Does anyone know enough about pigment flow characteristics to advise me how best to address a process problem?

I work in a grinding/milling unit where a particular pigment is internittently refusing to flow.

The equipment line consists of a conical hopper fitted with a rotating mixer arm, which feeds a helical feedscrew which in turn feeds a hammer mill then an extraction filter (under negative pressure). From here the pigment passes into a small tank then through a densifier (2 rotating drums, one of which operates under vacuum to squeeze the air from the milled powder), and out of a volumetric filling point.

Until recently the pigment - mostly in granular form before feeding to the mixer - flowed perfectly well through the process line. Recently we noticed that the discharge rate had dropped off, and this seems to be exacerbated by reducing levels of pigment in the tank.

Then the pigment stopped flowing from the tank. It was bridging badly in the tank. We reduced the negative pressure in the system and forced it to flow, albeit with an increased number of line blockages. We have also been running with the tank full to help matters.

Now the pigment has stopped flowing from the mixer - its bridging terribly.

So far the only direct correlation I've found in terms of physical properties is between tamped volume (density) and discharge rate, with an r-squared of 0.92.

I'm not sure whether I should be looking at shear strength, tensile strength/cohesion or sticking with bulk density as a measure of static flowability. Can someone please advise?

What is the best procedure to determine the critical parameters and their limits in this system? What equipment will I need? ■