Posted on November 16, 2000 at 02:01:08:

In Reply to: Caking of Crystallized Powders posted by Dr. Cesar Martinez on November 13, 2000 at 07:36:09:

‘Caking’ is usually the result of the formation of crystal bridges between the contact points of static particles in a bulk array due to the presence of minute moisture traces adsorbed or condensed from ambient vapour. Granular products loaded to storage in a warm condition, particularly if from a drying process where the air has a high relative humidity, are prone to suffer from free moisture deposition on the particle surfaces. This tends to aggregate as cusps at points of coordination within a structured mass of particles and lead to crystal growths that act as a strong bond to join the particles. The resulting product may appear ‘dry’ due to the bound nature of the moisture in the crystals.

Various approached can be made to minimise the behavior of mitigate its effects. Pre-cooling the material and venting prior to storage, or minimising ambient moisture, reduces the initial moisture content within the mass. Counter current gas flow or permeative ventilation can be used to drive off moist air during condensation, particularly if dry air can be used. Keeping the material above dew point, by insulation or trace heating, will also reduce the drop out of moisture.

As the intensity of crystal growth is dependent upon the time of contact at rest, the use of mass-flow type storage containers is essential to avoid extended and indeterminate residence periods of static product in storage. Where long periods of storage are necessary it is good practice to systematically draw off small quantities of the bulk material to affect minute particle to particle disturbances that will fracture weak, incipient crystal bonds.

The use of exploiting differing geometric forms and hopper inserts provides an invaluable tool for developing a number of advantageous features, such as: -

(a) Aid the generation of Mass Flow in containers that are subject to height or other constraints that inhibit the utilisation of classical Mass Flow forms.

(b) To prevent orifice blocking due to caked agglomerates of indeterminate size.

(c) To impose breaking stresses on weak, friable agglomerates.

(d) Reduce the size of agglomerated cakes by flow attrition.

Finally, with regard to the testing of powders, it is suggested that the attainment of quantified values of flow related and bulk strength properties offer positive and better design information than empirically achieved qualitative or phenomenological related information. To this end values of contact friction on differing wall contact material and shear strength measurements will provide the necessary data for designing Mass Flow hoppers and correlating agglomerate strength with formation conditions. Ajax supply such measuring tools and can offer advice on bin design, inserts and discharging devices for difficult flow products. On receipt of full application particulars a more detailed analysis can be prepared and it is expected that specific recommendations may be provided. Please see web site http://www.ajax.co.uk for more background. ■

Dear Dr Martinez,

We have just launched an instrument at the AAPS which allows you to accurately measure the caking/free flowing characteristics of powders after storage. The instrument allows us to compact the powder to a certain force (simulating the compaction during storage) and then allows us to measure the flowability of the powder after that compaction.

Please take a look at our website for more information (www.stablemicrosystems.com) or e-mail me directly (ben.proudlove@stablemicrosystems.com) ■