Hydrostatic pressure measurement for evaluation of particle dispersion and flocculation in slurries containing temperature responsive polymers

Abstract

The state of particle dispersion and flocculation was evaluated through hydrostatic pressure measurements in aqueous slurries of alumina with neutral and charged temperature responsive co-polymers of poly (N-isopropylacrylamide) (PNIPAM). Slurries were prepared and poured into test tubes at room temperature, and then heated in a hot water bath to a controlled temperature. The slurries were then cooled down to room temperature. The hydrostatic pressure at the bottom of the slurry was continuously measured. The effect of the fraction of charge on the polymer on the particle dispersion state was investigated by changing the ratio of acrylic acid (AA) to PNIPAM in the synthesized co-polymers. It was shown that the hydrostatic pressure of the slurry with 15% AA–PNIPAM co-polymer decreased the fastest at temperature above the polymer lower critical solution temperature (LCST), indicating that the largest flocs were formed due to a strong attractive force. However, the hydrostatic pressure of the 15% charged polymer did not recover (increase) after reducing the temperature below the LCST, revealing that the formed flocs did not re-disperse. On the other hand, the hydrostatic pressures of the slurries with 1% and 5% AA–PNIPAM co-polymer showed relatively quick decrease at temperature above the LCST and clearly recovered (increased) after changing the temperature to below LCST. The particles in these slurries can be changed from the dispersed to the flocculated state by temperature control. The hydrostatic pressure measurement has been shown to be a very useful tool to monitor how the particle dispersion and flocculation state are changed by changing the surrounding temperature for slurries with temperature responsive polymer.

Introduction

It is very important to evaluate the state of particle dispersion and flocculation in slurries in order to optimize slurry formulation in various industrial processes such as ceramic processing (Lange, 1989, Lewis, 2000) and solid–liquid separations (Sivamohan, 1990, Jarvis et al., 2005). In our previous reports (Tsubaki et al., 2003, Tsubaki and Mori, 2004, Mori et al., 2004, Mori et al., 2006, Mori and Tsubaki, 2008, Ohtsuka et al., 2011), we have developed a novel evaluation technique based on hydrostatic pressure measurement, and reported that this technique is very useful for characterization of fine particle slurries in which particles settle very slowly, and slurries containing dark-colored particles which make the slurry interface difficult to observe. In addition, it was also found that one of the causes of the mismatch between the apparent viscosity of the slurry and the packing density of the sediment is the change of particle dispersion state as a function of time in slurry, which was discovered by the use of the hydrostatic pressure measurement technique (Tsubaki et al., 2001). Hydrostatic pressure measurement can give us useful information about particle dispersion and flocculation state and monitor its dynamic change continuously.

The dynamic change of particle dispersion and flocculation state during settling is often caused by the particle interaction when decreasing the surface distance between particles. On the other hand, the particle dispersion and flocculation state can be changed by applying an external stimulus such as change in temperature. Some researchers (Sigmund et al., 2000, Balzer et al., 1999, Franks et al., 1995, Bergstrom, 1994, Graule et al., 1997) reported novel ceramic fabrication processes where a green body with high density can be easily obtained by changing the particle interaction from repulsive to attractive using responsive additives. It was also reported that a temperature responsive polymer was used to develop a core-shell particle for drug delivery systems (Medeiros et al., 2011). Recently, we used the temperature responsive polymers based on poly (N-isopropylacrylamide) (PNIPAM) to overcome the disadvantage of conventional polymer use for thickening, that is, the high water content in sediment and difficulty of further compaction (Franks, 2005, Franks et al., 2009, O'Shea et al., 2010, O'Shea et al., 2011, Li et al., 2009). The main concept of this method is that (1) heating a slurry containing PNIPAM above the lower critical solution temperature (LCST) can cause particles to flocculate by hydrophobic attraction (Burdukova et al., 2010, Burdukova et al., 2011), (2) subsequent cooling to temperature below the LCST allows the polymer to partially dissolve back into solution, and hence, the particle–particle attraction is reduced causing the particle volume fraction in the sediment to be increased. For optimizing this method, it is very important to know how to change the particle dispersion and flocculation state by changing the temperature. Therefore, the objectives of this paper are to monitor the change of the particle dispersion and flocculation state when changing the temperature continuously using the hydrostatic pressure measurement, and to obtain useful information to optimize this novel solid–liquid separation process.