Effect of the degree of hydrolysis on the chemical activity of polyacrylamide flocculant

Abstract

The use of hydrolyzed polyacrylamide (HPAA) to intensify clarification of suspensions greatly reduces the consumption of flocculant in comparison with unhydrolyzed polyacrylamide (PAA) with a simultaneous improvemerit in the clarification rate and purity [1][2][3], but in some cases the effect obtained by means of HPAA is either negligibly small or zero [4]. The optimal degree of hydrolysis of PAA, determined by various procedures, ranges from 20 to 50~There is as yet no agreed theory of the mechanism of attachment of PAA molecules to solid particles or of its flocculating effect during clarification of suspensions.Hydrolyzed PAA is a polyelectrolyte. A general characteristic of electrolytes is their dissociation constant, the value of which governs their chemical activity in solution. The value of this constant enables one to assess the state of an electrolyte in an equilibrium system and to determine both the possibility of effecting the process in question and its direction.Literature reports in this field deal mainly with the conditions of use and the degree of hydrolysis of PAA, data on its chemical activity being scanty. Thus one can find the following values given for the dissociation constant of the hydrolyzed part of PAA (carboxylic acid) :2.9-10 -.5 for PAA with a degree of hydrolysis 20.6~ [5], 3.8.10 -s for PAA with a degree of hydrolysis 12.8~ [6], and 1.35-10 -s for polyacrylic acid (100~ hydrolysis of PAA) [7].The hydrolyzed part in HPAA is present as carboxyl groups (carboxylic acid), or as the salt of weak carboxylic acid with a weak (NH +) or strong (Na +, Ca 2+) base, markedly hydrolyzed in aqueous solution. During hydrolysis of PAA a change is observed in the proportion of amide groups and the hydrolyzed part, i.e., in the composition.When a mixture of solntions of a strong acid (HCI) and commercial PAA with various different degrees of hydrolysis was titrated with a strong base (NaOH), we noted [8] that if the degree of hydrolysis is low, differential titration of the strong acid from the hydrolyzed part of PAA in H form cannot be effected: at the equivalence point for titration of the hydrolyzed part of PAA in H form, with decreasing degree of hydrolysis of PAA the inflection and the pH jump on the potentiometric titration curves increase. These characteristics are manifested only in those cases when the dissociation constant of the carboxyl groups ofHPAA varieswith its degree of hydrolysis.The variation of the dissociation constants of the hydrolyzed part of HPAA with the degree of hydrolysis was determined on weakly hydrolyzed ammoniacal polyacrylamide flocculant of the Leninsk-Kuznetsk Semicoking Factory. Specimens of HPAA with high degrees of hydrolysis were obtained by artificial hydrolysis of this flocculant in 1 ~ solutions with different NaOH contents. All the HPAA specimens (1 ~/~ solution) were purified by the procedure in [9]. The content of the polymer and the hydrolyzed part were determined in solutions of these purified HPAA specimens [6]. The dissociation constant Kk of the ca...