Revealing cracking and breakage behaviours of gibbsite particles

Mitigating gibbsite particle cracking and breakage during industrial alumina production can increase the quality of smelter grade alumina product by reducing the ultrafine particle content. Therefore, it is essential to investigate the particle cracking during static calcination and the breakage of calcined gibbsite particles under external force. In this work, we investigated the impact of the calcination ramping rate and the crystallite size on gibbsite particle cracking during static calcination. A slow ramping rate and a large pristine crystallite size tend to increase particle cracking. Apart from the study of particle cracking behaviour, we also investigated the breakage of calcined gibbsite particle under external force. Cracks on the particle surface can initiate breakage within the crystallite and along the grain boundary under external force. The breakage within crystallite occurs as the cleavage of the crystallite, while the breakage along the grain boundary leads to the shedding of a whole crystallite. We further explored the factors influencing the strength of calcined gibbsite particles. With increasing calcination temperature, the strength of particle increases when gibbsite converts to boehmite, and then decreases when boehmite converts into amorphous alumina. Particles containing smaller crystallites and calcined with fast ramping rates exhibit higher resistance to breakage.     

Modeling and simulation of a fully air swept ball mill in a raw material grinding circuit

A raw material grinding circuit was modeled using plant data. Samples were collected from around the circuit and, following a crash stop, from inside the mill. The size distributions of the samples were determined down to a few microns. Using the data from inside the mill a modeling approach, based on perfect mixing, was developed. The modelling approach implicitly assumes that the mixture of feed materials broken is homogenous from the breakage point of view. The air classification around the circuit was modeled using the efficiency curve approach. In order to measure the success of the method the circuit performance was predicted by simulation studies while it was operating at different conditions. The results were then compared with the measured data. It is concluded that modeling gives a useful quantitative indication of what may occur in fully air swept mills.     

Toward a unified framework for the derivation of breakage functions based on the statistical theory of turbulence

The evolution of droplet or bubble size distribution in turbulent flow is of great significance in a variety of technological fields. Modeling this evolution by employing a population balance approach requires knowledge of the so-called breakage functions (rate and kernel). Over the years, a large number of phenomenological breakage functions with various degrees of sophistication have been proposed in the literature. Among them, those based on the statistical theory of turbulence are of particular interest, in that they attempt to take into account the structure of the flow field responsible for breakage. The purpose of the present work is to present a unified framework for developing this type of breakage functions and to show how existing models can be derived in a systematic and consistent way. The key parameters in this modeling approach are identified, which have to be determined by comparison with experimental data. It is also shown that the breakage functions, obtained within the framework presented here, lead to predictions of a droplet size evolution whose main features are consistent with experimental observations. It is suggested that this framework is an important step toward the development of a standard approach for modeling droplet size evolution in turbulent flow.     

Implementation of the quadrature method of moments in CFD codes for aggregation-breakage problems

In this work the quadrature method of moments (QMOM) is implemented in a commercial computational fluid dynamics (CFD) code (FLUENT) for modeling simultaneous aggregation and breakage. Turbulent and Brownian aggregation kernels are considered in combination with different breakage kernels (power law and exponential) and various daughter distribution functions (symmetric, erosion, uniform). CFD predictions are compared with experimental data taken from other work in the literature and conclusions about CPU time required for the simulations and the advantages of this approach are drawn.     

Experimental tracking of silica dispersion into silicone polymer

This work deals with the experimental tracking of a dispersion of very fine silica particles in PDMS using a new, easily implemented methodology. The dispersion experiments are carried out in an 8-l capacity mixer equipped with a torquemeter. Using a Couette analogy, the torque-rotational speed data can be converted into process viscosity-effective shear rate data. Simultaneously, the Particle Size Distribution (PSD) evolution is followed through Laser Light Scattering measurements on diluted samples. The PSD evolution allowed us to interpret the dispersion process in terms of different mechanisms, such as breaking-up and erosion of aggregates, as well as, in some cases, re-agglomeration (or flocculation) of particles.     

