Breaking up Wetted Clumps

In a powder that is dry (nothing beyond a monolayer of surface moisture) or non-saturated (interparticle regions not filled with water) the individual particles are held together in clumps by forces acting at or near the points where the particles contact one another:

• Polarizability attraction

• Surface tension of the liquid located at these contacts

• Sinter bonds due to pressure on malleable materials

• Precipitation bonds from drying of solution after solid-liquid separation

The strength of bonds holding clumps together may increase with storage time due to the surface migration of material into the high energy contact region as a result of solubilization and mobilization due to adsorbed vapors or surface treatment chemicals or out of the high-pressure contact region to a low-pressure non-contact region. Some powders sold commercially have been coated with a solid or liquid to decrease the likelihood of such changes and to keep the clumps "soft" (easy to deagglomerate).

Breaking clumps to fully disperse the submerged and wetted clumps as individual particles (often called "deagglomeration") may be achieved by high shear stress or mechanical impact (Ref 14, 15, 16). This generates only a small amount of new surface area (from breaking the contact points) and thus uses only a moderate energy per unit mass of material. The surface area newly created by deagglomeration may have higher adsorptivity and reactivity than the previously existing surface of the clump. Note: Deagglomeration is distinct from "milling," which employs similar equipment to fragment the fundamental particles, thus generating significant new surface area and requiring a significantly higher energy per unit mass of material.

High shear stress can be achieved in high-viscosity systems (for example molten plastic or systems with very high solids loading) using:

Slowly turning mixing paddles

Close clearances between the shearing elements

• High rotational speed of the moving element

For intermediate-viscosity systems (for example highly loaded inks) the dispersion may be:

• Passed through the gap between two mill-rolls that are rotating with different surface velocities

• Circulated past alternately up- and down-turned teeth at the periphery of a rapidly spinning disk impeller

For low-viscosity systems (for example mineral ore in water) dispersions may be:

• Circulated through the small gaps between a slotted stator outside a close-fitting slotted rotor moving at a high RPM or

• Pumped through an annular gap around a cone rotating in a housing machined so that the clearance decreases from entry to exit

• Forced at high pressure through a very small clearance in a spring-loaded valve

At any viscosity the heat generated by the shear may be a factor limiting the extent of the operating region and thus the deagglomeration that can be achieved.

Mechanical impact can be achieved using grinding media (usually spheres or cylinders) set in motion by:

• Stirring using cross-bars or disks attached to a driveshaft

• Vibration of the container holding the liquid-powder-media mixture

• Cascading by gravity within a container rotating on a horizontal axle

• Cascading by centrifugal force within (several, for balance) rotating containers mounted on a centrifuge with their axles parallel to and offset from the axis of centrifugation

Conventional mills have a grind limit (minimum achievable particle size) about 20 the diameter of the media, but mills with high energy input can achieve a grind limit of 200 the size of the media. It is hard to achieve high-energy impacts in a viscous medium, so impact methods are restricted to low-viscosity systems.

Autogenous Milling. If contamination by wear fragments from the media is a problem, autogenous milling may be preferable. In this process particles are ground through impact with one another caused by:

• Directing one high-velocity jet of dispersion against another

• Vigorously stirring a highly loaded dispersion

The number of collisions is maximized if the solids loading is above 40 volume percent so that there is little liquid separating the particles.

Ultrasonic baths or probes are commonly used to prepare laboratory dispersions in low-viscosity liquids. Piezoelectric crystals attached to the container wall produce pressure waves that alternately create a vacuum (that can pull the liquid apart to form evacuated cavities) and a pressure (causing the cavities to collapse) at 104-107 Hz. The cavities formed next to particle surfaces are hemispheric in shape. The jets of liquid that form as these hemispheres collapse impinge on the particle surface and focus stress that can shatter the bonds holding a clump together. Because the cavitation energy is strongly attenuated as it propagates through a dispersion, cavitation occurs in only a limited volume (the "active zone") near the vibrating wall. It is important to pass all the dispersion through the active zone and to have a sufficient total residence time there to achieve full deagglomeration without overheating the dispersion or breaking the fundamental particles. Since the presence of vapor in the cavity will decrease the intensity of collapse, the closer the liquid is to the boiling point, the less effective is ultrasonication.

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