Flocculation of homoionic sodium palygorskite, palygorskite-montmorillonite mixtures and palygorskite containing soil clays

Flocculation of palygorskite has not been studied in detail. We have investigated flocculation of reference palygorskites, palygorskite-montmorillonite mixtures, and palygorskite containing Soil clays. The flocculation values (FVs) of the days were measured in a series of test tubes according to van Olphen (1977). The flocculation of the clays was determined visually by settling after 24 h of standing. Very dilute clay suspensions (0.066%) were taken for the tests. The FV of palygorskite at near neutral pH was significantly lower than that of montmorillonite. The FVs of four Na-palygorskites were in the range from 0.2 to 2.5 meq/L NaCl and the FV of Na-montmorillonite from Wyoming was 13.3 meq/L NaCl at pH ~ 7. FVs of both palygorskite and montmorillonite increased with pH. The effect of pH on FV of palygorskite was more significant than that of montmorillonite. At pH ~ 10.5, the FVs of all palygorskite samples were 25.0 meq/L NaCl, and the FV of the mont, morillonite sample was 50.0 meq/L NaCl. Settling of clay particles was affected by concentration of clay in suspension. Settling was clearly observed in test tubes at a clay concentration of 0.066%. At a clay concentration of 0.666%, however, settling was not observed. The apparent contradiction between low FVs of palygorskite suspensions, obtained in the present study, and low rheological susceptibility of palygorskite muds to salts, known from industrial experience, can be explained by differences in clay concentration used for FV measurements and for industrial applications. FVs of palygorskite-montmorillonite mixtures at pH ~ 7 increase with montmorillonite percentage in the mixture. At a specific concentration of montmorillonite in the mixture, the FV of the system attains the FV of pure montmorillonite and does not change with further montmorillonite addition. This montmorillonite concentration varied among the palygorskite samples and was in the range of 40 to 60%. FVs of three smectite- palygorskite- kaolinite-containing soil clays (from which carbonates, iron oxides and organic matter had been removed) at near neutral pH values were the same as FV of reference montmorillonite.