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.

Flowability is a complex issue with particle size distribution, shape, humidity, and friction/tribology all playing a role. Consultation with one of the industry’s leaders in flow issues will certainly be beneficial and we’d recommend Jenike and Johanson (Jim Prescott is a very helpful contact there). Other companies in this field can be found with a web search.

Carbon black has small primary particle size (and forms chains) and bridges gaps very easily, thus causing the issues of which you write. Concentration of a dilute solid-in-gas to powder is a problematic issue in many cases like this - alternative transport and delivery systems will need evaluation; it seems that there is no alternative to the dilute phase transport. If you have tried (as it seems) pneumatic conveying then the hole will need to be larger to prevent bridging/blockage. Is the air perfectly dry? Has the system been grounded (static is yet another issue)?

I’m not sure that I understand your definition of flow angle - has the cone become steeper or more shallow? Believe it or not we have seen, in graphite plants, an automatic hammer that bangs the side of the hopper every 30 seconds to keep the powder from bridging - perhaps that isn’t too wild a possibility?!”

You may be referring to sieving or laser diffraction, but since the latter is more interesting, let’s focus on that. We must first assume that both the dry and wet accessories are performing to the manufacturer’s specifications. If the material is in the same state of dispersion, then the result should be the same whether wet or dry - all other things being equal. So the first possible reason is inadequate dispersion in either the wet or dry accessory, but we do need to take a step back, as the measuring device can only be as good as the sample given to it, bringing sampling into question.

Let us assume that we are dealing with identical subsamples of the same material, or at least have allowed for this in the comparison. Typically, the sampling issue raises its ugly head when material greater than 75 or 100 µm is present in a system. If in doubt, divide the powder on a spinning riffler or using a liquid sampler (called a Burt sampler). For coarser material, it is important to use a wet disperser that has a larger capacity to ensure representative sampling. If there is material smaller than around 25µm present in a system, we look at dispersion. In order to look at dispersion we have to examine the effect of input energy on (reported) size. In our wet methodology we should sonicate to stability, and in our dry analysis we conduct what is termed a pressure-size titration, where we look for stable regions in the distribution.

Earlier I stated ‘all things being equal’. Some things that may not be equal:

Optical Model choice: With dry dispersion we know the medium is air, with an Ri of 1. With wet dispersion, it is important to choose the right value for the medium liquid.

Dry measurements made with excessive air pressure may cause the material to break-up or comminute

Choosing the wrong liquid medium could cause dissolving, swelling, or a reaction. A microscope next to the dispersion unit and removal of a drop of material at the appropriate stage may be a very useful guide
here.

Cleanliness of the disperser and optical surfaces (are windows stained or scratched?)

For additional references, I suggest you consult ISO13320, the international standard for laser diffraction, and Terry Allen’s book “Particle Size Measurement” (now in its 5th Edition).