Floc Stability Research Articles

Particle adhesion in coagulation and bridging flocculation

The adhesive forces between solid particles mutually attached during coagulation or bridging flocculation are important for modelling floc stability. Results are presented in this study which are obtained from experiments on the adhesion of glass or quartz particles to a flat glass substrate (centrifugal method) or to the wall of a glass capillary through which an aqueous electrolyte solution was passed. Coagulation experiments carried out in 10−2 mole/1 MgCl2 showed the action of hydration layers on hydrophilic surfaces, whereas surface methylation is associated with adhesion in the inner potential minimum. In addition essential ageing effects interpreted as interparticle gelation were observed, especially on the interaction of alkali glass surfaces. Adhesive strength in the case of flocculation with hydrolysed polyacrylamide and a cationic Praestol mainly depends on the polymer concentration and on the preadsorption conditions before the particle-substrate attachment. A significant strengthening of adhesion due to reconformation of the bridging agents was not observed for a contact time greater than 3 min. The effect of steric stabilization with polymer overdosing could be proved by a special preparation technique.

Floc stability during Coulter-counter size analysis

Floc stability during Coulter-counter size analysis

A model of floc strength under hydrodynamic forces

The strength of an idealised model of a floc has been examined with a view to calculating the effect of hydrodynamic forces on floc stability. Various spatial arrangements of spherical primary particles were considered joined by elastic polymer links, in accordance with the accepted bridging mechanisms of flocculation. An available structural frame analysis computer program was used to find the forces in the links as the flocs were considered stressed by simple shear fields. For a given shape of floc a dependence of maximum stable size on the shear gradient was established. There is general agreement with published experimental findings of the variation of the maximum size of an aggregate for a given shear gradient.

The hydrodynamic forces on touching spheres along the line of centers exerted by a shear field

The flow near two touching spheres of radius a 1 and a 2 in a general linear flow is considered. It is shown that the force on either sphere along the line of centers ( z direction) is determined by that part of the flow field which is of the form V ∞ = Vi z + G{zi z − 1/2w ∼ i w ∼ } . For a uniform shear flow G is simply related to the shear rate and doublet orientation. For a freely suspended doublet, the force along the line of centers is F 2 = 6πμa 1 2Gh · ( a 1 a 2 ) = −F 2 , where h is a tabulated function. It is hoped the results will find application to the analysis of floc stability at high shear rates.