Flocculating power

Hardy-Schulz rule phys chem An increase in the charge of ions results In a large increase in their flocculating power. hard-e shults, rtil )... 

Potato starch is inherently anionic, since it contains phosphate groups. The presence of both anionic and cationic groups in cationized potato starch enhances its flocculation power and renders it active over a wider pH range. Com starch has to be rendered amphoteric by the introduction of phosphate groups. Charge modification has also been applied to waxy maize starch. 

Equation (3.6) confirms what is already known from experience Ca2+ and Al3+ have greater flocculating power than Na1+ and K1+. In the other direction, polysaccharide polyanions are less inclined to disperse in water containing multivalent ions than in water containing monovalent ions. 

The flocculation power of polymers depends on the number of ionic groups attached to the long chains of the polymers. Multi-valent flocculants possessing a number of ionic groups in their molecules will adsorb more on oppositely charged mineral surfaces, suggesting the validity of the Schultz-Hardy rule as shown in Table 5.28. 

Hofmcistcr Scries. Cations arranged in order of decreasing flocculating power for clay slips H+, AP+, Ba2+, Sr +, Ca2+,... 

With the advent of flotation as a useful treatment process in water and waste water treatment, made possible by the creation of finer bubbles by dissolved air precipitation, and electrolysis, the concept of bubble flocculation can be reconsidered. In the flotation process in water treatment, flocculation takes place prior to bubble flotation, usually by mechanical (paddle) flocculator with a coagulant salt (such as alum). It seems possible that the flocculation and flotation processes could be combined with the bubbles providing the flocculating power. 

Operating Costs Power cost for a continuous thickener is an almost insignificant item. For example, a unit thickener 60 m (200 ft) in diameter with a torque rating of 1.0 MN-m (8.8 Mlbf in) will normally require 12 kW (16 hp). The low power consumption is due to the very slow rotative speeds. Normally, a mechanism vi l be designed for a peripheral speed of about 9 m/min (0.5 ft/s), which corresponds to only 3 r/h for a 60-m (200-ft) unit. This low speed also means veiy low maintenance costs. Operating labor is low because little attention is normally required after initial operation has balanced the feed and underflow. If chemicals are required for flocculation, the chemical cost frequently dwarfs all other operating costs. 

With any chemical treatment system, the main task is one of getting the chemical thoroughly mixed with the solids without degrading the floes which are formed. For those slurries that are relatively fluid, the chemical can frequently be added and mixed satisfactorily using a relatively wide spatula. However, for those thick, relatively viscous slurries, a power mixer will be required. In this case, the mixer should be stopped about one second after the last of the flocculant is added. Should this approach be required, it means that a suitably designed addition system must be supplied with the full-scale instaUation in order to do an effective job of flocculation. 

In a series of experiments on the flow of flocculated kaolin suspensions in laboratory and industrial scale pipelines(26-27-2Sl, measurements of pressure drop were made as a function of flowrate. Results were obtained using a laboratory capillary-tube viscometer, and pipelines of 42 mm and 205 mm diameter arranged in a recirculating loop. The rheology of all of the suspensions was described by the power-law model with a power law index less than unity, that is they were all shear-thinning. The behaviour in the laminar region can be described by the equation ... 

Table 5.2. Power-law parameters for flocculated kaolin suspensions...

A highly concentrated suspension of flocculated kaolin in water behaves as a pseudo-homogeneous fluid with shear-thinning characteristics which can be represented approximately by the Ostwald-de Waele power law, with an index of 0.15. It is found that, if air is injected into the suspension when in laminar flow, the pressure gradient may be reduced even though the flowrate of suspension is kept constant, Explain how this is possible in slug flow and estimate the possible reduction in pressure gradient for equal volumetric flowrates of suspension and air. 

Incorporated in paints, P.Br.41 exhibits equally high hiding power, which makes it a less attractive product for use in metallic finishes. Its tinctorial strength in these media is comparatively poor. However, P.Br.41 is completely fast to overcoating. The commercial grade shows considerable flocculation in various paint... 

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