Precipitation, particle size reduction

Two classes of grinding equipment are used to prepare dispersions. The first, the coUoid mill, does not effect a particle size reduction but does break down aggregates of fine particles. CoUoid mills are used for such powders as clays, precipitated whiting, etc. Sometimes these mills are used to process zinc oxide but for dipped mbber products that is not satisfactory. 

High-energy milling should not be used during particle size reduction. Microcrystals are best formed by controlled precipitation techniques or shock cooling. 

The first factor to be considered for particle size reduction is the supersaturation profile. For very low concentrations of the nonvolatile solutes in antisolvent crystallization, supersaturation during the initial precipitation stage is characterized by the maximum attainable supersaturation, a. This... 

Dangaran, K.L., Cooke, P., Tomasula, P.M. The effect of protein particle size reduction on the physical properties of C02-precipitated casein films. J. Food Sci. 71, 196—201 (2006)... 

Physico-chemical methods for particle size reduction comprise methods wherein the substance is first dissolved and then precipitated in a finer form during further processing. This has the advantage that it is independent of the fineness of the starting material and that the formation of small agglomerating particles is largely prevented. Two methods are distinguished the solution method and the precipitation method. 

The techniques that have been reported for preparing finely dispersed drug particles can be broadly categorized as either particle size reduction or controlled precipitation. There are variations on each theme, some of which are described later. 

With the principle of this process being antisolvent addition, the process does not differ significantly from the SAS technique. As in SAS process, the drug or polymer solution is sprayed into a bulk of SCF, typically carbon dioxide, for a fixed period of time. This step is then followed by passing supercritical carbon dioxide to extract and remove the solvent and dry the precipitated product (18). Reports on the application of this technique focus on particle size reduction (18-22). 

Falope etal. (2001) extended the MSMPR model of agglomerative crystal precipitation based on the Monte Carlo simulation technique to account for particle disruption by considering two alternative particle size reduction mechanisms - one representative of particle sphtting into two parts of equal volume, the other representative of micro attrition. 

Many crystalline products, including fine chemicals, foodstuffs and pharmaceuticals, require a final particle size that is significantly smaller than that produced during the crystallization or precipitation step. One way of achieving the required particle size is to employ a subsequent size-reduction step using some form of comminution device, frequently a mill. 

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