EFFECT OF MULTIVALENT SALTS CALCIUM AND ALUMINUM ON THE FLOCCULATION OF KAOLIN SUSPENSION WITH ANIONIC POLYACRYLAMIDE

The effects of calcium and aluminum ions on the interaction behavior o f anionic polyacrylamide (anionic PAM or HPAM) and kaolin are importan t in determining the efficient flocculation of kaolin. While kaolin su rfaces are negatively charged in media such as water, they exhibit the positive surface charge characteristic at pH below 3.2 in the solutio n of calcium cation, and at pH below 9.2 in the solution of aluminum c ation. The experimental results show that both calcium and aluminum io ns suppress the kaolin flocculation process. The results are analyzed and explained by molecular orbital theory, solution chemistry, infrare d spectra, and electronic probe examination of kaolin treated with and without anionic PAM. Analysis results indicate that the specific adso rption of Ca2+, Al3+ and their hydroxo complexes on anionic PAM causes the loss of anionic character in the low pH range and leads to a low flocculation efficiency. In the high pH range, the poor kaolin floccul ation can be attributed to precipitation of calcium and aluminum hydro xides on active functional groups, which inhibits the hydrogen bonding between anionic PAM and kaolin surfaces. At neutral pH values, the tr ivalent aluminum ion has more significant adverse effect on the kaolin flocculation than the divalent calcium ion. (C) 1994 Academic Press, Inc.