Controlling Pitch Substances Of Paper Furnish By Magnesium Aluminum Hydroxide

Magnesium aluminum hydroxide is a typical layered double hydroxides (Layered Double Hydroxides, referred to as LDH). A series of magnesium aluminum hydroxides with high crystallinity were prepared via a coprecipitation method at a constant pH of 10.5~11 by varing the molar ratios of magnesium to aluminum as well as interlayer anions, and then transformed into calcined hydrotalcite ( layered double oxides, LDO) by thermal treatment at different temperaures. The magnesium aluminum hydroxides and their calcined products were used for removing pitch substances in pulp furnishes. Laser particle analyzer, x-ray diffraction (XRD) technique and high-resolution transmission electron microscope (HRTEM) were used to characterize the particle size distribution, crystal structure, and micromorphology of the samples.The results show that magnesium aluminum hydroxide colloids have higher positive charge in neutral medium. Most of the samples are complete crystals with hexagonal structure, and the particle size distribution is narrow, and most of the particles have the typical particle size of 0.1μm ~ 1μm. After calcination, the original layered structure of the magnesium aluminum hydroxides disappear and become the magnesium-aluminum composite oxides. The LDOs have larger surface area, higher charge density and the particle size are more uniform than the magnesium aluminum hydroxides.Magnesium aluminum hydroxides show higher adsorption capacity to the model pitch and stickies at neutral pH. The double hydroxides of nitrate form bear higher adsorption capacity to model pitch and model stickies than that of other forms. Increasing the calcining temperature of magnesium aluminum hydroxides from 100 oC to 550 oC gradualy improve their adsorption capacity to model pitch and stickies and reduce the discrepancy of the adsorpion capacity between the magnesium aluminum hydroxides of nitrate form and that of carbonate form. After adsorbing model contaminants, the LDO can be recovered by re-calcination and reused for adsorption. It is a kind of environment-friendly adsorption material. The XRD and HRTEM analysis indicate the recovery of the layered structure by re-hydration of LDO, in other words, the "memory effect", which is the main the reason of the adsorption capacity of LDO more than its precursor magnesium aluminum hydroxide. The experimental isothermal adsorption data of model pitch and stickies on LDO fit well to Freundlich isotherm model and Dubinin–Radushkevish isotherm equation, and the adsorption kinetics accord with the pseudo second order kinetic equation. Using the charge titration technique to establish the model compounds (polygalacturonic acid and lignin sulfonate) of DCS in high yield pulp adsorption on LDO, and the results show that LDO had an excellent performance in adsorbing anionic model compounds.The turbidity, cationic demand and conductivity of DCS water of CTMP can be reduced largely when magnisum alyminum hydrodixe and its calcined product are employed as adsobants. While the calcined magnisum alyminum hydrodixe may eliminate the DCS completely as its addition level is 0.4%. The DCS removing effect of calcined magnesium aluminum hydroxide is also rather high when CTMP is in present in DCS water. Therefore, Using the calcined hydrotalcite to control the pitch substances of paper furnishes has good application prospects.