Suspensions fine, coarse

Suspensions are coarse dispersions of finely divided solids in a liquid. The solid particles have a mean particle size greater than 0.1 pm in diameter. Pharmaceutical suspensions are administered orally, topically, and parenterally and should avoid the following problems sedimentation, caking, flocculation, and particle growth. Physicochemical principles in the solid/liquid interface will be discussed in this section as they pertain to the preparation of good pharmaceutical suspensions. 

Coagulation by inorganic electrolytes has been found to be effective only for fine colloids and not for suspensions of coarse particles, for example, of size over 0.074 mm. 

On the other hand, an analysis of the extreme case of coarse dispersions is more difficult, in a sense, than an analysis of the opposite extreme of fine suspensions. This is due to the mere fact that particles in Ene suspensions interact only hydro-dynamically. Although this means that there is no need to consider direct particle collisions, the problem of formulating both the conservation and rheological equations remains difficult because hydrodynamic interactions involve many particles simultaneously in fine particle suspensions. A sophisticated statistical theory of Brownian suspensions is now being developed by Brady and his co-workers that might help to tackle this problem [11-13]. An attempt to take into account pseudo-turbulent fluctuations in finely dispersed suspensions is described in [14,15]. It is quite evident that any generalization of these models of fine collisionless suspensions to coarse collisional suspensions involves, first of all, the necessity to account for direct collisions, and this is certainly a matter of some difficulty. 

Addition of Inert Filter Aids. FUtet aids ate rigid, porous, and highly permeable powders added to feed suspensions to extend the appheabUity of surface filtration. Very dilute or very fine and slimy suspensions ate too difficult to filter by cake filtration due to fast pressure build-up and medium blinding addition of filter aids can alleviate such problems. Filter aids can be used in either or both of two modes of operation, ie, to form a precoat which then acts as a filter medium on a coarse support material called a septum, or to be mixed with the feed suspension as body feed to increase the permeabihty of the resulting cake. 

Strictly speakiag, these correlations apply only to the cases where flocculation is absent, such as for coarse mineral suspensions. Suspensions of fine... 

A method for physically separating turbostratic carbon and graphite involves shaking a sample into suspension in ethylene bromide of sp gr 2.17 and centrifliging. The method is unreflable except where fine carbon and coarse graphite are admixed it can be an aid in quaUtative examination. 

Coal (hard) and coke are used in water filtration, primarily for the removal of coarse suspensions, care being taken to prevent them from scouring or washing away, because of their relative lighmess and fine division. Coal is principally composed of carbon, and is inert to acids and alkalies. Its irregular shapes are advantageous at times over silica sand. 

One method for improving the size distribution still further is to employ fines destruction (Jones etal., 1984 Jones and Chianese, 1988). In this technique, a classified stream of suspension, i.e. containing only fine not coarse crystals, is removed from the crystallizer in an elutriation leg (Figure 7.6(a)). 

Suspensions are conveniently divided into two broad classes—fine suspensions in which the particles are reasonably uniformly distributed in the liquid and coarse suspensions in which particles tend to travel predominantly in the bottom part of a... 

There have been several studies involving the use of media consisting of fine dense particles suspended in water for transporting coarse particles. The fine suspension behaves as a homogeneous fluid of increased density, but its viscosity is not sufficiently altered to have a significant effect on the pressure drop during turbulent flow, the normal condition for hydraulic transport. The cost of the dense particles may, however, be appreciable and their complete separation from the coarse particles may be difficult. 

This result can also be applied directly to coarse particle swarms. For fine particle systems, the suspending fluid properties are assumed to be modified by the fines in suspension, which necessitates modifying the fluid properties in the definitions of the Reynolds and Archimedes numbers accordingly. Furthermore, because the particle drag is a direct function of the local relative velocity between the fluid and the solid (the interstitial relative velocity, Fr), it is this velocity that must be used in the drag equations (e.g., the modified Dallavalle equation). Since Vr = Vs/(1 — Reynolds number and drag coefficient for the suspension (e.g., the particle swarm ) are (after Barnea and Mizrahi, 1973) ... 

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