Basic aluminum sulfate can be prepared by homogeneous precipitation starting from admixtures of ammonium aluminum sulfate and ammonium bisulfite solutions. Powder agglomeration problems usually arise in wet precipitation processes mainly at high concentration of precursors. Some organic additives have been used in order to modify the physical and chemical properties of the final product during precipitation processes. In this work, the effects of cationic, anionic and neutral organic additives on the physical and chemical characteristics of basic aluminum sulfate (BAS) obtained by homogeneous precipitation were investigated. Dodecylethyl-dimethyl-ammonium bromide (DEA) and chitosan were used as cationic additives, whereas dodecyl lithium sulfate (DLS) was selected as anionic additive. Furthermore, poly(vinyl alcohol) (PVA) and poly(ethylene glycol) (PEG) were used as neutral additives. The products were characterized by Fourier transformed infrared spectroscopy (FTIR), powder x-ray diffractometry (XRD), thermal analysis (TG and DTA) and scanning electron microscopy (SEM). The chemical composition and crystallinity of the final product were not modified by the presence of the studied additives in this reaction media. The basic aluminum sulfates were amorphous, with chemical composition 2Al2O3·SO3·9 H2O. However, the morphology and agglomeration state of the final product were modified by the nature of the used additives. Anionic additive, DLS, produced irregular agglomerated particles. On the other hand, neutral additives, PVA and PEG, provided particles with low agglomeration state. The highest effect on dispersion was observed when cationic additives were used. Fine well-dispersed particles of basic aluminum sulfate were obtained by addition of chitosan during the precipitation process.
Keywords

Organic additives, agglomeration, basic aluminum sulfate
Introduction

Aluminas have been in use for many years as catalyst, adsorbents, desiccants, abrasives, plastic fillers as well as fire retardants for many chemical process industries. Aluminas for water adsorption, which are traditionally known as desiccants, were first introduced in 1932 [1]. Since long time ago, synthetic aluminas have been used in the chromatographic purification of biological compounds. Recently aluminas have found widespread usage in applications as diverse as municipal wastes treatment as well as in polymer and pharmaceutical industries [2].

Alumina chemicals have been applied to treat industrial and municipal waters [3]. On the other hand, aluminum sulfate is useful as a coagulant for removal of various metals, particles and undesirable organic compounds from industrial waste water. Alumina trihydroxide (gibbsite) and monohydrate (pseudoboehmites) also have been accepted as flocculants in drinking-water. Transition forms of aluminas have been used as adsorbents for removal of undesirable contaminants in both municipal and industrial waters.

As a result of more stringent conservation requirements large alumina adsorption processes have been developed for water treatment. Successful new environmental applications of alumina adsorbents include the removal of phosphate, mercaptan, arsenic, fluoride and colloidal silica compounds in ground and drinking water [4].

The most important source of aluminum hydroxides is the Bauxite refining plant. More than 94% world alumina production is accounted for by the Bayer process for Bauxite refining. The alumina product from the Bayer process is usually 99.5% pure with soda (Na2O) as the mayor impurity [5]. Therefore, when a pure alumina product is required, it must be obtained by another chemical process that uses pure aluminum salts as a raw material.

The interest to find alternative routes for the preparation of ceramic powders that avoid the inconveniences of the traditional techniques has been growing. For this reason, the development of new synthetic routes is required that allow to obtain ceramic powders with specific properties, such as small particle size, spherical in shape, narrow particle size distribution, absence of agglomerates and high chemical purity.

One of the most interesting recent developments in the preparation of aluminum hydroxide and alumina has been the introduction of BAS as monosized alumina precursors with uniform and controllable morphology. Matijevic [6] originally produced particles that contained an appreciable amount of sulfate but this contaminant could be exchanged for hydroxide species, in much the same manner as the one described by Gordon et al. [7], converting the BAS to hydrous aluminum oxide.

Other researchers have also used basic aluminum salts as high purity alumina precursors. Cornilsen and Reed studied amorphous basic aluminum succinate and BAS as potential precursors [8]. Whereas Sacks et al. [9] and Blendell et al. [10] used spherical BAS as the specific starting material for conversion to alumina.

The BAS studied by the above mentioned researchers, were obtained by homogeneous precipitation of BAS in aqueous medium. In this case, agglomeration problems of the BAS particles generally arise, especially when the precipitation process is performed at high concentrations of aluminum salt. Therefore, under this condition it is difficult to obtain monosized and non-agglomerated particles.

The use of low aluminum concentration solutions in the precipitation process allows the preparation of non-agglomerated BAS particles. However, at these low alumina concentrations the amount of product obtained is too low, from a practical point of view.

In one alternative process to prevent agglomeration, organic compounds are added to the precipitation medium. Generally, the preparation of mono-dispersed alumina hydroxide by sol-gel process was achieved, using high alumina concentration and hydroxyl-propylcellulose as a steric agent to prevent agglomeration.

In this work, the effects of cationic, anionic and neutral organic additives on the physical and chemical characteristics of BAS obtained by homogeneous precipitation were investigated. DEA and chitosan were used as cationic additives, whereas DLS was selected as anionic additive. Furthermore, PVA and PEG were used as neutral additives. The products were characterized by XRD, TA (DTA and TG), FTIR and SEM.