Abstract
A new method of Na-activation of raw bentonite, rich in Ca-montmorillonite, consisting of combined thermal treatment at 200 °C, followed by immediate impregnation with aqueous solution of Na2CO3 of concentration corresponding to 0.5, 1.0, 1.5, or 2.0 cation exchange capacity (CEC) of clay, was investigated. Structural and compositional evolution of the activated solids after 1, 2, 3, and 4 weeks of storage was monitored by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). XRD analysis indicated that within the investigated period of ageing transformation to Na-rich montmorillonite required Na2CO3 concentration of at least 1.0 CEC. FTIR spectra showed that, depending on the Na2CO3 concentration and ageing time, formation of Na-rich montmorillonite was accompanied by precipitation of poorly crystalline calcite, amorphous calcium carbonate, gaylussite (a double calcium-sodium carbonate), and portlandite (Ca(OH)2).
Highlights
The discovery was prompted by the observation that only bentonites rich in Na-smectites were suitable for the use as binders in foundry moulding sands, while bentonites containing Ca-smectites proved inadequate for this purpose [4]
Na-rich bentonites are characterized by higher cation exchange capacity, higher swelling properties and better dispersiveness in water than Caforms
The TG/DTG/DTA curves of the studied Ca-rich bentonite, shown in Figure 1, are similar to those reported in the literature for Ca-montmorillonites or Ca-rich bentonites [27,28,29,30]
Summary
Bentonites occupy a special position among clay raw materials due to the unique properties of smectites, their main mineral component. Most common smectites found in bentonites are calcium and/or sodium montmorillonites. Partial substitution of Al3+ oct with Mg2+ generates a negative layer charge, compensated by Ca2+ and/or Na+ cations located in the interlayer. The layer charge of 0.2–0.6 e per half unit cell determines the cation exchange capacity, swellability, and viscosity of bentonite [1], the features of key importance for its use in manufacturing of adsorbents, binders, drilling muds, catalysts, materials for civil engineering, etc. Sodium and calcium montmorillonites display significantly different properties. Na-rich bentonites are characterized by higher cation exchange capacity, higher swelling properties and better dispersiveness in water than Caforms. The patent from 1935, based on invention by German scientists Hofmann and Endell, described the method of transforming Ca-rich bentonites into Na-forms by treatment with
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