The link between the strength of filler flocs formed with retention aids and retention shear sensitivity

The Dynamic Drainage Jar and related laboratory retention tests are routinely used to evaluate the shear sensitivity of retention aids based on the assumption that retention aids giving stronger bonds are more shear resistant and thus give better retention. For the first time, the strength of individual precipitated calcium carbonate (PCC) flocs formed with retention aids was measured by a microscopic tensile test and the results were compared with conventional laboratory retention and flocculation tests. PCC flocs with a variety of properties were prepared from two families of retention aids both based on polyethylene oxide -In one case a commercial phenolic cofactor (ACS) was used with PEO whereas in the second case an experimental copolymer, poly(vinyl phenol-co-sodium acrylate) (MVA) was employed. PCC flocs about 15 μm in diameter required about 100 nN to rupture the flocs and the ACS flocs were stronger than the MVA flocs. On the other hand, the MVA flocs showed roughly twice the elongation at break (∼800%) than did ACS. Comparison of individual floc strength results with laboratory flocculation tests revealed that the strongest flocs did indeed give the highest first-pass retention values. On the other hand, subtle effects on floc strength such as the ratio of cofactor to PEO were not reflected in the laboratory flocculation tests. A range of floc properties was achieved by changing the type and concentration of cofactors. Highly elastic flocs were less capable of surviving turbulent mixing than were stronger but more brittle flocs.