by melt solidification. In the formation of nanopowders, however, evaporation of small grains followed by homogeneous nucleation from the vapor phase and grain growth seem to be the dominant processes.     

Agglomerate breakage and adhesion upon impact with complex-shaped particles

Understanding the breakage and adhesion of an agglomerate upon collision with a target particle is a primary step to fathom the adhesive mixing process. While the effect of several variables, such as collision velocity and particle interface energy, on collision behavior has been explored, the effects of target particle morphology have yet to be revealed. In this work, we generate three-dimensional particles with controllable shape and texture using Fourier harmonics and, using the discrete element method, we examine the collision of an agglomerate that impacts each target particle. Results show that the agglomerate breakage depends on the local curvature in the impact zone. We observe that the asperity and elongation factors of the target particle largely contribute to the extent of the deposition of fine particles and the size and number of generated fragments after impact, respectively. These results reveal the large potential error when approximating real particles as smooth spheres in fragmentation studies.     

The influence of organic additives on the crystallization and agglomeration of gibbsite

During the crystallization of gibbsite Al(OH)₃, which leads to the synthesis of alumina in the Bayer process, crystals go through a step of agglomeration. In this work, we present a study concerning the influence of different organic compounds, polycarboxylic acids and polyalcohols on the crystallization and agglomeration of gibbsite crystals. It has been determined that they act as crystal habit modifiers and inhibitors of the agglomeration occurring during the formation of alumina crystallites. Simply by following the kinetics of the reaction, it has been observed that polyalcohols are stronger inhibitors than polycarboxylic acids, which can be linked to the structure of gibbsite crystals. The influence of the characteristics of the additive, such as the number of carboxylic groups and the distance between them for polyacids, the stereochemistry and the length of the carbon chain in the case of polyalcohols, is demonstrated to have a significant effect.     

裂解炉出口温度控制技术的优化

通过对大庆乙烯装置80-U型裂解炉出口温度(COT)测量方式进行优化及改进,提高了对裂解深度的控制程度,进而改善了裂解炉日常的运行状况,达到了提高乙烯产品收率的目的。     

Impact breakage of fertiliser granules

Systematic data are presented for the single impact failure of 3.2, 5.3, and 7.2 mm fertiliser granules over a wide range of impact speeds and angles. The probability of failure was found to change only slowly between 90° (normal) impact and 50°, but decreased rapidly below 50°. The probability of failure increased with increasing size of granules. The effect of impact velocity on the mean, median and the proportion of the largest-sized fragments were examined. Two distinct forms of normal impact damage were identified, corresponding to low and high impact velocities, and the mechanisms of failure are discussed.     

Impact breakage of spherical, cuboidal and cylindrical agglomerates

A numerical study of the micro-mechanics of breakage of agglomerates impacting with a target wall has been carried out using discrete element simulations. Three agglomerates of different shapes are examined, namely spherical, cuboidal and cylindrical. Each agglomerate consists of 10,000 polydisperse auto-adhesive elastic spheres with a normal size distribution. The effect of agglomerate shape and impact site on the damage of the agglomerates under an impact velocity of 1.0 m/s for an interface energy of 1.0 J/m² is reported. It is found from the simulations that cuboidal edge, cylindrical rim and cuboidal corner impacts generate less damage than spherical agglomerate impacts. The cuboidal face, cylindrical side and cylindrical end impacts fracture the agglomerates into several fragments. Detailed examinations of the evolutions of damage ratio, number of wall contacts and total wall force indicate that the size of the contact area and the rate of change of the contact area play important roles in agglomerate breakage behaviour. Internal damage to the agglomerate is closely related to the particle deceleration adjacent to the impact site. However, the local microstructure may not be a decisive factor in terms of the breakage mode for non-spherical agglomerates.     

An Unreacted Shrinking Core Model for Calcination and Similar Solid-to-Gas Reactions

A variation on the unreacted shrinking core model has been developed for calcination and similar non-catalytic solid-to-gas decomposition reactions in which no gaseous reactant is involved and the reaction rate decreases with increasing product gas concentration. The numerical solution of the model has been validated against an analytical solution for the isothermal case. The model parameters have been tuned using literature data for the thermal dehydration (calcination) of gibbsite to alumina over a wide range of temperatures, from 490 to 923 K. The model results for gibbsite conversion agreed well with the published experimental data. A reaction order with respect to water vapor concentration of n = ?1 was found to give a good fit to the data and yield activation energies consistent with literature values. Predictions of the non-isothermal unreacted shrinking core model compare well with a more complex distributed model developed previously by the authors.     

Analysis of breakage kernels for population balance modelling

The accurate prediction of droplet sizes is fundamental in many industrial applications. In order to be able to simulate the evolution of a size distribution, suitable kernels should be used in the population balance. In this paper we make use of four different breakage kernels in order to predict a size distribution and compare the results among them as well as with experimental data. Two breakage kernels are derived from inverse problems and the other two are derived from physical or empirical interpretations.The Sauter mean diameter, a moments-based error and Gaussian shape factors were used to compare between the resulting distributions.     

