Manipulating Particle Shape

To ensure the same washing efficiency on large scale as on lab scale, it is important to learn about the washing behavior of the filter cake. In case the filter cake tends to crack, this may be not critical on lab scale if the crack is closed, for example, with a spatula, but on large scale this happens to a much higher extent and the cracks may cause the wash to preferentially flow this way and the rest of the cake will not be washed properly. 

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Particle behavior is a function of particle size, density, surface area, and shape. These interact in a complex manner to give the total particle behavior pattern [28], The shape of a particle is probably the most difficult characteristic to be determined because there is such diversity in relation to particle shape. However, particle shape is a fundamental factor in powder characterization that will influence important properties such as bulk density, permeability, flowability, coatablility, particle packing arrangements, attrition, and cohesion [33-36], Consequently it is pertinent to the successful manipulation of pharmaceutical powders that an accurate definition of particle shape is obtained prior to powder processing. 

From the practical point of view, the change of filtration pressure, filtration area, filter medium, filtrate viscosity and solids loading is limited . To increase the filterability of the cake, most practical way, therefore, is to decrease the average specific cake resistance a. This could be accomplished by changing the particle shape, increasing the particles size and narrowing the size distribution of the particles in the cake. The latter two factors can be manipulated by polymer flocculation. 

Particle shape can be manipulated if adequate seeds are used. If needle-hke crystals are micronized to a compact shape and these particles are used in a large amount (e.g., 10% of the expected yields), the resulting product crystals are forced to have an aspect ratio of less than 10, even if they would grow only in the longitudinal direction. 

Amalgams made with spherical particles may predominate ia use over those made with flake-shaped particles because the desirable plasticity is obtained with a lower mercury content, satisfactory compaction is achieved with lower packing pressures, and there is less influence of manipulative variables upon values for appropriate physical properties. 

The types of intrinsic dissolution profiles obtainable through the loose powder and constant surface area methods are shown in Fig. 19. Oxy-phenbutazone was obtained as the crystalline anhydrate and monohydrate forms, with the monohydrate being the less soluble [129]. The loose powder dissolution profiles consisted of sharp initial increases, which gradually leveled off as the equilibrium solubility was reached. In the absence of supporting information, the solubility difference between the two species cannot be adequately understood until equilibrium solubility conditions are reached. In addition, the shape of the data curves is not amenable to quantitative mathematical manipulation. The advantage of the constant surface area method is evident in that its dissolution profiles are linear with time, and more easily compared. Additional information about the relative surface areas or particle size distributions of the two materials is not required, since these differences were eliminated when the analyte disc was prepared. 

Resolution is without question a key element in accurate and detailed particle characterization. Particle populations that cannot be resolved cannot, in any sense, be distinguished from one another. While deconvolution techniques can provide particle size distribution curves from low resolution systems, the deconvolution must be based on assumptions about instrumental band broadening and band shape. In general, any detailed information lost because of poor resolution cannot be recovered by mathematical manipulation alone. In all cases, the quality of a size distribution curve will increase with the intrinsic resolution exhibited by the system. 

The final product in all cases is a-Fe203, but its hue can range from orange to pure red to violet through manipulation of particle size, shape, and surface properties. The four processes yield a range of physical properties. Density can vary from... 

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