There are several approaches to this and the most common are Gamma ray devices eg Berthold, Ultrasonic or vibrating element. For vibrating element try http://www.Mobrey.com or http://www.viscoanalyser.com.
Vibrating tube types have been used with china clay slurries and the fork sensors with chalk slurries, coal tailing slurry, granite wash slurry and in classifiers, underflow measurement and as part of underwater towed arrays for measuring river mud density (and viscosity).
In the china clay industry a typical application is where they blend different clays together by blending the slurries containing them. In each stream they have a magnetic fowmeter and a vibrating tube density meter. They thus have the mass rate of flow of each clay and can then blend them in the correct ratio. (Note: Using the density with the volume flow would give the mass flow of the slurry, what the density meter does is calculate the % mass of slids and thsu you get the mass flow of the solids, not the solids plus carrier).
