Interfacial chemistry, particle interactions and improved dewatering behaviour of smectite clay dispersions

In this work, the influence of hydrolysable metal ions, Mn(II) and Ca(II), on the interfacial chemistry, particle interactions and dewatering behaviour of smectite clay dispersions has been investigated at the pH range 3-12. Metal ion specific adsorption, accompanied by a marked reduction in particle zeta potential, was observed to have a profound influence on the colloid stability, rheology and dewatering behaviour of unflocculated and flocculated smectite dispersions. The performance of anionic polyacrylamide-acrylate copolymer (PAM) used for flocculation strongly depended upon the initial degree of dispersion, the metal ion type and its concentration and the pulp pH. Prior addition of Mn(II) ion species significantly enhanced flocculant adsorption at pH 7.5 but not at higher pH (e.g. 10.5). A similar improved flocculation effect was observed in the presence of Ca(II) ions at pH 10.5 than at lower pH (e.g. 7.5). This observation is consistent with metal ion hydrolysis and specific adsorption occurring at these pH values and had a significant impact on slurry settling rate and extent of consolidation. An optimum flocculation and dewatering conditions of 0.05 M metal ion and flocculant dosage of 500 g/ton solid which induced partial (∼50%) rather than complete particle surface coverage by a monolayer, was determined. The flocculant adsorption mechanism appears to involve hydrogen bonding between the amide functional group (-CONH2) of the polymer and the hydroxyl groups of the smectite particle surface as shown by spectroscopic analysis. Metal ion and flocculant adsorption, which synergistically acted to reduce the electrokinetic zeta potential and the high yield stress between particles, produced faster clarification rates and greater dispersion consolidation. The results demonstrate clear links existing between surface chemistry, particle interactions and pulp dewatering and enable some of the physicochemical treatment issues towards improving smectite clay tails dewatering behaviour, beyond what is currently observed in plant practice, to be addressed.