The Mixing of Liquids and Solids

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Chapter: Pharmaceutical Engineering: Mixing

Examples of solid-liquid mixing are found in the operations of dissolution and crystallization and in the control of chemical reactions between solids and liquids.


THE MIXING OF LIQUIDS AND SOLIDS

Examples of solid-liquid mixing are found in the operations of dissolution and crystallization and in the control of chemical reactions between solids and liquids. Alternatively, randomization of materials for subdivision and presentation may be the object, as, for example, in the production of a toothpaste.

The flow properties of a liquid-solid mixture alter markedly with change in the ratio of the two phases. At low solid disperse phase concentrations, flow properties are Newtonian and mixing by impellers is satisfactory so long as the components of flow oppose settling. Under such conditions, it may be desirable to increase the size and decrease the speed of the impeller. For a given power input, improved flow patterns will be produced at the expense of turbulence. Unless the difference in density between solid and liquid is small, paddles are ineffective for suspending solids. Otherwise, the discussion presented for the mixing of liquids may be applied.

Anomalous flow characteristics are exhibited at higher disperse phase concentrations in which the apparent viscosity is a function of the rate of shear. The apparent viscosity may increase or, more commonly, decrease as the speed of the impeller is increased. Mixing will be achieved by suitable impellers, notably the turbine, as long as adequate flow patterns in the entire volume of the mixing vessel are created. Turbulence is less effective as a mechanism of mixing and regimes of laminar flow will be extensive.

Further increase in apparent viscosity occurs at higher disperse phase concentrations. This is often associated with the development of a yield value. Unlike true liquids, shear forces must exceed a certain level before deformation occurs. Since the shear forces developed by particles suspended in liquids are small, sedimentation does not occur and the mixture may be classified as neu-tral. Mixing by impellers is precluded if the apparent viscosity is very high because the projection of adequate flow patterns is impossible. Alternative methods must be used in which the mixing element visits all stations in the mixing vessel. For thin pastes, machines typified by the domestic food mixer are used. Imposition of planetary movement on the rotation of the mixing element causes all parts of the mix to be sheared at intervals. Very high shear rates are produced as the element sweeps out zones close to the wall of the container. In other machines, the containing vessel rotates.

For thicker pastes and plastics, a kneading, stretching, and folding action is employed. The sigma blade, mounted axially in a trough, is a commonly used mixing element. Intense shear is induced by the close clearance between element and container. Simultaneous transport around the trough occurs so that all portions of the mass are periodically deformed. Considerable variation in theological properties may occur during mixing, and robust construction of the mixer is essential.

The differential speed of the rolls of the roll mill induces high shear rates in the material. This machine is suitable for paste mixing. With more fluid dispersions, the ball mill and the colloid mill may be used. Solids that aggregate may be successfully dispersed, although subsequent stability may require the addition of a deflocculating agent.

Blending solids with very small quantities of liquid, an operation com-monly used for granulating powders, presents extreme problems of uniformity. If the material does not become plastic and pasty, it will not mix by shear deformation in the manner described above. Mixing is then best achieved by spraying the liquid as fine droplets onto a highly mobile powder, which is continually and rapidly developing new surfaces. In this way, all the particles can be exposed to the spray. A closed ribbon mixer, planetary mixer, or sigma blade mixer can be used. Alternatively, tumbler mixers can be fitted with a spray device. If the solid is itself a mixture, the material must be completely mixed before the liquid is added. Otherwise, homogeneity will be difficult to achieve.

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