Simplified model for particle collision related to attrition in pneumatic conveying

This paper presents a simple method for predicting particle attrition during pneumatic conveying. The model calculates the changes in the particle size during pneumatic conveying (as a result of the collisions between the particles and bend walls) by using empirical correlations for both the machine and material functions. The method does not require the use of complicated simulations such as DEM–CFD. Furthermore, the computational model was written in MATLAB, and the results agree well with the experimental results for salt particles. The computation time was very short: a few seconds for the first collision (particles passed through one bend), and below one minute for six collisions. The experimental results and parametric study show that higher bend radius ratios caused less damage to the conveyed material. Moreover, higher air velocities and larger pipe diameters caused more damage to the conveyed material.     

DETERMINATION OF TIME-DEPENDENT CONSTITUTIVE MODEL PARAMETERS FOR MICROCRYSTALLINE CELLULOSE

ABSTRACT

Material parameters of a time-dependent elasto-viscoplaslic constitutive model, proposed by Adachi and Oka, have been determined for microcrystalline cellulose. Adachi and Oka model has eight material parameters, with five of them (i.e., consolidation index, swelling index, critical index, shear modulus, and Poisson's ratio) same as those in the more commonly used in time-independent modified Cam-clay model, and remaining three parameters account for time-dependency. To measure the time-dependent parameters, constant strain-rate conventional triaxial compression (CTC) test was designed and conducted using Penn Slate's flexible-boundary cubical triaxial tester (CTT) with microcrystalline cellulose as the test material. To deposit the powder, the sprinkle filling method was used in a layer by layer fashion with a small spoon. The three time-dependent Adachi and Oka parameters were determined from these tests. Adachi and Oka model was verified by comparing the predicted values from the model with measured values from the CTT at 0.05 level of significance. Results showed that: 1) there were no significant differences (p > 0.05) between measured and predicted HTC responses at all loading rates; 2) failure values were predicted accurately (p > 0.05); and 3) there were significant differences (p < 0.05) between measured and predicted CTC responses at all loading rates.     

温度对成品烟丝造碎率及尺寸分布均匀性的影响

为研究温度对配方烟丝造碎率及尺寸分布均匀性的影响,采用烟丝结构分布方程分析了不同温度烟丝破碎后尺寸分布规律.结果表明:在试验温度范围内,①烟丝的造碎率随着温度的升高而降低,相同温度下烟丝的造碎率是:三类烟＞二类烟＞一类烟;温度对不同等级烟丝的影响程度不同:一类烟＞二类烟＞三类烟;②不同温度烟丝,其破碎后尺寸分布的差异性...     

Investigation of the ultimate particle size distribution of a carbonate sand

A series of ring shear tests were conducted to investigate the ultimate particle size distribution of a carbonate sand. The tests were carried out under different stress levels, on three types of specimens: 1) uniformly graded specimens made of dry natural sand 2) remoulded specimens of the crushed sand after first shearing to large strains 3) specimens made of natural sand grains but with the same grading as in (2). The first series of tests on type (1), carried out to very large strains, led to apparently stable gradings, distinct for each stress level. Only limited additional particle breakage could be induced by remoulding the specimens after shearing (type (2)) and subjecting them to more shearing. Tests on specimens created at the apparently stable gradings (type (3)) but from the intact sand particles however led to significantly greater breakage. For the three types a stable, fractal grading was achieved. Analyses of the soil particles’ shape showed that the aspect ratio, sphericity and circularity reach a steady value at large strains, in parallel to reaching a stable grading. The mobilized angle of shearing resistance however was not significantly different in the different types of samples, suggesting the final grading dominates the behaviour.     

轴向振动钻削中麻花钻的失效形式分析

基于硬铝和不锈钢的振动钻削试验,对振动钻削中的麻花钻的失效形式及其产生原因进行了研究,并与普通钻削中的失效形态进行了对比分析.研究结果表明:与普通钻削相比,在合理的振动参数下,振动钻削时麻花钻的前后刀面磨损现象明显减弱,且更均匀化,横刃和外缘点磨损很少发生;钻头出现折断、主刃的崩刃和剥落等破损的几率也大大减小,但是若振幅过大,易出现横刃处的崩刃现象.＃