Achievements in Size Reduction

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Author Last Name: 
J A H de Jong
Report Type: 
Research Area: 
Size Reduction
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Size reduction is a widely used process in industry. In the mineral industry it finds application on a large scale, but in other industries like the chemical, pharmaceutical and pigment industry, where it is performed on a much smaller scale, it is a big issue. There is even wider interest in the closely associated problem of attrition, inadvertent breakage, which occurs in almost all particle transport processes and can lead to extensive costs to control the fine dust produced, as well as interfering with the flow properties and subsequent processing of the main product.

Key questions arise:

- Can we understand what is happening during breakage?

-Can we quantitatively predict the breaking process?

-What can be done with the available knowledge in industrial situations?

These and other questions have been addressed in an extensive IFPRI program of research over the past ten years. This review attempts to bring together the achievements of this program and to identify what questions may now be posed or remain unanswered for any reason.

This review is divided into seven chapters, covering Breakage Phenomena, Problems and solution methods available, Material Properties, Breakage in Industrial Equipment, Industrial Requirements and concludes with a summary of achievements and remaining questions.

Fracture mechanics, Discrete Element Simulation, materials measurement as well as improved experimental techniques have done much to improve our understanding of the processes involved in Comminiution and Attrition. The IFPRI program has contributed a major element in this improvement. Attempts at quantification, whilst much improved have been unable to leap the barrier between laboratory measurement and industrial equipment. The main reason is that we have an inadequate knowledge of the forces developed in industrial machinery nor yet any successful method for predicting a successful comminution event in a multi particle situation in real machinery. Additionally our materials property assessment assumes uniformity of material property and idealised fracture behaviour, which is contrary to practical experience. Changes in property for nominally the same material brought about by a change in previous processing of raw material source will add to the variability of processing and unpredictability of output.

There are reasons, therefor, as a long-term requirement to improve on our ability to predict particle breakage from material properties. This needs to be extended to include agglomerated and porous materials and to define more precisely existing flaw structures, inhomogeneity and shape effects. The role of repeated collisions also need clarification, as do the breakage mechanisms brought about by shear and compression.

A major problem is the definition and description of the processes that occur in industrial equipment. This needs to include flow regimes, the occurrence (scale and frequency) of breakdown events, and the type of comminution or attrition event. At this time, the best way forward may be by simulation to provide a guide to what events to look for, followed by carefully planned experiments for confirmation. It remains an open question as to whether simulation can provide the hoped for guidance and the experimental program will not be cheap.

Until these further objectives are achieved, empirical methods will still be required, but even here improvements will be slow unless the problems posed above are tackled vigorously.