PROPERTIES OF FLOCCULATED SYSTEMS.

Different measuring techniques (eg:- turbidity, settling rate, sediment volume, floe size and size distribution) have been used for estimating the efficiency and optimum dosage for a given flocculant. Approaches using optical parameters (eg:-

transmittance), are not sensitive to floe structure. Healy and La M er (1963) pointed out that filtration rate measurements are a better method of characterising flocculated systems since they are influenced by the floe properties and the unflocculated fines. Since floes are not homogenous with respect to size or structure, it is only the average sizes or distributions that are measured.

Under given conditions, one of the major factors which influences floe structure is the mechanism by which flocculation occurs. Floes formed by bridging have been shown to be completely different from those formed by charge patch neutralisation (Buscall et a l, 1984). The floes formed by charge patch neutralisation are similar to those formed by inorganic coagulants; they tend to be small and consist of randomly orientated particles. Bridging flocculation produces floes which are large and sediment rapidly, though there is little particle/particle contact (due to the bridging). The structure and properties of concentrated, flocculated suspensions are of primary importance with respect to their separation and dewatering behaviour in bioprocessing operations. Flocculated suspensions tend to be more viscous than stable suspensions of the same size. An understanding of the behaviour of flocculated

materials and control of their structure should allow effective matching of process design to material properties. The suspension characteristics are dominated by a limited number of factors, namely the primary particle size and shape and the solids content (Stewart & Sutton, 1984). The techniques which have been used to study morphology include optical microscopy, light scattering, sedimentation methods and electronic (Coulter) particle counters,* however these techniques are only suitable for dilute suspensions. Rheological techniques have been used (Stewart & Sutton, 1984; Ward, 1990) to study concentrated flocculated suspensions, but structural information concerning the floes has to be inferred and hence is model dependent. Freeze etch microscopy is an alternative technique of probing floe morphology and the ultra- structural detail of concentrated aggregated suspensions (Stewart & Sutton 1984). This has allowed quantities such as particle coordination numbers and particle/particle contact areas to be directly measured. The effect of solids content and particle size and shape have been examined in order to predict processing behaviour on a larger scale. Whilst dilute systems tend to have a simple rheology, flocculated dispersions tend to be highly non-Newtonian. At low solids contents, the sedimentation rate is increased by flocculation, but at higher solids concentrations, the sedimentation rate can be retarded by the formation of a network structure of floes (the gel point). The extent to which the solids content can be increased is dependent upon this gel point. Processing operations such as filtration and centrifugation exert considerable pressure upon the particle network, hence the compressibility of the network is important. Characterisation procedures have been developed (Buscall et a l, 1984) to determine the effects of solids loading and particle size and shape on the network compressibility above the gel point. These procedures can be used to predict the efficiency of dewatering processes in large scale equipment. Characterisation can be effected by equilibrium centrifugation, where the equilibrium sedimentation volume of samples is noted at various speeds. These are used to construct a series of curves of equilibrium sedimentation volume against acceleration; these are then converted into mean pressure against mean solids content. The slope of this function is known as the network modulus, and is given by:

K = <|) — = - V — (1.9)

where K is the network modulus, P is the applied pressure, V is the volume of flocculated network and <f> is the volume fraction. The network modulus (K) is a measure of the resistance of the disperse phase to densification. The advantage of using the network modulus is that it has dimensions of an elastic modulus. Since concentrated suspensions are visco-elastic, it is possible to measure other plastic moduli which are also characteristic of the particle network, for example, the instant shear modulus. The value of the instant shear modulus appears to be identical to the network modulus K, for all systems studied (Stewart & Sutton 1984). The instant shear moduli for flocculated suspensions are measured using a pulse shearometer, with the modulus being derived from the velocity of propagation of a transient pulse. There is a clear particle size effect in flocculated systems, with curves for larger species being moved to higher solids contents. The effect may be attributed to the number of particle/particle contacts per unit volume of sediment outweighing the decrease in strength of each individual particle/particle contact on moving to smaller particle sizes. There is a specific critical concentration at which the suspension starts to show solid-like behaviour. There is a rapid increase in the network modulus (K) with solids content. This has consequences for dewatering because once the critical concentration has been exceeded, substantial increases in concentrating pressure are required to effect small increases in solids content. The primary particle shape has a profound effect on the properties of flocculated suspensions. With non-spherical particles, a space filling network can be obtained at progressively lower solids contents {ie> as aspect ratios increase). The gel point at which the sediment starts to form a cohesive structure, and the rate of increase of the network modulus determine the response of the material irrespective of the dewatering equipment selected. Slow speed centrifugation can be employed for predictive purposes.

The flocculation mechanism using polyelectrolytes affords additional control over the suspension characteristics. The flocculation of anionic latex with PEI (50 kDa) showed that the floes appeared to be more compact than in electrolyte (BaClg) coagulated systems (Stewart & Sutton, 1986); the mechanism of flocculation appeared to be charge-patch, with direct inter-particle contact. The floe network strength was greater than in the electrolyte coagulated case, especially during polyelectrolyte overdosing conditions. However solids content and particle size were still the dominant influences on material properties. From the resulting floe structure,

bridging was thought to occur to a small degree, however double-layer compression (charge-patch) was thought to be the dominant mechanism.