Tuesday 21 May 2019
Of the many factors that influence powder behaviour, moisture, or humidity, is perhaps one of the most instantly recognised. Adding even small amounts of water to a powder can transform its properties. This is evidenced in a positive way in the process of wet granulation, which is often used to agglomerate fine, difficult to handle powders into free flowing granules. Water can also be used beneficially to lubricate the flow of certain materials, and because it conducts charge it will effectively ‘ground’ a powder, reducing any electrostatic-related behaviour. Elsewhere, however, uncontrolled levels of moisture can cause significant problems.
In storage, for example, humidity levels that are ill-suited to the powder are a primary cause of caking, an issue that can adversely affect in-process and end-use performance. Poor flowability downstream of a process where water is deliberately added, such as crystallisation, wet ball milling or froth flotation, can also be detrimental to processing efficiency. Where moisture is a problem, there is usually a solution - the storage area can be maintained at lower humidity, for example, or the process stream can be dried – but these solutions are often associated with significant cost. Drying in particular is an energy intensive process that is often avoided where possible.
The challenge for formulators and process engineers is to understand the extent to which a powder will take up moisture when exposed to a humid atmosphere and, more importantly, how this moisture will affect the characteristics of the powder and its performance in any given process. Such understanding supports the development of effective strategies for process optimisation and the realistic economic assessment of measures to control moisture.
This paper explores the application of dynamic, shear and bulk property measurements to assess in detail the impact of humidity and gather relevant knowledge. It includes experimental data that illustrate the breadth of response of different powders to the presence of moisture.