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Spiral jet mills utilise high-pressure gas jets, which induce particle breakage by collisions. Appropriately angled jets force particles to circulate in dense phase near the wall due to the centrifugal field. Moving radially inwards towards the mill centre, the two-phase dense flow is transformed into a lean phase and fine particles are entrained out by the fluid drag. Here we analyse the effect of grinding nozzles pressure on the particle dynamics and fluid flow by numerical simulation. The average velocity gradient in the radial direction is very steep for low mass loadings, indicative of rapid shearing, but it decreases significantly as the particle mass loading is increased. The dissipated collisional energy, accountable for size reduction, is strongly influenced by the jet penetration through the circulating bed, which in turn depends on the mass loading and operating pressure of grinding gas nozzles. As a result of jet penetration, the grinding gas nozzles pressure becomes less effective at high pressures. The analysis developed here elucidates the coupled effect of mass loading and grinding nozzles pressure on the dissipated collisional energy, which accounts for particle breakage.

Crystal growth rates are notoriously difficult to predict and even experimental data are often inconsistent. By allowing for mass and energy diffusion through the molecular and thermal layers surrounding a growing crystal and for the heat effect of crystallization, a new model of crystal growth from solution is proposed and applied to crystallization of potassium chloride from aqueous solution. The driving force for crystal growth was calculated using the solubility at the interface temperature in contrast to the conventional one based on bulk temperature. A positive heat effect at the crystal interface as well as the resistances to the mass and energy transfer processes to and from the crystal surface can reduce the conventional driving force for crystal growth by more than 20 %.

Particle size reduction in spiral jet mills is induced by high velocity gas jets, causing interparticle and particle wall collisions leading to breakage. Despite extensive research on the design and operational parameters, the underlying mechanics of size reduction is still poorly understood. Discrete Element Method and Computational Fluid Dynamics are used here to analyse particle and fluid motions. A fast shearing dense particle bed is formed on the wall, with a transition to lean phase towards the centre of the mill. This necessitates four-way coupling of fluid and particle interactions for analysis. It is shown that increasing the depth of the particle bed reduces the fluid phase tangential velocity in the proximity of the classifier, as momentum is exchanged with circulating particles. The energy dissipation through particle collisions occurs mainly along the bed surface and in front of the grinding jet nozzles.

Characterisation of anodic aluminium oxide coatings and measurement of their thickness using microscopic techniques is valuable for analysing the effectiveness of the prior anodising process. Three different methods for preparing samples to view the coating cross-section (mechanical fracturing, cryogenic fracturing and metallography) were trialled and assessed for speed of implementation, simplicity and achievable measurement accuracy. Cryogenic fracturing was found to be destructive to samples. Mechanical fracturing yielded relatively accurate coating thickness measurements and coating structural information. Metallography provided the most accurate coating thickness measurement at the expense of coating structural information.

A spiral jet mill was simulated using Discrete Element Method modelling and Computational Fluid Dynamics. The particle behaviour and fluid motion were analysed as a function of hold-up and grinding pressure. Particle collision energy was predicted to be prevalent along the bed surface and in front of the grinding jets, as shown through the collision data recorded. The bed itself affects the fluid flow field, as momentum is transferred to the particles. Increasing the grinding pressure does not result in a proportional increase in the kinetic energy of the particle system, as the high pressure jets begin to penetrate the bed with greater ease. The particle bed moves as ‘plug-flow’, with the layers of the bed closest to chamber wall.

Crystallization in solutions under steady cooling is considered in the case of instantaneous nucleation, in which all crystallites appear at once in the solution and grow in the absence of crystallites born subsequently. Expressions are obtained for the total crystallite volume as a function of the steadily increasing undercooling. These expressions are employed for determining the dependence of the relative critical undercooling u(c) for crystallization on the cooling rate q. The resulting u(c)(q) formula reveals the physical meaning of the parameters in the linear relationship, often reported experimentally, between u(c) and q in double logarithmic coordinates. The results obtained are also directly applicable to overall crystallization of melts at sufficiently small undercoolings.