Flocculation of hydrophobic and hydrophilic colloidal particles in near-critical pyrrolidone-water mixtures

The unusual solution behavior of a series of N-alkyl-2-pyrrolidones has been exploited for efficient flocculation of hydrophobic and hydrophilic particles in aqueous dispersions. These pyrrolidones are surface-active low vapor pressure solvents of increasing industrial interest. The lower alkyl chain length pyrrolidones are completely miscible with water but aqueous solutions of higher alkyl chain length exhibit lower consolute temperatures (LCT), some close to ambient temperatures. It is shown here that pyrrolidones at concentrations and temperatures close to phase separation are outstanding reversible flocculants for graphite and coal suspensions. Flocculation with these surfactant solvents can be more effective than commercial polymeric flocculants and is reversible by manipulating the phase behavior. Furthermore, flocculation by alkyl pyrrolidones results in bigger and more compact flocs than those formed with high molecular weight polymers. In some instances, the dispersed colloidal particles were compacted into a single floc with ∼35% solid content. The flocculation efficiency of these compounds correlates with their phase behavior and is discussed in detail.