Any connection between a Froude number and a dimensionless mixing coefficient would be purely coincidental.  No such formula or relationship exists for ribbon mixers or any other type of mixer.
Understand that the Froude number represents a ratio of inertial forces to the gravitational force.  The inertial forces would be related to the force applied by the ribbon on the powder in the mixer.  Gravity, obviously keeps the powder in the ribbon mixer.  However, if for some interplanetary project, the gravity were only half what it is here on earth, the effect on mixing time probably would be undetectable for most materials.  The only effect, might be caused by compaction in the lower part of the blender, and while it could certainly increase the force on the ribbon blades, it would have little effect on mixing time.  Froude number might be relevant for describing wave height of vortex depth on a liquid surface, but not mixing time.
One of the big problems with predicting mixing performance for any type of powder mixing equipment is that the effect of powder properties on mixing is extremely difficult to define.  Unlike liquids, which are essentially incompressible materials, compression and other factors can have a significant influence on powder bulk density and flow characteristics.  Liquids have measurable viscosities, while the definition of powder flowability depends on the type of mixing equipment.  In all cases, to accomplish the powder motion required for mixing, the powder must undergo some "dilation" (local expansion) to allow the particles to move relative to each other.

As with liquid mixing, the factor most closely related to the degree of mixing uniformity is the number of rotations of the mixing device.  With larger mixers, the rotation speed (revolutions per minute) is less and therefore the mixing time is longer.  That relationship is often described by a dimensionless mixing coefficient.  Although, that coefficient depends on the mixer geometry and the mixing requirements, to include degree of uniformity, powder flow properties, and relative quantity of ingredients.

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