Flocculation Induced By Magnesium Aluminum Hydroxides Based Microparticle Retention Systems

Magnesium aluminum hydroxide (MAH), a mixed metal hydroxide, is the most common hydrotalcite–like compound. In experiment, positively charged magnesium aluminum hydroxide was synthesized by co-precipitation of magnesium chloride and aluminum chloride with dilute alkaline solution. It was characterized by TEM, particle analyzer and zeta potential tester. Then, the retention and flocculation effects of the colloidal magnesium aluminum hydroxide as a single coagulant and cationic microparticulate component with cationic polyacrylamide (CPAM), anionic polyacrylamide(APAM) or cationic/anionic polyacrylamide complexes on fibers and fillers were investigated by means of Dynamic Drainage Jar(DDJ) and photometric dispersion analyzer (PDA2000), respectively. The retention mechanisms and flocculation characters of magnesium aluminum hydroxide based systems were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).The results indicate that the magnesium aluminum hydroxide (MAH) particles are complete crystals with hexagonal structure. The size of largest particle is 200nm. The average particle size is 100nm. The zeta potential of MAH colloid is increased gradually with the increase of peptization time. Consequently, its retention effects on stocks and flocculation effects on fillers are improved significantly with peptization time. In addition, appropriate shearing force may improve the dispersibility of MAH and in turn to enhance the retention of fiber and the flocculation of filler. However, MAH itself causes a weak flocculation of kaolin clay and shows a little retention effect on paper stocks. The kaolin clay is flocculated by bridging large kaolin clay particles with small loosely connected MAH particles while the talc powders are flocculated by the charge neutralization with MAH.MAH shows good synergistic retention effects with APAM on both stock retention and filler flocculation. Higher amount of MAH needs more APAM to achieve the best synergistic retention/flocculation effect. The flocculation of both stocks and fillers induced by MAH/APAM shows certain shear resistance. Higher stirring speed after MAH is immediately added, often results in higher final stock retention.CPAM/MAH system displays good retention effects on paper stocks. The addition sequence of CPAM followed by MAH commonly shows higher retention effects on paper stocks, and displays higher retention efficiency under neutral/alkaline pH. For the flocculation of kaolin clay, both addition sequences of MAH followed by CPAM and CPAM followed by MAH show significant synergistic flocculation effects. However, the addition sequence of MAH followed by CPAM may achieve its biggest flocculation effects on kaolin clay at lower addition levels of MAH. The flocculation mechanisms of kaolin clay caused by the different addition sequences of MAH and CPAM are different. When CPAM is first added to the clay suspensions, the clay particles flocculate to form dense flocs with the addition of MAH. When MAH is added to clay suspensions followed by CPAM, the clay particles are loosely connected together.In the system of the MAH with APAM/CPAM complexes, increasing the mass ratio of APAM/CPAM will enhance the first pass retention of wheat straw pulp. The APAM/CPAM complexes with cationic charges induce stronger flocculation than that with anionic charges. However, the MAH only shows very limited synergistic retention effect with APAM/CPAM complexes of higher CPAM/APAM mass ratios.