Flocculation kinetics of an ochre suspension in salt(NaCl)-containing aqueous media in the presence of anionic and cationic acrylamide copolymers

Abstract

The sedimentation kinetics of an ochre suspension in salt (NaCl)-containing aqueous media was studied in the presence of ionogenic (anionic, A, and cationic, C) acrylamide copolymers with high molecular weight (M > 2 × 10⁶) using a VT–0.5 torsion balance. The ionic strength of the dispersion medium varied in the wide range from 0.001 N to 0.4 N. The flocculation proceeded predominantly by a `bridge' mechanism, and the fraction of macromolecules inactive in the acts of floccule formation was significantly higher for C copolymer as compared with A copolymer. A drastic fall in the flocculating activities of A and C copolymers when passing from salt-free to salt-containing media is caused mainly by two following events:

1. The change in the conformational state of macromolecules, primarily, in their effective dimensions

2. The participation of a certain part of electrolyte in the formation and modification of an electrical double layer around disperse phase particles

After introducing binary compositions of A and C flocculants into salt-containing media their resultant flocculating effect depends on the introduction mode of polymeric components. A strong difference in the magnitudes of the flocculating effect for A and C copolymers is observed in water. In the region of high ionic strengths (0.1–0.4 N) this difference becomes far less distinct. The flocculating activities of A and C copolymers were compared when introduced as the first (λ_A and λ_C) and the second (λ_A ^′ and λ_C ^′) additives. It was shown that λ_A/λ_A ^′>1 and λ_C/λ_C ^′>1.

Such relationship between λ_A and λ_A ^′, λ_C and λ_C ^′ indicates that selective interactions between A and C copolymers play an essential role in the flocculation processes. The last statement was indirectly confirmed in the present work by the data of electrochemical and viscosimetric studies. When using C copolymer as the second additive in the region of low ionic strengths its main function undergoes reversal, and the copolymer begins to operate not as a flocculant, but as a stabilizer of disperse phase particles (λ_C< 0).