A diverse range of unit operations are employed to process and manipulate powders. These invariably subject materials to a wide range of conditions – from the relatively high and quasi-static compaction stresses observed in hoppers, to the low stress, dynamic conditions experienced during fluidisation. A thorough understanding of how a bulk material will behave over a range of conditions, and in various phases of flow – whether stationary, in motion or about to flow– is therefore vital for designing and monitoring the unit operations and transfer systems that make up many given process.
Despite the extreme variations that exist between different operations, there is still a tendency to identify a simple, single parameter with which to characterise a powder. Single number characterisation, such as those categorising performance from ‘cohesive’ to ‘free flowing’, is unlikely to be sufficient in fully evaluating and predicting the performance of materials across a range of processes.
The key is to ensure compatibility between processing equipment and the characteristics of the powders it will handle. Such an approach requires a comprehensive understanding of the bulk behaviour of the materials in order to feed relevant information into process design and optimisation activities.
Case Study - Assessing Batch-to-Batch Variability of Bulk Chemicals
Titanium Dioxide has been used as a pigment and opacifier in a wide range of industries for many years, due to its bright white colour and high refractive index. However, despite such widespread use, processing Titanium Dioxide in its powdered form is often extremely challenging due to the powder’s high cohesivity. Special measures often need to be implemented when managing this material in operations such as dispensing from hoppers, feeding into unit operations and blending with other powders.
Identifying and quantifying which powder properties are conducive to efficient processing allows new formulations to be optimised without the significant cost of running samples through the process to assess suitability, making considerable savings in terms of time and raw materials, and minimising wastage due to out of specification products.
Despite meeting the existing specifications, three batches of Titanium Dioxide demonstrated significantly different behaviour when used in the same process, resulting in unacceptable variation in final product quality. A range of traditional characterisation techniques were employed, but failed to differentiate between the three batches, partially due to the high degree of variability in the test results.
Samples of the batches were analysed using an FT4 Powder Rheometer®, which demonstrated clear and repeatable differences between them that rationalised the variations in process performance and enabled the user to reliably assess the quality of incoming batches in process-relevant terms.