Factors effecting a loss of flocculation activity of polyacrylamide solutions: Shear degradation, cation complexation, and solution aging

Abstract

SynopsisPolyacrylamide and poly(acrylamide-co-acrylic acid) flocculant solutions were subjected to Shear degradation in a rotating cone instrument. At constant shear rate, shear degradation was minimid (highat limiting intrinsic viscosity) by the use of a lower solution concentration and high ionic strength of the polymer solution. Resistance to shear degradation also increased with increasing anionic character of the polyacrylamide. Sheared polymers showed reduced performance as flocculants of coal preparation plant tailings, their major commercial application in the U.K. Partially anionic polyacrylamide solutions were completely inactivated in the presence of certain multivalent cations. Aluminium, ironlIl, lead, copper, and zinc ions formed complexes with the carboxyl group on the polymer, resulting in zero flocculation activity. Nonionic polyacrylamides were unaffected. Viscosity-aging of polyacrylamide solutions was observed over a period of several months, but was insu5cient to affect the flocculation activity. In the presence of 3% ethanol or methanol, no aging was observed in solutions stored for over a year.

Ageing behaviour of polyacrylamide-based excavation fluids

“…The main conclusion that can be drawn from these results is that the PHPA fluid, at least under the test conditions described herein, did not display an initial rapid reduction in viscosity as described by some previous researchers (Kulicke and Kniewske, 1981;Shyluk and Stow, 1969). Rather, the viscosity-time profiles shown by tests 1 and 2 are more in line with those reported by Henderson and Wheatley (1987) and to a lesser extent those by Gardner et al (1978). The cause of the gradual reduction in viscosity is possibly a conformational change of the polymer molecules leading to chain disentanglement as suggested by Narkis and Rebhun (1966).…”

“…These range from an initial rapid decline in viscosity (Kulicke and Kniewske, 1981;Shyluk and Stow, 1969), through negligible change (Mächtle, 1982), to a significant rise in viscosity (Likos et al, 2004). Other behaviours such as a slow decline (Henderson and Wheatley, 1987) and an inverted V-shape viscosity profile (Gardner et al, 1978) also have been reported. As the sample preparation procedures, test methods and the materials used for these studies vary considerably, it is not possible to draw any consistent conclusions that can be applied in civil engineering.…”

“…Each of these tests lasted for between 23 and 33 d. The slightly different test periods were due to the availability of the instrument in the authors' laboratory. On the basis of the previous test results of Henderson and Wheatley (1987), these times were considered sufficient for any important longer-term trends to be established, and certainly any trends likely to occur within the timescale of a typical fluid-supported excavation project. Furthermore, any initial rapid loss of viscosity, such as that observed by Kulicke and Kniewske (1981) and Shyluk and Stow (1969), was expected to develop over the first 5-10 d after mixing.…”

The agglomeration of high molecular mass polyacrylamide in aqueous solutions

“…However, early studies 1,2 observed decreasing solution viscosities with time. A closer examination showed that PAAm solutions experienced a rapid increase in viscosity to a maximum after 24 h, before a slower viscosity decrease to a local minimum after 160 to 180 h. 3 Explanations for viscosity changes have included chain scission due to the action of residual catalyst 2,4,5 or microorganisms, 6 crosslinking, scattering studies. 10,11 Biocides have been shown to have little effect, 11 while hydrolysis and crosslinking have only been observed at elevated temperatures.…”

Synthesis, properties and performance of organic polymers employed in flocculation applications

“…Flocculation is a complex process and the flocculation efficiency is significantly dependent on a variety of factors, including but not limited to pH and ionic strength of the solution, agitation rate, particle size, charge density, molecular weight and dosage of the polymer. 10, 46,47 In addition, the mineral composition, and type and addition sequence of the flocculating agents also play an important role in successful destabilization of the colloidal particles. 9 Any attempt at comparing these flocculants against one another would be inadequate as there are many inconsistencies in flocculation performance analysis across different studies.…”