INFLUENCE OF ADDITION OF A POLYELECTROLYTE, NONIONIC POLYMERS, AND THEIR MIXTURES ON THE RHEOLOGY OF COAL WATER SUSPENSIONS/

The effect of addition of a polyelectrolyte, a nonionic surfactant; AB A block copolymers, and mixtures of the polyelectrolyte with the block copolymers on the theology of coal/water suspensions was studied usin g steady state and oscillatory theological measurements. The polyelect rolyte was sodium lignosulfonate (Ufoxane 3A). The nonionic surfactant (EL1602P) was hexamethylenediamine with 4 tails of 10 propylene oxide (PO) and 55 ethylene oxide (EO) units. The block copolymers (Synperon ic PE) consisted of 55 PO units and two tails of 4-147 EO units per ch ain. The addition of the polyelectrolyte caused a rapid reduction in t he complex modulus (G) above similar to 0.1% on the basis of the coal . This indicated deflocculation of the suspension above this concentra tion. This deflocculation was caused by high adsorption of the polyele ctrolyte that was accompanied by an increase in the negative zeta pote ntial. The deflocculation was also reflected in the sedimentation beha vior of the suspension which showed a decrease in sediment volume at t he same concentration at which deflocculation became substantial. The results obtained using the nonionic surfactant were significantly diff erent from those using the polyelectrolyte. They showed an initial inc rease in the modulus, yield value, and viscosity as the concentration of the surfactant was increased. A maximum was reached at a critical c oncentration, above which there was a rapid reduction in G and viscos ity. These results were explained in terms of the adsorption character istics of the surfactant. Initially, the surfactant adsorbs with the P EO chains pointing toward the solid, and this causes flocculation by h ydrophobic interaction. At higher surfactant concentration, adsorption occurs via the hydrophobic groups, leaving the PEO tails dangling in solution, and this leads to restabilization. The PEO-PPO-PEO block cop olymers showed a gradual decrease in flocculation with an increase in PEO chain length. This was attributed to the increase in adsorbed laye r thickness with an increase in PEO chain length. Energy-distance curv es show an attractive minimum whose depth becomes smaller as the adsor bed layer thickness increases. Addition of the PEO-PPO-PEO block copol ymers to coal suspensions stabilized by the polyelectrolyte showed a s mall effect when the PEO chain length was <37 units. However, with the largest PEO chain studied (147 units per chain), the addition of the block copolymer caused an initial increase in flocculation (accompanie d by a rapid increase in G), reaching a maximum at an optimum concent ration, after which there was restabilization of the suspension. These results were explained in terms of the orientation of the molecule. I nitially, the molecule probably adsorbs with the PEO chain toward the surface (on the hydrophilic batches), resulting in flocculation by scr eening the charge and possible hydrophobic interaction. At higher conc entration a second layer is produced with the PEO chains dangling in s olution, resulting in restabilization of the suspension. These mixture s of polyelectrolytes and nonionic block. copolymers may find applicat ion in the preparation of stable coal/water suspensions.