Rheological properties of sepiolite ground in acid and alkaline media

Abstract

Effects of different acids (HCl, H₂SO₄ and citric acid) on rheological properties of brown sepiolite from the Eskisehir region of Turkey have been determined. The optimum apparent viscosity value was obtained at the natural pH of the suspension. Below the natural pH, partial collapse of the structure due to the release of Mg ions causes a significant decrease in viscosity values. However, below pH 1, there is a substantial increase in viscosity values owing to gel formation. On the other hand, above the natural pH of suspension, increased amounts of OH ions lead to a decrease in viscosity values and inhibit gel formation. The reversible nature of sepiolite was tested by changing the pH of acid and alkaline treated sepiolite suspensions back to its natural pH by washing with water and acid, respectively. In summary, from a practical point of view, there is no favorable effect of acid treatment of sepiolite on its rheological properties. On the contrary, reducing the pH to natural pH after grinding in a basic environment led to an improvement in rheological properties.

Introduction

Layer structured smectite, needle structured sepiolite, and palygorskite type clays are utilized in a variety of rheological applications (Santaren, 1993). The mechanism of gel formation for each clay mineral differs because of their unique structures. The layer composition and high ion exchange capacity are the main factors dictating the extent of gel formation in montmorillonite. Activation is usually done to change the clay without deforming its structure through chemical or physical processes. As has been previously proposed by Gonzales et al. (1988), the main objective of acid activation in sepiolite is to loosen fiber bundles in order to increase the mesoporosity. Acid activation leads to the dissolution of octahedral sheets with a resultant increase in micro porosity built up within the tetrahedral silicate sheets, but the mineral skeleton consisting of silanol groups is maintained (Inukai et al., 1994, Lazarević et al., 2007).

Silanol groups are developed by the acidic hydrolysis reactions leading to an increase in both surface area and pore volume and a concomitant increase in adsorption capacity (Jimenez-López et al., 1978, Aznar et al., 1996). However, increases in acid concentrations and treatment periods cause modifications of the silanol groups resulting in a decrease in adsorption capacity (Valentin et al., 2007, Valentin et al., 2006). Corma et al. (1987) applied acid activation using HCl on sepiolite and palygorskite and stated that acid activation affected mineral outer surfaces but acid could not penetrate into the channel structure. Different types of clays responded differently indicating that the amount of inaccessible pores has a greater effect on activation than the chemical composition of each mineral. Gonzales et al. (1988) focused on the two structurally different types in palygorskites; one rich in aluminum and the other rich in magnesium. Acid activation with different acid (HCl) concentrations revealed that the highest activation occurred with the magnesium rich palygorskites. Differences in the behavior of magnesium-rich sepiolite were ascribed to the concentration of Mg in the octahedral layer, viz. the smaller size of fibers and higher outer surface area (Corma et al., 1990). Acid activation at high acid concentrations typically resulted in amorphous silicate formation and the destruction of fiber structure (Barrios et al., 1995).

Sepiolite with its unique structure has a variety of applications in which rheological properties play a significant role (Sabah, 1998, Alvarez, 1984, Galan, 1996). The needle like fibers holding the bundles of sepiolite control not only the surface characteristics but also the rheological properties (Santaren, 1993, Simonton et al., 1988). The ability of sepiolite to give high viscosity values at low solid concentrations provides a major advantage over other clays (Mart et al., 2003).

The aim of this study is to determine the rheological properties of sepiolite, to identify its flow characteristics and to understand the mechanisms for its viscosity development in the absence and presence of acid and alkaline media.