Wet Milling Comminution

Several methods may be appHed for measuring particle size distribution, and these will be described in detail in Chapter 19. 

Several factors affect the efficiency of dispersion and milHng, including the volume concentration of the dispersed particles (i.e., the volume fraction) the nature of the wetting/dispersing agent and (iii) the concentration of the wetting agent/dispersant (which determines the adsorption characteristics). 

The effect of particle interaction at relatively low particle concentration c can be taken into account by expanding the diffusion coefficient into a power series of concentration, 

PCS is a rapid, absolute and nondestructive method for particle size measurements. It has some limitations the main disadvantage Is the poor resolution of particle size distribution. Also it suffers from the limited size range (absence of any sedimentation) that can be accurately measured. Several instruments are commercially available, e.g. by Malvern, Brookhaven, Coulters, etc. The most recent instrument that is convenient to use is HPPS supplied by Malvern (UK) and this allows one to measure the particle size distribution without the need of too much dilution (which may cause some particle dissolution). 

This method is based on the use of fiber optics, sometimes referred to as fiber optic dynamic light scattering (FODLS) and it allows one to measure at high particle number concentrations. FODLS employs either one or two optical fibers. Alternatively, fiber bundles may be used. The exit port of the optical fiber (optode) is immersed in the sample and the scattered light in the same fiber is detected at a scattering angle of 180° (i.e. backscattering). 

The above technique is suitable for on-line measurements during manufacture of a suspension. Several commercial instruments are available, e.g. Lesentech (USA). 

Milling or comminution (the generic term for size reduction) is a complex process and there is little fundamental information on its mechanism. For the breakdown 

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Nanoparticle size, as accomplished by size reduction techniques, has been an area of focus using wet milling comminution processes to achieve submicron particle nm (Liversidge et al.,... 

Surfactants are essential for the preparation of solid/liquid dispersions (suspensions). The latter are generally prepared using two main procedures (7) Bmlding up of particles from molecular units. (2) Dispersion of bulk performed powder in a liquid followed by dispersion and wet milling (comminution) to produce smaller particles. An example of the first system is the production of polymer latex dispersions by emulsion or dispersion polymerization. The monomer is emulsified in an aqueous solution containing a surfactant to produce an emulsion of the monomer. An initiator is added to initiate the polymerization process. In some cases, initiation occurs in the micelles that are swollen by the monomer. The number of particles produced and hence their size is determined by the number of micelles in solution. In dispersion polymerization, the monomer is mixed with a solvent in which the resulting polymer is insoluble. A surfactant (protective colloid) and initiator is added. The surfactant prevents flocculation of the polymer particles once formed. Again the size of the particles produced depends on the nature and concentration of the surfactant used. 

The dispersion of aggregates and agglomerates and subsequent reduction of particle size by wet milling (a process referred to as comminution) also requires... 

Dispersion methods. In this case one starts with preformed particles of the bulk chemical, which are then subdivided into smaller partides by a suitable dis-persion/comminution process. An example of this method is the production of suspensions by wet milling (using grinding equipments). Emulsification of oils (using high speed stirrers or homogenisers) is also a kind of dispersion method. 

The primary particles produced after dispersion are subdivided into smaller units by milling or comminution (a process that requires rupture of bonds). Wet milling can be achieved using ball mills, bead mills (ceramic balls or beads are normally used to avoid contamination) or colloid mills. Again the milling conditions must be adjusted to prevent heat and/or foam formation. The role of the dispersing agent (surfactant) in... 

FIGURE 17.5 Scanning electron micrograph of steroid crystals comminuted in a wet ball mill to submicron dimensions. (Data generated by NanoSystems, Elan Drug Technologies, a member of the Elan Corporation, pic.)... 

A machine for the comminution, or size reduction, of minerals or other materials. Such machines crush the input material by wet grinding in a cylindrical rotating bin containing grinding balls. These mills can produce colloidal-sized particles. 

Kwade, A. Wet comminution in stirred media mills— research and its practical application. Powder Tech. 1999, 105, 14-20. 

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