Influence of interface energy of primary particles on the deformation and breakage behaviour of agglomerates sheared in a powder bed

In this paper, the parameters that affect the deformation and breakage of agglomerates embedded in a bed of particles subjected to rapid shearing are identified and analysed. The influences of interface energy between the primary particles and the size ratio (between agglomerates and particles of the bed) on the deformation characteristics of the agglomerate are addressed. The study is based on computer simulations using the distinct element method (DEM). It has recently been shown that for agglomerates having a size ratio greater than about 7, the nature of stresses experienced by the agglomerates when sheared inside a particulate bed is predominantly hydrostatic, hence it is difficult to break them (Hassanpour et al., 2007). However, the role of the interface energy between primary particles coupled with the effect of size ratio on the breakage and deformation characteristics of agglomerates during shearing has not been analysed. This feature is of great interest in the agglomeration process and is hence addressed in the present study. It is found that despite the predominantly hydrostatic nature of stresses responsible for retarding the breakage, agglomerates with size ratio greater than about 7 could undergo macroscopic deformation when the surface energy between the primary particles is decreased below a critical value. Furthermore, a failure map of agglomerates is presented in terms of their size ratio and the value of interface energy of the primary particles.     

High-pressure catalytic and thermal cracking of polyethylene

The thermal cracking and catalytic cracking processes of low-density polyethylene were studied in a closed autoclave. The compositions of gaseous and liquid products were analysed by means of GC/FID and GS/MS chromatographic methods. The fractional composition of liquid products was found by distillation. Increased temperature of PE depolymerisation process increases the production of gaseous products and low-boiling liquid compounds; more aromatic hydrocarbons are formed instead of alkenes. When a lower temperature and longer time are adopted for the process to reach the assumed conversion, more straight chained hydrocarbons are produced. The acidic aluminosilicate catalyst yields more low-boiling liquid fractions, more isoalkanes and more aromatics. The neutral alumina is favourable for the production of alkenes and vacuum gas oil fraction in comparison to a non-catalytic process. The Ni–Mo/Al₂O₃ catalyst is efficient in hydrogenation of depolymerisation products. The reaction products contain only saturated compounds then and no aromatics are formed.     

A 1-D non-isothermal dynamic model for the thermal decomposition of a gibbsite particle

A 1-D mathematical model describing the thermal decomposition, or calcination, of a single gibbsite particle to alumina has been developed and validated against literature data. A dynamic, spatially distributed, mass and energy balance model enables the prediction of the evolution of chemical composition and temperature as a function of radial position inside a particle. In the thermal decomposition of gibbsite, water vapour is formed and the internal water vapour pressure plays a significant role in determining the rate of gibbsite dehydration. A thermal decomposition rate equation, developed by closely matching experimental data reported previously in the literature, assumes a reaction order of 1 with respect to gibbsite concentration, and an order of −1 with respect to water vapour pressure. Estimated values of the transformation kinetic parameters were k₀ = 2.5 × 10¹³ mol/(m³ s) for the pre-exponential factor, and E_a = 131 kJ/mol for the activation energy. Using these kinetic parameters, the gibbsite particle model is solved numerically to predict the evolution of the internal water vapour pressure, temperature and gibbsite concentration. The model prediction was shown to be very sensitive to the values of heat transfer coefficient, effective diffusivity, particle size and external pressure, but relatively less sensitive to the mass transfer coefficient and particle thermal conductivity. The predicted profile of the water vapour pressure inside the particle helps explain some phenomena observed in practice, including particle breakage and formation of a boehmite phase.     

Application of CFD and breakage modelling for predicting the size reduction of protein precipitates during transport

Particle breakage due to fluid flow through various geometries can have a major influence on the performance of particle/fluid transport and separation processes. Whey protein precipitate dispersions were used as a case study to investigate the effect of flow intensity and exposure time on the breakage of precipitates during transport. Computational fluid dynamic (CFD) simulations were performed to evaluate the turbulent energy dissipation rate (?) and associated exposure time along various flow geometries. A breakage model, incorporating the CFD output and experimentally determined parameter values, was found to provide a satisfactory capability for predicting the breakage of the protein precipitate particles. The breakage modelling approach was then applied to particles formed under different agitation intensities during the precipitation process. The formation history of the precipitates had a significant effect on their structure and strength and hence different breakage rates were observed. The precipitate dispersions were propelled through a number of different geometries such as bends, tees and elbows. The shape of the flow geometry was found to have an important effect on particle size reduction. This predictive particle breakage modelling approach was then applied to larger-scale flow geometries with cross-sectional area of 150 times greater than the experimental.     

Effect of grain size on the synthesis of active alumina from gibbsite by flash calcination and rehydration

Two series of alumina have been prepared by flash calcination (FCAL) and subsequent rehydration of coarse (> 50 m) and fine grain size gibbsite (< 50 m). The initial grain size of the gibbsite was found to determine the degree of amorphization, water content, rehydration ability, composition and pore structure of FCAL products. Active alumina materials having pore structure parameters similar to those of commercial alumina adsorbents and catalyst supports were obtained by FCAL and subsequent rehydration of fine grain size gibbsite.     

活性石灰回转窑用耐火砖损坏原因及对策

分析了活性石灰回转窑用耐火材料的损坏原因,认为炉衬的附着物质与氧化硅和氧化铁的反应有关,硅酸钙与CaO是结圈的主要成分.改善耐火材料可以提高其使用寿命.