Hello Sergio,

there are several mechanisms causing caking, which is also known as time consolidation. Time consolidation can cause flow obstructions (arching, piping) in silos, but also caking problems in BigBags etc.

Some typical mechanisms are:

- Deformation of the particles at their contact points causes a decrease of the distance between the particles. Thus van der Waals forces increase (important for dry fine particles, say < 100 µm, where van der Waals forces are the most important adhesive forces. But also for moist materials the adhesive forces due to liquid bridges between the particles can increase if the distance decreases).

- Moisture migration: At long term storage moisture can collect in the contact areas between the individual particles thus increasing (or creating) liquid bridges and the corresponding adhesive forces. The moisture can also lead to local solution of the particles (e.g. salt, sugar). This can be the reason for a following crystallization causing solid bridges between the particles. Solid bridges cause much higher adhesive forces than van der Waals forces or liquid bridges.

Moisture migration can also happen if the particles contain some moisture. In this case a drop of the ambient temperature causes the moisture to migrate towards the surface of the particles. Then the effect described above (solution/crystallization) can happen.

- Solid bridges can be caused also by chemical bondings (if reactive materials are present) or sintering effects (if the material is stored at a temperature not much smaller than the melting temperature.

If problems with caking shall be investigated, shear tests and time consolidation tests should be performed. These tests will give quantitative numbers on the time consolidation effect. A solution could be to modify the product, e.g. by changing the production process or by adding flow aids, and to check the effect with shear tests.

Best regards

Dietmar Schulze ■

Hello Sergio,

further information on powder testing (shear testing) you will find on my website www.dietmar-schulze.com. For general information there are some essays on powders and powder handling. If you have further questions, please do not hezitate to contact me.

Best regards

Dietmar ■

The Wolfson Centre For Bulk Solids Handling at the University of Greenwich has just concluded a four–year program that examined the mechanisms of ‘caking’, segregation and particle attrition. It has developed a ‘toolkit’ for measuring the effects of these phenomenon and software for dealing with applications. Whilst all the information derived will not enter the public domain until the end of 2005 certain parts and services are available. One instrument used to measure ‘caking’ strength is the Ajax Tensile Tester. This device was originally based upon research at Warren Spring Laboratories but has recently been re-designed to increase its resistance sensitivity, resistance to contamination and measure much higher bulk strengths. ‘Caking’ is the term applied to the formation of crystal bridges between the contact points of soluble particles that have been wetted on their surface and dried out. There are a host of other bonding and sintering mechanism that cause a similar massive gain in strength of particulate masses due to chemical, thermal, molecular, electrostatic and other effects.

These phenomena are usually associated with periods of static contact under a degree of compaction stress and in certain ambient conditions. They tend to develop in storage containers, because periods of undisturbed particle contact under storage stresses and varied ambient conditions allow such bridges to form and grow.

Apart from measuring the strength that does develop, a holistic approach to the situation addresses the conditions of ‘caking’ formation and consideration of techniques and equipment design to mitigate the consequences. In general, if bulk material ‘cakes’ in a gravity flow hopper is an operational disaster and the material has to be cleaned out by other means for the recoverable volume of storage to be achieved. ■

Dear Sergio,

We have so called treatment silo, in which the caking can be preventing.

For the right design of the treatment silo we need the an analysis of the caking process. This analysis we perform with our shear testers.

Please send us near data and requirements for the silo. Thereafter we can make you an proposal.

Please open our website WWW.IPT-ONLINE.COM where you can get some information about our treatment silo and our shear tester and services.

We hope to hear from you

Regards

Dr. Ivan Peschl ■