Formation of spinel from hydrotalcite-like minerals and destruction of chromite implanted by inorganic salts

Abstract

Thermal decomposition of natural hydrotalcites (HT) and the role of hydrotalcite-like materials in the destruction of solid with a spinel-type structure, after grinding with solid phase Na₂CO₃ additives, are investigated by means of infrared (IR) spectroscopy and X-ray powder diffraction (XRD) methods. The thermal treatment of hydrotalcite results in intermediates with contents and structures, which essentially depends on impurities in natural samples, conditions of treatment, as well as the environment. In this study, chromite implanted by inorganic salts is used to illustrate the influence of the mechanical activation of the process to convert spinel to hydrotalcite-like structures. It has been found that the small concentration (4 wt.%) of solid-phase admixture, implanted in the layers near the surface, is favorable for enhancement of the structural changes.

Introduction

Hydrotalcite (HT), Mg₆Al₂(OH)₁₆CO₃·4H₂O is a hydrated hydroxycarbonate of magnesium and aluminum with a modified brucite-like structure in which the cations occupy the octahedral position Allman and Jepsen, 1969, Dritz et al., 1987, Arakcheeva et al., 1996. Hydrotalcite-like materials, which can be represented by the general formula:

$$\text{[M}{}_{\mathrm{1-x}}{}^{\mathrm{2+}}\text{M}{}_{\mathrm{x}}{}^{\mathrm{3+}}\text{(}\text{OH)}{}_2\text{]}{}^{\mathrm{x+}}\text{A}{}_{\mathrm{x/n}}{}^{\mathrm{n-}}\text{m}\text{H}{}_2\text{O}\text{,}$$

where M²⁺=Mg²⁺, Ni²⁺, Zn²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Fe²⁺, Co²⁺, Cd²⁺, Cu²⁺, Mn²⁺; M³⁺=Al³⁺, Fe³⁺, Cr³⁺, Ga³⁺; x is atomic ratio of M³⁺/(M²⁺+M³⁺); Aⁿ⁻=F⁻, Cl⁻, ClO₄⁻, OH⁻, NO₃⁻, CO₃²⁻, SO₄²⁻, organic or complex anion, form a large group of natural and synthetic clay minerals Dritz et al., 1987, Aramendia et al., 1999, Hansen and Poulsen, 1999, Hernandes-Moreno et al., 1985, Titulaer, 1993, Bail et al., 1987, Kloprogge and Frost, 1999a, Kloprogge and Frost, 1999b. A charge deficiency in the positively charged layer of di- and trivalent metal hydro-oxides is compensated by hydrated anions in the interlayer region. Hydrotalcites find applications as antacid or antipeptin in pharmaceutical chemistry, as a catalyst support, flame retardant, absorbing agent and anion exchanger, stabilizers for polymers, separator of optical isomers, etc. The catalytic significance of these layered double hydroxides lies in the ease with which the composition of the cations and anions can be controlled.

In recent years, the hydrotalcite–spinel conversion has been widely discussed Mackenzie et al., 1993, Hibino and Tsunashima, 1997, Valcheva-Traykova et al., 1993, Kloprogge and Frost, 1999a, Kloprogge and Frost, 1999b. It is known that natural hydrotalcite (as well as phlogopite, pargasite, amesite) is a product of the magnesian skarn transformation, during which spinel and fassaite play a role as a source of Al₂O₃ Kolesnikova, 1980, Shabinin, 1973, Strubel and Zimmer, 1982. According to the last authors, hydrotalcite is a product of spinel transformation (Strubel and Zimmer, 1982). On the other hand, the hydrotalcite thermal decomposition may result in the spinel formation Mackenzie et al., 1993, Hibino et al., 1995, Hibino and Tsunashima, 1997, Hibino and Tsunashima, 1998, Valcheva-Traykova et al., 1993, Braithwaite et al., 1994, Kloprogge and Frost, 1999a, Kloprogge and Frost, 1999b, Aramendia et al., 1999. Such a process was detected by Mackenzie et al. (1993) at 1200°C for hydrotalcite synthesised with a Al/(Al+Mg) ratio of ≈0.36. For hydrotalcite with a ratio Al/(Al+Mg) of ≈0.33, this temperature was above 800°C (Hibino et al., 1995). Calcination at 800°C leads to samples with powder X-ray diffraction peaks due to ZnO and a spinel (Barriga et al., 1999). Infrared emission spectroscopy indicate spinel, MgAl₂O₄, formation at 350–400°C (Al/(Al+Mg)=0.25), CoAl₂O₄ at 300–350°C for Al/(Co+Al)=0.25 (Kloprogge and Frost, 1999a,b). The transformation of reconstructed Mg–Al–CO₃ hydrotalcites with only 55–70% of carbonate anions required by stoichiometry to spinel at 400°C is the result of a reaction occurring between the edges of crystallites Hibino and Tsunashima, 1997, Hibino and Tsunashima, 1998. After thermal decomposition of Mg–Al hydrotalcites, MgO, and Al₂O₃ (various modifications) are also observed depending on the calcination temperature, the initial hydrotalcite content, impurities and by-products in samples (Valcheva-Traykova et al., 1993). “Memory effect” of hydrotalcite, which allows the original material to reconstitute upon contact with water or water vapor, disappears for T>400°C (Sychev et al., 2001).

In the present work, the hydrotalcite-like mineral–spinel structure transformation has been studied for minerals from Ural, Angara-Ilim and Khabarovsk region (East Siberia, Russia). For reference, the transformation of the spinel-like mineral chromite was included in this study. In addition to calcination, the effects of other factors, such as chemical and mechanochemical treatment are also considered.