Suspensions deflocculation

Monovalent cations are good deflocculants for clay—water sHps and produce deflocculation by a cation exchange process, eg, Na" for Ca ". Low molecular weight polymer electrolytes and polyelectrolytes such as ammonium salts (see Ammonium compounds) are also good deflocculants for polar Hquids. Acids and bases can be used to control pH, surface charge, and the interparticle forces in most oxide ceramic—water suspensions. 

Air injection may sometimes be an alternative to deflocculation. In general, defloccu-lated suspensions flow more readily but they tend to give much more highly consolidated sediments which can be difficult to resuspend on starting up following a shutdown. Furthermore, deflocculants are expensive and may adversely affect the suitability of the solids for subsequent use. 

By introducing surfactants, which lower the interfacial tension, it is possible to reduce the work necessary to deflocculate agglomerates. In liquid suspensions the introduction of an interfacial tension depressant facilitates wetting of the solid by the liquid and the displacement of adsorbed gases from the solid surface. Certain solids have adsorbed films whose adhesional forces are so great that they resist all mechanical efforts to displace them. Upon the addition of a surfactant, the Aims are displaced and a solid-liquid interface is achieved (1). 

The degree of flocculation, / , is defined as the ratio of the sedimentation volume of the flocculated suspension, F, to the sedimentation volume of the suspension when deflocculated, F,yj. It is expressed as ... 

Hydrocyclones are 20-500 mm in diameter, with the smaller units giving a much better separation. Typical values of length to diameter ratios range from about 5 to 20. Because of the very high shearing stresses which are set up, floes will be broken down and the suspension in the secondary vortex will be completely deflocculated, irrespective of its condition on entry. Generally, hydrocylones are not effective in removing particles smaller than about 2-3 rm. 

Deflocculating Agent An adjuvant which inhibits precipitation or the settling of solids in the suspension fluid. 

Schofield, R.K. and Samson, H.R., 1953. The deflocculation of kaolinite suspensions and the accompanying change over from positive to negative chloride adsorption. Clay Miner., 2 45-50. 

In the slip-casting process the ceramic powder is suspended in a fluid vehicle, usually water. The suspension, or slip , has a high solids content, typically 50vol.%, and the individual particles are fine, usually less than 10 /mi. Deflocculants, which modify the electrical environment of each particle so that they repel one another, are added to the slip. The fineness of the powder and consequent high surface area ensure that electrostatic forces dominate gravity forces so that settling does not occur. When exceptionally heavy powder particles... 

Sediment in an aged dispersion may be collected and measured in a crudely quantitative test. To distinguish between a deflocculated, a flocculated, and an aggregated suspension, a weighed amount of solid is uniformly suspended in a small quantity of liquid, the suspension is transferred to a graduate cylinder, the volume of sediment during a stated period of time is measured, and the specific sediment volume (milliliters per gram) vs time is plotted. In... 

Although the production of highly deflocculated suspensions is a primary objective for formulation of suspension concentrates, these systems tend to settle under gravity forming dilatant sediments (clays). The latter must be prevented either by controlled flocculation or by the addition of a second disperse phase to the continuous medium (1). One method which may be applied to sterlcally stabilised dispersions, is to add a free (ie. non-adsorbing) polymer to the continuous medium. 

Effectiveness of deflocculants in dilute suspensions may be studied by light absorption. If the optical density of an agitated suspension falls with time this is an indication of either floeculation or dissolution, whereas an increasing optical density indicates breakdown of floes. 

Flocculation comes from the Latin word flocculate meaning loose and woolly. Flocculated systems result in rapid rate of settling because each individual unit is composed of many particles and is therefore larger. However, due to the loose packing of floes they are easily dispersible on shaking. Deflocculated systems on the other hand are made up of smaller particles whose settling rate is slower, but the settled particles tend to form an irreversible compact and are difficult to redisperse. This phenomenon is called caking. For coarse suspensions, a deflocculated suspension will have better uniformity of dose but poorer stability... 

If a suspension is to be produced by a dispersion technique (as opposed to precipitation techniques), surfactants may be used in the formulation to aid dispersion of the solid particles in the liquid. This is particularly important if the powder is not readily wetted by the liquid vehicle. Surfactants can reduce the interfacial tension between the solid particles and the liquid vehicle. The advancing contact angle is reduced, and wetting of the solid particles promoted. Such a system is said to be deflocculated. The inclusion of a surface-active agent to improve powder wettability can often improve the bioavailability of the formulation. 

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