This is a highly complex issue and one that can’t be adequately answered with a few glib words or without the need for more information such as the amounts of the materials involved and their relative proportions. Caking issues revolve around the particle size and shape distributions, the chemistry of the constituents and around storage conditions involving thermal fluctuation and humidity. Solutions may be found in both the generic areas listed below:

• Formulation. It may be stating the obvious but if there is a problem then changes in the formulation will have to take place; for example increase in the proportions of flow modifier. Calcium carbonate is sparingly soluble in water and various silicas can be envisaged as picking up water (fumed silica is often added as a flow enhancer, silica gel picks up moisture). The key here is the word ‘powder’. It may be that reformulation as a granule may keep the desired properties (such as dissolution) but will aid transport. Some form of ‘pre-packaging’ may help to exclude moisture. This could include tabletting or providing a more ‘finished’ product. Most, if not all, potential solutions are likely to add to the cost. In terms of particle size distribution, all the materials need to be within a range where there is substantial overlap of the distributions; adding micronized material to millimetre sized material usually isn’t advisable. The rule of thumb is that powders should be mixed in the range 0.3 – 1.4D where D is the median size of the major ingredient. In general terms, the finer the material, then usually the greater the problem — hence, the suggestion to granulate or agglomerate the material prior to transport. Sub-5micron material for primary particle size will invariably create problems and the modern day trend is to smaller primary size of material.
• Storage conditions. Humidity and temperature cycling are key (see: http://www.indicizer.com/files/EliminatingCakingProblems.pdf). Obviously routes to minimizing the thermal shock and humidity need to be seriously considered. If ‘long transportation at environmental temperatures of 430C and above is causing the problem, then clearly one needs to find ways to avoid that practice.

Consultation with experts in the field such as Jenike and Johanson or Materials Flow Solutions will almost certainly prove beneficial.