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?!”