This paper describes a method for producing submicron high-purity alumina. In this method amorphous basic aluminum sulfate (BAS) was prepared by homogeneous precipitation of aluminum bisulfite solution containing chitosan. This cationic polymeric compound was used as an electrosteric dispersant. Submicron BAS was derived from an aluminum bisulfite solution (0.4 M in aluminum). The BAS powder was transformed into crystalline basic ammonium aluminum carbonate (ammonium dawsonite) by treating BAS with 1 M ammonium carbonate solution at 60°C for 4 h. Alpha alumina with mean particle size of 0.42 μm was obtained by heating ammonium dawsonite at 1150°C for 1 h.
Keywords

Submicron α-Alumina Powder, New Production Method, Ammonium Dawsonite, Chitosan
Introduction

Alumina generally is one of the chemically purest ceramic powders commercially available with submicron particle size. Submicron BAS has been proposed as alpha alumina precursor because of its purity and spherical morphology [1]. Submicron BAS has been prepared by homogeneous precipitation of aluminum sulfate with urea a precipitating agent [2-4]. In another method, formamide was used to precipitate BAS at a lower pH than urea in order to obtain high purity BAS [5]. Enzymatic decomposition of urea at low temperature has been recently proposed to obtain BAS with monodispersed spherical particles [6].

Submicron alpha alumina has been prepared by heating BAS in the range 1050°C to 1300°C. When BAS was heated up to 1300°C, it decomposed evolving sulfur trioxide to the atmosphere. Furthermore, undesirable hard agglomerates were formed due to the high calcination temperature [7].

The purity of alpha alumina obtained from BAS depended on the chemical purity of the precursor aluminum salt and precipitating agents, in addition to the conditions of the precipitation process. Blendell et al. [5] obtained high purity alumina containing only sodium, iron and gallium above 10 μg/g on a cation basis. In this case, a high sodium level was present in the starting material, aluminum sulfate, and was not separated by precipitation. On the other hand, a high purity alumina was obtained by using a high-purity form of recrystallized aluminum salts (sulfates, chlorides, nitrates). When a pure recrystallized ammonium alum was calcined, a 99.99% pure, fine crystalline alumina was obtained [8]. Therefore, high purity alpha alumina can be obtained by using high purity ammonium aluminum sulfate as BAS precursor, instead of aluminum sulfate.

In this work, ammonium aluminum sulfate is used as a precursor of BAS and biopolymer “chitosan” is used as electrosteric dispersant in the precipitation process to prevent BAS agglomeration. Furthermore, BAS is transformed into crystalline basic ammonium aluminum carbonate (ammonium dawsonite) in order to eliminate the sulfate and to avoid the formation of hard agglomerates, caused by heating the precursor at high temperature. Ammonium dawsonite is obtained by treating SBA with aqueous ammonium carbonate at 60°C. The compound is characterized by X-ray diffractometry (XRD) and Fourier transform infrared (FTIR) spectroscopy.
Experimental Procedure

The BAS used in this work was prepared by precipitation in homogeneous solution by heating an aqueous solution of aluminum bisulfite (0.4 M in aluminum) and chitosan (1.0 wt%). The aluminum bisulfite solution was prepared by dissolving previously precipitated basic aluminum sulfate with sulfur dioxide in aqueous media. All chemicals used in this study were reagent-grade products from J. T. Baker. The ammonium carbonate (AC) solution was prepared by mixing appropriate amounts of ammonium bicarbonate and ammonium hydroxide so that the final concentration of both reactants in solution was 1.005 M.

The formation of crystalline basic ammonium aluminum carbonate was studied by mixing BAS with varying amounts of ammonium carbonate solution, and heating the samples at 60°C for 6 h, in a water bath. The solids were separated from the supernatant liquid via vacuum filtration. Then, the solids were dried at 70°C for 24 h prior to analysis by FTIR spectroscopy.

The crystallization rate of ammonium dawsonite was determined by pouring 1.5 g of BAS and 14 ml AC solution in an Erlenmeyer flask. The suspension was heated in a water bath at 60°C, for 1, 2, and 3 h. The solids were separated from the supernatant liquid via vacuum filtration and dried at 60°C for 24 h before analysis by FTIR spectroscopy.

The phase transformation sequence during ammonium dawsonite calcination was determined. One gram of sample powder was put in platinum crucibles and was heated at different temperatures, for 30 min. The crystalline phases of alumina in the calcined samples were determined by FTIR spectroscopy and XRD.

Alpha alumina was prepared by heating ammonium dawsonite at 1150°C, for 60 min. The powder was attrition milled for 2 h, before particle size analysis.

The solids were characterized by XRD (Model D-500, Siemens, Germany) using Ni-filtered CuKa radiation. Infrared analysis was performed using KBr pellets and the samples were run on a FTIR spectrometer (Model 1600 series FTIR, Perkin Elmer, Norwalk, Connecticut, USA). The median particle size and the particle size distribution of the powder were determined by particle size analyzer (Model LA-300, Horiba, Irvine, California, USA).