Precipitation of Nanoscale Hydrous Alumina from Sodium Aluminate Solutions
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Precipitation of Nanoscale Hydrous Alumina from Sodium Aluminate Solutions Biman Das, Mats Graeffe2, Alicia Toscano3, Chris Brancewicz3, Don H. Rasmussen3 Department of Physics, State University of New York, Potsdam, New York, 13676 2 Department of Physical Chemistry, Åbo Akademi, Finland 3 Department of Chemical Engineering, Clarkson University, Potsdam, New York, 13699 ABSTRACT Sodium aluminate liquor was diluted and neutralized with water and oxalic acid in the presence of surface-active polymers polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA) and sodium carboxymethylcellulose (NaCMC). Dynamic light scattering experiments showed that the final precipitate had diameters between 80 – 300 nm when precipitated in the presence of PVP, and 600 – 800 nm when precipitated in the presence of NaCMC. The initial sols prepared using oxalic acid without surfactant had diameters of 200 – 300 nm. Scanning Electron Microscopy, SEM, Energy Dispersive Analysis by X-rays, EDAX, and X-ray diffraction were used for imaging, chemical and structural analysis of the final dialyzed particles. Transmission Electron Microcopy, TEM, images of the sols trapped and dried between nitrocellulose membranes showed that the initial particles were nearly spherical and bimodal in particle size. Larger particles had diameter of about 176 nm but were not pure solid alumina. A small fraction of initial particles had diameters much smaller than 176 nm. The discrepancies between initial and final precipitates indicate a complex precipitation pathway.
INTRODUCTION Aluminum trihydroxide, hydrous alumina, is odorless, non-carcinogenic and is widely used in industry such as an extender and filler in polymeric systems and as a primary ingredient in a variety of ceramic materials. The most common crystalline forms of hydrous alumina are Gibbsite, Bayerite and Nordstrandite. The hydrous alumina particles that are used in industry for a desired application must be manufactured in a cost-effective manner, be of sufficient purity and exhibit desirable properties. The Bayer process, [1] that is used for the manufacture of hydrous alumina for industrial applications, can be a source of nanoscale hydrous alumina. This work modifies the precipitation pathway of the crude hydrous alumina with the goal of generating nanoscale particles directly from sodium aluminate solutions (Bayer liquors). Various factors (hydrolysis/neutralization pathways, surfactant additives and processing parameters) are being evaluated to achieve precipitation of nanoscale hydrous aluminas and sodium aluminate ionomeric gels [2]. Precipitation of several forms of alumnium trihydroxide and AlOOH (depending upon temperature) are possible from rapidly diluted and/or neutralized NaOH – Al(OH)3 solutions. The size and structure of the particles formed depends on temperature, W6.20.1
supersaturation, and rate of precipitation, on the presence or absence of surface-active species in the dilution medium and on the concentration of residual sodium ion. Extensive research has focused [3] on unders
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